RLmmm<* \W 9 V<<, * Division of Agricultural Sciences s .',v >* Op ~V» .t^ f Of \^\ * i * 1 fc*«5 aisr >!;*!>** : >y >: a**** '% -v » 2 ! •" wAv i**r w ' ?:s 1 Thd U.C. System for Producing Healthy- ^ Container- Grown Plants UNIVERSITY OF CALIFORNIA Edited by KENNETH F. BAKER * . *> «!*«> I CAlfft l:xp I ■ x t e n $ i o n Service T ». U.C.D. LIBRARY ► - r 1 • . w The U. C. System for Producing Healthy Container-Grown Plants through the Use of Clean Soil, Clean Stock, and Sanitation Edited by KENNETH F. BAKER PHILIP A. CHANDLER, RICHARD D, DURBIN, JOHN FERGUSON, J. W. HUFFMAN, O. A. MATKIN, DONALD E. MUNNECKE, CHESTER N. ROISTACHER, WARREN R. SCHOONOVER, and R. H. SCIARONI UNIVERSITY OF CALIFORNIA DIVISION OF AGRICULTURAL SCIENCES AGRICULTURAL EXPERIMENT STATION— EXTENSION SERVICE THIS MANUAL is one of a series published by the University of California College of Agriculture and sold for a charge which is based upon returning only a portion of the production cost. I>\ this means it is possible to make available publications which, due to relatively high cost of production, or limited audience, would otherwise be beyond the scope of the College publishing program. CONTENTS Page 1. The U. C. System: a General Summary, by Kenneth F. Baker 3 The problems; the answers; the future. 2. Today's Nursery Problems, by Kenneth F. Baker 28 The industry; plant diseases; adopting the U. C. system. 3. Damping-off and Related Diseases, by Kenneth F. Baker 34 Dynamics and prevention of nursery diseases. 4. The Salinity Problem in Nurseries, by Warren R. Schoonover and R. H. Sciaroni 52 Nature; plant injury; cause; prevention. 5. The U. C.-Type Soil Mixes, by O. A. Matkin and Philip A. Chandler 68 Preparation and uses of soil mixes and fertilizers. 6. Components and Development of Mixes, by O. A. Matkin, Philip A. Chandler, and Kenneth F. Baker 86 Selecting physical and chemical components; history. 7. Nitrogen in Nursery Soils, by O. A. Matkin and Philip A. Chandler 108 Types and utilization of nitrogen sources. 8. Heat Treatment of Soil, by Kenneth F. Baker and Chester N. Roistacher 123 Benefits; practical procedures; cost. 9. Principles of Heat Treatment of Soil, by Kenneth F. Baker and Chester N. Roistacher. ... 138 How soil is heated; factors in equipment design. 10. Equipment for Heat Treatment of Soil, by Kenneth F. Baker and Chester N. Roistacher. . 162 Selecting equipment; steam generators; fuel. 1 1. Chemical Treatment of Nursery Soils, by Donald E. Munnecke 197 Materials and methods; soil drenches. 12. Treatment of Nursery Containers, by Kenneth F. Baker, Chester N. Roistacher, and Philip A. Chandler 210 Heat; chemicals; self-disinfesting containers. 13. Development and Maintenance of Healthy Planting Stock, by Kenneth F. Baker and Philip A. Chandler 217 Significance; obtaining and maintaining stock. 14. Beneficial Soil Microorganisms, by John Ferguson 237 Types; activities; controlled colonization. 15. Importance of Variation and Quantity of Pathogens, by Richard D. Durbin 255 Variability; inoculum potential; longevity in soil. 16. Grower Experience with the U. C. System, by R. H. Sciaroni and J. W. Huffman 263 Report on 12 examples of 7 types of nurseries. 17. Mechanization and the U. C. System, by J. W. Huffman and R. H. Sciaroni 271 Steps in production; flow diagrams. Appendix 285 References; glossary; weights and measures; sources of equipment, materials, fungicides, and chemicals. Index 307 THE U. C. SYSTEM THE AUTHORS Kenneth F. Baker, Professor of Plant Pathology, and Plant Pathologist in the Experiment Station, Los Angeles. Philip A. Chandler, Principal Laboratory Technician, Department of Plant Pathology, Los Angeles. Richard D. Durbin, Senior Laboratory Technician, Department of Plant Pathology, Los Angeles. John Ferguson, former Research Assistant, Depart- ment of Plant Pathology, Los Angeles; now at the Soil and Plant Laboratory, Orange, California. John W. Huffman, Assistant Agriculturist, Agricul- tural Extension Ssrvice, Los Angeles. O. A. Matkin, former graduate student in Horticul- tural Science, University of California, Los Ange- les; now at the Soil and Plant Laboratory, Orange, California. Donald E. Munnecke, Assistant Professor of Plant Pathology and Assistant Plant Pathologist in the Experiment Station, Los Angeles. Chester N. Roistacher, Principal Laboratory Techni- cian, Department of Plant Pathology, Citrus Ex- periment Station, Riverside, California. Warren R. Schoonover, Agriculturist, Agricultural Extension Service, Berkeley. R. H. Sciaroni, Associate Agriculturist, Agricultural Extension Service, Half Moon Bay. Aids in Adopting the U. C. System here are several logical steps in con- sidering the adoption of the system, whether in converting an old nursery or establishing a new one. Decide whether the U. C. system for producing healthy plants is to be adopted in your nursery. The experiences of twelve growers who have done so are described in Section 16. The advantages of such adoption are summarized on p. 30 through 33, 49, 51, and 270. If the system is to be adopted, the cultural prac- tices, present and contemplated, should be surveyed on the following bases: 1. Compare the U. C.-type soil mixes and nutrients with other possible soils on the bases indicated in figure 65 and in the related discussion in Section 6. A summary may be found on p. 90, 93, and 94. 2. The comparative ease of avoiding salinity injury by methods outlined in Section 4 should be studied for each of the cultural systems considered. 3. Compare results with untreated soil and that treated with heat or chemicals. The experiences of the twelve growers described in Section 16, and the benefits reported on p. 49 and 51 show the neces- sity of soil treatment. 4. Compare treatment of soil by chem- icals and by steam to determine which fits best into your operations. See table 13 and the accompanying discussion in Section 8. 5. If steam is selected for soil treat- ment: a. The boiler size required for a given amount of soil may be estimated from table 14 and the discussion in Section 9. b. The type of equipment best suited for steaming in your nursery may be se- lected by consulting table 15 and the data in Section 10. c. Possible ways to integrate such equipment into the mechanization pro- gram are shown in figure 126 and the text of Section 17. 6. If chemicals are to be used for soil treatment, the one best suited to your needs may be determined through table 17 and the discussion in Section 11. Ways to integrate such a treatment into a me- chanization program are shown in figure 126 and the text of Section 17. 7. Possible methods for treating con- tainers are discussed in Section 12. Whenever possible, however, the soil should be treated in the containers. 8. Pathogen-free stock or seed is neces- sary. Methods for obtaining and main- taining it are outlined in figure 115 and in the text of Section 13. 9. Sanitary precautions necessary to prevent contamination of clean soil and stock are outlined in "A Nursery Sanita- tion Code" in Section 1. The entire oper- ation should be reviewed, step by step, from the viewpoint of eliminating all possible sources of contamination. 10. Some possibilities of mechaniza- tion are presented in the flow diagram (fig. 126) and in the accompanying text in Section 17. Such a chart might well be prepared for the contemplated plan. You should also visit mechanized nurseries and perhaps consult an engineer who specializes in materials handling. 11. The technical assistance and ad- vice of a well-trained person will reduce errors and ease the transition while the system is being adopted. In some coun- [i] ties your farm advisor will be able to stand why the various practices are per- help during this "shake-down" period. formed. This manual should prove 12. The employees should be trained helpful in such training. in the system used so that they under- No endorsement of products or equipment referred to by trade names in this manual is intended, nor is criticism implied of similar products which are not mentioned. No responsibility is assumed for commercial soil mixes or fertilizers sold as formulations presented in this publication. SECTION The U.C. System: A General Summary The problems The answers The future Kenneth F. Baker .he most urgent need of the Califor- nia nursery industry, within the limits of its present market, is for lowered cost of production. This is best achieved by re- ducing plant losses and by lowering labor cost through mechanization. These in turn require modification of many exist- ing practices. Production must be de- pendable, uniform, and largely free from unpredictable failures due to diseases, salinity, insects, or weather. The U. C. system of soil mixes, soil and plant treatments, and handling op- erations has been developed since 1941 by the Department of Plant Pathology, University of California, Los Angeles, to practically eliminate the principal cause of such failure — diseases caused by those organisms and factors which involve the soil. It was evolved during a time when mechanization was empha- sized and, therefore, provides the neces- sary adaptability and dependability for the success of such a program. (Sec. 2.) 1 1 These numbers in parentheses give the sec- tion where further information on the topic may be found. Growers have generally found that they can produce better plants faster, easier, and more dependably by the U. C. system than by previous methods. Conse- quently many growers in California, as well as in other areas, have adopted the system or parts of it, and have contrib- uted in turn to its development. The large, year-round demand for nursery stock in California has given added im- petus to the program. The real key to the development and adoption of the system, however, has been the effective disease and salinity control it has provided. (Sec. 16.) The magnitude of the over-all problem is indicated by the quantity of soil used by the California nursery industry each year. It is estimated that this is about 350,000 cubic yards, or the top foot of soil from 217 acres of land. (Sec. 2.) Advances in disease control There has been close parallel develop- ment of disease-control practices in plants and animals. Both have progressed from superstitious practices, through the [3] use of antiseptics (on animals) and sprays (on plants), to aseptic proce- dures, to use of antibiotics (on animals and plants) and retardant or antagon- istic organisms (in soil). With plants this has emphasized pathogen-free soil and propagative material, and cultural techniques to keep them that way. This, the central core of the U. C. system, is expressed in the motto, "Don't fight 'em, eliminate 'em." (Sees. 3, 13, and 14.) Successful prevention of a disease usu- ally involves the application of more than one treatment. For this reason, sev- eral concurrent procedures are often sug- gested for a given trouble in this manual. Objective of this manual The objective of this publication is to assemble and synthesize information from many sources into a unified plan of action for nurseries. Many results of our research are reported for the first time. In addition, selected and tested in- formation from many other sources has been included to give a reasonably com- plete picture up to January, 1957. This manual is based on, and perhaps will prove most useful to California nurseries, but it should be helpful to growers in other areas who have shown interest in the U. C. system. While specific recommendations of present general application are made in this manual, it is not a "cookbook" of exact instructions. Nurseries vary so widely in type, size, and location, and in the number, kind, and age of crops grown, that detailed recommendations are often of limited application. Further- more it is impossible to anticipate the course of future developments in nursery practice. If growers understand the sci- entific bases underlying their practices, they can adapt the system to their own conditions and to future improvements in methods and equipment. The dual purpose of the manual, then, is to provide a plan for present action, with background information that will help the grower shift with the improvements that will surely come. One of the principal uses of this manual probably will be as a reference work to look up some specific fact, and the format has been arranged with this in mind. A fairly complete index, num- erous headings, frequent cross-refer- ences, tabulated data, and a general summary are provided. The same infor- mation may appear more than once, in order to reduce the danger that the refer- ence user may acquire partial facts or facts out of context. It has also some- times been necessary to repeat points in order to develop a line of reasoning. It is hoped that the annoyance to the gen- eral reader from such repetition will be outweighed by its general utility. THE PROBLEMS Diseases Diseases are a luxury Nurserymen are coming to realize that diseases are neither a necessary evil nor a trivial factor in the gamble in- volved in production of a crop. Diseases are, in fact, important in determining The pathogens, and what the grower does, how and when he n$Wtn«iy jsproad docs it, and why. The viewpoint is gain- ing acceptance that diseases are a luxury modern nurseries can ill afford, because I lies caii-c unnecessary losses. Because of the trend toward crop specialization, it is also well to remember that the fewer the crops grown, the less can one afford erratic disease loss in them. (Sees. 2 and 3.) Damping-off (figs. 1, 12 through 18, 30, 33, and 34) and related diseases (such as seed decay, top rot, cutting and stem rot, and root rot of mature plants) I 4 of nursery crops are most frequently caused by Rhizoctonia solani, but also by water molds (Pythium and Phytoph- thora spp.). Less important are the cot- tony-rot fungi (Sclerotinia sclerotiorum and S. minor) and the gray mold (Bo- trytis cinerea) , as well as host-special- ized parasites, such as the aster-wilt Fu- sarium. (Sec. 3.) The losses caused by a disease are de- termined by the interaction of several factors: (1) the susceptibility of the host, as well as its carbohydrate-nitrogen status, and the vitality of the seed planted; (2) the abundance (inoculum potential) 2 and virulence of the patho- gen in the soil; (3) the favorableness of the environment (for example, the levels of salinity, moisture, and temperature in the soil, the light available to the plant, the depth of planting). (Sec. 3.) The destructive Rhizoctonia and water molds produce no important air-borne spore stage, and their spread is, there- fore, largely dependent on the scattering 2 Many of the technical terms used in this manual are explained in the Glossary (p. 298). CAUTION: Many of the < :hemicals mentioned in this manual are poi- sonous and may be harmful. The user should carefu lly Follow the pre- cautions on the 1 abe Is of the con- tainers. of soil or plant fragments in which they are present. Spores of the water molds may be spread in water stored in a tank or reservoir, but not in that from city mains or wells. Spread of any of these fungi may occur when: 1. Soil is spattered by drops or a jet of water, as from irrigation or rain; 2. The pathogen is spread by dipping cuttings in water or in hormone solutions; 3. Soil gets in the end of the hose when it is dropped on the ground, and is expelled into the bench with the next watering; 4. Soil is carried over on flats, pots, benches, or other containers be- tween plantings; Fig. 1. Rhizoctonia damping-off of pepper seedlings. Flat at left showing damping-off pro- gressing inwardly from an infested container. Next to the flat the seedlings were infected first, and rotted before emergence. Farther from the edge, the seedlings were successively bigger before postemergence damping-off occurred. At right is shown the wire-stem phase of damping- off (see arrow) of large seedlings, causing the plants to fall over. Fig. 2. Which is the most important leg of a three-legged stool? 5. Soil is carried over on tools, cloth covers for flats, and on workmen's hands; 6. The grower walks over treated soil or flats; 7. Flats are placed on infested ground; 8. Infected seed, cuttings, or seedlings are planted. Therefore, emphasis in control is placed on clean soil, clean stock, and sanitary procedures to keep them that way (fig. 2 ) . Once the pathogen has penetrated into a plant it is not economically pos- sible to eradicate it except, with valuable planting stocks, by heat treatment. (Sec. 13.) Chemical treatments generally are ineffective. For these reasons prevention is emphasized in plant disease control, rather than cure, as in medical pro- cedures. (Sec. 3.) These fungi are not restricted to juvenile plants Fungi which cause damping-off and related seedling diseases may kill a tree or shrub some years later when the loss is greater, or may infest for all time a clean piece of ground. This fact imposes on nurserymen the obligation to produce plants free of disease organisms, rather than merely free of symptoms. Cultural suppression of these soil fungi in the nursery is only likely to postpone dis- ease until later, costlier losses occur. (Sec. 3.) Two strains of one species may differ greatly There is a widespread misconception that, because Rhizoctonia solani (or some other microorganism) is naturally pres- ent in a given soil, it is unimportant whether more of it is introduced with planting stock, soil, or manure. Evalu- ating the similarities of the disease po- tential of two organisms is a specialist's job, and a grower can with safety only assume them to be different until proved otherwise, even though the same scien- tific or common name is applied to them or to the diseases they cause. Strains of a single species may differ in: (1) the hosts which they can attack (fig. 124) ; (2) the virulence of attack (fig. 124), which on a given host may range from nonpathogenic to highly potent; (3) the temperatures at which attack will occur ; (4) the ability to develop in the lower levels of soil and to withstand appreci- able concentrations of carbon dioxide. (Sec. 15.) Although one strain of a fungus may be present in a field without causing disease, the introduction of another strain of the same species may produce an epidemic. Sometimes the introduc- tion of an organism may even increase the loss produced by a different one al- ready present. Prompt severe losses are usually sustained when infested stock is planted, and clean soil may be perman- ently infested, or the inoculum potential of it may be increased. One of the most dangerous features of transmission of organisms with seed or vegetative parts is that the constant association of the virulent strains of a pathogen and the host is thus assured. A second serious factor is that many soil organisms will persist for many years in a soil, once introduced and established there. These facts all emphasize the importance of 6 1 eliminating disease organisms in the nursery, rather than merely suppressing or fighting them. (Sec. 15.) Don't fight 'em, eliminate 'em Among the many advantages of free- dom from disease in nursery crops are the following: It is an aid to easier, more certain, and less expensive crop produc- tion. While growers can eventually learn to live with a disease, they would almost always be better off without it, because of the enlarged growth potentialities for the host. Thus, V erticillium wilt of chrysanthemum can be controlled by using resistant varieties, but more, and in some cases better, varieties are avail- able if this restriction is absent. Simi- larly, losses from Phytophthora root rot of heather can be reduced by minimal watering, but plant growth is retarded and more skill is required in watering than when the disease is absent. (Sec. 3.) The degree of financial benefit from disease control is relative to the cultural proficiency of the grower; the greater the return per plant, the greater will be the profit if disease in it is prevented. Buying or growing pathogen-free nurs- ery stock frequently is cheaper than fighting the disease. The danger of national panics in the nursery industry, such as those concern- ing rose mosaic in 1929-1932 and chrys- anthemum virus stunt in 1947-1950, would be considerably reduced if patho- gen-free stock were generally used. All nursery practices must mesh Disease in the nursery should never be viewed as an isolated phenomenon, unrelated to other phases of growing. Actually it is one of a series of interre- lated problems which must be solved simultaneously rather than piecemeal, for maximum effectiveness and perman- ence. A case in point is the perennial dilemma of southern California nursery- men, who lose their seedlings from sa- linity if they water lightly, and from damping-off if they water heavily. To successfully mechanize such a nursery and introduce mechanical watering, the soil pathogens must be eliminated, so that the salinity may be held down by copious watering, without aggravating damping-off. Again, when soil is steamed, the frequency and quantity of watering must be modified because of altered water retention. (Sees. 2 and 3.) A disease-control program must either fit into the current cultural methods, or these must be modified before it can be adopted. Often both develop together. For example, the control of carnation diseases has developed along with sweep- ing changes in methods of growing: (1) year-round glasshouse culture; (2) di- rect benching of cuttings; (3) the single pinch; (4) continuous forcing; (5) use of cuttings from the new disease-free seedlings. These situations illustrate the basic unity in proper nursery production pro- cedures. If an advance is made in dis- ease control, its successful adoption necessitates changes in other practices, and its benefits may project into other aspects of nursery culture. Salinity Small amounts of various salts are necessary to plant development, but ex- cessive concentrations cause injury or death, just as small amounts of table salt are necessary to man but large amounts may aggravate high blood pressure and very large amounts may kill. Plant injury varies Plant susceptibility to an excess of water-soluble salts varies widely from almost no injury (carnation, stock; fig 3) to severe injury (plants with soft large leaves, such as begonia and fern; figs. 54 and 55). Salinity may produce sev- eral types of effects: [7] Fig. 3. Salinity injury to leaves of three ornamentals. A, Maranta leuconeura, showing leaf with burned tip at left, normal at right. B, Apex of leaves of Cordyline terminalis, showing tip- burn at left, normal at right. C, Leaves of stock, Matthiola incana, showing minor tipburn of old leaf at left, and more serious injury of young leaf at right. Such injury to stock is important mainly because it is readily infected by the Botrytis gray mold. l.No apparent symptoms at moderate concentrations, reduced growth at higher levels (carnation I ; 2. Leaf burn, usually at margins I azalea I or tips (cymbidium, Maranta, Cordy- line; fig. 3), which makes the plant un- sightly and weakens it; 3. Root corrosion and killing (azalea, gardenia; fig. 56) which may, depend- ing on severity of injury, cause chlorosis [8] of the leaves or wilting and collapse of the plant; 4. Little or no germination of seed; 5. Prompt wilting, desiccation, and death after transplanting seedlings into saline soil; 6. Localized injury to leaves from salts accumulatively deposited on the surface by irrigation water (begonia) or from contact with a salt-saturated flower pot ( Saintpaulia) . The weakening of the plant renders it more susceptible to attack by organ- isms; thus seedlings may be made more susceptible to damping-off, and stock leaves with tipburn become susceptible to Botrytis infection. (Sec. 4.) Sources of salinity Salts may accumulate from those in- troduced into soil with: (1) irrigation water (fig. 38), which at times is quite saline; (2) application of fertilizers (fig. 40) in excessive amounts or of types which leave substantial unused residues in the soil; (3) use of manure or leaf mold (fig. 42) gathered in places where they have accumulated large amounts of salt, a condition common in southern California. (Sec. 4.) Salts added in any of these ways may: (1) be flushed on down by subsequent large applications of water, and thus leached beyond the root zone or out of the container; (2) be absorbed by plants and either used in their metabolism, or accumulate and finally reach a toxic level (as in leaf-margin or tipburn) ; (3) ac- cumulate in the soil, being washed down a few inches with each irrigation, only to be carried back to the surface with the water and deposited there as a residue when it evaporates. It is necessary to apply more water than is used by the plants and evaporated from the soil sur- face, in order to prevent the accumula- tion of salts. The greater the salinity of the water, the greater must this excess be. Repeated light sprinkling leads to trouble unless an occasional heavy leach- ing irrigation is practiced. (Sec. 4.) The salts may accumulate in a clay pot in the same way, exposing the roots, which are most plentiful next to it, to a high concentration of salts. (Sees. 4 and 12.) Because the concentration of salts in soil water increases as the soil dries out, plant injury is aggravated by either growing plants "on the dry side" or watering only when the soil is dry. A consistently moist soil will give least in- jury. (Sec. 4.) Measuring salinity When salinity is suspected, because of plant injury or the type of culture, it may be tested for by measuring the elec- trical conductance of a saturated soil extract with an inexpensive simple in- strument, the Solubridge. The salt con- tent of the water supply should be determined, as well as that of the soil, peat, leaf mold, manure, and so on, be- fore they are mixed for use. This elimi- nation of a source of potential trouble, rather than waiting for it to develop, is as necessary for mechanized nursery operation as is the establishment of qual- ity tolerances for parts used in an auto- mobile assembly line. (Sec. 4.) Toxicity Use of the U. C. system of soil mixes and nutrients has eliminated the toxic effect from steaming or chemically treat- ing soil. Because of this simple solution, the problem seems to be unnecessarily causing concern. Plants grown in con- ventional treated soil mixes may develop injury (stunting, dropping of leaves, root corrosion, death ) from toxins. These materials, which may be tempo- rary or persist for months, result from the formation or release of various chem- ical materials (such as ammonium, or- ganic matter, manganese, soluble salts). Soil mixtures high in readily decom- [9] posable organic matter (manure, leaf mold, compost) are most likely to give injury from such treatment. Growers who do not use mixes of the U. C. type may reduce injury by leaching or aging the treated soil before use, application of gypsum in some cases, or by plant- ing immediately after treatment. (Sec. 6.) Injury may also result from a persist- ent residue of the chemical used in soil treatment (for example, on carnations planted in soil treated with methyl bromide). This is a different problem, and may be prevented by using a dif- ferent chemical, by altering the condi- tions of its use, or by aging of soil after treatment. (Sec. 11.) THE ANSWERS Disease control in the nursery is most effective when preventive treatment aims at eliminating the causal organisms from the soil, from the seeds, cuttings, or other planting material, and from the bits of soil on tools, flats, hoses, and other equip- ment, or in places where it may be readily splashed by water (fig. 4). This approach demands that preventive meas- ures be planned in advance, rather than waiting until trouble arises. (Sec. 3.) The U.C. System of Soil Mixes and Nutrients One of the commonest erroneous ideas in nursery practice is that a special soil is required for each type of plant. Actu- ally, most plants of necessity had to have a wide tolerance of different soils in order to survive. The basic fact that many kinds of plants can be successfully grown in a single soil mix, or in slight modifications of it, was demonstrated by the John Innes Horticultural Institution in 1934-1939. This was an important contribution to nursery practice, and the J. I. mixes have been widely adopted in recognition of this fact. They fail, how- ever, to eliminate several serious inher- ent disadvantages common also to conventional soil mixes. The U. C.-type soil mixes have corrected these objec- tions. (Sec. 6.) Advantages of the U. C.-type soil mixes The soil serves four principal func- tions for plants: (1) it provides me- chanical anchorage and support; (2) it stores and makes available a supply of water; (3) it stores and regularly sup- plies mineral salts essential to the plant; (4) it provides aeration for the roots. These functions are well served by the U. C. soil mixes. (Sec. 6.) Certain advantages over the multi- plicity of mixes are also provided by the U. C.-type mixes: 1. They may be heat- or chemically treated without producing injurious toxic residues. 2. The variability from the use of leaf mold, animal manure, turf, and composts, as well as from differences in the degree of their decomposi- tion, is reduced, and more uniform results are possible. 3. The salinity problem is reduced by eliminating some of its common sources and by providing a medium that may be readily and effectively leached. 4. Labor requirement is reduced. 5. The space utilized for compost piles and storage of raw materials and Fig. 4. Diagram of nursery soil problems considered in this manual, showing their sources and answers. The numbered boxes on the source lines indicate the appropriate preventive measures, named at the right. I 10] SOIL P R O 6 L EMS \ D T s / \ 1 O A / \ s X L / \ E 1 1 / \ A \ s c 1 N / 1 / \ E T T / V s Y Y / 1 / s o u R c E S 10 Fertilizer Organic Matter — Planting Stock Containers Recontamination ANSWERS 1. U. C. Type Soil Mix 2. Good drainage 3. Moist soil and air 4. Leaching 5. Treating soil 6. Good water 7. Frequent light fertilizing 8. Clean planting stock 9. Treating container 10. Sanitation [in various mixes is saved, an import- ant item where land is expensive. 6. Loss of volume from shrinkage during composting of organic matter is avoided. 7. Odors and flies from the compost piles, likely to prove restrictive in resi- dential areas, are eliminated. 8. The problem of the scarcity of leaf mold, animal manure, and turf is avoided. (Sec. 6.) Physical components The physical base of the U. C.-type mix consists of an inorganic material (fine sand, perlite, vermiculite) and an organic fraction (sphagnum peat moss, rice hulls, sawdust, shavings, bark). The two components presently suggested for use in California nurseries are fine sand (particle size ranging from 0.5 to 0.05 mm) and sphagnum peat moss. These components satisfy the greatest number of desirable features (fig. 65) : 1. They are readily available in uniform grade. 2. They are chemically uniform and relatively inert. 3. They are not broken down by steam or chemical treatments used to free them of disease organisms. 4. They are easily made into a uniform mix. 5. They provide good aeration and wa- ter drainage. 6. The peat retains mineral nutrients against leaching, although the fine sand is less effective in this. 7. Their fertility is low, furnishing a known low starting point for add- ing nutrients. 8. They are relatively inexpensive. ( ). When mixed they have good water retention. 10. They arc light in weight. 11. They have proved adequate in micro- nutrients, but should instances of deficiency arise the elements can easily be added. 12. They have negligible shrinkage in storage and use. Characteristics 1 through 4 are essen- tial, and their deficiency cannot be made up by adding or substituting other mate- rials, as can features 5 through 12. This fine-sand— peat mixture has most of the good features of clay soils without their disadvantages. (Sec. 6.) The sand ingredient may have 12 to 15 per cent (preferably less) coarse sand, must not have more than 15 per cent (preferably less) clay or silt, or both, and should have 70 to 85 per cent or more of fine sand. A method is available for nurserymen to determine particle size of soil samples. Five dif- ferent proportions of sand to peat are suggested for different purposes: Mix A. 4:0, sometimes used for certain crops and bench stocks; Mix B. 3:1, the most commonly used ratio, for bedding plants and general nursery planting; Mix C. 2:2, for plants grown in pots or benches; Mix D. 1:3, for pot plants that are large in relation to their containers, and for cymbidiums; Mix E. 0:4, for growing azaleas and similar acid plants, sometimes mixed with wood shavings. (Sec. 5.) Chemical components These U. C. mixes are purposely low in nutrients, so that mineral elements may be added to this known base with pre- dictable results. Sometimes fertilizers are omitted from the mix and applied in solution after planting. Usually, how- ever, they are added, at least in part, at the time of mixing the soil. Phosphorus, because it is applied as a slowly soluble 12 | NOTE: Urea and urea-formaldehyde fertilizers may contain biuret, a by-product toxic to many plants. Unless labeled biuret- free, these materials should be used only after thorough testing on each crop. superphosphate, does not contribute ap- preciably to the salinity problem while supplying crop needs for this element. Potassium is presently supplied as water- soluble potassium nitrate or, less com- monly, as potassium sulfate or potassium chloride, and thus contributes to the salinity problem. Calcium and magne- sium are presently supplied as dolomite lime in the soil mix. This does not con- tribute appreciably to the salinity prob- lem, while neutralizing the acidity of the peat moss and supplying these necessary elements. (Sec. 5.) Nitrogen is generally supplied in the organic form (hoof and horn meal, urea- formaldehyde resin, blood meal, cotton- seed meal, castor-bean pomace), but sometimes as inorganic nitrogen (po- tassium or calcium nitrate), or both. (Sec. 5.) Nitrogen availability The organic nitrogen, which is un- available to the plants, is converted to ammonium (available to plants) by bac- teria and fungi that are quite resistant to soil steaming or chemical treatment; the ammonium is changed to nitrate (available to plants) by nitrifying bac- teria intolerant of most of these treat- ments. The latter step is adversely affected by an acid medium and low soil temperature, and therefore ammonium tends to accumulate under these condi- tions. Also, bacteria tend to be sparse in fine sand obtained at a depth of a foot or more (as much of it is in California) , and in this way such material resembles treated soil. (Sees. 6, 7, and 14.) Under average conditions treated soil containing organic nitrogen does not, for a variable period (about 1 week), pro- vide nitrogen in a form available to plants, then presents it in the form of ammonium for another period (about 2 weeks), and thereafter as both am- monium and nitrate. Seedlings grown in a newly treated soil containing only organic nitrogen are often deficient in nitrogen, a situation readily corrected by watering with a calcium nitrate starter solution at time of planting. (Sec. 7.) Nursery crops differ in their response to ammonium. Many types of crops (for example, foliage plants) utilize the am- monium nitrogen without ill effect, and these are successfully grown with the organic source. Other plants (for ex- ample, sweet alyssum, clarkia, and car- nation) show injuries varying from leaf burn and root injury to death, when supplied ammonium nitrogen. Petunia and snapdragon seedlings may show ex- cessively soft growth and iron chlorosis. This injury is worst to plants in the seedling stage. In such cases the organic nitrogen should be eliminated or reduced in amount, and nitrate fertilizer should be supplied. (Sees. 5, 7, and 14.) The quantity of organic nitrogen sup- plied varies with the size of the plant in relation to the volume of soil in the con- tainer, and is greatest for large plants in small containers. It may be possible to inoculate treated soil with nitrifying bacteria in order to lessen the ammonium increase, but the effect may not be evident for 10 to 20 days. If this is done, nitrifiers should be introduced without ammonifiers, or the situation may be worsened. (Sees. 5, 7, and 14.) Because the breakdown of organic nitrogen occurs in a stored U. C.-type mix, the high content of water-soluble forms of nitrogen may cause plant injury if the soil is held for several weeks be- tween mixing and planting. If such storage is planned, one of the fertilizer variants should be used that does not [13] include the organic form of nitrogen; this form should be applied as a top dressing after planting. (Sees. 5, 7, and 14.) Selecting nitrogen fertilizers The organic forms are slowly available and, therefore, present a continuous low supply rather than the varying quantities resulting from occasional applications of inorganic forms. In descending order for rate of conversion and ascending order for nitrogen content, the organic nitro- gens tested were: castor pomace; fish meal; cottonseed meal; blood meal and hoof and horn meal; urea-formaldehyde resins. The urea-formaldehyde resins are apparently hydrolyzed by steaming and may therefore give rapid ammonium build-up; these materials must be free of biuret to be safe for use on nursery plants. Hoof and horn meal and blood meal are presently considered best for the mix. (Sees. 5 and 7.) Surface dressings of these organic ma- terials do not lead to injury from am- monium, because of the slow rate of penetration of ammonium through soil; it is normally converted to nitrate before reaching the root zone. (Sees. 5 and 7.) The nitrate forms are immediately available and eliminate the hazard of ammonium accumulation. They are therefore useful (1) in mixes when nitri- fying bacteria are absent (sees. 7 and 14) ; (2) in starter solutions for seed- lings in treated mixes containing organic nitrogen (Sec. 7) ; (3) when soil mixes are to be stored (Sec. 5). Fertilizer application Where plants are to be carried in containers or benches for an extended period of time, it becomes necessary to replace fertilizers which are lost through plant uptake and leaching. Dry fertilizer may be broadcast over the surface and watered in, or liquids may be applied in the irrigation water itself. (Sec. 5.) I sually dry fertilizers should include an organic source of nitrogen, in order to prolong the effective period and reduce the salinity hazard. There is no problem with ammonium from such application, even though it may be formed. Am- monium does not move readily down past the surface layer of soil until broken down by microorganisms to nitrate. The other components of the fertilizer should be superphosphate and potassium sulfate. (Sees. 5 and 7.) Liquid fertilizers may contain any one of several nitrogen sources. Ammonium nitrate or urea is commonly used, as they can readily be mixed with any other ingredients. Calcium nitrate should be used where an all-nitrate form is desired, but this source of nitrogen should not be mixed with sulfates or phosphates in con- centrates because it forms insoluble salts of calcium phosphate and calcium sul- fate. Phosphate is easily supplied as mono-ammonium phosphate, and potas- sium as potassium chloride. Other mate- rials can be used, and examples of rates and formulas are outlined. (Sees. 5 and 7.) Preventing Salinity Injury Excessive concentrations of salts may be avoided through detection of accumu- lation at an early stage by measuring the electrical conductance of soil samples, and taking active preventive measures. Once the plant is crippled by root injury or leaf burning, rehabilitation is slow, and seldom economic for a nursery crop. Preventive measures include: 1. Use excess water over that required for plant absorption and evapora- tion from the soil, assuring drainage from the root zone with each water- ing. 2. Use the best possible quality of water (low salinity; conductance less than 1.0, or 650 ppm), and avoid saline waters, particularly on sensitive plants. 3. Leach with a considerable excess of water whenever salts reach a dan- I 14] gerous level (conductance of satura- tion extract about 3.0, or 1,950 ppm for most crops) ; the higher the salinity of the water or soil the greater the quantity of water that must be used. 4. Use deionized water for some of the more expensive crops. 5. Expose soil and plants to rainfall (salt-free) when feasible. 6. Maintain excellent drainage (unob- structed drainage holes in pots and cans; open cracks in bottoms of benches; avoid hardpan soils below ground beds; install drain tiles when needed; use porous soil) so that salts may be readily flushed from the soil. 7. Use a U. C.-type soil mix with the organic matter in small pieces; avoid loam (even if Krillium- treated) , as it does not leach readily. 8. Avoid soil, manure, leaf mold, black peat, and kinds of sewage sludge high in soluble salts or, if they must be used, leach them heavily before- hand. 9. Apply fertilizer in frequent small, rather than in a few heavy, applica- tions. 10. Avoid mixed fertilizers that include minerals not needed at all or not in amounts supplied. 11. Grow plants with the greatest shade and humidity compatible with good culture of the crop, so as to decrease salt concentration and injury. 12. Keep soil as uniformly moist as is compatible with good culture of the crop, avoiding alternating wet and dry soil conditions. 13. Add organic matter to the soil to stabilize moisture content and reduce injury. 14. Soak old clay pots before re-use to remove accumulated salts. 15. Avoid overhead sprinkling of plants (begonia) subject to leaf burn from salt left from evaporated water drops. The U. C. system of soil mixes is com- pletely compatible with these measures, either incorporating them or making it possible to follow the procedures. (Sec. 4.) Treatment of Soil by Heat Heat treatment is used to free soil of organisms which cause plant disease, as well as of weed seeds and insects. Steam treatment of soil remains the best method of disinfestation for all fungi, bacteria, nematodes, weed seed, and insects. (Sees. 8 and 9.) Temperature requirements and soil preparation Heat treatments in which a temperature of 180° F is maintained for 30 minutes are adequate; with many methods, how- ever, the process cannot be stopped short of 212°. With the U. C. system of soil mixes there is no residual toxicity from heating in either case. The soil should be in good planting tilth, well mixed, free of clods, and with sufficient moisture so that after being squeezed in the hand it will crumble easily. For economical treatment, the soil should not be soggy, because five times more heat is required to heat water than soil. After cooling, the soil is in good planting condition, with- out excessive moisture. (Sec. 8.) Methods Treatments are preferably applied after the soil is mixed and placed in tlie containers, since this method reduces the recontamination hazard from handling. Some of the bulk methods of soil treat- ment are, however, satisfactory if mech- anized to minimize the handling. (Sees. 8, 9, and 10.) Steam is most economically used in the free-flowing form. When steam is released into soil under pressure it [15] immediately reverts to 212° F and the nonpressure (relative to atmospheric pressure) condition; therefore, there is little advantage in using it. Since super- heated steam provides a little more heat per pound and contains somewhat less entrained water, there are some slight advantages in using it, although equip- ment of this type is not commonly available in this country. (Sees. 8 and 9. ) The quantity of steam needed to raise the soil temperature to 212° F varies with many factors, but a generally accepted working average is 6.5 pounds per cubic foot of soil, or 42 B.t.u. per cubic foot per degree of rise. The amount of soil that can be steamed in a given time using boilers of various sizes is given in table 14. One of the advantages of steam for soil treatment is that it may be used near living plants without injuring them; it is neither toxic nor unpleasant to work- men. (Sees. 8 and 9.) Equipment Many kinds of equipment have been designed for steaming soil, and these have been grouped in this manual into thirty-five types. Nine of these (types 2, 4a, 4b, 5, 6, 7, 18, 19, and 29) seem well adapted to California conditions. (Sec. 10.) For treatment of bulk soil, stationary or mobile steam boxes (type 4, fig. 5) or the mobile bin (type 2, fig. 80), both with perforated pipe grid and a sta- tionary soil mass, are excellent. The feature of continuous output may be combined with the advantages of having a stationary soil mass in the continuous- batch modification of types 2, 4, 6, or 9 (fig. 6) . For a continuous output of bulk soil, the rotating-screw type with injected steam (type 29, fig. 100) is satisfactory. For treatment of soil in containers, the Thomas method (type 5), the vault (type 0. fig. L31), and the multipurpose tank (type 7, fig. 85) arc recommended. The Thomas method (type L8, fig. ( )\ I and the inverted steam pan (type 19. fin. 92 I are the most convenient for treating soil in benches and beds, but are not depend- able below 8 to 9 inches in depth. If deep- er treatment is required, the buried per- forated pipe (type 20, fig. 93) or the permanent buried tile (type 22, fig. 95) may be used. (Sec. 10.) Equipment in which a stationary soil mass is treated raises the temperature to 212° F when steam is used, and even higher if electric immersion heaters are employed. When steam is used, equip- ment with a stationary soil mass is prob- ably best, although some with a moving soil mass are satisfactory. When heat from immersion units, hot plates, and other dry sources of heat is used, it is de- sirable that a type with a moving soil mass be employed, since there is charring of organic matter in types with a station- ary soil mass. The use of hot water for soil treatment is suitable only for propa- gating sand. One should use table 15 to determine the types of equipment with the necessary features for a given instal- lation, and then refer to the text for details. The size of the boiler required may be estimated from table 14. (Sees. 9 and 10.) Natural gas is the cheapest fuel in California. Oil and butane are more ex- pensive but better adapted to portable equipment. Electricity is very expensive to use for soil treatment, but is very convenient. (Sec. 10.) Steam versus chemicals Steaming soil requires about an hour, plus another hour to cool before plant- ing; methyl bromide is used in a 24- to 48-hour treatment, plus a 24- to 48-hour aeration; chloropicrin is used in a 48- to 72-hour treatment, plus a 7- to 10-day aeration. Steam is effective against all organisms except a few types of weed seeds; methyl bromide is only partially effective against Verticillium, and leaves a residue toxic to some plants (carna- tions; snapdragon seedlings); chloro- picrin is generally effective against [10] Fig. 5. The removable-front steam box for stationary bulk soil, and for soil in containers (Sec. 10, type 4b). See also fig. 82. Fig. 6. Pressure autoclave for treatment of flats of soil (Sec. 10, type 9). The flats are placed in a special rack and rolled into the unit for steaming. (Photo courtesy of American Plant Growers, Lomita, California.) See also fig. 86. [17] organisms except those in root masses, and leaves no toxic residue unless im- properly used. The cost of treatment with steam (including cost of boiler) is less than 2.0 cents per cubic foot; for methyl bromide it is about 2.9 to 3.2 cents, and for chloropicrin about 1.9 to 3.0 cents; labor is excluded in these cal- culations. (Sec. 8.) Regardless of the method of treatment, if clean soil is dumped in bulk piles on the floor, the surface should previously have been wet down with a formaldehyde solution (1 gal. to 18 gal. water). (Sees. 8 and 11.) Treatment of Soil by Chemicals When a source of steam is not avail- able, or when an area of field soil is to be treated, chemical applications are often used. Fungi are harder to kill in the soil with chemicals than are nema- todes, insects, or weed seeds. When soil fungi are involved, the chemical and dosage must be adequate to kill them; this dosage will usually also kill the nematodes, insects, and weed seed. Nematocides (DD, EDB) often have little fungicidal value, and insecticides, weed killers, and soil conditioners are also largely ineffective against fungi or bacteria. (Sec. 11.) Methyl bromide Nursery soil in containers is commonly treated with gaseous methyl bromide in California. The stacked flats, pots, cans, or small piles of soil are covered with a tight plastic tarpaulin, and the gas re- leased under it from pressurized cans or cylinders (figs. 106 and 107) at the rate of 4 pounds per 100 cubic feet of en- rlosrd space If tin 1 temperatures are low, the gas is passed through a copper pipe immersed in hot water to volatilize any Liquid methyl bromide. The soil is left covered for 21 to 18 hours, and aerated for 24 to 48 hours before use. This is an effective and commonly used treatment, but should not be applied to soil to be used for carnations or snap- dragon seedlings because of residual toxicity, or for chrysanthemums because of its ineffectiveness against Verticillium. (Sec. 11.) Chloropicrin Chloropicrin is more generally useful and cheaper, but because it is less con- venient and takes longer to aerate, it is less commonly used in California. It has not been commonly used for treating stacked containers of soil. On beds it is injected 6 inches deep with special equipment (fig. 105), at the rate of 2 to 3 cc per 10 inches square of soil, and is confined either by wetting the top inch of soil or by covering it with a plastic tarp. Bulk soil may be treated in bins, drums, or any gasproof receptacle at the rate of 3 to 5 cc per cubic foot. Treat- ments with chloropicrin are for 48 to 72 hours, plus a 7- to 10-day aeration before planting. (Sec. 11.) Vapam This new water-soluble material is be- coming widely used, especially for ground beds and field soil. It is applied at 1 to 2 quarts per 100 square feet, either on the surface with irrigation water or injected into the soil. Soil may be cultivated after 7, planted after 14 days. (Sec. 11.) Nematocides DD is used for nematode control in the field at 200 to 400 pounds per acre, and the soil may be planted after 1 to 2 weeks. Ethylene dibromide (EDB) is used against nematodes at the rate of 3 to (> gallons per acre, and soil may be pi an led after 2 to 3 weeks. Neither of these last two materials is recommended against fungi or bacteria. (Sec. 11.) [18] Spot treatments Spot treatment of limited areas of fungus infection is desirable to prevent spread to other plants, but it should never be made the complete control pro- gram, as it is in some nurseries. Such spot treatments are not eradicative and merely aim to make the best of a bad situation. The disease suppression af- forded is temporary and in many cases not very satisfactory. Materials used in- clude ferbam, thiram, and captan (1 tbsp. per gal. of water, per 8 to 16 sq. ft.), Semesan (1 tbsp. per gal. of water, per 24 sq. ft.), nabam (1 fl. oz. per 4 to 8 gal. of water, per 64 sq. ft.), and Ter- raclor (1 oz. of 75 per cent wettable powder per 42 to 63 sq. ft.). In treat- ment of flats the whole surface should be drenched; in the field the removal of dis- eased plants may be desirable and the drench should be applied to an area 1 to 2 feet beyond the margin of disease. (Sec. 11.) Treatment of Containers Flats, pots, cans, benches, and pallets should be treated with heat or chemicals before being re-used. Preferably the raw soil mix is placed in the container and both are treated together, but treating the containers and soil separately is often practiced. Heat If steam is used for treatment of con- tainers, a temperature of at least 180° F should be maintained for 30 minutes, the same as for soil. With clay pots perhaps the best method is to soak thern in water of 180°-212° for at least 30 minutes, since this removes the accumulated salts and kills pathogens and algae in the sur- face slime as well. The prevalent practice of going over a bench with a blow torch is without disinfesting value; heat of this type is so concentrated as to char the wood, but so short that heat does not penetrate. Steam provides the steady penetrating heat needed for this work. (Sec. 12.) Chemicals Chemicals may be effectively used for treatment of containers. Methyl bromide may be used for fumigation by treating stacked flats exactly as for soil above; they should be aerated for 1 day before use. Formaldehyde ( 1 gal. of 37 per cent commercial formaldehyde per 18 gal. water) may be used for dipping con- tainers or may be sprayed on them, using a coarse nozzle. In either case they should be stacked wet and covered with a tar- paulin for 24 hours and then should be aerated for at least 4 to 5 days, being kept wet to prevent the formation of the slowly volatilizing paraformaldehyde. Tools may be dipped for a few minutes in a crock of this solution, rinsed with water, and used at once. Copper naphthe- nate applied to flats affords a self-disin- festing surface residue for at least a year; there is no carryover of damping- off organisms on containers treated in this way. It is least expensive when pur- chased in the 8 per cent concentrate, and diluted (1 gal. to 3 gal. of Stoddard solvent per 800 to 1,600 sq. ft.) for dipping or painting on wood containers. It is an excellent wood preservative. Such containers may be steamed, but need not be if they are freed of lumps of soil and roots. Since there is some root injury, the material should not be used on seed flats or benches where plants are set less than 2 to 3 inches from the sides. It is excel- lent for treated benches, shelves, and timbers on which containers are set, and may be used for disinfesting an empty bench in a house full of plants, if the vents are kept open for a day. (Sec. 12.) Development and Maintenance of Healthy Planting Stock For the preceding soil treatments to be really effective it is necessary that healthy stock be planted in such soil, because [19] diseased plants are centers of infection for healthy ones, this spread of disease often being more rapid in treated than in untreated soil. This unfortunate effect from treating soil is actually more of a mental than a practical obstacle at pres- ent, and it may be possible eventually to eliminate it through the use of antago- nistic or retardant organisms. (Sec. 14.) Pathogens introduced into a planting are more dangerous if they infest the soil than if they do not, and the longer they are able to persist in it the greater the danger. Most of the pathogens with which we are concerned in this publica- tion more or less permanently infest the soil. (Sec. 13.) Obtaining clean stock Clean stock or seed may be initially ob- tained in the following ways (Sec. 13.) : 1. From a specialist propagator who has maintained it. It is as much a duty in the nursery world to report to the propagator any stock that carries disease as it is to vote in the political world, and for much the same reason. 2. From a few healthy plants that may be available. 3. By using practices which enable a few plants to grow away from the pathogen. Tip cuttings produced 12 inches or more above the soil may be taken from plants grown in areas as nearly free of disease microorganisms as possible, and without overhead sprinkling. Cane- producing and trailing plants may be trained up off the ground to achieve this. In exceptional cases aseptic culturing of tiny apical growing points may be used to pro- vide a nucleus of healthy stock. 1. By using cultured-cutting techniques (fig. 109) to obtain a nucleus of disease-free plants. This has been done commercially for chrysanthe- mums, carnations, roses, and gera- niums. It will not eliminate virus infections, however. The technique involves culturing from the base of each cutting; those that are found to be clean form the nucleus for further propagation. This involved technique has been most effectively used by specialist propagators, less so by growers. 5. By heat treatment of planting stock. This has been applied to a wide range of ornamentals including foliage plants (figs. 112 and 114), succulents (fig. Ill), seeds, corms, bulbs, plants, and cane. It is based on the greater heat sensitivity of the pathogen than the host, and each treatment is therefore spe- cific. Methods must be evolved for each parasite-host combination and must be carefully followed. The central idea is to obtain a nucleus of clean stock, and to do this even high mortality from the heat treatment is justified. Actually, however, there is very satisfactory survival in most cases. The cleanest most vigorous stocks or seeds available should be used, and they should be in a state to withstand treatment or be conditioned for it. The key to success in many cases lies in using heat-resistant plant material. Temperatures range from 115° to 131° F for 10 to 40 min- utes for different plants; details are given in Section 13. 6. By chemical treatment. This has limited use in eradicating an or- ganism in host tissue (calla rhi- zomes treated with formaldehyde, mercuric chloride, or New Im- proved Ceresan ; chrysanthemum sprayed with parathion against foliar nematode), for purposes here considered. It is primarily useful for protective purposes, to 20 prevent an organism from invad- ing the coated tissue, especially with seeds. 7. By sanitary practices. Though rarely capable of providing clean stock, these will do so in the cases of camellia and azalea flower blight through elimination of carryover sclerotia. 8. By aging of seed. This sometimes frees the seed of an organism; celery seed 3 years or more old is thus freed of Septoria late blight. 9. By prolonged roguing of diseased plants from a stock. This will gradually free it of a disease that does not spread in the field (for example, rose yellow mosaic in a mother block). 10. By growing plants from true seed rather than from vegetative parts. This is particularly effective against viruses (ranunculus, free- sia, anemone, yellow calla). 11. By selecting areas for producing seed or propagative material where the climate exerts a naturally restric- tive effect on the organism in ques- tion. This will help in obtaining pathogen-free stock. (Sec. 13.) Maintaining clean stock The maintenance of the healthy status of a stock must involve certain safe- guards. 1. New stock brought in should be iso- lated in an introduction "pest house" to determine its state of health before it is planted in clean production houses. 2. Propagation operations should be iso- lated from all possible sources of in- fection; they should not be con- ducted in weedy areas or known dis- ease centers. 3. The production and merchandising of plants on the one hand, and the maintenance of the basic stock for future propagation on the other, must be handled as independent and isolated activities (fig. 115). This mother-block principle is effective because it is easier to care for and protect a small block than the large production areas. It is a practice worthy of far greater adoption by the nursery industry. 4. The sanitary procedures outlined in sections 3 and 14 should be care- fully followed. (Sec. 13.) Preventing Recontamination The natural soil contains a vast popu- lation of roots of higher plants, algae, fungi, bacteria, actinomycetes, insects, nematodes, protozoa, and other organ- isms. These exist in a state of dynamic equilibrium or fluctuating balance, com- peting with each other for food and space. One may become temporarily dominant due to some change (for ex- ample, a favorable food, temperature, moisture), but soon is submerged by the parasitic, competitive, or antibiotic ac- tivities of other organisms. (Sec. 14.) Parasitic fungi and bacteria generally have been weakened for such competi- tion by the very specialization required for their attack on higher plants, and are unable long to withstand the survival pressure in the soil. The more highly de- veloped the parasitic activity, and the more able the organism is to attack a vigorously growing plant, the less able it usually is to survive under natural competition. When the host is present, competition is evaded by utilizing the specialized food source (living plant) which the saprophytic forms cannot at- tack. Specialized parasites, such as the geranium leaf spot and stem rot bac- terium and the fungus that causes flower blight of azalea, survive for a relatively short time in natural soil free of the hosts. Generalized parasites that only in- fect weakened plants (the bacterium that causes soft rot of many hosts, and many [21] of the water molds) remain as part of the soil flora, even without the pres- ence of a host. (Sec. 14.) Effect of eliminating soil microorganisms The first organisms to return after an eradicatory soil treatment obviously will luxuriate. If a crop is planted in treated soil it will generally grow better than in an untreated soil, because many soil microorganisms are largely crop-antago- nistic, even though they may not produce disease. Similarly, the first microorgan- ism which returns develops abundantly, sometimes visibly, in treated soil. If this organism is capable of causing disease, a severe outbreak may occur. This is the re- contamination problem; the risk is highest in the first week, before relatively harmless air-borne bacteria and molds reinfest the soil. (Sees. 3 and 14.) If a soil fungicide is used at lower or considerably higher than recommended rates, or if one is used that is effective against most organisms but not some given pathogen (for example, methyl bromide against Verticillium) , the same effect may be attained. Therefore, one should use recommended dosages of a soil fungicide known to be adequate for the job. (Sees. 11 and 14.) The common potential sources of re- contamination in the nursery have been outlined above ("The Problems — Dis- eases") and in Section 3. The preventive procedures are summarized below in "A Nursery Sanitation Code." A Nursery Sanitation Code 3 Treat all soil used with either steam or chemicals. The chance of recontamination will be reduced if all soil in a given glass- house area is treated. (Sees. 8 and 11.) Segregate the clean treated pots and flats in definite areas of the workroom, iso- lated from used untreated containers. Don't place an untreated pot or flat in a pile of clean containers. Never store clean ones on the ground. (Sec. 3.) Treat floors with formaldehyde solution before dumping treated soil on them. Pal- lets for flats and pots should also be treated. (Sees. 8, 11, and 12.) Don't plant clean seed or stock in un- treated soil. Don't transplant infested seed- lings into treated soil. Don't place clean soil in untreated containers. (Sec. 3.) Either use pathogen-free stock from a specialist propagator, or treat your own selected material with heat or chemicals to insure its health status. (Sec. 13.) Use top cuttings from the cleanest plants you have, grown on supports up off the f Since it is impossible to foresee all possible transgressions, only those actually encountered in commercial production have been listed. Specialist propagation of pathogen-free plant- ing stock is not considered here. ground. Don't use cuttings taken from plants at or near soil level unless you are certain of their freedom from disease. Don't use root divisions unless absolutely neces- sary. (Sees. 3 and 13.) In taking cuttings, break them off when possible, rather than pinching or cutting them. If knives are used, soak one in disin- fectant while using the other, alternating them at frequent intervals. In mother blocks, knives should be treated before starting each new plant. (Sees. 3 and 13.) Place clean planting material only on treated surfaces of benches, flats, baskets, etc., or on previously unused newspaper or wrapping paper; never place it on the ground. (Sees. 3 and 13.) Don't dip clean planting material in water unless absolutely necessary. Never dip clean planting material into a tank used previously for infested stock. Don't use hormone solutions on cuttings of uncertain health; dust hormone powders onto the cut ends of stems. (Sec. 13.) Discard seed flats with any diseased seedlings, or if it is absolutely necessary to use them, transplant only from spots remote from diseased areas. (Sec. 3.) Segregate propagation activities and mother blocks from crop production, and [22] Fig. 7. A 20-gallon crock containing a solution of 1 gal. of commercial formaldehyde to 18 gal. water. Before use, tools are dipped into the solution for a few minutes and the excess drained or rinsed off, or allowed to volatilize. Fig. 8. (Right) The nozzle of the water hose should be kept off the ground when not in use. isolate them from commercial areas. Main- tain them as separate operations. (Sec. 13.) Avoid handling treated soil unnecessarily. Treat soil directly in the containers when- ever possible. Don't nervously dip hands into bench soil while conversing near by. Don't unnecessarily feel the soil for moisture content, or knock plants out of pots, unless the hands are clean. (Sees. 3, 8, 1 1, and 12.) Avoid splashing infested soil particles into treated soil. Don't walk over treated flats of soil, expose treated soil to blowing dust, or kick dust into treated soil. (Sec. 3.) Treat tools with disinfectant before using in treated soil (fig. 7). Use clean or treated cloth or papers to cover seed flats. (Sec. 3.) Place flats on 2 x 4 timbers treated with copper naphthenate or on polyethylene sheets, for hardening plants outdoors. Never place flats of plants directly on soil. (Sees. 3 and 12.) Steam benches or beds after each crop is removed, even if it was grown in pots or flats. If the glasshouse is free of plants, chemical fumigation may be used. This is sanitation insurance. (Sees. 8 and 12.) Hang the hose nozzle on a hook on the side of the bench when not in use (fig. 8); never drop it on the ground. (Sec. 3.) Wash the hands after working with any soil or planting stock not known to be clean, before handling clean materials. (Sec. 3.) Use ditch or surface irrigation with slow stream of water (no lateral flow) on plants used for propagation material. Don't use overhead sprinkling on mother blocks. (Sees. 3 and 13.) Place all new planting material of un- certain health in a special isolation ward until you know it is healthy. Never place it in the middle of or near clean plantings. (Sees. 3 and 13.) Don't underrate the danger of introduc- ing an organism into your nursery or fields because it "sounds" like one you already have. Consider organisms as different- even though they are called by the same name— until they are proved otherwise. (Sec. 15.) Remember that many organisms which cause seedling diseases also attack mature plants, perhaps years later, reducing yield or killing them. Don't use palliative meas- ures (soil drenches, sphagnum cover, etc.) against seedling diseases. (Sec. 3.) Use clean stock in treated soil and containers, and practice sanitation to keep them disease-free. Don't fight diseases, eliminate them. [23] The sanitation requirement is not unreasonable One of the principal objections by growers to the whole program is that an impossible degree of hospital cleanliness is demanded. This is not the case; it would be more accurate to say that or- dinary household cleanliness is expected. The housewife fully cooks pork to avoid giving her family trichinosis, and uses only pasteurized milk to protect them from undulant fever; the nurseryman treats soil to prevent infecting his plants with nematodes and fungi. The house- wife washes and peels fruits and vege- tables to prevent disease; the nursery- man should use only pathogen-free stock for the same reason. In washing dishes she is reducing the danger of spreading colds among her family; in disinfesting the flats, pots, and so on he is only taking comparable precautions for his plants. As she usually does not use food that has dropped on the floor, so he should not place clean flats of seedlings on infested ground. It is not as fussy to avoid walk- ing across flats of treated soil as it is to avoid tracking mud into a clean kitchen. It is accepted practice to sneeze or cough into a handkerchief to prevent spreading colds; similarly a grower will avoid scattering infested soil in watering and handling. One is at least as likely to spread disease among plants by scooping up soil from infested beds or ground in the hose nozzle as to spread sickness by using another's drinking glass. Growers should use treated tools to prevent dis- ease spread in the nursery for the same reason that we use only our own tooth- brush and towel. One does not visit friends while suffering from mumps, nor should one introduce new plants of un- certain health into the middle of a large planting of painstakingly acquired healthy stock. As modern civilization evolved, these accepted modem health precautions were first unknown or ignored, then scorned as Fussy, then grudgingly adopted, and finally accepted as standard procedures. Even today primitive people ridicule many of them. Nursery practice for dis- ease control is similarly developing. At which evolutionary level do you stand in your attitude toward these inevitable de- velopments? Retardant organisms Although much more information is required before the use of retardant organisms is commercially feasible, its potentialities for nurserymen are great. (Sec. 14.) Rather than leaving to chance the re- contamination organism and the time of its introduction, it may be possible to introduce, after soil treatment, a specific organism. This would be selected for its ability to inhibit the development of sub- sequently introduced pathogens by pro- ducing an antibiotic in place, by compet- ing for available nutrients, or by parasit- izing the pathogen. This controlled colon- ization of the soil may perhaps best be ac- complished by using a selected group of organisms, retardant to pathogens, including some which develop at the various soil temperature and moisture ranges to be encountered. At least one of the organisms would always then be ac- tive. Because of the delicate balance be- tween the inhibitory action on pathogens of the antibiotic produced, and its toxic effect on other beneficial organisms and on the crop itself, the more uniform, duplicable, and controlled the soil condi- tions are, the better the chance of success. The adoption of the U. C. system of soil mixes thus brings closer the possible commercial application of this method by stabilizing many of the conditions. A suspension of the beneficial organisms could be atomized over the surface of the treated soil in flats prior to planting. The use of antagonistic soil organisms and the adoption of some uniform soil sys- tem may enter general nursery practice together, reinforcing and modifying each other in the process. I Sec. 14.) [24] Rhizoctonia damping-off has been ex- perimentally controlled in a U. C.-type soil mix under conditions entirely com- parable to those of a California bedding- plant nursery. Species of Myrothecium, Penicillium, Trichoderma, Streptomyces, and Plicaria prevented Rhizoctonia damping-off of pepper seedlings, even when inoculated at the same time as the parasite (figs. 121, 122, and 123). Under some conditions the presence of these re- tardant organisms was slightly injurious to the host as well as the parasite. (Sec. 14.) Nitrifying bacteria might be inocu- lated into the soil at the same time as the antagonists, perhaps reducing the time before nitrate becomes available to the plants. (Sec. 14.) No success has been attained from adding these organisms to raw soil under natural conditions. The microbiological flora of a given natural soil is well bal- anced and has increased to the capacity of nutrients and space under the given environmental conditions. In a word, it is biologically buffered against any or- ganisms that are subsequently intro- duced. Some shock is necessary to upset this balance before effective addition of beneficial organisms is possible. This shock may be achieved by soil treatment, by addition of a nutrient particularly favorable to the organisms added but not to others, and perhaps by modifying soil moisture or temperature. (Sec. 14.) Mechanization in the Nursery The California nursery industry has not been slow to capitalize on the adapt- ability to mechanization of the whole U.C. system of soil mixes and handling. Every nursery has special features around which mechanization must be designed. Certain general ideas of wide utility have been developed, however (fig. 126). (Sec. 17.) Among these are the ones described in the following para- graphs. Mixing, filling, and treating operations A skip-load tractor is used to transport the soil ingredients from the storage piles to a large concrete mixer, in which they and a measured amount of water and fertilizers are mixed. The mix may be dumped directly into a mechanical flat filler (figs. 9 and 127), can filler (fig. 10), or pot filler (fig. 135), after which the containers are stacked on wooden pallets. Alternately, the soil may be dumped into a bulk soil treater and from this into treated containers as above. The untreated containers and soil are loaded into a steam cooker (figs. 6 and 131) or stacked in piles for chemical treatment (fig. 107), the handling being done by a fork-lift tractor. After treatment the containers of soil may be stored for a few days. The flats are coming to be subdivided by insertion of smaller re- movable containers, each holding a dozen plants. (Sec. 17.) Planting and transplanting operations Flats have been planted by machinery when large seeds were used, but there are some difficulties with tiny seed ( snap- dragon, petunia). Attempts to increase the size of small seed by pelleting have been only partially successful because of reduced germination. Successful equip- ment to space-plant tiny seed may soon be available. At present, seed is sown in flats for later hand-transplanting; indeed it is possible that this laborious process may not soon be abandoned, because it provides the opportunity to group uni- form-sized plants together. To balance this point, however, there is the delayed development (10 to 20 days) resulting from the transplanting operation. (Sees. 16 and 17.) Plants for field use (celery, peppers, eggplant, tomato, or plants grown for flower-seed companies or cut-flower growers) have been grown from me- chanically seeded flats. When this is [25] :■■.;.■ Fig. 9. Equipment for filling flats with soil. Fork-lift tractor for conveying stacks of flats on pallets to treating equipment. (Photo courtesy of American Plant Growers, Lomita, California.) See also fig. 127. done, the seed may be sown by a vacuum plate and then covered with tissue paper and sterile sand. The flats are then watered, stacked, and placed in a germi- nation room of high humidity and con- stant temperature. Some growers cover them with polyethylene sheets, or place individual flats in bags of this material. When the seedlings are just emerging, the flats are moved into the glasshouse on temporary lines of steel rolls. Such handling requires absolute freedom from Fig. 10. Automatic filler for 2- or 5-gal. cans which places soil in containers and forms a central depression in which the liner is planted. (Photo courtesy of Oki Nursery, Perkins, Cali- fornia.) See also fig. 135. damping-off, and must be confined to large lots of the same variety. It saves using the glasshouse for as much as 10 days during germination, and provides more uniform moisture conditions with less labor than can be maintained in the open glasshouse. Adaptations of these methods have been made for planting in cans and pots, and such equipment is presently being tested by growers. (Sees. 16 and 17.) Watering Watering of the containers in the glasshouse may be done mechanically with overhead sprinklers or other meth- ods. In some cases fertilizers have also been applied in this way, perhaps fol- lowed by a water rinse. The excellent drainage and aeration of the U. C.-type mix greatly facilitates this method of watering, as there is slight danger of overwatering. The freedom from disease reduces the possibility of spread of or- ganisms during this operation. Further- more, it has been found that seedlings de- velop faster (5 to 11 days for celery) in a U. C.-type mix than in standard ma- nure formulas. (Sees. 16 and 17.) THE FUTURE A unified positive system for the pro- duction of healthy nursery stock is out- lined in this manual. Maximum benefit will be obtained from the adoption of the complete integrated program, and its potential merits cannot adequately be judged from the results of using one phase of it. Some parts (for example, the soil mixes) will prove helpful if used alone. Others possibly could lead to ap- preciable losses (for example, soil treat- ment, followed by planting with infested stock). In order to achieve maximum effectiveness and economic benefit, the grower must alter his procedures to mesh with the U. C. system. In this, as in re- ligion, partial conversion is likely to lead to backsliding. The shift of a nursery to the U. C. system is not as difficult as it may sound. While numerous production practices may require modification, the changes develop logically and progressively. Change in the attitude of the grower may be more troublesome, often presenting the biggest obstacle to success. For this reason, the easiest and most rapid adop- tion of the method has usually been by intelligent, resourceful growers new to the California nursery business, and without preconceived ideas. Established growers who have made the change, how- ever, usually agree that the results have been worth the effort. As one elderly nurseryman phrased it, "It's what you learn after vou think von kn ow it all that counts. " Minds, like parachutes, function only when they are open! With the increasing use of uniform soil mixes, soil treatments, and healthy seed or planting stock, and the trend toward mechanization and the package marketing of plants, any plan for the future must take them into account. It is probable that in some cases a cen- tralized service for providing a uniform treated soil may develop, as it has in England. The soil may then be delivered into bins at each nursery in a treated condition ready for use. Alternatively, the bins may be fitted with a perforated pipe grid, and the soil steamed in place, the steam being supplied by the grow- er's boilers or by a portable steam- generating service. In any case, nitrify- ing organisms and those antagonistic to pathogens can be added. It is certain that changes will come rapidly. Sometimes these may be neces- sary in order to cope with disease prob- lems, sometimes because of competitive pressure for cheaper production, at times for other reasons. The only certainty is that the trend will always be toward the least expensive production of the best possible plants. [27] SECTION Today's Nursery Problems Kenneth F. Baker The California nursery industry Effect of economic changes Mechanization and disease control Causes of disease .he INFORMATION developed in this manual concerns the complex problems of nurserymen and flower producers who grow crops in prepared soil confined in flats, pots, cans, beds, or other contain- ers. Several of the features of California and its nursery industry have such an important bearing, directly or indirectly, on disease control in these crops that they are considered here. THE CALIFORNIA NURSERY INDUSTRY The California nursery industry is an important part of the agricultural econ- omy of the state and nation. According to the 1950 U. S. Census of Agriculture, the state leads in the production of all nursery stock. The wholesale value of the crops grown in 1949 by the 535 produc- ing nurseries (out of 2.500 in the state) that filed returns was $10,789,239, or 15.2 per cent of the national total.' This was more than the next two states com- bined, California also led in the produc- tion of ornamental nursers slock (15.0 'That this figure is very conservative is shown l.\ the L954 farm valuation ($33,324,980) of nursery stock in \2 southern California coun- ties. This was tenth among 66 farm commodities for the area. per cent of the U. S. total) ; floricultural plants, rooted cuttings, and other ma- terial for growing on (17.8 per cent) ; bedding and vegetable plants ( 16.5 per cent) ; and lining-out stock (10.9 per cent) . There were 6,676 nurseries and other outlets licensed for plant sales in the state in 1954-55. There are about 300 growers of bed- ding plants, farm plants, and cacti in the stale. About 80 per cent of the bed- ding and farm slock is produced in and south of San Luis Obispo, Kern, and San Bernardino counties, and about 52 per cent of the stale total is grown within 5 miles of Garden a, in Los Angeles I 28 ] County. It is estimated that 2,500,000 flats of bedding and vegetable plants were produced in the state in the year ending July 31, 1953. Of this number 1,580,000 flats were sold in the state. 2 It was estimated 3 that 10,000,000 1- gallon and 500,000 5-gallon cans were used in California nurseries in 1952. In order to produce such quantities of container-grown nursery stock in Cali- fornia an enormous volume of special soil mixes must be used. The bedding- plant growers use an estimated 39,000 cubic yards annually, and the can nur- series an additional 55,000 cubic yards. It is estimated 4 that the 2,500 nursery growers in the state had an average of one acre per nursery in container pro- duction. Using a conservative 150 cubic yards of soil per growing acre per year, 375,000 cubic yards would be required annually for container growing. A conservative estimate of the amount of soil mixes used for container-grown stock would be 350,000 cubic yards, with the possibility that it may reach 500,000 cubic yards. This represents the top soil (1-foot depth) of 217 to 300 acres of land. This does not include flori- cultural stock grown in beds and benches, a use which involves additional large amounts of soil. Diseases are important Because much of the propagative ma- terial produced in California is distrib- uted over the United States, its disease status is very important and of more than local interest. Increasing attention has, therefore, been devoted to devising 2 Figures in this paragraph are based on data kindly supplied by J. L. Mather, Manager of the former Bedding Plant Advisory Board, Bureau of Markets, California State Depart- ment of Agriculture. 3 Figures prepared by E. J. Merz, Executive Secretary, California Association of Nursery- men, for an O.P.A. investigation of can use. 4 By W. F. Hiltabrand, Nursery Service, Cali- fornia State Department of Agriculture. means for producing pathogen-free plants. Wide variety of crops The nursery industry in the state pro- duces a wide variety of ornamental crops, including bedding plants, propa- gating stock (for example, poinsettia and geranium cuttings), started plants (roses, azaleas, palms), foliage plants, succulents and cacti, trees and shrubs, and herbaceous ornamentals. Pathogen- free seedlings (pepper and tomato, for example) are also grown for the vege- table industry. Thus nurserymen here grow hundreds of kinds of plants and in all sorts of combinations. The term "nursery" as used in California includes many more kinds of crops than in other areas of the country, where it generally refers to woody stock, largely grown in the field. This diversity of crops makes it im- possible to reduce nursery practice to a rule-of-thumb. In this manual, therefore, the general methods for disease preven- tion are outlined, and facts are given from which a sound program can be de- veloped for the specific crops, layout, and location of a given nursery. Semiarid coastal climate Nursery production is concentrated in the counties of Los Angeles (6.9 per cent of the national total), Alameda, San Bernardino, San Diego, Riverside, Santa Clara, Orange, Merced, Tulare, Ventura, San Joaquin, and Kern, in descending order. Most of the ornamental crops ex- cept roses are produced in the cool coastal zone. The sunny, generally mild, semiarid, coastal climate enables the nurseryman to modify his environment more com- pletely than is possible in other compa- rable growing areas. It is more efficient to control soil moisture through irriga- tion than by building rain shelters, to shade plants from the sun than to use supplemental lights, and to accept re- [29] gional temperatures than to heat against subzero weather. This climate also ef- fectively limits certain plant diseases. For example, azalea flower blight causes severe losses in southeastern states but is important in southern California only where a "southeastern climate" is created by the continuously moist conditions un- der lath. The possibility of climatic con- trol in California is great, and the disease-control problem is simplified by this fact. Salinity There is, however, another side to the picture. Because of the semiarid climate, agriculture in California, particularly the southern part, continually faces a salinity problem. Most other growing areas have trouble with salinity only from accumulation of fertilizers in the soil due to deficient leaching. Nursery plants in the southwestern states may be injured by naturally occurring salts in the irrigation water or soil, as well as by too much manure or decomposed leaf molds and composts (Sec. 4). Although the nursery industry uses irrigation and special soil mixes in containers, it has constant and sometimes severe losses from excess soluble salts. One of the ad- vantages of the methods described in this manual is that they reduce this salinity problem. Year-round demand The climate has still another effect on the nursery industry here. This is one of the few regions in the country where gardening and the demand for nursery stock continue through much of the year. Although there is considerable seasonal variation (fig. 11), the market is pro- longed and stable as compared, for ex- August Fig. 11. Distribution of wholesale sales of bedding plants in California, by months, 1951-52. Nearly 75 per cent of the stock is sold in the 6 months from February through June, and in October. (Figures from the former Bedding Plant Advisory Board, Bureau of Markets, California State Department of Agriculture.) I 30 1 ample, with that in the northeastern states with freezing winter temperatures. This fact has made it profitable to mecha- nize operations to a greater degree than in most other areas. Smog injury This problem is most acute in the Los Angeles area, but is appearing elsewhere as industrialization and urbanization proceed. Even if reduction of injury to crops eventually is possible, it will be expensive, directly or indirectly, and will be an added economic burden on the nurseryman. The effects are so serious that many growers are moving to smog- free rural areas. When a nursery thus moves, an opportunity is afforded to de- velop the new unit along lines of a mech- anized U. C. system. Pathogen-free planting stock There is a general trend toward the use of pathogen-free propagating mate- rial. This is evidenced by increasing use of mother blocks, of certification of bud- wood, plants, and seeds by various state agencies, and by the appearance of spe- cialist propagators. The grower response to this control of disease at source is illustrated by the situation of chrysanthe- mum cuttings. In 1943 a method was developed by A. W. Dimock for selecting chrysanthemum stock plants free of Ver- ticillium wilt, and this was adopted by an Ohio propagator. In 1949 nearly 26V2 million cuttings were produced in Ohio (69.5 per cent of the U. S. total) , largely by this concern ! The use of healthy pro- pagative stock eliminates one of the im- portant sources of disease organisms, and is fundamental to the U. C. system. Unit containers for marketing For more effective retailing of plants. California growers developed the use of unit containers. Bedding plants are pro- duced in thin wood veneer, aluminum, or molded-asphalt packages holding about a dozen plants. This method of growing requires a uniform, well-aerated soil, and freedom from disease organ- isms. The use of cans for woody plants in California and Florida has made such stock available all year for transplanting, in contrast to the usual short planting season for stock dug and balled in burlap or heeled in. EFFECT OF ECONOMIC CHANGES Recent economic changes in California are making it increasingly important to reduce the cost of fighting disease, and to avoid the occasional heavy losses they may cause. These changes also make it more important to find ways of cutting labor cost and saving space, and make mechanization more necessary. Land values, tax rates, and zoning restrictions are increasing Increasing population pressure in the state is bringing about real-estate devel- opment, rising land values, higher taxes, and zoning restrictions. From 1940 to 1950 the population increased by slightly more than one half, and one third of the state's dwelling units were constructed. In the Los Angeles and San Francisco- Oakland areas the population increased 37.6 and 41.6 per cent respectively, dur- ing that period. Growers are finding that these developments collectively create one of the w r orst pressures they face to- day, and many are contemplating mov- ing to rural areas. On the other hand, the larger popula- tion will provide an expanding local market and eventuallv reduce out-of- [31] state shipment. The immediate effect, however, is to sharpen the interest in techniques, such as soil treatment, that will reduce cost of production. In addition to escaping the population pressures and the increasing air pollu- tion, a move may be advantageous in other ways. It provides an opportunity to clean up a nursery and make a fresh start along the lines described in this manual. It is also an ideal time to mecha- nize, for this almost always involves re- design of the layout, and is best done on a new site. In some cases the increased value of the city land pays a large part of the c«st of establishing a new, modern, mechanized nursery. The mechanization in turn brings reduced labor cost, in- creased efficiency, and lower production cost. The new rural location need not be on the best level land. Indeed, there are some advantages, such as lower initial cost and possible utilization of gravity- flow operation, in side-hill locations (Sec. 17) . Present grower experience in- dicates, furthermore, that distance from market is not as much of an obstacle as it once was. Eventually the various pres- sures may partially offset each other and hasten the modernization of the Califor- nia nursery industry. Labor costs are increasing Labor costs are increasing rapidly because of both increased pay scale and decreased work output. This has led directly to a growing interest in labor- saving methods and devices, mechaniza- tion, and reduction of erratic crop losses. Returns are decreasing There are many indications that com- petition is intensifying in the nursery business, and that the financial returns to growers are decreasing. There are two ways nurserymen can meet this situation. 1. They may reduce production cost through improved culture, mechaniza- tion, and reduction of erratic unneces- sary losses from diseases and similar factors. 2. They may reduce competition by growing plants that other nurserymen find difficult to produce profitably, rather than those found in most establishments. The "difficult" crop is usually one that requires such painstaking, specialized, or highly skilled techniques for success that most growers are unwilling or un- able to produce it profitably. This may be due to the necessity of controlling some serious disease, or to the develop- ment and exclusive retention of a supe- rior crop variety. Some specialists, for example, grow pathogen-free propaga- tive stock of chrysanthemums, carna- tions, geraniums, or foliage plants. If the propagator produces healthy stock at reasonable cost, growers come to de- pend on him as a source of supply. Other specialists are developing F 1 hybrid flower and vegetable seed that may be purchased only from the originator. However accomplished, such speciali- zation leads to a limited natural monop- oly and reduced competition. The more difficult the problem, the better the job is done, and the more reasonable the charges for it, the greater the chance of thus reducing competition. MECHANIZATION AND DISEASE CONTROL For reasons already mentioned, mech- anization is a present and future fact in the nursery industry. This in turn im- poses certain demands, most of which are in themselves beneficial. Scheduled production at low cost demands dependa- ble results. Just as assembly-line manu- facturing requires that no phase of the 32 process break down, so scheduled mecha- nized production of plants demands that all possible chances for error or failure be removed. Uncontrolled losses from diseases, such as damping-ofl in seed flats, must be eliminated or reduced to unimportance, or the rest of the growing procedure may be stopped for lack of material. Mechanization may lead to bigness, since some kinds of equipment (for ex- ample, flat-making or can-filling ma- chines) can profitably be added only when the volume has reached a certain level. This bigness eventually may intro- duce other problems, as it becomes im- possible for the man who built the suc- cessful enterprise to maintain personal supervision. The daily application of his knowledge, experience, and foresight often is the price of maintained success, and very large nurseries may exhibit slackness or inefficiency for this reason. Increase in size places ever greater em- phasis on assured control of diseases, insects, and soil problems. CAUSES OF DISEASE Many explanations are offered for failure of a nursery crop, and these often confuse rather than clarify. Thus it is said that flats of seedlings have been wa- tered too much or too little, the seedlings were planted too deep or too shallow, the weather was too hot, the plants were too soft or too hard, or came from a poor lot of seed, or were grown in the wrong soil mix. In most cases investiga- tion has revealed that pathogens had caused the disease, and that the condi- tions blamed had merely aggravated the trouble. This confusion is understandable in view of the complexity of some of the situations encountered. Perhaps the com- monest example in southern California is the damping-off-salinity complex. A grower who uses untreated composted soil that is high in both soluble salts and damping-off fungi truly faces a dilemma. If he keeps the soil on the dry side to reduce fungus attack, the plants will be injured by the increasing concentration of salts. If he tries to avoid salinity in- jury by leaching out the salts, or by keep- ing the soil wet to dilute them, favorable conditions may be created for damping- off fungi (fig. 35). Indeed, this situation sometimes is seen in a single flat contain- ing poorly leveled soil. The water runs to the low parts and leaches them, and the seedlings there grow well until they damp-off. In the high parts of the flat, the seedlings are stunted and hardened by salt concentration, but have little damping-off. The only real solution to this situation is to keep the salinity at a low level, and to treat the soil to free it of pathogens. Growers may be further confused be- cause many factors that may aggravate disease once it appears, will not initiate it. It is one purpose of this manual to explain the action of the several factors in this disease complex so that growers will have a rational basis for its under- standing and prevention. [33 1 SECTION Damping-OfT and Related Diseases Kenneth F. Baker D Development of methods for disease control Nursery diseases and their pathogens The Rhizoctonia story The water molds, Pythium and Phytophthora Other organisms that cause nursery diseases Control of nursery diseases iseases OF California nursery crops range from seedling damping-off to leaf spots and stem cankers, fire blight and crown gall, root rots and flower blights. Since this manual is concerned with dis- eases in relation to nursery soil in con- tainers, its coverage is largely restricted to damping-off and related diseases of propagative material. These pathogens necessarily involve soil, live in it, or use it as a base for attacking crops. Excluded are diseases specific to single crops, those in which the causal organ- isms are only incidentally associated with soil, and those that parasitize so slowly that symptoms are largely shown in the post-nursery phase. Included, how- ever, are some of the most insidious, omnivorous, tenacious, and destructive pathogens, causing some of the worst headaches of the nurseryman. DEVELOPMENT OF METHODS FOR DISEASE CONTROL The parallel development of disease control in plants and man is instructive. In early times (and in backward areas even today) lack of understanding of disease led to invocation of supernatural causes. Treatment involved atonement to angry deities, and therefore involved application of unpleasant noxious mix- tures to man and plants. Surgery, per- formed only when imperative and with- out regard to sanitation, was usually fatal. During the mid-nineteenth century, after microorganisms were shown to cause disease, antiseptics came into use to destroy pathogens on or in plant or man. The mortality from disease and surgei \ fell sharply. 'Flic comparable use of sprays on plants developed rapidly, and still provides one of the principal methods of plant disease control. Toward the end of the nineteenth cen- tury the concept was gradually accepted that preventing the introduction of or- ganisms was better than trying to destroy them on or in the host by antiseptics. This gave rise to the aseptic surgery of today, with emphasis on sterilization of equipment and on general cleanliness. Preventing the introduction of pathogens is also the thesis of this publication on nursery diseases, expressed in the motto, "Don't fight 'em, eliminate 'em." This emphasizes using soil and plant mate- rials free from disease organisms, and using cultural techniques which will keep [34] them that way. This is the central core of the U. C. system. In the present day, antibiotics have revolutionized many aspects of medicine and surgery. They may conveniently be injected into animals, but this has not been very successful with plants. They have only a local fungicidal effect when sprayed on plants. Since antibiotics rapidly break down when introduced into soil, it appears that best results there may come from the production of them by the appropriate organisms, in place. Difficulties must be resolved before this method of disease control comes into wide use, even under the uniform con- trolled conditions of the nursery. This, then, provides another reason for uni- formity of soil mix, soil treatment, and handling. The use of antagonistic soil or- ganisms and the adoption of some uni- form soil system may enter general nursery practice simultaneously, rein- forcing and modifying each other in the process; these topics are discussed fur- ther in sections 5 through 7, and 14. NURSERY DISEASES AND THEIR PATHOGENS Types of Diseases Damping-off of seedlings The several types of damping-off may occur separately or simultaneously in a seedbed. 1. Seed may decay before it germinates (the seed-decay phase) or the seedling may rot before it emerges from the soil (preemergence damping-off, fig. 12). Losses of these types are usually blamed on defective seed because of the poor emergence, but are often caused by a number of different fungi. 2. Seedlings may develop a stem rot near the soil surface and fall over (fig. 13). This postemergence phase, the most conspicuous type of damping-off, is caused by Rhizoctonia or by the water molds. 3. Some seedlings may be so tough, or the environment so unfavorable, that the stems may only be girdled, and the plants remain alive and standing (figs. 1 and 14). Although this wire-stem or sore- shin type is less striking than the former, it is just as destructive because the plant is stunted and eventually dies. It usually is caused by Rhizoctonia. 4. Another postemergence variant, commonly caused by water molds, is that in which the rootlets rot from the tips (fig. 15) ; the fungus usually progresses up to the stem, and the plant dies. Top rot of seedlings or cuttings Under moist conditions Rhizoctonia, or sometimes Phytophthora, may spread from leaf to leaf, or stem to stem, through the tops of seedlings or cuttings (fig. 16). It may rot the tops down to soil level, and frequently the crown and roots are uninjured. The fungus may originate from the soil, spreading up the first plants, but remain aerial thereafter. Cutting and stem rot Cuttings may rot progressively from the cut end (fig. 17), the root bases, the wounds of disbudding (rose) or remov- ing of basal leaves (gardenia), or from dead leaf bracts (geranium) . A variant of this is the rot of the propagative piece of Dieffenbachia cane by Pythium or by soft-rot bacteria carried over in tiny lesions from the mother cane. Similarly, the organisms causing bacterial stem rot of geranium, bacterial wilt of carnation, and bacterial blight of chrysanthemum may be transmitted unnoticed in the vascular system of the cutting and cause its eventual decay. [35] Root rot of mature plants The roots of nursery plants in pots, flats, or gallon cans may rot (much as in fig. 15) and cause the death of the plant. The tops may die slowly, with yel- lowing and dropping of leaves beginning at the base, or the leaves may rapidly wilt and die, remaining attached to the plant. The rate of symptom expression in the tops is dependent on the rapidity of root decay. The water molds usually cause this disease, and the soil moisture is important in determining the rate of root decay. Time and Severity of Losses The loss to the nurseryman may be immediate, as in damping-off or top rot of seedlings. It may, however, be delayed and cause slow loss of infected plants, as in root rot caused by water molds when plants are grown under relatively dry conditions. It may cause the death of plants in 5-gallon cans several years after propagation and infection, as is often the case of Choisya infected with water molds. Loss may even be so delayed that it will occur after the plant has been sold and planted in a home yard. Nurserymen often do not learn of such a loss, or may feel that it is beyond their responsibility. Since the soil becomes infested from such a plant, other clean replacements are likely to become diseased and likewise die. Although a very few nurserymen may take the view that these repeated losses will promote plant sales, the gross effect undoubtedly will be harmful to the industry. The buyer may become a dis- couraged gardener and poor customer, may change nurseries in order to find better stock, or may learn the facts and become actively antagonistic. It is cer- tain that the sale of vigorous pathogen- free plants is one of the best ways to build a good reputation and promote sales, both for a single nursery and the industry. Thus the direct or immediate [36] losses and the indirect or delayed ones are of equally great importance to nur- serymen. Losses from these types of disease in nurseries may be very great. Frequently 50 per cent or more of some types of plants (Choisya, succulents, cacti, Cali- fornia "natives") die before sale. When it is considered that the margin of profit in the nursery business is fairly small and becoming smaller, a severe disease loss may be disastrous. An average loss of 1 to 10 per cent from diseases would be a conservative estimate. This must be a severe drain on the slender margin of profit in many nurseries, and probably causes many of them to operate at a loss, at least on some items. Relative Importance of Pathogens In California, as apparently in much of the world, the principal cause of the diseases mentioned is a fungus (Rhizoc- tonia solani) commonly, though not af- fectionately, called "rhizoc." In the past 10 years this fungus has become increas- ingly important as a pathogen of flower, nursery, vegetable, and field crops in southern California. While the reason for this increase is not known, the pathogen is rapidly becoming the most important single fungus causing crop disease in that area. Of less general importance, the water molds (Pythium and Phytophthora spp.) may be locally damaging. Other fungi cause infrequent losses to seedlings and cuttings, but are of minor importance. It is the purpose here to explain and illustrate the nature of these various fungi, and how they survive and spread, so that a grower may better understand what is happening, and better plan his preventive program. Rhizoctonia is dis- cussed in detail as the principal example, and discussions of less important organ- isms refer to it. [37] THE RHIZOCTONIA STORY Recognition of Rhizoctonia Diseases It is reasonably certain that rhizoc is the cause of the disease when (1) the decay originates near the soil surface (fig. 13) , rather than at root tips as with the water molds (fig. 15) ; (2) coarse brown fungus mycelium is seen, with the aid of a good 10 to 15x hand lens, on the decayed parts (figs. 17, 18, and 27) ; (3) there are soil particles clinging to the tough fungus strands after the seed- ling is shaken to remove the soil mass from the roots (fig. 18) . Few other fungi {Helminthosporium cactivorum on cac- tus is one of the exceptions) have been found to simulate these features of Rhizoctonia; the fungi can of course easily be differentiated by growing them on culture media. Cuttings may rot progressively up- ward when infection has occurred below soil level (fig. 27). In humid propagat- ing cases Rhizoctonia often spreads as a coarse web through the tops of some plants (for example, Araucaria cuttings, azalea cuttings and grafts) , matting them together (fig. 16). The Rhizoctonia Fungus Rhizoctonia solani is a simple plant consisting of brown threadlike branch- ing mycelium. The presence of this characteristically branching mycelium (fig. 19, top) on, and particularly in, diseased tissue affords a rapid labora- tory microscopic diagnosis for the fungus. In the soil these filaments grow between the particles and in bits of or- ganic matter (highly magnified in fig. 20). Sometimes these strands fuse to- gether and form visible clumps (sclcro- lia. fig. 19. bottom) that are long-lived and survive drying. Thus, infested soil and flats ma\ be stored dry for 6 months 01 more without killing the parasite. When the fungus grows into contact with a seedling (figs. 12 and 13), its my- celium grows over the surface and pene- trates the tissues (fig. 28), digesting them for its own nutrition, and thus pro- ducing the disease. The fungus is a relatively unspecial- ized parasite, able to attack many kinds of plants, although there are important differences in this ability between strains of the fungus (Sec. 15). Spread of the Fungus, and Preventive Measures Under conditions of plant propaga- tion, Rhizoctonia produces spores ex- tremely rarely or not at all. For all practical purposes there is no air-borne stage, and spread occurs by mechanical transfer of mycelium and sclerotia in in- fested soil particles and infected plant tissue. These bits of mycelium resume growth in the new location. This is of great significance in control procedures. Rain or watering During rain, overhead irrigation, or watering of flats and beds in which the fungus occurs, bits of soil containing its strands commonly are spattered to near- by uninfested plantings (fig. 21). Dipping cuttings Similarly, spread may occur from dip- ping cuttings in water or in hormone solutions. Since water is an efficient car- rier of many kinds of pathogens, it should not be used as a dip in cutting treatments; any materials should be either dusted or sprayed on instead. Soil in watering hose The soil under benches in greenhouses often is infested with Rhizoctonia. When the hose is dropped on the ground after use, bits of infested soil may get into the [38 1 [39] open end (fig. 22) and be washed into a clean planting when next used. This is the principal reason why heavy disease losses frequently occur in beds at points near the faucet. Nozzles should be hooked on the side of benches when not in use to keep them up off the ground (fig. 8). Infested containers Rhizoctonia very commonly lives over between crops in bits of soil on the wood and in the corner joints of flats (fig. 23) . When clean or treated soil is placed in such a flat, and irrigated after planting, the fungus resumes growth and causes damping-off in the corners (fig. 23) or along the sides. It is carried over in benches and cold frames and on pots in the same manner. Flats and benches should be treated with steam or chemi- cals (Sec. 12) after each use to prevent carryover of pathogens. Infested tools and equipment Exposed surfaces and cracks in tools and equipment such as shovels, trowels, dibbles, replanting tools, and wheelbar- rows also afford a place for survival and spread of the fungus (fig. 24). After use in infested soil, tools should be dipped for a few minutes in a crock con- taining 1 gallon of commercial formal- dehyde to 18 gallons of water (fig. 7) ; they may be rinsed in water and used without delay. Grower's hands and feet The fingers of the grower may also carry bits of soil and the fungus from flat to flat while testing for moisture or knocking plants out of pots for root ex- amination. A green thumb may actually be the black hand for seedlings! This hazard is minimized by having only treated soil in clean flats in the green- house range. Equally dangerous and unfortunately common is the practice of walking on the edges of flats in ground beds while [40] watering, for infested soil particles often drop from the shoes into clean flats. Placing containers on ground It is poor practice to place clean flats or cans on ground beds at all; infested soil may be kicked or splashed into flats, or the roots grow through the bottom and become infected, the fungus then spread- ing to the plant above. Outdoor beds where flats are to be placed may be drenched with formaldehyde (1 pint of commercial formaldehyde to 6 1 / 4 gal. water) at % gallon per square foot of surface. They must be kept moist and aerated for 10 to 14 days before use (Sec. 11). Two by four timbers treated with copper naphthenate (Sec. 12) placed flat on the ground will elevate the flats sufficiently to provide inexpensive and fairly effective protection. If flats are placed on polyethylene sheets laid on the ground, protection is also af- forded. Some nurseries pave the area with asphalt. When treated flats are stacked on un- treated ground the bottom one should be discarded; at the minimum it should never be stacked among other clean flats. A good practice is to place stacks of treated flats on clean wooden pallets to keep them off the ground. Unsterilized covers The use of old unsterilized canvas or sacking over seed flats is a common source of infestation of clean soil (fig. 25). Unsterilized lath frames placed over flats may also be dangerous. All such materials should be steamed or treated chemically before re-use. Infected plants or seeds Rhizoctonia may also be carried to clean soil by infected (but healthy ap- pearing) plants or seed. With some crops (tomato, eggplant, pepper) the fruit in contact with the soil may be slightly de- cayed by the fungus, which then develops on and in some of the enclosed seeds 27 CLEAN SOIL 28 [41] I fig. 26). Zinnia seedheads piled to dry on canvas on the ground may similarly be invaded and the seed infected. Such transmission of Rhizoctonia fortunately is not known for most kinds of seeds. It is usual practice to salvage seedlings or cuttings from the margins of an area of damping-off in seed pan or propagat- ing bench, in the mistaken notion that only decayed plants are infected. Ac- tually infection, under some conditions, may occur a few days before symptoms appear. To transplant such stock to clean soil is to transfer the Rhizoctonia fungus (fig. 27). A wise precaution is to cover the diseased areas with inverted tin cans before beginning to transplant, making sure that the can extends well beyond the margins. Root divisions or basal cuttings of plants such as chrysanthemums grown in infested soil commonly carry the fungus to the new planting or propaga- tion bed (fig. 27). This carryover can largely be avoided by using only cuttings from tips of stems a foot or more above the soil, since the fungus is not carried to that height by splashing water. The parasite invades the stems of the seedling or cutting, and once inside the tissues is very well protected from any fungicide (fig. 28). Because of this, it is not possible to cure a diseased seedling by fungicidal application. Em- phasis must be on prevention of infec- tion. It is possible, however, to prevent spread of Rhizoctonia from a small in- fection in a flat by "spot treatment" of the diseased area (Sec. 11). Conditions Affecting Disease Severity The severity of attack by Rhizoctonia is conditioned by the susceptibility of the host, the inoculum potential, and a number of environmental conditions. Some examples of these factors follow. In general, those conditions unfavorable to the plants without being loo detrimen- tal to the fungus will give severe disease losses. The more unfavorable the condi- tions are to the plant, without drastically reducing growth of the fungus, the worse the disease will be. Susceptibility of the host Peppers consistently suffer heavier losses from Rhizoctonia damping-off than do tomatoes, and pansies or stocks are more sensitive than calendula. The sensitivity of the plant to the environ- mental conditions given below must also be considered. Inoculum potential The quantity of Rhizoctonia in the soil determines the potential severity of the disease and, to some extent, the effective- ness of control procedures. For example, if the fungus is present in sufficient quantity in soil, it is almost impossible to control damping-off by chemical treatment of the seed or by treatment of the soil by the dilute-formaldehyde method (sees. 11 and 15). The environ- mental conditions determine whether the potential severity is attained. Soil salinity Soil salinity (Sec. 4) causes, at dif- ferent concentrations, suppression of germination, stunting of plants, killing of the margins or entire blades of leaves, or death of seedlings. Experiments at Pennsylvania State University showed that sublethal salinity increases severity of Rhizoctonia damping-off. This may ex- plain the increasing importance of this fungus in southern California in the dec- ade after 1944, when deficient rainfall greatly increased the salinity problem. Nitrogen and carbohydrate status of plant The higher the relative level of soil nitrogen, the softer the plant growth will be. Above a certain level this increases susceptibility to Rhizoctonia damping- off. Favorable light for the plant enables [42] it to produce sufficient carbohydrates for thickened cell walls and sturdy growth. To a considerable extent, nitrogen sup- ply and carbohydrate level (sunlight) offset each other. A plant grown at a barely adequate nitrogen level in sub- dued light would be nitrogen-deficient in bright light, and one with adequate nitrogen in full sun would have too much in reduced light. Thus, light must be considered in determining an adequate nitrogen level. Generally, high-nitrogen or low-carbohydrate seedlings are sus- ceptible, whereas the hard plants of low nitrogen or high carbohydrate are more resistant. Watering Application of water affects plant suc- culence and susceptibility, in part through affecting nitrogen intake. At- tack by water molds can be reduced in severity by maintaining the soil as dry as will permit plant growth, but this will not inhibit Rhizoctonia. Rhizoctonia con- trol through reduced watering probably is mainly operative on the plant rather than on the fungus. Soil temperature The growth of both plant and fungus is affected by soil temperature, but the effect is often unequal in degree and range. Thus, a strain of Rhizoctonia may severely injure peas (a low-temperature crop) at high, but do little or no damage at low soil temperatures, and attack beans (a high-temperature crop) at low but only slightly at high soil tempera- tures. Depth of planting Deep planting of seed delays emer- gence and keeps the seedling in a sus- ceptible state (devoid of light and there- fore low in carbohydrate) for a long period of time. This naturally favors incidence of damping-off. Reduced vitality of seed . . . causes delayed emergence of the seed- ling, with much the same effect as deep planting. Old weak seed may have more trouble from damping-off than new seed of high rapid germination. Rhizoctonia Infections on Mature Plants Mature plants as well as seedlings are affected. It is a mistake to regard damp- ing-off and cutting-decay fungi as limited to juvenile plants, although their damage may be greatest there. Rhizoctonia causes, in addition to seedling damping- off, serious losses from wire-stem gir- dling of mature stocks, peppers, cabbage, and other plants in the field, as well as from stem rot of mature carnations, pansies, and petunias. It has also caused serious rot of the deep roots of nursery roses, asters, and camellias. The water molds may even assume their greatest importance in the postnursery growth. Transplanting apparently healthy, but in- fected, seedlings to the field or bed does not end the matter, for they frequently die later and give an irregular stand (fig. 29), as well as infest the soil with the fungus. Temporary suppression is not control Some measures aim at suppression of the fungus in seedbed or propagation frame by the use of: 1. A sand or sphagnum moss surface layer; 2. Reducing watering; 3. Increased aeration of seedlings; 4. Reduced use of nitrogenous fertilizers: 5. Increased light; 6. Fungistatic drenches (for example. PCNB), or even fungicides having poor soil penetration (Arasan, cap- tan). [43] These palliative treatments may be quite effective when skillfully applied, and therein lies the danger. The fungus may be suppressed under the controlled conditions of the seedbed or flat, only to appear again when the plant is in the pot, the 5-gallon can, or in the largely uncontrollable environ- ment of the commercial or home plant- ing. Under these conditions the suppres- sive measures may be ineffective or un- economical, and it finally becomes evident that the loss has been merely postponed until the investment is greater. (see also "The Water Molds," below). Strains of Rhizoctonia Rhizoctonia strains differ in response to various environments and hosts. As pointed out in Section 15, it is unsafe to assume that all strains of Rhizoctonia solani, which are fairly common and widespread, are alike and that the dis- tribution of diseased stock is therefore unimportant. In comparison with highly specific soil organisms, such as the wilt fusaria, which attack only a single species of plant, this fungus is unspecialized ; but there are differences in pathogenicity among strains of Rhizoctonia sufficient to be of great economic importance. There are saprophytic forms of R. solani in most field soils, but this is no excuse for bringing in virulent pathogens on the stock to be planted. In addition it must be considered that diseased stock, because it is already in- fected, will suffer more rapid and severe injury than that infected in the field. Planting diseased stock also serves to increase the quantity of pathogen present and to distribute it more uniformly through a field. For all of these reasons it is important that the stock produced not be merely disease-free (that is, healthy appearing), but that it be pathogen-free as well. THE WATER MOLDS, PYTHIUM AND PHYTOPHTHORA Types of disease Damping-off caused by Pythium or Phytophthora usually starts at tips of main or lateral roots but may rapidly involve all parts below ground (fig. 30) and thus cause the seedlings to fall over. These organisms also commonly cause decay of seeds or seedlings before they emerge from the soil (as in fig. 12). The fungi The fungus mycelium on the roots is fine, colorless, and difficult to see with a hand lens; it is so delicate that it does not hold soil particles, as does that of Rhizoctonia. The mycelium grows be- tween soil particles and in organic mat- ter l as in fig. 20) through the top several inches of soil; it is therefore in a posi- tion to invade root tips. These fungi generally are damaging to plants only when the soil is very wet, hence the name "water molds." Rhizoctonia, on the other hand, develops best under conditions of moderate soil moisture. Under favorable conditions the water molds produce in soil and on its surface microscopic saclike structures (zoos- porangia),from which emerge numerous swimming spores (fig. 31). These swim about in water for a time before develop- ing into mycelium, which penetrates a seedling root. In diseased plants these fungi com- monly develop thick-walled oospores (fig. 32), which are long-lived and very resistant to drying. These spores are use- ful in rapid laboratory microscopic ex- [44] amination of suspected roots for this group of fungi, which frequently are difficult to isolate in culture. Spread The water molds are spread in the various ways just described for Rhizoc- tonia, and in addition the swimming spores may be scattered in splashing drops, irrigation water, and so on. These motile spores may develop in standing water in small reservoirs and irrigation canals, and be spread by using such in- fested water. They are not, however, found in city water supplies in Cali- fornia. The oospores are released to the soil by decay of the plant, and may function there in the same way as sclerotia of Rhizoctonia. Therefore, the water molds, despite the sensitivity of the mycelium and motile spores to drying, may survive in dry soil for several months and be spread in dry soil or on tools or flats. The mycelium, motile spores, or the resting spores of water molds are not carried by air currents. The water molds may be introduced to clean soil with in- fected seedlings and cuttings. Phytoph- thora also is known to be carried in the seed of some types of plants. Effects on mature plants The water molds are important in postnursery phases of growth as well as in the seedbed. There is increasing evi- dence that water molds, as well as some other organisms, may retard root de- velopment (and thus plant growth) without invading tissues. Because of this effect, as well as actual root decay, many of the water molds are dangerous both in the nursery and postnursery phases. Phytophthora cinnamomi may cause de- cay of tiny heather cuttings and the death of large plants in cut-flower fields; it may cause loss of avocado rootstock seedlings in the nursery, and of large trees in the grove. [45] Gravatt (1954) 1 has recently called attention to the fact that Phytophthora cinnamomi is an introduced danger to native stands of chestnut, shortleaf pine, Douglas fir, and Port Orford cedar, as well as many cultivated plants, and that it is widely spread with nursery stock. Control procedures should be employed that eliminate these fungi, rather than those that temporarily suppress them. OTHER ORGANISMS THAT CAUSE NURSERY DISEASES Other organisms than Rhizoctonia and the water molds may sometimes cause diseases of nursery crops. In addition, some other seedling diseases, such as downy mildew of snapdragon or bac- terial leaf spot of delphinium, may be confused with damping-off. It is possible to discuss here only representatives of such seedling diseases. Because of the difficulty of distinguishing some of these less common troubles, the grower should, when in doubt, consult the local farm advisor. The policy, already discussed, of eliminating the pathogen should be adopted whenever possible. Gray mold Under certain circumstances Botrytis cinerea may cause losses in flats of a wide variety of plants. The fungus is able to infect only through dead or dying plant parts under continued cool, moist conditions, and works from the top downward. It starts in (1) seedlings in- jured by other causes, such as fertilizer or salinity burn or water dripping from the greenhouse, or (2) from foreign plant parts (for example, petals) that have fallen on the seedlings. The fungus spreads to adjacent seedlings, which finally become covered with a woolly gray growth (fig. 33). This fungus can be identified by this growth, by its re- striction to cool, moist conditions, and by the fact that it begins with injured or dead parts. ' Citations given in the lex! by author and • late will be found, listed by sections, under "References" in the Appendix. It is possible to prevent this disease by growing seedlings under either drier or warmer conditions and avoiding in- juries to the plants, despite the fact that the fungus is extremely common and is air-borne. The cottony-rot fungi Sclerotinia sclerotiorum and S. minor sometimes cause damping-off of seed- lings. They attack healthy plants and under cool, moist conditions spread very rapidly through the flat or seedbed. They are easily recognized by the dense, white, cottony growth in which are found numerous hard black sclerotia (fig. 34). These fungi most commonly occur as soil organisms, being spread and controlled in much the same way as Rhizoctonia. The sclerotia may also sometimes be spread with the seed. A shooting-spore stage is rarely formed under greenhouse conditions, so the air-borne spores may be ignored there. Under field conditions, however, they are rather commonly de- veloped, and infection by the air-borne spores gives rise to the white blight of aerial parts of some crops (for example, stock, petunia). The aster-wilt fungus Fusarium oxysporum f. callistephi may cause damping-off in seed flats or beds. It attacks no other plant than the China aster and causes damping-off only in very heavily infested or warm soil. The seedling rots from the root upward and falls over, without showing any fungus growth. Recognition of this [46] trouble may require the assistance of the farm advisor, but it should be suspected if losses are restricted to aster, if the soil is known to be infested with aster wilt and has been held at temperatures above 70° F, or if untreated seed has been planted. Control of this disease is by soil and seed treatment. Nematodes These animals may cause (1) swell- ings on the roots (root-knot nematodes, Meloidogyne spp.) , (2) roots to be killed or lesions produced on them (Pratylen- chus spp. and many other surface-feed- ing types), (3) stem enlargement or necrosis (stem and bulb nematode, Dity- lenchus spp.), and (4) dead areas in leaves (foliar nematodes, Aphelenchoides spp.). They are tiny, colorless, wormlike animals barely visible to the naked eye, and are spread in the ways already de- scribed for Rhizoctonia. There are many types of soil-inhabiting nematodes, and a great deal of specialized information has been published about them. For our purposes here, however, it is important that the same general control procedures (clean soil — see sees. 8 and 11; clean stock — see Sec. 13; sanitary practices — see sees. 1, 12, and 14, as well as this one) outlined for fungi in this manual will prevent their damage. Other seedling diseases Other diseases are frequently confused with damping-off. For example, the downy mildew ( Peronospora antirrhini) of snapdragon may attack the leaves and stems of seedlings, killing them to the ground. The leaves have a dull green color, become rolled, and have a dense, dirty white, mealy fungus mass on the undersurfaces. Seedlings killed to the ground frequently will resprout and de- velop new tops, a condition that does not occur with damping-off. The disease oc- curs only under very moist cool condi- tions, and may be prevented by growing in a drier or warmer greenhouse. The application of a Parzate dust has proved effective in control. The bacterial leafspot of delphinium (caused by Pseud omonas delphinii) sometimes kills seedlings. Infections start as tiny watersoaked spots in the leaves that may become black or, under very moist conditions, may spread through the whole plant. The disease spreads down- ward from the top, and is thus distin- guished from damping-off. If the disease is controlled by a spray of dilute Bor- deaux mixture, the seedlings will sprout again. [47] CONTROL OF NURSERY DISEASES Disease organisms may be introduced into a clean planting through the medium of (1) the soil, (2) the seeds, cuttings, or other planting material, and (3) bits of soil on such things as tools, flats, and hoses, or splashed by water. Damping-off and related nursery diseases are most effectively combated when preventive treatment aims at elimination of the fungus from the above sources before beginning the given oper- ation. Such a complete program demands foresight and planning, but has been successfully adopted in several nurseries. Control Measures Needed Treat soil Steam or chemically treat all soil mixes and propagating media to destroy disease microorganisms and weed seeds in them. This preferably should be done in the container (for example, flat or bed) where it is to be used. See sections 8 and 11 for details of methods. Use pathogen-free seed and planting stock There is no point in planting infected stock in treated soil, nor is there any excuse for planting healthy stock in in- fested soil. Seed may be heat- or chemi- cally treated to free it of pathogens. Vegetative propagative material can be freed of disease organisms in various ways, and such stock is already available for a number of commercial crops. See Section 13 for details. Follow a sound sanitation program To prevent contamination of the clean plant in treated soil it is necessary to disinfest tools, flats, and other con- tainers (sees. 1 and 12). Handling prac- tices that spread disease organisms should be discontinued or modified (see 'The Rhizoctonia Story," above). More direct ways for coping with this recon- tamination problem are under study (Sec. 14). Spot treatments for limited infestations When a small area of seedling disease appears in a valuable bed or flat, its spread may be stopped by application of a chemical drench (Sec. 11) and the un- infested plants saved. In all stock that is to be grown on for extended periods or is to be planted in uninfested field soil, this method is beset with many hazards, as already explained. The most intelli- gent use of spot treatments is for confin- ing disease to a specific area in a grow- ing crop as a means of reducing loss, particularly when the land in which the seedlings are planted is to be subse- quently treated to destroy the infestation. Relation to Certification Programs In California, nurseries which meet the high standards of freedom from dis- eases and pests required by Rules of the State Director of Agriculture may use intercounty nursery stock certificates ("pinto" tags) on shipments within the state. Shipments bearing these certifi- cates need not be held for inspection at destination, as would otherwise be neces- sary. One of the requirements of the Director is that certain plants must be grown in treated soil and adequately protected from reinfestation in order to be eligible for movement in shipments bearing such certificates. In effect, these certificates provide an indication that the nursery is using ap- proved disease-prevention practices. The program is voluntary, but high standards must be maintained if the certification is [48] to be kept. Many nurseries treat all soil used, whether or not it is required, and this permits continued use of the certi- ficates. Clean soil is important in any certification program. Benefits from Elimination of Diseases When a disease is eliminated from a nursery, production becomes easier, more certain, and less expensive. Increases growth potential of crop Many of the "secrets" of growing various crops are simply practices, ar- rived at by costly trial and error, which enable a grower to live with a disease. In almost all cases, however, the produc- tion would be improved if the disease were not present, because of the in- creased growth potentialities of the crop. Thus, V erticillium wilt of chrysanthe- mum can be controlled by using resistant varieties, but more and in some cases better varieties are available if this is not the determining factor. Control of this disease has in this way undoubtedly speeded the adoption of year-round pro- duction. Some growers have been able to produce susceptible varieties in in- fested soil by very careful and costly watering, but are now adopting the preferable soil treatments. Similarly, losses from Phytophthora root rot of heather can be reduced by minimal watering, but plant growth is retarded and more skill is required in watering than when the disease is absent. Increases environmental tolerance of crop The presence of a disease often dras- tically restricts the range of variation in some environmental factor that is toler- ated by a crop. Elimination of the disease in such cases therefore permits use of the full growth range of the crop, and makes for easier, less restricted culture. In southern California nurseries, the problems of salinity and damping-off illustrate the confusion which often arises when disease prevention is at- tempted through controlling the soil moisture (fig. 35). In the absence of salinity and of parasites, seedlings grow over a wide moisture range. If salinity exists, the soil should be kept moist (Sec. 4) and, if water molds are present, it should never be watered excessively. The presence of Rhizoctonia complicates the situation, since it generally is favored by intermediate moisture levels, the same as the seedlings. Because uncertainty fre- quently exists as to the exact problem or problems involved, it is understandable how confusion has arisen from attempts to solve these problems by water control. The only real answer to this complex situation is the elimination of disease, rather than trying to "live with it". A similar difficulty involves the pro- duction of China asters (fig. 36). Fusa- rium wilt is favored by soil temperatures of 60° to 85° F, and most favored at about 75° to 80°. Growers who at- tempt to reduce wilt losses by growing in the cool season or along the coast have sustained heavy losses from Botrytis and Rhizoctonia crown rot. The plants re- main in the rosette stage under short-day conditions, the shaded lower leaves die, and moisture is favorable for Botrytis and Rhizoctonia. The best answer is to eliminate Fusarium and Rhizoctonia, and to grow the plants during warm weather and long-day conditions. Benefits are greatest with best culture The better the culture of a crop the greater will be the benefit from elimina- tion of disease, because each healthy plant will produce greater financial re- turn than under poor culture. For this reason new methods of disease control are usually taken up first by, and prove most beneficial to the better growers. [49] No parasites or salinity Salinity , Rhizoctonia , Water molds Salinity only O O z < Salinity, water molds Water molds only Salinity, Rhizoctonia Rhizoctonia only Water molds, Rhizoctonia Dry Moist Dry Moist SOIL MOISTURE SOIL MOISTURE Fig. 35. Diagrams illustrating some of the difficulties of preventing damping-off and salinity in seedling production through controlling the soil moisture. The shaded areas indicated the soil- moisture levels which result in best growth because of least damage from salinity and damping- off due to Rhizoctonia or water molds, separately and in various combinations. Each of these three factors restricts the levels of moisture that may be maintained with safety, and the presence of all three essentially precludes useful control in this way. No parasites O C£ O Fusarium wilt only ^rsj Botrytis crown rot only Fusarium and Botrytis Fig. 36. Diagrams illustrating some of the difficulties of preventing Fusarium wilt of China aster in southern California by planting in the cool winter season. The shaded areas show the soil temperatures that result in best growth be- cause there is least damage from the indicated diseases. Each disease restricts the range of temperatures at which the crop can be grown. 40 50 60 70 80 90 SOIL TEMPERATURE ( F) [50] Makes possible the evaluation of cultural practices A diseased plant cannot grow as well as its environment permits. This may be, for example, because a deficient root system restricts absorption, a deficient leaf area reduces carbohydrate forma- tion, or because an injured stem impedes movement of water, nutrients, and foods between them. For these reasons, a dis- eased plant gives little indication of the growth potentiality of its performance if it were healthy. The only true indicator of the value of any given cultural practice is provided by a healthy plant with a sound root system (Sec. 5). Plants with defective roots may show responses to fertilizer application rang- ing from none to nearly normal, depend- ing on the degree of root damage. Fer- tilizer trials with plants of this nature usually show no gains, whereas healthy plants would have benefited from them. It is difficult, therefore, for a grower to determine, through experience, the best fertilizer procedure for a crop unless he deals with reasonably healthy plants. Similarly, nutritional investigations that use infested stock (even though it appears clean), untreated soil, or both, may provide no valid indication of the potential response of the crop. Nutri- tional research should be conducted with healthy plants in order to be generally applicable. It is not necessary for a large part of the root system to be injured to produce serious effects. Plants sometimes show severe injury from the mere loss of the young white root tips, sometimes re- ferred to by growers as loss of "root action." This is because most of the ab- sorption occurs in that zone of the roots. Furthermore, the combining of the nitro- gen absorbed by the roots, and the carbo- hydrates formed by the leaves, into amino acids may occur in the roots. These acids are later used in forming the proteins of the plant. Perhaps growth- regulating substances are also formed in the roots (Jackson, 1956). The whole plant is thus seriously affected by partial loss of roots. If studies on irrigation practices are conducted with plants that have diseased roots, an entirely erroneous idea of the water requirements of the crop will be obtained. Furthermore, plants with in- jured roots may wilt when exposed to light of an intensity necessary for ade- quate carbohydrate formation in the leaves. If there is insufficient carbohy- drate formed and conducted to the roots they will be further weakened. Light re- quirements may also be determined only on healthy plants. Reduces cost of other disease controls The use of disease-free stock fre- quently reduces the cost of other disease- control procedures. Thus, a California celery grower found that seedlings grown from hot-water-treated seed in steamed soil were free from late blight, and when planted in the field required spraying only toward the end of the season, where- as plants grown from untreated seed had to be sprayed throughout the season. Reduces danger of disease panics Periodic disease panics, such as those concerning rose mosaic in 1929-1932 and chrysanthemum virus stunt in 1947- 1950, would be considerably reduced if disease-free stock were more generally used. Knowledge is the best defense against these disturbing upheavals, par- ticularly when it is put into practical use. For additional benefits see sections 2 and 16. [51] SECTION The Salinity Problem in Nurseries Warren R. Schoonover R. H. Sciaroni How the problem of salinity arises Salt injury to ornamental plants Detection of salts in the soil moisture What can be done about salinity The U.C.-type soil mixes and the salinity problem R Iurserymen are increasingly aware of the fact that excess soluble salts in the root zone have been responsible for the failures of many ornamental and horti- cultural plants. Excess soluble salts in the soil, known as salinity, are not confined to nurseries, but are quite common in soils of arid and semi-arid regions. Studies of the salinity problem with agri- cultural crops in the laboratory and field have yielded much information that is directly applicable to the solution of the problem in nurseries. This section has been prepared so that growers will have a better understanding of the salinity problem and what can be done about it. What are salts? Salts are chemical compounds consist- ing of an acid part, or ion, and a basic part, or ion. For example, common table salt — sodium chloride — consists of one acid-forming ion, chloride, and one basic ion, sodium. The two combine in chemi- cally equivalent quantities to form a neutral salt. Some common acidic ions are sulfate, nitrate, phosphate, and bi- carbonate. Common basic ions are cal- cium, magnesium, potassium, sodium, and ammonium. Any basic ion may com- bine with any acidic ion, a great variety of salts thus being formed. HOW THE PROBLEM OF SALINITY ARISES All nutrients needed for plant growth arc; absorbed by the plants in the form of salts or their ions. Some salts contain no plant nutrients, others contain nu- trients essential to the plants, and are Ix-ncficial in proper amounts. All salts, however, are harmful beyond the small quantity needed for growth. The prob- lem arises when the concentration of soluble salts in the soil moisture reaches levels that are harmful. Salts may come from fertilizers, water, or soil. [52] Excess chemical and organic fertilizers In nursery soils the origin of harmful salts is most frequently from chemical and organic fertilizers which have been used in excess (fig. 40). Chemical fer- tilizers such as ammonium sulfate, am- monium nitrate, and potassium sulfate are already in the form of salts soluble in water. Organic materials such as dried blood, hoof and horn meal, and leaf mold become mineralized through decay processes, and the nutrients are finally converted into salts. Nitrogen in the organic form (for example, in hoof and horn meal, manure, and leaf mold) breaks down slowly under cool condi- tions (Sec. 7). Growers may add such a fertilizer, and when it does not become available, add some quickly available in- organic nitrogen. With return of warm weather, accelerated decay processes may produce a sudden excess of water-soluble nitrogenous compounds which will cause salinity injury. Improper irrigation practices Irrigation without proper attention to leaching may cause a build-up of salts in the root zone. Practically all irrigation waters contain salts, sometimes in in- jurious amounts (fig. 38). This accumu- lation, added to that from fertilizers, may result , in dangerous levels if periodic leaching is not practiced. Injury is ag- gravated by permitting a saline soil to become somewhat dry (fig. 44). There is evidence that salinity injury may occur on some plants, such as begonias, from the sprinkling of saline water directly on the foliage (fig. 39). In addition, steam and chemical soil sterilization may bring about the release of certain chemicals, such as ammonium, at levels toxic to plants; this topic is discussed further in sees. 6, 7, and 14. Poor drainage and soils initially high in salts The use of soils with poor drainage, especially those which already have a high content of soluble salts (fig. 37) is hazardous. Poor drainage restricts leach- ing and may therefore lead to salt ac- cumulation. On several occasions serious financial losses have been incurred when soils, sedge peat, leaf mold, compost, manure, and similar materials, high in content of soluble salts, were used in growing mix- tures. Some animal manures may be particularly dangerous. In most feedlots, manure is scraped up, dried, and sold. Many feedlots use salt-grain mixtures to fatten livestock. This salt, added to the urine accumulated in surface manure by the evaporation of water, may result in extremely high salinity levels (figs. 41 and 42). During the past few years nurserymen have been shifting towards a standard soil mix such as the U. C. type. One of the reasons for this shift has been the danger involved in using many unreliable ingredients in a soil mix, some of which might be initially high in salts. Clay pots and salt injury It has been observed that used clay pots may also contribute to the salinity problem. This is because moisture evaporation on the outside surface leaves behind the soluble minerals in concen- trated form (fig. 45). Roots which come in contact with the pot may be injured and even killed. It has also been shown that soluble salts may cause rotting of leaves of Saintpaulia which come in con- tact with the salt crust on the rims of clay pots. These problems may be avoided by soaking the pots in water before using them again (Sec. 12). They may be eliminated by using cans or plastic con- tainers. [53] 37 40 38 41 -£:£4 ] SALT INJURY TO ORNAMENTAL PLANTS Salinity injury occurs over a con- tinuous range extending from no ap- parent damage to rapid death. In the lower concentrations of salinity there may simply be reduced growth without any visible symptoms. With somewhat higher concentrations, the plant may absorb considerable quantities of salt, which tend to accumulate at the leaf margins or tips and there will cause actual burning when, through evapora- tion, the concentration finally reaches a lethal point (figs. 3, 46, 54, and 55). Of course, this slow attrition may, with sus- ceptible soft-leaved plants or plants grown under full sun and under dry con- ditions, lead eventually to death of the plant. The more usual thing, however, is to render it unproductive and unsightly (figs. 53 and 58). If a plant is trans- planted into a sufficiently saline soil, it may collapse within a matter of hours. There is a great variation among types of plants as to salt tolerance. Some plants (carnation) will tolerate fairly high concentrations of soluble salts in the soil moisture. The only symptoms may be a slight yellowing and slow decline in vigor. In contrast, some plants (gar- denia) are so sensitive that root corro- sion and scorching of the leaves will develop shortly after exposure (figs. 56 and 57) . Some saline conditions may cause con- siderable injury to the roots (fig. 56). Root injury due to salinity may lead to chlorosis of foliage. Salinity injury to the foliage of plants is accentuated under conditions causing high transpiration water loss; for example, in bright sun- light under hot, dry conditions as op- posed to humid, cool conditions. Shad- ing greenhouses and humidifying the atmosphere reduces transpiration and hence the rate at which salt injury to the foliage occurs (fig. 51). Salinity may aggravate the losses from seedling damping-off (Sec. 3). With stocks in field plantings, salts accumulat- ing in the tips of leaves may increase the severity of attack by the Botrytis gray mold (fig. 3, C) . Nematodes and fungi may produce plant symptoms easily con- fused with salinity injury. In general, certain symptoms are typical of the injury caused by excess soluble salts. All or part of the following may develop under conditions of high salinity in the soil moisture. Poor Germination of Seeds Poor germination is particularly im- portant in the growing of bedding plants started from seed. Salts may build up (fig. 43) to a point where germination is greatly reduced. Seeds that do germinate may produce plants that are stunted and may be killed suddenly. Many times the surface 1 /4- to 1-inch layer of soil may accumulate more soluble salts than the second or third inch. This situation may develop as a re- sult of overfertilizing or insufficient watering to induce leaching, or both. Evaporation from the surface of the soil will leave salts behind in a concentrated form (fig. 43) . Therefore, the entire root systems of small seedlings may be ex- posed to high salt levels. When the sur- face of the soil is allowed to dry slightly, the concentration of salts in the soil moisture increases and the seedlings die quickly (compare figs. 43 and 44). Injury to Tops and Roots of Plants Plants grown in containers or in raised or ground benches under highly saline conditions may develop all or part of the following symptoms: plant stunting (fig. 53),. yellowing, wilting, or shedding of leaves (figs. 57 and 58), tip or marginal [55] 43 • ' ■ -T ~-» 45 ACCUMULATION AND CONCENTRATION OF SALTS Fig. 43. Salts concentrate in soil surface from evaporation of water there, leaving a deposit of salts in upper layers. This is the zone in which seeds germinate, and in which roots of seed- lings and shallow-rooted annuals develop. Fig. 44. Salts become concentrated in soil when the water content reaches low levels. Plant injury from salinity is thus aggravated by the practice of "growing plants dry." Fig. 45. Salts accumulate in clay pots because water evaporates from the surface, leaving a crust of salts. With many soils the roots are most abundant next to the pot, exposed to saline conditions. Metal or other nonporous containers may be preferable in California. Fig. 46. Salts accumulate in leaves, particularly at margins and tips, because water evaporates from them, leaving salt accumulation behind. When these reach toxic levels the tissue may be killed. (Based on a chart by K. F. Baker.) I r >6 1 47 !/i / riin t -ni'r m/lllllilllllii m i/l!lliim/ /llll/TTTiJTi •50 =T^— - s^ii^S ?5> 3T-C 48 s 1 'ft 49 . ': . PREVENTION OF SALINITY Fig. 47. Rainfall or irrigation with deionized water will wash the salts down or (in containers) leach them from the soil. Because no other salts are introduced, this treatment is very effective. Fig. 48. Leaching soil with water of the best quality available, washes the salts from the bottom of containers. The poorer the water used in irrigation, the greater the excess over plant require- ments that must be used, and the more frequently leaching must be done. Fig. 49. Keep the soil moist so that the salts are diluted and plant injury minimized. Fig. 50. Provide good drain- age so that salts may be washed away. Keep holes open in pots and cans, and the bottom cracks in benches; avoid hardpan soils. With poor drainage and high water table, salts remain in place and concentrate from surface evaporation. Fig. 51 . Provide shade and high humidity for salinity-sensitive plants, to reduce water loss from leaves and salt accumulation therein. Fig. 52. Use fertilizers in small quantities as often as needed, interspersed with liberal watering. This will keep salts at a low concentration. (Based on a chart by K. F. Baker). [57] \ Fig. 53. Soluble-salt in- jury to camellia. The plant on the left is healthy; the one on the right was grown under conditions where soluble salts accumulated. Symptoms were wilting and severe leaf burn in some cases. "~*tm.£ m Fig. 54. Salinity injury to Roosevelt fern. Note the marginal leaf burn on the left caused by excess soluble salts in raised benches. Healthy leaf at right. leaf burn (figs. 3, 46, 54, and 55), de- creased root activity and sloughing of roots (fig. 56), and complete collapse of the top of the plant (fig. 57). Experience with ornamentals has shown that they react variously to excess soluble salts. The symptoms described below were observed under actual grow- ing conditions in commercial nurseries. Carnation, stock, and amaryllis . . . will tolerate relatively saline conditions. A ^li^rht yellowing, particularly of older leaves, and slow decline in \ Igor are typi- cal. Production drops off slowly as salt content increases. Camellia, rhododendron, and Roosevelt and Boston ferns Tip or marginal burning of leaves de- velops (figs. 54 and 55). There is also decreased root activity and partial de- foliation (fig. 53). When conditions which have contributed to excess soluble salts are removed, recovery may be very slow. By the time the tops show visible symptoms, considerable damage has al- ready been done to the root systems. I 58 1 ? f ' ; * 1 i § Fig. 55. Individual leaflets from Roosevelt fern, showing salinity damage. The two leaflets on the left are healthy; the others show various stages of leaf scorch. Fig. 56. Decreased root growth of gardenia caused by salt build-up resulting from too much chemical fertilizer. The root system at the right is normal. Fig. 57. Excess soluble salts caused wilting and collapse of tops of the three gardenia plants at the left. The healthy plant at the right was grown under conditions of low salinity. Fig. 58. The azalea plant on the left was killed by excess soluble salts. Poor drainage and lack of aeration may cause similar symptoms on azaleas. The plant on the right is healthy. Cymbidium orchids . . . show a leaf tipburn and dieback. Cattleya types become yellow, and new growth is stunted. Gardenia and azalea Decreased root activity and sloughing off of roots occur. There may be wilting of leaves and sometimes a spectacular collapse of the entire top of the plant (figs. 56, 57, and 58). Excess salts are often blamed for these conditions on gardenia and azalea, however, when ac- tually poor drainage and lack of aeration are the real culprits. Azaleas have a high requirement for good aeration and drainage. DETECTION OF SALTS IN THE SOIL MOISTURE The Agricultural Extension Service, commercial soil-testing laboratories, and many growers are using instruments called Wheatstone bridges to study sa- linity problems. Salt solutions conduct electricity to an extent approximately proportional to the concentration, and this conductance may be measured by such bridges. Simplified forms of these instruments called "Solubridges" are available on the market. Two types are in common use, the RD-26 and RD-15. They measure electrical conductivity in terms of reciprocal-ohms or mhos/cm. Since the mho/cm is a large unit, a scale reading in the range of salinity injurious to plants would be expressed as small decimals. Therefore, the units have been divided by 1,000 into units called mil- limhos or by 100,000 into unnamed i<>0] units. The scale on the Solubridge RD-26 gives readings in millimhos. Readings obtained on the RD-15 Solubridge can be converted to millimhos by dividing by 100. The U. S. Regional Salinity Labora- tory, and the Agricultural Extension Service and the Department of Soils and Plant Nutrition, University of Califor- nia, have adopted the saturated-soil-ex- tract method for determining the salinity of soils. An approximate value for the concentration of soluble salts in the saturation extract expressed as parts per million (ppm) can be obtained by mul- tiplying the reading on the RD-26 by about 650, and the RD-15 by about 6.5. Brief instructions for making soil- salinity measurements using the extracts from saturated soils are given on p. 62. Detailed directions can be obtained from U. S. Department of Agriculture Handbook 60 or the offices of the Agri- cultural Extension Service. When exam- ining soils and potting mixtures, it is very important that the extract on which the salinity measurement is made should be directly related to the soil solution which is actually in contact with the roots. The best procedure would be to extract some of the moisture with its dis- solved salts from the moist soil in the root zone of plants. Unfortunately, this is too difficult to be practical. The next best procedure is to saturate a soil sample with distilled, salt-free water and then take out some of the water by suc- tion. This is easily done. The amount of water required to saturate a soil is directly related to the amount it will hold under natural conditions of good drainage. If larger amounts of water are used, such as 2 parts or 5 parts of water to 1 of soil, there will be no fixed relation between the concentration in the extract and the concentration in the soil solu- tion. The results will be meaningless when comparing one soil with another, unless water-holding capacity is con- sidered. For example, two greenhouse soils containing equal amounts of solu- ble salts were examined. No. 1 was a sandy soil which would hold 24 per cent water when saturated; no. 2 was a finer- textured soil holding 64 per cent water when saturated. Saturation extracts both contained about 6,000 ppm of total salts, a harmful amount, and gave conduc- tivity readings of about 8 millimhos/cm. When a suspension of 1 part of soil to 2 parts of water was examined, the first soil gave a reading of 1.0 millimho/cm and the second 4.5. By ordinary stand- ards, the first soil would have been con- sidered safe and the second quite in- jurious. Both were actually salty to the point that tender plants would have been damaged severely. Unfortunately, many laboratories still report results on soil extracts of varying soil/water ratios without including any data from which one might predict the probable salinity to which plant roots are exposed. If growers use the satura- tion-extract method it will be easier to interpret their results, and all work done will contribute to a common pool of knowledge. With agricultural crops in field soils, it is pretty well known how much total salinity can be tolerated and still secure reasonable yields. This is not yet the case with many ornamental and flower plants. Additional experience is needed with ornamentals, but experience to date indicates that little or no difficulty will be encountered if the concentration of the saturation extract is around 2 mil- limhos/cm. A concentration somewhat lower than this will allow for ample quantities of nutrient salts. [61] Procedure for Determining Salinity by Saturation Extract Method 1 1. Collect a soil sample which is representa- tive of the root zone. 2. Prepare a saturated soil paste as follows: Fill a pint container half full of the sample to be tested. Add distilled water slowly until the whole soil mass appears wet. Mix thoroughly with a small stiff spatula or the handle of a spoon, adding more water or more soil as may be needed to reach the saturation point. When this point is reached, the soil paste should show the following characteristics: A. Will be somewhat plastic and will tend to shift or flow slightly when the con- tainer is tipped. B. Will slide freely from the spatula or spoon, except in the case of a heavy clay soil (may not be true of mixes containing a very high percentage of peat) . C. Will show a very little free water in surface depressions upon standing a few minutes. 1 Adapted from methods in "Diagnosis and improvement of saline and alkali soils," U. S. Dept. Agr., Agr. Handbook 60, 1952. When the saturation point apparently has been reached, allow the sample to stand 15 minutes or longer, then restir, and recheck it according to the three criteria above. 3. Remove an extract from this saturated soil as follows: Set up a suction filtering assembly (fig. 59; equipment listed in Appendix). A convenient assembly consists of a size 1-A or size 2 Biichner funnel fitted by a rubber stopper onto a 500-ml side-neck suction (Erlenmeyer) flask. It is advisable to catch the extract in a test tube, 25 by 150 mm size, placed within the flask. Vacuum is provided by connecting the side neck of the flask to a filter pump. Place a dry 7-cm hard (Whatman No. 50) filter paper in the clean, dry Biichner funnel and fill with the saturated soil paste. Apply suction and con- tinue until enough filtrate is obtained to de- termine conductivity. This determination will require about 6 milliliters for rinsing a small conductivity cell and making the test. This amount of extract would fill the test tube de- scribed to a depth of about 1 inch. Do not continue suction after the soil dries and cracks, and air starts passing through. 4. Determine electrical conductivity of the sat- uration extract. The conductivity measurement should be made according to the directions furnished with the instrument you are using. Fig. 59. A suction filtering assembly for making extracts of saturated soils. A Solubridge is at the left. A filter pump, connected to a water faucet, provides a vacuum to speed the filtering operation. ! \p»«-.: If the reading is in terms other than millimhos/ cm, convert to these units by shifting the decimal point. The shift most commonly re- quired is two places to the left, to convert from the common unit EC x 10 5 . The conductivity cells with which tests are made are not always accurately adjusted to give direct readings. They can be tested with N potassium chloride solution (see Appendix), which should give a reading of 1.41 millimhos/cm. The amount of the reading above or below this figure should be added to or subtracted from the normal setting for the observed temperature in each case. It is important to test the solution at the exact temperature for which the instrument is designed, or to make a temperature adjust- ment. Many instruments provide a dial for ad- justing the temperature. Test the extract you have prepared, as fol- lows: If enough extract is available, rinse the cell twice, discard the rinsings, and fill the cell, being sure there are no air bubbles. If in- sufficient extract is available for rinsing, wa added to each cubic yard These mixes may be stored indefinitely Fertilizer I (B) Fertilizer IV (B) 6 oz. potassium nitrate 10 oz. potassium sulfate 4 oz. potassium sulfate 23^ lb. single superphosphate 2^2 lb. single superphosphate 43^ lb. dolomite lime 4^ lb. dolomite lime 134 lb. calcium carbonate lime 134 lb. calcium carbonate lime 134 lb. gypsum 134 lb. gypsum Contains moderate amount of available No available nitrogen included. Will re- nitrogen but will require supplemental feed- quire feeding as soon as planted. Good for ing within a short time. Very good for holding plants back. Same uses as formula bedding plants and can growing. KB). Approximate cost 20 cents Approximate cost 17 cents These mixes should be planted within one week of preparation Fertilizer II (B) Fertilizer V (B) 23^ lb. hoof and horn or blood meal 23^ lb. hoof and horn or blood meal 6 oz. potassium nitrate 10 oz. potassium sulfate 4 oz. potassium sulfate 23^ lb. single superphosphate 23^ lb. single superphosphate 43^ lb. dolomite lime 4.Y lb. dolomite lime 134 lb. calcium carbonate lime 134 lb. calcium carbonate lime 134 lb. gypsum 134 lb. gypsum Contains available nitrogen plus moderate Moderate supply of reserve nitrogen with nitrogen reserve. Good for fast-growing none immediately available. Same uses as rooted cuttings, transplants, or liners. Also formula II (B). used for potting-on. Approximate cost 38 cents Approximate cost 35 cents Fertilizer III (B) Fertilizer VI (B) 5 lb. hoof and horn or blood meal 5 lb. hoof and horn or blood meal 6 oz. potassium nitrate 10 oz. potassium sulfate 4 oz. potassium sulfate 23^ lb. single superphosphate 23/> lb. single superphosphate 4'^ lb. dolomite lime 4 Yi lb. dolomite lime 134 lb. calcium carbonate lime 134 lb. calcium carbonate lime 134 lb. gypsum 134 lb. gypsum Contains available nitrogen plus high nitro- High nitrogen reserve with none immedi- gen reserve. Good for potting-on where ately available. Same uses as formula plants are quite fast growing or where small III (B). amounts of added soil are used. Approximate cost 55 cents Approximate cost 52 cents L72J Table 4. Chemical Ingredients for U. C. Soil Mix C (50 Per Cent Fine Sand, 50 Per Cent Peat Moss) Use these fertilizers only with mix C Amount of materials to be added to each cubic yard These mixes may be stored indefinitely Fertilizer I (C) Fertilizer IV (C) 4 oz. potassium nitrate 8 oz. potassium sulfate 4 oz. potassium sulfate 23^2 lb. single superphosphate 23^ lb. single superphosphate 73^ lb. dolomite lime 73^ lb. dolomite lime 23^2 lb. calcium carbonate lime 23^2 lb. calcium carbonate lime Contains moderate amount of available ni- No available nitrogen included. Will re- trogen but will require supplemental feed- quire feeding as soon as planted. Good for ing within a short time. Good for rooted holding plants back. Same uses as formula cuttings and growing-on. Easily rooted 1(C). cuttings may be rooted and started in it. Approximate cost 22 cents Approximate cost 20 cents These mixes should be planted within one week of preparation Fertilizer II (C) Fertilizer V (C) 23^ lb. hoof and horn or blood meal 23^2 lb. hoof and horn or blood meal 4 oz. potassium nitrate 8 oz. potassium sulfate 4 oz. potassium sulfate 23^ lb. single superphosphate 23^ lb. single superphosphate 73^2 lb. dolomite lime 73^ lb. dolomite lime 23/2 lb. calcium carbonate lime 23^2 lb. calcium carbonate lime Contains available nitrogen plus moderate Moderate supply of reserve nitrogen with nitrogen reserve. Excellent for greenhouse none immediately available. Same uses as pot plants, fast-growing rooted cuttings and formula II (C). liners. Very good for potting-on. Approximate cost 40 cents Approximate cost 38 cents Fertilizer III (C) Fertilizer VI (C) 5 lb. hoof and horn or blood meal 5 lb. hoof and horn or blood meal 4 oz. potassium nitrate 8 oz. potassium sulfate 4 oz. potassium sulfate 23^2 lb. single superphosphate 23/2 lb. single superphosphate 73/£ lb. dolomite lime 73^ lb. dolomite lime 23/£ lb. calcium carbonate lime 2}/2 lb. calcium carbonate lime Contains available nitrogen plus high nitro- High nitrogen reserve with none immedi- gen reserve. Good for potting-on where ately available. Same uses as formula plants are quite fast growing or where small III (C). amounts of soil are used. Approximate cost 57 cents Approximate cost 55 cents [73] Table 5. Chemical Ingredients for U. C. Soil Mix D (25 Per Cent Fine Sand, 75 Per Cent Peat Moss) Use these fertilizers only with mix D Amount of materials to be added to each cubic yard These mixes may be stored indefinitely Fertilizer I (D) Fertilizer IV (D) 4 oz. potassium nitrate 8 oz. potassium sulfate 4 oz. potassium sulfate 2 lb. single superphosphate 2 lb. single superphosphate 5 lb. dolomite lime 5 lb. dolomite lime 4 lb. calcium carbonate lime 4 lb. calcium carbonate lime Contains moderate amount of available ni- No available nitrogen included. Will re- trogen but will require supplemental feed- quire feeding as soon as planted. Good for ing within a short time. Good for trans - holding plants back. Same uses as formula planting and for seed germination. 1(D). Approximate cost 16 cents Approximate cost 14 cents These mixes should be planted within one week of preparation Fertilizer II (D) Fertilizer V (D) 2 ! ■_, lb. hoof and horn or blood meal 23^ lb. hoof and horn or blood meal 4 oz. potassium nitrate 8 oz. potassium sulfate 4 oz. potassium sulfate 2 lb. single superphosphate 2 lb. single superphosphate 5 lb. dolomite lime 5 lb. dolomite lime 4 lb. calcium carbonate lime 4 lb. calcium carbonate lime Contains available nitrogen plus moderate Moderate supply of reserve nitrogen with nitrogen reserve. Very good for growing-on. none immediately available. Same uses as Reduced water requirement will enhance formula II (D). nitrogen efficiency, resulting in a lower supplemental feeding requirement. Approximate cost 34 cents Approximate cost 32 cents Fertilizer III (D) Fertilizer VI (D) 5 lb. hoof and horn or blood meal 5 lb. hoof and horn or blood meal 4 oz. potassium nitrate 8 oz. potassium sulfate 4 oz. potassium sulfate 2 lb. single superphosphate 2 lb. single superphosphate 5 lb. dolomite lime 5 lb. dolomite lime 4 lb. calcium carbonate lime 4 lb. calcium carbonate lime Contains available nitrogen plus high nitro- High nitrogen reserve with none immedi- gen reserve. Dangerous to use except for ately available. Same uses as formula very fast-growing crops owing to greater III (D). efficiency of nitrogen added. Approximate cost 52 cents Approximate cost 50 cents [74] Table 6. Chemical Ingredients for U. C. Soil Mix E (1 00 Per Cent Peat Moss) Use these fertilizers only with mix E Amount of materials to be added to each cubic yard These mixes may be stored indefinitely Fertilizer I (E) Fertilizer IV (E) 6 oz. potassium nitrate 6 oz. potassium sulfate 1 lb. single superphosphate 1 lb. single superphosphate 23^ lb. dolomite lime 23^ lb. dolomite lime 5 lb. calcium carbonate lime 5 lb. calcium carbonate lime Contains moderate amount of available No available nitrogen included. Will re- nitrogen but will require supplemental feed- quire feeding as soon as planted. Good for ing within a short time. Good for starting holding plants back. Same uses as formula rooted cuttings and for potting-on and 1(E). bedding. Approximate cost 15 cents Approximate cost 11 cents These mixes should be planted within one week of preparation Fertilizer II (E) Fertilizer V (E) 23^ lb. hoof and horn or blood meal 23^ lb. hoof and horn or blood meal 6 oz. potassium nitrate 6 oz. potassium sulfate 1 lb. single superphosphate 1 lb. single superphosphate 23^ lb. dolomite lime 23^ lb. dolomite lime 5 lb. calcium carbonate lime 5 lb. calcium carbonate lime Contains available nitrogen plus moderate Moderate supply of reserve nitrogen with nitrogen reserve. Nitrogen supply should be none immediately available. Same uses as sufficient for considerable period of time. formula II (E). Used for potting-on. Approximate cost 33 cents Approximate cost 29 cents Fertilizer III (E) Fertilizer VI (E) 5 lb. hoof and horn or blood meal 5 lb. hoof and horn or blood meal 6 oz. potassium nitrate 6 oz. potassium sulfate 1 lb. single superphosphate 1 lb. single superphosphate 23^ lb. dolomite lime 23^ lb. dolomite lime 5 lb. calcium carbonate lime 5 lb. calcium carbonate lime Contains available nitrogen plus high ni- High nitrogen reserve with none immedi- trogen reserve. Nitrogen may be excessive ately available. Same uses as formula except where small amounts of mix are m (E). used in potting-on. Approximate cost 50 cents Approximate cost 46 cents [75] It would be desirable if investigators and growers used the same system of soil mixes so that information might be more readily transmitted between them. For example, the specific method used to pro- duce blue hydrangeas, as opposed to pink, can be followed by anyone using the same reliable soil system. Here the procedure might be to reduce or elimi- nate superphosphate and lime, and to add some aluminum sulfate. If long-term crops are grown, or if mixes containing little reserve fertilizer are used, fertilizer supplement must be added as either liquid or dry material during the growing period. APPLICATION OF FERTILIZERS Dry Fertilizers liver the quantity of fertilizer desired As much as possible it is desirable to into each container by some simple use for dry-fertilizer application, ma- tri gg er mechanism. Suggested fertilizer terials which are not readily soluble, in formulas and rates for containers are order to avoid the danger of temporary given in tame i. excess. Organic nitrogen and the super- Liquid Fertilizers phosphates fall into this classification. r™ j . .1 £. r . r .,. ., there are numerous advantages in the since most potassium iertihzers are avail- £ ■,. .j £ ..-.* T 1 • x . , 1 - l 1 1 r 1 1 use 01 liquid iertilizers. Labor is tre- acle only in soluble form, thev must be t1 j j 1 «.i_ x _«.«v •• , n -i . 1 . aii ill quently reduced because the fertilizing handled with caution. A slowly soluble , . i 1 1 • • „ . , . J ., can be carried out during a normal lrri- potassium irit has recently become avail- c 1 r . £ -r r . , /c , r . ,, .11 .i 1 gation. such application is sate, it rea- able (bee. 6). Materials that might be 11 : .. j a v , , r 1. • sonable concentrations are used. Apphca- used, and common rates 01 application .. - , j t ' p r 1 1 lif tions can be made at more trequent m- per 1(JU square feet tor bed or bench ier- 1 • j • i .... ^ . .. tervals in order to maintain nearly con- tihzing are as follows: u 1 i r • -i u«i«* t'u D stant levels ot nutrient availability, lhe Hoof and horn meal 1 to 3 pounds disadvantage is that the nutrients added Blood meal... 1 to 3 pounds ag R i( j are algQ more readn logt Cottonseed meal 1 to 3 pounds , 1 i 1 • A 1 .,, Castor pomace 1 to 3 pounds through leaching. Again, the possible Fish meal 1 to 3 pounds combinations of materials are numerous. Ammonium sulfate % to 1 pound Table 8 provides a simple set of all neces- Calcium nitrate V 2 to 1 pound sary var i ants providing the major ele- Ammonium nitrate V^to 1 ^ pound Single superphosphate 2 to 4 pounds . Double superphosphate 1 to 2 pounds lhe most commonly used liquid for- Potassium sulfate V\ to 1 pound mulas are L-2 and L-7. Normal practice Potassium chloride V± to 1 pound i s to use the liquid fertilizer in place of Potassium nitrate % to 1 pound a regu i ar i rr i ga tion. These materials can Potassium frit (Dura-K) 2 to 5 pounds , ,. , . , . . , , P ■.. be applied without rinsing the toliage Where containers are to be fertilized, afterward under all but extreme condi- it is common practice to use spoon meas- tions of bright sunlight. Vegetable dyes ures. These are available from any va- may be added to fertilizer concentrates riety store in sizes ranging from % tea- as indicators of injection, spoon up to I tablespoon. There is a Concentrates are more readily made definite need for a more efficient dis- with hot water or by introducing steam penser which could be adjusted to de- while dissolving. The solutions may be I 76 I Table 7. Supplementary Dry Fertilizers for Container-grown Plants Suggested rates are for 6-inch pots, gallon cans, and beds or benches about 8 inches deep. Use proportionately more for larger, and less for smaller soil volumes. The ingredients should be carefully mixed before application. The rates suggested are substantial. If light watering is practiced, it may be necessary to reduce the amounts used. Numerous combinations other than those listed may be found useful by trial or soil testing. In the average sand- peat mix the element required in greatest quantity is nitrogen, next is potas- sium, and last is phosphorus. SUPPLYING NITROGEN ONLY: Fertilizer VII 1 heaping teaspoon 1 to 3 lb. per 100 sq. ft. Hoof and horn or blood meal Particularly useful for first applications to plants in mixes with fertilizers I and IV, and for extra nitrogen supplement in forcing. SUPPLYING NITROGEN, PHOS- PHORUS, AND POTASSIUM: Fertilizer VIII 2 heaping teaspoons 2 to 5 lb. per 100 sq. ft. 4 lb. hoof and horn or blood meal 4 lb. single superphosphate 1 lb. potassium sulfate or chloride May be required after plants have grown for some time in the same container. SUPPLYING NITROGEN AND POTASSIUM: Fertilizer IX 1 heaping teaspoon 1 to 3 lb. per 100 sq. ft. 6 lb. hoof and horn or blood meal 1 lb. potassium sulfate or chloride Most useful in mixes A, B, and C, contain- ing high proportions of sand, as these ele- ments are most rapidly lost through leach- ing. SUPPLYING NITROGEN AND PHOSPHORUS: Fertilizer X 2 heaping teaspoons 2 to 5 lb. per 100 sq. ft. 1 lb. hoof and horn or blood meal 1 lb. single superphosphate Most frequently used in mixes D and E containing very high proportions of peat moss, where the nature of the material re- sults in most rapid removal of phosphate from solution (phosphate fixation). SUPPLYING PHOSPHORUS AND POTASSIUM: Fertilizer XI 1 heaping teaspoon 1 to 3 lb. per 100 sq. ft. 6 lb. single superphosphate 1 lb. potassium sulfate or chloride Useful where plants are to be held back by allowing nitrogen deficiency to occur. Well suited to legumes. [-!'] Table 8. Liquid Fertilizer Formulas for Use with U. C. Soil Mixes Where an applicator is used, the liquid can be made up in concentrated form and diluted through it to give the concentrations listed. A dilution ratio of more than 1:200 is not practical as these solutions cannot be made much more than 200 times as concentrated as listed. Solutions may be stored for extended periods without deterioration. Urea — 45 to 46 per cent nitrogen (may be dangerous to use if biuret is present). Ammonium nitrate — 33.5 per cent nitrogen. Mono-ammonium phosphate (technical grade) — 12 per cent nitrogen, 61.5 per cent phosphate (P O ). Potassium chloride — 60 per cent potash (K..O). Calcium nitrate — 15.5 per cent nitrogen. Amounts per 100 gallons of water SUPPLYING NITROGEN ONLY, EX- CEPT FOR L-3 WHERE CALCIUM IS ALSO SUPPLIED: L-l: 1 lb. urea L-2: 1 lb. ammonium nitrate L-3 : 2 lb. calcium nitrate Particularly useful for first applications to plants in mixes with fertilizers I and IV, and for extra nitrogen supplement in forc- ing. SUPPLYING NITROGEN AND POTAS- SIUM: L-8: 12 oz. urea 12 oz. potassium chloride L-9: 12 oz. ammonium nitrate 12 oz. potassium chloride Most useful in mixes A, B, and C containing high proportions of sand, as these elements are most rapidly lost through leaching. SUPPLYING PRIMARILY PHOSPHO- RUS AND POTASSIUM, BUT IN- CLUDING A SMALL AMOUNT OF NITROGEN: L-10: 12 oz. mono-ammonium phosphate 12 oz. potassium chloride Useful where plants are to be held back by allowing nitrogen deficiency to occur. Well suited to legumes. SUPPLYING NITROGEN AND PHOS- PHORUS: L-4: 12 oz. urea 12 oz. mono-ammonium phosphate L-5: 12 oz. ammonium nitrate 12 oz. mono-ammonium phosphate Most frequently used in mixes D and E containing very high proportions of peat moss, where the nature of the material re- sults in most rapid removal of phosphate from solution (phosphate fixation). SUPPLYING PRIMARILY PHOSPHO- RUS, BUT INCLUDING A SMALL AMOUNT OF NITROGEN: L-ll: 1 lb. mono-ammonium phosphate Useful when phosphate is low. SUPPLYING NITROGEN, PHOSPHO- RUS, AND POTASSIUM: L-6: 8 oz. urea 8 oz. mono-ammonium phosphate 8 oz. potassium chloride L-7: 8 oz. ammonium nitrate 8 oz. mono-ammonium phosphate 8 oz. potassium chloride May be required after plants have grown for some time in the same container. SUPPLYING POTASSIUM ONLY: L-12: 1 lb. potassium chloride Useful when potassium is low. [78] stored for extended periods without de- terioration. Examples of application of these formulas are given below. Biuret Injury Many commercial ureas and urea-for- maldehyde preparations have been found to contain biuret, a chemical by-product formed in the manufacture and prepara- tion of these nitrogen fertilizers. Biuret is toxic to most plants, the typical symp- toms being stunting, leaf burn, chlorosis, and even death of the plant. Unless the manufacturer labels the bag or con- tainer, the only means of determining the biuret content of the urea fertilizer is by analysis or biological test. Since this toxic ingredient may be a serious hazard in plant production, the grower is advised to exercise extreme caution in the use of fertilizers containing or de- rived from urea. Unless labeled biuret- free, these materials should be used only after thorough testing on each crop. SUGGESTED USES OF THE U. C. SOIL MIXES The type of growing operation will largely dictate the choice of soil prepara- tion and handling. The following are typical procedures for several types of growing. It is assumed that in all cases the soil mix will be steamed or fumigated for weed and disease control prior to planting. When a single growing procedure is altered it usually affects other operations, and a general adjustment to new meth- ods may be required. Adoption of U. C- type soil mixes is no exception to this. Watering procedures must usually be modified for best results. Some growers have found that, because of more rapid crop growth, the production schedule is altered. While planting dates may need to be altered to accommodate schedules, faster production will lower cost and in- crease volume. Flats Mix B will be used for most bedding plants with the possible exception of some of the shade plants, such as be- gonia and primula, where mix C might be used. Seed flats will normally be of the same soil preparation as growing flats. The appropriate fertilizer I, II, IV, or V is added to the basic mix. Since some [ danger from ammonium excess exists where organic nitrogen is present in quantity, many growers avoid this pos- sibility by using fertilizer I or IV throughout. If organic nitrogen is omitted, fertilizer is normally applied soon after transplanting. If fertilizer I is used, application of subsequent material is normally delayed 1 to 2 weeks, then a broadcast application of blood or hoof and horn meal may be made, or a pro- gram of using liquid nitrogen as formula L-2 or L-3 may be used at 1- to 2-week intervals. If plants are held for an ex- tended period, fertilizer VIII may be used for broadcast application. In a few words, one procedure would be: Physical mix B. Fertilizer I (B) . After 2 weeks use liquid L-2 at 10-day intervals on transplants. Use no additional fertilizer on seed flats. One week prior to sale of trans- plants, apply fertilizer VIII at 4 pounds per 100 square feet of flat area. Pots Pot plants may start with rooted cut- tings planted in 2%- or 3-inch pots with one or more subsequent shifts up- 79] ward in pot size before the product is ready to sell. Many variations are prac- ticed. Some growers actually place the rooted cuttings in 6-inch pots, carrying them in these to salable size. By proper selection of fertilizer formula this is both possible and entirely practical, saving considerable labor in transplanting. The following suggestions, however, are in- tended for the common transplanting and shifting procedure. Rooted cuttings will be placed in mix C fortified with fertilizer I (C), II (C), IV (C), or V (C). When well rooted in the new medium, dry fertilizer might be applied as fertilizer VII, or liquid L-l, L-2, or L-3. When ready for shifting to larger pots, the soil preparation will be mix C or D, with the appropriate ferti- lizer III or VI. If the plants are held for a short period no further fertilization should be required. If held for a. long time, dry fertilizer VIII or a program of liquid fertilization with liquid L-6 or L-7 may be used. In brief, one procedure might be: Plant rooted cuttings in 2V2- incn pots of mix C, fertilizer I (C). After 10 days start using liquid L-2 at 10-day intervals. Shift 2 1 X>-inch liners up to 6-inch pots using physical mix D and fer- tilizer III (D) . If plants are held for more than 2 months in 6-inch pots, apply 1 heaping tablespoon fertilizer VIII to each. Cans In can-grown nursery stock the cost of the soil preparation is a very important factor, and in many cases the shipping weight is also economically important. For these reasons, nurserymen may use materials in the physical mix which will reduce both cost and weight of ingredi- ents. Shavings, sawdust, bark, and even rice hulls are currently in use as substi- tutes for part of the peat moss. Nursery- men should acquaint themselves with these and other possibilities, keeping in mind that any substitute material must conform to the standards outlined in Section 6 if reliability is to be retained. For this type of growing, physical mix B might well be used as the base, with fertilizer I (B), II (B), IV (B), or V (B). If soil is to be stored for any length of time, as is frequently done, the fertilizer formulas are limited to I (B) or IV (B). The liners which are to be planted into gallon cans may be handled the same way as rooted cuttings under the discussion of pot plants. If mix B and fertilizer I (B) are used, either dry fertilizer VII should be applied about 2 weeks after planting, or liquid L-2 or L-3 should be applied at that time and repeated at approximately 10-day in- tervals. After plants are well established, dry fertilizer VIII may be applied or the liquid program shifted to liquid L-6 or L-7. When plants in gallon cans are moved up to egg cans (3-gallon size) or 5-gallon cans, the same procedure of fer- tilizing may be carried out as was out- lined for liners into gallon cans. A typical procedure would be: Grow liners as described for pot plants. Transplant liners into gallon cans using mix B, fertilizer I (B). After 2 weeks begin applications of liquid L-2 every third irrigation. After 2% months shift to liquid L-7. Just before plants are sold, or in preparation for the winter rainy sea- son (when liquid fertilizer cannot be applied), use dry fertilizer VIII. Benches and Beds Plants grown in benches or beds would include cut flowers as well as stock plants from which cuttings are pe- riodically taken. Usually this type of growing is carried out in the glasshouse, but in warmer climates it may take place outdoors. The inilial cost of bed preparation may be 80 ] substantial, but when one considers the potential useful period in terms of seasons or years, it is obviously unwise to economize on important ingredients. Mix B may be satisfactory, but mix C offers greater insurance of desirable physical properties for optimum growth. As discussed under "Cans," it may be possible to substitute other organic mate- rials for part of the mix, to provide the best possible physical conditions at re- duced cost. In commercial establish- ments the U. C.-type soil mixes (25 per cent or 50 per cent peat) have given good lateral distribution of water applied to the surface by the porous-hose or drip system. Beds might be prepared by using mix C and fertilizer II (C) or V (C) if they are to be planted at once. If planting is to be delayed, fertilizer I (C) or IV (C) should be used. If fertilizer II (C) or V (C) is used, subsequent application may begin 4 to 6 weeks after planting. Regular applications of dry nitrogen sources will be required for a few months, and then it may be necessary to use one of the mixed dry fertilizers from time to time. Because of the length of the growing period, it is impossible to out- line a reliable long-term procedure here. Liquids may be used in place of the dry applications, keeping in mind that nitro- gen will be required at first, with mixed materials later. If fertilizer I (C) or IV (C) is used, application should start 1 to 2 weeks after planting, with subse- quent procedure the same as above. An example might be as follows : Prepare bed with mix C plus fer- tilizer IV (C). Apply starter solution as liquid L-2. Apply liquid L-2 every third irri- gation for 6 weeks, then shift to liquid L-7 every other irrigation. When the beds are renewed or re- planted more peat can be added. [ Planter Boxes and Dish Gardens Present-day landscaping makes com- mon use of planter boxes both indoors and outside. Standard soil mixes of the types described are useful for a wide range of plants. For large boxes with adequate drainage use mix B or C, with the appropriate fertilizer I, II, or V, fol- lowed by the same feeding program as for bench- or bed-grown crops. Dish gardens or beds with obstructed drain- age may use mix D or E with the appro- priate fertilizer I, II, or V, but should receive little or no subsequent fertilizing. Home-Yard Planting Where the natural soil is of a fine sandy texture, a U. C.-type mix may easily be prepared. Where the natural soil is not of this type, an expensive alternative is to remove and replace the existing soil with the more desirable kind. When setting out plants grown in con- tainers of fine sand and peat into soil of different texture, an effort should be made to blend the soil of the container with the existing soil so that a transition zone is produced. If proper planting pro- cedures are followed, plants raised in the fine sand and peat mixes will grow very well even in heavy clay soils. The exten- sive root systems produced in these mixes favor more rapid establishment when transplanted. Frequently it is necessary in landscape work to bring in top soil. The U. C.-type mixes have proved successful, simple to handle, and relatively inexpensive, par- ticularly if cheaper organic materials such as redwood sawdust or shavings, or rice hulls are used in place of peat. For Research It is common practice in a research glasshouse to steam or otherwise treat soil for growing test plants, to protect them from soil-borne diseases. The need for healthy, vigorous, and uniform plants 81] is obvious whatever phase of botanical or agricultural science is under investi- gation (p. 51). The importance of soil in the production of experimental plants is often underrated, and wholly unsuita- ble soil types used. Poorly grown and ex- ceedingly variable plants result. The U. C.-type soil mix answers the demands for growth of many kinds of experimental plants. In studies of soil pathogens it may be used (1) immedi- ately after steaming, before it becomes extensively recolonized, or (2) after the soil has again developed a stable flora from air contaminants, contact, or in- oculation with a specific flora. For routine tests of the pathogenicity of or- ganisms on underground tissues, the U. C.-type mix in such a biologically buffered state provides conditions similar to those in a natural fertile sandy loam. When tests of pathogenicity and the manifestation of symptoms demand vigorously growing and uniform plants, a U. C.-type soil mix is generally excel- lent as a growing medium. There are also examples of research for which the desirable attributes of the U. C.-type mix disqualify it. Thus it would be unsuitable for studies on re- sistance to a Phytophthora root rot which normally occurs in a heavy, poorly aerated soil. Wherever the qualities of the soil itself constitute an important factor in the problem under study, a natural soil, or a soil mix with suitable properties, should be given preference. Apart from numerous uses in the study of diseases induced by fungi, bacteria, and nematodes, the U. C.-type mix has proved to be excellent for growing plants for virus research. In this field the re- quirements for uniformity are often as demanding as in the most accurate physiological studies on soil-grown plants. Turkish tobacco plants have been grown l>\ J. G. Bald and P. A. Chandler 3 in 4-inch pots from seed to a height of aboul L5 inches, bearing 15 to 18 ex- panded leaves, within a period of 7 weeks from seeding. The average rate of increase in leaf area in lots of 600 plants has reached 25 per cent per day. From the emergence of the first true leaf above the cotyledons until near the end of this period the leaf-area growth rate was logarithmic, provided greenhouse conditions remained uniform. Percentage increases in leaf area from day to day were the same whether the plants were small or relatively large. For much of that time fresh weight of the tissues was almost linearly related to leaf area ; later the relation was more complex, but regular and predictable. The logarithmic growth rate was reduced by the crowd- ing of roots in the 4-inch pots before the physiological changes preceding flower- ing could take effect. By growing herba- ceous plants in larger containers of a U. C.-type soil mix it should be possible, for a particular set of conditions, to maintain uniform and unrestricted growth until seeding. It was possible with this soil to transplant tobacco seedlings in the cotyledon stage without loss, or even a noticeable check to growth. The physical characteristics are so good that the first roots of the tiniest seedling or of the largest plant pass directly through the soil mass without being diverted to the edge of the con- tainer (fig. 61) . In addition to rapid growth, experi- mental plants in a U. C.-type mix, if correctly selected and handled, exhibit remarkably uniform size and habit. There are several factors unrelated to the soil mix which affect uniformity of plant growth. Among them are: (1) heritable variability between seedlings, even within a horticultural variety; (2) rapidity of seed germination; (3) dif- ferences in extent of roots, caused in part by development of recolonized non- pathogenic organisms in any treated soil; (4) other random sources of vari- 1 Department of Plant Pathology, University of California, Los Angeles. [82] Fig. 61. Root development of Croft lily in U. C. mix C (50 per cent peat). Plant at left in the soil ball, at right washed free of soil. Note the size of the root system and its development throughout the ball. ability. A group of plants grown in a U. C.-type soil mix may superficially appear more variable than those grown in other soil because the more rapid healthier growth accentuates the inherent differences. For example, a plant arising from a seed that germinated 3 days later than its neighbors will appear relatively smaller in a rapidly growing series than it will in one of poor growth. Plants grown under optimum conditions in a soil mix of the U. C. type may be sorted, accurately matched, and practically all undesirable plants eliminated, with the assurance that the maximum variability is revealed. Such selected plants are often comparable, leaf by leaf, from the cotyledons to the growing tip, and they will remain so through the period of the experiment. On the other hand, poorly grown plants cannot be matched in this way. Their apparent similarity may mask sources of variation that eliminate all chance of obtaining accurate information from experiments in which they are used. The U. C. mixes will prove very useful in the research greenhouse wherever the production of uniform, well-grown plants is desired for experimental studies. The uniform growth rate may be main- tained during the experiment if other conditions are favorable. PRACTICAL CONSIDERATIONS Preparation of the Mixes The mixing of the fine sand, peat or other organic material, and fertilizer components can be very simple. Peat should be wetted before being mixed, preferably 1 or 2 days before use. The fine sand, peat or other organic material, and the mixed fertilizers should be in [83] piles convenient to the mixing operation. Complete mixing is essential. If mixing is done by hand, the proper amounts of the various ingredients should be placed in a low, level pile with the fertilizer components broadcast evenly over the surface. This should be turned with a shovel, progressively work- ing through the mass from one side and forming a second pile as the shovels of soil are turned over. This second pile is then turned back again, and the process repeated until blending is complete. If mixing is by machine, it is generally done with a concrete mixer (Sec. 17) or a ribbon mixer. In the smaller nursery a small cement mixer may be used and ingredients added by hand. In larger operations, the major ingredients (fine sand, peat) are generally placed in the mixer with a skip loader and the fertilizer components added by hand. Moist, but not excessively wet, base ingredients are essential to uniform mix- ing and to reliability of subsequent treat- ment. Watering Practices Since fine sands do not have cementing properties (Sec. 6), the particles can be readily dislodged. If heavy streams of water are applied to fine sand and or- ganic mixes, the surface is churned up, and the sand settles first. The organic matter therefore collects on the surface. Where mulches are undesirable, it is necessary to use water breakers during irrigation to avoid this effect on U. C- type mixes. Some adjustment in watering prac- tices is generally desirable when the grower first uses a U. C.-type mix if he has been accustomed to a clay soil. Dump Soil In any necessary operation a certain amount of used soil is apt to accumulate. This is probably more true of the bed- ding-plant operation than of any other. J. L. Mather" found that in 15 repre- sentative California bedding-plant nur- series an average of 16 per cent of the flats were dumped. The problem of dis- posal of this material sometimes assumes major proportions. Since the physical texture and structure of used U. C.-type soil will be quite acceptable, the problem is centered on its chemical properties. If it is to be re-used, it is necessary to know whether to add fertilizer, and if so, how much. For obvious reasons no standard pro- cedure can be proposed which would take into account all the possible varia- tions which will exist. Several sugges- tions, however, may be made: 1. Sell the material as top soil for landscaping or similar use where physical, rather than chemical, properties are of prime importance. 2. Check the salinity of the soil by the saturation-extract conductance method (Sec. 4) and, if it is satis- factorily low, use a portion of this soil as a substitute for the fine sand in the standard mix. 3. Determine by adequately complete analysis the exact nutrient status of the used soil, and calculate exactly what to add in order to bring it up to standard. Cost of the Soil Mixes Cost of a soil mix will be determined by the cost of the soil, transportation to the nursery, organic ingredients, ferti- lizers, and labor in mixing. A major dif- ference in cost between the older type of composted mixes and those of the U. C. type is that of labor. With a compost the materials are handled at least 2 or 3 times before treatment, whereas mixes of the U. C. type require only one opera- tion. There is also the lower cost of the materials themselves. Fine sands will 8 Manager of the former Bedding Plant Ad- visory Hoard, Bureau of Marketing, California Stale Department of Agriculture. [84] usually be cheaper than top soil because of the more efficient machinery that can be used in digging the material, often to a considerable depth. Top soil often is more costly because only the surface layer of limited areas is removed, using less efficient machinery. In many areas the price for top soil is greater than for fine sand because of competition for it. Soil-conservation practices also often forbid the removal of top soil. Some of the fine sands that are suitable for use in U. C.-type mixes are actually waste products from the screening and washing of building materials. Probably the main factor in any soil cost will be the transportation charge. Every nurseryman should try to locate a source as close to his nursery as possible. Assuming an average 33 per cent shrinkage of leaf mold, manure, or other compost materials, the cost per useful unit volume should be increased by one half over the purchase price. One cubic foot of baled peat, on the other hand, yields 1.5 to 1.6 cubic feet of loose material. Besides the lower labor requirement mentioned above, comparative cost of ingredients, mixes, and composts shows that the U. C.-type mix is less expensive than other common soil preparations. Computed at 1955 wholesale prices, de- livered to a near-by nursery in the Los Angeles area, the cost per cubic yard of some ingredients is as follows: Fine sand S 2.00 Top soil $ 3.00 Peat moss, loose $ 6.75 ($4.50 per 12 cu. ft. bale) Sawdust $ 1.00 Leaf mold $13.50 ($9.00 per cu. yd. delivered; 33 per cent shrinkage sustained) Steer manure $ 9.00 ($6.00 per cu. yd. delivered; 33 per cent shrinkage sustained) Using these materials, the cost per cubic yard of various nursery soil mix- tures may be computed as shown in table 9. Table 9. Comparative Cost of Common Soil Mixes Soil mix STANDARD U. C. MIXES A (100% fine sand) B (75% fine sand, 25% peat) C (50% fine sand, 50% peat) D (25% fine sand, 75% peat) E (100% peat) U. C. MIXES USING SAWDUST B (75% fine sand, 25% sawdust) C (50% fine sand, 50% sawdust) D (25% fine sand, 75% sawdust) E (100% sawdust) COMPOST 75% top soil, 25% leaf mold. . . . 50% top soil, 25% leaf mold, 25% manure 50% top soil, 50% leaf mold 100% leaf mold Cost of ingredients per cubic yard of mix Soil $2.00 1.50 1.00 0.50 1.50 1.00 0.50 2.25 1.50 $1.50 Peat $1.69 3.37 5.06 $6.75 Saw- dust $0.25 0.50 0.75 $1.00 Leaf mold $3.37 3.37 6.75 $13.50 Manure $2.25 Total $2.00 3.19 4.37 5.56 6.75 1.75 1.50 1.25 1.00 5.62 7.12 8.25 $13.50 [85] SECTION Components and Development of Mixes O. A. Matkin Philip A. Chandler Kenneth F. Baker Functions of the soil Disadvantages of multiple soil mixes Attempts to improve nursery soil mixes Soil toxicity in relation to treatments Criteria for physical ingredients of soil mixes Selecting ingredients for U.C. mixes o ne OF the commonest erroneous ideas in nursery practice is that a special soil, resembling as closely as possible the soil of its native habitat, is required for each type of plant. This involves the fal- lacious assumption that distribution of wild plants is determined by soil type, whereas actually the temperature, rain- fall, day length, light intensity, soil salinity, the point of origin, as well as other factors, are at least as important in determining where plants grow. It may actually be misleading to assume that the best soil for a plant is that of its native habitat, since the plant may have had to "tolerate" that soil because another factor, such as frost, may have limited it to that particular area. Most plants of necessity must have a wide tolerance to soil types in order to survive. The soil used may simply be a matter of tradition. Some growers plant verbena in straight leaf mold although it is not a native of dense woodlands, and lilies in black adobe although they require good drainage. The surprisingly good results sometimes obtained in such media testify to the tolerance of plants in this regard. The John Innes Horticultural Institu- tion (Bayfordbury, Hertfordshire, Eng- land) demonstrated in 1934-1939 that many kinds of plants could be grown in a single soil mix, or in slight modifica- tions of it. As this concept has been recognized by growers, there has been a trend away from specialized mixes for each type of plant. FUNCTIONS OF THE SOIL Any good growing medium must pro- vide for the basic requirements of the plants in it. Since all green plants have the same basic requirements, the prob- lem is simplified. The growing medium supplies only the following functions. [86] Support Most crops require some means of physical support. Unless artificially pro- vided, this is a function of the growing medium; support is not a factor of major concern unless the plant is large and the growing medium of very light-weight material such as peat moss. In nursery growing it is common to use stakes and ties of various types to support plants in small containers. Moisture The living plant is largely composed of water, which must be obtained from the soil in which it grows. A good grow- ing medium should have a reasonable ability to hold moisture in sufficient supply for plant requirements between irrigations. Water is more often limiting to plant growth than such items as ferti- lizer, salinity, or alkalinity, which are so often blamed. High salinity (Sec. 4) may virtually make soil water unavailable to the plant because dissolved salts increase the os- motic pressure in the soil solution. If the concentrations outside the root approach those within it, owing to dissolved salts, water movement into the plant is re- stricted. Since containers have limited depth, a boundary exists at the bottom in con- trast to a continuous soil column in the field. This boundary constitutes a restric- tion to free drainage (Baver, 1956; Huberty, 1945). Thus, soil in a con- tainer will retain more moisture avail- able to plants after an irrigation than it would in the field. Large quantities of water are lost by the plant through transpiration; when the plant wilts, this indicates that loss is greater than the supply from the roots. Although this is the major plant use of water, it is bv no means the only im- portant one. Water is the solvent in which minerals are taken into and transported through the plant. The two elements comprising water, hydrogen and oxygen, play individually important roles in plant metabolism. All of the organic materials of plants contain large quantities of each. The fact that plants can be grown in water (culture-solution growing) in- dicates that there is no such thing as ex- cessive water where the other basic re- quirements are satisfactorily met. On the other hand, plant growth unquestionably can be restricted by conditions which subject the plant to increasingly deficient moisture. Frequently this point is over- looked by the grower unless he happens to have a comparison available. The ac- cumulative stunting effect is shown dia- grammatically in figure 62. Aeration The roots of a plant obtain the raw materials, water and mineral nutrients, which are carried upward through the stem to the leaves. The tops act as fac- tories, synthesizing the compounds re- quired for growth and reproduction from these materials and carbon dioxide from the air. For roots to function normally they must be supplied with a source of energy and an environment favorable for utilizing it. The top of the plant provides the sugars and other carbohydrates, which are transported through the stem down to the roots, where, through respiration, they supply the energy necessary for root function. Respiration, as in the case of animals, requires oxygen and produces carbon dioxide and water. Oxygen is also re- quired for respiration in other parts of the plant, but the supply there is nearly always adequate. Because of the tiny pore spaces in soil through which the gases move, aeration (oxygen supply and carbon dioxide removal) of the roots can readily become limiting. A good soil mix must insure the best possible aera- tion consistent with other requirements. The additional moisture retained by soil in a container reduces the air space. It is. therefore, important that container soils [87] Plant Growth Available Soil Moisture Unavailable Soil Moisture CONSTANTLY AVAILABLE WATER SUPPLY INTERMITTENTLY AVAILABLE WATER SUPPLY Time Time Fig. 62. Diagram of plant growth in relation to moisture availability. The plant constantly sup- plied with water grows continuously. The plant exposed to occasional water deficit grows inter- mittently, and is smaller. Fig. 63. Diagram of plant growth in relation to mineral nutrient supply. The plant uniformly supplied with fertilizer grows continuously and is larger than the one intermittently supplied. An excessive application of fertilizer (at right) killed the plant. Plant Growth Tissue damage Slight excess Favorable supply Deficiency CONSTANTLY FAVORABLE MINERAL NUTRIENT SUPPLY VARYING MINERAL NUTRIENT SUPPLY Death f Fertilizer Application Fertilizer Application s \ p^v \N. Time Time [ 88 | have a maximum porosity. It is pri- marily by diffusion that gases move into and out of a soil, though applications of water may also be effective in displacing soil air, particularly in containers (Sec. 9) . If the soil pore spaces are very small, water will fill them and reduce aeration until the water content has been lowered by evaporation or transpiration. Of additional importance is the fact that a soil through which air does not diffuse readily will also be difficult to treat efficiently by fumigation or steam (sees. 8, 9, and 11) . Mineral nutrients At the present time most green plants are known to require at least twelve chemical elements (nitrogen, phospho- rus, potassium, calcium, magnesium, sulfur, iron, zinc, manganese, copper, boron, and molybdenum) that are ob- tained from the growing medium by the roots. Foliar feeding may be used to sup- plement root absorption. A fertile soil is one in which all of these elements are present in adequate but not excessive quantity. A good soil mix must therefore contain them, or the growing procedure must provide for their supply during plant growth. This function is made possible in part through the breakdown of organic mat- ter, native mineral soils, and fertilizers in the complex activities of soil microor- ganisms, as well as fixing atmospheric- nitrogen to make it available to the plant (Sec. 14). To this extent they are prop- erly considered as a necessary part of the soil environment of the plant. As with moisture, it is important that the supply of these minerals be continu- ous rather than intermittent (fig. 63). The greatest problem occurs in main- taining proper nitrogen supply (Sec. 7). DISADVANTAGES OF MULTIPLE SOIL MIXES It is still the practice in some Califor- nia nurseries to have a separate bin or compost pile of a special soil mix for nearly every crop grown. This is a costly procedure beset with several serious dis- advantages. Labor requirement Preparation of many small lots of soil costs vastly more than does preparation of a single large batch, and makes un- economic the mechanization of handling. If compost piles or bins are maintained for each, the labor requirement becomes very large. Space utilized Land area is required for the piles of raw materials or compost, mixing areas, and storage bins. It is not uncommon for large nurseries in southern California to use 1 to 2 acres for these purposes. Be- cause of the real-estate pressures pre- viously mentioned (Sec. 2), and the in- creasing tax rates, land area must now be used with greatest efficiency. Variability of composts Composts containing leaf mold, animal manure, or turf will be highly variable in composition because these materials are themselves far from uniform. Fur- thermore, if these ingredients are com- posted, the degree of decomposition will not be uniform in all lots at different seasons, and the mixture thus would be even more variable than before. With these inherently variable mixtures, the plant response often becomes so un- predictable as to prevent scheduled pro- duction. Shrinkage in composting Manure, used to supply organic matter and nutrients, is not a good source of either for nurseries. In California it is [89] likely to be more dehydrated than de- composed, and the buyer accordingly as- sumes the shrinkage. "Leaf mold" in California usually means partly decayed leaves, subject to a considerable loss in composting. Because peat is largely de- composed before it is dug, composting is unnecessary and shrinkage during use is comparatively minor. Odor and flies during composting In residential areas unpleasant odors and flies are likely to bring zoning re- strictions. Compost piles are already being discontinued for this reason in some areas. Scarcity of composting materials Because of the scarcity of deciduous forests in California, the semiarid cli- mate, and destruction from fires, leaf- mold deposits are rare and generally protected by law. Animal manure may become scarce, and the nurseryman must compete with mushroom growers and the package-manure trade to get it. For these reasons many nurserymen have prac- tically ceased using these two materials. In England and the humid parts of this country, sod from turf or meadowland is used in compost piles. In California, turf can rarely be used because there are few natural meadows, and land and water are too valuable to be used only for a turf crop. Salinity problem This serious problem, discussed in Section 4, is important in the selection of composting materials. During the decom- position of leaf mold in place, in the compost pile, or in the container, the mineral content is made soluble, which increases salinity. Consequently, Califor- nia "leaf mold" is commonly high in soluble salts, whereas in areas of high rainfall these have been removed by leaching. In one instance, leaf mold that had been used for growing cattleya or- chids in benches, was sold to nurserymen for use in bedding plants. This decom- posed material had excellent physical properties, but because it had been watered lightly without leaching, the salts from the water and from fertilizers had accumulated to an extremely high level. When it was used in nursery soils, seedlings were quickly and severely in- jured by the soluble salts. Manures, by their very nature, are always saline. Post-treatment toxicity This important disadvantage of con- ventional soil mixes is discussed below. Comparison with U. C.-type mixes By comparison with multiple soil mixes, the use of a single one of the U. C. type (Sec. 5) presents definite advan- tages: it requires less labor and can be more economically mechanized in the handling operation; less storage space is needed since compost piles are elimi- nated; greater uniformity of mixture and predictability of plant growth will result; loss by shrinkage and leaching during composting is reduced; scarce materials, such as manure and leaf mold, may be eliminated by using readily available peat instead; some of the sources of soluble salts may be avoided. ATTEMPTS TO IMPROVE NURSERY SOIL MIXES Progress in the art of compounding artificial conditions imposed by man. nursery soil mixtures is but an expres- Empirical additions of different mate- sion of the developing fundamental rials, or the formulation of various philosophies of plant culture under the mixes, are of less permanent importance [ 90 1 to nurserymen than is the evolution of the ideas behind them. Viewed from this angle there has been slow but steady progress in the subject. Man's earliest cultivation of plants was undoubtedly in some sort of field plots, and he found at a fairly early period that growth was enhanced by ap- plication of some type of fertilizer. Per- haps he discovered that plants would grow in containers when he was faced with the necessity of moving some of them to a new area during migrations. It was then but a step to find that they could be grown in that way very suc- cessfully if fertilized and properly han- dled. It is known that trees were grown in large "boxes" or "pots" cut in rock and filled with special soil in Egypt about 4,000 years ago. Frankincense trees were brought from Punt (Somali Coast) to Egypt in containers to be grown in gar- dens about 3,500 years ago; that this was carefully recorded as an outstanding achievement suggests that it was one of the earliest instances of plant nursery operations (fig. 64 J. 1 When man began growing plants for ornamental use it was natural that some should be grown in containers to be taken into his home or gardens. When a practice is slowly developed, there usually is little critical examina- tion of the methods employed. Growers of plants in containers adopted a very complex series of soil mixtures for dif- ferent crops. Although many growers thought they had independently de- veloped the ideal soil for a given crop, there was no consistency between them, as there should have been if the con- clusions were valid. Sometimes this mix was based on the soil that had been used for an especially successful crop, the 1 The Temples of Neb-hepet Re' Mentu-hotpe (2061-2010 B.C.) (Winlock, 1942), and Hat- shepsGt (1520-1479 B.C.), Deir el-Bahri, near Thebes (Naville, 1913). The Metropolitan Mu- seum of Art, New York, directed the writers to these examples. possibility that some other factor may have been largely responsible for the superior results being ignored. In other cases this was based on the fallacious as- sumption, already discussed, that the only proper soil for a plant is one similar to that of its native habitat. The J. I. Composts Tests at the John Innes Horticultural Institution in England demonstrated that a single soil mixture could, with minor modifications, be used for growing a wide range of plants. The usual battery of special mixes was, therefore, unneces- sary. The importance of this finding has slowly been appreciated by growers in England, Europe, and this country. This development constitutes one of the im- portant conceptual landmarks in the sub- ject of nursery soils. For the English grower this has been crystallized in the J. I. composts, as follows: Seed compost 7 parts composted medium loam, by volume 3^j parts peat, by volume 3^/2 parts coarse sand, by volume To each cubic yard of the above is added, with thorough mixing 2 pounds of superphosphate (18 per cent phosphoric acid) 1 pound of chalk (calcium carbonate) Potting compost 7 parts composted medium Loam, by volume 3 parts peat, by volume 2 parts coarse sand, by volume To each cubic yard of the above is added, with thorough mixing 2 pounds of hoof and horn meal (13 per cent nitrogen) 2 pounds of superphosphate (18 per cent phosphoric acid) 1 pound of sulfate of potash (48 per cent potash) 1 pound of chalk (calcium carbonate) The medium loam consists of the com- posted residue of a 4- to 5-inch layer of turf removed from pastures or meadows; it contains 2 to 7 per cent humus and only enough clay "to be slightly greasy when smeared". Sandy, heavy, or cal- careous soils are said to be unsatisfac- tory. [91] i>A>jf», r mm -ifif] w,\\v\\ yyiStl I / ///V^7/ / / /AC//// / / / / '2Jjlj.ll )j)iU, J/ / / / / /jhA^ufij / / / / / / / / / / //_.v '/'// //// //////'/) //)//////////// a?) i In order to avoid toxic residue from this mix it was found necessary to steam the composted loam before adding the peat, then to mix them and add the lime and fertilizers. The following disadvantages are in- herent in the J. I. composts: (1) some variability necessarily results from the use of composted nonuniform turf soil; (2) the necessary composting of the turf loam before using in the mix takes both time and space; (3) there is a high labor requirement in handling the composting operation; (4) meadowland turf is not commonly available in most areas in this country; (5) they contain coarse sand, which unnecessarily increases the weight; (6) a toxic residue is apparently produced if the soil is steamed after mix- ing. If toxicity is prevented by aging after steaming, the recontamination problem is increased and a storage prob- lem is created; if it is prevented by steaming the components separately and then mixing, the recontamination prob- lem is still increased. Evolving the U. C.-Type Mixes Work was begun in the Department of Plant Pathology, University of Califor- nia, Los Angeles, in 1941 to find a better soil for growing plants in containers than that then available. Because of the scarcity of turf for composting, the toxic residue after steaming, and the salinity problem, it was not possible to use the J. I. compost system. A substitute was therefore sought. At first a fine sandy loam was selected, and to this was added leaf mold and horse manure in the ratio of 10:2:2. Because of the variability of these organic materials, and the potential danger of excess salts in them, Canadian peat was soon substituted for them in the ratio of 3 parts of peat to 7 parts of sandy loam. Mineral fertilizers were added before steaming. No toxic effect was observed in any of a wide range of plants grown in this mix, even when planted immediately after steaming. Work to this stage of development was reported in 1948 (Baker, 1948). This mix was used for several years. Further developments of the mix were undertaken by the Department of Plant Pathology in 1949 when Philip A. Chandler, who had been at the John Innes Horticultural Institution in 1935- 1937, joined the group. 0. A. Matkin co- operated in the work after 1950, particu- larly on physical and chemical aspects of the soils. Five representative formula- tions of two ingredients, fine sand and peat, with a number of variations of fertilizer additions are now suggested (Sec. 5). Collectively these have been named the U. C. soil mix, and sometimes erroneously called "the UCLA blend," "Cal-Mix," "light soil mix," and "Calsoil Mix." The concepts behind this system, how- ever, are of greater permanent value to the industry than the formulas. Toxicity after steaming has been eliminated, probably through the use of fine sand without appreciable clay or organic- matter fraction, and of peat free of readily decomposable organic matter, and through maintenance of low con- centrations of soluble salts by using low- conductance ingredients and fertilizers not readily decomposed by steaming. The mixes are reproducible because they use only ingredients that are readily available in uniform quality. Compost- ing is eliminated because the organic Fig. 64. Culture of plants in containers was perhaps first practiced by the Egyptians 3,500- 4,000 years ago. Top, frankincense trees growing in pots. Middle and bottom, frankincense trees in containers being introduced to Egypt from the Somali Coast. This is one of the early recorded instances of plant introduction. Recorded in the Temple of Hatshepsut, Deir el-Bahri, near Thebes. (From Naville, 1913.) [93] matter used is already largely broken down, and the fertilizer is uniformly dis- tributed through the mass by mechanical means rather than "weathering". Plant growth in the mixes is uniform in size and time. Many kinds of plants obtain the necessary nutrients in uniform sup- ply from these mixes for several weeks before additional fertilizer is needed. When organic nitrogen is used, plants are supplied a minimum level of nitro- gen below which the supply does not fall. The organic nitrogen is in a form only slightly decomposed by steaming. It is quite possible to procure and mix the uncomposted ingredients, steam them, and use the soil for planting, all in the same day. This fact, plus the uniform results, makes possible for the first time truly scheduled production and mechani- zation. In England, the pasteurized J. I. com- posts have been placed on the market by commercial suppliers in quantities varying from a bag upward. This may be the ultimate development with the U. C- type mixes, with the grower no more "involved in the soil business" than he is presently in the seed business. Expe- rience in California has shown, however, that there are several problems yet to be solved before this goal can be success- fully achieved here. The Einheitserde Still a different solution of the prob- lem of soil treatment toxicity has been developed in Germany. The Einheitserde (Standardized Soil) developed by Dr. A. Fruhstorfer of Hamburg and introduced in 1948, is now marketed by several companies in that country. The mix is half peat and half well-aggregated sub- soil clay, to which are added ammonium sulfate, superphosphate, and potassium sulfate. Half as much fertilizer is added to seed soil (P-Erde) as to potting soil (T-Erde). Lime may be added to main- tain pH 5 to 6. These materials are mixed together and used without treatment, be- cause the peat and the subsoil are largely free of weeds, organisms, and decom- posable organic matter. This single mix is used for a wide range of plants. This seems to be a drastic method of avoiding the treatment-toxicity problem, and still involves the disadvantages of clay soils described in this section, and of ammonium accumulation (Sec. 7). The assumption is unwarranted that peat is free of organisms capable of causing disease. Recent German studies have shown that the mix may be infested with pathogens during storage (Danhardt and Ramsch, 1955) and handling. Neither is the organism carryover on containers prevented. Granting that this method is better than using ordinary untreated composts, it still leaves much to be de- sired. In England, a patent application for a compost similar to the Einheitserde has recently been published (Allerton and Ray, 1954). The mix consists of 1 to 2 parts by volume of sphagnum peat moss, 1 to 2 parts of fine vermiculite, 1 to 2 parts of heavy clay, plus fertilizers. An example of the fertilizers is to use 8 ounces per bushel of a mixture of 2 parts by weight of magnesium sulfate, 2 parts potassium nitrate, 3 parts ammonium sulfate, and 4 parts single superphos- phate. This compost apparently is not yet being sold. SOIL TOXICITY IN RELATION TO TREATMENTS A drawback sometimes encountered from chemical or steam treatment of soil il the resulting injury to plants grown in it. The type and severity of the injury varies with the soil, the treatment, the plant, the time and handling of the soil after treatment, the environmental con- ditions, and perhaps other factors. Symp- toms may be stunting of the plant, dis- coloration, necrosis, and abscission of leaves, death of the plant, or reduced seed germination. The toxic effect may be temporary or last several months, particularly if the soil is kept sterile or dry. When a proper soil mixture is used this toxicity does not appear, and there is no reason to have this problem. Substances Involved The many investigations that have been concerned with the nature of the toxin warrant the conclusion that several injurious agents may be involved. Briefly these are as follows. Accumulation of ammonium Bacteria that decompose ammonium are non-spore-forming and more sensi- tive to heat and many chemicals than the ammonifying organisms which convert organic nitrogen to ammonium. Treat- ments may thus cause the accumulation of ammonium, since there is a delay in conversion to nitrate. As discussed in Section 7, accumulation of ammonium may reach toxic levels in 2 weeks and last for 6 to 8 weeks or more. If leaching of the soil reduces toxicity, ammonium may not be the factor in- volved because: if leaching is successful soon after steaming, the ammonium usually has not yet reached toxic levels; if leaching reduces toxicity in soil treated some weeks before, the am- monium may largely remain in the soil instead of being removed. Ammonium may, however, bs involved if prompt planting after treatment prevents injury, since the transplants might be carrying nitrifying bacteria that would convert the ammonium, and the plants might keep the ammonium at a low level by absorbing it as formed, both of which would prevent accumulation. Water-soluble organic matter Organic matter is rendered water- soluble (broken down) in varying de- grees by heat and chemicals, but the nature of the process or the products are little understood. These materials are removed by leaching immediately after steaming and may be involved where benefit is derived from such treatment. Our experience has confirmed reports that soil mixtures high in readily decom- posable organic matter (manure, leaf mold, compost, some black peats) gen- erally give greatest toxicity from steam- ing. Highly organic soils also give greatest residual toxicity to bromine- sensitive plants (for example, carna- tions) from methyl bromide fumigation. Available manganese Soils, particularly of the acid lateritic type, may release toxic amounts of man- ganese when steamed. In Hawaii this causes severe injury to some crops (let- tuce, cowpeas) in steamed soils. Leach- ing of treated soils removes this in- jurious factor. Increase of total soluble salts Some soil ingredients may release enough adsorbed salts when steamed to produce plant injury. Thus, our tests in 1944 showed an increase in conductance (EC x 10 5 at 25° C on a 2:1 extract) from 117 and 151 to 228 and 213, re- spectively, in two series using a highly organic mix plus cow manure, after 45 minutes' steaming at 212° F. Leaching removes these toxic salts and is com- monly practiced for this purpose. Other agents Other agents that have been reported as resulting from soil steaming are: in- crease in water-soluble salts of calcium, copper, magnesium, potassium, zinc, phosphorus, and aluminum; altered pH; decreased iron and nitrate; osmotic con- centration of soil solution: modified ab- sorptive capacity of soil for water, gases, and salts. [95] Avoiding Post-Treatment Toxicity Several methods of dealing with this toxicity problem have been discovered and utilized under commercial condi- tions. Choice of soil mixture The best method is to avoid the trouble by using a soil mixture which does not form toxins after steaming or chemical treatment (sees. 5 and 7). All the types of toxicities resulting from heating soil, with the possible exception of ammonium accumulation, are eliminated by using a U. C.-type soil mix. The ammonium problem may easily be kept under con- trol by methods explained in sections 5 and 7. The J. I. composts and the Einheit- serde mentioned earlier evade treatment toxicity at the expense of increased risk from disease loss; the U. C. system both eliminates the problem and enhances protection from disease. Since there are also other advantages in a soil mix of the U. C. type, there is little valid reason for using a soil that requires corrective measures for post-treatment toxicity. In- terest in this toxicity problem is now largely academic, since the choice of proper ingredients makes it of no prac- tical concern. Leaching of the soil Leaching is the present most common method of reducing soil toxicity, and is often quite effective. However, it puddles the soil, creates a flood in the green- house, removes soil nutrients and makes fertilization necessary, increases the re- contamination hazard, delays planting operations, and increases cost of labor, water, and fertilizers. It is an expensive and messy solution of an unnecessary problem. It is not very effective against accumulated ammonium. Gypsum (cal- cium sulfate) may be added to facilitate the leaching of ammonium from the soil (Sec. 7). Aging of the soil Steamed soil is sometimes left for several weeks after treatment to reduce toxicity, presumably through reestab- lishing a biological balance in the soil. In some cases it is steamed in the fall, left all winter, and used in the spring. Among the obvious disadvantages of this method, however effective, are the ex- treme recontamination hazard, the delay in operations, cost of storing idle soil, and the expense of additional handling. Planting immediately after steaming This recent method may have merit when ammonium accumulation is the toxic factor involved. There are, how- ever, some plants that are sensitive to ammonium (Sec. 7), and these should not be used in this way. See also "Ac- cumulation of ammonium," above. CRITERIA FOR PHYSICAL INGREDIENTS OF SOIL MIXES Because of the foregoing facts, it is necessary to select ingredients which will perform the required functions and satisfy certain other practical and eco- nomic requirements of the growing op- eration. Characteristics of each potential component of the soil mix are discussed below and summarized in figure 65. The first four characteristics concern factors which cannot be improved by mixing with one or more other ingredients. If a component fails to satisfy any one of these conditions it is a hazard to the use- fulness of the mix. The last nine char- acteristics are, however, subject to altera- tion or improvement by proper mixing of ingredients. Mixtures of fine sand and peat moss approach the ideal. [ % i Characteristics E o o —J a u o U 4) > O 6 4> 4> a. "5 u 'e > 13 O E 4) 4) C O o E 4) a. o Q. E o u 3 T3 O lO o -Q "a B 3 o 6 3 0) u Si > o 00 o o c o<5 c l/l d) c LL. X 'e 2 Readily available in uniform grade? !K2g 1 Chemically uniform? Stable to steam and fumigation? 9 9 • 9 • 9 9 9 Easily made into uniform mix? ■ j Good aeration assured? ? ■ 9 9 9 9 Resistant to loss of nutrients by leaching? 9 Fertility low? 9 9 9 • 1 Relatively inexpensive? Moisture retention reasonably good? 9 Light in weight? (Low Density) 9 Shrinkage in storage negligible? 9 1 1 ■ 9 Gypsum or lime required? ? 9 • 9 • 9 ' Micronutrients adequate? •* 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 yes unknown or unpredictable intermediate ~o a a. -c _q -Q I 4) O . — . -5 'x o t -a -£ 200 _g '5 > < 100- Steamed 8 hrs. NH + NO 10 20 Time (days) 7o Steamed 8 hrs. NH 4 +NQ 3 NO a / ,-^ 10 Time (days) 20" 7o Fig. 68. Ammonium and nitrate nitrogen production in steamed and unsteamed U.C.-type soil j mix, with and without inoculation by nitrogen-converting organisms, held at 70°F. The figures represent an average of three series. See p. 113 and 115 for explanation. [ 114] 3. There was no significant difference between the effect of V2" an d 3-hour steaming on organisms which pro- duce ammonium and nitrate from the organic nitrogen. The treatment intervals inhibited activity to about the same degree. 4. Inoculation reestablished the ni- trate-producing power of the soil, but nitrification was delayed for about 10 days. The nitrate produc- tion in the unsteamed soil was also enhanced by inoculation, indicating that the fine-sand subsoil used was low in nitrifying bacteria. 5. Inoculation enhanced ammonium production in all cases, but did so to a lesser extent in the unsteamed soils. In the nursery, nitrate is produced in a steamed soil as a result of accidental inoculation. Since ammonifiers are both numerous and of many types they are better able to reestablish under a variety of conditions than the nitrifiers (Sec. 14). If inoculation is to be used as a means of reducing ammonium toxicity, it will be desirable to exclude ammo- nium-producing organisms from the inoculum. Otherwise the enhanced am- monium production could defeat the purpose of the inoculation. A steamed soil will normally become reinoculated with nitrate-forming bac- teria in a few days to a few weeks. When plants are sensitive to ammonium, and conditions are favorable for its accumu- lation, procedures must be adopted that will avoid damage. At present the pos- sibility of artificial inoculation with nitrifying bacteria is insufficiently ex- plored to be practically useful. Presently the only other approach is to control am- monium production by adjusting the amount and type of organic nitrogen used in relation to the conditions which affect its rate of release. To compare the rate of breakdown of different organic nitrogens under a variety of treatment and environmental conditions, another series of tests was run. Production of Available Nitrogen from Organic Sources The experiment was designed to de- termine the effects of different forms of organic nitrogen, rate of addition, steam- ing, and storage temperature of the soil mix upon release of available nitrogen. Organic nitrogen sources were: 1. Castor pomace (5.75 per cent nitro- gen) ; 2. Cottonseed meal (7 per cent nitro- gen) ; 3. Fish meal (11 per cent nitrogen) ; 4. Blood meal (13 per cent nitrogen) ; 5. Hoof and horn meal (13 per cent nitrogen) ; 6. Urea-formaldehyde resin (35 per cent nitrogen) ; 7. Steer manure (2 per cent nitrogen) . Also included were : 8. Leaf mold in place of peat moss, without other nitrogen source; 9. Control; peat moss, but no other organic nitrogen source. Soil mix B (25 per cent peat) was used in all except no. 8 above. Potassium, phosphate, and lime were added to all. Hoof and horn was added at the rate of 4% and 6% pounds per cubic yard; the other nitrogen sources were added to supply the same amount of actual nitro- gen. One set of samples of each form of organic nitrogen was steamed (212° F for 30 min.), another set not steamed, and both were stored at 50° or 70° F. Soils were protected from recon- tamination, and samples taken at 0, 11, 24, 38, 52, 81, 102, 133, and 149 days. Ammonium and nitrate nitrogen were determined on each sample, using a 1:5 soil-water extraction ratio in the pres- ence of excess calcium sulfate. Effect of treatments on release of available nitrogen Three of the above nine series (steer manure, leaf mold, and the control) [115] c d) Ui O J) _Q _D '5 > < STEAMED NH 4 + NO 3 — T— 15 50 Time (days) NOT STEAMED NH 4 + N0 3 s"^*'* //'MO , / I / 1 / t / / / • 1 1 — i 100 15 50 Time (days) 100 Fig. 69. Diagrams showing general character of the effect of steaming of U.C. mix B on con- version of organic nitrogen to available forms. See p. 115 and 116 for explanation. produced little or no nitrogen ; the others produced substantial amounts. The check showed little available nitrogen under any conditions; toward the end of the experiment there was a slight tendency for available nitrogen to increase, prob- ably from the peat moss, which normally contains about 1 per cent nitrogen. In the mix containing leaf mold, the avail- able nitrogen level was higher than in the control but was affected by treatment only when the unsteamed mix was stored at 70° F. In this lot an appreciable amount of available nitrogen accumu- lated during five months' storage, but the amount produced did not compare with that from nitrogen sources 1 through 6. Steer manure showed some active release of nitrogen at 70° after 50 days or more. The amount was less than that released from leaf mold, how- ever, and is considered to be of little consequence from a practical standpoint. Initial tests indicated that the manure mix was very saline. Organic nitrogen forms 1 through 6 were sufficiently similar in their pattern of breakdown to be discussed together. Two diagrams (fig. 69) show this general pattern. Whether steamed or not, nitrogen was more rapidly released dur- ing the first month than in the next two months. When the mixtures were un- steamed, nitrate was produced; but there was a delay before the rate was sufficient to prevent ammonium accumulation. When the mixtures were steamed, no nitrate was produced in 100 days, in- dicating that reinoculation by nitrifying bacteria had not occurred. In unsteamed soil the time interval before nitrification began was greatly in- creased by low temperature and by low organic nitrogen. The results are sum- marized in the tabulation below. One may conclude that these condi- tions retarded the activity of nitrifying organisms. Further indication of the effect of these factors upon rate of nitrogen re- lease is shown by the average time re- quired for available nitrogen to reach 50 Storage temperature Time required for nitrification to begin (unsteamed soil) With 4'j lb. rate With B\-i lb. rate 50 F 70 F 60-130 days, av. 95 days 10-40 days, av. 30 days 60-110 days, av. 88 days 10-25 days, av. 18 days [116] Storage temperature Soil treatment Average storage time to reach 50 ppm of available nitrogen With VAVa. rate With SH lb. rate 50° F f Steamed \ Unsteamed f Steamed \ Unsteamed 37 days 27 days 18 days 16 days 28 days 19 days 12 days 10 days 70° F ppm of dry soil, a moderate supply for plant growth in soil mix B; the data are summarized above. An increase in the storage temperature resulted in an increase in the rate of available-nitrogen production. Increas- ing the amount of source material also increased this rate. On the other hand, steaming had a retarding effect. The rate of release of available nitrogen was re- duced by steaming the soil, by low storage temperature, and by low rate of addition of organic nitrogen. These con- ditions evidently retarded the activity of organisms producing ammonium nitro- gen from organic sources. At the time of the first sampling (11 days) there was, in this expsriment, an appreciable increase in the ammonium concentration in all but the steamed soils stored at 50° F. In these, ammonium production was only slight, except for urea-formaldehyde, to be discussed later. In the previously described (fig. 68) experiments on inoculation, ammo- nium production was appreciable at 5 days. It seems that if the temperature is high enough, activity of ammonifiers be- gins very quickly, certainly within a few days. Still another indication of the in- fluence of these environmental conditions on rate of nitrogen release is shown by the concentration of available nitrogen at given times in the soil mix; the data are given in table 11. Again, the rate of release of available nitrogen was reduced by steaming, low storage temperatures, and low rate of nitrogen addition. In all cases the amount of available nitrogen evolved Table 1 1 . Effect of Storage Interval on Concentration of Available Nitrogen in Soil Mix Average of six organic nitrogen sources Storage temperature, °F Treatment Concentration of available nitrogen,* in ppm of dry soil With \y 2 lb. ratef With 6% lb. ratet 15 days 50 days 100 days 15 days 50 days 100 days 50 f Steamed (Unsteamed { Steamed \ Unsteamed 20 33 45 61 74 86 80 120 80 96 95 159 33 52 63 83 88 94 108 180 96 126 108 214 70 * Average of total available (ammonium plus nitrate) nitrogen from organic nitrogen sources 1 through 6. t Hoof and horn meal added at 4' 2 and 6 1 2 pounds per cubic yard. Other nitrogen sources were added to supply an equivalent amount of nitrogen. [117] was substantially greater in the first than in the second 50-day period. This ten- dency for leveling-off was greater in steamed than in unsteamed soil, as shown in figure 69. Nitrogen supplied by specific materials There were marked differences be- tween the available nitrogen released from the six different organic sources (table 12). The materials (castor pomace, cotton- seed meal, and fish meal) that most rapidly produced available nitrogen, were, surprisingly enough, those with the lowest original nitrogen content. Of the three, castor pomace seemed to break down most rapidly. Blood meal, hoof and horn meal, and urea-formaldehyde all have a much lower rate of release of available nitrogen, and the values are considerably lower than those of the first three. Steaming enhances the breakdown of urea-formaldehyde, whereas the effect was just the opposite in all other mate- rials. In the more complete data it is apparent that there was an immediate build-up of ammonium whenever the urea-formaldehyde mix was steamed. Possibly the effect of steaming was to partially hydrolyze the compound to pro- duce some free urea, which was quickly converted to ammonium. After this first surge, the rate of nitrogen release was very slow. In all cases where the mixes were not steamed, the rate of release was slower from urea-formaldehyde than from any of the others. Thus, urea-for- maldehyde has certain desirable prop- erties, but a serious drawback of initially releasing ammonium when steamed. As a result of these tests the materials might be listed in the descending order of their activity as follows: castor pomace; fish meal; cottonseed meal; blood meal and hoof and horn (equal) ; urea-formaldehyde. This information does not necessarily Table 12. Available Nitrogen Released by Various Organic Nitrogen Sources in a Soil Mix After 30 Days' Storage Two rates of organic application, steamed and unsteamed, and stored at two temperatures Storage temperature, op Treatment Rate, lb. per cu. yd.f Concentration of available nitrogen,* in ppm Castor pomace Cotton- seed meal Fish meal Blood meal Hoof and horn meal Urea- formal- dehyde resin 50 < Steamed Unsteamed Steamed Unsteamed UA Wa Wa Wa Wa Wa Wa Wa 60 100 93 128 73 132 160 160 54 75 108 116 60 98 95 140 60 92 80 130 66 114 104 160 25 48 47 60 45 60 63 105 25 37 47 63 50 60 70 95 80 73 42 46 78 64 60 64 70 Average steame d 91 130 72 115 83 119 45 69 43 69 74 53 Average unstea] ned. * Total available nitrogen (ammonium plus nitrate) in ppm of dry soil. In steamed (212° F for 30 min.) series all nitrogen was ammonium. t Nitrogen addition equivalent to 4 1 £ lb. or 6 1 jj lb. hoof and horn meal per cu. yd. hold when these fertilizers are applied broadcast or as top dressings (see "Sur- face Application versus Mixing," below) . Hoof and horn meal and blood meal constitute the most desirable sources of organic nitrogen for the soil mix, in that they are slower in their release of avail- able nitrogen than are castor pomace, cottonseed meal, and fish meal. If the problem of initial ammonium release upon steaming can be overcome or mini- mized, urea-formaldehyde might be superior to any of the natural sources — assuming, of course, that biuret toxicity can be avoided (Sec. 5). Until further investigations are carried out, the effects of thorough chemical treatment must be assumed to produce results similar to those found where steaming was used. Since heat is prob- ably a major factor in the hydrolysis of the urea-formaldehyde, chemical treat- ment might prevent the rapid breakdown to ammonium by this material. Application of This Information Bedding plants Identification and clarification of the problems discussed above led to certain changes in bedding-plant procedure. The use of organic nitrogen in the mixes was greatly reduced or eliminated. This re- duction in nitrogen supply was offset by the use of nitrate forms. Calcium nitrate solution was used as a liquid supplement and in many instances potassium nitrate has been substituted for potassium sul- fate in the mix. In addition, hoof and horn or blood meal was applied as a top dressing, this procedure supplying slowly available nitrogen over an ex- tended period without the dangers of ammonium toxicity (see "Surface Appli- cation versus Mixing," below). It was especially important to avoid large amounts of organic nitrogen in the mix during periods of hot weather. Pot plants While the work with bedding-plant growers was progressing, attention was given to similar problems in greenhouse- pot and nursery-can culture. In pot-plant growing the only period of high sen- sitivity seemed to be in the early growth of rooted cuttings or small seedlings. Where plants were vigorous, there was seldom any setback if reasonable amounts of organic nitrogen were used. No problems developed where plants were moved up from a smaller to a larger container. There are two primary reasons for the increased safety in "pot- ting-on": 1. The amount of soil used for each plant is substantially less in proportion to plant size than when potting seedlings or rooted cuttings with no root ball. The established plant will require much more nitrogen and must get it from a relatively smaller amount of added soil. 2. The root ball which is placed in the new soil has had time to become reinocu- lated with nitrifying bacteria. The added soil will, therefore, become quickly inoculated and nitrate will be produced. In some cases, growers are practicing subirrigation for such crops as Saint- paulia. In this procedure there is almost no leaching action and the added nitro- gen is not so readily lost as with surface watering. Under these conditions the upper limit of added hoof and horn has been about 2% pounds per cubic yard of soil. Nursery-can culture In nursery-can growing the problems are similar to those encountered in the bedding-plant industry. Rooted cuttings or small liners are transplanted into gal- lon cans where growth of many items may be relatively slow. This provides ample opportunity for added organic nitrogen to supply an excess of available nitrogen. Although no indications of specific toxicity due to ammonium have [119] been noticed, there have been instances where the build-up of nitrogen was suf- ficient to produce a salinity effect. For this reason organic nitrogen is com- monly omitted or used in relatively small amounts. Another reason for reducing the organic nitrogen is that soils are fre- quently prepared ahead of time and held for several weeks to several months be- fore planting (Sec. 5). Bed or bench culture When crops are grown for several seasons or years in beds or benches, or- ganic nitrogen may be completely omitted in the mix and a program of fertilizing, with or without organic nitro- gen, followed from the start. There is little advantage in delaying the initiation of the long-term program by initial mix- ing of organic nitrogen into the soil. FACTORS AFFECTING NITROGEN RELEASE FROM ORGANIC SOURCES In well-aerated soils such as those ob- tained in the physical medium of the U. C.-type mix, the following factors may affect the amount and rate of release of available nitrogen from the organic form. Microorganism population In natural soils the population of microorganisms will be much higher in the surface 6 to 12 inches than at greater depths. Subsoils are frequently low in organism population. This is to be ex- pected since the moisture, air, and or- ganic matter necessary for most organ- ism activity are more available in surface soils. The difference in the effect on rate of nitrogen release by untreated surface and subsoils may be substantial. Where heat or chemical treatment is practiced, the difference between surface and subsoils is reduced, since all nitrify- ing bacteria are killed and the population of those which produce ammonium is initially reduced (Sec. 14). Heat and chemical treatment As suggested in the preceding para- graph, the procedure of treating the soil to rid it of pathogens necessarily affects the whole microorganism population. Nitrifying bacteria will be eradicated where treatmenl is effective. Ammonium production may be reduced but not en- tirely eliminated. Furthermore, ammo- nifiers will more rapidly repopulate the soil. Thus ammonium may be produced, but not converted to nitrate, until the soil becomes reinoculated with nitrifiers. Under normal conditions, inoculation will occur within a period of several days to several weeks. This is an uncertainty which in the future may be eliminated by the use of inoculation cultures. Soil reaction (pH) Highly acid media generally have little effect on the activity of ammonium pro- ducers, while inhibiting the activity of nitrifiers (Sec. 14). This was shown in a nursery test in which azaleas were grown in beds of pure peat plus organic nitrogen. Hydrated lime was worked into a test bed before planting. After several months, plants in the limed area were noted to be darker in color than the re- mainder. Tests of the growing medium showed the differences in acidity and nitrogen concentration as tabulated on page 121. It has been demonstrated by Tied j ens and Robbins (1931) that some crop plants are unable to utilize the ammo- nium form of nitrogen when the pH is low, but do so readily when the pH is in the neutral to alkaline range. The use of lime in recommended mixes should de- crease difficulties of this nature. [120] Test bed PH Nitrate nitrogen, ppm of dry peat Ammonium nitro- gen, ppm of dry peat Limed Nonlimed 5.9 3.9 350 26 155 225 Soil temperature In general, the warmer the soil, the greater will be the activity of micro- organisms in it. There is an upper limit, of course, beyond which an increase in temperature will reduce and even kill them. When temperatures are low, ac- tivity is reduced. The nitrifiers are more critically affected than are the ammoni- fiers (Sec. 14) . Thus during cool weather there will be a reduced rate of ammo- nium production, and an even greater reduction in the rate of nitrification. When soil temperatures are below 40° F, applications of blood meal or other organic sources may be quite ineffective and the rate of release of nitrogen too slow to meet the plant requirements. Under these conditions the grower should use soluble materials such as calcium nitrate. Concentration and source of organic nitrogen As already shown, an increase in amount of organic nitrogen added to a soil mix will increase the rate of release as well as the total amount of available nitrogen. Therefore, the higher rate of addition does not necessarily mean that the period of release will be lengthened. The sources of organic nitrogen are quite varied in chemical composition. Some organic sources of nitrogen seem to be more readily assimilated by organ- isms which decompose them than do others. Urea- formaldehyde, hoof and horn meal, and blood meal are among the slowly decomposable forms of or- ganic nitrogen. Cottonseed meal, castor pomace, and fish meal are quite rapidly decomposed. Since the primary objective in adding an organic nitrogen source to the mix is one of prolonging the period of release of available nitrogen, it is most reasonable to add those sources which are slowest in rate of breakdown. As explained below, there are substan- tial differences in rate of release of avail- able nitrogen when organic materials are broadcast over the surface of the soil as compared with being mixed into it. Presumably the more finely divided and more "palatable" materials would result in more rapid release of available nitro- gen with surface application. Moisture and aeration A soil that is dry will not support plant growth and, as might be expected, will retard microorganism activity. It is, however, unsafe to assume that a stored soil mix is dry enough to prevent the re- lease of available nitrogen from organic sources. In some cases oven-dried and stored samples have been found to pro- duce available nitrogen. Lack of ade- quate moisture would probably be more damaging to the growing plant than any side-effect it might have on nitrogen relations. Lack of oxygen supply (aeration) is an inhibiting factor in plant growth and alters microorganism activity. It has commonly been accepted that poor aera- tion will result in nitrite accumulation (Sec. 14) . It now seems that many of the troubles blamed on nitrite may have been due to ammonium toxicity and salinity. There is recent evidence ( Duis- berg and Buehrer, 1954) that nitrite toxicity may have been highly overrated. In any event, a U. C.-type soil mix pro- vides a medium with excellent aeration, and chemically stable to soil treatment. Nitrite accumulation has thus far not been found to occur in it. [121] SURFACE APPLICATION VERSUS MIXING Surface application of organic nitro- gen can be substantially less dangerous from the ammonium-toxicity standpoint than when it is mixed into the soil mass. This greater safety results from: 1. The slower production of available nitrogen at the surface, owing to less intimate contact with the soil mass and to environmental condi- tions intermittently unfavorable to soil organisms. 2. The restricted ability of ammonium to move through the soil into the root zone (see "Ammonium nitro- gen," above) . 3. More rapid inoculation with nitri- fiers at the surface of treated soil, converting to nitrate the ammonium produced. Since top-dressing mate- rials are usually not treated with heat or chemicals, they may have a better balance between ammonify- ing and nitrifying microorganisms. The restrictions on the use of organic nitrogen in the soil mass discussed in this section do not necessarily apply to their use as surface dressings. Features of surface application that are less desirable than soil-mix inclusion are: 1. Practical inability to make uniform application ; 2. Danger of plant damage from rapid decomposition at high temperatures in contact with the seedling ; 3. Objectionable residues, odors, and flies. Under some circumstances it is better to mix the organic nitrogen into the soil than to apply it as a later top dressing. The dangers discussed in this section from mixing organic nitrogen into the soil mass encourage cautious and judi- cious use of it in this way. On the other hand, there are also problems in using the materials in surface application. ADJUSTING TO SPECIFIC SITUATIONS A grower is never relieved of the necessity of thinking by this or any other system of soils and fertilizers. The fore- going discussion illustrates the complex- ities of nitrogen application in plant growing. The problem of ammonium toxicity is an important consideration where organic forms are used, but they may greatly prolong the usefulness of the soil mix and prevent the occurrence of extreme nitrogen deficiency. In some growing operations (for example, pot plants) organic nitrogen included in the mix reduces labor and variability from top-dressing application. Where the U. C. system is carefully followed the only nutritional problems which can develop in the first several months of use must be related to nitrogen supply. It is im- portant whether nitrogen is supplied as ammonium or nitrate, and in what quan- tity. The information in this section should aid in crop programming and analysis of nitrogen nutrition problems. r 122 ] SECTION Heat Treatment of Soil Kenneth F. Baker Chester N. Roistacher Comparison of commonly used treatments Benefits from heat treatment of soil Sanitary precautions in soil treatments Treatment temperature and time The form of steam used Efficient soil steaming Volume of steam required Preparing soil for steaming Uneven heating Cooling the treated soil Water content after steaming Steam-treating home yards Cost of steaming soil .HE soil AND the host plant are the two ultimate sources of organisms that cause plant disease. Pathogens may be eradi- cated from the soil by treatment with heat (this section and sees. 9 and 10) or chemicals (Sec. 11), and from the planting stock (Sec. 13) and containers (Sec. 12) in various ways. These prac- tices, along with the use of a U. C.-type soil mix (sees. 5, 6, and 7) and sanitary practices (sees. 1, 3, and 14). are the supports for economic modern plant production through the U. C. system (frontispiece). COMPARISON OF COMMONLY USED TREATMENTS Heat treatment of soil may be done with steam, a dry source of heat, or hot water. Each has its place in nursery prac- tice, the same as chemical treatment. Steam versus chemicals The comparative advantages of steam and the two most commonly used chemi- cals are shown in table 13. Treatment [123] Table 1 3. Comparative Advantages of Steam and Chemical Treatments of Soil in Common Use in California Nurseries Characteristic Steam, 180°-212° F for 30 min. Methyl bromide, 4 lb. per 100 cu. ft. Chloropicrin, 5 cc per cu. ft. Time required for treatment About 1 hr. 24-48 hr. 2-3 days Time between treat- ment and planting . . About 1-2 hr. to cool 24-48 hr. 7-10 days Kills all pathogens, weeds, and insects?. Yes, best treatment; a few weeds sur- vive Most, but not Ver- ticillium; a few weeds survive Yes ; a few weeds survive When can penetration of material be deter- mined, as a measure of effectiveness? .... At once, by measur- ing soil tempera- ture Later, by noting re- duction of disease or pathogen Later, by noting re- duction of disease or pathogen Toxic after-effect to crops? None with U.C.- type soil mixes Yes, for carnations and some others None, when properly aerated Use near living plants? Yes Within 3 ft. if ade- quately ventilated Only with excellent ventilation Destroys organisms in unrotted crop refuse? Yes Yes Poorly Can it be used any- where? Only if portable boiler used Yes Yes Is its use limited by environment? Time and cost in- crease with cold or wet soil, but can be so used Not recommended below 60° F Dosage increased if soil below 65° F or wet Ease of application .... Easy Easy Obnoxious and time consuming Dangerous to work- men? No Yes Yes Is large capital outlay required? If boiler unavailable No No Cost per cu. ft. of soil, exclusive of labor . . Less than 2 cents, including equip- ment cost About 2.9-3.2 cents, excluding equip- ment cost About 1.9-3.0 cents, excluding equip- ment cost [124] with steam is faster, easier, cheaper, and more effective than with these materials. It remains the best general method of disinfesting soil, destroying fungi, bac- teria, nematodes, weeds, and insects. It is the standard for judging new chemical treatments. For these reasons it is the treatment emphasized in this section. Steam offers further advantages. It is more dependable and its effectiveness is more readily determined than most chemical treatments. The penetration of steam can be quickly and easily assessed by measuring temperature and time. This is so dependable a measure of effective- ness that temperature is practically used for its evaluation, instead of the measur- ing of disease as must be done with chemicals. Treatment by steam is funda- mentally a transfer of heat from the boiler to the soil, and all factors (such as soil moisture, porosity, volume, and temperature) affect a single variable, the temperature, measured by a thermom- eter. The effectiveness of chemicals is modified by many of the same factors, but the result (concentration of active chemical in the soil) is not readily de- termined. Thus, it is commercial practice to judge penetration of a chemical by the control of pathogens. Practically, this measurement is only possible weeks or months later, and by the occurrence of disease. Soil may be steamed, furthermore, within 1 or 2 feet of living plants without injury to them. This is a distinct advan- tage in a planted glasshouse, where it may be necessary to treat a single bench or a localized area of it. Chemicals can be used in this way only with excellent ventilation and even then there is some risk (Sec. 11) . There is no hazard from steaming soil in the headhouse room or other places where men are working. In closed areas the heat may be uncomfortable, but no noxious or dangerous gases are given off. There have been no complaints from neighbors when steam has been used, whereas serious difficulties have some- times arisen from chemical fumigation. There are, however, some conditions where steam is impractical (for example, large field areas used for low-value crops), and some where it is initially expensive (for example, if a boiler must be purchased). In such cases chemical treatments are often used (Sec. 11). Some chemicals are occasionally injected with steam (Sec. 10, type 26) , and others are applied as supplements to it (Sec. id. Steam versus other heat treatments A dry source of heat (for example, metal heated by a flame or electricity) may be used for treating soil. One of the worst disadvantages of this type of heat is that it is necessary to apply intense heat (high temperature) in a limited area in order to impart the required quantity (B.t.u.). Usually intensity is high, quantity is small, and distribution through the soil is poor. Steam, in con- trast, imparts a large quantity of heat at low intensity (212° F), and flows through the soil to the cold area. It is almost as though successive heat sources were turned on along the advancing front as the steam moves forward. One of the principal advantages of steam is that the B.t.u. are released at the point to be heated. Furthermore, there is a natural ceiling of 212° when soil is heated by flowing steam. This is a safety feature shared by no other heat treat- ment except by hot water. From these facts it is concluded that a dry source of heat may be used for treating soil that moves past the heater. When handled in this way, satisfactorily uniform progressive heating of the whole soil mass results. It should not be used to treat a stationary soil mass. In con- trast, steam is most efficiently used for treating a stationary soil mass. When a dry source of heat is used with a moving soil mass, it is necessary to treat con- [125] tainers in some other way (sees. 10 and 12). The principal disadvantage of hot water, even when boiling (212° F). is that it releases so much less heat per pound to the soil than does steam (Sec. 9) . Hence much more of it must be used to raise the soil temperature to the same level. Soil may be puddled by such treat- ment, and the quantity of water draining from the soil is messy and troublesome, particularly in glasshouse beds. Hot water is less efficient and convenient than steam for treating soil. About the only compensating advantage is that salinity is reduced to a low level by the leaching provided. Hot water is some- times used for treating propagating sand (Sec. 10, type 25). It may be used for leaching of salts from propagating beds, which may then be steamed. Its use is decreasing. BENEFITS FROM HEAT TREATMENT OF SOIL The primary reason for most soil heat treatments is the elimination of fungi, bacteria, and nematodes that cause plant disease. There are, however, other benefits. Heavy soils become more granular, with improvement of drainage and aera- tion. Much of the steaming of glasshouse rose soils in the United States is done for this purpose, rather than for disease con- trol. This same effect, however, often causes trouble for bedding-plant growers when they begin steaming and do not properly adjust watering operations, since the stock may not be kept suffi- ciently moist. The formation of toxins from steaming such heavy soils is dis- cussed in Section 6. Improvement of plant growth not definitely associated with elimination of known disease sometimes occurs. This may result from increased availability of nutrients (Sec. 6), change from nitrate to ammonium nutrition (Sec. 7), or im- proved physical structure of the soil, but is often due to a biological change that may or may not be directly associated with disease. This corresponds to the "increased growth response" from chem- ical treatment of soil (fig. 119; and sees. 11 and 14). Elimination of weeds is the benefit from heat or chemical treatment of soil over which many growers are at first most enthusiastic, perhaps because of the spectacular results. Many bedding-plant growers have stated that this feature alone pays for the treatment. Since some have spent 3 to 4 cents a flat for weeding, this claim is well founded. The practice of composting, done in part for the elimination of weeds, is rendered un- necessary when soil is treated (Sec. 6). Very few weeds survive heat treatment of soil (fig. 70 and Sec. 9). and even these may be largely eliminated if germination is started by keeping the soil moist for a few days prior to mixing and treatment. SANITARY PRECAUTIONS IN SOIL TREATMENTS To reduce the possibility of infesting treatment. This eliminates handling and treated soil with pathogens, it should be h a ndled as little as possible after steam- ing or oilier trealment. It is desirable, therefore, to place the soil in the con- tainers (pots, fiats, cans, benches) before insures that the containers are ade- quately disinfested. The fact that soil trealment in bulk is discussed in this manual (sees. 10 and 17) does not imply that it is equally satisfactory. The method 120 | is given because some nurserymen do not find it feasible to adapt in-container treatment to their operations. When bulk treatment is used, the containers must be separately treated and special care taken to avoid contamination before and during filling operations. If new cans, flats, and pots are used, and these have been carefully handled to prevent contamination after delivery, there is less reason to treat after filling. This is also true for new insert unit con- tainers for flats. If the flats in which they are placed have been used before, how- ever, there is still some risk. Further- more, the possibility of contamination during the actual filling operation exists, unnecessarily, in each of these practices. It is a wise general practice to place the soil in containers and then to treat them as a unit. Mixing of fertilizers or other in- gredients into the soil after treatment should be avoided; this may readily be accomplished with the U. C.-type soil mixes (sees. 5, 6, and 7). TREATMENT TEMPERATURE AND TIME A temperature of 180° F for 30 min- utes is adequate to free soil of pathogens, weeds, and insects (fig. 70, and Sec. 9). For reasons explained in Section 9, heat- ing cannot be stopped at 180° in some kinds of equipment. A final temperature of 180° for 30 minutes is possible and satisfactory for treatment of a moving soil mass with either a dry source of heat or steam, because the rate of movement and the amount of steam or dry heat ap- plied can be regulated. If the English steam-air mixture system (Sec. 9) should prove generally satisfactory, this temperature could also be used for steam- ing a stationary soil mass. With present methods, however, steam will not heat a stationary soil mass to less than 212°. Hence, a final temperature of 212° for 30 minutes is recommended for steam treatment of soil in containers or in a stationary bulk mass. These temperatures take into account such practical considerations as the re- duced rate of heat penetration into clods and pockets of organic material and Fig. 70. Temperatures necessary to kill path- ogens and other organisms harmful to plants. Most of the temperatures indicated here are for 30-min. exposures under moist conditions. °F O 212 200 190 180 170 160 150 140 130 120 110 100 M Few resistant weed seeds Resistant plant viruses Most weed seeds _ All plant pathogenic bacteria Most plant viruses .Soil insects Most plant pathogenic fungi — Most plant pathogenic bacteria Worms, slugs, centipedes ~ Gladiolus yellows Fusarium — Botrytis gray mold — Rhizoctonia solani _ Sclerotium rolfsii and Sclerotica sclerotiorum ~ Nematodes — Water molds [127] cc /?• *V X US ^ tr '/I' 8 * /|W us S-con us °>. us - • . — ^^T /-/l v I 212° tr J cc \ 7 US - — " X 2» IHHUM es 212° o cc z^con ss / / Or plant residue. The timing should begin when the coldest point in the soil mass has reached 180° to 212° F. Usually a slight steam flow must be maintained after this time in order to hold the de- sired temperature for 30 minutes. Equip- ment for treatment of bulk soil often will have cold corners at the bottom. These should be located by trial, and used as the temperature test points. The worst cold corners should be eliminated by fitting the box with triangular pieces of wood (see Sec. 10, type 4). Because steam thus moves through the soil as an advancing front, temperature rise at a given point is usually quite sudden under efficient operating condi- tions (fig. 71 and Sec. 9). Steam con- denses on the nearest cold soil particles and does not penetrate farther until it has heated them to 212° F. For this rea- son, several thermometers may be placed Fig. 71. Diagram showing movement of steam from the orifice (or) in the buried pipe (p) through a stationary soil mass. Steam ex- pands as a spheroid with an elongated top. If the distance of movement (d) above the out- let is 1, then that below it is approximately Vi, and that to the sides is Vi to %. An advancing boundary or zone of condensation (con) sepa- rates the unheated soil (us) from that at 212°F. The same unit at the same instant of heating is shown in end view in A and in bottom view in B. The shape of the spheroid would be the same in side view as shown in A. When the steam from the top pipe reaches the soil surface (ss), it escapes (es). Steaming should then continue at a reduced rate of flow (the "trickle finish") until the cold corners (cc) are heated. The same unit as in A and B is shown in C a few minutes later, with nearly all the soil heated to 212°. The zones do not overlap as the steam flow continues, but the steam then tends to flow into the cold corners. Note that the cold corners are easily eliminated by a triangular piece of wood (tr) placed in the vertical angle of the box. 28 I at varying distances from the steam in- puts to plot the advancing heat front. Ac- curate chemical thermometers must be used, since inexpensive ones are likely to give inaccurate readings (see Appen- dix). Convenient pellets called Tempil Pellets are available (Appendix) that melt at indicated temperatures from 113° to 250° F or more. A series that melts at 163°, 188°, 200°, and 213° may be placed in various parts of the soil mass; the highest one melted indicates the highest temperature attained at that point. It is safe to heat a U. C.-type soil mix to 212° F without the soil's becoming toxic to plants. With these soils it is better, therefore, to overcook than to risk incomplete heating. When possible, other types of soils should be heated to only 180° until the grower finds that the particular soil may be safely heated to 212°. Temperature requirement for pinto-tag certificates In August, 1954, the California De- partment of Agriculture ruled that vege- table plants grown in flats could be moved, without inspection at destination, between counties participating in the intercounty nursery stock certificate agreement. To qualify, the soil and flats must be either "steamed in a closed chamber until temperature of all soil reaches 180° F" or treated for 24 hours with methyl bromide at 4 pounds per 100 cubic feet, and protected from recon- tamination. The certificate may be used for shipments by authorized nurseries of plants so handled. These "pinto-tag" cer- tificates are becoming recognized as in- dicative of superior nursery stock. THE FORM OF STEAM USED Free-flowing or pressureless steam Of the three forms of steam used for soil treatment, the free-flowing is the most commonly used and probably, con- sidering equipment cost, the most prac- tical. The steam is at atmospheric pres- sure or very slightly above. This means that large-diameter, light-weight, quick- coupling aluminum irrigation pipe can be used for mains, and that a lighter and less expensive boiler is possible. There are other advantages for this type of steam (sees. 9 and 10). If a boiler must be purchased for soil steaming, one op- erating at little or no pressure probably is the best investment. Many boilers used for glasshouse heating deliver steam through the mains under pressure. When such steam is used for soil treatment, it drops back to at- mospheric pressure as it enters the soil. The steam supplied is slightly drier than the free-flowing type and carries a little more heat. Its effect on the organisms is the same as the free-flowing type, since they are subject to no pressure. Steam under pressure Soil may be subjected to steam at about 15 pounds' pressure in tight containers (autoclaves or cannery retorts). Al- though there is about 1.4 per cent more B.t.u. per pound in this than in free- flowing steam, there is no real gain in treatment efficiency (Sec. 9). Steam at 15 pounds' pressure has a temperature of 249.8° F. Since no worthwhile pur- pose is served, and the excessive heat may be detrimental to the soil, the use of pressure steamers is not recommended. Furthermore, the increased cost of equip- ment over the free-flowing tvpe is ap- preciable. [129] Superheated steam Steam which is heated to 300° F de- livers only 4.4 per cent more B.t.u. per pound than does pressureless steam (Sec. 9). It has the advantage of being drier, but this is insufficient to justify the greater boiler cost. If the soil is at the right moisture content before treating, even free-flowing steam does not leave the soil too wet for planting. Superheat boilers are, furthermore, not used for heating glasshouses. EFFICIENT SOIL STEAMING To use steam efficiently, the rate of flow should be adjusted to the volume of soil treated, or the volume of soil should be altered to the steam flow available. The efficiency of the operation depends on the proper balancing of all the factors by the grower. Balanced steaming and the trickle finish There are both upper and lower limits to the flow rate for efficient use of steam (fig. 74 and Sec. 9). If the rate is too high for the volume of soil surrounding each steam input, the steam will escape or "blow out" from the surface (if the input is buried) or escape through open- ings (if released into a chamber contain- ing soil) before some of the soil reaches 212° F. Steam will then be wasted. If the flow rate is too low, the treatment period will be unduly prolonged, there will probably be excessive loss of heat from the soil surfaces before the mass is heated, and the soil may become too wet. "Balanced steaming" is achieved by adjusting soil volume and steam flow to the range of maximum efficiency (Sec. 9 ) . This range is affected by so many factors that no definite recommendation can be made. It can be readily found by trial for each set of conditions, since there is a fairly wide acceptable range of flow rates and input spacing distances. The rate should be such that the tempera- ture rise at any point is rapid, once it has started. The best flow rate approximates, but remains just below, that which per- mits steam to escape. When only a small quantity of soil remains at a temperature below 212° F there is an increasing tendency for steam to escape. It is, there- fore, desirable to reduce the steam flow rate at this time, the so-called "trickle finish." After the coldest corners have reached 212°, the steam need be kept on only enough to prevent the temperature from dropping below 180° during the next 30 minutes. Preventing "blow-out" Several additional precautions are helpful in preventing "blow-out" and ex- cessive steam loss. The soil surface should be level, and of uniform height above the steam out- lets, so that steam will not reach the sur- face at one point and escape, which would decrease penetration elsewhere. The soil should be well mixed and of uniform moisture and compaction. Those areas of beds which have been walked on, or which are wetter than surrounding soil, will heat more slowly than the rest. Steam will, therefore, escape at the other points before these are heated. The introduction into soil of steam under fairly high pressure may cause it to form a channel to the surface. In such cases, placing a pressure reducer in the line will be helpful. Depth and spacing of steam outlets The depth and spacing of steam out- lets in a stationary mass of soil is im- I L30 I portant for the most efficient treatment. They are determined in part by available steam flow. The lower the flow rate, the shallower should be the soil layer, and the closer the spacing of pipes and the holes in them. In a unit with a single-layer steam grid (Sec. 10, type 1) the pipes should be so placed that the distance upward to the soil surface is at least twice that to the bottom of the container. In many such units it is placed on the bottom. This is in accordance with the tendency of heat to rise. If the pipe is placed too close to the surface, the steam will escape be- fore most of the soil is heated. In a unit with a multilayer steam grid (Sec. 10, type 4) the distance between layers should be 1% times that of the depth of the top layer, and the distance from the lowest pipes to the bottom of the soil should be % the depth of the top layer. This is because steam moves into soil from the outlet in the shape of a spheroid, enlongated at the top, as shown in figure 71 and explained in Section 9. When the zones of condensation in the expanding spheres meet they do not over- lap, and the steam probably then flows toward the unheated corners. When all soil is heated to 212° F, the steam will diffuse out of the soil mass at every pos- sible point. The distance between steam outlets able. The examples given in Section 9 will provide a basis for estimating proper spacing of outlets for a given boiler capacity. Steam applied to soil surface When steam is applied to the sur- face of soil by either the Thomas or in- verted-pan method (Sec. 10, types 18 and 19) the dependable depth of steam pene- tration is about 8 inches. If this depth is exceeded, steam escapes and does not move efficiently through soil. This depth is adequate for most benches. It may be insufficient for ground beds to be used for deep-rooted plants; in such cases it is better to inject steam into the soil through buried perforated pipes, buried tiles, or moving rakes (Sec. 10, types 20, 22, and 23). Steam applied to soil in chambers When pressureless steam is released into a chamber surrounding soil in con- tainers (Sec. 10, types 4b, 5, 6, 7, 8, 10, 11, 12, and 13), it is important that the containers be separated by about % inch in each direction. A method for stacking flats is shown in figure 104 that permits unrestricted steam flow without use of separator strips. Many and between pipes should not be more nurseries prefer to place wooden sepa- than 25 per cent greater than the depth of the pipe (or of the top pipe in a multilayer grid). The pipes may be closer together than this, but they should not be so close that the soil bridges on them when it is dumped. Decreasing the steam flow rate and in- creasing the distance between steam out- lets have the same retarding effect on temperature rise in the soil mass (fig. 76 and Sec. 9). If a boiler of small output volume is used, therefore, efficiency will demand a closer spacing of pipes and orifices in the soil mass than will be necessary if a high-output boiler is avail- rator strips between horizontal layers of flats, and to leave a small crack between vertical piles. All chamber steamers have little escaping steam as long as there is cold soil for it to condense on. As ex- posed surfaces are heated, however, and steam must penetrate farther through soil before condensing, it escapes in- creasingly from the chamber. For this reason, it is inefficient to steam soil in very large containers (such as tubs or large planter boxes) by this method. There is satisfactory efficiency in flats up to 4 inches deep, since the steam then only penetrates 2 inches through soil. [131] Efficiency will be improved by reducing steam flow when excessive steam loss from the chamber indicates that the con- densation rate of the soil has been ex- ceeded. When steam is maintained under pres- sure in a chamber (Sec. 10, type 9) there is no problem of its escaping when the condensing power of the soil is ex- ceeded. Before beginning soil treatment, the air must be fully displaced from the autoclave by steam, in order to prevent cold pockets. If this is carefully done it will not entail excessive steam loss. VOLUME OF STEAM REQUIRED The quantity of steam produced by the boiler and delivered to the soil largely determines the quantity of soil that can be heated, and the time required to do it. It is, therefore, desirable to have a boiler and steam mains of such ca- pacity that the flow rate of steam does not seriously limit the operation. The in- formation supplied in table 14 and Sec- tion 9 makes it possible to estimate the size of boiler required for a given opera- tion. The services of a heating engineer will be helpful in calculating the capacity of the boiler required for a specific situa- tion, taking into account the efficiency of equipment used. If the operation is fairly simple, or if the adequacy of an existing boiler is in question, the capacity needed can be roughly determined from the volume of soil to be heated at one time. The follow- ing examples give approximate calcula- tions of steam requirements for various types of operations. More exact methods of calculation are presented later in this section. From table 14 in Section 9 it can be calculated that approximately 6.5 pounds of steam will be required per cubic foot of a U. C.-type soil mix to raise the temperature 150 degrees F in equipment of about 50 per cent efficiency. Such ef- ficienc) is attained or exceeded by equip- ment in which steam is injected into the stationary soil mass (sees. 9 and 10). Examples are the Rudd type (type 1 ). steam boxes (4a and 4b), buried per- forated pipe (20), and buried tile (22). The mobile bin (2) and combined bin and potting bench (3) may attain such efficiency if heat loss from the sides and bottom is not excessive. Thus, if a steam box containing 25 cubic feet of soil is to be heated from 62° to 212° F, about 163 pounds of steam (25 x 6.5 x ) will be required. A 150/ boiler delivering 200 pounds per hour will be ample if long pipe and hose con- nections are avoided, and if almost an hour can be devoted to the heating. If steaming needs to be done in a half hour, a boiler delivering 400 pounds might be required. Similarly it can be calculated that ap- proximately 10.8 pounds of steam will be required per cubic foot of mix in equipment of 30 per cent efficiency. Such efficiency levels may be reached or ex- ceeded with chamber-type equipment when steam is released into space sur- rounding containers of soil (sees. 9 and 10). Examples are the Thomas method (types 5 and 18), vault (6), multipur- pose tank (7), vertical cabinet (8), and inverted pan (19). Thus, if a bench of soil 3 x 10 feet x 8 inches deep is to be steamed by the Thomas method, and the soil tempera- ture must be raised 170 degrees F from 42°, about 245 pounds of steam will be 170\ required ( 3 x 10 x % x 10.8 x 1 50 boiler delivering 300 pounds of steam would be adequate to treat the soil in an [132] hour, and 600 pounds would be needed for a half-hour treatment. Similarly, a pile of 100 flats, each 18 x 18 x 3 inches, heated 140 degrees F from 72°, would require about 567 pounds of steam calculated as follows: fiy 2 x 1% x 14 x 100 x 10.8 x l^\ . A boiler delivering 600 pounds would heat these in an hour, and a 1,200-pound unit in 30 minutes. Among factors that the grower must consider in selecting a boiler and treat- ing equipment is the amount of soil that must be steamed per day. Soil can be treated in successive batches, but one should be certain that enough batches can be treated in a working day. If it takes an hour to bring the soil to 212° F, plus the required half-hour exposure, and an hour to cool, unload, and reload the unit, only 3 batches can be treated in an 8-hour day. In designing the steam- ing operation, the amount of soil per load and per day is the proper starting point in calculations. PREPARING SOIL FOR STEAMING Moisture content The moisture content of soil to be heated is very important in determining the efficiency of the operation. It requires about five times as much heat to raise the temperature of a pound of water 1 degree F as it does a pound of soil (Sec. 9) . On the other hand, heat plus moisture is much more effective in killing patho- gens than is heat alone. Seeds start to germinate if kept wet a few days prior to treatment, and even the more resistant weeds (Sec. 9) are then killed by treat- ment. These objectives are served if the soil to be treated is moist enough for plant- ing. After it is squeezed in the hand it should crumble freely. Dripping benches during steaming in- dicate that the soil was initially too wet, that the steam carried condensed water, or perhaps that the flow rate was insuf- ficient. If the soil is very wet when steamed it may come out in a soggy condition, be- cause the water held increases the B.t.u. requirement and more steam must be used, which adds more water from con- densation, in a vicious spiral. This will be aggravated if the excess water does not drain off readily, as in some ground beds or heavy soil where evaporation is not rapid. It is always advisable to bleed the water and moist steam from the pipe line and hose near the point of connection to the soil before treatment begins. If this is not done, water is initiallv discharged into the soil, causing increased B.t.u. re- quirement, uneven soil heating, and wet soil. For the same reason it is desirable to use a water trap in the steam line near the point of entry into the soil, so as to continuously drain off the condensate. This is particularly important if long mains are used. Freedom from lumps U. C.-type mixes use fine sand of a type which does not form hard clods (Sec. 6). Even these, however, need to be well mixed to avoid pockets of peat. If lumpy soil is used, it should be pulver- ized or screened before being steamed. This is because clods are not readily penetrated by steam I Sec. 9). Uneven packing of the soil in the container also makes for uneven heating. [133] UNEVEN HEATING If soil heats unevenly during treat- ment, one of the following factors may be the cause. Uneven compaction Areas of beds which have been walked or driven over will heat more slowly than the rest, owing to reduced porosity of the soil. The trouble is aggravated at high rates of steam flow. The presence of clods, just mentioned, also affects heat penetration. Soil in benches or beds should be well prepared and free of clods to eliminate areas of compaction. In boxes and bins one should not tamp the soil, but let it settle firmly into place. Uneven moisture Soil with uneven moisture distribution will heat unevenly, because of the greater specific heat of water than soil. Soil should never be watered after prepara- tion and before steaming. Proper prepa- ration will do much to make moisture satisfactorily uniform. Cold corners "Cold corners" in boxes or bins have been discussed above and diagrammed in figure 71. The problem may be elimi- nated by filling each corner with a tri- angular wood block. Excessive spacing of pipes If pipes in a grid are too far apart, relative to the steam flow rate, heating will be so retarded that unevenness may result, particularly if a clod or compact or wet area occurs at the cool spot. Uneven mixing If the peat or other organic material is not evenly mixed with the fine sand, uneven heating may occur. Pockets of dry peat are relatively impenetrable by heat, owing to their insulating properties. Insufficient steam If the flow rate is insufficient for the volume of soil treated, trouble with un- even heat is aggravated. See "Efficient Soil Steaming," above. Partially frozen soil Attempting to steam partially frozen soil leads to uneven heating. It should be thawed throughout, and mixed before steaming. Unexpelled air in autoclaves Uneven heating in autoclaves results when the air is not expelled before in- creasing the pressure. The air should be forced out of the exhaust valve or par- tially open door by incoming steam for several minutes, before closing the auto- clave and beginning the treatment. COOLING THE TREATED SOIL Our experience is that flats may drop to temperatures suitable for planting within 1 or 2 hours, the time depending upon exposure, air movement, and air temperature. The process of evaporation accelerates the cooling of the soil. In uncovered beds the temperature has been reported to drop from 212' F to 160° in 1 minutes at the surface, 2.6 hours at the 2-inch level, and 8.3 hours at the 7-inch level. A layer of canvas may extend the time of cooling at the surface to 1 1 minutes. Whenever treated soil is dumped and piled on the floor, the surface should previously have been wet down with a formaldehyde solution (1 gal. to 18 gal. water). I K?41 WATER CONTENT AFTER STEAMING Growers often ask whether steaming does not make the soil excessively wet to use. It may be calculated that, to raise the temperature of 1 cubic foot of soil with 15 per cent moisture 150 degrees F, would add 4.1 pounds of water, or 6.8 per cent, through condensation of steam (assuming 80 per cent efficiency in heat transfer). Senner (1934) showed that the moisture in soil after steaming in- creased by 2.3 to 8 per cent. Bunt (1954- 55) found an increase of 2.0 to 5.6 per cent, and Morris (1954a) reported 5 to 7 per cent for light and 7 to 12 per cent increase for heavy soil. Actually much of this added moisture is lost by evaporation as the soil cools. Thus, in our tests with U. C. mix B (25 per cent peat) the final soil moisture was increased by only 2.1, 3.6, and 3.9 per cent after cooling. Even if evaporation is reduced by stacking the flats or covering the soil mass, the moisture content of a U. C.-type mix has never been excessive after treatment. For this reason we rec- ommend that the soil to be steamed should be moist enough to plant prior to treatment. There is no consistent difference in the final moisture content of soil treated by free-flowing and pressure steam, and this is unchanged by rate of steam flow ex- cept at very low, inefficient levels, where it increases. Soil treated with super- heated steam will be slightly drier (Sec. 9). STEAM-TREATING HOME YARDS The fact that soil may be treated in proximity to plants suggests the pos- sibility of the use of steam in home yards against infestations of the oak-root fungus, aster-wilt Fusarium, water molds, and similar persistent soil fungi. A porta- ble steam boiler and a steam rake with pan (Sec. 10, type 21 1, could be used to insure depth of penetration. Such a practice would enable the progressive nurseryman to cope with the problem of existing infestation of soil in home yards, so that the healthy plants he sells will not be killed as previous ones have been when planted there. This investment in good will might also prove to be profitable. COST OF STEAMING SOIL Because of the variation between nurseries in many operating conditions that greatly influence the cost of soil steaming, no exact figure can be given for all situa- tions. A hypothetical conservative example is given in detail so that the grower may compare his own operations with it, and calculate his approximate cost. Heat requirements Assuming a soil with a specific heat of 0.2, 15 per cent moisture content, a weight of 60 lb. per cu. ft., and the temperature to be raised 150 degrees F (for example from 62° to 212° F), and water with a specific heat of 1.0: B.t.u. for 1 cu. ft. soil = 60 lb x 0.2 specific heat x 150° F = 1.800 B.t.u. for water = 60 lb. x 0.15 moisture x 1.0 specific heat x 150° F = 1.350 B.t.u. requirement per cu. ft. 3.150 [135] Assuming gas fuel of 1,100 B.t.u. per cu. ft. heat value and different levels of efficiency in the total heat exchange: At 100 per cent efficiency, the gas used per cu. 3150 ft. of soil is _ ■,-■.. — z— - = 2.86 cu. ft.; at 70 per cent it is 4.09 cu. ft.; at 50 per 1100 x 1.00 cent it is 5.73 cu. ft.; at 30 per cent it is 9.55 cu. ft. Cost of fuel Using the most expensive (winter) gas rates (G-40 schedule), and assuming a nursery of 60,000 flats per year, operating at 3 levels of efficiency in heat exchange: 60,000 flats == 1,000 cu. yd. = 27,000 cu. ft. of soil. At 70 per cent efficiency: 27,000 cu. ft. soil x 4.09 — 110,430 cu. ft. gas required. 100,000 cu. ft. at $0,564 per thousand rate = $56.40 10,430 cu. ft. at $0,514 per thousand rate = 5.36 Cost for 27,000 cu. ft. soil = $61.76 Cost per cu. ft. soil =$ 0.002237 Similarly, at 50 per cent efficiency: Cost per cu. ft. soil = $ 0.003130 Similarly, at 30 per cent efficiency : Cost per cu. ft. soil = $ 0.005094 Cost per cu. yd. soil = $ 0.13754 Thus, with gas, the cost of fuel for steaming soil ranges from about 0.23 cent per cubic foot at 70 per cent efficiency up to 0.51 cent at 30 per cent efficiency. Fuel oil, costing about 0.078 cent per 1,000 B.t.u., as against about 0.055 cent for natural gas, would make the cost about half again as much for fuel. Cost of equipment If a boiler is already used for heating glasshouses, this cost may be prorated and will be less than the figures quoted below. These calculations are based on the pur- chase of a small boiler, distribution lines, and soil-steaming equipment specifically for this operation. A 25-day working month is assumed, with 3 batches of soil per day. 27,000 cu. ft. per yr. = 2,250 cu. ft. per month 2,250 cu. ft. — ^r— j = 90 cu. ft. per day 2d days 90 cu. ft. TTi j — = 30 cu. ft. per batch, which must be heated in 1 hr. o batches This would require 324 lb. of steam per hr. at 30 per cent efficiency (30 cu. ft. x 10.8 lb. per cu. ft.; see "Volume of Steam Required"). Since efficiency has already been calculated, the boiler would need to be 324 lb. — — — -, or 9.4 horsepower. A 10 to 15 horsepower boiler that would produce around 34.5 lb. 400 lb. steam per hr. would thus be adequate for this job. [136] Equipment cost of $3,000, with a 10-year life, and a 6 per cent interest rate is assumed to provide the needed equipment: ... S3000 Principal, cost per year (depreciation) - = $300 10 yr. Interest at 6 per cent, average per year = 99 Equipment cost per year = $399 $399 Cost of equipment per cu. ft. - = $ 0.014778 H ^ F 27,000 Total cost Any additional labor introduced by the steaming operation is here omitted because of the extreme variability in various nurseries. Furthermore, most of the handling in a bedding-plant or gallon-can nursery will be necessary whether the soil is steamed or not. Cost of equipment per cu. ft = $0.014778 Cost of fuel (at 30 per cent efficiency) per cu. ft = $0.005094 Cost per cu. ft. of soil, exclusive of labor === $0.019872 It should be emphasized that these figures are very conservative, being based on only 30 per cent heat efficiency, on gas at the winter rates, and include cost of the boiler and equipment. The calculated cost of steaming is appreciably below that of methyl bromide or chloropicrin fumigation (table 13), even when no equipment cost is included for those treatments. Since the labor would be approximately the same for each of the three methods, it may be disregarded. It is clear that soil steam- ing is cheaper than fumigation, even when a boiler and equipment must be purchased. The argument of the initial cost of the boiler, usually cited as the reason for fumigat- ing rather than steaming, is shown to be economically unsound, even if money has to be borrowed to purchase the equipment. [137] SECTION Principles of Heat Treatment of Soil Kenneth F. Baker Chester N. Roistacher Temperature and time necessary for treatment Objectives and definitions Treatment of soil by heat Treatment of soil by hot water Treatment of soil by steam HE IMPORTANCE of the soil in plant culture has long been recognized by com- mercial growers and botanists. It actually supplies materials which make up about 85 per cent of the weight of green plants ( 80 per cent water, 2 per cent minerals, and 3 per cent as hydrogen and oxygen in carbohydrates), with only 15 per cent taken from the air (carbon and oxygen in carbohydrates). The justifiable early practical interest in soils and in "root action" quickly extended to include root parasites after 1850-1880, once it had been clearly established that plant dis- eases could be caused by microorgan- isms. Attempts were soon made to free the soil of organisms injurious to plants by treating it with heat or chemicals. Heat sterilization had been demon- strated to destroy fermentation organ- isms as early as 1776, and to destroy fungi in living plant tissue by 1883. \bout L890, aspetic surgery, involving heat sterilization of equipment, came to the fore. About the same time (for exam- ple, by B. Frank, 1888, in Germany) steam treatment of soil was experi- mentally used. W. N. Rudd (1893) in Mt. Greenwood, Illinois, commercially injected steam through perforated buried pipes in the bottom of a bin of soil to kill fungi, weeds, and insects, and similar methods were soon adopted by other growers. The Wutrick Brothers, Cleve- land, Ohio, are said to have used the steam-pan method about this same time. It is to be noted that florists and nursery- men were only a few years behind the medical profession in the adoption of steam sterilization. It has now become a standard procedure in glasshouse opera- tions the world over in order to reduce losses from diseases, weeds, and insects. This section presents the principles and data supporting the practices out- lined in Section 8 and the equipment I 1^8 3 described in Section 10 for the heat treatment of soil. This background in- formation will provide a better basis for present use of the suggestions outlined in the preceding section, and is neces- sary to the understanding of new de- velopments. For example, the factual basis for the use of steam-air mixtures being developed in England is necessary to understand and evaluate this method. TEMPERATURE AND TIME NECESSARY FOR TREATMENT Extensive studies by many workers in various parts of the world have demon- strated that exposure to moist heat at 150° F for 30 minutes will destroy the important plant pathogens, insects, and weeds (fig. 70) . Fungi . . . are relatively sensitive to heat. Rhizoc- tonia may be eradicated from living plant tissue by hot-water treatment at 125° F for 30 minutes (Sec. 13) ; the most rigorous treatments recommended have been 122° for 60 minutes. Water molds are even more sensitive, Pythium ultimum being killed in Aloe and Haw- orthia plants at 115° in 20 to 40 minutes (Sec. 13). The Botrytis gray mold is killed at 131° for 15 minutes. The gladiolus-yellows Fusarium is killed in cormels at 135° for 30 minutes. The cottony-rot Sclerotinia is destroyed at 122° for 5 minutes. Sclerotium rolfsii is killed in 30 minutes at 122° in caladium tubers and iris rhizomes (Sec. 13). Most other pathogenic fungi are also destroyed by time-temperature relations below 140° for 30 minutes. Bacteria Most bacteria that cause plant disease are killed at 140° F for 10 minutes, and probably all at 160°, since they do not form the heat-resistant spores of some animal pathogens and food-spoilage forms. The data in figure 68 also show that steaming at 212° for 30 minutes was as effective as 8 hours in destroying the spore-forming ammonifiers. Nematodes . . . are also quite susceptible to heat. The root-knot nematode is killed at 118° F in 10 minutes and is easily eradicated in living plants (Sec. 13). The most resistant foliar nematodes are killed at 120° for 15 minutes. The stem and bulb nematode is killed at 127° for 11 min- utes. The resistant, cyst-forming potato root nematode, not known in California, is killed at 118° for 15 minutes. The lesion nematodes are killed at 120° for 10 minutes. Insects and mites . . . are also susceptible to heat, even in the egg stage, and cannot long survive 140° to 160° F. Worms, slugs, centi- pedes, and similar animals are ap- parently destroyed by moist heat at 140° for 30 minutes. Weeds . . . for the most part, are destroyed at temperatures of 158° to 176° F for 15 minutes. In California nursery expe- rience, however, three weeds survive tem- peratures approaching 212°; these are the button weeds (Malva), bur clover {Medicago), and Lotus strigosus. In other areas, shepherd's purse {Cap sell a ) . Klamath weed (Hypericum), lambs quarter (Chenopodium) , wild oat (Avena), and some mustards are re- ported to be quite heat-tolerant. Expe- rience has indicated that seed of these plants is not numerous in soils of the type used in California. A fairly satis- factory index of effectiveness of treat- [139] ment used by California nurserymen is whether weeds other than the first three mentioned appear after steaming. Viruses . . . of nursery plants do not persist in soil, but some may survive in undecom- posed plant refuse for a time. Thus, the virus of chrysanthemum virus stunt will live over in dried infected tissue for at least 2 years, and will survive 200° F for 10 minutes. In spite of this, soil carryover of this virus is adequately eliminated in commercial operations by removal of most of the plant residue and decay of the rest, and by steaming. Tobacco mosaic, a similar virus with respect to carryover, has been intensively studied in the Department of Plant Pathology glasshouses at the University of California, Los Angeles, during the past 7 years; the above procedures have been so successful that there has been no soil carryover, despite the fact that 212° moist heat for 15 minutes is required to destroy the virus in dead stems. The majority of viruses are destroyed by tem- peratures of about 160° for 30 minutes and do not survive in the soil or refuse. The use of a U. C.-type soil mix also virtually precludes any virus carryover because no host plants of troublesome viruses occur in the source-areas of the ingredients. Experience has shown that growers following the U. C. system of soil mixes and soil treatment have no trouble with virus carryover. Recommendations Under ideal conditions, the organisms of concern to growers may be killed by heating to 140° F for 30 minutes, a fact championed by A. G. Newhall at Cornell University over the last two decades. Since; commercial operations do not sup- ply ideal conditions, a compromise with reality is necessary. For example, clods or lumps do not heat through as quickly as loose soil, and there may be "cold corners" in the equipment. To give a working margin of safety we have, for the past 16 years, recommended a mini- mum temperature of 180° for 30 min- utes. Before the development of the U. C.-type mix, there was some danger of soil post-steaming toxicity (Sec. 6), and it was therefore desirable to keep the temperature-time as low as possible. Now that this is no longer a factor, even higher temperatures are safe. The present recommendation is to heat a soil mix of the U. C. type to 212° F for 30 minutes, except that with equipment in which uniform heating can be stopped at 180° and held there for 30 minutes, it is safe to do so. The temperature of 212° is specified for steam because: (1) it is often the only possible final temperature for the process; (2) it is a temperature that can be easily controlled (soil tem- perature is not raised above that point by steam, except under superheating or pressure) ; (3) a U. C.-type mix develops no post-steaming toxicity for plants and may safely be heated to 212°; (4) the extra cost of heating soil from 180° to 212° is less than 2.9 cents per cubic yard 1 and therefore economically not important. With a U. C.-type mix it is better to overcook than to risk incom- plete heating. Other types of soils should, when possible, be heated to only 180°, until the grower finds that it is safe to heat to 212°. In continuous types of soil-steaming equipment it is possible to control final soil temperature to 180° F. In bulk types of equipment it is practicallly impossible to stop short of 212° if the soil is held in the unit for the full time. If the soil is dumped for "after-cooking", and is mixed in the dumping, a uniform tem- perature of 180° may develop. ' Based <»u L5 per cent soil moisture, 30 per ccul Ileal olVwionry, and natural gas of 1,100 B.t.u. per cu. ft., costing $0,537 per 1,000 cu. ft. [ 140] OBJECTIVES AND DEFINITIONS Heat treatment of soil is basically a problem of transfer of heat from a source, such as a boiler or heater, to the soil particles. The objectives are to heat the mass uniformly to 180° to 212° F, to retain this temperature for 30 minutes, and then to cool as rapidly as practicable. Heat transmission through soils, like biological phenomena of soils, is very complex, difficult to resolve experi- mentally, and therefore still imperfectly understood. Background information on the dynamics of heat flow through soil is given to enable more efficient planning and practical use of soil heat treatment. As far as we are aware, this is the first unified statement of the principles in- volved in the various methods of heat treatment of soil. Heat is that form of energy resulting from molecular motion, whose intensity is measured by the temperature rise of the receiving body, and quantity by the B.t.u. (British thermal unit) received by that body. The heat capacity of a sub- stance is the quantity (calories) of heat necessary to raise the temperature of 1 gram of it 1° C. This "heat storing" capacity of water is very important in the heat treatment of soils. Steam is the vapor phase of water, which releases heat as it recondenses to water. Heat and steam move through soil in very different ways, and must be clearly distinguished. Because equipment used for soil treat- ment employs either dry heaters or steam, the movement of both heat and steam through the soil is here considered. (For steam, see p. 146 through 161. ) TREATMENT OF SOIL BY HEAT Manner of Heat Distribution The distribution of heat from a hot to a cooler object is by conduction, con- vection, and radiation. The importance of each in soil is not fully clarified be- cause of experimental difficulties. Con- duction of heat is transmission through a solid, liquid, or gas by those molecules with greater energy transferring some of it, without any mass motion, to their neighbors of lower energy. The flow of heat through a metal bar is an example. Convection is the transfer of heat in a liquid or gas by movement from the hotter to the cooler area, as in the flow of heat from a floor furnace. Radiation is the transfer of heat through space from one body to another not in contact with it. Much of the heat from a fireplace is of this type. Conduction Heat conduction through a porous material is much less than through the same material without pores, because flow is greatly reduced by the contained air, a poor conductor. On the other hand, conduction is improved by the presence of water in the spaces. Thus, dry sand- stone conducts heat about 14 times better than dry sand, about 7 times better than water, and about 175 times better than air, at temperatures with which we are concerned. It follows that heat moves by conduction through each soil particle, and from soil particle to particle through their numerous points of contact. It also moves efficiently by conduction from particle to particle across the water film, and then has an enlarged area of contact. Although water improves the contact be- [141] tween particles and so increases heat transfer, if too much is added the heat capacity of the soil is so increased that there may be a decreased rate of tem- perature rise. Patten (1909) reached this point at about 18 per cent water con- tent (dry-weight basis) for sand, about 10 per cent for fine sandy loam and silt loam, and about 63 per cent for muck soil (25 per cent organic matter). This factor probably is not often important in heat treatment of nursery soil, since moisture is usually at lower levels. 2 Conduction through actual contacts and through water films is very im- portant in heat transfer through soil. Convection in the air spaces seems also to be important, though less clearly demonstrated. Generally considered to be less significant are radiation from par- ticle to particle through the pore air, con- duction through the pore air, and con- vection in the water film. Thermal conductivity through various types of mineral soil particles probably varies little. Porosity, and therefore par- ticle size and compaction is, however, strongly related to transmission. Thus, the percentage porosity in dry soil in- creases, and the heat transmission de- creases, in the following order: sand, loam, clay, peat. Soil with particles of several sizes tends to compact readily owing to wedging of small pieces be- tween large ones, which increases the points of contact and therefore the con- duction. Radiation across pores is also 2 In steaming soil, conduction is relatively much less important, since heat is released by condensation of steam directly on the soil par- ticles (see "Treatment of Soil by Steam," be- low). For this reason, in heating with steam there is probably no improvement comparable to that observed with dry heat, from the addi- tion of water to the soil. At all but low mois- ture levels, water actually tends to reduce Bteaming efficiency by partially plugging the pores and by increasing the total heat capacity. This situation illustrates the necessity of clearly distinguishing between steam and heat, already mentioned. increased, but convection is probably de- creased. Compaction generally improves move- ment of heat. This, and several other lines of evidence, indicate that conduc- tion and radiation are more important than convection in such movement. On the other hand, compaction reduces movement of steam and of chemical fumigants by diffusion through the pores. The addition of organic matter to soil reduces heat transmission by in- creasing porosity. Thus, Newhall (1940) found that immersion heaters raised the temperature of loam to 125.6° F in 3 hours and 147.2° in 4 hours, whereas muck reached only 98.6° and 123.8° F, respectively. Morris (19546) provided a comparison between the rate of heat transmission through soil by conduc- tion, convection, and radiation, as against steam flow. Insufficient heat pene- trated through 1 inch of undisturbed soil to raise its temperature to 160° F from a steam grid resting on it, although dur- ing the same period 15 inches of loose soil above the pipe was raised above that temperature. Steam flowed through the soil above and released its heat, whereas the temperature rise in the soil below was probably largely from heat trans- mission. Another aspect of conduction in the heating of soil is the distribution of heat by the metal container or cooker. Steel and iron are about 20 times better heat conductors than moist soil, 150 times better than dry soil, and 75 times better than water. Consequently, the soil in con- tact with the metal container will have heat indirectly transmitted to it. Metal liners are sometimes specified for treat- ment equipment using immersion heat- ers in order to take advantage of this dis- tribution effect. Steaming of soil in gal- lon cans involves very complex heat ex- changes, since conduction by the can to the soil, and steam flow into the soil sur- face both occur. [142] Convection Movement of heat through soil by con- vection is directly related to pore size. It is particularly effective in highly porous soil, and probably is important in heat transfer through a U. C.-type soil mix. Bouyoucos (1913, 1915) has em- phasized the importance of air convec- tion currents in the transfer of heat through soil. The magnitude of such movement through the pore system is shown by the normal exchange of soil carbon dioxide and air with the atmos- phere. It has been computed that the air in some soils is completely renewed to a depth of about 8 to 12 inches every hour, and that this is largely by gas diffusion through the pores ( Baver, 1956). The rapid movement of methyl bromide and other gases through soil pores during treatment is further indirect evidence for the importance of pores and convection currents in heat transfer. Although the percentage porosity of a soil is inversely related to transmission of heat by conduction and radiation, it is misleading for estimating permeability to gases. The number and size of the large pores are of great importance here, although much of the pore space is not significantly involved in convection or diffusion. Thus, coarse sand ( 37.9 per cent porosity) is about 1,000 times more permeable to air than is fine sand (55.5 per cent porosity) (Baver, 1956). Simi- larly, a granular loam was 50 to 100 times more permeable than it was in the powdered state. Only a small amount of clay is neces- sary in a sandy soil to reduce the pore diameters and greatly reduce permea- bility. Buehrer (1932) found that addi- tion of 10 per cent clay to coarse sand decreased air flow to about one fourth, 20 per cent clay to about one tenth, and 30 per cent clay to about one twentieth. Clay soils may have 50 per cent porositv and still be poorly aerated, whereas sand with 30 per cent porosity may be well aerated. He found that only part of the spaces are involved in gas movement, a considerable part of them being blind alleys or so small as to greatly restrict flow. Only in soil with coarse particles does air flow reach levels to be expected on the basis of percentage porosity. It was concluded that only the larger and continuous pore systems were significant in air passage through soil. These would be best provided by coarse or granular soils with large bits of organic matter. It would appear that conduction of heat would decrease in the order sand, loam, clay, peat, but that convection and diffusion would be greater in peat and sand than in clay and loam. When added to a soil, water has the effect of reducing pore size and permeability to air, de- creasing convection and increasing con- duction. This may contribute to the slow heating of very wet soil by steam. Gases increase in viscosity as the tem- perature rises. Thus, the time in minutes to pull equal amounts of air through soil columns of moist sand, sandv loam, clay, and peat, respectively, was found by Bouyoucos (1915) to be as follows: 50° F = 1.50, 2.00, 15.00, and 16.40; 86° = 2.12, 3.37, 26.00. and 20.40; 122° = 2.50, 10.35, 33.00, and 38.40. The viscosity of steam likewise in- creases from 125.5 at 212° to 144.5 at 302° (54 lb. pressure), but since it moves through soil at 212° F or less, this is not a factor. This may contribute to the greater efficiency of heat distribution through soil by steam than by dry hot air. Air convection would decrease as the soil became hotter, and thus decrease the effectiveness of the dry type of heat transfer. Since the rate of heat inter- change between a gas and porous object through which it flows is proportional to the temperature difference between them, this also causes the rate of tem- perature increase to fall off as the soil gets hotter. On the other hand, water convection would increase with rising [143] temperature, and thus aid heat transfer in moist soils. Radiation Radiation of heat from the surface of a soil particle occurs in a straight line across the air space to another particle. It is probably not very important in heat transmission in soil, but has not been evaluated. The efficiency decreases as the pore size increases, and varies with the nature of the exposed particle surface. The radiation from the surface of the particle is improved if it is moist, but does not attain the efficiency of water itself. It also improves with increasing temperature. Tests with soils As the previous discussion has in- dicated, the three methods of heat trans- fer are not always affected in the same direction by a given soil condition. The conduction of heat by soil im- proves as the percentage porosity and pore size decrease, and as water content increases. Convection is favored by many large pores, but decreases in wet soil and with increasing temperature. Radiation is favored by small pores, wet soil, and increasing temperature. It is thus ap- parent that only by trial can the actual heat transmission of a given soil be de- termined. The results of such tests may be instructive. Von Schwarz (1879) placed soils of 61.7° F in contact with a heat source (140°) and noted the temperature reached in 15 minutes. Temperatures reached by dry soils (increasing order of transmission) were: peat 65.7°, clay 82.4°, loam 90.0°, sand 95.5°; for moist soils these were: peat 69.6°, clay 102.6°, loam 120.7°, sand 133.2°. Bouyoucos (1913) measured the time required for heat to pass from a source (92.3° F) 7 inches through a column of soil and cause the temperature to start rising. For air-dry soil the figures (in- creasing order of transmission) were: peat 55.2, loam 49.7, clay 44.2, and sand 38.0 minutes. Similar tests with soils in the field gave the following times (in- creasing order of transmission) at 6 inches from the heat source: peat (148.6 per cent water) 9 hours, loam (36.6 per cent water) 6.5 hours, clay (25.9 per cent water) 6 hours, sand (3.6 per cent water) 4 hours. In another test the time at which temperature began to rise at different distances in similar cores of dry quartz sand and of moist loam, respec- tively, were: 1 inch, 2 and 1 minutes; 3 inches, 15 and 16; 4 inches, 27 and 26; 5 inches, 34 and 35; 7 inches, 40 and 44. From these and other data it would appear that transmission of heat through dry soil is best for sand, poorest for peat, and intermediate for clay and loam soils. Transmission of heat is improved in all of these soils by the addition of water, but the general order of transmission is unchanged. It is probable, therefore, that most of the transmission of heat is by conduction, but that convection is also important. Rate of Heat Distribution Dry soil The rate of distribution of heat through dry soil is as important as the manner of transmission just discussed. When dry soil is exposed at some point to a constant heat source such as an im- mersion heater, the temperature of the soil directly exposed to the heat rises fairly quickly. At a distance of 2 inches, the temperature will begin to rise some- what later, will rise more slowly, and does not rise as high as at the source. At 4 inches it will rise later, slower, and to a lower maximum than at 2 inches, and so on. At any given distance there is more heat flowing into the soil on the "hot" side than is flowing out from the cool side. This differential results from the insulating effect of the dry soil, the heat absorbed in warming the given spot, and from lateral transmission of I 144] heat to surrounding particles or to the atmosphere. The lateral flow is im- portant because, as the sphere of heat en- larges, the B.t.u. are being diluted into an ever greater volume of soil (see "Movement of Steam through Soil," be- low) . Wet soil When the soil is wet, the advance of heat is much the same, except that water is evaporated near the source of heat and condenses on the cool son" farther out. This evaporation somewhat lowers the temperature near the heater, and in ad- dition the dry soil thus produced con- ducts heat less efficiently than when wet. Since the heat capacity of wet soil is higher than dry, the temperature should rise more slowly. These factors tend to make the temperature of wet soil rise more slowly than dry, at any given time or distance near the heater. They are offset, however, by the excellent heat transfer by the steam formed, and by the improved heat conduction of the soil at a distance, so that the total effect of limited additional moisture may be to slightly increase the rate of heating. For example, we found that an electric heater that produced 120 B.t.u. per hour from P/i inches of the tip exposed to a dry sandy loam (0.88 per cent water) re- quired 45 minutes to raise the tempera- ture 32 degrees F at a 2-inch distance. The same soil moistened to 7.11 per cent water required only 39 minutes for this temperature increase. Correcting for the heat capacity of the water, the times would be 43.1 and 28.8 minutes, respec- tively. This again illustrates that conduc- tion is the principal means of heat trans- mission in soil. When heat is applied to moist soil and the water at a given point has been evaporated, the temperature rise follows that described for dry soil. Thus, heat applied to a moist soil is intermediate between dry heat and steam, more nearly approximating the former. The steady and the unsteady states The above conditions apply when heat is advancing through dry or wet soil — the so-called unsteady state. If the heat has been constantly applied for a suf- ficiently long time, and the surrounding conditions are also constant, each point in the soil remains at a given tempera- ture — the so-called steady state (fig. 72) . At that time the temperature decreases uniformly with distance from the source. Temperature Steady state Distance from heat source Fig. 72. Temperature gradients with distance from a heat source, in the advancing (unsteady) and steady states. (Based on Patten, 1909.) [145] For a given heat input and surrounding conditions affecting temperature loss, there is a distance beyond which there is insufficient heat transmitted for the soil to reach 180° or 212° F. In practical heat treatment of soil the steady state is almost never reached, and the heat dis- tribution through the soil is improved by increasing the number of heat sources per volume of soil. One of the worst disadvantages of a dry source of heat in soil treatment is that intensity (temperature) is high, quantity (B.t.u.) is small, and distribu- tion is poor. Steam, by contrast, imparts a large quantity of heat at low intensity (212° F) and flows through the soil to the cold areas. One of the principal ad- vantages of steam is that the B.t.u. are released at the point to be heated. TREATMENT OF SOIL BY HOT WATER Heat distribution by hot water in- volves different factors than those out- lined above. The water is applied to the surface and flows by gravity through the soil pores, heat being transferred to the soil particles by conduction. The first water displaces the air and fills the pores, additional hot water pushes the cooled water downward. The temperature de- creases from top to bottom, and the low- est level approximates the temperature of the draining water. There is some lateral spread, but not enough to heat very far. Because even boiling water has only 212 B.t.u. per pound, minus the existing soil temperature, the tempera- ture rise is very slow and a great deal of water is required. Only 142 B.t.u. are re- leased, for example, in cooling 1 pound of water from 212° to 70° F, whereas 1,112 B.t.u. are released in similarly cooling 1 pound of steam. Thus, with soil at 70°, 7.8 times more moisture must be added with boiling water than with steam. Because of the effectiveness of hot water in leaching salts from soil, flood- ing the propagating bench with it before use is an excellent practice. If the messi- ness of flooding with sufficient water to raise the temperature to 180° F makes heating in this way impracticable, the leaching may well be followed with a steam treatment to free the soil of patho- gens. TREATMENT OF SOIL BY STEAM Condensation of Steam in Soil As the steam moves into the pores of a soil it mingles with the air held there, the ratio of steam to air rising with in- creased time. The condensation of such water vapor is determined by the tem- perature difn rential between the vapor and the soil particles, and by the ratio of steam to air. as pointed out by Hoare (1953), Morris (1954a), and Bunt (1955). The dew point is the highest tempera- ture at which the quantity of water vapor in the air is sufficient to saturate it and cause condensation. The lower the con- centration of water vapor (relative hu- midity), the lower the temperature at which condensation occurs. For ex- ample, at 70° F all water vapor in excess of 1 pound to each 63.3 pounds of dry air will condense (fig. 73); at 100° the critical ratio is 1:23.2; at 130°, 1:9.0; at 190°, 1:0.9; at 211°, 1:0.03; and at | I 10 Lbs. dry air Fig. 73. Maximum pounds of dry air per pound of water vapor at which condensation occurs at various temperatures. The ratio of steam to air at a given temperature may be determined by reading down from the intersection of the temperature and the condensation lines. Thus, the ratio for 90°F is 1:32. (Calculated from data of Zimmerman and Lavine, 1945.) 212°, 1:0 (that is, pure steam). This is true whether it refers to condensation of humidity in the glasshouse or of steam in a bench of soil. These facts strongly affect the manner in which steam heats soil. Relation of steam/air ratio to condensation When steam is released into soil of 70° F, it expands if it has been under pres- sure in the pipe, and drops to approxi- mately atmospheric pressure and soon to 212°. It condenses quickly on the cool particles at its point of entry be- cause of the low steam/air ratio (1 :63.3 l at that temperature. As the temperature of the soil and soil air rises from the heat released by condensation, an ever richer mixture of steam and air must be reached if condensation is to continue (for example, at 90°, this is 1:32.1). On a given soil particle steam would con- tinue to condense until the temperature became too high for the existing steam/ air ratio. Since, however, steam is con- tinuously released, and some air is pushed ahead of the incoming steam, the ratio rises steadilv until all air is dis- placed. Condensation is, therefore, con- tinuous until 212° is reached. The steam then flows on to the cooler advancing zone. Incidentally, this principle explains why an autoclave type of soil cooker op- erating at 15 pounds' pressure must have air displaced by steam (through operat- ing with the exhaust or door open for a time) before the temperature can reach the expected 249.8° F. [147] Treating soil with steam-air mixtures From the above discussion on steam: air ratio it is apparent that the steam temperature may be reduced by injecting and mixing air into the line near the out- put. Recent investigations by Morris (1954a) and Bunt (1955) in England have ultilized this principle to heat soil with steam to a final temperature below 212° F. By employing a venturi tube in a steam line carrying 40 to 50 pounds' pressure, enough air has been drawn in to reduce the steam temperature to 160°. If low-pressure steam is used, however, a pump is necessary to inject air into the flowing steam. When 180° steam is in- jected into soil, the steam condenses on the particles as before. The heated air is pushed ahead, forming a band 3 to 4 inches wide, the temperature of which remains between that of the heated and unheated soil for 2 to 4 minutes. The zone of condensation of 212° steam in soil may be 1 inch wide or less (see be- low) and remain for only 10 to 20 seconds. Savings of up to 15 per cent in fuel have been reported. As already men- tioned in this section, the calculated sav- ing from heating soil to 180° instead of 212° is less than 2.9 cents per cubic yard. The method is not yet ready for com- mercial application. When it is available it can be attached to equipment types 1 to 8, 18 to 23, and 26 to 28 (Sec. 10). Its application to equipment with a mov- ing soil mass, or to pressure chambers, is more doubtful. Treating soil with a steam-air- chemical mixture The combination of a volatile fungi- ride with a low temperature steam-air mixture instead of with 212° F steam (Sec. 10, type 26) would appear to offer possibilities for effective cheap treat- ment of soil. Since the velocity of chemi- cal reactions increases two to three fold CAUTION: Many of the chemicals mentioned in this manual are poi- sonous and may be harmful. The user should carefu lly Follow the pre- cautions on the 1 abe Is of the con- tainers. with each 18 degrees F rise in tempera- ture, it is evident why the combination of heat and chemicals is substantially cheaper than steam alone. Beachley (1937) found that the combination of formaldehyde with 212° steam reduced by one third the time necessary for in- verted pan treatment with steam, and the cost by one fifth; it was one third cheaper than a formaldehyde drench. Soil conditions affecting steam penetration The air in the labyrinth of pores is ex- pelled at a rate proportional to the volume of steam injected; the rates are not equal because of the volume reduc- tion from condensation of the steam. When large volumes of steam are used, the air is displaced so rapidly by the mass flow that it may not be appreciably heated. With the usual smaller steam quantity, the air is also heated as it is pushed ahead, and transfers its heat in turn to cooler soil particles. Since the heat capacity of air is about half that of steam (see Appendix), the quantity of heat transferred in this way by air is not large. It may contribute, however, to the gradual temperature rise at a given point in soil heated by inefficiently small volumes of steam (figs. 75 and 76) . Cer- tainly this air movement would reinforce convection and diffusion in the continu- ous soil pores. When air is injected into the steam (see above) the importance of heat transmission by the air is relatively greater than when steam is used alone. As already indicated, much of the soil pore space is not involved in movement of gases, there being many pockets and plugged or partially blocked channels. [148] The steam flows freely along the open channels, condensing on the adjacent particles. The heat thus released is trans- mitted to the surrounding particles by conduction, radiation, and convection within the sealed pores (see "Treatment of Soil by Heat — Manner of Heat Dis- tribution," above). A clod of soil may be surrounded and by-passed by steam before it has been heated to 212° F. In a soil consisting of uniform lumps, the thickness of the layer being heated at any moment depends on the rate of heat supply and absorption. If both the lumps and the steam volume are large, the vapor will be lost through the surface during much of the opera- tion. The same might apply to soil con- taining many large pieces of organic matter, or having cracks either in it or between it and the container. In such cases the width of the zone of condensa- tion is determined by the depth of soil treated (see also "Soil structure," be- low). Steam penetration of dry soil may tend to be slower than for moist soil, owing to the greater tendency to com- paction and smaller pores of the former, as well as reduced conductivity from low moisture content. Water apparently af- fects the flow of gases through soil largely by its effect on pore size and soil structure. As mentioned earlier, when soil is too wet, the high heat capacity of water re- tards heating. Because of this, by the time the water is heated to 212° F, so much steam will have condensed that the water-holding capacity of the soil will be exceeded and water will drain from the bottom of the bench (Sec. 8). This may produce a broad temperature gradient in the lower soil levels. Movement of Steam through Soil Steam moves through the air spaces of the soil in the same way as any other gas (see "Manner of Heat Distribution — Convection," above). This certainly involves diffusion, and probably also eddy currents, through the continuous pore system of the soil. Several facts in- dicate that steam does move through soil in this manner. It moves very slowly through compact soil (for example, into clods and subsoil — see "Soil structure," below). It moves more rapidly through a soil to which organic matter is added, or which is lumpy. Thus, we found that at 5.54 per cent moisture and with a single steam input of 3.5 pounds per hour, a sandy loam required 33 minutes for the temperature to rise 142 degrees F at a distance of 5 inches, whereas U. C. mix C (50 per cent peat) required but 18.5 minutes. Within limits, the larger and more numerous the pores, and the more continuous the system they form, the better the penetration of steam. The pores, however, must restrict the flow enough that steam does not "blow out" through the surface before condensing. When all of the soil has been heated to 212° the steam does not condense, but seeps out of the exposed surface; this is visible evidence of the ready flow of steam through soil. Pressure flow is probably not involved in such move- ment, as steam has little or no more than atmospheric pressure in the soil. With a small flow of steam The way in which steam moves through soil, and the rate of temperature rise produced by it, are both strongly affected by the volume of steam used, and the distance from the point of in- jection (see below, and figs. 74 and 76) . When a small flow of steam is used, the mixture of steam and air in the soil pores at a given point becomes progressively richer with increasing time and with proximity to the input, as already ex- plained. The pores of the soil around the input are soon filled with pure steam, and the temperature reaches 212° F. At the moment steam was injected into soil, the temperature in one of our tests was 188.4° (a steam air ratio of 1:1) 2 [ 149] Wide o E o d) Q) c o to N D) «-»- c o t/5 _c c TD ~o £ c o Narrow Efficiency Low High Low Steam Flow Rate Ratio of Steam to Air High Low Fig. 74. Diagram showing the relations of steam flow rate to the steam: air ratio, width of zone of condensation in soil, and efficiency. See p. 152 through 154 for explanation. inches away, at 2% inches it was 148.2° (a ratio of 1:5), at 3% inches 105.4° (1:20), at 4 inches 76.8° (1:50), and at 5 inches 71.5° (1 :60) . Ten minutes later the same relations would still exist, but at greater distances from the input. Ap- proximately the same situation exists for higher steam injection rates at points in the soil distant from the input (fig. 76). That the ratio of steam to air is the principal factor involved in the wide condensation zone at low steam flow rates is shown by data of the type graphed in figure 75. In these tests with U. C. mix C (50 per cent peat) we meas- ured the temperatures at several dis- tances from the steam input, and im- mediately collected a large soil sample from around each thermocouple to de- termine, by oven-drying, the condensed steam they contained. The temperature curve and the percentage of moisture both decrease with distance from the steam input. The faster temperature rise than would be expected from the amount of condensed steam may have been partly due to experimental limitations. The suggestion is strong, however, that there was some conduction of heat in ad- dition to t lu- steam flow. It is largely to preserve efficiency, by reducing the dis- tance of steam travel, that perforated pipes and steam rakes are placed at in- tervals in a stationary soil mass in most types of equipment (see below and Sec. 10), or that soil is broken into small volumes by being placed in flats or pots. It is apparent from these facts that there is a wide zone (several inches) of condensing steam in the soil under con- ditions of reduced steam flow, both close to and distant from the input. The width of this zone in a given soil at a certain distance from the steam input narrows as the steam flow rate is increased. The importance of heat transfer by conduction, convection, and radiation after condensation, in the case of steam with a small flow rate, is not known. It would be expected, however, that they would become more important as the in- put flow reached very low levels. When a point is reached at which the heat lost from exposed soil surfaces balances that introduced by the steam, no further tem- perature rise will occur (the steady state) . Transmission through soil of the heat released by a small flow of steam would fall off rapidly with distance. This is be- cause of the B.t.u. required to heat a given point of soil, and because of 1 1 ™ ] lateral transfer of heat to surrounding cool particles. Because of the increasing volume of the spheroid of steam (see below) , the number of cool particles and the number of the plugged pores not penetrated by steam increases in each successive layer. Thus, there is the double effect of the increasing volume of soil for condensing steam, and the de- creasing rate of transmission of heat with distance. These two effects are well illustrated by MacLean's (1930) data on the heat penetration of green pine logs in a steam autoclave. In his tests, heat but not steam penetrated the timbers. The tem- perature at a given depth from the sur- face decreased as the log size increased, although other conditions were uniform; the increasing volume thus presented corresponds to the expanding spheroid of soil discussed below. The temperature also decreased progressively toward the center of each log, despite the fact that the volume became progressively less. Thus the temperature decreased whether volume increased or decreased. This in- dicates that the decreasing transfer of heat with distance through a solid is more important in causing the lowering 12 3 4 5 Distance (in.) from steam input Fig. 75. Relation of soil temperature and percentage moisture (condensed steam) with increas- ing distance from the steam input. U. C.-type soil mix C, moisture-free at beginning of test, injected with steam of 0.6 to 1.1 lb. per hr. for 12 to 20 min., when data were taken. Average of three series. The temperature of the soil (broken line) and the per cent moisture (solid line) it contains rise together. Both also decrease with distance from the steam input. This is because the amount of air in the steam increases with distance from the input. [151] temperatures than is the increase of volume from the enlarging sphere of heat penetration. From this considera- tion, the rate of temperature rise in soil from transmitted heat may be described as decreasing rapidly with distance be- cause of resistance to heat flow, and this decrease is reinforced by the enlarging volume of the spheroid. There is a further tendency for steam of small flow rate to contain more en- trained water than that of high rates, because of greater relative condensation in the lines. Low input rates often tend because of this to diminish the pore size in the soil and to further restrict steam movement, particularly when the perfo- rated pipes are on the bottom of a tight bin of soil with poor drainage. With intermediate and high flow rates Morris (1954a, 19546) has carefully studied the movement through soil of steam at intermediate and high flow rates. It moves rapidly through porous or lumpy soil or when a large volume of steam is used, and more slowly in uni- formly fine soil or with lower steam volume. There is a maximum rate at which a given soil can condense steam. Beyond this rate the condensation front becomes wider as the steam rushes by particles without condensing, the steam "blows out" through the surface, and ef- ficiency is very low. At a somewhat lower range the steam condenses in a narrow zone whose thickness decreases with the steam quantity. Morris and Winspear (1957) were able, by using a quick-acting thermocouple recorder, to demonstrate this advancing condensa- tion zone in soil. In one test, a point 2 1 /i> inches above the steam source increased from 38.7° F at 2.12 minutes to 212° 0.75 minute later, a point 7% inches above the source increased from 38.3° at 9.25 minutes to 212° 1 minute later, and a poinl L'3 inches above increased from 39.0° at 17.38 minutes to 212° 0.5 minute later. Thus, the 212° front moved 10% inches in 15.01 minutes, or about 0.7 inch per minute. The front passed the three points in 0.5, 0.75, and 0.25 minute, respectively (an average of 0.5 minute), and from this the thickness of the front was computed to be only 0.35 inch. They concluded that, for efficient use of steam, the thickness of this zone should be 1 inch or less, and that the total surface area of the soil particles in the zone must be sufficient to condense the steam supplied. In this range the ef- ficiency is high. It is apparent from the above facts that at very low steam flow rates the zone of condensation in soil is broad, there is much mixing of the soil air with steam, and efficiency is low. As the flow rate increases, the condensation zone narrows, the amount of air mixed with the steam lessens, and efficiency rises. Finally a flow volume is reached at which the condensation zone is at its narrowest, there is very little mixing of air with the steam, and efficiency is at its highest. An opposing factor begins to operate at about this point: as the maximum con- densing power of the given soil is reached, there is an increasing tendency for steam to "blow out" of the soil sur- face. Once the condensing power of the soil is exceeded, the efficiency falls rapidly, because the mass flow of steam rushes by the particles without condens- ing. In this situation there is insignificant mixing of air with the steam, the air be- ing pushed out ahead of the steam, and the condensation front again widens. Thus, as the steam flow rate increases from very low to high the width of the condensation zone passes from very wide to narrow, and again to very wide, as different factors come into play. This is schematically shown in figure 74. Be- cause of the multiplicity of factors in- volved, it is not possible to define the limits of each of these levels of steam efficiency accurately. It is obvious, how- ever, that there is a lower, as well as an [152] upper limit for efficient use of steam. Best commercial practice is to use a steam flow just below the rate that gives surface "blow out." Expanding spheroids of steam Morris (19546) investigated the proper spacing of steam outlets in soil. "It is a safe rule .... that the sterilising effect can reach to 1V2 times the depth of the pipes .... and the steam should be injected at not less than % of the total depth .... The space between the hori- zontal pipes should not exceed the depth of the holes by more than 25% and the spacing .... along the pipe should be about equal to the hole depth." From these specifications, the findings of Bunt (1954-55), and our observa- tions, it is concluded that steam moves out from an orifice into soil as a sphe- roid with an elongated top, the margins laterally and downward being approxi- mately half that of the upward limit. This is in accordance with the tendency of heat to rise. The lateral movement may exceed the downward flow by as much as one fourth. This is the status during the advancing state. When two expanding spheroids overlap, the steam probably flows toward the unheated corners, because of the pressure reduc- tion there caused by condensation of the steam, and the fact that all of the pore space at the point of overlapping would already be filled with steam. When the soil mass including all of the corners and spaces between spheroids is heated, each injection point will have heated a rectangular volume whose dimensions will be in the approximate ratio: dis- tance from outlet to soil surface (or lower limit of next rectangle above, in a multilayer pipe grid) = 1.0; distance from outlet to bottom (or to upper limit of next rectangle below) = 0.5; distance to lateral sides (or to lateral limit of ad- jacent rectangles) = 0.5 to 0.625. This relation is shown diagrammatically in figure 71. The farther out the steam flows from the orifice the greater is the volume of soil into which it passes. The approxi- mate volume of the spheroid when the steam has advanced 1 inch horizontally from the input is 6 cubic inches, at 2 inches it is 50, at 3 inches 170, at 4 inches 402, at 5 inches 785, and at 6 inches 1,357 cubic inches. Because of this sharp increase in the volume of soil into which the steam passes in its out- ward flow, as well as the factors already mentioned, the rate of extension of the spheroid of steam rapidly falls off with distance from the source. It would thus take about 215 times as long to heat a 6-inch as it would a 1-inch spheroid of soil. Increasing the distance from a fixed steam flow has the same retarding effect on temperature rise as does decreasing the rate of steam flow at a fixed distance (fig. 76), and for the same reasons. Thus, in our tests a U. C. soil mix C (50 per cent peat) with 5.54 per cent mois- ture reached 212° F in 8 minutes at a point 6 inches from the input of 6.7 pounds of steam per hour, but only 3 inches from the input of 2.18 pounds per hour. Distance and steam volume are to this extent mutually compensating. Efficient Rates of Steam Flow Steam may be efficiently used over a considerable range of flow rates and penetration distances into soil. Neither the steam volume nor the lateral distance should be so great that steam escapes from the surface before the mass is heated. At the other extreme, the steam volume should not be so small, nor the distance between outlets so great, that the heat losses from the surface and sides offset the input, and necessitate extended steaming periods. In other words, the condensation zones should be neither too wide nor too narrow, ap- proximating 1 inch or slightlv less. Efficient steaming thus requires that the [153] °F 210" 200- 190- 180- 170- 160- 150- 140- 130- 120- 110- 100- 90- 80^ 70 U. C. Mix 6.7 lbs. per hour 2 in. 3 in. A . 4 in. 5 in , . 6 in 1 1 1 1 1 U. C. Mix 1.45 lbs. per hour '5 in. / 6 in. 10 20 30 40 50 60 Time (min.) U. C. Mix 3.5 lbs. per hour / / k 1 — i — i — i — i — i U. C. Mix 0.93 lb. per hour U.C Mix 2.18 lbs. per hour /6 Sandy loam 3.5 lbs. per hour 10 20 30 40 50 60 Time (min.) —1 1 10 20 30 40 50 60 Time (min.) Fig. 76. Temperature gradients of U. C.-type soil mix C (5.54 per cent water content) at five distances from the steam input and at five steam flow rates. A chart for one comparable series with sandy loam illustrates the effect of organic matter on steam penetration. See p. 153 and 155 for explanation. [154] temperature rise at any point should be rapid, once it has started (fig. 74). In tests at Los Angeles, U. C. soil mix C (50 per cent peat) with 5.54 per cent moisture was injected at a single point with varying quantities of steam. The times for the temperature to start to rise from 70° F, and to reach 212° there- after, respectively, 5 inches from the in- put were as follows: 6.7 pounds steam per hour, 3 and 4 minutes ; 3.5 pounds, 10.5 and 6.5 minutes; 2.18 pounds, 10.9 and 21.7 minutes; 1.45 pounds, 22.5 and 52.5 minutes. At the indicated rates of flow for the total time to heat each 5-inch spheroid, the B.t.u. required would be 754, 962, 1,149, and 1,758, respectively. This series shows an increasing efficiency with increasing steam flow into the spheroid. Probably heat was transmitted to the surrounding soil by conduction, convec- tion, and radiation, and this played an increasing role as steam flow was de- creased. Since the upper limit of the condensing capacity of the soil was not exceeded, there was no falling off at the higher volumes. Bunt (1954-55) also found, for soil in bins, a decrease in the amount of steam per cubic foot of soil, and in time, to reach 212°, as the steam flow was in- creased. When a given volume of soil was treated in 34 minutes, 7.35 pounds of steam per cubic foot was required; when the flow was increased so that only 8 minutes were required. 5.40 pounds was used. He attributed the inefficiency at low flow rates to heat losses from the soil surfaces. In our test the lower limit of practical efficiency probably was the 2.18 pounds per hour flow per orifice. At the 1.45- pound rate, the temperature rise was so slow (52.5 min.) that heat transmission by conduction, convection, and radiation probably came into play. One of the principal advantages of steam (the re- lease of B.t.u. at the point to be heated) was, therefore, diminished. Bunt (1954-55) found, on the other hand, that thermal efficiency was greater with moderate rather than with large steam flow rates for soil in ground beds. This is because there is less opportunity for heat loss from exposed surfaces than there is in benches or bins. Steam "blow-out" The upper limit of steam flow rate for a given soil is recognized by the ten- dency to "blow out" from the surface before most of the soil is heated to 212° F. Obviously this rate should not be exceeded for efficient operation. It would appear, furthermore, that the flow rate should not fall far below this level for maximum efficiency. The time for the temperature to rise to 212° F several inches from the steam input is a useful measure of this range. In other words, the steam flow should utilize fully, but not exceed, the condensation capacity of the given soil. Since this "balanced steaming" is determined by so many factors, it is best found by trial for the given soil and conditions. The tendency for steam to "blow out" before the soil is treated may be mini- mized by: (1) reducing the rate of steam flow; (2) reducing the steam pressure at the point of injection into soil; (3) hav- ing the soil surface level, and of uniform height above the steam outlets, so that steam will not reach the surface at one point and escape, decreasing penetration at other points; (4) having soil well worked and of uniform moisture and compaction. In any case, efficiency is in- creased by reducing the steam floiv to a low level when 212° F is reached and it begins to escape, the so-called "trickle finish." Spacing of steam outlets The temperature rise to 212° F at a point removed from the steam input is [155] never instantaneous, though in a prac- tical sense it may appear to be. Even in a case reported by Morris and Winspear (1957) in which the temperature rose from 38.7° to 212° F in 0.5 minute, the reading at 0.25 minute was 135.5°. The time increases with increasing distance or decreasing steam flow. In one of our tests it required 1.0 minute to heat U. C. mix C (50 per cent peat) to 212° at 2 inches from a source injecting 6.7 pounds of steam per hour, and 7.5 min- utes at 6 inches; at 3.5 pounds of steam 1.5 minutes is required at 2 inches, and 43 minutes at 6 inches. Thus, the proper spacing of perforated pipes in the soil is determined in part by the available steam flow. Up to a point, greater dis- tance between steam inputs is possible without lessened efficiency, if the steam flow rate is increased. With a flow of about 7 pounds per hour from each ori- fice, the spacing might well be 12 inches or a little more without loss of efficiency. However, if the flow is as low as 2 pounds per hour, the spacing should not exceed 6 inches for a comparable effi- ciency and time (fig. 76). The soil mass settles during steaming, presumably from the increased weight of the water and from expulsion of air. The settling may be as much as 4 inches in 20 inches of soil, and it commonly is 1 inch or slightly more. This compaction affects steam distribution. In a steam-box soil treater with a rigid pipe grid having holes on the underside of the pipes (for example, type 4, Sec. 10), this settling of the soil leaves an open space along the underside of the pipe after steam has been applied for a time. Steam fills this space and thus diffuses into the soil from a line, rather than a series of points. The outward flow of steam probably begins from the several orifices and gradually extends to become a linear source along each pipe. This would minimize the im- portance of exact spacing of the holes in the pipes in such equipment. Phis situa- tion is not likely to occur with buried perforated pipes or tiles because they would settle with the soil. Characteristics and Forms of Steam Water is an extremely efficient medium for the transfer of heat. It changes form from ice to water at 32° F, and above that point stores heat at the rate of 1 B.t.u. per pound per 1 degree F rise, up to the boiling point (212°). Thus, boil- ing water contains 180 B.t.u. available for soil heating above the freezing point (fig. 77). At 212° another change of form occurs, from water to steam, and for this 970 B.t.u. per pound are neces- sary, with no temperature increase (fig. 77). As is well known, water may be brought to the boiling point much more quickly than it can be boiled away, due to these heat requirements. Thus steam transfers its heat (970 B.t.u) in addition to that of the condensed water (180 B.t.u.), or about 6.4 times as much as does boiling water. About 6.4 times as many pounds of water must be used as steam to bring soil temperature from 32° to 212°. This explains the principal disadvantage of the hot-water treatment of soil (see "Hot-Water Drench of Propagating Sand," Sec. 10). The prin- cipal heat transfer by water occurs when it changes to steam, and vice versa. The existence of an advancing front of steam must be appreciated in taking tem- perature readings during soil steaming. (See "Movement of Steam through Soil," above.) The temperature rises rapidly at a given point under efficient operating conditions, and cold spots are likely to be untreated and at the original temperature. Readings should be taken at points of slowest heating, and the tim- ing started when these have reached 180° to 212° F. If the soil is adequately protected from heat loss, it may not be necessary to keep the steam on after this time. Thermometers and Tempil Pellets for measuring temperature are described in Section 8 and the Appendix. 156] 1300 1200- 1100- 1000- o "o 900 o 1107 800- 0) CO C o CO o a) i_ D to CO d) Steam and water 180 _o _g '5 > a CO 00 o ai" i— D CO CO 0) k_ D_ _Q J) _Q '5 > D CO O CN O © CO CO o _Q _g 'a > o o o l_ CO CO a> D_ o 00 1086 x x* Hot water y - / Ice 32 100 300 _o '5 > o CO CN O <0 i_ CO CO > o o CO »o CN O I— D to CO O CO s / CO CN CN O O O ^O / / / .X" 400 500 212 TEMPERATURE (°F) Fig. 77. Relative amount of heat (B.t.u.) released by hot water, free-flowing steam, steam superheated to four different temperatures, and saturated steam at six pressures. The available B.t.u. for soil heating are indicated in each case. (Based on data of Keenan and Keyes, 1936; Morris, 1954 b, has a similar graph.) [157] Free-flowing or pressureless steam The conversion from water to steam is accompanied by a 1600-fold increase in volume. If this change occurs in a boiler, the degree of pressure may be regulated by controlling the rate of fuel supply or of steam flow. Free-flowing steam without pressure or superheating may deliver 1,150.4 B.t.u. per pound to an object at 32° F on which it condenses. Of this amount 180.07 B.t.u. represents the heat residual in water at 212°, and 970.3 B.t.u. the heat released when steam condenses to water (fig. 77). When steam is injected into soil at 212° the heat is not trans- ferred from the water since the soil itself is raised to 212°, heat flowing only from a warm to a cooler object. Only 970 B.t.u. per pound are therefore available; but when soil is heated to only 180°, there are 1,002 B.t.u. available. Steam delivered from the boiler through pipes is commonly under pres- sure in order to deliver it in adequate amounts. At 15 pounds' boiler pressure the temperature of steam is 249.8° F. However, when such steam is released into the soil, the pressure is immediately lost and the temperature drops back at once to about 240° and then to 212°. As this steam condenses it will yield only about 14 B.t.u. more per pound of dry steam than at 212° (fig. 77). The extra heat content from the pressure is briefly converted to superheat, and tends to dry the steam by evaporating the water drops it contains. The end result is usually, therefore, to supply slightly more and drier steam to the soil. Its effect on the organisms is the same as the free-flowing type, since they are subject to no pres- sure. Steam under pressure I he flow may be restrained so that boiler pressure will be built up, with some increase in the heat available for soil steaming. Thus, at 80 pounds per square inch boiler pressure, there are 1,006 B.t.u. available (fig. 77). Steam under pressure is most com- monly used in cannery retorts or auto- claves operating at 15 pounds' pressure. Although this is an effective type of equipment, the gain in heat transfer does not justify the cost of a steamtight sys- tem. Only 14 B.t.u. more per pound of dry steam are delivered in such auto- claves than by free-flowing steam (fig. 77) . If the pressure is increased to more than 15 pounds, the equipment becomes excessively expensive and there may be restrictions to its operation. Further- more, there is no effective decrease in time of treatment or gain in efficiency, because the autoclave must be operated with the exhaust valve or door open for a time to free it of air pockets before pressure is built up (see "Relation of steam/air ratio to condensation," above) . Superheated steam Steam may be superheated by passing it through the fire box to heat it, much as a furnace heats air. Because the specific heat of steam is about half that of water, there is a gain of only about 47 B.t.u. for each 100 degrees of super- heat (fig. 77). At 300° F the gain would be only about 43 B.t.u. This 4.4 per cent gain causes almost no noticeable de- crease in time of treatment, but con- siderably increases the cost of equipment. Steam superheated to about 450° was used for a time in one commercial soil- treatment operation in southern Califor- nia; this gave an increase of about 114 B.t.u. (about 11.8 per cent) available for soil treatment per pound of steam over the free-flowing type. In general, however, superheating is more effective than high pressure in in- creasing the heat content of steam; at 400° F it is 3.8 per cent, and at 500° 8.3 per cent better. Superheated steam contains no unvaporized water and, therefore, does not make the soil quite as wet as does steam under pressure. I L58 I This, however, is not a critical factor with good nursery soils. It is questionable whether there is enough gain over free-flowing steam at 212° F from either steam under pressure or superheated to justify the increased cost; in the ranges shown in figure 77 the gain is only 1.4 to 14.1 per cent. Volume of Steam Required It is desirable to have a boiler and steam pipes of sufficient capacity that quantity of steam will not be seriously limiting at any time in soil treatment. This means that the higher the boiler horsepower rating (steam-producing capacity) the faster a given soil mass can be heated, or the larger the mass that can be heated in a given time. One boiler horsepower is the capacity to con- vert 34.5 pounds of water at 212° F per hour into steam at 0-pound gauge; it equals 33,475 B.t.u. per hour. It is now customary to rate boilers in pounds of steam generated per hour. Boiler ca- pacity bears no relation to steam pres- sure, and a satisfactory boiler may be of the flash type, without pressure, pro- vided the steam does not have to travel a long distance through pipes. Since there is little gain in heat transfer from using steam under pressure or in superheated condition (fig. 77), the best way to obtain the necessary soil-heating capacity is to use a boiler of adequate size. Steaming too much soil for the boiler capacity is inefficient owing to heat losses through radiation, transmission, and convection, as is permitting exces- sive steam loss because of too small a load. A balance must be worked out for each piece of equipment, between too much and too little steam for the volume of soil treated. The assistance of a heat- ing engineer is helpful in calculating the required boiler capacity for a given soil- steaming operation. However, a grower can, to a large extent, adjust to the capacity of a given boiler by: 1. Decreasing the area (in benches) or volume (in bulk steaming equipment) of soil treated, if it requires more than 1 hour to raise the temperature to 212° F. « Table 14. The Time Required to Bring a U. C.-Type Soil Mix to 212° F, and the Amount of Soil That Can Be So Heated in 1 Hour For 7 different boiler capacities and 3 levels of efficiency in heat exchange* Boiler capacity Equivalent kilowatt- hours t Time per cu. yd. to raise temperature to 212° F at 3 efficiency levels; in min. Maximum soil heated in 1 hr. at 3 efficiency levels; in cu. yd. Lb. steam per hr. Calculated boiler horse- power t 30% 50% 70% 30% 50% 70% 100 2.9 28.4 175 105 75 0.34 0.57 0.80 200 5.8 56.9 88 53 38 0.68 1.14 1.60 300 8.7 85.3 58 35 25 1.03 1.71 2.39 500 14.5 142.2 35 21 15 1.71 2.85 3.99 1,000 29.0 284.4 18 11 8 3.42 5.70 7.98 2,500 72.4 710.9 7 4 3 8.55 14.26 19.96 5,000 144.9 1,421.9 4 2 2 17.11 28.51 39.92 * Computed on basis of 15 per cent water content in soil, 150 degree F rise in temperature, and specific heat of 0.2. t Computed on basis of 33,475 B.t.u., or 34.5 lb. steam, per boiler horsepower at 100 per cent efficiency. Because boilers are often rated on the basis of area of heating surface, without regard to efficiency, these figures may bear little relation to commercial horsepower ratings. t Calculated on basis of 3,411 B.t.u. per kilowatt-hour at 100 per cent efficiency. [159] 2. Increasing the area or volume of soil, increasing the number of steam out- lets in the mass, or simply reducing the steam flow with a valve, or the pressure with a regulator, if steam is escaping in- stead of condensing. The efficiency of the operation de- pends on the proper balancing of all of the factors by the grower. Table 14 gives data on the time re- quired for soil steaming with boilers of various sizes, and for heat-exchange sys- tems of different levels of efficiency. It also presents data on the amount of soil that can be treated in each case. Cal- culated for a soil mix of the U. C. type, this gives a steam requirement of 10.8 pounds per cubic foot of soil at 30 per cent efficiency, 6.5 pounds at 50 per cent, and 4.6 pounds at 70 per cent efficiency to heat 150 degrees F. Bunt (1954-55) found the requirement to be 5.40 to 8.45 pounds per cubic foot (average 6.51) to heat clay loam 158 degrees. A soil mix of the U. C. type with 15 per cent moisture would require 70 B.t.u. per cubic foot per degree rise in tem- perature at 30 per cent efficiency, 42 B.t.u. at 50 per cent, 30 B.t.u. at 70 per cent, and 21 B.t.u. at 100 per cent effi- ciency. Morris (1954a) obtained figures ranging from 24 B.t.u. per cubic foot per degree for compact, dry, light soil at 9 per cent moisture, up to 53 B.t.u. for compacted heavy soil at 58 per cent moisture. A safe working figure for steam re- quirement in heating soil would appear to be 6.5 pounds per cubic foot, or 42 B.t.u. per cubic foot per degree F. Proper Soil Condition for Steaming Moisture content The moisture content of soil to be heat treated is of great importance in three different ways. I. // requires about five times as many B.t.u. to heat J pound of water as it does 1 pound of soil. The specific heat of a light sandy soil has been reported by Morris (1954a) as 0.192 and a heavy clay soil as 0.202 (both oven-dried), in comparison with approximately 1.0 for water. A peat soil had about the same value as the heavy clay soil. An average of 0.2 is generally used for soils. Thus, to raise 1 pound of an average soil 1 degree F requires about 0.2 B.t.u. A soil with 20 per cent moisture requires about as many B.t.u. to heat the water as it does the soil, despite the fact that the water accounts for only about one sixth of the total weight. Ex- cessively wet soil often requires twice as long to reach 212° F as one in good planting condition, and it is therefore uneconomic to steam soil while it is soggy. 2. Heat plus moisture is much more effective in killing microorganisms than is heat alone, and soil should, therefore, not be excessively dry when treated. 3. Heat conduction of soil improves with increasing moisture content, and treatment therefore becomes slightly more efficient. As a practical compromise between these opposing conditions, the soil to be steamed should have sufficient moisture to be in good planting condition, that is, after being squeezed in the hand it will crumble easily. When such soil is steamed it comes out in good condition for planting, without wasteful heating of excess water, and with satisfactory de- struction of microorganisms. Soil structure The structure of soil is important in heat treatment because it affects the pas- sage of steam and the conduction of heat (see "Manner of Heat Distribution," and "Movement of Steam through Soil," above) . A clod is compressed, has reduced L60 | pore size, and therefore presents special problems. It may be surrounded as the steam margin advances, since it reaches 212° F throughout its mass more slowly than does the porous soil. The time re- quired increases with the size and com- pactness of the lump, since steam diffuses inwardly, and air is expelled simulta- neously through the reduced pores. Morris (19546) found that a lump 3% inches in diameter did not reach 160° at the center while the surrounding soil reached 212°; one 5% inches in diam- eter took 50 minutes to reach 212° at the center; one 7 inches in diameter took more than 1 hour to reach 160° at the center. If lumps are covered by at least 2 inches of porous soil, they may be heated through from transferred heat during the after-cooking. They should, however, be broken up as much as pos- sible or removed by screening, to reduce the chances of imperfect heating and of "blow-through" by steam. Hoare (1953) considered that the heating of clods, even by this slow diffusion of steam, was rapid as compared with the rate of heat transfer by conduction. Because uneven packing of the soil in the container also makes for uneven heating, particularly at high rates of steam flow, it should be avoided (see "Movement of Steam through Soil," above). Soil must be as free of clods as pos- sible for fast, successful, and economical steaming. A U. C.-type mix uses soil of such texture that resistant clods are not formed (Sec. 6), and the whole problem is thus avoided. If lumpy soils must be used, they should be pulverized or screened before being steamed. In heating dry soil the picture is quite different; heat is transmitted from par- ticle to particle, and the smaller the pore space the better. However, this treatment method is inefficient for other reasons, and is now little used. In treatment of moist soil with heat, part of the thermal transfer is by steam, as explained under "Treatment of Soil by Heat — Rate of Heat Distribution," above. The greater moisture capacity of clay than sandy soils is more important in determining the greater number of B.t.u. required to heat them than is the porosity or weight per cubic foot. [161] SECTION Equipment for Heat Treatment of Soil Kenneth F. Baker Chester N. Roistacher Considerations in the choice of equipment Stationary soil mass treated in batches Stationary soil mass in benches or beds Moving soil mass in continuous output Moving soil mass treated in batches Equipment for generating and distributing steam Soil treatment in a mechanized nursery ROM THE discussions in the two pre- ceding sections it is evident that there can be no one type of soil heat-treating equipment that is best for all purposes. There are almost as many different special models of equipment as there are growers treating soil. The grower must decide whether to use a continuous type with a moving soil mass or a batch type with static soil mass, whether soil is to be treated in bulk or in the containers, whether the unit is to be stationary or mobile, whether steam or dry heaters are to be used, and whether the fuel is to be natural gas, butane, oil, propane, or electricity. It is a matter of finding the type best suited to the given operation. To provide the facts on which such a choice must rest, the principles have been discussed, and specific equipment involving them is now presented. Growers often develop equipment them- selves, and rediscover designs already abandoned by others. All basic types are therefore described, even though some are not recommended. References are given so that further details may be obtained. CONSIDERATIONS IN THE CHOICE OF EQUIPMENT Stationary versus moving soil mass All of the types of equipment used by growers may be grouped into two classes, according to whether the soil mass is stationary or moving. On the basis of efficiency, dependability, low cost, and minimum recontamination hazard we consider that the best type is that in which the soil is treated by flow- ing steam in planting containers (flats, pots, beds) or /'// stationary piles (steam [162] chambers, autoclaves, Thomas method). It does not follow, however, that all nurseries should use one of these types. // the heat source is steam, types of equipment with a stationary soil mass will prove best in most cases. If the source is a dry heater of some sort, thermal transmission will be much more efficient and better controlled, with either moist or dry soil, if equipment using a moving mass of soil is employed. Conse- quently, equipment of this type is con- sidered best when dry heaters and either dry or moist soil are used. As explained in Section 9, it is presently impractical with stationary- type equipment involving the use of steam to heat a soil mass uniformly to less than 212° F, although this can be ac- complished with equipment having a moving soil mass. If heat is applied to dry soil, the temperatures may rise well above this point. With equipment treat- ing a moving soil mass it is possible to terminate the process at any desired temperature by varying the heat input (through control of the steam, gas, oil, or electricity) or the time of exposure (through regulation of the speed with which soil is moved). Provision for after-cooking Merely heating soil to 180° to 212° F is not sufficient; it must be kept at that temperature for 30 minutes. Equipment for continuously treating a moving soil mass rapidly heats a small quantity of soil and then dumps it. Provision must always be made to keep the soil tempera- ture at 180° to 212° F for at least 30 minutes. This can be accomplished by quickly stacking the filled flats and covering with a clean heavy canvas, or by similarly covering the pile of soil. Batch equipment for treating a sta- tionary soil mass usually provides for this after-cooking, but separate arrange- ments must be made for the continuous output from moving-soil equipment. Is a boiler already available? In general, there are two types of California nurseries with reference to the use of steam. One group operates glass- houses and has a large steam boiler for heating the range, or plans to install one. The other and more common type does not have steam-generating equipment, either because only lath houses and out- door plantings are used, or because the glasshouses are heated by vented gas stoves or unit heaters. The first group may use their existing facilities, but the latter must either procure a boiler or use the self-generating or dry-heater types of equipment discussed in this section. Permanent versus mobile equipment A question that frequently arises is whether to place the soil treatment and handling equipment in a permanent loca- tion in a nursery, or to maintain a mobile unit. There are many modifica- tions of each equipment type in use, and the decision must rest with the grower. Experience has indicated, however, that there is less trouble with the boiler, mechanical parts, and recontamination when the soil is taken to the equipment, rather than vice versa. With present conveyer systems an efficient mechanized nursery can be built around a single soil-treating installation (Sec. 17). Separating various operations It is better if the various operations in soil preparation are physically separated in some way, so as to minimize recon- tamination. For example, several nur- series have divided them as follows: 1. A soil yard where storage and mix- ing take place. The flats or cans may be filled there if treating is to be done in the container, and a mobile bulk cooker may also be filled there. 2. The treating should be done at another location or preferably in a building so as to minimize wind-blown [163] Table 15. Summary of Characteristics of Equipment + = yes; - = no; — + = both yes and no apply because of dual function; Characteristics Equipment types* with soil stationary, handled in ' latches External steam source Generates own steam 3 Pi 1 a A © A O S 2* A o a B a >x a '£ o p. 1 c m 3 M •3 A M O Jo a § w n 4a* 3 A K 2 a ■°-s 8a a o 01 o 4b* M £ m . hi M J) * s 11 5* 3 ■8 > 6* 4) N O o. o, Sis 7* 4) a A d B "3 u B 4) > 8 0) > 73 o 3 < 9 45 t) S O ■ -o It °A 2 <« c > o o XB 10 e c A el @ *3 s E 4) > 11 C 6 12 1 a o ■ o X 13 Soil treated in containers - + - + - - - + + + + + + + + + - + Treats containers separately + + + - + + + + + + + + + + Powered moving parts used Steam efficiently used + + + + + + + + + + +? + + + Equipment inexpensive + + + + + + + - + - + + + + Steam put into or surrounding soilf IS IS I I IS S S S S S s s S S Soil heated above 212° Ft Easy to load and unload - + + + + - + + - - - + - - - Type of steam used§ . . FPS FPS FPS FPS FPS FPS FPS FPS FPS P F F F F Minimized recontamination hazard - + - + - - - + + + + + + + + + - + Portable unit^ - + + - + + + - - - - - - - - + Also used for hot-water treatment of stock - - - - - - - + - - - - - - Fits into mechanization - + + + + - + + - - + + - - - Type of fuel or electricity used|| GP GE GP GPCE Danger of burns or shocks - - - - - - - - - + - - - - Treat around posts easily Effective deep treatment of soil Useful in small or large operations**. . . . SL SL SL SL S SL SL S S L S S S S L64 | for Heat Treatment of Soil. See Text for Details ? = variable or uncertain; blank = does not apply. Equipment type * Soil in benches or beds Soil moving Continuous type Dry source of heat Steam Hot water Steam- formal- dehyde Steam Dry source of heat Batch type bo B a -B M st 5 S u E « a w» 14 ■ & 9 ■ 3 15 !2 M a _o o 3 •a c 16 bo e S a » 17 ■ s o J3 H 18* a 03 O. ■ • a 19* B N O B a Q. ■ 3 & «a 20 s M a M M ■ — w 21 • •a V = m 22 a M OS N bo a '> o s 23 •a a M a t» a a E is w ft 24 i h a S a flj 03 « a 25 "3 I fi hi Oi — •" o W [> 26 o a a Cm £j MS 27 Bo a ■ %* I? 3 * ei.5 28 * a g H ■ bo a 3 o PS 29* 8 a c "3 '3 a *» a II 30 as a E £ a ^c be— a <* 11 Sa K a 31 a o s a a a S 32 £ a hi a ■ bo Bo 3o O - — - 33 *a £0 - oS m c Si K a 34 B a = = bo_ C =« B a — a 9 M tf a 35 - - - - + + + + + - + + + + - - - - + + - - - - + + + - - - - - - - - - + - - - + + + + + + + + + + + + + + + + - + +? +? + + + - — + + + + - - - + +? S S I I I I S I S I I I + - + + + + + + + - + + + - + + + + + + + + +? FPS FPS FPS FPS FPS FPS F FPS FS FPS FPS F - - - - + + - - + + + + + + - - - - + + + + - + - + + + + + + + - - + - - - - - +? + + + + +? +? +? + E E E GCP E PG GP P GP E E GP P - + - - - + + + - - + + + - - - + + - - - - + + - - - + +? + + - - + S S S S SL SL SL SL L L S SL SL S S S S S S S S S * Numbers refer to equipment types in text; asterisks indicate best types for California conditions. t I = into; S = surrounding. X May, of course, be heated above 212 3 F if superheated steam is applied for a long enough period. § F = no pressure, free-flowing; P = pressure; S = superheated. ^ Unit may be portable, but cannot be used away from special electric wiring (for example, 14, 15, 16). If units are portable when pro- vided with a mobile steam source, they are so classed here. I! P = type of petroleum; E = electricity; G = gas; C = coal. ** S = small operations only; L = adaptable to large operations. [165] dust. Filling of treated flats or cans with treated soil may be done at the same place or in the planting shed. 3. Planting of the containers usually is done in another room. With this compartmentalization of the process there is small chance of dust from the soil yard blowing into treated soil. Care should be exercised, however, to see that adhering untreated soil is not carried on the mobile cooker from phase 1 to 3. There should be a concrete or wood floor for phases 2 and 3, and this might well be hosed down every day. When treated soil is dumped in bulk piles on the floor, the surface should pre- viously have been wet down with a formaldehyde solution (1 gal. to 18 gal. water) . Adapting batch equipment to continuous operation It is possible to capitalize on the effi- ciency inherent in equipment treating a stationary mass of soil, and yet operate on a continuous-batch system. This was done by a large commercial unit in southern California that operated with the method of the steam box (type 4) . Two steam boxes were operated in se- quence, one being filled with soil while the other was cooking. Since the soil was dumped directly into flat-filling equip- ment, an almost continuous flow of treated filled flats was produced. Something of the same sort might be done with the mobile bins (type 2), one mobile bin being taken after treatment and dumped into the flat- or can-filling equipment while the other was being filled and steamed. Two vault units (type 6) could be similarly operated in se- quence, and the work mechanized by placing the containers on pallets han- dled by fork-lift tractors. This would keep the boiler and the crew almost con- tinuously active if the equipment were properly designed. The same could be done with two autoclaves (type 9) operating in sequence, but the initial cost would be high. The important point is that it is not necessary to use equipment with a mov- ing soil mass in order to achieve con- tinuity of operation, with the attendant benefits of mechanization (Sec. 17). Use of superheated steam There is slightly greater heat trans- mission and decreased water content in superheated than in free-flowing steam (Sec. 9). While it is not commonly used because of the greater cost of the boiler, it can be used with many of the types of equipment described here, without their modification in any way. Superheated steam may be used with equipment types 1 to 8, 18 to 23, and 26 to 28 described below. Summary of Equipment A summarized statement of the char- acteristics, advantages, and disadvantages of the 35 types of equipment is given in table 15. One should use this tabulation to determine the types of equipment with the necessary features for a given instal- lation, then refer to the text for details. STATIONARY SOIL MASS TREATED IN BATCHES Other things being equal, the sta- tionary types are cheaper, easier to main- lain, do not require power to drive moving parts, and are less likely to break down at crucial times, than are those will) a moving ^<>il mass. In general, they are also more efficient in the use of steam because it is less likely to escape from inside a static soil mass (before it has reached 212° F) than from a moving mass intermittently exposed to air. Types I I through 17 have a dry source of heat. [166] Pressureless Steam from External Source Released into Soil 1. The Rudd type . . . first used in Illinois in 1893, repre- sents the simplest form of this equip- ment. It has been redescribed by several stations in England, Europe, and the United States, often as a new develop- ment. It consists essentially of a bin con- structed of wood, cement, or brick, lo- cated on the ground so that it can be loaded and unloaded from either the top or one side (fig. 78). It is preferably covered with a hinged lid. The dimen- sions vary considerably, but the soil depth should not greatly exceed 12 in. Pipes of 1-in. diameter are laid in open channels in the floor about 9 in. apart, and are drilled on the underside with %-in. holes 9 in. apart. The channels may be covered by boards with V2 _m - holes. Alternatively, the pipes may be partly imbedded in the concrete floor, with the holes at the top; the pipes must then lead into a condensate header from which the soil may be blown before use. The floor should slope slightly toward one corner for drainage of condensate. Such a unit may be used with free-flow- ing, pressure, or superheated steam, operating in all cases without pressure in the box. If used for soil in containers, this becomes the vault type (type 6). In Norway a box with a steam grid in the bottom is pivoted at the ends so that the box may be easily tipped and dumped after steaming. This idea might be ex- tended to the steam box (type 4a) with benefit. Advantages: very efficient use of steam, simple and inexpensive; de- pendable; no moving parts; can be used for steaming pots and flats; may be loaded by machinery. Disadvantages: soil must be shoveled out, greatly in- creasing recontamination hazard and labor cost. Does not fit well into a fully mechanized schedule. Best use: treating soil in containers; treating containers. References: Rudd 1 1893) ; x Fosler (1950); Bewley (1948, p. 13-15, fig. 11); Lawrence and Bunt (1955). A modification of this is to use tiles instead of pipe, and use a deeper mass 1 See Appendix for complete references, cited here by author and date. Fig. 78. The Rudd type of steamer for stationary bulk soil (type 1). Fig. 79. The fixed-front steam box for stationary bulk soil (type 4a). See also fig. 81. [167] of soil. References: Ball (1942, p. 3) ; Roll-Hansen (1949, p. 7-8). *2. The mobile-bin type... used by several California growers elimi- nates the above disadvantages. The bin in this case may be the body of a dump truck or a two-wheeled cart (fig. 80) that can be coupled to a tractor. The pipe grid is on the bottom of the body as before (temporary mounting in dump truck), and a tarpaulin is used for a cover. May be filled with a skip loader, or conveyer belt from the mixer, and emptied after steaming by tipping up the front end and removing or tilting the back panel. May be used with free-flow- ing, pressure, or superheated steam, operating in all cases without pressure in the bin. Two units may be used in a continuous-batch operation. Advantages: very efficient use of steam; simple and * One of the types considered best for Cali- fornia conditions. dependable; flexibility — may be ma- chine-filled in soil area, connected to boiler elsewhere, and dumped into flat- or pot-filling equipment, on floor, or in bins where needed; recontamination risk reduced; may also be used for pots and flats, or soil in containers; fits well into mechanization. Disadvantage: some re- contamination hazard. Best use: treating bulk soil for bin storage, or for use with flat-filler; also for pot and can growing. References: Anonymous (1952) ; Morris (1953). 3. The combined bin and potting bench . . . is a four-wheeled table with the ends and one side fixed, and one side re- movable (fig. 132). It is similar to the mobile bin (type 2). A perforated pipe grid is placed on the smooth metal floor of the bin, which is then filled with soil, covered with a tarpaulin, brought by a tractor to the boiler, and steamed. The Fig. 80. A mobile bin (type 2) soil steamer, trailer variant, for stationary bulk soil. Dump trucks are sometimes fitted with perforated steam grids to give larger units of this type. A small portable steam generator is shown at the rear. [168] pipe grid may then be lifted out, the vehicle pulled to the potting site, one side removed, and the potting done directly on the bench. The unit may be used with free-flowing, pressure, or su- perheated steam, operating in all cases without pressure in the bin. Advantages : efficient use of steam; simple and de- pendable; labor saving. Disadvantages: considerable recontamination hazard because whole load is exposed to in- festation from the potting operation when soil is first used; extreme sanita- tion necessary; fair integration with other mechanization. The contamination hazard could be minimized by construct- ing on the wagon bed a tapered soil bin, from the bottom of which the soil flows through a variable gate onto the potting table. A permanent perforated pipe steam grid could be built in the bin. The treated soil would be thus protected from recontamination until used. Best use: pot-plant growing. Reference: Anony- mous (1954). *4. The steam box . . . is a wooden or metal box equipped with a hinged lid for filling with soil, and a dump bottom for emptying. It is elevated from the floor, and is stationary ( or may be mounted on wheels for mobility, if fabricated of metal). If it is made of wood, resin-impregnated marine plywood 2 must be used, as others will deteriorate when steamed. The "cold" edges and bottom four corners should be fitted with triangular pieces of wood to expedite uniform heating. These four corners are usually the coldest spots in all batch treating equipment. In the box is a perforated grid of pipes arranged in rows 9 in. apart each way; on the under- side Vs-in. holes are drilled about 9 in. * One of the types considered best for Cali- fornia conditions. 2 Phenolic resin glueline and coating on sur- faces; may be called High Density Overlaid Plywood. apart. This arrangement insures that no soil particle is more than 5 in. from several steam outlets, which provides maximum efficiency in distribution of steam and speed of heating. It is filled from above, emptied below. There are several methods for operating the bottom doors; among the best are controlled lowering by a chain hoist, rack and pinion gears, sprockets and chains, or by a cable wound on a racheted shaft; a mechanical trip-catch on free-falling doors may also be used. It may be made with a fixed front (type 4a; figs. 79 and 81) for bulk soil only, or with the front panel removable (type 4b; figs. 5 and 82), so it may be used for bulk soil or for flats and pots, empty or filled with soil, these being placed on the pipe shelves. The units were developed by the authors in the Department of Plant Pathology, Univer- sity of California, Los Angeles. A Los Angeles company will fabricate such boxes on special order (see Appendix). When used for flats, steam is released into only the bottom layer of pipes, the valve closing off the other pipes. The bulk-soil type is useful for nurseries with fairly large soil requirements, since the box may be dumped once an hour, or oftener if a large steam flow is used. Continuous-batch operation may be achieved by having two boxes in tandem, one being filled while the other is steam- ing. Such a unit was successfully used on a contract basis in southern California for several years, turning out about 16 cu. yd. per day. When such a dual unit is coupled to an automatic flat-filler (Sec. 17), an essentially continuous output of flats is obtained. Anv of these units may be used with free-flowing, pressure, or superheated steam, operat- ing in all cases without pressure in the box. The front-opening tvpe is probablv the best available for the small nursery, because of its flexibility and efficiency. These boxes are probably the most effi- [109] Fig. 81. The fixed-front steam box for sta- tionary bulk soil (type 4a). See also fig. 79. cient. dependable, and convenient of the bulk-type equipment, and fit admirably into mechanization. For example, a Toledo, Ohio, nursery is said to use boxes of this type holding 3% cu. yd., arranged so that the soil drops directly into dump trucks. Advantages : very effi- cient use of steam; rapid and uniform heating of soil; simple and fairly inex- pensive to construct; very dependable, with few moving parts; highly adaptable in the various forms, fitting well into mechanization; cannot be overheated (unless used with superheated steam) ; ease of handling; containers also treated (in open-front type only). Disadvan- tages: since containers are not treated (except in open-front type), this must be done separately; some recontamina- tion hazard with bulk type, as soil must be handled. Best use: bulk-soil type, single or dual, with or without attached flat-filler, is excellent for large nurseries. Open-fronl type nearly ideal for small nursery. Reference: Roislacher and Baker (1956). I I Fig. 82. The removable-front steam box for stationary bulk soil, and for soil in containers (type 4b). See also fig. 5. PressureSess Steam from External Source Released around Soil *5. The Thomas method for steaming soil in containers . • . should be more widely used. The prin- ciples and methods are the same as described for the surface Thomas method (type 13; see fig. 91). Flats or pots are stacked on the concrete floor with wood separators between layers, or in special steel racks, and covered with a rubberized canvas or similar material. The tarp is held down by pipes or sand bags along the edges. Steam is released under the cover, which then acts as a steam chamber. The method may be used with free-flowing, pressure, or super- heated steam, operating in all cases without pressure in the soil. Advantages: inexpensive, simple, efficient; materials readily available; used for any con- tainer, empty or full; also used for bulk soil (type 18) ; adaptable to mechaniza- * One of the types considered best for Cali- fornia conditions. 70 | tion by use of pallets and fork-lift trac- tor. Disadvantages : cover may wear out, particularly if paper; takes up work area unless a separate floor is provided. Best use: small nursery; larger opera- tions should use permanent equipment. Reference: Dimock and Post (1944). *6. The vault type . . . has been used for at least 25 years, and is still one of the best for soil in flats and pots. It consists of a vault built of reinforced concrete, planks, or heavy resin-impregnated marine plywood, 3 with a concrete floor continuous with that outside (figs. 83, 128, and 131 ) . A swing- ing wooden door on one end is sealed around the edges with a gasket of rubber steam hose, and held tightly closed by several refrigerator-type door clamps. The unit is not steamtight and will not build up pressure. The steam will con- dense on the soil, with only slight leak- age from the vault, until the inside temperatures approach 212° F, when increasing amounts will be lost. The steam is admitted to the vault from a perforated pipe on the floor against the sides and back end, with the holes toward the inside of the vault and down- ward on a 45° angle with the floor. Such a vault may be loaded manually, flats being stacked with %-in. separator strips or as shown in figure 104, and pots set on top of each other. A better system, however, is to pile the flats or pots on pallets and load these into the vault with a fork-lift tractor. While such a vault can be used for bulk soil in large boxes, it is less efficient than the perforated pipe grid for this purpose. These units may be built in any size, depending on the needs of the nursery and on the boiler capacity. Two vaults may readily be used in a continuous-batch operation. The equipment may be used with free- flowing, pressure, or superheated steam, operating in all cases without pressure in the vault. Advantages: soil steamed directly in containers, reducing both the handling after treatment and the recon- tamination hazard; quite efficient use of steam; convenient; dependable, without moving parts; quite inexpensive to build; adaptable to any type of con- tainer, empty or full, and even bulk soil (with lowered efficiency) ; fits well into a mechanization program; heating soil above 212° F is possible only with superheated steam. Disadvantages: may be awkward to load and unload man- ually, with some chance of getting burned; if soil load is excessive for boiler capacity, treatment may take several hours; poorly adapted for treat- ing bulk soil. Best use: excellent in general growing operations, manually handled and of small size in small nur- series, mechanically handled and large for bigger ones. Reference: Newhall, Chupp, and Guterman (1940, p. 33-34). *7. The multipurpose-tank type . . . is a modification of the vault steamer designed so that it may be used for soil and container disinfestation, and also as a hot-water tank for treatment of seeds, bulbs, and planting stock (Sec. 13), as well as a soaking and steaming tank for pots, cans, and flats. This combination was suggested by C. E. Scott, California Agricultural Extension Service, and the equipment designed in 1940 by H. Gor- don, Department of Agricultural Engi- neering, University of California, Davis. It has been extensively used by the De- partment of Plant Pathology, Los An- geles, for 16 years and has proved to be the "work horse" of our treating equip- ment. It consists essentially of an in- sulated horizontal metal shell, with a hinged lid (figs. 84 and 85). The lid is loosely sealed with a gasket of rubber steam hose. Steam is released through a perforated pipe in the bottom of the tank. * One of the types considered best for Cali- fornia conditions. 3 See footnote 2, above. [171] 83 S-** c ^^S^^ ^^S^S^SSSk ^^S^r^S^S 84 Fig. 83. (Top) The vault-type steamer for soil in stationary containers (type 6). See also figs. 128 and 131. Fig. 84. The multipurpose tank (type 7) for steaming soil in stationary containers, for hot-water treatment of planting material, and for soaking salt from pots. See also fig. 85. I 172] Fig. 85. The multipurpose tank (type 7) for steaming soil in stationary containers, for hot-water treatment of seeds and planting material, and for soaking salt from pots. Note the outlet pipe for circulating water in left photo, and pump and input in the view on the right. The input pipe is fitted with a valve, and also serves as the drain pipe. A thermometer is placed in the input line. The steam input pipe (white), and method of stacking flats are shown. See also fig. 84. The unit is not steamtight and cannot build up pressure. It may be operated on free-flowing, pressure, or superheated steam. Into this container may be stacked soil in flats, cans, or pots, with the layers separated by wood strips. It may also be used to steam empty con- tainers prior to use. The tank may be filled with water and heated by passing steam through a pipe in the bottom, with the condensate passing out of the tank, or steam may be released directly into the water. The temperature may be ac- curately controlled by adjusting the steam valve. The circulating pump re- moves the water from the top at one end and returns it to the bottom at the other, providing excellent water circulation. Used in this way the unit has proved very efficient for the hot-water treatment of seeds, corms, and whole plants. With or without the circulating pump oper- ating, it provides an excellent tank for soaking the salt out of pots (Sec. 4). Such units can be built in a size suitable for the given nursery. Advantages: soil is steamed directly in the containers; quite efficient use of steam (it should be turned very low when condensation has nearly ceased and much steam is escap- ing) ; dependable, without moving parts; quite inexpensive to build; adaptable to any type of container, empty or full; heating soil above 212° F is possible only with superheated steam; extremely flexible, triple-function unit. Disadvan- tages: awkward to load and unload; poorly adapted for treating bulk soil; does not fit well into mechanization. Best use: excellent for general growing operations, particularly where heat treatment of seeds and propagative ma- terial is planned; best suited for triple function in small nurseries and in experi- ment stations. 8. The vertical-cabinet type . . . consists of a wood (marine or resin- impregnated plywood 4 ) or metal cup- board, on the shelves of which are placed flats, pots, or cans of soil to be treated. 4 See footnote 2, above. [173] Steam is released into the bottom of the cabinet and condenses on the cool con- tents as outlined for the vault type (type 6). Apparently this was first adapted from self-generating equipment used for dairy utensils. The usual form is a wall cabinet with pipe shelves on which flats and pots may be placed. The door should have a gasket of rubber steam hose. Similar to the vertical-cabinet type (type 11; fig. 88). May be used with free- flowing, pressure, or superheated steam, operating in all cases without pressure in the cabinet. Advantages : soil steamed directly in the containers; quite efficient use of steam; dependable, without mov- ing parts; inexpensive; adaptable to various containers, empty or full; heat- ing soil above 212° F is possible only with superheated steam; soil not han- dled after treatment. Disadvantages : not adapted to large volume or to bulk soil; awkward to load and unload; does not fit well into mechanization. Best use: small nursery using flats, pots, or cans, and not requiring bulk soil. Reference: Johnson (1930, p. 4-5). Pressure Steam from External Source Released around Soil 9. Autoclaves or cannery retorts . . . (figs. 6 and 86) are sometimes used for soil treatment. Usually they are second- hand, as cost would otherwise be pro- hibitive. When used at 15 lb. steam pres- sure, the exhaust valve or the door must be left open for a time to displace the air with steam. Sometimes operated as pressureless containers, when the unit becomes the vault type (type 6). Heat transfer by steam at 15 lb. pressure is only 14 B.t.u. per lb. greater than when free-flowing, and the temperature only 37.8 F higher (fig. 77). May be mech- anized l>\ stacking containers on pallets thai are loaded into autoclave with a lork-lifi tractor. Two autoclaves could be used in tandem in a continuous-batch operation. Advantages: slightly faster heating (about 1.4 per cent) than with flowing steam; soil treated in contain- ers; efficient; dependable, without mov- ing parts; fits well into mechanization program; adaptable to any type of con- tainer, full or empty. Disadvantages : re- quires pressure boiler to operate it; high initial cost; difficult to load and unload, with some chance of getting burned; soil heated to unnecessarily high tempera- ture; if air is not displaced by steam be- fore closing the door very uneven heat- ing may result. Best use: general grow- ing operations in medium to large nursery. Reference: Newhall, Chupp, and Guterman (1940, p. 33). Pressureless Steam from Built-in Generator Released around Soil Several kinds of soil-treating equip- ment may be modified in such a way that water may be converted to steam inside the unit by heat supplied from gas, oil, butane, or propane flame, or by electricity. Usually a metal pan of water is heated until sufficient water is evapo- rated to raise the temperature to 180° to 212° F. To be effective, therefore, such units must contain that amount of water, plus an excess to avoid boiling dry. To raise the soil temperature of 1 cu. ft. of soil with 15 per cent moisture 150 degrees F will require that about 6V2 lb. (about 6 1 / 4 pints) of water be con- verted to steam, assuming 50 per cent efficiency. Equipment of this sort usually contains up to twice this amount of water. 10. Horizontal type with removable hood . . . is used by several nurseries in Califor- nia. It consists of a metal pan contain- ing water underneath a rack on which flats or pots are stacked. A large metal hood is lowered over the stack by a hoist, to form the steam chamber (fig. 87). Heat is applied by gas or oil burners [174] 86 g g *?* Fig. 86. Cannery retort (autoclave) for steaming soil under pressure in stationary containers (type 9). See also fig. 6. Fig. 87. Horizontal type of steamer with removable hood (type 10). Steam is generated from the water in the metal pan below, and containers of soil are stacked on a wooden frame above it. Fig. 88. The vertical cabinet type of steamer (type 11) for steam- ing soil in stationary containers. Steam is generated from the water in the metal pan below. Steam may be released into the cabinet (type 8); the metal pan is then omitted. [175] under the pan, or by electric immersion heaters in it. If externally generated steam is released into such a unit, it es- sentially becomes a vertical-cabinet type (type 8). Two such units could be used in a continuous-batch operation. Ad- vantages: soil treated in containers; in- expensive; cannot be overheated; fairly efficient use of steam; easy to load and unload; fits fairly well into small-scale mechanization. Disadvantages: not use- ful for bulk soil; slow operation; im- practical for more than 16 flats per load. Best use: small nursery having little need for bulk soil; home garden or hobby greenhouse. Reference: Califor- nia Dept. Agr. (1944). 1 1. Vertical-cabinet type . . • is a modification of type 8 above, in which a metal pan in the bottom holds water which is heated by a gas burner or electric elements (fig. 88). Uses 1.2 to 2.0 k.w.h. per cu. ft. of soil. In Eng- land the "saucepan" and "trough" meth- ods are small-volume variants of this type. None of these types fits into a mechanization program. References: Peterson (1942) ; Newhall (1940, p. 19- 25) ; Lawrence and Newell (1950, p. 83- 84) ; Lawrence (1956, p. 47-49). 12. Modified-oil-drum type . . . is sometimes used by very small nurs- eries in California. An oil drum is mounted vertically on a brick base so that gas or oil burners may be placed under it. Inside there is fitted a metal base to keep the stack of flats above the water that is placed in the bottom. A wooden lid is used. Advantages: very inexpensive; soil treated in containers. Disadvantages: very small capacity; dif- ficult to load and unload; does not fit into mechanization. Best use: very small nursery, home garden, or hobby green- house. 13. Horizontal-tank type . . . is a variant of the multipurpose tank (type 7), apparently little used in this country. Two modifications are used in England. In one the tank has water in the bottom and either soil in bulk or in containers supported in stacks over it; heat is applied from beneath by coal, oil, or electricity. In another type, the tank is mounted on wheels for porta- bility, and has in the understructure a coal-fired furnace to heat the water in the tank. Reference: Lawrence and New- ell (1950, p. 85-87, 145-57). Dry Source of Heat Thermal transfer from dry heaters to a stationary soil mass is effected in one of two ways. 1. If the soil is dry there is slow and inefficient transmission from particle to particle, the air spaces between acting as insulation. It is necessary, therefore, to use high temperatures for long periods to get penetration of heat, and this usu- ally means that the soil is excessively heated at the point of contact, with char- ring of the organic matter. 2. If the soil is moist, the soil water at the heat source is converted to steam, which moves outward in a zone, as ex- plained in Section 9. It is evident, how- ever, that in the zone where the moisture has been evaporated, the soil will be heated to excess, as in the case above. There is an outer moving zone of steam, followed by an inner expanding sphe- roid of drying soil, and at the center an area of very high temperatures (not un- commonly reaching 400° to 500° F in electric heater types) and charred or- ganic matter. If continued long enough, the mix would be entirely desiccated and perhaps charred. 14. Box with electrical heating elements in soil . . . is now less commonly used than before. Also called the New York, immersion, or indirect type; it has been known since L931 . It consists of a wooden box similar to the steam box (type 4a above) in r i7<> i which are mounted special electrical heating elements (fig. 89). These heavy- duty immersion heating elements are of the strap, plate, or rod types. Current consumption about 1 to 1.5 k.w.h. per cu. ft. The "cold" edges and bottom four corners should be fitted with tri- angular pieces of wood to expedite uni- form heating. A small (Va cu. ft.) tubu- lar model with single central heating unit is used in England by amateurs and gardeners. Advantages: simple, safe op- eration; constant power load; quite ef- ficient; inexpensive to build. Disadvan- tages: requires quite moist soil to func- tion without burning; chars organic matter next to heating elements; uneven heating; expensive to operate; used for bulk soil only, with containers untreated; very slow heating; does not fit well into mechanization. Best use: in small nurs- eries with other means of treating con- tainers, but without steam source. Ref- erences: Newhall (1940, p. 5-8, 14-18) ; Hardy and Dillon, Inc. (1953) ; Brown and Wakeford (1947). 15. Electrode type with soil heated by resistance to electric current . . . sometimes called the Ohio or direct type, is seldom used in California, but ap- parently is in England. It has been avail- able since 1921. It consists of a box similar to the steam box (type 4a), in which are mounted metal electrodes (fig. 90). The current passes through the soil solution between electrodes due to the presence of dissolved salts, gen- erating heat in the soil in the vicinity of the plates as a result of the resistance presented. Current consumption varies from 1 to 4 k.w.h. per cu. ft. of soil. A transformer is required for best results. Soil is dried in treatment. In England a harrow-type grid of rod electrodes has been successfully used, with a trans- former, on ground beds; this required 3 to 4 k.w.h. per cu. ft. to reach 158° F. Advantages: shuts off automatically at 180° to 200°, as water boils away from electrodes; relatively uniform heating. Disadvantages: serious shock hazard; transformer is usually necessary and makes equipment expensive; current load varies widely as soil dries along plates, with firmness of soil packing, temperature, and with salt content of soil; in some areas (probably not in California) addition of epsom salts or potassium nitrate is necessary; difficult to insulate, particularly in ground beds; not adapted to mechanization schedule. Best use: little used; suitable only for small nursery. References: Canham (1951) ; Newhall (1940, p. 3-5) ; Taver- netti (1935). 16. Box type with soil heated by electrical induction grid . . . has very rarely been used. It consists of a box similar to the steam box (type 4a), in which an iron pipe grid enclos- ing coils of copper wire is buried in soil. Alternating current passes through the coils, sets up currents in the pipes, heating them. Advantages: as in type 14. Disadvantages: high initial cost; localized heating, as in 14; insulation of wire deteriorates; not adapted to mech- anization program. Best use: limited to small nurseries. Reference: Newhall (1940, p. 13-14). 17. Baking or burning of soil . . . an ancient method, is now obsolete be- cause it is inefficient and destroys the soil organic matter. Since organisms are more resistant to dry than to moist heat, the biological efficiency is low. Because of low heat conduction by soil this method is expensive and inefficient. Usu- ally soil is placed in containers in an oven, but in England special brick struc- tures are used. Not recommended. Ref- erence: Bewley (1939, p. 22-27). [177] STATIONARY SOIL MASS IN BENCHES OR BEDS The preceding seventeen types, as well as types 27 to 35, treat soil in bulk or in containers. Nurserymen frequently must treat soil in benches or beds, and the methods now to be discussed have been used for this purpose. Surface or pan types (types 18, 19, 24) heat the soil downward from the surface, whereas buried pipes or tiles, or the spike or rake methods (types 20 to 23) heat upward from the bottom. It is reasonable, and is supported by abundant experience, that the buried-pipe or -tile or the mov- ing-rake methods give the deepest soil treatments. Where penetration below 8 to 9 in. is not required, the Thomas method (type 18) may be used, but where penetration to 12 to 18 in. or more is required, the buried-pipe or -tile or moving-rake method is necessary. Pressureless Steam from External Source Released around Soil 18. The Thomas or surface method of steaming . . . (fig. 91) has been widely adopted in the past 13 years in this country and abroad. A canvas hose, aluminum pipe, metal downspout, perforated pipe, or simply a series of croquet wickets set in the soil, is placed lengthwise on the top and center of the bed to be steamed. Over this is spread a plastic sheet (Visqueen, Duratex, Stericover, Velon Fumicover), rubberized cloth, treated fabric (Fiber- thin, Tufedge) , treated fiberglass (Steril- tex), or paper (Sisalkraft) . These may be fitted and tied around glasshouse posts, or two strips may be laid length- wise and joined at a wire stretched down the line of posts, by folding the edges together and clipping with spring-type wooden clothespins (fig. 91). Sonic of * One of the types considered best for Cali- fornia conditions. the materials automatically seal them- selves to the sides of the bed with the condensate ; others may be held down by 2x4 timbers (hot metal angle iron or pipe may injure some plastics) placed on top of sideboards. Wood strips may also be held by C clamps against the ma- terial on the inside of the sideboard, but this reduces effectiveness of treatment of sides. Lath may be tacked over the ma- terial to the outside of the sideboards. The arrangement should be nearly steamtight and arranged so that it may inflate with steam to a height of 5 to 6 in. Steam should be turned on slowly to avoid blowing off the cover. Covers may be used many times if properly cared for. Plastic covers should not touch hot steam pipes and should not be exposed un- necessarily to sunlight, nor stored wet. These covers act like a metal inverted pan (type 19). The soil should be loosened to the bottom of the bench or the desired depth of penetration, and be in good planting condition, free of clods. The end of the bench where steam is in- troduced may be difficult to heat because of condensed water expelled there from the pipes, or because of a dead air pocket. The condensate and moist steam should be drained from the pipes before attaching the hose. A short perforated side pipe from the main at this point will provide steam distribution at the input end. The cover should be left on for 30 min. after a final temperature of 180° to 212° F is reached. The method may be used with free-flowing, pressure, or superheated steam, operating in all cases without pressure in the soil. The method may be modified to treat bulk soil in the headhouse. Pile the soil 7 to 8 in. deep on floor or workbench, cover with tarp and place boards on the edges to hold it down, then handle as above. It may also be used for soil in [178] 89 90 Fig. 89. Box type of heater, using electrical heating elements immersed in the stationary bulk soil (type 14). Fig. 90. Electrode heater, in which the stationary bulk soil is heated by resistance of the soil to passage of electrical current (type 15). Fig. 91. The Thomas or surface method of steaming a bed of soil (type 18). The same method may be used for steaming soil in containers (type 5). [179] Hats or pots, stacked with separators, in the same way (type 5). Another variation has been used for ground beds in lath houses. A flat frame- work of 1 x 4 lumber is placed on the ground and covered with Velon. The cover is held down with 2x2 lumber clamped to the frame. Steam is released under the cover through a canvas hose. To move the unit, the steam is shut off and the structure skidded by pulling it with attached ropes, so that it slightly overlaps the previous setting. The boards are then pressed down into the soil. It is not necessary to walk on treated soil in this operation. Advantages of the Thomas method: simple, inexpensive, with parts readily available; light labor requirement; effi- cient; best practical way of treating around posts and irregular areas; only slight danger of getting burned in mov- ing sets; permits treating field soil with- out walking on it. Disadvantages: ap- parently effectiveness below 8 in. is not dependable; covers wear out (particu- larly if of paper) ; some difficulty in moving covers to new beds. Best use: excellent for all bench treatment under glass or outdoors; less effective on ground beds requiring treatment depth beyond 8 in. References: Dimock and Post (1944) ; Seeley (1954) ; Ball (1953 [5]: 1-5; 1954). * 19. The inverted-pan method of surface steaming . . . has been in successful general use for about 60 years. The pan (fig. 92) is best made of aluminum alloy or other metal, is 6 to 9 in. deep, and of a size determined by the size of beds, ease of handling (by hand or with mechanical aids), and by boiler size. The pan is pressed into the ground 4 to 5 in. and, if high-pressure steam is used, may re- quire weights to hold it down. Soil must l>c well worked up. of good planting * One <»f the types considered best for Cali- fornia conditions. moisture, and free of clods. Tempera- ture may be brought to 180° to 212° F at 8 to 9 in. if soil is well worked up, but does not penetrate a firm soil layer. Pan is left 30 min. after reaching tempera- ture, or it may be moved to next posi- tion (overlapping the former) and the hot soil covered by rubberized canvas to hold in heat. About 2.7 sq. ft. per hr. per boiler horsepower have been treated with the pan method. If pans are to be used in benches or beds, both should be designed for the most efficient size and proper fit. Pans may be used with free- flowing, pressure, or superheated steam, operating in all cases without pressure in the soil. Advantages: simple, inexpen- sive; efficient; soil not handled after treatment, as in the case of types 20 and 21. Disadvantages: difficult to work around posts or in irregular areas; dif- ficult to move large pans (mechanical aids are available — see Newhall, Chupp, and Guterman, 1940) ; danger of getting burned while moving pans. Best use: ex- cellent for bench or bed treatments under glass or outdoors. References: Newhall, Chupp, and Guterman (1940, p. 24- 30) ; Newhall (1930, p. 31-39). Pressureless Steam from External Source Released into Soil 20. The buried-perforated- pipe method . . . a development from the Rudd type, en- joyed long popularity but is now less used because of the labor requirement. Modified forms, the "Hoddesdon pipe" in England and the "long pipe" in Europe, are extensively used, but probably will not replace the simpler Thomas method here. About 2Vi> sq. ft. per hr. per boiler horsepower have been treated by the pipe method. Pipes 1 in. in diameter with %-in. holes 9 in. apart, are laid 9 in. apart and 9 to 15 in. deep. Morris (1954) found that the cross-sectional areas of the pipe should be 1 Vi2 to 2 limes that of all the holes in it to insure [180 | Fig. 92. The inverted-pan method of surface steaming of soil in benches or beds (type 19). This unit may be fitted with a water pan and electric heating elements, to make a self-generating unit (type 24). Fig. 93. The buried-perforated-pipe method of deep steaming of soil in benches or beds (type 20). Fig. 94. The spike method of deep steaming of soil in benches or beds (type 21). The pan covering the soil surface is to increase efficiency. Fig. 95. Permanent buried tile method of deep steaming of soil in benches or beds (type 22). [1811 uniform flow of steam to all of them. The distance between pipes should not exceed the depth they are buried by more than 25 per cent. They may be single or joined in a grid framework (fig. 93). Pipes should be buried in trenches in well-worked, clod-free, mod- erately moist soil, and covered with rub- berized canvas. When the temperature is reached, pipes are pulled out and moved to the next setting, and the hot soil covered with a tarp. One Virginia grower uses perforated 2-in. aluminum downspout for the buried grid in out- door beds. Winch-drawn adaptations called "steam plows" are used in Den- mark and England, which eliminate digging up the pipe for each new setting. Buried pipes may be used with free- flowing, pressure, or superheated steam, operating in all cases without pressure in the soil. Advantages: deep treatment of Fig. 96. The moving-rake method of deep-steaming a field soil (type 23). The rake is pulled by a winch at the end of the field, the slanted blades penetrating to 14-in. depth. Fig. 97. Device for heating water with steam as it is injected into the soil (type 25). [182] soil: very efficient use of steam. Disad- vantages: high labor cost in burying and digging up pipes: soil must be handled after treatment, with recontamination hazard: danger of getting burned while moving grid. Best use: ground beds re- quiring deep penetration. References: Xewhall. Chupp, and Guterman (1940, p. 21-24); Morris (1954, p. 11-13); Bewley 1 1939, p. 4-11 I : Xewhall (1930. p. 26-29); Schmitz (1954); Coates (1954); Hansen (1953-54); Lawrence (1956, p. 119-20). 21. The harrow, spike, or rake method . . . is now little used. It consists of a risid pipe frame with vertical teeth on the lower side, which are plunged into soil. Steam is released through holes near tip of each tooth. In England this is mounted in a steam pan to reduce steam loss (fig. 94). Morris (1954) found that the space between pipes should not ex- ceed the depth of steaming by more than about 25 per cent, and that the spacing of spikes along the pipe should about equal the depth of treatment. The pipe cross-sectional area should be IV2 to 2 times the area of the holes fed bv it to insure uniform distribution of steam. A single pipe with vertical teeth, called a comb type, is also used in England. May be used with free-flowing, pressure, or superheated steam. Advantages: rapid and easy to use: inexpensive; fairly ef- ficient use of steam, particularly when enclosed by a pan. Disadvantages: serious steam "blow out" along spikes; holes plug with soil: awkward to move: danger of getting burned while moving grid. Xo longer recommended. Ref- erences: Xewhall (1930, p. 29-31); Bewley (1939, p. 11-12. figs. 6-7): Lawrence (1956, p. 109-10). 22. Permanent buried-tile method . . . has clay drain tiles buried end to end in ground beds 13 to 16 in. deep and in rows 18 in. apart 1 fig. 95). Tile is left permanently in place; used for deep steaming, as well as drainage, leaching, and subirrigation. Tiles may be placed in benches for steaming and then re- moved, but this is too laborious, and exposes the treated soil to handling. Soil is covered with rubberized canvas dur- ing steaming. About 1.3 sq. ft. per boiler horsepower per hr. have been treated. The connecting hoses should be removed when the steam is shut off, to prevent mud being sucked into the line as the steam condenses. May be used with free- flowing, pressure, or superheated steam, operating in all cases without pressure in the soil. Advantages: triple-function permanent installation; gives deep soil treatment. Disadvantages: very high initial cost; laborious installation; soil profiles disturbed; tiles must be reset after several years to remain functional. Best use: permanent ground beds under glass or lath. Reference: Xewhall, Chupp, and Guterman 1 1940, p. 11-21). 23. The moving-rake method . . . has recently been introduced in Florida for use on outdoor beds. The rake con- sists of a 4-in. header 12 1 / / o ft. long, on which are mounted, at 10-in. intervals, blades 16 in. long, set on a 20° forward angle (fig. 96). Descending immediately behind each blade, and bent to trail 14 in. to the rear at blade depth, is a 1 2^ n - steam pipe connected to the header. As the unit is pulled steadily forward by a motor-driven winch, the blades dig in. Forward progress is at about 25 ft. per hr.. treating about 320 sq. ft. (370 cu. ft. ) per hr., nearly 3 sq. ft. per boiler horsepower. A trailing canvas skirt covers the treated soil for about an hour, maintaining the temperature to a depth of 14 in. An adequate stationary, high- pressure boiler provides the steam, which is carried, preferably over the untreated soil ahead of the unit, by a 2-in. steam hose. Could also be used with free-flowing or superheated steam or [183] with steam-air mixtures. This unique method has many possibilities of devel- opment. It is suggestive of the winch- drawn "long-pipe" method (a buried- perforated-pipe type; type 20) used in Denmark and England. Advantages: deep treatment of field soil much easier than with buried perforated pipe; very efficient use of steam; cost per acre ap- parently no greater than for some fungi- cidal treatments. Disadvantages: initial cost of equipment; large steam boiler required; has moving parts to be main- tained; slow operation. Best use: field beds for high-valuation crops. Refer- ences: Ball (1955); Coates (1954); Anonymous (1955, 1956, 1957); Web- ber (1956). Pressureless Steam from Built-in Generator Released around Soil 24. The electric-inverted-pan method of surface steaming . . . is a steam pan (type 19, fig. 92) with an enclosed tray of water which is boiled by electric heaters. Uses about 1.8 k.w.h. per cu. ft. of soil. Advantages: much as for type 19. Disadvantages : messy opera- tion; high cost of electricity; limited to small operations because of power load; expensive heavy wiring required in glass- house; overheats soil under electric elements. Best use: small nursery using no bulk soil or containers, and without a steam source. Reference: Newhall (1940, p. 25-30). Hot-Water Drench of Propagating Sand Hot water transfers a maximum of only 152 B.t.u. per lb. to soil at 60° F, whereas steam yields 970 B.t.u. per lb. at 212°. Thus, at least V/ 2 to 2 gal. of water per cu. ft. of dry sand is needed in order to raise the temperature from 60° to 180°. Hot-water drenches of propagating sand leach out accumulated salts and, if continued until the sand reaches 180° F and remains at that temperature for 30 minutes, will destroy pathogens. If the necessary quantity of water to do this is troublesome, the sand may be leached and then treated with steam. 25. Equipment for converting steam to hot water . . . as it is applied to the bed is available (fig. 97), or hot water directly from a hot-water boiler may be used. Advan- tages : method is useful when only a hot- water boiler is available; leaches sand of soluble salts; may be used where steaming gives toxic effect. Disadvan- tages: extremely messy; not entirely de- pendable; if used on soil, may puddle it; not used on ground beds unless very well drained. Best use: sand in propagat- ing benches. Reference: Ball (1942, p. 12-16, 19, 23-25). Combined Steam and Formaldehyde Vapor 26. Equipment to volatilize water and formaldehyde . . . was described 19 years ago, but has been little tested. It is possible to drive formal- dehyde as deeply into soil as the steam, with increased effectiveness or shorter required treatment time and reduced cost of treatment. Most seeds may be sown within 24 hr., and the necessity of seed treatment is reduced because of the slight soil residue. Formaldehyde is added to water at rate of 1 pint per 40 to 80 gal. (0.4 to 0.2 fl. oz. per gal.) in- jected into flash-type boiler or into the steam line, and the vapor passed into 200 sq. ft. of soil; with steam pan this will penentrate to 10 in. depth. Growers near Toledo, Ohio, have injected formalde- CAUTION: Many of the < :hemicals mentioned in this manual are poi- sonous and may be harmful. The user should carefu lly Follow the pre- cautions on the 1 abe Is of the con- tainers. 184] hyde into the steam line leading to per- manent buried tiles in ground beds. They reported decreased time and cost, and increased efficiency over steam alone. This system has many possibilities, and should be further explored. The prin- ciple involved is discussed in Section 9. Advantages: cheaper treatment than steam alone. Disadvantages: cannot be used near living plants; may delay use of soil; special boiler or accessories re- quired. Best use: outdoor ground beds, or in houses completely emptied of plants; much experimental work needed. Reference: Beachley (1937); Anony- mous (1940). More recently Thomas began studies on the combined action of methyl bro- mide and steam against nematodes. Reference: Thomas (1954). MOVING SOIL MASS IN CONTINUOUS OUTPUT Because this type of treatment equip- ment applies heat to a moving soil mass, it is possible to terminate the process at any desired temperature by varying the heat input (through controlling the steam, electricity, gas, or oil) or the time of exposure (through regulation of the speed with which soil is moved). The principal disadvantages also arise from this feature: power is required for the soil movement; higher cost initially and in maintenance of moving equipment; greater risk of breakdown due to mov- ing parts; lower heat-transfer efficiency because of losses to air during move- ment. Pressureless Steam from External Source Released into Soil In this type of equipment the steam condenses more or less uniformly on the soil particles as they are tumbled about until their temperature reaches 212° F. From that point on the steam simply escapes to the surrounding air, and the soil temperature is not raised above 212° if continued longer. 27. The continuous-knife-injector type for flats . . . in which flats of soil are pulled under the injecting knives, was designed by the Department of Agricultural Engineering, University of California. Davis. Though not much used, this represents an inter- esting new approach to soil steaming (fig. 98) . The knives slice the soil in one direction as the flats are pulled under them, then a second set cut across the flat as it is drawn at right angles. Power is supplied by moving belts driven by an electric motor. Advantages: soil treated in containers; continuous operation; uses efficient external steam source; fairly efficient use of steam. Disadvan- tages: small output; expensive to build; power cost and mechanical upkeep of moving parts; cannot be used for bulk soil; probably insufficient soil volume for mechanized schedule. Best use: small bedding-plant nursery. 28. The horizontal-rotating- drum type . . . with steam released into the soil mass by knife injectors is a possible variant of type 30. The drum rotates on four rollers and is driven by a large sprocket and chain much as in type 30; in fact the basic machine of that type (available without burner) could well be modified for this. Soil is fed in through a hopper at one end, is carried through the ro- tating drum at a rate controlled bv the adjustable slope. In the center of the drum is placed a steam pipe on which are welded at right angles, 10 to 12 in. apart, flat hollow knives of sufficient [185] Fig. 98. The continuous knife-injector steamer for moving flats of soil (type 27). Fig. 99. The horizontal rotating-drum type of steamer with knife injectors (type 28). A continuous flow of bulk soil passes through the drum and into flats or other containers. [186] length to reach nearly down to the drum (fig. 99). These slice through the tum- bling soil and inject steam into it. The steam pipe is closed and welded to the hopper at the input end, and the open end is fastened to the frame at the other end of the drum. The steam is obtained from an external boiler, and may be of the free-flowing, pressure, or super- heated type. Some concrete mixers used for mixing nursery soils might be fixed with similar steam pipes. Advantages: continuous operation; heat can be con- trolled to any level above 160° F de- sired, by varying time in drum; steam is the heat source; may be used for simultaneous treating and mixing of soil; fits well into mechanization sched- ule. Disadvantages: containers not treated; initially expensive; power cost and mechanical upkeep of moving parts. Best use: if different sizes were available would be useful in many nurseries re- quiring bulk soil. Pressureless Steam from External Source Released around Soil *29. The rotating-screw type . . . for propulsion of soil through a pipe into which steam is injected. A com- mercial unit of this type is available in California. Gas or butane burners gener- ate steam in a tank beneath the soil pipe ; the steam bathes the pipe and is intro- duced into it through holes in the input end of the shell. The soil is propelled through the pipe by a screw rotated by an electric motor (fig. 100). Advan- tages: continuous operation; steam is heat source; degree of soil heating con- trollable; fits fairly well into mechaniza- tion. Disadvantages: containers not treated; initially expensive; power cost and mechanical upkeep of moving parts; propelling screw may wear badly. Best use: general nursery use for providing :,: One of the types considered best for Cali- fornia conditions. bulk soil. (1953). Reference: Anonymous Dry Source of Heat In equipment with a moving soil mass and dry heaters the soil particles are tumbled about so that they are heated uniformly by direct transmission during contact with the heating element. The heating is uniform through the mass, whether the soil is moist or dry. With dry soil there is some transmission from particle to particle when not in contact with the heat source, and this makes for uniformity through the mass. In moist soil there is the additional heat transfer by steam produced from water films around particles in contact with the heat source. This steam condenses on cooler particles, heating them uniformly. When all particles reach 212° F, the steam escapes to the surrounding air; if long enough continued the soil is desiccated. With either situation there is no danger of charring organic matter unless treat- ment is continued beyond 212°. 30. The horizontal rotating drum with internal blowtorch . . . also called the flash-flame pasteurizer, is used in the eastern states. It consists of a rotating drum of adjustable slope, into which soil is thrown at the high end, coming out at the low end. Into the drum from the low end is introduced a flame from a large kerosene blowtorch, which heats the soil (fig. 101) . The commercial unit is said to use 2% to 6 gal. of kero- sene per hr. and to turn out about 2 cu. yd. of soil per hr. at 175° to 190° F. A possible modification of this equipment (see type 28) answers most of the dis- advantages. Advantages: light weight, portable, convenient; continuous opera- tion; temperature of soil controllable by varying time in drum; fits fairly well into mechanization program. Disadvan- tages: containers not treated; kerosene flame directed into soil may leave an oily residue which is injurious to some [187] ?>>>»)>} >>>>>>>>>>>>> >>>>>>>>>>i{ kM — W ^ vl ^c c c u c k j^n r *M***y *M****p M»**M W^M W W I¥ a ^ ^^^ 100 Fig. 100. The rotating-screw type of steamer, continuous bulk output and self-generating (type 29). The steam prevents overheating in the tube, and is injected into the unit at the soil-input end. Fig. 101. The horizontal rotating drum heater with internal blow torch (type 30). The bulk soil is thrown in at one end, comes out continuously at the torch end. 188 1 102 &m_ ^^ 103 104 Fig. 102. The electric hot-plate soil heater giving a continuous output of bulk soil (type 32). Fig. 103. The rotating screw type of electric soil heater (type 33). A continuous flow of bulk soil is supplied. Electric heating elements are wound around the tube. Fig. 104. Method of stack- ing flats of soil to permit unrestricted steam flow without the use of separator strips. [189] plants (this might be avoided by using a gas flame) ; power cost and mechanical upkeep of moving parts; dries soil. Best use: small nursery using bulk soil and having some means for treating contain- ers; should first check for possible resi- dual toxicity to the crop bsing grown. Reference: Newhall and Schroeder (1951). 31. The horizontal rotating drum with external flame heat . . . has been little used. It consists of a rotary drum (sometimes an oil drum with the ends removed), through which the soil moves; heat is applied as gas or oil flame to outside of drum. Advan- tages: continuous operation; tempera- ture of soil controllable by varying time in drum; moderate initial cost; may be used for simultaneous mixing and treat- ing of soil; fits fairly well into mechani- zation. Disadvantages: containers not treated; power cost and mechanical up- keep of moving parts; dries soil; small capacity; heat torsion of drum. Best use: small nursery needing bulk soil and having means of treating containers. 32. The electric-hot-plate type . . . has been used in New York, where it is called the Hutchings type. Soil is pro- pelled by a chain drive in a thin layer along an elongated electric hot plate; it is fed into a hopper at one end and drops out at the other (fig. 102). Uses about 1.5 k.w.h. per cu. ft. of soil. In- frared lights have been used to impart additional heat to the soil moving on the plate; they would not provide sufficient heat by themselves for the purpose. Dia- thermy has been suggested, but the energy available is also insufficient for this purpose. Advantages: continuous operation; uniform, controllable, rapid heating. Disadvantages: dries soil; con- tainers not heated; small capacity; high cost of electric power (but could be op- erated with gas burners) ; power cost and mechanical upkeep of moving parts; no stones or lumps can be tolerated; capacity too low for effectively mecha- nized nursery. Best use: small nursery requiring only bulk soil and having means of treating containers. Reference: Newhall (1940, p. 30-32). 33. The rotating-screw type with electric heat . . . has been little used. It consists of a 4-in. pipe 6 ft. long wound on the outside with electric heating elements and housed in an insulating shell (fig. 103). A revolv- ing screw forces the soil from the hopper at one end through the tube and out at the other end, about 2 min. being re- quired. Uses about 1 k.w.h. per cu. ft. of soil. Advantages: continuous operation; uniform heating of soil; efficient use of heat; temperature controllable by chang- ing speed of screw; moderate initial cost. Disadvantages: soil containers not treated; power cost and mechanical up- keep of moving parts; sticks and stones must be removed from soil; screw wears badly; high cost of electric power; necessity of power wiring; dries soil; small capacity (4 to 7 cu. ft. per hr.) limits use in mechanization program. Best use: small nursery using bulk soil and having means of treating contain- ers. Reference: Tavernetti (1942). 34. The rotating-screw type with external gas heat . . . has been little used although a com- mercial unit was available for a time. The pipe, through which the screw forced the soil, was heated by a gas or oil flame. Advantages, disadvantages, and use as for type 33; heat produced torsion of equipment. 90 MOVING SOIL MASS TREATED IN BATCHES Dry Source of Heat 35. The horizontal rotating drum with external flame heat . . . has recently been introduced as a com- mercial unit in the eastern states. It con- sists of a drum which holds about l 1 /^ cu. ft. of soil. Soil is tumbled during heating, and the unit shuts off when soil is heated. Uses bottled gas for fuel, and treats about % cu. yd. of soil per hr. at 180° F. Fuel cost said to be about $1.10 per cu. yd. Advantages: final soil temperature automatically controlled; simultaneously mixes and treats soil. Disadvantages: containers not treated: power cost and mechanical upkeep of moving parts; small capacity. Best use: small nursery that has means of treating containers. Reference: Tarrant Mfg. Co. (1955). EQUIPMENT FOR GENERATING AND DISTRIBUTING STEAM Certain aspects of the generation and distribution of steam having special ap- plication to its use in soil treatment are here discussed. For more detailed in- formation consult standard reference books on the subject, or a heating engi- neer. Some of the principles involved should be understood by the grower for maximum results. Types of Steam-Generating Equipment There are many kinds of boilers that may be used for this purpose, but we shall here consider some of their general characteristics rather than specific types. High-pressure versus very low-pressure steam It was customary up to about the last decade to use high-pressure steam for soil treatment, and many growers even considered this to be essential for suc- cess. Now, however, it is recognized that: (1) such steam expands and the pressure is lost when released into soil, the temperature dropping to essentially that of free-flowing steam; (2) steam at 80 pounds transfers only 36 B.t.u. (about 3.7 per cent) more heat per pound than does free-flowing steam and is, therefore, not a significantly more effective me- dium of heat exchange (fig. 77); (3) the principal benefits from high-pressure steam are the faster distribution through the pipes (see below), and the evapora- tion of droplets of water through ex- pansion of the steam during the drop in pressure along the distribution line; (4) the use of larger distribution mains achieves the same improved flow without the need of pressure. From these con- siderations and from the experience of many growers has developed the present more economical practice, both in this country and abroad, of using low-pres- sure boilers with well-insulated mains of suitable size and the shortest length pos- sible. There are other reasons for using low- pressure equipment. Because such boil- ers do not require heavy pressure-resist- ing shells or pipes, they are less expen- sive and lighter in weight. The distribu- tion lines may also be of lighter con- struction I although larger in diameter), and there is less difficulty in making the system steamtight. Water may be fed di- rectly into low-pressure boilers from water mains, and controlled automati- cally by a float valve. If the boiler op- erates at above 15 pounds' pressure [191] there may be operational restrictions im- posed in some localities, and the boiler insurance is somewhat more costly. Be- cause of the higher temperature of steam under high pressure (323.9° F at 80 lb.), there is more heat loss in the lines than at lower pressures (227.1° F at 5 lb.), and greater insulation is required. In some cases, low-pressure steam is superheated to attain greater heat ex- change (47 B.t.u. more per 100° F; see fig. 77). Boiler costs may be about the same or a little less than for high-pres- sure steam. The lines must be larger and, because of the higher temperature, well insulated and as short as possible. Furthermore, such boilers are not used for heating glasshouses. For these rea- sons, superheat boilers are used spe- cifically for soil treatment. They may be permanently located at the site of treat- ment or may be portable, thus decreas- ing the length of main. Regular boilers versus "flash" steamers The so-called "package" and "flash" steamers are small portable units that include both boiler and burner, and de- velop steam in a short period of time. Generally they operate as free-flowing units, water being injected into one end of a pipe coil in a firebox, and steam coming out the other at very low pres- sure. The volume of steam may be large or small, according to the size of the unit. Types with a steam dome that collects and returns some of the en- trained water droplets are to be pre- ferred to those in which the steam comes directly from the end of the generating pipe, since they supply drier steam. Types that use part of the water in an external jacket may be more efficient in the use of heat. Such steamers are excel- lent in small nurseries without other sources of steam. The several types presently on the market may be operated on gas, oil, butane, propane, or elec- tricity (see Apnendix). They are de- signed for continuous output, and are not used in return-type steam heating systems. In southern California they are subject to inefficiency due to rapid scal- ing of the steam pipes from the high salt content of some water, particularly be- cause scale is collected from the large volume of new (rather than recirculated) water that is heated. In areas where scale is troublesome it may be desirable to use a water softener (Sec. 4). The "flash" steamers are initially much less expen- sive than regular types, but their useful life is generally less. There are many types of boilers on the market, both new and secondhand. These range from cast-iron sectional and steel fire-tube boilers for small opera- tions, to steel water-tube boilers for large installations. They may be vertical or horizontal types. They are usually in- ternally fired (for example, steel firebox and marine types), but may require construction of an expensive external brick firebox. Most of this type give long dependable service in supplying steam both for soil treatment and heating glass- houses. Wherever practical such boilers are preferable to less expensive package units. With regular boilers, as with the "flash" steamers, the problem of scaling is increased when steam is bled from the system for soil treatment rather than re- circulated, as in a heating circuit. It is possible to modify some (but not all) hot-water boilers so that they will generate steam for soil treatment. An automatic water-level feeder, a 15-pound safety valve, a water gauge, and a series of valves are necessary. The water level is lowered, and the upper part of the boiler serves as a steam dome, or a separate steam drum may be attached. With cast-iron boilers, the injected water should be preheated. Consult a heating engineer for any particular installation. It is usually better to run separate lines for the steam than to try to use the hot- water mains. [192] Stationary versus portable units Some types (for example, bricked-in boilers) are completely nonportable. Most other types may be portable in the smaller sizes, but not in the larger. Finally, the small "package" steamers referred to above are designed for port- ability. It is generally most convenient to restrict the use of portable boilers to small operations, or to divide the large job up into small parts that can be han- dled by such equipment. Both types of operation have a definite place in the California nursery industry. Aside from the low initial cost, there are other potential advantages in a portable unit. With such a unit, the large boilers need not be fired up for a small soil-treatment job during warm weather. The steam is generated at the job rather than being conducted there through mains (with loss of steam, and increased steam condensation in the process), or than taking the job to the steam source. In England it is possible to rent boilers for soil steaming. Custom steam- ing is done in England, Europe, and in New York state. Some English growers have also grouped together to purchase a boiler, much as farmers in the eastern part of this country have formed "spray rings" to buy equipment for their or- chard or potato spraying. Similar ar- rangements might be advantageous among smaller California nurseries. It is possible that someone could operate a profitable business supplying steam or renting boilers to small nurseries. For a number of years in southern California, a complete portable service unit for steaming soil in flats operated success- fully in a number of nurseries. A possible variation of this would be to provide small nurseries with uniform soil bins containing a pipe grid. When one was empty, the nurseryman would refill it with the desired soil mixture. He might then have the operator steam the soil. An additional kind of work available to the operator of a portable boiler would be the treatment of outdoor or lath-house soil beds for nurseries, and even in home yards (Sec. 8). Another variation would be to have a centralized soil service which would mix the specified soil and place it in a dump truck with a per- forated pipe grid (mobile bin; type 2) which would be connected to a large boiler for steaming. The soil would be tightly covered with a tarp and delivered, still hot, into the bins of the nursery, much as ready-mixed concrete is today. Variations of these types are operating in England, and presumably could do so here. Size of boiler required to steam soil Because soil steaming may be done in small or large quantity at each run, there can be no general statement of boiler size related to size of nursery. When a boiler is used solely for steam- ing soil, there is a fairly consistent re- lation between the time required to do a given job and the required size of boiler. However, in nurseries using steam heat there is a definite relation between size of boiler and area to be steamed, and the time required to do the job is more uniform. The question of the size of boiler re- quired is related to the volume of steam already discussed (table 14). Because the requirements vary with soil moisture, temperature, and type, with the over-all efficiency of generation and distribution, and with the distance from the boiler, general figures may be quite misleading. Furthermore, the rating of boilers in horsepower may still be based largely on area of heat exchange between the fire and the water 5 rather than on demon - 5 Even this method of calculation varies from 8.2 to 10 sq. ft. of heat-exchange area per boiler horsepower. [193] strated production of pounds of steam per hour. Because of these facts, the data are quite variable. Newhall (1953) gives l 1 /*? to 6 cubic feet of soil per boiler horsepower per hour as the range in commercial practice, with 3 cubic feet as a working average. If one assumes that each rated horsepower of a boiler gives 33.475 B.t.u., the over-all efficiency is then only 28.2 per cent for the average, and 56.5 per cent for the maximum vol- ume of soil treated. Published figures by other workers on commercial operations fall within the above range of boiler requirements. From these data it is pos- sible to estimate the size of boiler that will be required to treat a given area or volume of soil in the permissible time. The efficiency of the heat-exchange system from boiler to soil is highly variable. The boiler efficiency may range from 40 to 90 per cent, and is the main point of lost energy. There are further losses of heat in the distribution piping, these increasing with distance from the boiler and with decreasing pipe size; this may range up to or above 8 per cent of the boiler output. There may be large losses during injection of steam into soil, from escaping vapor, heating of structural material of beds, and so on; loss may range from 9 to 56 per cent of that introduced into the soil. Morris (1954) calculated that an over-all ther- mal efficiency of 41 per cent in the heat- transfer system from coal to soil was a good target for English growers effi- ciently using present equipment and a buried perforated-pipe grid; this would represent about 68 per cent efficiency in the part of the system; following the boiler. Table 14 presents data on the volume of soil that can be heated per hour, and the time required to heat I cubic yard, for several boiler capacities and thermal efficiencies. The efficiency of the boiler, distribution system, and soil-injection processes may be approximately calcu- late! (see Appendix). Such data should be useful in analyzing the heat-exchange process for possible increased efficiency. Type of Fuel or Power Used to Generate Steam Natural gas . . . is the least expensive fuel in California, but it is not always available without excessive piping cost. It may be used for stationary boilers but not for portable ones, and should never be piped into a glasshouse filled with plants in order to operate a portable unit. It is sold on a cubic-foot basis, each delivering 1,100 B.t.u. Fuel oil .. . is commonly used for steam boilers be- cause it is fairly inexpensive. It requires a storage tank and fairly expensive burners, particularly for the heavier cheaper oils. It is quite readily used in portable units. It is sold on a gallon basis, providing about 141,000 B.t.u. per gallon for the No. 3 grade. Kerosene . . . is used by one commercial portable unit for soil treatment (rotating-drum type with internal flame; type 30), but is otherwise not commonly used. It is sold on a gallon basis, each yielding about 130,000 B.t.u. Coal . . . (bituminous) is used solely for sta- tionary boilers in this country, and both stationary and portable units in England and Europe. It is both dirty and incon- venient to use and requires a great deal of attention, even with automatic stokers. It is sold on a ton basis (2,000 lb.), yielding about 14,000 B.t.u. per pound. Butane . . . is used where natural gas is not available and the boiler is not equipped for oil. It can be used for portable units. It yields about 102,000 B.t.u. per gallon. [194] Propane . . . or bottled gas, is usually the most ex- pensive of the fuels. Uses as for butane. It is sold on a gallon basis, yielding 91,800 B.t.u. per gallon. Electricity . . . is the most convenient, cleanest, and most expensive source of power. Its use is restricted to small boilers where for some reason other sources of power can- not be utilized, and to self-generating soil-treatment units (types 10, 11, 14 to 16, 24, 32, 33). It is sold on a kilowatt- hour basis, each k.w.h. yielding 3,411 B.t.u. Because of the heavy power de- mands of any sizable unit, very heavy wiring must be provided, and with some units (for example, electrode type; type 15) a transformer may also be required. Power requirements reported for the various types of equipment range from 1.0 to 4.0 k.w.h. per cubic foot of soil, with a working average of 1.5 to 2.0 k.w.h. Distribution of Steam The objective is to deliver the steam at some distant point from the boiler with a minimum loss of heat, pressure, and rate of flow, and a minimum of condensation. Among the many factors that influence the flow of steam in the pipes are the following. Length of the pipe Lengthening the pipe sharply in- creases the rate of pressure drop and heat loss. Thus, Morris (1954) calcu- lated the heat loss for an uninsulated 1%-in. steel pipe carrying 25 lb. steam per min. at 100 lb. pressure to be 3.1 per cent at 100 ft., 5.8 per cent at 200 ft., and 8.3 per cent at 300 ft., the pressure drop was 21, 46, and 95 lb. in the same distances. A 2 1 /^-in. uninsulated steel pipe with 10 lb. pressure and the same flow lost 2.3 per cent of its heat in 100 ft., 4.6 per cent in 200 ft., and 6.9 per cent in 300 ft.; the pressure drop was 2.8, 5.9, and 9.3 lb., respectively. By con- trast, an uninsulated 2^-in. aluminum pipe carrying 25 lb. steam per min. at 10 lb. pressure lost 1.2 per cent of its heat in 100 ft., 2.5 per cent in 200 ft., and 3.7 per cent in 300 ft. Diameter of pipe Increasing the diameter of the pipe decreases the pressure drop and heat loss; there may be a 4- to 6-fold increase in the rate of flow by increasing the pipe size from 1 inch to 2 inches. Senner (1934) found, furthermore, that a main used for soil steaming would carry ap- proximately 4 times as much steam as one of the same size used for glasshouse heating. This was due to the free flow of steam, unrestricted by the back pressure of a closed system. This factor should be considered if the mains are to be used only for soil steaming, cannot be if they also serve as heat mains. Type of pipe The kind of pipe greatly influences the heat loss, as the above figures show. Morris (1954) has suggested the use of light alloy aluminum irrigation pipe for steam mains in low-pressure systems, because the heat loss is about half that from steel pipe and it is much lighter in weight. Such pipes, with light insula- tion, are also used in the Scandinavian countries for connecting portable boilers to outdoor beds for steaming. Steam pressure The rate of flow increases approxi- mately 4-fold with increases in steam pressure from 10 pounds to 100 pounds, with constant pipe size. The past practice of using high steam pressure and rela- tively small pipes is giving way to low pressure and large pipes. The larger pipe size does not increase heat loss owing to the larger exposed surface, be- cause this is offset by the lower tempera- ture involved. If insulated aluminum [195] pipes are used the efficiency of steam distribution will be greatly improved over the old method (3.7 per cent heat loss in 300 ft. against 8.3 per cent; see above) . Internal roughness of pipe The flow of steam is decreased by the internal roughness of the pipe and by valves, elbows, tees, and reducers in the line, since they increase the friction. Quality of steam Steam quality also affects the rate of flow, decreasing it as the number of water droplets increases. Since super- heated steam is relatively drier than saturated steam, it flows faster. Water in the steam lines Water condensation in the steam lines should be prevented for the above reasons, because it represents lost heat, and because it affects the efficiency of soil steaming. The condensed water car- ried into the soil has a temperature of 212° F and, therefore, adds no heat; the 970 B.t.u. per pound (fig. 77) from the steam has been lost in transit. Further- more, the soil is made wetter than it would otherwise be. The steam line should always be drained and bled until dry steam appears, before it is connected to the treatment equipment. This will prevent the injection of cold water into the soil; in this connection it should be recalled that it takes five times as many B.t.u. to heat a pound of water as a pound of soil. Furthermore, a water trap should be placed in the line at a point just preceding the treatment equipment, the condensate either going into the re- turn line or being wasted. If there is considerable condensation in the line this procedure becomes particularly de- sirable. Designing the steam distribution system Many of these factors can be resolved for lines used solely for soil treatment by operating the boiler at pressures be- low 10 pounds, and using large, well-in- sulated pipes (perhaps of aluminum alloy) that are as short as possible. However, if the pipe is too large, the heat loss from the increased surface exceeds that saved by reduced friction. The losses may also be reduced by using a portable boiler close to the soil being treated. A heating engineer should be consulted for the specific design. SOIL TREATMENT IN A MECHANIZED NURSERY Mechanization of every practicable nursery procedure is of increasing in- terest to the industry, due to rising wages for labor and smaller margins of profit. Any successful mechanized nursery pro- gram must include, indeed must be built around, soil treatment, for reasons out- lined in sections 2 and 3. A number of methods and pieces of equipment pre- viously described for soil treatment integrate very well into mechanized nur- sery practice, and 13 of these are shown schematically in figure 126. By following the arrows in that figure, the routing of a particular procedure may be visualized. It should be emphasized that untreated flats, cans, or pots may be used in the procedure prior to soil treatment, but that treated soil should not be dumped into untreated containers or on an un- treated floor (Sec. 12). Mechanization is further discussed in Section 17. [196] SECTION Chemical Treatment of Nursery Soils Donald E. Munnecke Fungicides Nematocides Insecticides Soil drenches around living plants s OIL may BE treated with chemicals to rid it of fungi, bacteria, insects, nema- todes, and weeds. The effectiveness of chemical soil treatment generally de- creases as the size of the treated area increases. Although early trials of soil fumigation in the field attempted to eradicate pathogens and insects, experi- ence has shown that this is impractical if not impossible; a few chemicals are, however, effective in reducing field in- festations, and some of them are men- tioned briefly in this section. On the other hand, pathogens can be eradicated from soil in containers such as flats, where the chemicals do not need to penetrate large soil masses. Steam treat- ment is usually more satisfactory, but there are many occasions where chemi- cals can be used more cheaply and effectively and their use has increased tremendously. The cost of chemical fungicide treat- ment using methyl bromide or chloro- picrin varies from approximately 1 to 3 cents per cubic foot, according to the method and chemicals used (table 16). The present practice is to use the chemi- cal and dosage which will give the greatest net return to the grower, as determined by practical experience. It is difficult to recommend single dosages of chemicals for all soils and all condi- tions. In general, the lower dosage recommended herein is for use with a U. C.-type mix; the higher dosages are recommended for use on clay soils or soils containing undecayed organic matter. An ideal chemical for treating soil is one that kills a variety of fungi, bacteria, insects, and weeds; is inexpensive and harmless to the operator and equipment ; is quick-acting and effective deep in the soil as well as on the surface ; is harmless to near-by plants; and is nontoxic to subsequent plantings in the soil. None of the presently known chemicals fulfills CAUTION: Many of the chemicals mentioned in this manual are poi- sonous and may be harmful. The user should carefully follow the pre- cautions on the labels of the con- tainers. [197] Table 1 6. Comparison of the Cost in 1 955 of Methyl Bromide and Chloropicrin Used to Treat Soil for Fungus Control Exclusive of labor and cost of accessory equipment Cost Dosage Flats* Bulk Surface Chemical Cents per flat Cents per cu. ft. Cents per sq. ft. $1.65 per lb. 3 cc 1.8 1.8 (1-lb. lot) 5 cc 3.0 Chloropicrin $1.25 per lb. 3 cc 1.4 1.4 (25-lb. lot) 5 cc 2.3 $1.02 per lb. 3 cc 1.1 1.1 (100-lb. lot) 5 cc 1.9 $0.80 per lb. 4 lb. per (1-lb. lot) 100 cu. ft. 2.2 3.2 3.2 Methyl bromide $0.72 per lb. 4 lb. per (50-lb. lot) 100 cu. ft. 1.9 2.9 2.9 * Chloropicrin not recommended for flats. Flats 18 X 18 X 3 inches. These figures based upon 40 flats per cu. yd. of air space. In actual practice the cost varies with the way the flats are stacked and the dosage used. The dosages given are sufficient to eradicate pathogens in confined areas. The lower dosage of chloro- picrin is sufficient for this purpose with a U.C.-type mix; the higher dosage may be necessary for clay soils or those high in manure or other undecayed organic matter. all of these requirements, but many ful- fill enough of them for practical use. Much research is being done on this problem and it is probable that some materials will be available in the future that approximate this ideal. The most common chemicals, dosages, and treat- ments used to control diseases and nema- todes in nursery soils are summarized in table 17 at the end of this section. Special chemicals and dosages required to kill fungi Fungi, such as Rhizoctonia, Fusarium, Armillaria, and Verticillium are, in gen- eral, more difficult to kill in the soil with chemicals than are insects, nematodes, and most weed seeds. This fact must be kept clearly in mind when a chemical is chosen for soil treatment. For example, it is a waste of time and money to use ethylene dibromide, an excellent noma- tocide, to try to control damping-off, which is caused by fungi. Also, low dosages of methyl bromide, an excellent soil fungicide, may be used for weed and nematode control, but two to four times as much may be required for fun- gus control. Growers often erroneously believe that if weeds and nematodes are controlled, the fungi are also. If fungi are controlled, however, insects and nematodes are usually eliminated. // the main problem is fungus control, suitable fungicides at recommended dosages must be used. Nematocides or herbicides can- not be used to control fungi. Soil preparation, temperature, and aeration The condition and temperature of the soil must be considered in using chemi- cals for soil treatments. A good rule to follow is that the soil should be in good planting condition before treatment. The soil should be in good tilth, and there [198] should be no lumps or clods. Too much soil water prevents thorough diffusion of the gas (Sec. 9), whereas too little moisture on the soil surface allows the gas to escape. Best results are obtained at soil temperatures of 65° to 75° F. Where soil is likely to be cold and wet for long periods, chemical treatment may be facilitated by storing it in bins in a heated shed. It may then be treated in place. After treatment the soil must be thoroughly aerated so that all trace of the fumigant is gone before planting. In general, the higher the temperature and the lighter the soil, the shorter the aeration period. In all cases the chemi- cals should be handled with care, as should any poisonous substance. After-effects of treatment In some cases an increased growth response is apparent in chemically treated soils (fig. 119). This is not be- cause the chemicals act as fertilizers, but probably because the soil-borne patho- gens and pests are eliminated and a more favorable balance of the other micro- organisms is obtained (Sec. 14). Occasionally soil treated with chemi- cals is toxic to subsequent plantings. This toxicity is usually due to insufficient aeration of the soil after treatment. Soils which are high in some organic ma- terials or clay, excessively wet, or treated at low temperatures may contain toxic amounts of the chemicals several weeks after application. These ill effects are due to a residue of the gas, to a breakdown product of it, or to secondary reactions of soil microorganisms causing such re- sults as ammonium accumulation. Usu- ally this toxicity may be avoided by applying the chemicals correctly and by delaying planting until all odor of the chemicals is gone. The use of a U. C.-type soil mix greatly reduces the chance of injury from chemical treatments, another reason for adopting such a system. Recently the chemicals containing bromine (methyl bromide, ethylene di- bromide, chloro-bromo-propene) have been found to leave a residue which is extremely toxic to certain plants (espe- cially carnations) ; these compounds should not be used on soil to be planted to carnations. Although a large number of bedding plants and other crops have been planted in chemically treated soil without harm, it is a good precaution to use chemicals on a small scale on an un- tried crop and note the results before treating large quantities of soil. Formaldehyde treatment of floor Whenever treated soil is dumped in bulk piles on the floor, the surface should previously have been wet down with a formaldehyde solution (1 gal. to 18 gal. of water) . FUNGICIDES A wide variety of chemicals is mar- keted for soil treatment of one, sort or another. The various chemicals are here grouped according to the purpose to which they are suited. First let us con- sider fungicides. The soil troubles caused by fungi are, in general, the most difficult to control with chemicals. Some fungi are able to survive in soil for many years in the ab- sence of their host plants. Most of them form thick-walled resting bodies which are resistant both to unfavorable en- vironmental conditions and to chemical treatments (Sec. 3) . A number are capa- ble of growing or persisting deep in the soil, well below the depths reached by the surface treatments of the soil. The most effective soil fungicides are fumi- gants which act as gases in the soil. [199] Chloropicrin, methyl bromide, and formaldehyde are the most widely used soil fungicides. These compounds are liquids whose gaseous phases diffuse through soil. When properly applied in adequate dosages, they control most of the fungi, nematodes, and weeds. The gases have to be confined in some way during the treatment period. After treat- ment the soil must be thoroughly aerated before planting. For sources of soil-treatment materials in California see the Appendix. It is not necessary to get a license to use these chemicals. Chloropicrin Chloropicrin, or tear gas, is extremely toxic to soil fungi, insects, weed seeds, and nematodes. It penetrates bulk soil readily, but does not readily penetrate plant tissue such as unrotted nematode galls. It is neither explosive nor in- flammable, but it is difficult to use since it is a potent tear gas. To avoid eye irritation, start working on the windward side of the planting with the back to the wind so that the fumes are carried away from the eyes. Do not rub eyes when affected; rather, turn and face the wind and let the effects wear off. Since it is very corrosive to most metals (but not to stainless steel), metal equipment must be washed with kerosene after using. It cannot be used in the vicinity of living plants; conse- quently, all plants in a glasshouse must be removed before treatment. It may be used in semienclosed areas, such as lath houses, provided there is ample air movement to dissipate the gas, and ad- jacent plants are over 3 feet away from the treatment area. Heavy soil absorbs large quantities of the gas and requires larger dosages and longer aeration periods than light soil. Chloropicrin may be applied by ma- chines in field applications, or by hand- operated injectors (fig. 105). For sur- T K> 1 Fig. 105. Hand-injector for applying soil fumigants, at left. Proper spacing shown at right for hand-injection of fumigants into soil in beds, benches, or fields. [200] face applications the dosage is 3 cc per 12-inch square, injected 6 inches deep; for bulk soils the dosage is 3 to 5 cc per cubic foot. Bulk soils may be treated in bins, drums, garbage cans, or any gas- proof receptacle that can be tightly sealed. The gas may be confined in fields by wetting the top inch of soil or by en- closing beneath a gasproof cover (poly- ethylene types, such as Visqueen, are most common) for 1 to 3 days. In gen- eral, plantings must be delayed 7 to 10 days after treating or until all traces of the gas have disappeared. Do not use if soil temperatures at 6-inch depth are below 60° F; best results are obtained at 70°. Chloropicrin is specifically recom- mended in soil used for chrysanthemums (dosage: 3 cc on 12-inch centers, 6 inches deep) and carnation plantings in California, since methyl bromide is sometimes unsatisfactory for these crops. With proper application and aeration, chloropicrin-treated soil gives excellent disease control and plant growth, and it is generally considered as the experi- mental standard when comparing soil fungicides. However, it is difficult to handle, it requires much labor to apply, and the treatment periods are long. Aeration of the soil after treatment is essential since a number of crops have been lost through faulty aeration. Con- sequently, it is not recommended for flat-soil operations nor for other circum- stances where soil must be used soon after treating. See table 16 for approxi- mate cost of chloropicrin. Methyl Bromide Methyl bromide is widely used, espe- cially for flat- and bulk-soil sterilization. It has been approved by the California Department of Agriculture for treatment of soil in which certain plants may be grown if they are to be shipped, using the Intercounty Nursery Stock Certifi- cate or "pinto" tag (sees. 3 and 8). It is effective against most of the soil pests, is simple to use, and has the shortest treatment and aeration period of the present soil fungicides. Although it is extremely toxic to man, there is little danger if it is handled with reasonable care. Take adequate precau- tion to prevent exposure of children and other persons to the poisonous gas. Post warning signs. It is marketed in several forms (pure methyl bromide, methyl bromide with 2 per cent chloropicrin, or as a liquid in various solvents), but the pure gas is almost exclusively used in California and the following recommendations apply to this gaseous form. Field soils, ground beds, flats, pots, cans, tools, and even large trucks and farm machinery may be satisfactorily treated with methyl bromide by enclos- ing them beneath a gasproof cover (polyethylene is cheapest; polyvinyl chloride and vinyl-coated nylon are more durable, but more expensive) and injecting the gas beneath the cover through special applicators from cans or cylinders (figs. 106 and 107). The cover must be examined carefully for holes; these may be sealed with masking tape. The cover must be sealed tightly with soil around the edges before apply- ing the gas. Never use the gas at soil temperatures below 50° F, preferably around 70°. For treatment of contain- ers see Section 12. Treating stacks of flats Up to 400 flats can be adequately treated in one stack as follows. Fill the flats with soil moistened in preparation for planting, but not pud- dled or rendered soggy. Place them on a level surface or hard dirt area and stagger them as shown in figure 107, or use lath spacers between flats to allow free circulation of the gas. Fasten the outlet of a rubber or plastic hose to the top of the center flat so that it points away from the soil. Place burlap or rags over the corners of the flats to prevent [201] damage to the cover. Put the cover loosely over the top and seal the edges with a dirt seal or with "sand snakes" (canvas or plastic tubes 3 inches in di- ameter filled with sand). The gas may be more effectively ap- plied by heating it in one of several ways before injecting beneath the cover. The simplest way is to immerse the cans in a bucket of water heated to 140° to 160° F, or to route the gas through a copper coil immersed in hot water (fig. 106) . If large cylinders are used, the gas may be circulated through heated coils in the same manner. A supplementary heating device may be used to keep the water hot. Avoid turning cans upside down while injecting. If cans are turned upside down, liquid methyl bromide will flow through the hose. This reduces effective- ness and may cause the soil to be toxic near the hose exit. To calculate the amount of chemical to use, multiply the length, breadth, and height of the stack in feet, including the air space beneath the cover. Use 4 pounds of methyl bromide per 100 cubic feet for fungus control (damping-off, root rot, etc.), or 1% to 2 pounds per 100 cubic feet for weed and nematode control. A special metering device (fig. 106) is advisable for use on cylinders, or a scale may be used to actually weigh the gas as it is being dispensed. Place the cylinder on a leveled scale, note its weight, open the valve and allow the gas to flow until the scale indicates that the desired amount of gas has been used. Close the valve and recheck the weight. The initial weight minus the final weight equals the weight of methyl bromide ap- plied. After a 24- to 48-hour treatment pe- riod, remove the cover and allow the flats to aerate for 24 to 48 hours, when they are ready for planting. Fig. 106. Equipment for applying gaseous methyl bromide to nursery soils. Left, gas metered from a cylinder, for large operations. Right, applicator for 1-lb. cans of gas for small operations. T 202 1 Fig. 107. Diagrams showing the methods for stacking, covering, sealing, and injecting flats of soil with gaseous methyl bromide. Treating bulk soil and ground beds Bulk soil and ground beds may be treated in the same way. Pile bulk soil 1 to 2 feet deep, be sure that it is not excessively wet or cold, and treat as shown in figure 107. For ground beds, measure the area beneath the cover and apply at 4 pounds per 100 square feet for fungus control, or 1% to 2 pounds per 100 square feet for nematode and weed control (fig. 125). Effectiveness Methyl bromide kills nematodes even in unrotted galls, but it does not kill V erticillium albo-atrum (which causes wilt of chrysanthemum and numerous other crops) and it may leave the soil toxic for carnations. Consequently, it is not recommended for carnations or chrysanthemums. It is recommended for bedding-plant operations where a rapid turnover of soil and flats is required and the short treatment period is neces- sary. The recommended dosage of 4 pounds per 100 cubic feet will control all of the common soil fungus diseases except V erticillium wilt. Excellent weed con- trol is obtained except for Malva and bur clover. The user should not think treat- ment unsuccessful if these weeds sur- vive, but if many other weeds survive, it indicates that the application was faulty. Lower dosages have been suc- cessfully used in warm sandy soils, and growers have found that dosages as low as 2 pounds per 100 cubic feet may be effective. Table 16 shows the approximate cost of methyl bromide based upon the maxi- mum dosage. Formaldehyde Formaldehyde is the chemical that has been in longest use for soil fumigation, and is still used as a drench for rooting [203] media and glasshouse benches. It is a water-soluble liquid that penetrates the soil as far as the water carrier and volatilizes rapidly, one of the principal advantages of this material. Since the fumes are very toxic to near-by foliage, it has restricted use in glasshouses with living plants. It has been successfully used in large glasshouses when the vents were fully open and the treatment area was somewhat removed from living plants. Formaldehyde is very irritating to eyes and nasal passages, and is ob- noxious to use for this reason. Commercial formaldehyde (37 to 40 per cent formaldehyde in water solu- tion) at 1 pint in 6 1 / 4 gallons of water is applied at the rate of % gallon per square foot of soil surface. Soil is cov- ered for 24 hours with plastic covers or gas-resistant paper and then it is aerated by thorough stirring. The aeration period is from 10 to 14 days. The residual effects are very dam- aging to seedlings and transplants and care must be taken that all odor of the gas is gone. A sure way to determine whether soil is safely aerated is to plant a few seedlings in it and see whether in- jury results. A dilute method has been prescribed for damping-off control which may be effective with lightly infested soils. It is ineffective in heavily infested soil. By this method 2 tablespoons (1 fl. oz.) of formaldehyde in % cup (6 fl. oz.) of water are sprinkled over a cubic foot of moist soil which has been spread in a thin layer; the soil is then mixed thor- oughly, and stacked in clean flats and covered with a tarpaulin or wet paper. After 24 hours seeds may be sown, pro- vided the flats are watered thoroughly afterwards. Vapam Vapam, sodium N-methyl dithio- carbamate dihydrate, has recently been released for commercial use as a general soil fumigant. It decomposes in soil to form methyl isothiocyanate gas, which is the active killing agent. It has been successfully used to control weeds, nema- todes, soil insects, and soil fungi, al- though it is not recommended for eradi- cative treatments. Since Vapam is solu- ble in water, it may be applied by sprin- kling or irrigating on the surface of the soil or by injecting into soil with stand- ard equipment. Consequently it is one of the most versatile of the soil fumi- gants. It has sufficient advantages to warrant use, especially on crops grown in the field or in ground beds. Although Vapam may be used for treating flat or potting soil, methyl bromide is usually more satisfactory for these purposes. Vapam is sold as a liquid containing 4 pounds of the active material per gal- lon of water. Dosages are usually ex- pressed as quarts or gallons of this for- mulation to be applied to a given area of soil. The dosage required depends upon the organisms which are to be con- trolled; for example, for weed control it is from 1 to l 1 /^ pints per 100 square feet and for fungus control it is from 1 to 2 quarts per 100 square feet. Treating beds or benches For small areas, such as ground beds or benches, 1 quart of Vapam in 2 to 3 gallons of water in a sprinkling can may be applied uniformly over the surface of 100 square feet of soil and immediately followed with enough water to wet the soil to a depth of 6 inches. This dosage is approximately equivalent to 100 gal- lons per acre, which is a good median application for fungus control. A hose proportioner may be used in the same way with a stock solution of 1 quart of Vapam in 1 quart of water. The proportioner ratio should not exceed 1:20. Treating fields For field applications either overhead sprinklers or soil injectors may be used. Overhead sprinkler applications may [204] be made by introducing Vapam into the lines. The field is sprinkled for 5 to 10 minutes, then the required amount of Vapam is introduced in the next 10 to 20 minutes. The sprinklers are then left on until the soil is wet to a depth of 6 to 12 inches. Vapam may be applied with stand- ard knife-blade soil injectors set 6 inches apart and 4 inches deep. The injection method is useful because rows or strips may be easily and inexpensively treated. The row-treatment method has been used with many field crops such as cot- ton and beans, but surface applications Lave been more widely used with orna- mental crops. As with all effective chemical soil treatments, the soil should be in seedbed condition at the time of treatment. Opti- mum soil temperatures at the 4-inch depth are from 55° to 65° F, although Vapam has been successfully used at temperatures of 45° to 70°. The tem- peratures of California soils at planting time do not ordinarily restrict its use, although high temperatures may vola- tilize the gas too rapidly, and surface ap- plications should not be made on hot days. Seven days after treating, or when soil has dried to a workable condition, the soil should be cultivated lightly to break the crust and facilitate aeration. Usually treated soil may be planted 14 days after treatment unless prolonged rains have prevented adequate aeration. In such cases it is desirable to wait until all odor of the gas is gone, or to make test plantings before planting the main crop. Terraclor Terraclor (PCNB), pentachloronitro- benzene, has recently been marketed as a soil fungicide. It differs from most soil fungicides in its specificity of action (for example, Rhizoctonia is con- trolled, Pythium is not), its low toxicity to many plants, and its relatively long residual activity in soil. Another peculi- arity of the compound is that it inhibits growth of Rhizoctonia, but does not kill it. Because of these factors it may be used in several ways for disease control, provided water molds are not present. The compound is new and it must be tried on many more ornamental plants before it can be generally recommended. The results on carnations obtained by Sciaroni and Raabe (1955) and Scia- roni (1955) are cited here as a guide for use on other crops as well as for carna- tions. As a preplanting treatment the 75 per cent wettable powder PCNB may be dusted or sprayed on the soil surface at the rate of 1 to 1% pounds per 1,000 square feet and mixed to a depth of 1 to 2 inches by raking. Carnation plants may be set immediately in soil treated in this manner without damage. Damp- ing-off caused by Rhizoctonia has also been effectively controlled in sweet-basil plantings by application of the 75 per cent wettable powder at 1 to 1% pounds per 1,000 square feet immediately before seeding. No seedling injury has been ob- served. In addition to the soil-surface method of application, PCNB has been applied as a strip treatment in the planting fur- row at the time of planting. A general recommendation for this method with beans and cotton is to use 5 pounds of the 75 per cent material in 10 gallons of water per acre. A unique use of PCNB has been its application as a protective fungicide on soil previously treated with steam or chemicals such as chloropicrin, Vapam, or methyl bromide. To protect against infestation (sees. 3 and 14) of such treated soil by Rhizoctonia, 1 to 1% pounds PCNB (75 per cent wettable powder) is applied to 1,000 square feet of soil and raked into the top 1 to 2 inches just prior to planting. [205] NEMATOCIDES Chloropicrin and methyl bromide eco- nomically control nematodes in con- tainer soils when other pathogens must also be killed. For controlling nema- todes alone in field soils, however, these materials are too expensive. If nematodes or soil insects are the problem one of these nematocides may be used; but it should not be used for control of damp- ing-off or other fungus diseases. For these reasons the recommendations given for nematocides deal with field ap- plications only. The same general soil preparation as outlined for fungicides is required for successful treatment for nematodes. Ethylene Dibromide Ethylene dibromide is a liquid which is applied to soil by a hand-injector or by a continuous-flow applicator at the rate of 3V2 to 7 gallons of actual ethylene dibromide per acre. Since it is not so volatile as chloropicrin or methyl bro- mide, special covers or seals are not re- quired for its use. This relatively low volatility necessitates long aeration pe- riods of 2 to 3 weeks before planting. It should not be used on soil to be planted to carnations. D-D Mixture D-D mixture (dichloropropane-dichlo- ropropene) was the first of the cheap effective nematocides. It is applied by hand-injection or continuous-flow appli- cators at the rate of 200 to 400 pounds per acre. Planting must be delayed 1 to 2 weeks. Nemagon and V-C 13 Nemagon (l,2-dibromo-3-chloropro- pane) and V-C 13 (0-2,4-dichlorophenyl 0, 0-diethyl phosphorothioate) are newly marketed nematocides which are said to be nontoxic to many plants when applied to soil around established root systems. They represent promising developments in nematode control, but cannot yet be recommended for general use. General Effectiveness of Nematocides These chemicals are very cheap and, although nematodes are not completely eradicated by treatment, sufficient con- trol is obtainable to return the cost of the treatment many times. Many field-flower growers are now using metering valves mounted above the plowshares to drip soil fumigants into the furrow during the plowing op- eration. This practice fits in well with the culture of most crops and has re- duced the cost of nematode control, since expensive injection machines are not necessary. INSECTICIDES The common soil insects are all killed problem, two recently published pest- by treatments which are effective against control guides (Jefferson and Pritchard, fungi and nematodes, hence the specific 1956; Pritchard, 1949) may be con- soil insecticides are not discussed here, suited for a description of the insecti- In the event that insects are the main cides and treatments. [206] SOIL DRENCHES AROUND LIVING PLANTS A few fungicides have been used as soil drenches. These are applied to the soil around the growing plant and are used primarily to prevent enlargement of an existing infestation. They are ma- terials of limited volatility which kill by direct contact with the parasites. They are not recommended as primary dis- ease-control materials, but they may be useful in checking the spread of disease that arises in a planting in spite of previ- ous precautions. Soil drenches of this type must penetrate the soil, kill or in- hibit the fungus parasites, and be non- toxic to the existing plants. Drenches may suppress pathogens but do not eradi- cate them, since none of the present ma- terials are able to penetrate soils more than a few inches. In addition, many of them are rapidly inactivated in soil. The most commonly used materials for soil drenches are ferbam (Fermate), Semesan, thiram (Arasan), captan (Ortho 406), nabam (Dithane D-14), and Terraclor (PCNB). Terraclor (PCNB) may be used as a soil drench for Rhizoctonia control on living carnation plants at the rate of 1 to 1% pounds of the 75 per cent for- mulation per 1,000 square feet. When used in this way the obviously infected plants should be removed and the chemi- cal applied to an area extending 1 to 2 feet past the infested area. Ferbam, thiram, and captan are usu- ally applied at 1 tablespoon per gallon of water at the rate of V2 to 1 pint per square foot. Nabam may be diluted to concentrations of 1:500 (V2 tablespoon per gallon) and applied at the rate of y<2. to 1 P mt P er square foot. Semesan at 1 tablespoon per gallon of water is used at the rate of % P mt per square foot. Do not use Semesan on roses in enclosed or poorly ventilated areas since the mercury may volatilize and cause severe injury. Semesan may be effectively used against Rhizoctonia damping-off on such plants as stocks, but plants such as pansy, petunia, and snapdragon may be stunted by its use. Consequently, be careful in using Seme- san if the tolerance of the crop to the chemical is unknown. In using drenches, remove all obvi- ously diseased plants, drench the in- fested area for 1 to 2 feet past the edges of the infestation and repeat 7 to 10 days later. Since nabam penetrates deeper than the solid materials, but has less residual effect, a combination of it with one of the other materials (for example, nabam, y± tablespoon, plus captan, 1 tablespoon per gallon of water) may be successful. It must be remembered that these spot treatments are not eradicative but tempo- rary inhibitory treatments at best.li soil- disease problems persist, they indicate that some major contamination exists in the operation of the nursery and efforts should be made to see that soil, plants and plant parts, and greenhouse are made free of contaminating organisms (sees. 8 through 13). [207] Table 17. Summary of Chemicals, Dosages, and Treatments Used to Control Diseases and Nematodes in Nursery Soils Chemical Recommendations SOIL FUMIGANTS— FUNGUS CONTROL Chloropicrin Formaldehyde Methyl bromide (gaseous) Terraclor (PCNB) Vapam DOSAGE: Field: 3 cc per hole on 12-in. centers, 6 in. deep. Bulk soil : 3-5 cc per cu. ft. TREATMENT PERIOD: 1-3 days, confine with cover, wet news- papers, or water seal. AERATION: 7-10days or until all odor of gas is gone. Recommended for carnation, chrysanthemum, field soil applications. DOSAGE: As drench: 1 pint in 634 gal- water, applied at the rate of Yz gal. per sq. ft. TREATMENT PERIOD: 24 hr. AERATION: 10-14 days. Stir thoroughly and do not plant until all odor of chemical is gone. DILUTE METHOD : 2 tbs. per % cup water. Sprinkle this amount over each cu. ft. of moist soil spread in thin layer, stacked and covered for 24 hr. Sow seeds after 24 hr. and water thoroughly. Formaldehyde may be used as a drench for rooting beds and for cleaning up greenhouse areas when plants are removed. Chloro- picrin or methyl bromide is better suited for most nursery needs. DOSAGE :41b. per 100 cu.ft.of bulk soil or 100 sq.ft. of soil surface. TREATMENT PERIOD : 24-48 hr. Must be confined beneath gas- proof cover. AERATION : 24-48 hr. Very effective and especially suited for flat and bulk soil fumigation. Do not use for carnations or chrysan- themums. DOSAGE: (for Rhizoctonia control) 1-1 y 2 lb. (75% wettable pow- der) per 1,000 sq. ft. TREATMENT AND AERATION PERIOD : May plant immediately after application. DOSAGE: 1-2 qt. per 100 sq. ft. TREATMENT AND AERATION PERIOD: 7-14 days. Longer period required if soil is cold and wet after application. GENERAL RECOMMENDATIONS : Use methyl bromide for soil in flats and containers, chloropicrin or methyl bromide for bulk soils, and chloropicrin, methyl bromide, or Vapam for other soils, whichever is most suited to your needs. 208 ) Table 17. (Concluded) Chemical Recommendations SOIL FUMIGANTS— NEMATODE CONTROL D-D mixture Ethylene dibromide Chloropicrin Methyl bromide (gaseous) DOSAGE: 200-400 lb. per acre. TREATMENT AND AERATION: 1-2 weeks. DOSAGE: 3-6 gal. per acre for the 85% EDB. Dosage must be al- tered according to the actual concentration of EDB in the material used, since it is marketed in several forms. TREATMENT AND AERATION: 2-3 weeks. Do not use for car- nations. This compound, when applied at dosages sufficient for fungus disease control, will also kill nematodes. Other nematocides are cheaper, however. When used for fungus disease control, nematodes are also con- trolled. It may be used at lower dosage for weed and nematode control. More expensive than standard nematocides. DOSAGE: 1^-2 lb. per 100 cu. ft. or sq. ft. TREATMENT : 24-48 hr. AERATION : 24-48 hr. GENERAL RECOMMENDATIONS : If only nematode control is desired use D-D mix- ture or ethylene dibromide, since they are cheaper and very effective. Do not use for fungus control. SOIL DRENCHES— SPOT TREATMENT FOR DAMPING-OFF CONTROL Captan, ferbam, thiram Nabam Semesan Terraclor (PCNB) Remove all diseased plants, drench area 1-2 ft. past the edges of infestation and repeat 7-10 days later. DOSAGE: 1 tb. per gal. water applied at the rate of M>-1 pint per sq. ft. Same as above except that dosage is M~3^ tb. per gal. Pene- trates further than solid suspended materials in soil, but has less residual effect. Same as above except that dosage is 1 tb. per gal. water applied at the rate of ^ pint per sq. ft. Since it contains mercury, do not use near roses. For Rhizoctonia control on carnation, remove all obviously infec- ted plants, apply 1-13^2 lb. (75% wettable powder) per 1,000 sq. ft. to an area extending 1-2 ft. beyond infected plants. GENERAL RECOMMENDATIONS : All have been used with varying degrees of success. These will not eradicate pathogens, but they may serve to check their advance. [209] SECTION Treatment of Nursery Containers Kenneth F. Baker Chester N. Roistacher Philip A. Chandler Heat treatment of containers Chemical treatment of containers .REATMENT OF NURSERY Soil to free it of pathogenic organisms is rapidly be- coming an accepted procedure in Cali- fornia. Such disinfestation, either by steam or chemicals, is best performed in the containers (for example, flats, pots, cans, or benches) in which the soil is to be used. There are some situations in which this is impracticable, however, and soil is treated in bulk (sees. 8 through 11) . There must then be some additional means of treating the containers if dis- ease control is to be achieved. Placing treated soil in infested containers per- mits rapid extension of pathogens and usually leads to severe disease losses (sees. 3 and 14). On the other hand, if containers are treated, bulk soil may be steamed and placed in them with no greater disease hazard than the possi- bility of contamination from handling. Since such an operation is commercially feasible, methods for container treat- ment are presented in this section. HEAT TREATMENT OF CONTAINERS Heat treatment of containers is not ap- preciably different from the steaming of soil discussed in Section 8, either in methods or temperatures required. After CAUTION: Many of the chemicals mentioned in this manual are poi- sonous anc may be harmful. The user should carefu lly Follow the pre- cautions on the 1 □ be Is of the con- tainers. steaming, flats or pots may be filled with treated soil and planted immediately, whereas those chemically treated require a period of aeration before use. Steam may also be used without injury to near- by living plants or irritation to workers, in contrast to most chemicals, which cannot safely be used in confined areas. Containers may be placed in a bench and covered with a tarpaulin beneath which steam is released. Equipment used for treating soil in containers (Sec. 10) may also be used for this purpose. [210] Flats The minimum treatment should be at least 180° F for 30 minutes, and there is no harm from higher steam tempera- tures. Flats may be steamed in stacks either covered with heavy tarpaulins, or placed in a chamber with flowing steam or in a pressure container (Sec. 10). Flats should be separated from each other horizontally by %-inch strips, or stacked in a staggered manner with ver- tical spaces of about 1 inch (fig. 104). This will permit the free flow of steam, and make for faster, more economical heating. This process can be mechanized by stacking flats on pallets for handling by fork-lift tractors (Sec. 17) ; such a practice would decrease the chance of recontamination by keeping treated flats off the ground and by reducing han- dling. In any case, the pallets should be treated before re-use. Continuous steam tunnels have been used in canneries and fruit packing- houses for sterilizing of lug boxes be- fore re-use. In general, it has been found that exposure of 2 minutes to flowing steam of 212° F is necessary to destroy molds in corners of the boxes. Results with nursery flats should be comparable. The time might be shortened somewhat by using superheated steam. From the standpoints of efficient use of steam and avoiding moving parts, the use of space steaming in piles described above is to be preferred. Benches Benches are satisfactorily treated by steaming them under a tarpaulin. As with flats, the minimum treatment should be 180° F for 30 minutes. It should be pointed out that the use of intense localized heat on benches, flats, and so on, may not be satisfactory. Sometimes nurserymen rapidly go over their benches with a blowtorch. This practice is of little use because it would be necessary to char the surface in order to heat in the cracks. Somewhat better is the use of a jet of flowing steam directed on the surface; if continued long enough to heat the ma- terial it is satisfactory. So much steam is lost in this method, however, that it is prohibitively expensive. By confining the steam, one obtains the efficient space treatment described above. Clay pots Empty clay pots present a special problem in California nurseries. Water is continually evaporating from the porous surface and leaving a deposit of soluble salts that in time becomes clearly visible (Sec. 4). In most nursery soils the roots tend to grow out to the pot and then to spiral, forming a more or less hollow cone. Most of the roots are then located in the highly saline zone, with resultant stunting of plant growth. There are three ways of reducing this hazard: 1. Using a nonporous container (for example, cans, plastic pots, or painting the inside of clay pots) as discussed in Section 4. The success of this method is shown by the common use of cans in California nurseries. 2. Using a light porous soil mix of the U. C. type. This provides favorable aera- tion conditions for roots throughout the soil mass instead of just at the sides, as occurs with heavy soils (fig. 61). Such a low-salinity, well-aerated soil there- fore reduces the seriousness of salt ac- cumulation in clay pots, but cannot eliminate it. 3. Soaking of clay pots for 24 hours or more in water, then washing them in the usual way. We have found that this practice has largely solved the salt prob- lem. It is possible, therefore, to achieve a reduction of salinity and to disinfest the pots in one operation by heating the water in which they are soaked to 140° to 180° F. This will destroy the algae and mosses, as well as disease organisms that commonly persist from one plant- [211] ing to the next. The pots may then be washed if desired. It is considered that hot-water treat- ment of clay pots is preferable to steam- ing in California, because of the salinity problem. If, however, the empty pots are not soaked, they definitely should be steamed before re-use, heating to 180° F for 30 minutes. They can be efficiently handled when nested in horizontal rows, and may be handled on a pallet equipped with sideposts (fig. 134). Metal or plastic containers Metal or plastic containers that are so tapered that they nest may be steamed but must be stacked vertically; it is un- necessary to soak them, however. CHEMICAL TREATMENTS OF CONTAINERS Containers disinfested with some chemicals must be aerated to dispel the materials before use. With some ma- terials (for example, methyl bromide and copper naphthenate) the delay is short and reasonably convenient, but with others (for example, formaldehyde) it may last for several days. Some of these materials are harmful to plants and annoying to workmen, and must be han- dled accordingly. Copper naphthenate is retained by wood for a year or more and thus pro- vides a self-disinfesting surface. Methyl Bromide Methyl bromide is used at the same dosage, 4 pounds per 100 cubic feet of space, and in the same way as for soil (Sec. 11). Containers should be aerated for 1 day before use. This is a very satis- factory method for the nursery with types of steaming equipment that make no provision for container treatment. It is of limited value to a nursery that uses chemicals for soil treatment, since the soil should in such cases be treated after being placed in the container. Methyl bromide is expensive to use for empty flats or pots because the dosage is the same as if they were filled with soil; see Section 1 1 for approximate costs. This material effectively disinfests the flats or pots, but, unlike copper naphthenate, leaves no residue that will reduce recon- tamination. In New York it has been found (Lear and Mai, 1952) that methyl bromide at 4.6 pounds per 100 cubic feet of space may be used to treat bales of burlap bags, farm machinery, and other equip- ment to free them of nematodes. Trucks were covered with polyethylene tarps and treated without sustaining damage. It is desirable first to wash dried mud from under fenders and wherever it has accumulated. Formaldehyde Formaldehyde is an excellent fungi- cide, although an obnoxious one and therefore not frequently used for con- tainer treatment. It is used at a dilution of 1 gallon of commercial formaldehyde (37 per cent concentration) to 18 gal- lons of water. Application is made by dipping or spraying flats, pots, or other containers. They may be immersed for a few minutes in the material, permitted to stand until the excess has drained back into the tank, then stacked. For- maldehyde in the above concentration may be sprayed on containers until they are thoroughly wetted, using a very coarse nozzle to minimize volatilization. In either case the containers should be stacked while still wet, and preferably covered with a tarp for 24 hours. When they are uncovered they should not be permitted to become dry at any time until the odor of formaldehyde is gone (usually less than 5 days). The water in [212] a formaldehyde solution will evaporate before the formaldehyde volatilizes, and the chemical then passes over to a white powder, paraformaldehyde. From this state it volatilizes at a much slower rate than from a water solution. For example, flats kept wet were free of formaldehyde in 4 to 5 days, whereas those permitted to dry were found to require 10 days to reach comparable levels. It should be noted that the rate of volatilization from paraformaldehyde may be so low as not to be detected, but that if the flats are again wetted it may appear in amounts toxic to plants. It is a wise precaution to check care- fully for the presence of formaldehyde before planting flats. To do this, wrap moist sample flats in a piece of poly- ethylene plastic for 24 hours, then open one side to see whether the odor of the fumes can be detected. An even safer method is to fill several flats with soil, plant sensitive seedlings (for example, petunia) in them, then cover with a poly- ethylene tarp for 24 hours. Injury will be evident on seedlings near the sides of the flat within 2 days as white dead areas in the leaves. The highly irritating nature of for- maldehyde fumes makes indoor use ob- jectionable. It should never be used in the same room with living plants. Con- tainers are best treated and aired out- doors, down-wind from the growing areas. Workers should remain up-wind from the material, and in still weather a fan might be used. They should wear rubber gloves when handling the treated flats, and tight goggles when spraying the chemical on flats. Use of a full-face gas mask may be desirable. Formaldehyde is extremely useful as a rapid disinfestant for tools (Sec. 3). If it is used on benches, lath shade- frames, or glasshouses, the same pre- cautions regarding application, keeping wet, and aeration should be observed to prevent injury to plants or workers. It should be noted that formaldehyde, like methyl bromide, leaves no perma- nent residue that will reduce recontami- nation. Formaldehyde-steam A southern California mushroom grower has successfully used formalde- hyde-steam mixtures (Sec. 10, type 26) for disinfesting his houses. This tech- nique is worthy of trial for treating glasshouses between crops. Copper Naphthenate Methods of application Copper naphthenate is an excellent material for treatment of wooden con- tainers. It prevents fungi from growing on or into wood which it thus protects from decay, as well as rendering the con- tainers self-disinfesting for a time. It is available in bulk as a concentrate having 8 per cent copper (see Appendix) that may be diluted with Stoddard solvent (1 gal. of the napthenate to 3 gal. of sol- vent) to a concentration of 2 per cent copper. This is applied by dipping for 5 to 30 minutes either the finished flats or the shook for making them, in a tank of the chemical. The excess is drained back into the tank, and the containers then aerated for a day. The material may also be brushed on. A gallon of material covers from 200 to 400 square feet of surface by either method of application, according to the smoothness of the wood treated. Diluted copper naphthenate sold under several trade names may be much more expensive than the above but equally satisfactory. The corrosive effect of treated wood on iron or galvanized nails has been eliminated by using aluminum nails in constructing flats, benches, and so on. Effectiveness Of the materials presently available, copper napthenate most nearly fulfills [213] B \v Mi Mat •* ;• [214] the requirements of a residual flat disin- festant. It kills pathogens near or in contact with the wood by slowly leaching into the soil for a distance of 1 to 2 inches from the wood. Although it is injurious to roots that enter this zone, for many nursery purposes it is reason- ably satisfactory. It is still effective after a year's contact with moist soil. That it is relatively inexpensive is shown by the fact that nurserymen have found it economically justified even when used solely as a wood protective. In repeated tests, treated flats have Dot carried damping-off fungi from a plant- ing of infested soil to a subsequent one of steamed soil 'fig. 108, A I . even though emptied and refilled six times. The flats should be rinsed lightly with water before re-using, in order to re- move the attached soil. Injury to seedlings Injury to roots that enter the zone usually is seen as darkening, or even killing of the tips i fig. 108. B i . Such in- jury occurred on seedling roots of all plants tested, but some of them I pepper. pea. nasturtium, calendula ' show no corresponding injury to top growth. Others | tobacco, larkspur. Iceland poppy, snapdragon, petunia, pansy. Lo- belia. Coreopsis. } show varying degrees of in- jury around the edges of the flats i fig. 108. C). It is apparent that copper napthenate is somewhat more soluble than necessary to achieve the self-disinfesting properly; on the other hand. Wolman salts and Erdalith • see below i are too insoluble. Perhaps an intermediate level of solubil- ity mav later be achieved that will be self-disinfesting but less injurious to roots. Tests have shown that the Stod- dard solvent is not the source of residual toxicity, for it volatilizes rapidly. Uses It should be emphasized that many growers are unaware of the injurious effect of this material, and that it should be decided in each instance whether its usefulness exceeds its injuriousness. As a suggestion, the following uses may be cited. Copper naphthenate may safely be used on benches and shelves on which flats or other containers are to be placed. It is excellent for treating timbers to be placed on the ground for supporting flats I Sec. 3 I . It is verv useful for disinfecting a glasshouse bench before filling it with soil, under conditions where living plants must remain in the house. If the ventilators are kept open until the Stod- dard solvent has evaporated 1 12 to 24 hours | there is little danger to surround- ing plants. It should be noted that some sensitive plants, such as maidenhair fern, may be injured by the volatile sol- vent under conditions of inadequate ventilation. The copper naphthenate resi- due is not volatile and does not injure adjacent plants. The chemical mav safely be used on flats or benches which are to be filled with soil and used for large plants, or in which seedlings are planted 2 to 3 inches from the sides. It should not be used on seed flats or on containers in experimental work. Apparently it is toxic to large geraniums Fig. 108. Effect of copper-naphthenate treatment of flats on carryover of damping-off fungi, and on seedlings grown in them. Untreated flats at left, treated at right in each case. Steamed soil used in all cases. A, Effect on carryover of R" zcc^cr'a on a flat from a previous diseased planting, as shown by damping-off of pepper seedlings. Note elimination of marginal damping- off by t-eatment. B, injury to pepper root tips from a flat planted with four changes of soil in 4 months since treatment. C, Toxicity to tobacco seedlings from a flat treated and aerated for 10 days before seeding; half flats are shown. [ 215 ] grown in clay pots treated with it, and its use on such containers is inadvisable. Other Materials Several chemicals have been used at one time or another for container treat- ment, without general adoption. Among these are the following. Ethylene oxide gas (Carboxide: contains 10 per cent ethylene oxide in 90 per cent carbon dioxide) used at 0.5 pound of active ingredient per 100 cubic feet has been effective in destroy- ing the bacteria that cause potato ring rot in 16 hours in fumigation chambers. It has also been used for soil at some- what higher dosages. Sodium hypochlorite solutions have been successfully used for disin- festation of bulb trays, fruit boxes, and similar equipment. Commercial solu- tions (for example, Clorox, Purex) usually have about 5 per cent available chlorine, and are diluted to 0.4 per cent (1 quart to 3% gallons) or 0.2 per cent (1 quart to 6 1 / 4 gallons) . They are either used as dips or sprayed on the containers, and may be covered with a polyethylene tarpaulin for 24 hours. Fixed wood preservatives such as Wolman salts, Erdalith, and Celcure are so firmly stabilized in the wood as to be valueless in producing self-disinfesting flats. They are, however, excellent wood preservatives and are not generally harmful to plant roots. [216] SECTION Development and Maintenance of Healthy Planting Stock Kenneth F. Baker Philip A. Chandler Importance of clean propagating material How to obtain clean seed or stock Maintaining clean stock I T has already been pointed out I See. 3 i that plant-disease organisms may get into a planting from I 1 | the soil. I 2 I the seeds, cuttings, bulbs, or other propagative stock, or 1 3 1 from con- taminated containers or tools, or from infected material splashed, blown, or otherwise dispersed. Soil infestation is discussed in sections 3. 14. and 15, and methods for controlling it in sections 3 and 8 through 11. The recontamination problem and its control are considered in sections 1, 3. 12. and 14. There re- mains the propagative material to be discussed in this section, explaining how such stock may be produced and used, and the benefits to be derived from us- ing it. Almost anyone associated with the nursery business will agree that patho- gen-free planting stock is desirable. There may be differences of opinion on methods for producing such material, the price one can justifiably pav for it. and the best ways to use it in commercial growing. The proper use of clean seed and stock probably will be profitable anytime, and might prevent bankruptcy. That we fall far short of the ideal in production and utilization of such stock is shown bv the rapid increase in fungi- cide sales to nurseries in recent years. Two types of nurservmen are inter- ested in pathogen-free planting stock: (1) one who produces plants or flowers for market. He wishes clean stock as a means toward an end, the cheapest, most dependable possible production of plants, \\hen possible, clean stock should be obtained by such a nurseryman from a specialist propagator. L nf ortunatelv. pathogen-free stock of onlv a few crops is presently available, and the grower must usually develop his own supply along the lines discussed here. This sec- tion is presented primarily to assist such a grower. As he begins to provide clean stock to others he often gradually be- [217] comes a (2) specialist propagator, to cultural operation, most of which must whom the production of pathogen-free be locally evolved as needed. Therefore, stock is the end desired. This business it is expected that this section will be involves much highly specific informa- less useful at the specialist level than for tion for the given crop, disease, and the general grower. IMPORTANCE OF CLEAN PROPAGATING MATERIAL Only a small percentage of seed or propagative stock ordinarily is infested, but this is of the greatest importance because it serves as a center of infection. Whether the pathogen spreads with soil, as in the case of most of the fungi con- sidered in this manual, or by spores, the end result is to start a series of "spot ires. Rapid increase of pathogens Other conditions being equal, the greater the number of spores produced by a fungus, or eggs by a nematode, the more effective and rapid is its spread. Although the dissemination process is prodigiously wasteful, its effectiveness cannot be doubted. From a single sclero- tium of the cottony-rot fungus (Sclero- tinia sclerotiorum) are produced several tiny, cup-shaped structures said to form as many as 310,000,000 spores. A single leaf spot of celery Septoria late blight may produce more than 200,000 spores. A single female root-knot nematode com- monly lays 500 to 1,000 eggs and may even reach 2,800; a single gall may con- tain many such adults. Snapdragon rust has been conservatively estimated to be able to increase from a single rust pus- tule in a shipment of 10,000 plants to an average of 4,600 lesions on each plant 90 days later. The grower who believes that diseases "suddenly appear" is ob- serving only the final stages in their build-up. Because of such rapid increase it is particularly important that initial infections be prevented. Epidemics, like fires, are best stopped while they are small. Importance of initial infections The relative importance of each infec- tion is determined by its newness in that location, and by its permanence. The ascending progression of danger may be given as follows: 1. Least important are those organ- isms that are already present in the area and do not infest soil (for example, snapdragon rust in California). The centers of disease serve as temporary foci of infection and may be controlled or reduced in severity by fungicidal treatment. 2. Somewhat more serious are organ- isms already present in an area and able to infest soil. If the organism infests the land for only a few years, as with Alter - naria disease of zinnia, bacterial blight of stock, and bacterial stem rot of del- phinium, it is bad enough. 3. If, however, the pathogen is present and remains permanently in the soil, as with Fusarium wilt of aster, Phytoph- thora root rot of heather, and Rhizoc- tonia diseases, it is a serious matter. 4. Those organisms introduced to new areas but not infesting soil (for example, the appearance in 1952 of snapdragon rust in Australia) may create much ex- citement and become very important. 5. A still worse situation is the intro- duction to a new area of an organism that will persist in the soil. As in 2 above, this is serious if the organism infests soil only temporarily (for example, mov- ing crown-gall infected cuttings into virgin land) . [218] 6. When the new pathogen perma- nently infests the land, as with the intro- duction of Phytophthora cinnamomi on young heather plants from southern to central California, the worst situation of all is reached. The organisms discussed in this manual fall into categories 3 and 6, de- pending on whether or not they are new to an area. Since not all types of a given fungus are identical in response to either environment or host plants (Sec. 15), we are here concerned almost entirely with category 6, the worst of all. Benefits from use The practical benefits from using clean propagating stock are essentially those outlined for the complete disease- control program (sees. 1, 2, 3, 16, and 17). Among these benefits are reducing production cost and possibly reducing competition, as well as aiding easier, more certain, less expensive production. HOW TO OBTAIN CLEAN SEED OR STOCK The first step in a program of this sort is to obtain the initial healthy seed or stock. In some cases (for example, seed of garden stock; chrysanthemum cut- tings) these can be purchased from a specialist grower who maintains them. In most instances, however, the nursery- man must still develop his own clean stock. The methods used in either case are outlined here, with details of the second types. Since recommendations could not be given for each of the many nursery crops, even if they had b:en studied, the various methods are pre- sented, with applications indicated for some specific plants. Man's experience with his own fal- libility has often caused him to provide more than a single defense against ad- versity, as insurance companies are aware. This also applies to disease con- trol. Thus, a grower may discard all obviously diseased stock, treat the re- mainder, avoid overhead sprinkling or other cultural practice that favors the pathogen, remove any diseased plants that appear in order to reduce inoculum, and spray regularly to protect against any spores that blow in. Rarely is a single control measure sufficient to pre- vent a plant disease. This is true of the nursery diseases discussed in this manual, and particularly so in the de- velopment and production of pathogen- free stock. Hence several concurrent procedures are often suggested to achieve this objective. If a few healthy plants can be found, the grower can propagate from these by observing reasonable sanitary proce- dures (Sec. 1). These often cannot be found, however, and the grower must resort to other more involved techniques. Plant Grows away from the Pathogen It is possible in some cases, by mani- pulation of the environment or cultural practices, to obtain clean stock from diseased plants. The plant may be grown in such a way that the propagative parts remain uninfected because they develop in a position and under conditions un- favorable to the disease. Tip cuttings Thus, it is possible to reduce (but not commercially to eliminate) V erticillium wilt from chrysanthemum by taking tip cuttings from rapidly growing shoots at least 12 inches above the ground. If each cutting is then established in a separate pot of steamed soil, grown to maturity, only healthy plants used in propagation, and the process repeated, it should be possible to derive clean stock. It is now [219] done faster and more reliably by "Cul- turing Methods" (see below). With fungi (for example, Rhizoctonia and water molds) that do not invade the water-conducting system (as do Verti- cillium and Fusarium) this is more easily accomplished. It is possible 1 to get Choisya cuttings free from Phytophthora by taking them from large shrubs at a point at least 4 feet from the ground, and by rooting and growing them in steamed media to pro- duce gallon-can stock without disease loss. This is in striking contrast to the 50 to 90 per cent mortality in many nurs- eries. Furthermore, these plants have grown exceptionally well when planted in gardens. It is, of course, necessary that such cuttings be taken in the dry season, that the shrubs either not be splashed with mud or debris by rains, down-spouts, sprinklers, or other ways, or that the shrubs be so high that such spattering does not occur at the levels where cuttings are taken. It is further necessary that cuttings be placed on clean papers or in steamed baskets, never on the ground, to avoid con- taminating them. Cuttings should not be taken, unless it is absolutely imperative to do so, from known infected plants or infested areas. These same conditions apply equally to all other types of plant- ing material discussed herein. Similarly, the taking of azalea and camellia cuttings from high shrubs rather than low plants 6 to 12 inches from the ground, as is so often done, would greatly reduce carryover of Rhizoctonia and water molds. A similar method has been used in Oregon to ob- tain strawberry plants free of the Phy- tophthora that causes red stele. The same method would free heather cuttings of Rhizoctonia and of Phytoph- 1 Demonstrated in 1954 by J. A. Bcutel (As- Bistanl Agriculturist, Farm Advisor's Office, Los Angeles County) in tin- Department of Plant Pathology, University of California, Los An- geles. thora cinnamomi, cause of one of the principal diseases of the crop. Application of this general system could also provide a source of planting stocks free from root-knot nematode and the oak-root fungus. By such careful manipulations, and strict avoidance of overhead watering, syringing, or spraying, it should be pos- sible to obtain cuttings of marguerite daisies, shrubs, and other plants, free of crown gall, and oleanders free of stem and leaf gall. Araucaria tip cuttings taken from plants grown to a height of 4 to 5 feet under conditions free from splashing, would be uncontaminated by Rhizoc- tonia. It is worth an attempt to develop a nucleus stock of Esther Read daisy free of bacterial fasciation by (1) training up the shoots, (2) scrupulously avoiding sprinkling, syringing, or spraying of the plants, (3) taking tip cuttings, and (4) rooting and growing them in individual pots free from all water splashing. Plants on frames A modification of this is to train cane- forming plants, such as Dieffenbachia and some Philodendrons, so that the tips are taken several feet above the ground. This will free them of Rhizoctonia and water molds, and if overhead watering and high humidity are avoided, will eliminate bacterial soft rot as well. Peperomia may easily be freed from Rhizoctonia and water molds by taking tip cuttings from high on the plant. We easily freed fuchsia and coleus plants from foliar nematode by growing them in a glasshouse without wetting the foliage, even when the diseased leaves were not removed, as they should have been. Tip cuttings taken from these plants* 6 to 12 inches above infected leaves remained healthy. By growing on wooden or wire up- rights or frames to get the plants as far off the soil as possible, and avoiding all [220] splashing water, it is easy to free such trailing plants as Pellionia, Fittonia, Nephthytis, Philodendron cordatum, and ivy from Rhizoctonia and water molds. Similarly, ivy may also be freed of bacterial leaf spot. It would seem a reasonable precaution to continue to raise all mother stocks of these plants in this way. Aseptic culturing of growing point A further refinement is to make tiny cuttings of the growing tip, to graft the tips onto healthy plants, or to aseptically culture the tiny apical growing point (much as plant breeders culture em- bryos) in order to free the plant of viruses. Holmes (1955, 1956a, 19566) and Martin (1954) have thus freed dahlia of the spotted-wilt virus, chrysan- themum of the aspermy virus, and sweet potato of another virus. Norris (1954) also eliminated virus X from potato in this way. The technique might well be extended to other kinds of plants by a trained specialist. It is based on the fact that some viruses, under certain en- vironmental conditions, move slowly through tissue and may not reach the growing point. Quak (1957) freed car- nations of viruses by culturing apical meristems grown under high tempera- tures. Environmental control In these techniques it is desirable to make the environment as unfavorable as possible for the pathogen without unduly checking the growth of the host. Rhizoc- tonia and water molds, under moderately dry conditions, are unable to spread much above the soil surface. Tempera- ture control might increase this differen- tiation between pathogen and plant. Culturing Methods This method apparently was first sug- gested by A. W. Dimock of Cornell Uni- versity in 1943 against V erticillium wilt of chrysanthemum, and was soon adopted by a commercial propagator. In 1949 nearly 26!/2 million mum cut- tings were produced in Ohio, 69.5 per cent of the national total, largely by this concern. This technique is best per- formed by the specialist, although it is now used by a number of growers in various parts of the country. It has also been adapted for use in securing Verti- cillium-iree sticks of rose budwood by Wilhelm and Raabe (1956), pathogen- free carnation stock by Tammen, Baker, and Holley (1956), and pathogen-free geranium stock by Munnecke (1956). It will undoubtedly be found useful for many other crops grown in nurseries. Although it is best adapted to use against pathogens of either the systemic or vascular type, it can be used on a wide variety of plants and diseases. As is often the case, the method is basically simple, but becomes complex from the number of unusual circum- stances that may arise. Briefly, the method (fig. 109) is as follows: Each 4-inch cutting (fig. 109, A) and a cor- responding tube of nutrient agar are assigned a number. A 1-inch piece (B) is cut from the base of the cutting and immersed in a Clorox solution (1 volume to 4 of water) for at least 1 minute (C) . The scalpel is flamed between cuts. The piece is placed on a paper towel to drain (D) for a few minutes, and the basal !/4 inch is removed. Four slices each y%2 inch thick are then cut off and re- tained; the remaining ends are dis- carded. The stem is cut on the paper towel with a flamed scalpel. The four thin slices are transferred to a numbered agar slant (or tube of broth, if bacteria are in- volved) and held at 75° to 80° F (E) . The cuttings are then held in polyethy- lene bags in cold storage, or planted in paper cups of moist sterile media (F) for 10 days or more until readings are taken on the tubes. If any of the four pieces in a tube show any fungus or bacterial growth, the corresponding cutting is de- [221] ^9 "73 Fig. 109. Culturing method for selecting chrysanthemum cuttings free of Verticillium. See p. 221 and 222 for explanation. stroyed. The remaining healthy cuttings have infested the soil and infected neigh- (G), when rooted and grown on, become boring plants. the nucleus block. The cuttings produced Modifications of this culturing process by this block are grown in an increase have been introduced. Plain agar plus block, from which cuttings are marketed without additional culturing. The principle is to determine rapidly whether any fungus or bacterium is present thai might not normally be de- ter led until later, when it would already sterile, dried, chopped plant tissue may be used instead of nutrient agar when bacteria are not involved. This is par- ticularly useful for detecting Verticil- lium. Petri dishes, rather than tube slants, are then used. 222 | Heat Treatment of Planting Stock This method, introduced more than 70 years ago in Denmark, has, except for cereal seeds, only recently come into commercial application. Its effectiveness is based on the principle that some or- ganisms are more easily killed by heat than is the crop plant. Heat has been ap- plied through the agency of water, dry air, moist air, and carbon tetrachloride. At present, water is the best for rapid treatments since its heat conductivity is more than twenty times that of dry air. The use of carbon tetrachloride is still experimental. For long treatments (for example, with some virus diseases), ex- posure to moist or dry air is most useful. Neither all kinds of plants nor all their parts (for example, seeds vs. leaves) are equally heat-susceptible, nor are all types of growth (for example, lush succulent tops vs. hard woody stems). The "trick" for successful treatment often lies in se- lecting the most heat-tolerant host tissue, and manipulating culture procedures so as to produce the most resistant growth. Failure to consider these basic facts has led to many "successful operations" from which the patients did not recover. injury may be avoided Plant injury from heat treatment ranges from induced dormancy of vege- tative parts (for example, gladiolus cor- mels), through delayed seed germina- tion, production of weak and deformed seedlings, to killing of some or all of the material treated (reduced stand). Injury may be reduced by using seed relatively free from mechanical breakage of the coat, and by avoiding seed over 2 years old. Nurserymen frequently overlook the fact that seed or propaga- tive material is one of their smallest expenses, and that increased rate of seeding will adjust for germination loss. It often would be better to heat-treat the stock and eliminate disease, even though it reduced the germination 50 per cent, than to plant it untreated and risk 100 per cent loss from disease. For a specialist propagator the economic limits of treat- ment injury are much higher; in many cases the survival of only a single plant from a given lot is justified if it provides the start for a healthy mother block. Heat treatment has limited direct use in retail production, but it is becoming an invaluable tool in propagative activities. Value of treatment Since heat treatment is eradicative, aiming at elimination of the organism from the tissue, it is used primarily for situations where the pathogen is internal and not, therefore, reached by external chemical treatments. Heat is effective, however, against spores and mycelium both on the surface and borne internally. The principal value of heat treatment lies in preventing contamination of the soil and the surrounding plants when set in the field. It does not, however, provide any protection of the plant against sub- sequent fungi to which it may be ex- posed. There is no reason from a disease standpoint, therefore, to heat-treat seed known to be free of pathogens. There are several stages in a treatment program; not all of these necessarily apply to each type of material. Selecting material to treat Select for treatment the cleanest seed, plants, or bulbs available. This would also include selecting fresh seed, rather than that several years old. In general, only the cleanest and most vigorous stock available is worth the effort the treatment requires. Heat treatment is a method for obtaining clean stock, not a means of utilizing low-grade or worth- less planting material! Conditioning the material Get the material into the best possible condition for treatment. With many foliage plants this may require growing them, if in soil, for 3 to 6 months with- out nitrogen fertilization and only [223] enough water to prevent wilting. A faster system might be to wash the soil from the roots and set the plants in fairly coarse sand in order to reduce both nitrogen and water intake. Humidifiers and shading should be used only as necessary to prevent burning. Neph- thytis, Dieffenbachia, Fittonia, and Pel- lionia benefit greatly from hardening prior to treatment. Such hardening of stock should be practiced before treat- ing to obtain the original pathogen-free material with which to establish a mother block. Further propagation should, of course, be from the un- hardened mother block, since the com- plete procedure is too costly and slow for general propagation. It has been found that gladiolus cor- mels produced in warm, relatively dry California soils are more tolerant of heat treatment than are those grown in cooler moister areas. Calla rhizomes will tolerate 122° F for 3 hours 2 when in the desiccated market condition, whereas freshly dug, actively growing rhizomes and fleshy stems may be killed in a half hour. Preparing the material for treatment Preparation for treatment involves trimming of dead leaves, roots, and other parts, washing to remove dirt, and any necessary dividing of clumps. No more wounds should be made than ab- solutely necessary, as water-soaking occurs at such points. Dieffenbachia cane is left in pieces up to 2 feet long, and is divided to single-bud pieces several weeks after treatment. In our experience plant material is not success- fully freed of a pathogen if left in the soil-ball or pot. The cooling effect of the soil and pot on the water, and the slow penetration of heat into the soil, prevent real effectiveness. Plants should always he liare-rooted for treatment. I apublished data of S. Wilhelm, obtained in the Departmenl of Plant Pathology, Univer- -iiv nf California, Loa Angeles. Presoaking A presoak is desirable in cool water for 4 to 12 hours (nasturtium seed) to displace air between layers, or for 48 hours in warm water (gladiolus cor- mels) to displace air and render fungus mycelium more susceptible. The use of a presoak should be considered whenever plant parts have loose coverings that trap air and cause material to float. Treatment equipment and temperature Heat treatment on a large scale is best performed in a tank holding at least 100 to 200 gallons of water, because the temperature is much more stable with such volume. Equipment of this type is commercially available as bulb-treat- ment tanks (fig. 110; see also Appen- dix), and one unit combines facilities for steaming soil and treating plant ma- terial (Sec. 10, type 7). Heat may be supplied by a small stream of water from a tap 20 to 30 degrees F higher than the treatment temperature; this can usually be achieved by turning up the thermo- stat on the water heater. Heat may also be maintained by immersion-type elec- tric heaters, by releasing steam into the water, or by gas burners under the bot- tom of the metal tank. With a little prac- tice, the temperature can be kept within a half degree above and below the de- sired temperature after initial adjust- ment; such regulation is necessary for the best results. Water circulation in a large tank is best achieved by a circulat- ing pump or by a submerged propeller. Circulation is essential during heat treatment to prevent temperature strati- fication and to promote heat penetration into the material. Thermometers must be accurate at the range 120° to 135° F (see Appendix) ; it is usually desirable to have two or three of them suspended in the tank to indicate degree of temperature uni- formity in the water. It is a worth-while precaution to have a precision-calibrated [ 224 ] Fig. 110. Commercial hot-water treatment tank for bulbs, ornamentals, and strawberries. Tem- perature is thermostatically controlled, and the water is agitated by a propeller. thermometer (accurate in the above range; see Appendix) , against which the others may be calibrated at the exact temperature used. Thermometers with expanded scales in the critical treatment ranges are available in brass protective cases (see Appendix). These provide precise temperature measurement several degrees either side of a specified point. Since they are reasonable in price, they should be more commonly used. For small quantities of seed or plants we have had uniformly good results us- ing a 20-gallon sink or tub, maintaining uniform temperatures by a trickle of hot water, and stirring with a wooden paddle. The material to be treated may be placed in open-weave plastic or cloth bags, or in screen boxes for ready han- dling. Seed in cloth bags should be gently kneaded by hand to expel air bubbles; material in screen boxes should be constantly agitated for the same pur- pose. The treatment should be accurately timed so that exposure to heat will be precise. Cooling Prompt cooling of the treated material is imperative in order to increase pre- cision of timing exposure. This may be achieved by turning a hose on the con- [225] tainer, or by dipping in a tank of cool tap water, again kneading for rapid heat transfer. This should be continued until the material reaches water temperature. The tank, and the water in it, must be free of pathogens; it should not have been used for soaking untreated material. Drying The seed or stock (except foliage plants) should be dried as rapidly as possible, certainly within a few hours. This is best accomplished by spreading on screens, treated cloth, or new papers over which a warm dry current of air is blown, and which are raised off the ground. Temperatures in this phase should not exceed 90° F, so that drying is accomplished by a large volume of dry air rather than by heat. The screen may be disinfested with a formaldehyde solu- tion (1 gal. commercial strength to 18 gal. water), followed by a water rinse, or by a sodium hypochlorite spray (0.4 per cent available chlorine) as explained in Section 12. Storage Handling and storage of the clean ma- terial must conform to rules of sanita- tion. Under no conditions should the material be placed in old contaminated boxes, trays, or bags, or handled with dirty tools; these implements should be sterilized before re-use (sees. 3 and 12). Growing in isolation The clean material must be grown isolated from the general propagation, to minimize opportunity for contamina- tion. Application to specific nursery crops The above methods are perhaps best presented in relation to specific crops and diseases. Only the treatment phases mentioned are needed in each case. Aglaonema* — against Rhizoctonia and water molds. Old cane will stand Previously unpublished data of the authors. treatment at 120° F for 30 minutes; cool; plant. Aloe — against water mold (Pythium ultimum) root rot. Plants of various sizes cleaned of debris, treated at 115° F for 20 to 40 minutes (the larger the plant the longer the time) ; cooled; planted. Almost completely effective in salvaging infected plants; injury mini- mal (fig. 111). Apium (celery) — against late blight (Septoria spp.) in seed. Treat seed at 120° F for 30 minutes; cool; dry; plant. Treatment of seed does not appreciably reduce stand. Healthy plants produced from treated seed require less spraying in the field, as the pathogen builds up slowly from field debris. Begonia, tuberous — against root-knot nematode. Tests by P. A. Miller 3 indi- cated that treatment of dormant tubers at 120° F for 30 minutes freed them of nematodes without injury. Cool and plant. Buxus (boxwood) — against root-knot nematode. Tests by P. A. Miller 3 indi- cated that treatment of bare-root plants at 118.4° F for 30 minutes freed them from the nematodes without serious in- jury. Cool and plant. Caladium — against Sclerotium rolfsii tuber rot. Florida studies found that treatment of dormant tubers at 122° F for 30 minutes eliminated the organism without injury. Cool and plant. In Cali- fornia, a bacterial soft rot of the rhi- zomes is not controlled by this treatment. Capsicum (pepper) — against Rhizoc- tonia in seed. Treat seed at 125° F for 30 minutes; cool; dry; plant. Com- pletely effective with almost no reduction of germination. Dieffenbachia* — against water molds, bacterial leaf spot, and bacterial soft rot. Important that hardened canes be used as they tolerate the necessary 125° F; others stand only 120° F; young leafy shoots will not survive treatment. Treat :| Department of Plant Pathology, University of California, Los Angeles. [226] Z^Sl-SjiWi'eySy ■';.''Zi'' ! 'Xi ! 'f^^f. Fig. 111. Control of Pythium root rot of Aloe variegata by hot-water treatment at 115°F for 20 min. A, Treated plants above and checks below, 20 days after planting. B, Treated plant at left and check at right, 100 days after planting, showing the rapid recovery after treatment and planting in treated soil. cane in pieces about 2 feet long at 125° F for 30 minutes; cool; hold canes in steamed sphagnum moss until roots or buds start; cut into pieces each with a single bud, plant immediately in perlite and peat; replant in soil when top is well started. Has given excellent control. In- creases percentage of buds of D. bausei and D. picta breaking dormancy, as for Syngonium podophyllum below. Tip cuttings from treated plants, grown with- out overhead sprinkling, should further insure freedom from pathogens. There is some evidence that soft-rot bacteria may be spread through the propagating bed by larvae and adults of fungus flies. Spraying the soil surface with dieldrin (wettable powder, 1 oz. per 7% gal. water) or malathion (wettable powder, 2 oz. per 7^2 gal. water) has given some promise in controlling these insects. Fittonia* — against Rhizoctonia (fig. 112) and water molds. Harden plant. * Previously unpublished data of the authors. Clean and treat whole plant at 124° F (120° if plant is unhardened) for 30 minutes; cool; make cuttings and divi- sions, removing any damaged leaves; plant at once in perlite and peat, where roots may form in less than a week. Leaves are very sensitive to treatment, but stems survive and new shoots de- velop. See Section 16 for the experience of one grower with this treatment. Gerbera — against root-knot nematode (Meloidogyne incognita) . Tests by S. A. Slier 4 indicated that treatment of bare-rooted plants at 118° F for 20 min- utes freed them from these nematodes. Treated plants started a little slowly, but surpassed the untreated ones after 5 months. Haworthia — methods and results simi- lar to those with Aloe. Lilium (Croft) — against Rhizoctonia, stem and bulb nematode, and root-lesion 4 Department of Plant Nematology, Univer- sity of California, Riverside. [227] Fig. 112. Stem cuttings of Fittonia verschaffeltii var. argyroneura in a treated rooting medium in a propagation case, showing effectiveness of control of Rhizoctonia by hot-water treatment. Above, cuttings treated at 120°F for 30 minutes; note the perfect stand and absence of leaf decay. Below, untreated cuttings from the same lot, showing leaf decay and poor stand. [228] nematode; ineffective against Fusarium basal rot. Tests by J. G. Bald and P. A. Chandler 5 showed that the following schedule was successful for obtaining useful commercial stock: Cure bulbs at 95° F and 95 per cent humidity for 1 to 2 weeks; presoak 2 days in cool water. Treat for 2 hours in water plus formalde- hyde (1 gal. 37 per cent commercial for- maldehyde per 200 gal. water) at 115°; cool. Aftersoak the bulbs in Puratized Agricultural Spray (1 pint per 125 gal. water) for 24 hours. Scale the bulbs and dust scales with ferbam; place on moist vermiculite at 75° and 95 per cent hu- midity. Remove bulbils and plant in treated soil. Matthiola (stock) — Seedsmen now treat seed they plant so as to produce seed free from bacterial blight. Seed is treated by the producer in plastic screen bags at 130° to 131° F for 10 minutes; cooled; and dried. Special techniques are necessary in handling because of the mucilaginous seed coat. Most stock seed is now free of bacterial blight owing to the success of the treatment. Pellionia* — methods and results simi- lar to those with Fittonia. Plants tolerate treatment well, develop strongly. Philodendron pertusum and P. corda- tum* — against bacterial stem rot of the former, and Rhizoctonia on the latter (fig. 114). No data available on effect of hardening, but both plants take 120° F for 30 minutes well. Discarded, hard- ened, conservatory plants seem to take the treatment very well. Treat; cool; root in steamed sphagnum (as with Dief- fenbachia) ; cut into propagating sec- tions of 1 to 2 buds. Rosa — against dagger (Xiphinema) and spiral (Helichotylenchus) nema- todes in Florida; wash roots; treat at 121° F for 3% minutes; cool; dry. If root-knot or root-lesion (Pratylenchus) 5 Department of Plant Pathology, University of California, Los Angeles. * Previously unpublished data of the authors. nematodes are present, 121° to 123° for 10 minutes is required. Strelitzia — against root-rot complex involving internally borne Fusarium moniliforme. Presoak seed 1 day in water at room temperature; treat at 135° F for 30 minutes; cool; dry; plant. Commer- cially effective in preventing loss of seed- lings without reducing germination. Syngonium auritum (Philodendron trifoliatum) — against black cane rot (caused by a specialized form of the fungus Ceratocystis fimbriata) . Harden- ing increases heat tolerance of plants. Treat whole bare-rooted plants at 120° F for 30 minutes; cool; plant in soil. Eliminates pathogen; some leaf injury, but plants quickly recover. Syngonium podophyllum* — canes are treated in pieces about 2 feet long ; toler- ate treatment without injury. Clean and treat at 120° F for 30 minutes; cool; handle as for Fittonia. This treatment breaks dormancy of the cane buds, greatly increasing the rapidity and suc- cess of propagation (fig. 113). Present procedure is to hot-water-treat as above, then hold in a humidity cabinet at 70° F for 2 to 3 weeks before planting. Weigelia — against root-knot nema- tode. Tests by P. A. Miller 6 indicate that treatment of bare-root plants at 120° F for 30 minutes freed them of nematodes without serious injury. Cool and plant. Zantedeschia aethiopica (white calla) — against water mold {Phytophthora richardiae) . Clean dormant rhizomes; treat at 122° F for 1 hour; cool; dry. Zinnia — against Alter naria disease (A. zinniae) and Rhizoctonia. Seed should not be more than 1 year old, or germination will be reduced by treat- ment. Treat seed at 125° F for 30 min- utes; cool; dry. Commercially effective in eliminating these pathogens from fresh seed, without serious injury. 6 Department of Plant Pathology, University of California, Los Angeles. [229] Fig. 113. Effect on growth of Syngonium podophyllum (Emerald Gem) stem cuttings of hot- water treatment (120°F for 30 min.) and holding under humid conditions and 70° for 2 weeks. Photos, after 7 weeks' growth in the glasshouse, show stimulation of bud and root development due to breaking of dormancy. Upper left, untreated check, planted at once in the glasshouse. Upper right, untreated check, held at 70°. Lower left, treated and planted at once in the glass- house. Lower right, treated and held at 70°. Chemical Treatment of Planting Stock This method is effective only against pathogens externally carried on the planting material where the chemical will come in contact with them. It is, therefore, of limited value to nursery- men. Nurserymen should realize that chemi- cal seed treatments are of two types (protective and eradicative), only one of which aims at elimination of disease organisms from the seed. It is a potenti- ally disastrous misconception that the "treated" seed sold in packets today is free of pathogens. The seed is treated with a protective mild fungicide (for ex- ample Spergon on sweet pea seed) to protect it from decay prior to emergence, or to reduce damping-off in slightly in- fested soil. Treatments that eradicate pathogens from seed usually reduce germination, and therefore are not ap- plied before sale. Three examples of eradicative chemical treatment may be cited. 1. Dormant white calla rhizomes are soaked for 1 hour in formaldehyde (1 pint to 6% gal. water), mercuric chlo- ride (one 7^/2 grain tablet per pint of water), or in a suspension of 8 ounces New Improved Ceresan plus 1 ounce Dreft per 25 gallons of water. This will free them of the root rot Phytophthora. Since growth may be somewhat delayed by these treatments, the heat method de- described above may be preferable. 2. Seed of China aster may be treated | 230 ] Fig. 114. Control of Rhizoctonia root rot of Philodendron cordatum plants grown on poles in pots. Plants shown above are in the soil ball; the same plants are shown below with roots washed free of soil. Plants at left as they were obtained from a commercial nursery, showing severe root decay; comparable plants at right 3 months after the root systems and the poles were washed free of soil, treated in hot water (120°F for 30 min.) and replanted in treated soil. [231] with an unheated mercuric chloride solu- tion (one l 1 /-! grain tablet per pint of water) to free it of the wilt Fusarium. A given volume of seed is placed in a jar, covered for 30 minutes with three times that volume of mercuric chloride and intermittently shaken. The solution is poured off, the seed rinsed with several changes of water, and dried. Some germi- nation reduction may result, particularly in seed with cracked coats. 3. The foliar nematode (Aphelen- choides ritzema-bosi) of chrysanthemum may be eliminated from a cutting bed by chemical treatment. A spray of para- thion (25 per cent wettable powder, 1 lb. per 100 gal. water, plus 4 to 6 fl. oz. Triton B-1956) will be absorbed by the plant and poison the nemas. Formerly sodium selenate applications to the soil were used for the same purpose. Sodium selenate or Demeton (Systox) are used to free Saintpaulia of foliar nematode. Sanitary Measures It is rarely possible to free stock of a pathogen by removal of diseased tissue. However, azalea plants may be freed of the flower-blight fungus, Ovulinia aza- leae, by carefully removing all dead flowers from the plant, and stripping off the top inch of rooting medium, or by bare-rooting the plant. Thus, carryover sclerotia are eliminated, breaking the pathogen's cycle. Similarly, the flower-blight fungus (Sclerotinia camelliae) of camellia may be eliminated by careful removal and burial or burning of flowers. If no flowers are permitted on young nursery stock, and not allowed to fall into the cans from large flowering plants, there can be no carryover of the fungus with the plants. Removal of the top inch of soil, as for azalea, will be similarly ef- fective. Aging of Seed Rarely the fungus will survive for a shorter period of time than will the plant seed in which it developed. It is possible to free celery seed of the late- blight Septoria by holding it for 3 years or more before planting. This procedure is applicable only on celery seed among the nursery crops. Prolonged Roguing of Diseased Plants from a Stock Diseases have periods when they may not be detectable in plants that later may show striking symptoms. This makes it impossible to eliminate infected plants by a single roguing. If, however, the causal agent has essentially no natural spread, it is possible by careful and re- peated roguing over a period of several years to clean up a stock and thus estab- lish a healthy mother block. This is true of many of the viruses of woody plants, as for example, rose mosaic. If the virus spread occurs only with budwood or scions it is possible to index plants and issue registered budwood from disease-free specimens. This is being done for citrus stock free of psoro- sis virus, and for some of the stone fruits. Virus-indexing Methods Recognition of virus infection is diffi- cult in plants which, for various reasons, temporarily or permanently fail to show symptoms. To find virus-free plants under these conditions, special indexing procedures are required. Such methods are highly technical, but are already in commercial use by specialist propa- gators. Brierley and Olson (1956) have described a method for graft-indexing chrysanthemums to highly susceptible healthy varieties in order to detect virus- infected plants. Only the virus-free plants are then propagated. Gasiorkiewicz and Olson (1956) have described a similar method for carnations. The presence of mechanically transmissible viruses of either plant may be detected by sap transfer, usually by the use of carbo- rundum powder, to the leaves of some host which shows symptoms. With some [232] viruses (aster yellows) the use of an ap- propriate insect vector is necessary. Growing Plants from Seed Disease agents frequently are not car- ried through the seed but rapidly build up through vegetative propagation. This is particularly true of viruses. Thus, raising ranunculus, anemone, and free- sias from claws, roots, or corms leads to serious losses from mosaic, whereas the loss is minimal when grown from seed. Similarly, yellow calla grown from true seed is free of bacterial soft rot, water molds, and Rhizoctonia, all of which are commonly carried on the rhizomes. Ap- parently it is possible to obtain ferns free of foliar nematode (Aphelenchoides fragariae) by starting with spores from leaves kept dry during growth. Selecting the Growing Region The semiarid California climate is an effective ally in eliminating many nurs- ery diseases because plants dry off quickly after rain or overhead watering. Many fungi require prolonged moist conditions in order to produce spores, which may in turn be spread only in water and must be wetted for a few to 48 hours to infect the host. Many diseases, important elsewhere, are essentially unknown here because of the climate, among them snapdragon anthracnose and Phyllosticta leaf spot, Phomopsis canker of aster, bacterial leaf spot of poppy, and Ascochyta flower blight of chrysanthemum. Septoria leaf spot of chrysanthemum is important only in propagating frames or plantings extensively watered from above. Seed of China aster free of Stemphylium calli- stephi, and zinnia free of Alter naria zin- niae may be produced in California by growing in dry inland, instead of coastal, valleys. Azalea and camellia flower blights largely occur in California under lath or shrubbery cover. Promot- ing air movement through the lath house by removal of the sides will reduce the disease by restoring conditions of rapid drying. Although there are few nursery dis- eases that can be eliminated solely through selection of the region, growers should exploit this climatic advantage of the state to the maximum in support of other controls. Seed or plants grown under dry condi- tions without overhead watering often are freest from disease. They should be grown-on without sprinkling or syring- ing whenever possible. Reporting Diseased Stock to the Propagator This point is important because it is sometimes the primary force in bringing about improvement. Propagators may actually be unaware of the situation and may be grateful for notification of it, or they may mistakenly think that since perfection is unattainable, a fairly clean stock is good enough. They can no more have "a little disease" than they can be a little dead! They will, in any case, cer- tainly respond faster to customer pro- tests than to advice from Agricultural Experiment Station workers or state nursery inspectors. In this democratic process of the business world, the grower who accepts diseased stock without pro- test is derelict of his duty in the system. Since in the long run results may be achieved in this way that apparently can- not otherwise be accomplished, there is little virtue in being a silent sufferer. Of the rare propagator who inten- tionally markets diseased stock little need be said, since he is self-exterminat- ing. If he clips the scattered galls of root- knot nematode from his rose roots or cuts out the swellings of bacterial fascia- tion from his divisions of Esther Read daisies before shipment, he is practicing fraud rather than disease control, and this will eventually be recognized by his customers. [233] MAINTAINING CLEAN STOCK Once healthy nursery stock is ob- tained, one faces the problem of main- taining it in this condition. In actual practice, clean stock is fairly commonly obtained but is so quickly infected that the grower does not become aware of its value. New seedling varieties are almost always free of virus diseases as well as most pathogens, and in this sense plant breeding is a primary source of healthy stock. Usually no effort has been ex- pended to maintain the variety in this condition, and it is soon discarded be- cause of disease; the rapid commercial disappearance of the King Cardinal carnation because of mosaic is but one of many instances of this sort. This loss more often results from lack of knowl- edge concerning the desirability of, and methods for, maintaining the healthy condition of the plants than from in- surmountable difficulties in the system. For example, a large eastern propagator is developing and introducing new seed- ling varieties of carnations that are free from most of the major diseases. They are maintaining stock of these varieties under conditions that largely prevent their becoming diseased. Growers buy- ing such a variety sometimes find that it develops "excessive growth" the first year (in large part because of freedom from disease) , and usually note that pro- duction drops in a few years under their own propagation. They then buy new cuttings of the variety and start over. This company keeps the variety disease- free, rather than permitting it to become infected (as the grower and the propa- gator have done in the past), so that there is now a clean stock to fall back on. The more important basic procedures for preventing contamination of stock are given below. Isolating New Stock Almost every nurseryman has known an instance in which a small quantity of new stock was purchased and set among healthy local plants, introducing disease to the whole lot. A few progressive growers have an "isolation ward" or greenhouse to which new stock is sent until freedom from disease is established. Such a practice has the full weight of scientific theory and bitter experience behind it. The marvel is that it is not more frequently used. This process is used in addition to any inspection and certification practices. Many diseases are not detectable at first by inspection because there is always a lag (called the incubation period) be- tween infection and symptom appear- ance. Snapdragon rust, for example, shows no symptoms for about a week and is not conspicuous for 10 to 14 days. Other diseases may appear only at certain stages of growth, as for example, azalea and camellia flower blights dur- ing blooming. Still other diseases (for example, geranium mosaic) may be evident for only brief periods and then disappear. The safe procedure is, there- fore, to isolate introduced plants until their health is established. Because of the complexities exemplified above, the grower should consult the local farm ad- visor or the Agricultural Experiment Station in arranging a program of this sort. Another reason for using pathogen- free stock is that it may be brought into a nursery without preliminary isolation. The use of a "pest house" is highly desirable in most nurseries, and is abso- lutely required in one that is raising pathogen-free stock for sale. Because some pathogens or viruses are carried by wind and insects, it is essential that the isolation house really be separated from the main plantings. The same principle may be involved in the handling of single plants. Thus, callas are commonly grown in pots I 234 ] rather than benches in order to prevent the spread of Phytophthora root rot and bacterial soft rot. Isolating Propagative Operations At first seedlings are quite free of virus infection, gradually accumulating viruses with time. It is important to de- lay such infection by not locating a seed or cutting bed near any source of infec- tive insects. Thus, raising delphinium seedlings near an old planting of delphinium or a growing celerv field is an invitation to leafhoppers. They carry the aster-yellows virus and. in feeding, infect the seed- lings; symptoms may not appear, how- ever, until the plants are growing in home yards. Such seedbeds might well be tightly covered with aster cloth or plastic screens. Seedlings of aster, tomato, and pepper raav be infected with the spotted-wilt virus through migration of thrips from surrounding infected plants. In one in- stance tomato seedlings were thus in- fected from old diseased dahlia plants grown bv the nurservman's wife for cut flowers just outside the glasshouse ventilators. Certain coastal areas have severe outbreaks of spotted wilt each vear. These "endemic areas" are located in the cool coastal fog belt, and have a persistent population of dahlias, nas- turtiums, callas, buttonweed, chickweed, and other hosts. Such endemic areas should be avoided for propagation of certain plants. Growing stock seedlings in an area surrounded by wild radish and mustard is another dangerous situation. Aphids carry mosaic to the stocks, which may not show symptoms until flowering. In general, propagative activities should never be conducted in a weedy- area or in one with near-by fields of virus-infected herbaceous crops. Because of the complexities involved, consult your farm advisor for details in specific cases. The Mother-Block Principle When one has purchased or developed healthy stock, it should be grown in isolation and with special care. To main- tain this material in a separate planting is the mother-block principle. It is ob- viously easier to prevent introduction of a pathogen into a small special block of stock than into large plantings. It is easier to isolate such a small nucleus planting, to inspect it carefully and re- move any diseased plants, and to control insects that may spread virus diseases. The job is, therefore, more likely to get done. There should be only one-way traffic with the established mother block; cut- tings or plants may be taken out but no plants should be brought into it (fig. 115). Above all. no buds or grafts should be placed in plants of the block; undetected viruses and V erticillium wilt can thus be introduced and ruin the planting. Visitors should not be per- mitted in the glasshouses containing the mother blocks. By carefully selecting within mother- block plants, one can prevent both loss of horticultural quality and increased variation, and may thus be able to effect distinct improvements. The maintenance of the mother block, and the selection of plants in it, should be under the personal supervision of the owner, never referred to routine help. Growers who do not maintain mother blocks often sell the best stock in an effort to please their customers, and then plant the remainder for next year's propagation. Because many diseases cause decreased plant size and vigor, such a practice may actually be selecting toward disease. The phe- nomenon of "running out" of horticul- tural varieties probably largely repre- sents such accumulation of disease and weak variants. The production and mer- chandising of plants, and the mainte- nance of the basic stock for future propa- gation, must be handled as independent [ 235 ] ' '*, a Pathogen-free stock obtained from: Chance healthy plants New seedlings Practices which enable plant to grow away from pathogen Cultured-cutting technique Heat treatment Chemical treatment Sanitary practices Selected growing areas or conditions >F Propagate and grow Careful check for freedom from pathogens Establish isolated permanent mother blocks Healthy — ^ propagating stock sold a o 3 TJ O E E o u Propagate, plant, and grow Crop "* Propagate, pro- ^ — plant, duction and grow Grow on V_ for crop ^_ __ _ Check for freedom from pathogens before each propagation Propagate from plants z. if healthy + Grow a single crop, and discard plants. Grow in isolation for single early propagation. Plant to establish temporary mother block. Must be isolated from commercial production. Grow for crop Fig. 115. Diagram showing the segregation of propagation and commercial production, and the sequences of operations that may be followed. Only in the last method (not recommended; used only in exceptional cases) is propagating material taken from commercial production stock. Commercial growing activities shown in boldface type, stock propagation in ordinary type. Treated soil and containers to be used throughout. See text for details and specific crops. and isolated activities. To do so is a long-term service to the nursery indus- try, the grower, and his customers. An example of the danger of not hav- ing a mother block of rootstock may be cited. A nursery budded one of its new hybrid rose seedlings on three under- stocks taken from a commercial field and, after the plants were established, the seedling was lost. Buds taken from these three plants were placed in a field increase-block. A large percentage of the plants developed variegated symptoms of yellow mosaic, and it was found that two of the three original plants had been infected from the rootstocks. It was CAUTION: Many of the chemicals mentioned in this manual are poi- sonous and may be harmful. The user should carefully follow the pre- cautions on the labels of the con- tainers. necessary to destroy all detectably in- fected plants and delay introduction for another year. The world thus narrowly missed losing what proved to be a fine Ail-American rose. Sanitary Procedures This principle for maintaining healthy stock is less specific than the foregoing, and therefore more difficult to apply. It is, however, of the utmost importance that careful sanitation be practiced. A number of procedures have been out- lined for this, and are summarized in "A Nursery Sanitation Code," Section 1. The essence of a successful sanitation program is the positive mental attitude of the grower. One should analyze the many nursery operations for potential "leaks" of contamination and eliminate them before trouble appears, rather than wait until the appearance of disease forces corrective measures. This manual provides information necessary for such analysis. [ 236 ] SECTION m i Beneficial Soil Microorganisms John Ferguson Microorganisms in the soil Transformation of plant nutrients by microorganisms Effect of soil treatments on microorganisms Controlled colonization: a future step Xhis section deals with the numerous biological, physical, and chemical proc- esses in the soil which result from the activities of microorganisms. An at- tempt is made to portray briefly the soil microbiological population, the interre- lations among microorganisms, the microbial transformation of various ele- ments essential for plant growth, the ef- fect on plant disease and nutrition of man's activities in modifying the soil population, and the general applications of this knowledge to the growing of better plants. One of the essential functions of soil organisms is decomposing organic mat- ter and converting to forms available to crop plants such elements as carbon, nitrogen, calcium, magnesium, potas- sium, phosphorus, sulfur, iron, and zinc. There are only limited sources of several of these elements, especially carbon, nitrogen, and phosphorus, in a form available to plants. These essential ele- ments must be returned to inorganic, available forms, a process carried out primarily by the soil population. Thus, through their various activities, soil microorganisms enable life to continue by keeping in constant circulation the elements most essential for plant and animal life. MICROORGANISMS IN THE SOIL Abundance terial in normal field soil there are about Untreated natural top soil has a vast 10 pounds of microorganisms. Soil fungi population of microscopic plants and alone consistently occur in quantities animals. The members of this extremely weighing 1,700 pounds per acre, and active population include many forms of they are only one of the numerous repre- life. For every 100 pounds of plant ma- sentatives of the soil community. Bac- [237] teria commonly occur in amounts of 30 tain plant parasites may persist in soil to 40 pounds per acre, each pound com- for several months, however, and some prising upwards of 500,000,000,000 in- dividual bacteria. (Five hundred to 1,000 bacteria placed in a line would extend across the head of a pin.) It is interesting that another group of microbes very similar to bacteria, the actinomycetes, occur in high enough concentrations to give soils their typical musty odor. Most of this vast microbial population is con- centrated in the upper 1 to 3 feet of soil, the depth depending upon the environ- ment and nutrient availability. Competition Soil may well be likened to a minia- ture jungle in which some species prey upon others and all compete for avail- able space and nutrients. All forms of microscopic life (including bacteria, fungi, actinomycetes, algae, nematodes, and protozoa) exist in the soil in a state of dynamic equilibrium, or ever-chang- ing balance. Any change, such as in food supply or environment, affects all mem- bers of the colony and, therefore, dif- ferent species become dominant from time to time. Any new addition to this population faces intense competition and often perishes or develops slowly until some factor shifts to its advantage. Thus for any so-called "microbial inoculant" to be effective, the environment must be favorable and a ready nutrient source must be available. Relation to Crop Plants Parasites and saprophytes In their relation to plants, micro- organisms can be considered as either parasites or saprophytes. The parasitic forms are capable of growing on the liv- ing plant and causing its decreased growth or death. Saprophytic microor- ganisms grow only on dead tissue or its decomposition products. The typical soil population consists of organisms which arc largely saprophytic in nature. Cer- [238] are capable of leading a normal exist- ence there. In water molds, for example, the parasitic activity may be relatively unimportant for the survival of the or- ganism, and only incidental to its normal saprophytic life. Within one species of the fungus genus Fusarium are purely saprophytic forms and highly specialized vascular parasites capable of saprophytic sur- vival. This illustrates the escape from competition achieved through the spe- cialization of parasitism. As a sapro- phyte an organism must compete with the majority of the soil population for the available organic matter and mineral elements, while as a parasite capable of penetrating a living plant, most of the competition is eliminated. The parasite damages the plant not only through its own activities, but also by providing an entrance for secondary organisms ca- pable of damaging the plant but unable to penetrate alone. Direct and indirect effects The number, kinds, distribution, and interrelations of soil organisms have a very decided effect on plant growth. Within this delicately balanced complex are organisms favorable to plant growth and those which affect growth adversely. Included in the group which directly favor plant growth ("beneficial," fig. 116) are, for example, microbes which aid in making essential nutrients avail- able, decompose toxic materials, or im- prove soil structure. Organisms which inhibit plant growth ("harmful," fig. 116) may do so either by direct attack, as with parasites or predators, by com- petition for space or nutrients, or by the release of toxic substances. As illustrated in figure 116, the harm- ful or the beneficial organisms are in turn affected, either favorably or ad- versely, directly by other microbes or < * Beneficial T Beneficial to the plant- beneficial organisms — thus beneficial to the plant. / + *< u o / _>> BENEFICIAL ORGANISMS 4 Ex. Ammonifying bacteria and fungi, Nitrifying bacteria u 0) l_ \ 1 - -C O u. O) •♦- c Q. Harmful Antagonistic to the plant- beneficial organisms — thus harmful to the plant. / + C o c (1) o / o 1c # c E IS) 'c o en o i o IS) 4- -i c r 4- O c D_ O i/> E o_ QJ w o 'c n JZ o c en c a oo o en a> o F c ^— E NITRITE NITROGEN NO 2 toxic to plants Nitrosomonas spp. Specific bacteria, Bacteria > 11/ AMMONIUM NITROi NH 4 available to plants Fig. 1 17. Schematic representation of the most important nitrogen transformations in soils. Nitrogen-fixing bacteria As seen in figure 117, atmospheric nitrogen is fixed into microbial cell material by free-living bacteria. This re- action does not produce appreciable amounts of nitrogen, especially in soils with growing plants. Symbiotic nitrogen-fixing bacteria, which live in the root nodules of legumes, cause the fixation of atmospheric nitro- gen, which becomes available to their host plant. Symbiotically fixed nitrogen is a very important source of nitrogen, but limited to the Leguminosae. Ammonifiers and nitrifiers Whatever the source, if the nitrogen is tied up in organic compounds (plant materials or animal materials) the ac- tion of soil organisms is required to convert it to ammonium or nitrate nitro- gen before the plant can utilize it. Some organisms first carry the reactions from ammonium to nitrite, and then others convert from nitrite to nitrate nitrogen. Denitrification (reduction of nitrate to nitrite and then to gaseous nitrogen) can be performed by many organisms, but is not important where soil is well aerated. The essentials of nitrogen conversion in soils are summarized as shown below. Organic nitrogen (unavailable to plants except in the rare uptake of amino acids) is broken down by various kinds of organisms to produce ammonium. Many common air-borne fungi, actino- mycetes, and bacteria (including spore- formers) are capable of causing this conversion. The steps from ammonium to nitrate, on the contrary, are performed by specific, non-spore-forming bacteria. Organic nitrogen (unavailable) Type 1 organisms Bacteria Fungi Actinomycetes Ammonium (available) Type 2 organisms Specific bacteria Nitrate (available) [245] These bacteria are among the most sen- sitive of soil organisms. Conditions and treatments (heat, chemicals, pH, and so on) readily withstood by the ammonify- ing organisms cause injury or death to the nitrifying bacteria. The specificity and sensitivity of the nitrifiers, as opposed to the abundant types and hardiness of the ammonifying organisms, account for the fact that am- monium is produced under a much broader range of conditions than is nitrate. Conditions and treatments which inhibit the nitrifiers often have little effect on ammonium production. For example, highly acid media inhibit nitri- fication much more than ammonium formation, as illustrated in the following data from Section 7. In a growing medium at pH 3.9 the total ammonium and nitrate nitrogen was 251 ppm, of which only 26 ppm was in the nitrate form. However, at pH 5.9, with a total of 505 ppm, 350 ppm ap- peared in the nitrate form. Thus the medium at pH 5.9 had about twice as much total available nitrogen but nearly fourteen times as much in the nitrate form as in the medium at pH 3.9. Low temperatures also reduce nitrifi- cation much more than ammonification. Under most environments some organ- isms included in the ammonifying popu- lation can grow at temperatures that inhibit the specific bacteria concerned with nitrification. In normal, untreated field soil, however, the bulk of nitrogen available to the plant appears in the nitrate form. Ammonium formation usually becomes a factor only after some type of soil treatment, as is considered later in this section. Carbon Carbon makes up an average of ap- proximately 50 per cent of the dry weight of all chemical elements in plant and animal tissues. Carbon dioxide gas ia the source of carbon for the growth of green plants. Animals derive their car- bon from plant materials. Thus, the primary source of carbon for plant and animal life is carbon dioxide gas. Carbon dioxide is present in the at- mosphere in a concentration of 0.03 per cent, and about %5 of the total carbon content of the atmosphere is consumed each year by the plant world. This sup- ply is never exhausted, however, largely because it is constantly being replen- ished by the microbial decomposition of organic substances in the soil, but also by means of plant and animal respira- tion and industrial burning. When mi- croorganisms completely decompose an organic material, the carbon goes off as carbon dioxide. Because of the activities of soil mi- crobes, the atmosphere of the soil con- tains from 20 to 200 times as much carbon dioxide as air. This high carbon dioxide content of the soil results in the formation of carbonic acid, which aids in bringing insoluble elements such as phosphorus into solution. The produc- tion of carbon dioxide from soil is, in fact, often used as a measure of the activity of the soil microbial population. The transformations of carbon are sum- marized in figure 118. Sulfur Sulfur is another element essential to plant growth that undergoes microbial transformation. Sulfur reaches soil as organic compounds (plant and animal residues), elementary sulfur (fertilizers, fungicides, soil amendments), or sul- fates (fertilizers, amendments, irriga- tion water) . The transformation of sulfur-contain- ing organic compounds resembles that of nitrogen. Instead of ammonium (NH 4 ), hydrogen sulfide (H 2 S) is formed, and through various reactions sulfate (SO.,) is produced. Bacteria known as "sulfur bacteria" are respon- sible for the rapid conversion of ele- mental sulfur to sulfuric acid. [ 246 ] /* Atmospheric Carbon Dioxide co 2 Photosynthesis t Respiration Microbial Decomposition Microbial Decomposition Respiration Fig. 1 18. The carbon cycle. Plants Organic Compounds Animals feeding on plants I Animals Organic Compounds j This change (oxidation) not only renders the sulfur available for plants but makes the soil reaction more acid. The reaction is made use of in reclaim- ing alkali soils, to reduce certain plant diseases such as potato scab, to increase iron and phosphate availability, and to make soil slightly acid or neutral for the growth of certain plants. One pound of soil sulfur when oxidized to sulfate by organisms produces about 3 pounds of sulfuric acid. Phosphorus Phosphorus, essential for plant growth, is found in soil organic compounds or as insoluble phosphates. Several bacteria and fungi can liberate phosphorus from organic compounds in an inorganic form. Phosphorus is a constituent of mi- crobial cells and, just as with nitrogen, may be rendered temporarily unavail- able to plants when materials low in phosphorus are rapidly decomposed. Insoluble phosphates are made avail- able mainly by the indirect action of microorganisms. Many of the organic and inorganic acids produced by soil microbes react with the insoluble phos- phates to form soluble compounds. Ger- retsen (1948) has shown that organisms in the rhizosphere have considerable solvent action on insoluble phosphates. In the past this has been attributed to the roots themselves. Other Essential Elements Other essential elements such as potas- sium, calcium, magnesium, and iron are affected either directly or indirectly by soil microorganisms. When organic sub- stances are decomposed by microbes they release potassium, which is then available to higher plants. Although potassium is usually added to the soil in a soluble form, organic acids from microorganisms help liberate it when it becomes fixed by the soil. Calcium, mag- nesium, and iron are affected indirectly by the actions of soil organisms, espe- cially through acid production. Many experimenters have shown that soil organisms can affect manganese nutrition of plants. Manganese-deficiencv symptoms can result from the action of certain microorganisms that render [247] manganese insoluble by oxidizing it. Treatments which eliminate the man- ganese-oxidizing organisms alleviate the deficiency. A similar situation was reported in California in the little-leaf rosette disease of peaches, attributable to zinc defi- ciency. Ark (1937) found high bacterial concentrations in the root zone of sus- ceptible trees, but overcame the trouble by applying zinc salts or by sterilizing the soil. Most elements essential to plant growth are also required by soil organ- isms. This direct competition, along with indirect effects, such as release of acids, so closely links the activities of these two groups that any change in the microbial population has an effect on plant nutri- tion. EFFECT OF SOIL TREATMENTS ON MICROORGANISMS One of the important considerations in the increasing use of soil treatments by chemicals and steam is their effect on the population of soil organisms, both harmful and beneficial. When soil is treated with the recommended chemicals (Sec. 11) or steam (sees. 8 and 9) many excellent results are achieved. Soil-borne plant pathogens are controlled, weeds and insects are eliminated, and very marked plant-growth increases result (fig. 119). Treatment of soils is neces- sary because of these many advantages, but the effect on beneficial soil organ- isms may be disadvantageous. decontamination Hazard When soil is treated, the number of soil microorganisms is greatly reduced for the first few days; then it rises and eventually exceeds that of untreated soil. Let us consider what occurs when the soil is treated, and disease organisms gain access to this soil. The treatment destroys a large part of the dense popu- lation of soil microbes, and the first organisms to return after treatment meet no severe competition. Thus, if plant pathogens are among the first to re- colonize the soil, they develop rapidly, .■:■■;:■ ,,; Fig. 119. Increased growth of tobacco plants in chloropicrin-treated soil as compared to un- treated soil (left). [ 248 ] Fig. 120. The effect of steaming field soil on plant growth and disease spread. A, Steamed soil; excellent stand and growth. B, Nontreated soil; poorer stand and growth than in A. C, Steamed soil with Rhizoctonia added at arrows, showing greater pre- and postemergence damping-off than in nonsteamed soil. D, Nonsteamed soil with Rhizoctonia added at arrows. and cause severe disease losses. It there- fore is important to the grower that pathogens do not gain entrance to treated soil. Benches, flats, equipment, seed, and the numerous other sources of pathogen introduction (Sec. 3) become Danger of Inadequate Treatment One source of trouble is the use of treatments which destroy a portion of the soil population, but leave pathogens unharmed. Severe losses often occur after such treatments, because of the in- potentially more dangerous when using crease of the surviving pathogens under treated as compared with untreated soil. The damping-off of seedlings in nur- sery soils in California caused by the soil fungus Rhizoctonia solani may be cited as an example (Sec. 3). This path- ogen can be effectively eliminated from soil by any of several treatments. If, however, the fungus then gets into this treated soil, the resulting loss is much more severe than that suffered in un- treated soil (fig. 120). the conditions of decreased competition. The two most common ways of creating the above condition are: 1. The use of fungicidal treatments at lower than recommended rates. 2. The use of treatments that control other pests or specific diseases and may markedly decrease the soil popu- lation, but not destroy certain plant pathogens. In both cases the loss results from the dis- ruption of the balance in the soil popula- [249] tion and the shift to conditions favoring the increase of a plant pathogen. Examples of damage due to treatments at too low a dosage are most often seen with treatment-resistant pathogens such as the soil fungus Verticillium albo- atrum. V erticillium is relatively hard to kill and therefore it is possible to apply treatments which remove much of the soil population, leaving V erticillium to infect more severely. Where V erticillium, Fusarium, or other more resistant fungi are a problem it is dangerous to use less than recommended dosages for their control. Also, if treatments for control of these diseases are done improperly or incompletely, the same severe loss can occur as from too low a dosage. Examples of the second case are also encountered where treatments for given pests enhance the damage due to other pathogens. Treatments used for the con- trol of nematodes have sometimes re- sulted in increased losses from Verticil- Hum wilt. One experimental fungicide recommended specifically for Rhizoc- tonia control has been reported to con- trol Rhizoctonia, but heavy losses due to water molds, not important before treat- ment, may then be sustained. These examples are cited to illustrate the role soil microorganisms play in disease control, and the importance of considering them in all control opera- tions. Increased Need for Sanitation The proved advantages and wide- spread use of soil treatments with the resulting increased hazard and severe effects of recontamination, point to the need of a more thorough knowledge of the sources and means of combating pathogen introduction. The most impor- tant means of eliminating sources of con- tamination is a vigorous and constant program of sanitation. To reap the full benefits of soil treatment the grower must ever be on guard to protect the treated soil. Every operation must be checked for the possible introduction of disease material (see "A Nursery Sanita- tion Code" in Sec. 1). The chain of suc- cessful growing, though containing strong links of adequate nutrition, disease-free soil, and clean stock, can easily break if a weak link, such as the presence of pathogens on flats or benches, exists. The alert grower can more effectively combat recontamination if he under- stands something about the sources of disease organisms and the practices which introduce them into soil. The two main channels of introducing pathogens into pathogen-free nursery soil are planting stock and infested soil or plant particles which may come into contact with the clean soil in many ways. The most important plant pathogens causing damping-off, root rots, and related diseases are not air-borne, but must de- pend upon the mechanical transfer of infested soil or water and infected plant tissue for their spread (Sec. 3) . This fact enables the alert grower to reduce or eliminate the transfer of infested mate- rial by careful practices. CONTROLLED COLONIZATION: A FUTURE STEP To Retard Pathogens Since soil treatment has proved so advantageous and is so widely used, and since contamination of treated soil may be severe, there is need for a method of protecting treated soil from introduced disease organisms. In an effort to meet this need, studies were conducted by the author, using organisms antagonistic to plant pathogens but without adverse effect on plants. The purpose of report- ing this work is not to make any recom- mendations at this time, but to provide advance information on some steps be- I 250 1 ing taken toward a future solution of soil-disease problems. The retardant organisms, singly or in groups, are added to soil immediately after treatment. Since they are the first to return, they make rapid growth, just as would pathogens if they were the first organisms returning after treatment. These beneficial or pathogen-retarding organisms colonize the soil and protect it from recontamination. They act either as antibiotic producers or as parasites or competitors of pathogens or in various combinations of these types. The con- tinual production of antibiotics in the soil by organisms appears more effective than the addition of antibiotics alone, since these chemicals break down rapidly. The effect of several different organisms in restricting the spread or completely stopping damping-off due to Rhizoctonia solani is illustrated in figures 121 and 122. A retardant organism was added to flats of steamed U. C.-type mix at the time of seeding. The flats were also inoculated with Rhizoctonia in heavy enough concentration to cause eventual 100 per cent loss in flats not protected with retardant organisms. Complications The results are very encouraging, but some of the complications should be dis- cussed. The Rhizoctonia-r etar ding effect appears to last for the entire susceptible seedling stage with some organisms, but is only temporary with others. The re- tarding effect of Myrothecium (a com- mon soil fungus) diminished after a month in flats with growing plants. In- creased concentrations of Myrothecium high enough to inhibit Rhizoctonia for longer periods stunted the pepper seed- lings. Both this stunting effect (fig. 123) and the Rhizoctonia-retarding effect could be enhanced by the addition of various organic amendments. Since Myrothecium is a rapid cellulose decom- poser, materials high in cellulose gave the greatest effect. This case serves to Fig. 121. The effect of adding Rhizoctonia with and without a retardant (Myrothecium) to flats of a steamed U. C.-type mix planted to peppers. A, (upper right) Rhizoctonia plus My- rothecium added at this point; damping-off prevented. B, (lower left) Rhizoctonia added at this point; damping-off severe and continu- ing. illustrate the sequence of organisms in soil resulting from the addition of or- ganic matter. When cellulose-rich mate- rial was first added, the Myrothecium population increased rapidly, with re- sulting retarding effect on Rhizoctonia. As the cellulose became decomposed the population shifted, and with other or- ganisms becoming dominant the retard- ing effect of Myrothecium on Rhizoc- tonia diminished. The soil pH has a marked influence on the organisms used in controlled coloni- zation. In the acid range, fungi such as species of Penicillium and Trichoderma are most effective, while, as the reaction approaches neutrality, species of the genus Streptomyces become more prom- ising. Competition escape accounts for one of the difficulties encountered in the con- trolled colonization work. A soil may be colonized with a Rhizoctonia-retarding organism to the extent that growth of Rhizoctonia is completely restricted. If this same soil contains growing plants, Rhizoctonia may spread from plant to [251] Fig. 122. Protection from Rhizoctonia damping-off achieved by the addition of a retarding organism to flats of a steamed U. C.-type mix planted to peppers. A, Control flat; no retardant or Rhizoctonia added. B, Trichoderma sp. added as a retardant to the whole flat, and Rhizoctonia added at arrow; resulted in complete protection comparable to control flat. C, Penicillium sp. added as a retardant to the whole flat, and Rhizoctonia added at arrow; resulted in a small area of preemergence damping-off, but no postemergence damping-off. D, Rhizoctonia alone added at arrow; resulted in complete loss. plant above the soil surface. Thus, under conditions of plant crowding and high humidity, Rhizoctonia may escape the retarding effect of the soil flora. The above complications do not mean that it will not prove feasible to use con- trolled colonization to protect against disease loss, but indicate the need for further study and knowledge of soil microorganisms in relation to plants. Prerequisites for the program The entire concept of biological con- trol of soil-borne plant pathogens shows increasing promise, and the nursery in- dustry is unique in having available all the features requisite for a successful program using beneficial organisms. Before the addition of organisms to soil can be effective in protecting the soil from subsequent contamination by path- ogens,, most of the existing soil micro- flora must be destroyed or the existing organism balance changed in some other way. This is already accomplished in most nurseries by either steam or chemi- cal soil treatment. Consistent protection by beneficial organisms is also dependent upon a uniform soil. This is most satisfactorily obtained by the use of a U. C.-type soil mix, which can be accurately duplicated. Finally, the controlled and stable con- ditions of nursery growing further add feasibility to obtaining positive results with biological control. California nur- [ 252 ] sery operations so closely parallel the laboratory conditions used in these studies that results in protecting soil against pathogens, such as those illus- trated above, appear to be entirely pos- sible. No major modifications of the ultimate techniques developed in the re- search laboratory should be necessary for commercial application. Untreated soil contains such an abun- dance of life that little is gained by adding organisms without some type of treatment to overcome the natural biological buffering capacity of the soil. This can be accomplished by treatment to alter the soil population, or by sup- plying a specific nutrient substrate to make a favorable environment for the desired organisms. Other environmental factors such as pH, temperature, and moisture can also influence the popula- tion and can be controlled. It is impor- tant that the grower realize that he cannot simply add an organism to the soil and expect it to have some desirable effect. To Improve Nitrogen Nutrition In addition to the effect on disease, removal of beneficial soil organisms can adversely affect the nitrogen nutrition of plants. Two points are considered: 1. An initially low level of available nitrogen after treatment. 2. The bulk of available nitrogen after treatment appears in the ammonium form. In general, plants continually sup- plied with ammonium synthesize organic nitrogen rapidly and thus tend to deplete their supply of sugars and starch (Sec. 7). An undesirable condition of carbo- hydrate deficiency may result in softer and more succulent plants, as in the case of snapdragon seedlings. Often plants in this condition cannot survive trans- planting. Nitrate is absorbed and used Fig. 123. The effect of amendments in increasing both the retarding effect on Rhizoctonia and the stunting of plants when added with Myrothecium to flats of a steamed U. C.-type mix planted to peppers. Left, sterilized wheat straw added to upper half, no amendment to lower half of flat. Note greater restriction of Rhizoctonia damping-off with the wheat straw amendment. Right, sterilized pepper seeds added to upper half, no amendment to lower half of flat. The three representative seedlings on each card show the greater stunting of pepper seedlings when sterile pepper seeds are added. [253] comparatively more slowly, and plants supplied this form of nitrogen usually form more lignin and cellulose, the major constituents of the mechanical tissues of plants. A few plants apparently require at least part of their nitrogen in the nitrate form for best growth. Am- monium can also be toxic if high enough concentrations are reached (Sec. 7). As explained earlier, organisms are needed to convert unavailable organic nitrogen into an available form. The initial reduction in all organisms im- mediately after treatment accounts for the initial lag in available nitrogen from organic sources. A partial alleviation of this low initial nitrogen level could be achieved by the introduction of organ- isms especially efficient in converting un- available organic nitrogen into available forms. Experiments with several soil fungi have shown this approach to be feasible. The use of nitrate starter solu- tions is also helpful (Sec. 7). Introducing nitrifiers After steaming, the predominant available nitrogen is in the ammonium form, owing to the abundance and re- sistance of type-1 organisms as compared with those of type 2 (p. 245). Further- more, the type-1 organisms include many common air-borne fungi, actino- mycetes, and bacteria. These organisms colonize treated soil with comparative rapidity and some of the spore-forming bacteria survive most treatments. In con- trast, the type-2 or nitrifying organisms are readily killed by most treatments and are much fewer in species and numbers. Thus, the nature of the organisms in- volved explains the ammonium accumu- lation, and also indicates a method of carrying the reaction on to the forma- tion of nitrate nitrogen by adding the appropriate organisms to soil. This has been accomplished in the University laboratories in Los Angeles by adding to the steamed mix a water suspension of soil containing nitrifying organisms. As discussed in Section 7, in 21 days the nitrate nitrogen in one series with in- oculated soil averaged 101 ppm com- pared to 8 ppm in soil not inoculated with nitrifying organisms. To be com- pletely practical for all cases the time required to reach the 21-day level should be reduced, but these tests show the po- tential value of controlling soil nitrogen by adding certain organisms. A Possible Future Program A future program for a grower of plants in containers may be envisioned from these facts. Flats, pots, and other containers filled with a U. C.-type soil mix are treated with steam or chemicals to remove all organisms, and then a suspension of organisms is sprayed on the flats before planting. This suspension would include organisms capable of sup- pressing accidentally introduced patho- gens. Also included would be organisms which would promote the early produc- tion of nitrate nitrogen. This program would greatly reduce or eliminate soil- borne disease problems and would give plants grown in this way a measure of protection even if set out for growing in untreated field soil. Plant nutrition when organic nitrogen is used would be greatly improved. With all the special features of nursery growing in a U. C.- type soil mix, this program is a definite possibility. [254] SECTION Importance of Variation and Quantity of Pathogens Richard D. Durbin The variability of plant pathogens The inoculum potential Infected seed and stock Longevity in soil Mixed infections Obligation of the nursery rowers often ask, "Since rhizoc is already present in my soil, what dif- ference does it make if I introduce more of it?" It is commonly assumed that be- cause a fungus is present in the soil, the inadvertent addition of more of it with infected nursery stock, seed, bulbs, or in- fested soil will make little or no dif- ference in crop losses. This mistaken rea- soning can have serious consequences; it may lead to loss of the given crop and infestation of the field, ruining it for cer- tain crops. THE VARIABILITY OF PLANT PATHOGENS The variability of living organisms is quite generally accepted. Every indi- vidual in a given species is different in many respects from all the rest, yet shares certain common characters with the other members of the species. Thus we say that plants and animals having the same scientific or common name are similar but not identical. All humans are classified as Homo sapiens, although we have only to look around to see that each is unique in some respect. In flower- ing plants man has taken advantage of some of the apparent differences within crop species to develop varieties which are outstanding in yield, quality, or adaptability to certain environments. Lower plants, such as fungi and bac- teria, also exhibit this characteristic variation, but it is not so evident because of their small size. Variation in these microorganisms may be evident in the things they are able to do, that is, in their physiological activities; for in- [255] stance, they may produce more or a slightly different form of a substance such as penicillin or streptomycin. While this fact has been of great benefit to in- dustry, it has seriously complicated the prevention of diseases in plants and animals. Since the ability to produce plant dis- ease involves the interaction of complex physiological systems of pathogen and host, it is not surprising that one also finds variation here. The extensive varia- tion among crop plants is not considered in this section, but some mention of parasite variation is pertinent. This sub- ject is of concern to both grower and pathologist. Host Range and Virulence Fungi The fungus Rhizoctonia solani con- tains individuals or strains which may vary in host range, pathogenicity on any one host, and response to the en- vironment. Each strain is able to attack a given group of plants, with the patho- genicity of many strains overlapping on a single host. One strain may be strongly virulent to pepper but unable to attack Tagetes, whereas another may attack Tagetes vigorously; some strains from tomato are virulent to bean while others are not. Many such examples exist. On any particular host plant the disease produced may vary from stunt- ing to complete loss from damping-off, according to the strain involved. In one test on variability in patho- genicity, eleven isolates of Rhizoctonia solani were compared in virulence to pepper seedlings; figure 124 shows some typical results. Although pathogenic to the source host in each case, many of the isolates were not pathogenic to pepper, while others were more damag- ing than was one originally from pepper. These differences in virulence among the isolates exemplify the danger of in- troducing additional Rhizoctonia into an already infested soil. Why take the chance? Some strains of Rhizoctonia solani are almost saprophytic and cause little plant damage, such as those commonly pro- ducing black sclerotia ("the dirt that won't wash off") on potato tubers. Dif- ferent strains are now known to cause a serious potato stem rot, although for many years they were considered to be the same as the tuber-attacking forms. Tubers were even treated with fungicides to eradicate the sclerotia in the hope of stopping the stem rot, until it was dis- covered that the diseases were different. Still other strains of R. solani are re- stricted to the above-ground environ- ment, and cause foliar blights of various crops in the southeast United States. In southern California we are most familiar with strains causing root rots and damp- ing-off of nursery crops. Stephen Wilhelm 1 has found that Verticillium albo-atrum includes forms varying from those apparently unable to invade the host, through those which may invade without producing the dis- ease, to those which invade and produce varying degrees of disease on a given host. Some forms that do little harm on one host may be severe on another. Thus, among isolates pathogenic to stock some are nonpathogenic on tomato, others in- vade tomato roots but cause no above- ground disease symptoms, while still other isolates are severely pathogenic to tomato. Furthermore, Wilhelm and Raabe (1956) have recently found a strain of Verticillium albo-atrum that produced wilt on the Manetti rose root- stock, formerly resistant to known strains of this fungus. The genus Fusarium includes types which live saprophytically, or cause cortical stem and root rot or vascular wilts. F. oxysporum includes sapro- phytes as well as about twenty-five named forms that are highly specific in 1 Department of Plant Pathology, University of California, Berkeley; unpublished data. [ 256 ] Fig. 124. Effect of several isolates of Rhizoctonia solani on pepper seedlings. Seeds were sown in treated soil and immediately inoculated at the lower end with equal amounts of the fungus isolate. The area inside the white line is the zone of preemergence damping-off. Photos taken 18 days after planting. Left to right, top row: uninoculated; inoculated with a gladiolus isolate (isolates from tung, soybean, cotton, Dieffenbachia, and alfalfa responded in the same way); inoculated with a mild isolate from pepper. Left to right, bottom row: inoculated with a poin- settia isolate; inoculated with an isolate from morning-glory (an isolate from lima bean re- sponded in the same way); inoculated with a virulent isolate from pepper. The mild pepper isolate caused post- but no preemergence damping-off, while the poinsettia isolate caused pre- but almost no postemergence damping-off. The virulent pepper isolate caused both the pre- and postemergence phases. their ability to invade the vascular sys- tem of plants. These forms are so spe- cific that one of them will attack only certain varieties of a given crop species, a fact utilized in controlling them through resistance. Thus, the form which attacks aster is limited to that crop and to certain varieties of it as well. Oc- casionally a form appears that is able to attack a crop variety (for example, of tomato or pea) previously resistant to the Fusarium wilt. In such cases this may account for the apparent breakdown of resistance, because in reality two dif- ferent diseases are present. As in Rhizoc- tonia, isolates of any one of these forms may vary widely in the severity of dis- ease they produce in a given susceptible [257] host variety. When a grower is indif- ferent about introducing a Fusarium wilt to his fields, he is ignoring the dis- ease potentialities. For example, he may not presently be raising asters and thus be unconcerned about the dumping of infested aster refuse or soil on his land. Many years later he or someone else may wish to plant asters in these fields, and find that by the second year of the at- tempt, the residual fungus has built up and increased disease losses to a ruinous extent. Some growers assume that all water molds are alike and that they occur everywhere. However, some strains of Pythium debaryanum are said to induce 100 per cent root rot of spruce seedlings, while other strains under the same en- vironmental conditions are purely sapro- phytic. Roth and Riker (1943) found that the damping-off of red pine varied from 36 to 87 per cent under a given en- vironment, again according to the strain of the fungus used. It is now clear that the worst disease problem of both heather and avocado in California is root rot caused by Phytophthora cin- namomi. Although water molds as a group may be generally distributed, this one is not widespread in California even yet, and those plantings suffering from it usually can trace their infestation back to the nursery source of the stock. There is also evidence for some biological spe- cialization within species of this genus. According to Tucker (1931), Phytoph- thora capsici and P. parasitica f. nico- tianae are the only Phytophthora species attacking pepper and tobacco, respec- tively. # Thielaviopsis basicola commonly causes black root rot, stem decay, or graft failure on ornamentals; it has a host range of over 120 species in 30 families. It is known to be comprised of races which cause varying amounts of disease on some crops while on other crops they may not be pathogenic at all. Isolates from poinsettia, Primula ob- conica, cyclamen, and tobacco, for ex- ample, were reported in some inocula- tion experiments as most pathogenic to the host from which they were originally isolated, while the reciprocal inocula- tions yielded less, or in some cases no disease. In other experiments isolates from tobacco were more virulent on Primula, but less virulent on cotton seedlings than isolates from Primula itself. Bacteria Variability in virulence has also been noted in the bacterial pathogens that cause fire blight and bacterial fasciation. Crown gall has been shown to have some degree of host specialization. Iso- lates from marguerite daisy will produce medium-size galls on tomato and rasp- berry, but rarely do so on apple. Rasp- berry and probably loganberry isolates are pathogenic to tomato and apple but not to marguerite, while some isolates from apple do not seem to be pathogenic to other hosts. Nematodes There are also host-restricted races in plant-parasitic nematodes. In the stem or bulb nematode, Ditylenchus dipsaci, some of the "races" are comparatively unspecialized and are parasitic on a wide range of hosts; other "races" are more specialized and able to attack only a few hosts, while still others may sur- vive only on one or two hosts. The "races" attacking hyacinth and narcissus bulbs fall into this last category. Ap- parently nematodes attacking narcissus cannot attack hyacinth, although they may attack onions. Recent work has shown that some of these "races" are actually distinct nematodes (species). This situation further illustrates the danger in assuming that two pathogens are identical because they are presently called by the same name. In what is generally called "the root- knot disease," as though caused by a [ 258 ] single nematode species, it is now recog- nized that several are actually involved, and that they differ in host range. Shalil peach rootstock in some areas has some- times "lost resistance" to the nematode when attacked by a different population of what was then considered as one species. Now we know that there are three species of root-knot nematode com- monly found in California, only one of which attacks Shalil rootstock. Popula- tions from Philodendron sp. are able to multiply on Persian clover, but nema- todes from Pothos aureus are not. Ac- cording to the species or race of the nematode present, some plants are at- tacked in one locale but not in others. It has even been suggested that populations of root-knot nematode can be identified on the basis of whether or not they attack peanuts, pepper, watermelon, and Lyco- persicon peruvianum, and by the reaction of snapdragons to them. Environmental Response Not only may different strains of a given microorganism exhibit differences in respect to host range and severity of attack, but they may exhibit differences in response to physical and physiological factors of the environment. In industry this characteristic of physiological vari- ation has been utilized to obtain strains which are more efficient in doing specific jobs, such as antibiotic production, alcoholic fermentations, and production of dairy goods, as well as an array of many organic compounds. Comparatively little is known about the interactions of parasitic organisms with other soil microorganisms and with the host, and of the effects of fungicides, soil atmosphere, light, nutrient or vita- min deficiencies, or root secretions upon them. Our knowledge in this field, how- ever, is rapidly expanding. Temperature relations It has been reported that strains of Rhizoctonia vary in temperature re- quirements for disease production. The optimum temperature may vary from 59° to 95° F. When two strains which differ in their response to temperature exist together in the soil, disease will occur over a wider soil-temperature range than if either is present alone. Soil depth and carbon dioxide content Work in progress by the author in- dicates that strains of Rhizoctonia solani differ widely in their tolerance to the concentrations of carbon dioxide found in the soil. The fast-growing aerial iso- lates are relatively intolerant of carbon dioxide and therefore probably are unable to compete successfully with other organisms underground, where the carbon dioxide often is 100 times the atmospheric concentrations. Isolates found at or near the soil surface are more tolerant than aerial strains, but are not as tolerant as some found attacking roots 3 to 18 inches below the surface. This subterranean type is thus able to escape competition with organisms found at the soil surface. This type has become increasingly common in south- ern California in recent years, perhaps being spread with planting stock. These relatively tolerant strains can more easily grow in soils which have poor aeration, as well as at greater depths in the soil, andean thus parasitize roots that might otherwise escape infection. Growers can with safety only assume that two organisms will prove different in disease potentialities in the field, re- gardless of similarity of names applied to them or to the diseases they cause (fig. 124). Evaluation of differences in their disease potential in advance, like the determination of their proper name, is a specialist's job. From the growers' standpoint each strain of a fungus should be considered as causing a distinct dis- ease, in that the introduction of a new and different strain may increase disease losses. [259] THE INOCULUM POTENTIAL Even if the strains of a pathogen in- troduced into the soil were the same as those already present, it would still be unwise knowingly to carry in more of them with infested planting stock. Such a practice usually builds up the amount of the organism (inoculum potential) in the soil, and would also distribute it more uniformly through the field. The inoculum potential is important, because the disease symptoms which we see are often the result of not one but many attacks on the plant by the parasite. Naturally, the more places the roots are injured the more will be the loss of normal root functions. Usually a higher inoculum potential increases the inci- dence and severity of the disease. If a soil is only slightly infested with the aster-wilt Fusarium, there usually will be only a little disease if the soil temperature remains below 60° F, but at 75° to 80° the losses will be severe. In heavily infested soil, severe losses will occur even below 60°. Thus, in a lightly infested soil a grower can usually escape severe losses in coastal California, but if he builds up the inoculum he will be unable to produce a crop in any season. It has also been found that if a soil is heavily infested with Rhizoctonia solani, it is not possible to control damping-off by the dilute formaldehyde soil treat- ment method (Sec. 11), or by coating the seed with protective fungicides; either method is fairly effective in lightly infested soil. INFECTED SEED AND STOCK Another point to be considered is that a given strain of a fungus introduced on or in the planting materials may cause greater and more rapid disease losses than a more virulent one in the soil. Be- cause the fungus is already in the tissue, or is so situated as to infect the host quickly, the rapidity and severity of loss may be more or less independent of the amount of the organism present in the soil. It has been stated that plant materials have been the source of some 90 per cent of the plant diseases and insect pests which have come to us from other countries. At least fourteen pathogens are carried by tomato seed; fourteen by iris rhizomes; and eight by pepper seed. It is very often true that the strains of an organism carried with the seed- ling or on the seed, bulbs, root divisions, or other plant parts are particularly virulent to it. Those strains best able to attack the host do so, and build up a high inoculum potential, and are there- fore most likely to be carried over with the stock. For example, the Rhizoctonia strains stimulated to high activity in pepper-seed fields, because of the pres- ence of this host, are also most likely to cause rot of the fruits in contact with the soil there. Such fruit decay leads to infection of the seed. The end result is, then, that pepper seed often carries strains of R. solani highly virulent to pepper, and that, when these seeds are planted, the strains are established in the seedbed and new planting. One of the most dangerous features of the transmis- sion of organisms with seed or vegetative parts is that the constant association of the virulent strains of a pathogen and its host is thus assured. Some organisms such as the oak root fungus, Armillaria mellea, cannot easily attack vigorous growing plants directly from the soil. r 260 i They can do so, however, by means of rootlike masses of mycelium (rhizo- morphs) that grow out from a previously infected plant part. In this case the intro- duction of infected plants may provide an especially effective center for the spread of the fungus. Many other fungi undoubtedly are in the same way pro- vided with an effective focus for spread in a planting. Using infected stock may more than nullify any benefit gained from soil treatment. For these reasons the operation of an isolation house, through which all new plant material passes be- fore being planted in production areas, is a sound idea (Sec. 13). LONGEVITY IN SOIL Many of these organisms are able to survive indefinitely in the soil. There- fore, introductions made today may not be evident until many years later when a proper host plant is cultivated. An example of this is afforded in the branched broomrape, Orobanche ra- mosa, a higher plant parasitic on roots, which appears to have been introduced into Alameda County, California, on nursery stock, a nonhost. Subsequent cultivation of a host, tomato, on the surrounding land has given rise to the disease which in recent years has reached serious proportions. This type of situation can be avoided if the nur- series steam the soil in which their stock is grown. Bacterial blight of stock carries over for 1 year in the soil and bacterial stem rot of geranium for about 3 months. Organisms of this type do not persist in the soil for long periods in the absence of their hosts because they apparently are unable to compete successfully with other soil organisms. On the other hand, Rhizoctonia, water molds, nematodes, Verticillium, and wilt fusaria can exist for many years saprophytically, on weed hosts, or as resistant structures in the soil. These facts must be taken into con- sideration when evaluating disease-con- trol programs, and growers should be aware of the far-reaching dangers in- volved when dealing with pathogens capable of existing many years in soil. MIXED INFECTIONS In some instances disease is the result of attack of not one, but two or more organisms. In these cases the losses may be greater than the injuries from each working alone. In Fusarium wilt of cot- ton, the association of the sting nema- tode, Belonolaimus gracilis, with the fungus will cause losses from wilt even in supposedly resistant cotton varieties. In the absence of the nematode, the fungus is able to cause much less disease on susceptible varieties and does not damage resistant varieties at all. A similar situation prevails in the inci- dence of the tobacco black shank disease caused by Phytophthora parasitica f. nicotianae in association with root-knot nematode. If growers introduce either the nematode or the fungus in a field in which the other part of the complex is present, increased disease results. This places an added responsibility on the grower: to avoid introducing into his growing areas a pathogen that may prove to be an aggravating agency of a dis- ease complex. [261] OBLIGATION OF THE NURSERY It is always potentially dangerous, and often an immediate risk, to intro- duce a plant pathogen into a nursery or field regardless of whether it is believed to be present there already. It is probable that the greater knowledge of tomorrow will show that we have suffered grave disease losses in our crops because of today's thoughtless spread of organisms believed to be "already present". There- fore, the nurseryman has a special obligation to produce stock free of dis- ease organisms, and to sell only those planting media (soil, leaf mold, manure) that are also free of such pathogens. To answer the initial question: There is no such thing as a safe soil-infesting disease organism ! [262] SECTION Grower Experience with the U. C. System R. H. Sciaroni J. W. Huffman Bedding plants Vegetable plants Pot and foliage plants Can-grown woody plants Bench and bed crops Cymbidiums in beds Landscape application of the U.C. system General experience HE implications and explanations of the U. C. system have been presented in preceding sections, and its application and mechanization (Sec. 17) in several types of California nurseries remain to be reported. To illustrate how the sys- tem is being adapted to varying condi- tions, this section describes the expe- riences and practices of twelve growers of bedding plants, pot plants, foliage plants, vegetable transplants for field use, benched flower crops, cymbidiums, and can-grown woody stock in five counties. These changes have been slow to come about, and were in the majority of nur- series brought about only when tradition and seemingly standardized practices were broken. Those who changed to the U. C. system have found that the tech- nical assistance of a well-trained person has reduced errors and eased the transi- tion. Numerous crops (snapdragon, carna- tion, stock, calla, delphinium, Esther Read and Majestic daisies, violet, gladi- olus) were being successfully grown in coastal southern and central California fields of the same fine sandy soil that is used in the mix. This gave additional confidence in the U. C. system and offered the possibility of transferring the benefits of this soil to pot plant and bench culture. The transition began in commercial nurseries in southern California in 1943, but became general after 1950. In the San Francisco Bay area apparently no establishments used the U. C. system [263] prior to 1953; instead they employed the native clay loam soils. One large green- house establishment in central California selected the location of its new range in southern California on the basis of the presence of the customary clay soil, ignoring areas of the fine sand. This range is now hauling in for its green- house benches the soil that had earlier been rejected. BEDDING PLANTS According to Mr. Jack L. Mather, Manager of the former Bedding Plant Advisory Board, California State De- partment of Agriculture, about 80 per cent of the bedding-plant growers in southern California were using some type of light soil mix in 1952-53. By contrast, many growers in central Cali- fornia are still using clay soils. Nursery A . . . is a small, well-established bedding-plant concern in central California. This was purchased by a young man with little prior growing experience, although he had sold retail stock. He was told by the people from whom he purchased the nursery and by other local growers that some crops (alyssum, phlox, verbena, peppers, and eggplant) could not be grown in that climate. Because farm ad- visors and agricultural inspectors em- phasized the importance of producing healthy stock, he fumigated his soil with methyl bromide to rid it of disease organisms and weeds. Because plant growth was still poor, he sought further assistance, learning that the nutritional and salinity levels of manures and leaf mold varied widely and were therefore unreliable. He adopted a U. C.-type soil mix, and has since been able to grow crops very successfully, including those which could not previously be produced. II i^ intelligent and practical approach, coupled with the use of the soil mixes and of disease control, have produced outstanding results. Nursery B . . . is a large establishment in southern California thai produces flatted stock for field planting by vegetable and cut- flower growers and seedsmen, as well as for retail sale. Several variations of the U. C. system have been used since the nursery was started in 1944. The soil has consistently been steamed, and healthy planting stock or heat-treated seed used. Weeding of flats has been eliminated, this benefit alone almost paying for the cost of steaming. The nursery is fully mechanized and, therefore, uses a mini- mum labor force. During a period when only peppers were grown, seed was machine-sown in flats which were stacked until germina- tion had occurred. This saved about 10 to 14 days of growing time in the glass- house, and an additional 10 to 14 days were saved because the seedlings were not set back by transplanting. Because of the large variety of crops presently grown, seedlings are trans- planted from seed flats rather than the seed sown in place. This successful nursery is probably the oldest user of the U. C. system. Be- cause damping-off has never been a prob- lem here, specialist growers have exten- sively used their services for starting plants. Nursery C . . . is a large southern California bedding and vegetable plant producer. In 1948 the grower was attempting to maintain many different compost soil mixes to suit the supposed needs of the many dif- ferent varieties of plants he was growing. He had, for example, a petunia mix, tomato mix, pansy mix, and others. His stock piles were large and scattered, r 264 ] making the job of mixing difficult and expensive. Soil sterilization was not practiced because these organic mixes broke down when they were steamed and released toxic amounts of soluble salts. Losses from soil-borne diseases were stagger- ing, the only control practice being that of reduced watering. A light soil mix seemed out of the question, for it was thought to be costly and inconvenient because of the neces- sity of soil hauling. Most important of all it was contrary to the generally ac- cepted beliefs that had been followed in 40 years of nursery business. In 1949. economic factors led to a break in the tradition, and the conver- sion to a L. C.-type soil mix got under way. A small concrete mixer was used for the first trials. Results were so out- standing that a large concrete transit mixer was purchased, and mechaniza- tion was begun. The soil storage area has been reduced from IV2 or 2 acres to about 1 -3 acre. Two or three men are now preparing as many as 2,000 flats per day, a job that formerly required eight or ten men. Soil-borne diseases, except for an occasional chance recontamina- tion, are unknown. X\ eeding has been eliminated. All jobs have been standard- ized and are well performed with semi- skilled labor under intelligent super- vision. This nursery is now a leader in mechanization and one of the strongest advocates of the L. C.-type soil mixes. The owner often states. "Each step leads to the next, and rapidly pays for itself: the only barrier to the light soil system is tradition." VEGETABLE PLANTS A large volume of celery seedlings is grown under glass in southern California for field planting in areas where mosaic is controlled by a celery-free period. Pepper, eggplant, and tomato are also started under glass for special purposes, sometimes in large volume. Nursery D . . . is a large producer of celery seedlings in Los Angeles County. Tests were con- ducted by the Agricultural Extension Service in this nursery in 1952-53, com- paring a treated L. C.-type soil mix with their conventional untreated composted soil. The compost consisted of % decom- posed manure, ^ black peat, and 1 a a soil-sand blend, and had been composted for a year or more. Decomposition was not uniform because of differences in temperature, moisture, amount of straw contained in the manure, and other fac- tors, evident both within a given batch and between batches and seasons. By contrast, the L . C.-type mix required no prior preparation, and the mix could be prepared for planting immediately upon delivery of materials. Storage space for compost piles was thus saved. Fertilizer top dressing of the seedlings growing in compost cost SO. 90 to SI. 10 per 1.550 flats, whereas application of calcium nitrate i 1 ^ oz. per gal. on a 10-day schedule) to the L. C. mix cost only SO. 35. an additional saving. It was found in four tests that the plants grown in the L. C. mix were salable 5. 6. 9. and 11 days sooner than by the old method. Time saved in this way may mean that an extra crop can be grown during the busy season. In these instances the differences probably were due both to damping-off and exces- sive salts in the compost. Conductance 1 Sec. 4 1 of compost initially, and after 2 [265] weeks, for the above series was: 6.4, dropping to 3.7; 10.9, dropping to 5.4; 8.3, dropping to 5.8; 12.1, dropping to 6.9. The conductance of the U. C.-type mix ranged from 2.2 to 2.6 initially and 1.7 to 1.9 after 2 weeks. In the compost the root systems were poorly developed and top growth was very irregular (height varying by 1 to 1% in. in a single flat) , whereas plants in the U. C.-type mix were uniform in size (maximum variation % in.) and color, and were larger. Production in the compost was com- plicated, requiring an experienced per- son to water the flats, shifting from heavy leaching to light applications as the problem changed from salinity to damping-off (fig. 35). Plant growth was undoubtedly depressed, in turn, by this practice, since celery is a "wet crop". With the U. C.-type mix, a regular irriga- tion schedule was set up that maintained proper soil moisture, since neither salinity nor damping-off was a problem. The seedlings were transplanted to com- mercial fields, where it was consistently found that those grown in the mix re- mained green and started growth more quickly than those grown in the compost. Despite the demonstrated effectiveness of the system in this nursery, the nursery management considered that it was easier to continue in the old method and too much trouble to change! His com- petitors, however, did make the change. POT AND FOLIAGE PLANTS There is a large business in raising pot plants both for retail sale in Califor- nia and for shipping out of state. Foliage plants have perhaps the fastest expand- ing market of any florist crop today, and the demand is largely filled from Cali- fornia and Florida. Pot plants are grown throughout the state, but there tends to be a concentration of foliage crops in southern and central California. Nursery E . . . is an important producer of pot and foliage plants in central California. The owners formerly used multiple soil mixes, sometimes changing the type for each crop, or for a given crop each year, in search of something better. In 1953 they changed over to the U. C. system of soil mixes and treatment. This has been so successfully applied that mech- anization has been adopted extensively; the details of some of these methods are presented in Section 1 7. Because of the labor saving effected, four men are able to do the work that formerly required twelve to fourteen. Two years of commercial experience has demonstrated that practically all types of foliage and blooming pot plants grow very well under the U. C. system. The cost of soil preparation has been greatly reduced, and certainty of results tremendously increased. The results have been at least as good as before, and in most cases superior. The changeover introduced some problems, however. Poinsettias planted at the regular time grew so rapidly that they were far too large for the average market. The following season this was easily corrected by delayed planting and double pinching, a saving in time and space then being realized. This nursery is a successful advocate of the U. C. system. Nursery F This southern California foliage-plant grower began operations in 1948. It was soon found that disease losses from soil organisms constituted the principal production problem. The losses from [266] Rhizoctonia were often complete, even though the foliage was kept dry. He tried to obtain clean stock from various sources without success, and was finally forced to produce it himself. The success attained with Fittonia ver- schaffeltii var. argyroneura under the U. C. system illustrates the effectiveness of the methods employed. The planting stock was treated in hot water by the methods explained in Sec- tion 13, and then grown in individual pots of steamed soil similar to a U. C- type mix. The plants were grown to a height of 6 inches under conditions of general sanitation and without ever wet- ting the foliage before cuttings were taken. The cuttings were dipped in Parzate and rooted in steamed sand. By the time this procedure had been fol- lowed for three generations, adequate clean mother blocks had been estab- lished. They have since been carefully maintained and have yielded consistentlv healthy stock. At first the cuttings were rooted in sand and planted in small pots, taking 8 to 10 weeks. Now the cuttings are planted directly in small pots of U. C. mix C (50 per cent peat). Automatic misting of cuttings is practiced without loss from diseases caused by Rhizoctonia or water molds. Production of the fin- ished plant ready for sale now requires only 5 weeks. Over a quarter million young plants are now raised annually in scheduled production by this method. Similar success has been obtained with Peperomia obtusifolia var. varie- gata, Peperomia sandersii, Pellionia pul- chra, Nephthytis sp., Diefjenbachia picta, D. bausei, and Hedera sp. (Glacier ivy) by following the recommended proce- dures for eliminating the diseases. Nursery G . . . is a small producer of foliage plants, particularly Philodendron and Dieffen- bachia, in central California. For many vears clav soils mixed with leaf mold J J and manure were used. Growth was ir- regular and consistent losses were ex- perienced because of overfertilization and poor drainage. Fortunately this nursery is located on a Colma fine sand deposit. The grower switched all of his foliage plant operation to a I. C.-type mix in 1953. and the above problems were solved. CAN-GROWN WOODY PLANTS A large volume of nursery stock is grown in cans (ranging from 1 to 5 gallons' capacity) in California, under lath or glasshouses, or outdoors. This stock is sold largely for home planting. Nursery H . . . a well-established wholesale nurserv in central California, had been using soils of various types, mainly clay. They changed to a U. C.-type soil mix and used methyl bromide fumigation, obtain- ing 25 to 50 per cent increased growth over their standard procedure in the first season. The soil did not shrink away from the can when irrigation was de- layed as did the clav. making for easier and more effective watering. After part of a season of successful growing, it was decided to try using rice hulls, an inexpensive and abundant or- ganic material in the area. The mix was altered by omitting potassium, which rice hulls supplied. The growth has been excellent, containers are lighter in weight, and cost of the mix has been reduced. [267] BENCH AND BED CROPS The flower crops grown in raised benches and ground beds have enormous value in California, among the more im- portant being roses, carnations, chrysan- themums, and gardenias. Because these plants are large and deep-rooted and do well in garden soils, it would be expected that the benefits from using a U. C.-type mix would be less than with the foregoing crops. Nursery I . . . grows gardenias in raised benches in southern California. Using a soil mix composed largely of clay and peat, they were having trouble with chlorosis and death of plants. The soil steaming was only partially effective, and salinity from overfertilization and a poor water sup- ply created an additional problem. They tried a planting in U. C. mix C (50 per cent peat) . Uniform, effective steaming was obtained, with little or no evidence of disease in the .2% years of the test. The plants have grown very well, with greatly increased production. The use of an iron chelate in the winter (2 oz. per 100 sq. ft.) completely controls normal cool-weather chlorosis. All of this firm's plantings are now in this mix. The rather poor-quality water is no longer a hazard. In fact, they have expanded into the field of foliage-plant growing, and now use the same mix for stock beds and potted plants. Nursery J . . . is a carnation grower in central Califor- nia. On the basis of comparative trials of a U. C.-type mix in raised beds as against his standard clay loam, he has shifted to the light mix for his entire culture. Several outstanding benefits have been observed at this nursery. Car- nations in the U. C. mix showed a very low percentage of calyx "splitting" as compared to those grown in clay loam. In addition, the frequency of irrigation for the clay loam beds was almost twice that for the U. C. mix. Further, the clay loam upon drying developed deep cracks which caused severe root shearing; this was not evident with the U. C.-type mix. Nursery K . . . is a rose grower in central California. The U. C.-type soil mix was tested in several raised beds in a glasshouse. The production was as good as in his usual soil, a clay loam that had been in the benches for twenty years. Because of re- peated steaming and additions of peat during this period the soil had been brought into a good physical structure. If the soil should have to be changed in the benches for some reason, the nursery plans to use a U. C.-type mix. The initial physical structure of a U. C.-type mix equals that attained by clay soil after several years' improvement through organic additions. CYMBIDIUMS IN BEDS Since World War II the growth of Nursery L .. . cymbidiurns under glass or lath has be- raises cymbidiums in ground beds in come important, particularly in southern southern California. The original soil California. mix was completely organic, consisting [268] of leaf mold, manures, and bean straw. The soil mix was placed in beds 12 inches deep on top of an adobe soil. Within 2 years or less the beds shrank to a depth of approximately 6 inches, owing to decomposition of the organic mate- rials. The nutritional level varied widely throughout the nursery; some beds were overfertilized, others were underferti- lized. Because the nutritional levels were unpredictable, it was impossible to set up fertilizer applications on a regular sched- ule. The over-all result was irregular plant growth and production. To correct this problem the nursery- man changed to a soil mix consisting of fine sand, peat moss, and pine shavings, CAUTION: Many of the i :hemicals mentioned in this manual are poi- sonous and may be harmful. The user should carefu lly Follow the pre- cautions on the 1 abe Is of the con- tainers. with known nutrient leveis. The applica- tion of fertilizer has been standardized and placed on a simple, regular schedule. The volume of the soil in the beds has remained relatively constant. The over- all results are reflected in uniform plant growth and production and a simplifica- tion of management practices. Cost of preparing beds has been reduced. LANDSCAPE APPLICATION OF THE U. C. SYSTEM A large race track in southern Califor- nia had a history of high weeding costs and expensive replacement of plants killed by disease. To overcome this yearly loss the infield area of approxi- mately 10 acres was treated with methyl bromide (fig. 125) and subsequently planted with disease-free stock pro- duced under the U. C. system. Prior to the field fumigation, even though dis- ease-free stock was used, as many as 1,500 new plants were needed each week Fig. 125. Left, methyl bromide fumigation of field soil in southern California for elimination of pathogens and weeds. This is a field adaptation of the technique developed for glasshouse soil treatment. Right, the same field 2 months after planting with seedling pansies which had been grown in treated nursery soil. This illustrates the advantage of using healthy plants in clean soil; a more vigorous planting results, and the soil is not reinfested by the planting stock. to replace those killed by damping-off fungi. After fumigation, only 400 plants were replaced in a 4-month season, and most of these were mechanically or chemically injured. This illustrates the necessity of both soil treatment and clean plants in disease control (fig. 125). As another benefit from soil treat- ment, weed control was reduced from $60 per acre to a minor figure. A landscape use of the U. C. system was the replacing of heavy disease- infested soil in confined beds with a fumigated U. C.-type mix. The disease control achieved was similar to that de- scribed for the infield planting. Plants grown in a treated sand-peat mix were two to four times as large as the same stock grown in the treated natural loam of the infield. The use of a U. C.-type soil mix in places where large beds are to be filled with hauled soil seems to be worthy of wider trial in public or private plantings. GENERAL EXPERIENCE The results of grower experience with the several aspects of the U. C. system have shown that: 1. It is uniquely adapted to mecha- nization. 2. It provides a continuous supply of a uniform growing medium. 3. It permits the use of fewer and less experienced laborers. These, how- ever, should understand the "why" as well as the "how" of what they do, and should be directed by well- trained supervisors. 4. It provides a means of avoiding salinity difficulties. 5. The soil provides good aeration and water drainage for root develop- ment. 6. The disease problems are essentially eliminated by the treatment of soil and containers, the use of healthy planting stock, and careful culture to maintain them in that condition. 7. The cost of weeding is eliminated. 8. Post-steaming toxicity of soil is avoided. 9. Tedious composting procedures are eliminated. 10. A single basic mix replaces the many formerly used. 11. Materials are employed that are easily and cheaply obtained. 12. It enables the production of plants that are more uniform, healthier, and larger, at lower cost, more re- liably, and faster than before. [270] SECTION m Mechanization and the U.C System J. W. Huffman R. H. Sciaroni Mechanizing an old nursery Planning for mechanization Stages in the flow of materials Watering and fertilizing in the glasshouse General comments ne OF the major advantages of the tion with the Agricultural Extension U. C. system for the nursery is the ease with which this type of production may be mechanized. Increased labor cost, higher taxes, and real-estate subdivision have made nurserymen anxious to attain greater efficiency (Sec. 2). Residential development has forced the discontinu- ance of composting manure piles, be- cause of their odor and fly problems. The Agricultural Extension Service realized that changes would have to be made if urban nurserymen were to con- tinue in business. The greatest potential contribution seemed to lie in standard- ization of soil mixes and mechanization through the U. C. system. During the developmental period of the U. C. system, perhaps partly because of it, intensive mechanization was begun in a few California nurseries. In coopera- Service and Experiment Station, they adapted ideas from materials-handling equipment in canneries, assembly lines, sand and gravel operations, concrete- mixing plants, and other well-engineered installations. These pioneering growers exhibited ingenuity and imagination in devising equipment for their needs in existing nurseries. The methods and equipment de- veloped have greatly reduced labor re- quirements and enabled the substitution of semiskilled for scarce highly skilled labor in many jobs. Nursery mechaniza- tion is still developing, and further im- provements will certainly be made. Since, however, there is such a pressing need for greater efficiency in the nursery industry, it is desirable that the tech- niques thus far developed be presented here. [271] SOIL TREATED IN CONTAINERS Constituents of soil mixture clumped in piles or bins on delivery I Skip loader I Soil blended and moisture added in concrete mixer I Dumped direct or conveyer belt c i . " c\\ , Untreated flats or pots Flat or can filler ' — - — — — Manual Stacked on pallets I Forklift tractor Steam treatments: Piles of flats or cans covered with tarpaulin (5). Horizontal chamber with hood (10). Horizontal steam vault (6). Autoclave (9). Chemical treatments: Piles of flats or cans covered with plastic sheet and treated with methyl bromide orchloropicrin aerosol. Forklift tractor Piles unstacked by hand I Conveyer belts or rollers Planted or seeded by hand Machine planted or seeded Conveyer belts or rollers I Growing area Fig. 126 (both pages). Diagrams of vaiious methods for mechanization of thirteen ways to treat soil with steam or chemicals in commercial California nursery practice. Numbers refer to equipment types (Sec. 10). (From a chart by K. F. Baker.) [272] SOIL TREATED IN BULK Constituents of soil mixture dumped in piles or bins on delivery I Skip loader 4< Soil blended and moisture added in concrete mixer Dumped direct or conveyer belt V / Mobile Mobile bin steam types i \ box (4) Pulled oy tractor \ \ Bin (2) Dumped durr iped direct Mobile units pulled by tractor Stationary steam box (4); may be in tandem Treated with chloro- picrin in movable or stationary bins or boxes, or with methyl bromide in piles covered with plastic sheet. Dumped direct or conveyer belt I Flat or can filler Manual Bin and potting bench (3) * Stacked on pallets Forklift I tractor Piles unstacked by hand f— Conveyer belts or rollers \r Horizontal revolving drum with steam injected (28) or with blow torch (30, 31). Screw type with injected steam (29). Flats or pots treated with steam or chemicals. Dumped into flats or cans, or into flat or can filler. Manual Planted or seeded by hand Machine planted or seeded Conveyer belts or rollers Growing area [273] MECHANIZING AN OLD NURSERY While the greatest benefit from mech- anization obviously is gained in a nur- sery designed for it, impressive savings have often been made by adoption in existing nurseries of many of the proce- dures outlined here. No matter how small or poorly designed a nursery may be, some of the mechanized methods de- scribed will prove adaptable and profit- able. The grower should study the flow diagrams of mechanization in nurseries (fig. 126), the summary chart of types of soil steamers (table 15), and the text and illustrations in this section concern- ing mechanization in California nur- series. He should observe the practices in several well-mechanized nurseries; his farm advisor or the authors can, if de- sired, suggest some to visit. From these several sources profitable ideas adapt- able to the specific nursery will be ob- tained. It will be found, furthermore, that many of these methods may be adopted independently and consecutively, with- out major upheaval or expense (see "Aids in Adopting the U. C. System," p. 1). Many nurserymen have demon- strated that the U. C. system can be adopted in progressive easy stages. It is important, however, that the process be continued until a complete program is established, rather than stopping at some intermediate level of partial benefits. PLANNING FOR MECHANIZATION The exact manner of mechanization must be developed for each nursery, preferably before it is built. Because of smog, population pressures, and tax rates, many California nurseries may be forced to move in the next several years, house equipment) might well be em- ployed. Some of the basic mechanization methods presently used in a few nur- series are outlined here and presented in chart form (fig. 126). Some of the factors which should be and this affords an excellent opportunity considered in planning a new nursery to properly design the new units for efficient management. For this reason, it is suggested that the central ideas of the II. C. system be tested now, and that thought be given to incorporating these principles into any new construction. Some of the books on mechanization of materials handling (see Appendix, References) should be consulted in the initial stages of planning to be sure that the besl modern methods are considered. If (he unit is to be a large one, the services of an engineer who specializes in materials handling or in the design of continuous processes I for example. assembly lines, cannery and packing- may be suggested. Careful attention should be given to the soil-treatment method and facilities to be used (sees. 8 through 11). Utilizing a natural slope If the land has a natural slope this may be utilized by placing the soil piles at the top, followed progressively down- hill by the mixing equipment, container- filling equipment, treatment facilities, planting operations, and the growing facilities. Gravity can then be made to do much of the transportation work, by using steel rollers. [ 274 ] Glasshouse arrangement and design Glasshouses should be arranged so that they branch off both sides of a central corridor through which tem- porary steel rollers may be set up, lead- ing from the soil area. Plans should in- clude openings for steel rollers to run into each house through the end wall, rather than the doors. Containers may then be taken almost directly to their places in the glasshouse without exces- sive lifting or carrying. The width of the glasshouse aisles should be considered in terms of the equipment to be pushed through them, and vice versa. One nursery decreased the labor of emptying glasshouses by designing them so that the sides were removable down to bench level, to per- mit the removal of flats through the sides of the houses. Flats were moved on steel rollers and loaded directly onto truck beds (at the same height) for transporta- tion to the area where the plants were hardened-off before sale. The size and orientation of the benches should be studied for greatest mechanization potential. If bench crops are to be grown, the width of the benches and any steam pans to be used should be the same. Structural pipes through beds should be avoided whenever possible, as they invariably increase cost of steam treatment. Paving of yard The area around all houses should be paved to expedite mechanization and reduce weed growth, from which insects, often virus-carrying, move into the glasshouses. STAGES IN THE FLOW OF MATERIALS Processing and Stockpiling the Materials Possible sources of the fine sand were discussed in Section 6. A statewide sur- vey for exact sources has been con- ducted, 1 and information on this may be obtained through your local farm ad- visor. It should be emphasized that the various truckloads of the fine sand should be checked on delivery for uni- formity in conforming to physical (Sec. 6) and salinity (Sec. 4) standards, if there is any reason to suspect variability. To facilitate this, it is well to order this material somewhat in advance rather than wait until it is actually needed. Be- cause of the storage space required, it is not usual to stockpile large quantities. Since no composting is necessary in pre- ^y M. H. Kimball, Ornamental Horticul- turist, California Agricultural Extension Serv- ice. paring the mix, no space is required for this process. Usually a supply sufficient for 2 to 3 weeks is kept on hand. The Canadian or German peat moss may be obtained from your local horticultural supply house. Both of these ingredients should be stored in bins under a roof, preferably on a large concrete slab, or at least in a well-drained area. This structure might well be large enough also to accommo- date the mixing operation. There would then be only short hauls between the dif- ferent steps of this procedure, and opera- CAUTION: Many of the < :hemicals mentioned in this manual are poi- sonous and may be harmful. The user should carefu Ily follow the pre- cautions on the 1 abels of the con- tainers. [275] tions could continue during rains. This building should be separated from the soil-sterilizing and planting facilities in order to reduce recontamination, and should be located for greatest conveni- ence in carrying the soil or the filled containers to the soil-treatment equip- ment and glasshouses. Storage of Soil The U. C. mixes may be stored in- definitely if organic nitrogen has not been added, but should not be held more than a week if they contain more than a small amount of such material (sees. 5 and 7). Soil is preferably stored before, rather than after treatment to reduce the recontamination hazard. Some growers, however, have satisfactorily stored treated soil in a tight building where wind-blown soil will not reach it (fig. 128). Such a structure should not be near the mixing operations. The build- ing should be humidified, perhaps by mist nozzles, to reduce drying of the soil if prolonged storage is contemplated. Raising humidity to prevent drying of flats is preferred to direct watering, which is extremely difficult since the flats are stacked on pallets. Watering be- fore use also has a tendency to compact the flats and cause the transplanter to go through an unnecessary step of loosen- ing the soil before transplanting. Excellent temporary storage is pro- vided by placing a tarpaulin over the stacked flats. The flats should be kept out of the sun. Making Flats In one large nursery, a sufficient num- ber of flats was used so that it was economic to purchase a box-making machine especially designed for south- ern California flats. This reduced hand- construction and hence costs. Several smaller nurseries might go together in purchasing such equipment, or a port- able unit might be taken, on a service basis, from one nursery to another to prepare the season's supply. Preparing the Soil Mix Equipment needed The presently accepted and generally adopted method for preparing nursery soils is to use a stationary concrete mixer for about a 10-minute period. These are available, new or used, in various sizes. The larger sizes have a loading apron into which the ingredients are conveniently dumped by a skip- load tractor. Used transit-type mixers (figs. 127 through 130) with a capacity of about 5 cubic yards are commonly utilized. They cost $400 to $800 and must be cleaned of cement, reconditioned, and painted before use. Small nurseries use 1- to 2-cubic-yard cement mixers. Several growers use two mixers to reduce the time of the operation. Some nurseries have tried mixing their soil with a skip loader and screens, but have found this unsatisfactory. An important benefit of the U. C. system is that no complicated equipment is required for breaking up clods. // hard lumps of soil are present in the fine sand, it is not of the proper type (Sec. 6). Several nurseries in central Califor- nia have each recently spent $800 to $1,000 for equipment to break up the lumps in the clay soil before mixing could begin. The only equipment used for prepar- ing the ingredients for a U. C.-type mix is a small shredder sometimes employed for breaking up the bales of peat. More frequently the peat is wetted and is broken up by a fork. If the soil contains debris or rocks it may be screened at this time (rare), or after mixing (see below). Mixing The fine sand and moistened shredded peat, as well as the proper weights of the desired amendments and fertilizers (sees. 5, 6, and 7) are dumped into the mixer in their proper relative propor- tions. During the rotations of the mixer [276] , .##&& Fig. 127. General view of a bedding-plant nursery, showing the mechanization of soil handling. Transit mixers (A) mix the soil ingredients, which are then conveyed (B) to a rotating screen (C) above the flat filler (D). The flats are carried on steel rollers (E) under the filler, and are stacked on pallets (F), which are transported by the fork-lift (G) to the steam chamber. Fig. 128. Another view of the same soil-handling operation. The steam chamber (H) is shown in use. If soil is to be stored temporarily, it is moved to the enclosed building (I) by the fork-lift. The peat supply and empty flats are shown in the foreground, the fine sand is to the immediate left (not shown). a measured amount of water is some- times added to bring the mix to a uni- form desired moisture content for steam- ing and planting (Sec. 8) . It is generally preferable, however, to moisten the in- gredients before mixing. A ribbon mixer of the type used in mixing plaster has proved excellent for soil. If adequate storage space is planned, the nursery may use the soil-mixing and container-filling equipment only 2 to 3 days a week, using two or three men. Note, however, that the mix containing organic nitrogen should not be stored for longer than a week before planting (sees. 5 and 7) . It requires about 30 minutes to han- dle a load in concrete mixers. Thus the capacity in cubic yards, multiplied by 2, will give the hourly capacity. If the mix is not to be screened, it may be taken from the mixer directly or car- ried by a conveyor belt or skip-load tractor (see "Transporting the Soil," be- low) directly to the flat filler, to the treatment equipment (if it is to be treated in bulk), or to a combined bin and potting bench (Sec. 10). Screening When the soil is mixed it is sometimes dumped directly onto a moving belt that conveys it to a revolving circular screen 8 to 10 feet high (figs. 127, 128, and 130). These units are custom-made for each nursery and cost about $1,500. The belt width is about 16 inches, and the unit is driven by a gasoline engine or electric motor. The screen is 3 to 4 feet in diameter and 5 to 6 feet long. Usually there is little debris in the ingredients and this procedure is done as much to continue mixing as to get rid of stones, sticks, rubbish, or large pieces of peat. The mix may fall through the screen di- rectly into the hopper of the flat filler, or into a pile. Fig. 129. Method of filling a transit mixer by means of a skip loader. The proportion of fine sand and moistened peat is determined by the number of loads of each. The tandem arrange- ment of mixers assures a continuous output of mixed soil. Filling the Containers It is generally desirable to place the soil in the containers prior to treatment. Sometimes, however, the sequence may at this point be reversed. The soil is then treated in bulk before it is placed in con- tainers that have been treated separately. Treated soil should never be placed in untreated containers. The flat filler (fig. 130) or container fillers (figs. 10 and 135) vary more in construction than other pieces of equip- ment used in the mechanized nursery. In general they consist of a wide-mouth tapered hopper, the soil flowing through the adjustable opening onto a variable- speed rubber belt. The soil drops from the end of this belt into a flat or other container below, that is carried on an- other belt moved by power or manually. The width of the lower opening of the hopper and the width of the belts may be varied for different types of containers. Flat fillers Some of the flat fillers use an auger gear or leveling bar to level the soil in the flats. Some of these devices will han- dle 900 flats per hour. One man is re- quired to place the flats on the input conveyor belt, and one to remove the filled flats at the output. Flats are usually stacked on hardwood pallets (figs. 127 and 130) (36 by 54 in.) to be transported by a fork-lift tractor (fig. 127). The price of this type of equipment is about $1,000; the hardwood pallets are about $2.75 apiece. This step may be difficult to mechanize if flats are not of uniform size or have wide bottom cracks. Paper liners are sometimes used in such old flats. In southern California the flats are uni- formly 18 by 18 by 3 inches; those in central California are quite variable in size. This latter situation is, of course, an obstruction that must be resolved if Fig. 130. Detail of screening and filling operation, showing transit mixer (A), conveyor belt (B), rotating screen (C), flat filler (D), and steel rollers (E). The worker is removing flats from the steel rollers and placing them on a pallet. Note the wood separator strips (arrow), which are placed between layers of flats. economical mechanization is to be adopted. Mechanical can and pot fillers Mechanical fillers are available (see Appendix) for 1-gallon cans and pots (fig. 135) and for 2- and 5-gallon cans (fig. 10). Soil may be conveyed to this equipment after being mixed, and screened and treated in bulk; the filled containers are planted at once. Hand-filling Some nurseries fill the containers by hand at the point of mixing. They may then be piled on pallets as before and carried by a fork-lift tractor. However filling may be done, the number of flats placed on the pallet will depend on the size of the tractor, the size and shape of the containers, and the dimensions of the treatment chamber. Mobile bin and potting bench In one nursery, the soil from the mixer is dumped onto a mobile bin and potting table (Sec. 10, type 3) . This flat- bed wagon (fig. 132) has a movable tongue hitch, and can easily be pulled by a light truck or tractor. Perforated pipe is permanently mounted in the bot- tom of the wagon and is easily con- nected to the generating source for steaming. It is covered by a tarpaulin during this process. After steaming it is pulled into the potting headhouse. In a few hours the soil is cool, and one side of the wagon is let down for a work table. Potting and planting are done di- rectly from the treated pile. The soil mix is easily worked into pots around bulbs or rooted plants. Treating Soil and Containers The details have already been given for steam treatment (sees. 8, 9, and 10) and chemical treatment (Sec. 11) of nursery soils. Equipment has been de- scribed (Sec. 10) for steaming of the -oil. Chemical treatment is generally done in stacks of staggered flats or on wooden pallets. In either case, treatment operations are preferably isolated from those of soil mixing or flat filling to re- duce possible recontamination. The cost of materials and labor for constructing a steam chamber (figs. 128 and 131 ) varies from $250 to $500, ac- cording to the type and size of installa- tion. Marine plywood (waterproof glue) sheets % inch thick and 4 by 10 feet in size should be used for sides, doors, and top. Exterior plywood is not suitable for this purpose. The frame is constructed of either metal or 2 by 4 lumber. De- tails on this and other types of equipment are given in Section 10. Transporting the Soil Untreated soil Untreated ingredients are most eco- nomically transported by a skip-load tractor (figs. 126 and 129) from storage piles into the soil mixer. From there the mix may again be transported by a skip- loader or be carried by a moving rubber conveyor belt to the flat filler, or car- ried by a rubber belt to the revolving screen, from which it is dumped into the flat filler. Alternatively, the mixed soil may be taken by skip-loader to a bulk steam treater, or the steamer (if mobile) may be brought under the mixer and screen and filled. Filled containers Containers are usually stacked on wooden pallets and carried by a fork- lift tractor (figs. 127 and 131). The cost of this piece of equipment is variable, depending on the size of the unit; a large unit with pneumatic tires used by some nurseries costs $3,200 to $3,500. Since the fork-lift is efficiently used in many nursery operations, its cost should not be completely charged to soil handling. At one large pot-plant nursery the fork-lift mounted at the rear [280] Fig. 131. Loading a pallet of filled flats into a wooden vault (type 6, Sec. 10) steam chamber by means of a fork-lift. The vault is unloaded in the same manner, and taken by fork-lift to the seeding or planting operation. Fig. 132. Mobile bin and potting bench (type 3, Sec. 10), showing the potting operation. The soil is dumped into the bin from an elevated transit mixer and steamed from pipes in the bottom of the wagon, which is then pulled to the potting area. With this type of equipment care must be exercised to avoid introducing pathogens into the exposed soil mass from oc- casional diseased propagating material during the potting operation. Fig. 133. Bicycle-wheel cart used for trans- porting potted stock into the glasshouse from the potting bench. This is adaptable to glass- houses with narrow aisles or where conveyor belts are not practical. Fig. 134. Rack for transporting pots to a vault-type steamer, or to be covered with a tarp and steamed or treated with methyl bro- mide. Unit is transported by a fork-lift tractor. of the tractor is used to transport steri- lized clay pots (fig. 134). The containers may be taken from the treatment chamber by the fork-lift trac- tor on the same pallets, to the place where they will be planted. They may, however, be carried either on steel rollers or a conveyor belt. After planting they generally are taken on portable steel rollers to their location in the glass- house, sections of rollers being removed as the house is filled. A special cart equipped with bicycle wheels (fig. 133) has been constructed for use with the mobile bin and potting bench mentioned above. The cart has a low center of gravity for good balance, and the narrow bed facilitates rapid transit of planted pots through glass- house aisles. Planting Transplanting seedlings Planting is an extremely varied opera- tion. It varies from transplanting of seedlings into flats by hand, an opera- tion that seems to defy mechanization, to planting them in pots. The latter op- eration has been mechanized by the Erdprinz planter in Germany (see Ap- Fig. 135. Automatic can or pot fillers for placing soil in 1-gal. containers and forming a cen- tral depression into which the liner is planted. (See also fig. 10.) (Photo courtesy of Oki Nursery, Perkins, California.) [ 282 1 pendix) and equipment for use with the U. C. mixes has been developed in Cali- fornia (figs. 10 and 135). Machine-seeding Equipment has been used from time to time for spot-planting the seed in con- tainers. One unit was used for several years for such planting of pepper seeds in flats. It consisted of a vacuum plate that fitted the inside dimensions of the flat, and had a hole drilled where each seed was desired. This plate was alter- nated between a tray of clean seed (with the vacuum on) and a flat (with the vacuum then released). It was possible to seed 150 flats per hour in this way. The seed and soil were covered with tis- sue paper, then by sterile clean sand, and watered generously. Similar planting plates are available on special order (see Appendix). Mechanical seeders of this type will not operate efficiently on tiny seed, and attempts to evade this by pelleting the seed to larger size plunge one into germi- nation difficulties. In one series of tests by P. A. Chandler (unpublished data), pelleted seeds of Theodosia Improved petunia gave best germination when they were placed on the surface, pressed lightly into the soil, and then sprinkled. All other methods were quite inferior. The germination difficulties and dif- ferences in seed size, vitality, and germi- nation time have all indicated a dubious future for pelleting of fine seed. An experimental mechanical seeder recently seen may solve these difficulties. Until one is perfected, fine-seeded plants will require hand-transplanting. For large-seeded plants machine-seeding has for several years been an accomplished fact. WATERING AND FERTILIZING IN THE GLASSHOUSE In some cases it has been satisfactory to water flats in the glasshouse by over- head sprinklers. Because of the excellent drainage of a U. C.-type soil mix and freedom from damping-off fungi, there is little danger from applying excess water. Thus, enough water can be ap- plied to satisfactorily wet nearly all of the flats, and the few that need more can be hand-watered. This procedure saves Fig. 136. Liquid-fertilizer injector for accurately diluting nutrients into the water stream during irrigation. (Photo courtesy of Smith Precision Products Co., South Pasadena.) [283] much labor, but can only be used when a whole glasshouse area is reasonably uniform. Some growers have applied fertilizer through such a system. One grower of Kentia palms successfully fertilized in this way for several years. Another grower of pot foliage plants has also ob- tained excellent results, using a fertilizer injector (fig. 136) in the water line. Care must be taken to flush the chemical from the pipes. A better procedure is probably pro- vided by hand application of liquid fertilizer by means of special injectors (fig. 136 and Appendix) which operate through the watering hose. Fertilizers may also be applied to the surface of soil in pots in the dry form (sees. 5, 6, and 7) . GENERAL COMMENTS The total cost of mechanizing the soil- mixing process is about $6,000 to $8,000, according to the size of the nurs- ery. To avoid this large initial invest- ment, most nurserymen with establish- ments already built, develop one or two steps at a time, beginning with the ce- ment mixer. Each subsequent operation is developed and designed for the spe- cific operation. This partly explains the wide variation in types of equipment used in California. Results from mechanization have gen- erally been outstanding in producing better plants at lower cost. It is often found that two to three men working 2 or 3 days a week are taking care of all soil mixing in a nursery that formerly required six or more working all week for this operation. Sometimes the saving is even greater. The nursery has yet to be built that fully utilizes all the potentialities of the U. C. system. Will yours do this? [284] N D I X REFERENCES Section 2 California State Commission of Housing 1954. California housing. California State Dept. Indus. Relations, Div. Housing, p. 13, 15. Los Angeles County Chamber of Commerce, Agricultural Department 1955. Southern California agriculture ; 1954 — the year in review, p. 4. Sciaroni. R. H.. and G. Alcorn 1953. Farm land disappears. 4 p. California Agr. Ext. Sen., San Mateo Co. U. S. Bureau of the Census 1942. Population: 1940. Vol. 1. Number of inhabitants, p. 129-31. 1950. Statistical abstract of the United States 1930: 35. 1951. Statistical abstract of the United States 1951: 31. 1952. Census of population: 1950. \ ol. 1, Number of inhabitants. Chapter 5. California, p. 21. 1952. Special reports. Horticultural specialties. 1950 United States Census of Agriculture 5 (1) : 67-68. 71. 437. 457. 472. 529. 541-43. Section 3 Baker. K. F. 1946. Observations on some Botrytis diseases in California. Plant Dis. Reptr. 30: 145-55. 1947. Seed transmission of Rhizoctonia solani in relation to control of seedling damping-ofl. Phytopathology 37: 912-24. 1948. Nursery seedlings. Improved methods for production possible with control of damping-off disease'. California Agr. 2 (10) : 10. 14. Baker, K. F., and R. H. Sciaroni 1952. Diseases of major floricultural crops in California. 57 p. California State Florists' Assoc, Los Angeles, Calif. Beach, W. S. 1949. The effects of excess solutes, temperature and moisture upon damping-off. Pennsvlvania Agr. Exp. Sta. Bui. 509: 1-29. Chitwood. B. G.. and W. Birchfield 1956. Nematodes, their kinds and characteristics. Florida State Plant Board Bui. 2 (9) : 1—49. Ellis. D. E.. and R. S. Cox 1951. The etiologv and control of lettuce damping-off. North Carolina Agr. Exp. Sta. Tech. Bui. 94: 1-33. Filipjev. I. N.. and J. H. S. Stekhoven, Jr. 1941. A manual of agricultural helminthology. 878 p. E. J. Brill, Leiden, Netherlands. Goodey, T. 1933. Plant parasitic nematodes and the diseases they cause. 306 p. E. P. Dutton and Co., Inc.. New York, N.Y. Gravatt, G. F. 1954. Potential danger to the Persian walnuts, Douglas-fir, and Port Orford cedar of the Pacific Coast from the cinnamon Phytophthora. Plant Dis. Reptr. 38: 214-16. HlLTABRAND. W. F. 1951. Soil treatment as an aid to the pinto tag program. Pacific Coast Nurseryman 10 i'8"> : 13. Horsfall. J. G. 1938. Combating damping-off. New York (Geneva) Agr. Exp. Sta. Bui. 683: 1—16 Jackson. W. T. 1956. Flooding injury- studied by approach-graft and split root system techniques. Amer Jour. Bot. 43 f 496-502. Leach. L. D. 1947. Growth rates of host and pathogen as factors determining the severity of preemergence damping-off. Jour. Agr. Res. 75: 161-79. 1 References include the key literature referred to in the text, as well as material for additional reading on the subject. [285] McClure, T. T., and W. R. Robbins 1942. Resistance of cucumber seedlings to damping-off as related to age, season of year, and level of nitrogen nutrition. Bot. Gaz. 103: 684-97. Steiner, G. 1953. Plant nematodes the grower should know. Florida State Dept. Agr. Bui. n.s. 131: 1-48. Tyler, J. 1944. The root-knot nematode. California Agr. Exp. Sta. Cir. 330: 1-30. Section 4 Baker, K. F., O. A. Matkin, and L. H. Davis 1954. Interaction of salinity injury, leaf age, fungicide application, climate, and Botrytis cinerea in a disease complex of column stock. Phytopathology 44: 39-42. Baker, K. F., and R. H. 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Senner, A. H. 1934. Application of steam* in the sterilization of soils. U. S. Dept. Agr. Tech. Bui. 443: 1-20. Tarrant Mfg. Co. 1955. Roto-therm soil pasteurizer. 2 p. Tarrant Mfg. Co., Saratoga Springs, N.Y Tavernetti, J. R. 1935. Characteristics of the resistance type soil sterilizer. Agr. Engineering (St. Joseph, Mich.) 16: 271-74. 1942. A continuous soil pasteurizer. Agr. Engineering (St. Joseph, Mich.) 23: 255-56, 261. Thomas, C. A. 1954. Some greenhouse soil pests and their control — nematodes. Pennsylvania Flower Growers Bui. No. 47: 1-5,8. Webrer, R. 1956. Mr. Jackson's soil steamer keeps on moving. The Grower (London) 45: 208-9. Section 1 1 Hamner, O. H., and F. C. Amstutz 1955. Apparatus for more rapid vaporization of methyl bromide. Down to Earth (Midland, Mich.) 11 (2): 11-13. Jefferson, R. N., and A. E. Pritchard 1956. Pest control guide for California floricultural crops. California Agr. Ext. Serv. Lflt. 66: 1-11. Kendrick, J. B., Jr., and G. A. Zentmyer 1957. Recent advances in control of soil fungi. In: Advances in pest control 1: 219-75. Inter- science Publishers, Inc., New York, N.Y. Lear, B. 1951. Use of methyl bromide and other volatile chemicals for soil fumigation. New York (Cornell) Agr. Exp. Sta. Mem. 303: 1^8. [293] Lear, B., and W. F. Mai 1952. Methyl bromide for disinfesting burlap bags and machinery to help prevent spread of golden nematode of potatoes. Phytopathology 42: 489-92. Martin, W. J., N. L. Horn, and J. A. Cox 1955. Fumigation of bell pepper seed beds for controlling damping-off caused by Rhizoctonia solani. Plant Dis. Reptr. 39: 678-81. Munnecke, D. E., and J. Ferguson 1953. Methyl bromide for nursery soil fumigation. Phytopathology 43: 375-77. Munnecke, D. E., and D. L. Lindgren 1954. Chemical measurements of methyl bromide concentration in relation to kill of fungi and nematodes in nursery soil. Phytopathology 44: 605-6. Newhall, A. G., and B. Lear 1948. Soil fumigation for nematode and disease control. New York (Cornell) Agr. Exp. Sta. Bui. 850: 1-32. Pritchard, A. E. 1949. California greenhouse pests and their control. California Agr. Exp. Sta. Bui. 713: 1-72. Sciaroni, R. H. 1955. Terraclor for Rhizoctonia disease of carnation. California Agr. Ext. Serv., Alameda and San Mateo Counties, Flower Notes No. 18 : 5. Sciaroni, R. H., and R. D. Raabe 1955. Rhizoctonia disease control in carnations with PCNB. California Agr. Ext. Serv., Alameda and San Mateo Counties, Flower Notes No. 16 : 6. Stark, F. L., Jr. 1948. Investigations of chloropicrin as a soil fumigant. New York (Cornell) Agr. Exp. Sta. Mem. 278: 1-61. Section 12 Baker, K. F., and F. D. Heald 1934. Investigations on methods of control of the blue-mold decay of apples. Washington Agr. Exp. Sta. Bui. 304: 1-32. Huber, G. A. 1935. The use of sodium hypochlorite solutions as disinfecting agents. Better Fruit (Portland, Ore.) 29 (12) : 5-6. Klotz, L. J., and T. A. DeWolfe 1952. Steam sterilization of citrus field and storage boxes. Citrus Leaves 32 (12) : 20, 35. Lear, B., and W. F. Mai 1952. Methyl bromide for disinfesting burlap bags and machinery to help prevent spread of golden nematode of potatoes. Phytopathology 42: 489-92. Roistacher, C. N. 1952a. Methods used in sterilizing nursery flats. Pacific Coast Nurseryman 11 (5) : 17, 34-36. 19526. Phytotoxicity of formaldehyde residue on flats. Phytopathology 42: 171-72. Roistacher, C. N., and K. F. Baker 1954. Disinfesting action of wood preservatives on plant containers. Phytopathology 44: 65-69. Wellman, R. H., and F. D. Heald 1938. Steam sterilization of apple boxes for blue mold. Washington Agr. Exp. Sta. Bui. 357: 1-16. Section 1 3 Baker, K. F. 1948. The significance of disease-free seed and propagating material. Florists' Exch. 110 (9) : 21, 30, 58-59. 1952. A problem of seedsmen and flower growers — seed-borne parasites. Seed World 70 (11) : 38, 40, 44, 46-47. 1956. Development and production of pathogen-free seed of three ornamental plants. Plant Dis. Reptr. Suppl. 238: 68-71. BAKER, K. F., and P. A. CHANDLER 1956. Development and product inn of pathogen-free propagative material of foliage and suc- culenl plants. Plant Dis. Reptr. Suppl. 238: 88 90 | 2. IS. 111. 123, 210, 252, 251, 2S0 aeration of treated soil, 199 best treated in containers, 48, 126-27 bulk soil, 273 [309] Chemical treatment of soil, continued causing salinity, 53 compared with steam, 1, 16, 18, 123-25, 210 correct dosage, importance, 22 cost, 197-98 dosage. 249-50 drenches, 19, 43, 48, 207, 209 effect on microorganisms, 18, 120, 248-50 effectiveness, 197, 201, 203, 204, 205 equipment for, 302 eradicative only in container soils, 197 ideal chemical, 197-98 importance of correct dosage, 207, 249-50 in containers, 201-2, 272 in "pinto tag" certification, 48, 129, 201 increased growth response of plants, 126, 199, 248 measurement of effectiveness, 125 N em agon, 206, 305 organic matter, effect, 197, 199 preparation of soil, 198-99 prior to placing containers on it, 41 selecting one to use, 1, 208-9 soil moisture, effect, 199 soil porosity, related to fumigation, 89, 142 soil temperature, effect, 148, 199 time required, 201-5 toxic residue in soil, see Chemical residue in soil after treatment treatment of soil in field, 18, 197, 201, 205, 206, 208-9 use near living plants, 124, 125, 200 V-C 13, 206, 305 weed control, 126, 197, 198, 200, 200-3, 204, 208, 248 when best used, 18, 197, 201, 208-9 See also Chloropicrin; DD; Ethylene dibro- mide; Formaldehyde; Methyl bromide; Terraclor; and Vapam Chemicals, sources, 304—6 Chenopodium, 139 Chestnut, 46 Chickwced, spotted wilt, 235 China aster, 111, 215; Botrytis gray mold, 49- 50; chemical seed treatment, 230, 232; Fu- sarium wilt, 5, 46-47, 49, 135, 218, 230, 232, 257-58, 260; Phomopsis canker, 233; Khizoc- tonia on, 43, 49-50, 260; spotted wilt, 235; Stemphylium leaf spot, 233 Chinese evergreen (Aglaonema), 226 Chloranil (Spergon), 230 Chloride in water, 64 Chloro-bromo-propene (CHI* 55), 199 Chlorophyll, 29«, 299 Chloropicrin, 200-1, 205, 206, 208, 248, 272-73 application, 16, 1H, 121,200-1,208-9 comparison with methyl bromide, 121, 198; with steam, 121 cost, 1H, 121, 137. 197-98 effectiveness, determination, 121; in crop re fuse, 12 1, 200; in soil. Mi, IS, 12 1 not recommended for stacked flats, 198, 201 sorption by soil, 200 sources, 304 temperatures for use, 124, 201 toxicity to crops, 124 treatment, of benches and beds, 18, 201; of bulk soil, 18,201 used for chrysanthemums and carnations, 201, 208 used near living plants, 200 Chlorosis, of gardenia, control, 8, 107, 268; iron, 13, 107, 113; result of biuret, 79; result of root-infecting fungi, 15; result of salinity, 8, 55 Choisya, water-mold root rot, 36, 37, 220 Chrysanthemum, 31, 32, 42, 203, 208, 219, 263, 268 Ascochyta ray blight, 233 aspermy virus on, 221 bacterial fasciation, 220, 233 bacterial stem rot, 35 chemical treatment of plants, 20, 232 crown gall, 220, 258 culturing technique, 20, 31, 221-22 cutting rots, 35 foliar nematode on, 20, 232 Septoria leaf spot, 233 Verticillium wilt, 7, 18, 31, 49, 201, 203, 219 virus stunt, 7, 51, 140 viruses, 232 Cineraria, 111 Citrullus (watermelon), 259 Citrus, psorosis, 232 Clarkia, 13, 111 Clay, 94, 97, 98-99, 100, 109, 142, 144, 161, 197, 199, 264, 267-68, 276 determination in fine sand, 103-4 disadvantages of, in plant culture, 12, 97; aeration impaired, 98-99, 143; cracks when dry, breaking roots, 268; drainage reduced, 65-67; leaching of soluble salts impeded, 65-67; mixing difficult, 98; movement of fumigants and steam im- peded, 89; shrinks from pots, 267; toxic after steaming, 93, 96; variable chemicallv and physically, 97, 99 effect on permeability of sand, 143 tolerances in fine sand, 103 Clay containers, hot- water treatment, 19, 53, 171-73, 211-12; salt accumulation on, 9, 53 Clods, see Lumps of soil Clorox (sodium- hypochlorite), 216, 221, 226, 305 Clover, Persian, root-knot nematode on, 259 Coal, fuel for boilers, 161-65, 176, 191 "Cold corners" in soil steaming, 128, 134, 140; eliminating, 128, 134, 169, 177 Colcus, foliar nematode on, 220 Collapse of plant, from damping-off, 35-36, I I, 10; from root rot, 36-37, 15; from salinity injury, 55, 59-60 [310] Colonization, controlled, see Controlled colo- nization Comb method for soil steaming (type 21), 183 Combined bin and potting bench, for soil steaming (type 3), 132, 148, 164, 166, 168-69, 273, 278, 280-81 Competition escape in microorganisms, aerial growth by Rhizoctonia, 251-52; in soil microorganisms, 259; in vascular parasites, 21, 238 Competition in nursery business, means of reducing, 32 Competitive retardant microorganisms, 21, 24, 238-40, 242, 300 Components of U. C.-type soil mixes, see In- gredients for U. C.-type soil mixes Compost, 91, 93, 97, 98, 99, 100; cost, 85, 100; source of salinity, 30, 53; source of toxicity, 95; source of variability, 10 Composting, avoided by U. C.-type mixes, 89- 90, 93, 270; disadvantages, 89-90, 100; odors and flies, 12, 90, 271; reasons for, 93-94, 109, 243; salinity problem, 90; scarcity of mate- rials, 90; shrinkage, 89-90; source of varia- bility, 10, 90; weed control in, 126 Computation methods, for soil particle sizes, 103-4; for steam data, 132-33, 135-37, 159- 60, 298-99 Concrete, particle sizes in, 99 Concrete mixers for soil mixing, 25, 84, 187, 265, 272-73, 276-79 Condensation, relation to steam/air ratio, 146- 51 Condensation zone in soil steaming, 128, 146, 149, 150, 152; relation to efficiency of steam- ing, 149-52; relation to steam/air ratio, 146- 51; width of, 150, 152 Condensing capacity of soil, 149-52, 154-55 Conductance, electrical, as measure of salinity, 60-61; method of measuring, 9, 14, 60-63, 84, 299, 300, 303; of soil, 9, 61; of U. C.-type mixes, 65, 70, 266; of water, 9, 14, 63 Conduction of heat, 149, 177; definition, 141, 299; effect of soil moisture on, 141-45; fac- tors affecting, in soil, 141-42; importance in soil heating, 150, 154-55; relation to porosity, 141-42; relative importance for heat and steam, 142; through metal con- tainer, 142 Constant water level culture, relation to sa- linity, 63 Container culture, advantages, 91, 234-35; earliest example, 91-93 Containers metal or plastic, 53, 211-12 relation to spread of pathogens, 5, 39-40, 210,217 treatment of (Section 12), 210-16; also 1, 11, 19, 48, 111, 123; required when soil treated separately, 19, 22-23, 48, 125-27, 210, 273, 279; unnecessary for new containers, 127; with blow torch, 19, 211; with copper naphthenate, 19,41,212,213-16,301; with formaldehyde, 19, 212-13; with heat, 1, 19, 211, 212; with hot water, 19, 211-12; with methyl bromide, 19, 212; with steam, 19,40, 133, 134,211-12 Contamination problem, see Recontamination problem Continuous-batch steaming equipment, 16, 166, 169, 171, 174, 176 Continuous knife injector for steaming soil in flats (type 27), 148, 165, 166, 185-86 Control of disease, 6, 48-49; benefits of, 7, 49- 51; multiple controls often needed in, 4, 219; must mesh with nursery practices, 4, 7; progress in, 3-4, 34-35; see also Chemical treatment of soil; Containers, treatment of: Dry source of heat; Hot-water treatment of soil; Pathogen-free planting stock; Sanitary practices; and Steam treatment of soil Controlled colonization, 27, 250-54, 300; effect of pH on, 251; possible future program, 24, 27, 254; relation to U. C.-type soil mixes 24, 35, 252-54; to control nitrogen nutri- tion, 13, 25, 113-15, 116, 253-54; to retard pathogens, 4, 20, 24-25, 35, 250-53, 254 Convection of heat, 149; definition, 141, 148; importance in soil heating, 143, 150, 154-55; relation to pore size, 143; relation to poros- ity, 143; soil factors affecting, 142, 143-44 Conveyers, 26, 168, 272-73, 278-80, 282 Cooling soil after steaming, 16, 134 Copper, 95; essential to plants, 89, 107 Copper naphthenate, 19, 41, 213-16; sources, 304 Cordyline, salinity injury, 8, 9 Coreopsis, 215 Cork, heat conductivity, 299 Corn, 240-41 Corrosive sublimate, see Mercuric chloride Cotton, 205; Fusarium wilt, 261; Rhizoctonia on, 205, 257; Thielaviopsis on, 258 Cottonseed meal, 13, 14, 76, 105, 115-19, 121 Cottony rot, see Sclerotinia cottony rot Covers, fitting to glasshouse benches, 180, 275; for chemical treatment of soil, 201-3, 208, 302; for soil in steaming, 170, 178-80, 303 Cowpea, 95 Crop-antagonistic microorganisms, 22, 238-40 Crown gall, 34, 218, 220, 239; variability in pathogenicity of bacteria, 258 Cultural practices, and disease control must mesh, 7; evaluated only on healthv plants, 51 Culture-solution growing of plants, 87 Cultured-cutting technique, 20, 221-22, 236 Cuprinol (copper naphthenate), 19, 41, 213-16, 304 Cutting rot, 4, 35, 37, 38; see also Damping-off Cuttings, rooted, 28, 79-80, 119, 217; soil mixes for, 71, 72, 73, 75 [311] Cyanamide, 244 Cyclamen, Thielaviopsis on, 258 Cymbidium, 12; salinity injury to, 8, 60; U. C- tvpe soil mixes for, 268-69 Dagger nematode (Xiphinema), 229 Dahlia, spotted wilt of, 221, 235 Damping-off of seedlings (Section 3), 34-51; also 4-6, 198, 202, 206 aggravated by salinity, 5, 7, 9, 33, 42, 49-50, 55, 266 causes, 4-5, 33, 37 control, 48-49; see also Control of disease effect of carbohydrate status of host on, 5, 42-43; of depth of planting on, 5, 43; of nitrogen status of host on, 5, 42-43; of controlled colonization on, 24-25, 250-54; of seed vitality on, 5, 43; of soil moisture on, 5, 33, 36; of soil temperature on, 5, 6, 43; of watering on, 43 factors in, 5, 42-43 infection sites, 35 losses produced, 37 not restricted to seedlings, 6, 23, 36-37, 43- 44,45 recontamination problem, see Recontamina- nation problem relation to mechanization, 26-27, 32-33 severity related to host susceptibility, 5, 42; to inoculum potential, 5, 42, 260; to soil treatment, 22, 248-50; to pathogen viru- lence, 5, 6, 260 types, 4, 35-38, 43, 249, 257 Day length, relation to plant distribution, 86 DD mixture, application, 18, 206, 209; effec- tiveness as nematocide, 206; sources, 304 Dealers of equipment and materials, 304, 306 Decomposition of organic matter, desirable before use, 100-1; effect of microorganisms on, 89, 95, 237, 240-44; effect of soil oxygen on, 240, 244; effect of soil temperature on, 240, 244; see also Organic matter Deionized water, for leaching soil, 15, 63-64; for watering plants, 15, 57, 63-64; not free of boron, 64 Delphinium, 111, 263; aster yellows, 235; bac- teria] leaf spot (black spot), 46, 47; bacterial stem rot, 218; .sec also Larkspur Demcton (Systox), against foliar nematode, 232; sources, 304 I) ninific ation process, 245 Deposit on leaves, salinity injury, 9, 15, 53-54, 64 Dew point . definition, 146 Di an thus, see Carnation Dial hei m\ for soil treatment, 190 Dieffenbachia, bacterial leaf spot, 220; bac- terial soft rot, 35, 220, 226; germinating cane, 227; hardening for heal treatment, 223-24; heal treatmenl of cane, 220-27; pre paring cane foi treatment, 224; Rhizoctonia on, 220, 257; U. C. system for, 267; water- mold stem rot, 35, 220, 226 Dieldrin, used against fungus flies, 227; sources, 304 "Difficult" crops, 32 Diffusion of gases through soil, 89, 143, 146- 49; relation to porosity, 143, 148-49; rela- tion to temperature, 143-44 Direct-tvpe soil heater (type 15), 165, 166, 177, 179, 195 Disease (Section 3), 34-51; also frontispiece, 11, 300; apparent vs. true cause, 33; develop- ment of concept of, 3-4, 34-35, 138; elimi- nation, benefits from, 7, 49-51; factors in, 5, 22, 33, 49-51, 256; importance in propaga- tive material, 29; importance to grower, 4, 29, 49-51, 219; relation to other grower problems, 4, 7; restricts growth potentiali- ties of crop, 7, 49 Dish gardens, fertilizer and soil mixes for, 81, 84 Distilled water, sources, 304 Distribution system for steam, 132, 136, 195- 96; aluminium irrigation pipe, 129, 191, 195; diameter of pipe, 191, 194, 195; efficiency, 299; heat loss in, 194; water in steam lines, 133, 152, 178, 196 Dithane D-14 (Nabam), 19, 207,209, 304 Ditylenchus, see Stem and bulb nematode Dolomite lime, 70-75, 101, 106 Don't fight 'em, eliminate 'em, 4, 7, 23 Downy mildew of snapdragon, 46, 47 Drainage of soil, 11, 183; effect of soil condi- tioner on, 65-67; improved by tiling, 183; in relation to aeration, 60, 99; in relation to salinity, 15, 53-54, 57, 64; restriction by container boundary, 64, 87 Dreft, 230 Drenches, fungicidal and fungistatic, 207, 209; captan (Orthocide 106), 19, 43, 207, 209, 304; combination of materials, 207; ferbam (Fermate), 19, 207, 209, 304; nabam (Di- thane D-14), 19, 207, 209, 304; salvage treat- ment, 207; Semesan, 19, 207, 209, 305; Ter- raclor (PCNB), 19, 43, 207, 209, 305; thiram (Arasan, Iersan), 19, 43, 207, 209, 305 Dripping benches from steaming, cause, 133, 149 Drum soil Heaters, see Horizontal rotating (hum; Oil-drum method I)r\ fertilizers, II, 14,76,77, 79, 80 Dry source of heat for soil treatment, frontis- piece, I, 16, 123, 162, 166, 176-77, 187, 189-91 besl with moving soil mass, 1(5, 125-26, 163 compared with steam, 125-26, 146 disadvantages, 125, 146 equipment using, 176-77 intense heal in limited area, 125 objectives, 1 1 1 temperature and time required, 15 temperature in moving soil, 127 [312] used with dry soil, 144-46, 161, 176; with moist soil, 145, 161, 176 Dump soil, methods for using, 84; quantity used for bedding plants, 84 Dump truck, 168 Dura-K potassium frit, 76, 106, 304 Duratex cover for soil treatments, 178, 303 Dyes indicating liquid fertilizer injection, 76 EDB (ethylene dibromide), 18, 198, 199, 206, 209, 304 Eddy currents of steam in soil, 149 Efficiency calculations for soil steaming, 299 Eggplant, 25-26, 41, 264-65 Egypt, nurseries in, 91 Einheitserde (standardized soil), 94, 96 Electricity, power for boilers, 16, 162, 163, 164— 65, 176, 177, 184, 185, 190, 192, 195 Environment, effect on disease, 5, 29-30, 42- 43, 44, 49-50, 300; effect on plant, 5, 42-43, 86-89 Environmental tolerance of crop, 49 Equipment, fumigation of, 138 Equipment for mechanized fertilizer applica- tion, 76, 78, 283-84 Equipment for mechanized watering, 27, 283- 84 Equipment for planting, Erdprinz planter, 282-83, 302; machine seeding, 25-26, 264, 283; pot and can fillers, 25, 26, 33, 166, 168, 272-73, 280, 302 Equipment for soil handling (Section 17), 275-82, also 25-26 bicycle-wheel cart for pots, 281-82 breaking up lumps, 98, 133, 161 can filler, 25, 26, 33, 166, 272-73, 280, 302 concrete mixer, 25, 84, 187, 265, 272-73, 276-79 conveyers, 26, 168, 272-73, 278-80, 282 enclosed storage building, 163-66, 275-76 flat filler, 25, 26, 166, 168, 272-73, 279-80 fork-lift tractor, 25, 26, 166, 171, 174, 211, 272-73, 277-79, 280-81 mobile bin and potting bench (type 3), 132, 148, 164, 166, 168-69, 273, 278, 280-81 moving belts, 26, 168, 272-73, 278-80, 282 pot filler, 25, 168, 272-73, 280, 282, 302 screen, 276-79 shredder, 98, 276 skip-load tractor, 25, 84, 168, 272-73, 276, 278-79, 280 soil treatment, see Equipment for soil heat- ing, below steel rollers, 25, 272-75, 277, 279, 282 wooden pallet, 22, 25, 26, 166, 171, 174, 211, 212, 272-73, 276-77, 279, 280-81 Equipment for soil heating (Section 10), 162- 96; also 1, 16 adapting batch equipment to continuous operation, 16, 166, 169, 171, 174, 176 autoclave (type 9), 16, 17, 25, 129, 132, 134, 147, 151, 158, 162, 164, 166, 174-75, 272 baking or burning (type 17), 165, 166, 177 box, electric heating elements (type 14), 1 12, 165, 166, 176-77, 179, 195, 303; tubular version, 177 box, electrode heating (type 15;, 165, 166, 177, 179, 195 box, induction grid (type 16), 165, 166, 177, 195 bulk units, 15, 16, 167-77, 185-91 buried perforated pipe (type 20), 16, 128, 131, 132, 138, 148, 150, 152, 156, 165, 166, 178, 180-83, 184, 194 buried tile (type 22), 16, 131, 132, 118, 156, 165, 166, 178, 181, 183, 185 combined bin, potting bench (type 3), 132, 1 18, 164, 166, 168-69, 273, 278, 280-81 containers, treated in, 162, 164-65 continuous knife injector for flats (type 27), 148, 165, 166, 185-86 deep steaming of benches or beds, 131, 178, 180-83 efficiency levels, 132 electric hot-plate type (type 32), 165, 189- 90, 195 free-flowing vs. superheated steam, 129-30, 156-59 horizontal tank (type 13), 131, 164 horizontal type with removable hood (type 10), 131, 164, 174-76, 195,272 hot water (type 25), 16, 148, 165, 182, 184 inverted pan, electric (type 24), 148, 165, 178, 180-81, 184, 195 inverted pan, steam (type 19), 16, 131, 132, 138, 148, 165, 166, 178, 180-81, 184, 275 mobile bin (type 2), 16, 132, 148, 164, 166, 168, 193, 273, 280-81, 303 mobile units, 162, 273 movable Thomas method for ground beds (type 18), 148, 166, 180 moving rake (type 23), 131, 148, 165, 166, 178, 182-84 multipurpose tank (type 7), 16, 131, 132, 148, 164, 166, 171-73, 176 oil-drum type (type 12), 131, 164, 176 permanent vs. mobile equipment, 163, 164- 65 rotating drum, external flame, batch (typs 35), 165, 191, 303 rotating drum, external flame, continuous (type 31), 165, 190,273 rotating drum, internal flame (type 30), 165, 185, 187, 189-90, 194, 303 rotating drum, knife injector (tvpe 28), 14S, 165, 166, 185-87, 273 rotating screw, electric (type 33), 165, 189- 90, 195; external flame (tvpe 34), 165, 190 rotating screw, steam (tvpe 29), 16, 165, 187- 88, 272, 303 [313] Equipment for soil heating, continued Rudd type (type 1), 132, 148, 164, 166, 167- 68, 180 self-generating types of steamers, 174-76, 184 shallow steaming of benches or beds, 131, 178-80 sources, 302-4 spike or rake type (type 21), 135, 1 18, 150, 165, 166, 178, 181, 183 stationary units, 162, 166-77 stationary vs. moving soil mass, 16, 162-63, 166, 185 steam box, bulk and containers (removable front) (type 4b), 16, 17, 131, 132, 148, 156, 164, 166,' 169-70, 303 steam box, bulk soil (fixed front) (type 4a), 16, 132, 148, 156, 164, 166, 167, 169-70, 176, 273, 303 steam-chemical (type 26), 148, 165, 166, 184- 85, 213 steam-generating equipment, see Steam- generating equipment steam plow, 182 steam vs. dry heaters, 125, 141 table for selecting suitable type, 164-65, 274 Thomas, for beds (surface) (type 18), 16, 131, 132, 148, 163, 165, 166, 171, 178-80 Thomas, for containers (type 5), 16, 131, 132, 148, 164, 166, 170-71, 179, 272 tipping steam box, Norwegian, 167 vault type (type 6), 16, 25, 131, 132, 148, 164, 166, 167, 171-72, 174, 272,277, 280-81 vertical cabinet, electric (type 11), 131, 174, 175-76 vertical cabinet, external steam (type 8), 131, 132, 148, 164, 166, 173-75, 176 vertical cabinet, self-generating (type 11), 131, 164, 174, 175-76, 195 Equipment for soil testing, 302 Equivalents, table of, 301 Eradication of pathogen, difficult in field, 197; from tools, 19, 22, 23, 40, 48, 201, 226; in host tissue, 20, 223; see also Chemical treat- ment of soil; Containers, treatment; Dry source of heat; Hot-water treatment of soil; and Steam treatment of soil Erdalith for wood preservation, 215, 216 Erdprinz planter for pots, 282-83, 302 Erica, see Heather Esther Read daisy, see Chrysanthemum Ethylene dibromide (EDB), application, 18, 206, 209; effectiveness, 198, 206; residual toxicit) to plants, 199, 206; sources, 304 Eth) lene oxide, 216 Euphorbia (poinsettia), 29, 257, 258, 266 Evolution ol plants, 86, 91, 109 lasuat ion, bacterial, 220, 233, 258 Ferbam (Fermate), 19, 207, 209, 22!), 304 Ferns, foliar nematode on, 233; salinity injury, 7, 58-59 Fertility, of clays and composts, 99-100; of fine sand-peat mixtures, 12, 99-100 Fertilizer burn, aggravates Botrytis diseases, 46 Fertilizer ingredients of U. C.-type soil mixes, 69-76 1,71-75,77, 78, 79,80, 81, 111 11,71-75, 79, 80, 81, 111 111,71-75,80, 111 IV, 71-75, 77, 78, 79,80, 81, 111 V, 71-75, 79, 80, 81, 111 VI, 71-75, 80, 111 cost, 71-75 mixing, 70 salinity from, 15, 53-54, 57 Fertilizers, 76-79, 87, 96 deficit, effect on plant, 88 dry, 14; application methods, 11, 14, 76-77, 79; formulas (fertilizers VII-XI), 76-77; use, examples, 79, 80 evaluation, 51 excess, cause of chlorosis, 107 inorganic, see Ammonium and Nitrate ni- trogen liquid, 14; application during watering, 14, 27, 76, 78; formulas (fertilizers L-l to L 12), 76, 78; proportioned for applying, 283, 302; use, examples, 78-81 mechanized application, 78 organic, mixed in soil, 71-75, 122; on sur- face, 13-14, 118-19, 121, 122 source of salinity, 9, 11, 15, 30, 53-54, 57, 64-65, 70 Fertilizing equipment, 204, 302 Fiberthin covers for soil treatments, 178, 303 Fillers, can, 25, 26, 38, 166, 272-73, 280, 282, 302; flat, 25, 26, 166, 168, 272-73, 277, 279- 80; pot, 25, 168, 280, 282, 302 Fine sand, see Sand, fine Fir, Douglas, 46; bark, 65-67 Fire blight, 34, 258 Fish meal, 13,76, 115-19, 121 Fittonia, hardening for heat treatment, 223- 24; heat treatment of plants, 227-28, 229; Rhizoctonia on, 220-21, 227-28, 267; U. C. system for, 267; water-mold root rot, 220- 21,267 Flash-flame pasteurizer for soil (type 30), 187— 88, 190 flash steamers, 192, 302 Flat -making machine, 33, 276 Flats, 126, 127, 150, 163, 196, 197, 201, 210, 219, 250, 254; chemical treatment, 1, 19, 10, 201, 203, 201, 208, 212-16; fertilizer schedule lor plants, 79; filler, 25, 26, 166, 168, 272- 73, 279-80; si/e, 279, 301; soil mixes for, 12, 69, 72, 75, 79, 264-65; stacking for soil treat- ment, 131, 173, 189, 201-4, 211: steam treat- ment, 19. 10, 133. 13 1, 162, 163, 170, 171, 173, 211; transporting, 22, 25, 26, 166, 171, 171, 21 1. 212, 272-73, 276-77. 279, 280-81 [314] Flies from compost piles, 12, 90, 271; from organic nitrogen, 122 Floricultnral plants, 28 Flower blights, 34 Foliage plants, 20, 29, 32; U. C. system for, 20, 29, 32, 266-67; utilize ammonium, 13 Foliar feeding, 89 Foliar nematode (Aphelenchoides), 20, 47, 220, 233; chemical treatment of plants against, 232; lethal temperatures, 139 Fork-lift tractor, 25, 26, 166, 171, 174, 211. 272-73, 277-79, 280-81 Formaldehyde. 200, 203-4, 208 dilute method for soil, 204, 208; relation to inoculum potential, 42, 204, 260 effectiveness, 203 paraformaldehyde formation, 19, 212-13 sources, 304 steam-formaldehvde for soil treatment, 148, 184; for glasshouse cleanup, 213 toxicity to plants. 204 treatment, of containers, 19, 212-13; of floor, 18, 22, 134, 166, 199; of planting material, 20, 228, 230; of soil, drench, 41, 148, 204, 208; of tools, 19, 22, 23, 40, 213, 226 Formalin, see Formaldehyde Fragaria (strawberry), 220 Frankincense trees, 91 Free-flowing steam for soil treatment, 15, 129, 135, 157-58, 164-65, 167-74, 178-84, 187, 191-92, 196, 300; advantages, 129; heat con- tent, 157-58; pressure in mains, 129 Freesia, mosaic, 21, 233 Frit, potash, 76, 106 Frost, relation to plant distribution, 86 Frozen soil, cause of uneven steaming, 134 Fuchsia, foliar nematode on, 220 Fuel for steam boilers, 16 amount required for soil treatment. 135-36 butane, 16, 162. 187, 192, 194 coal, 164-65, 176, 194 cost, 136 electricity, 16, 162, 163, 164-65, 176, 177, 184, 185, 190, 192, 195 heating value, 299 kerosene, 187, 194 natural gas, 16, 135, 136, 162, 163, 164-65, 174, 176, 185, 187, 190, 191, 192, 194 oil, 16, 136, 162, 163, 164-65, 174, 176, 185, 190, 192, 194 propane, 162, 192, 195 Fumigants, see Fungicides and Xematocides Fungi, 15, 18, 197, 198, 200, 204, 206, 221, 237- 38, 242, 254, 299; longevity, 199, 243: nema- tode-trapping, 240; variability, 255-58; see also Damping-off Fungicides, 259, 299 captan (Orthocide 406), 19. 43, 207, 209, 304 Celcure, 216 chloropicrin, see Chloropicrin copper naphthenate (Cuprinol), see Copper naphthenate Erdalith, 215, 216 ethylene oxide (Carboxide), 216 ferbam (Fermate;, 19, 207, 209, 228, 301 formaldehyde, see Formaldehyde mercuric chloride, 20, 230, 232 methyl bromide, see Methyl bromide nabam (Dithane D-14), 19, 207, 209, 30! New Improved Ceresan, 20, 230 Puratized Agricultural Spray, 228, 305 Semesan, 19, 207, 209, 305 sodium hypochlorite (Clorox, Purex), 216, 221-22, 226, 305 sources, 304-6 Spergon (Chloranil), 230 sulfur, 246-47 Terraclor (PCNB), 19, 43, 205, 207, 208-9, 305 thiram (Arasan, Tersan), 19, 43,207, 209, 305 Yapam, 18, 204-5, 208, 305 Wolman salts, 215, 216 Fungistatic, 299 Fungus flies, control, 227; spread soft-rot bac- teria, 227 Fusarium, 198, 220,250 basal rot, 227-28 cortical root rot, 229, 256 cortical stem rot, 256 effect of soil temperature, 260 lethal temperatures, 127, 139 longevity in soil, 261 saprophytes and parasites in soil, 238 saprophytic, 256 survival in soil, 261 variability, 256-58 vascular wilts, 139, 256-57 wilt, favored by nematodes, 261 Fusarium wilt of China aster, 5, 135, 230, 232, 257-58; conditions favoring, 47, 49, 260; in- oculum potential, 260; life history of causal fungus, 218; relation to Botrvtis crown rot, 49; relation to Rhizoctonia crown rot, 49; soil temperature, 260; symptoms, 46-47 Galvanized nails, corrosion bv copper naph- thenate, 213 Gardenia, 35, 69, 268; chlorosis, 8, 107, 268; salinity injury, 8, 55, 59-60; U. C. system for, 268 Gas, natural, fuel for boilers. 16, 135. 136, 162, 163, 164-65, 174, 176. 185. 187. 190. 191. 192, 194 Gases, viscosity related to temperature. 143 Geranium. 29, 32, 35, 215-16; bacterial leaf spot and stem rot of. 21. 35, 261: cultured- cutting technique with, 20, 221; mosaic of. 234 Gerbera, heat treatment of plants. 227; root- knot nematode, 227 German peat, see Peat, sphagnum [315] Germination effect of pelleting on, 25, 283; of salinity on, 9, 42, 55 in mechanization, 26-27, 264 rate, seed, 82; effect of soil toxicity on, 94 Gladiolus, 223, 263; effect of soil temperature on heat tolerance of cormels, 224; heat treat- ment of cormels, 224; presoaking cormels before heat treatment, 224; Rhizoctonia on, 257; yellows disease, 139 Glossary of terms used, 298-301 Glycine (soybean), 257 Gossypium (cotton), 205, 257, 258, 261 Graft failure, Thielaviopsis, 258 Gravel, 97, 98, 99, 100, 103-4 Gray mold (Botrytis), 5, 9, 46-47, 49-50, 139 Grower experience with U. C. system (Section 16), 263-70; also 1; bed flower crops, 208; bedding plants, 264-65; bench flower crops, 268; can -grown woody stock, 267; cymbi- diums, 268-69; foliage plants, 266-67; land- scape application, 269-70; pot plants, 266- 67; vegetable transplants, 265-66 Grower "secrets," 49 Growing-on, soil mixes for, 71, 73, 74 Growth regulators, 51 Gypsum, source of calcium, 71-72, 101, 106; used to reduce soil toxicity, 10, 96, 109; see also Calcium sulfate Hardpan in soil, and salinity, 57 Harrow method of soil steaming, see Spike method Harrow-type electrode heater (type 15), 165, 166, 177, 179, 195 Haworthia, heat treatment of plants, 139, 227; Pythium root rot, 139, 227 Heat, definition, 141; differentiated from steam, 142 Heat capacity, definition, 141, 299, 301 Heat requirement of soil, 299; of water, 299 Heat sterilization, development of, 138; of containers, 1, 19,211,212 Heat-tolerant plant parts for treatment, 223- 24 Heat transmission compared with steam movement, 1 11-45 conduction, 141-42, 111, 150 convection, 141, 142, 143-44, 148, 150, 151-55 radiation, 141, 142, 144, 150 rate, in dry soil, 144-45; in moist soil, 144-45 ) elation to compaction, 130, 134, 112; to or- ganic mattei content, 112; to particle size, I 12; to soil porosity, I 11-12; to soil mois line-. 111 1") Stead) and misleads states, 145-46, 150 through pine logs, 151-52 I leal treatment of planting material, 138, 236; against microdrganisms, 139, 223-31; againsl viruses, 221; See also Hot-water treatment of planting material of soil (Sections 8, 9), 123-61; also 11, 210; see also Dry source of heat for soil treatment; Hot-water treatment of soil; and Steam treatment of soil Heather (Erica), chlorosis, 107; cutting rot, 45; Phvtophthora root rot, 7, 45, 49, 218-19, 220, 258; Rhizoctonia stem rot, 220 Heating of soil, effect on organic nitrogen breakdown, 13, 53, 105, 112, 115-19, 120 Hedera (ivy), 220-21, 267 Helichotylenchus (spiral nematode), 229 Helminthosporium cactorum, 38 Herbaceous ornamentals, 29 Heterodera, see Potato root nematode and Root-knot nematode High vs. low pressure steam for soil treatment, 191-92 Hippeastrum (amaryllis), 58 "Hoddesdon pipe" for soil steaming, 180; winch-drawn, 182 Home-yard planting, fertilizer schedule, 80- 81; soil mixes, 81, 84; steaming soil, 135, 193 Hoof and horn meal, 13, 14, 53, 70-75, 76, 77, 79, 91, 105, 106, 110, 111, 112, 113, 115-19, 121,242,243 Horizontal rotating-drum soil heaters, exter- nal flame, batch type (type 35), 165, 191, 303; external flame, continuous type (type 31), 165, 190, 273; internal flame (type 30), 165, 185, 187, 189-90, 194, 273, 303; knife-injec- tor type (type 28), 148, 165, 166, 185-87, 273 Horizontal rotating-screw soil heaters, electric (type 33), 165, 189-90, 195; external flame (type 34), 165, 190; steam (type 29), 16, 165, 187-88, 272, 303 Horizontal tank-type soil steamer (type 13), 131, 164, 176 Horizontal-type steamer, removable hood (type 10), 131, 164, 174-76, 195, 272 Hormone solutions, relation to spread of pathogens, 5, 22, 38 Horsepower rating of boiler vs. pounds of steam, 159-60 Hose nozzle and spread of pathogens, 5, 23, 24, 38-40 Host, 299, 300 Host range of pathogens, 6, 256-59; variability in. 256-59 Hot-plate method of soil heating (type 32), 165, 189-90, 195 Hot-water treatment of containers, methods, 19; salt removal, 19, 211-12 Hot-water treatment of planting material. 20, Hi I 65,223-30,236 application to specific crops, 226-30 breaking dormanc s of stock. 227. 229, 230 conditioning the material, 20, 223-24 containers lor material. 225 cooling the material, 225-26 [ 316 ] drying the material, 226 equipment, 302 eradication of pathogens, 20, 223 methods, 223-26 multipurpose tank, 171-73, 224 not protective against reinfection, 223 preparing the material, 20, 224 presoaking the material, 224 selecting material to treat, 223 storing the material, 226 temperature-time relation, 20, 139, 225 treatment tanks, 224-25 value, 223 Hot-water treatment of soil, 123; compared with steam, 126, 146-48, 152, 156-57, 184; disadvantages, 126, 146; equipment for heating by steam (type 25), 16, 148, 165, 182, 184; excessive water, 126, 184; heat con- tent of hot water, 146, 156-57; heat transfer involved, 146, 156-57; salt removal, 126, 146, 184; use on propagating sand, 126, 146, 184 Humidity, atmospheric, 146-47; effect on dis- eases, 29-30, 42-43; on salinity injury, 11, 15 Humus, 99 Hutchings method for soil heating (type 32), 165, 189-90, 195 Hybrid seed, 32 Hyacinth, stem and bulb nematode on, 258 Hydrangea, blue vs. pink, 76 Hypericum, 139 Hypnum peat moss, 69, 104 Iceland poppy, 111, 215 Immersion-type soil heater (type 14), 142, 165, 166, 176-77, 179, 195, 303 Increase block, 222, 235-36 Increased growth response from soil treat- ment, 126, 199, 248 Incubation period of disease, 232, 234 Indirect-type soil heater (type 14), 142, 165, 166, 176-77, 179, 195, 303 Induction-grid type soil heater (type 16), 165, 166, 177, 195 Infected seed or stock, 43, 44, 260; increase of inoculum potential, 260; means of selecting virulent strains, 6, 260 Infection, 299; by Rhizoctonia, 38 Infest, definition, 299 Infrared lights for soil heat treatment, 190 Ingredients for U. C.-type mixes, 12-13, 69- 76, 96-107 aeration, 97, 98-99 availability, 12, 97, 98, 103 characteristics, 101-7 chemical, 69-76, 97, 98, 101, 105-7; see also Fertilizer ingredients cost, 97, 100 criteria for selection, 96-107 ease of mixing, 83-84, 97, 98, 105, 133, 276- 79 fertility, 97, 99-100, 105-7 moisture retention, 12, 69, 87, 97, 100, 270 physical, 69, 101-5; see also Peat, sphagnum; Redwood; Rice hulls; and Sand, fine proportions, 69-76 resistant to leaching of nutrients, 12, 97, 99 selection, 96-107 shrinkage, 12, 80, 85, 89-90, 97, 100-1, 241 soil used, low in organic matter, 103 sources, 85, 97-98, 101-3, 275 stability to steaming or fumigation, 9-10, 11, 12, 15, 90, 93, 96, 97-98, 124, 129, 140, 199,270 uniformity, 12, 97, 98 weight, 12,69, 80,97, 100 Injector, for applying fertilizers, 283, 302; for applying soil fumigants, 200, 201, 206, 302 Inoculation of soil, with ammonifying micro- organisms, 13, 113-15; with antagonistic microorganisms, 4, 20, 24-25, 35, 250-54; with nitrifying bacteria, 13, 25, 113-15, 116, 120, 253-54 Inoculum potential, 5, 42, 204, 260, 300 Insect screens on seedbeds, 235 Insecticides, 18, 20, 206, 227, 232, 304, 305 Insects and mites, 15, 18, 21, 138, 197, 198, 200, 204, 206, 248, 275; lethal temperatures, 127, 139 Inserts for flats, 25, 31, 127 Introduction of new disease, importance, 20, 218; see also Spread of microorganisms Inverted pan for soil steaming (type 19), 16, 131, 132, 138, 148, 165, 166, 178, 180-81, 184, 275; electric type (type 24), 148, 165, 178, 180-81, 184, 195 Ipomoea (morning-glory and sweet potato',, 221,257 Iris, 139, 260 Iron, 95; availability affected by microorgan- isms, 237, 247; chelate for chlorosis, 107, 268; chlorosis, 13, 107, 113; essential to plants, 89, 107 Isolation house, 21, 23, 24, 234-35, 261 Ivy, bacterial leaf spot, 221; Rhizoctonia on, 220-21; U. C. system for, 267; water-mold root rots, 220-21 John Innes composts, 10, 86, 91, 93; contribu- tion to nursery soils, 10, 91, 93, 110; disad- vantages, 93, 96; sold on market, 94 John Innes high-rate soil steamer (type 1), 167-68 Kentia palms, 284 Kerosene, fuel for boilers, 187, 194 Kilowatt-hours, 159, 300 Klamath-weed seed, heat resistant, 139 Krillium, effect on leachability of soil, 15, 65-67 L 317 J Labor, cost rising, 32, 96, 196, 271; reduced by U. C.-type soil mix, 10, 84, 89-90, 100, 270 Lactuca (lettuce), 95, 112 Lamb's quarter seed, heat resistant, 139 Landscape use of U. C. system, 269-70; soil fumigation in field plantings, 269-70 Land value rising, effect on nurseries, 31 Larkspur, 215 Lathyrus (sweet pea), 230 Leachability of U. C.-tvpe soil mixes, 65-67, 99 Leaching losses of soil nutrients, 65-67, 99, 105-7 to reduce salinity, 9, 11, 14-15, 183; effec- tiveness related to water salinity, 9, 14, 63; relative leachability of various soil mixes, 15, 65-67; with nutrient solution, 65 to reduce toxicity, 10, 95, 96 Leaf burn, from ammonium, 13, 111, 113; from biuret, 79; from salinity, 8-9, 42, 55- 56, 58 Leaflet on U. C.-type soil mixes, 68 Leaf mold, 86, 97, 98, 99, 115-19, 242, 262, 267, 268 aeration when decomposed, 99 cost, 85, 100 scarce in California, 12, 90, 98, 100 shrinkage, 85, 90 source of salinity, 9, 15, 30, 53-54, 90, 264; of toxicity, 95, 108; of variability, 10, 89, 93 Leaf spots, 34 Legumes, 77, 78, 245 Lesion nematodes (Pratylenchus), 47, 227, 229; lethal temperatures, 139 Lettuce, ammonia toxicity, 112; manganese toxicity, 95 Light on plant, 29, 51, 259; relation to damp- ing-off, 5, 43; relation to plant distribution, 86 Light soil mix, 93; see also U. C.-type soil mixes Lignin, 254 Lily (Lilium), 86; Fusarium basal rot, 227, 229; root development in U. C.-type soil mix, 83 (fig. 61); treatment of bulbs against nematodes and Rhizoctonia, 227-28, 229 Lima bean, Rhizoctonia on, 257 Lime, 70. 91, 101, 115, 120; calcium carbonate, 70, 72-75, 91, 101, 106; dolomite, 70-75, 101, 100; ovstei shell, 106 Liners, soil mixes for, 71, 72, 73, 80 Lining-out slock, 29 Liquidanibar , chlorosis, 107 Liquid fertilizers, 14, 27, 76, 78-79, 106; pro portioners for applying, 283, 302 Little-leaf of peach, 248 "Living uilh" a disease, 7, 49 Loam, 15, 65-67, 91, 97, 98, 99, 100, 1 12, 1 13, I 11, I") I 55 Lobelia, 111,215 Lobularia (sweet alyssum), 13, 111, 112, 113, 264 Loganberry, crown gall, 258 Longevity of organisms in soil, 261 Long pipe for soil steaming, 180, 184; winch- drawn, 182, 184 Lotus strigosus seed, heat resistant, 139 Lumps of soil, equipment for breaking up, 98, 133, 161; not formed by fine sand, 98, 133, 161, 276; relation to chemical treat- ments, 199; relation to steaming, 15, 127, 133, 134, 140, 149, 160-61, 190 Lycopersicon, see Tomato Machinery, disinfestation, 201, 212 Magnesium, 64, 95, 99, 109, 110; availability affected by microorganisms, 237, 247; es- sential to plants, 89, 106; from dolomite lime, 13, 70 Magnesium bicarbonate, 63 Magnesium carbonate, 106 Magnesium sulfate, 94, 177 Maidenhair fern, 215 Maintenance of pathogen-free planting stock, 21, 226, 234-36; by selecting growing area, 21, 233; of chrysanthemums, 31 Majestic daisy, see Chrysanthemum Malathion, against fungus flies, 227; sources, 304 Mains (apple), 258; see also Fire blight Malva (buttonweed), 139, 203, 235 Manetti, see Rose Manganese, availability affected by micro- organisms, 247-48; essential to plants, 89, 106-7; role in soil toxicity, 9, 95, 98 Manure, 97, 98, 115-19, 198, 241-42, 262, 264, 265, 267, 268, 271 aeration, when decomposed, 99 competition, 12, 90 cost, 85, 100 package trade, 90 poor source of nutrients, 99, 100; of organic matter, 89, 100 shrinkage, 85, 89-90, 100, 241 source, of salinity, 9, 30, 53-54, 90, 116; of toxicity, 95, 108; of variability, 10, 89, 93 Maple leaves, 100 Maranta, salinity injury, 8-9 Marguerite, see Chrysanthemum Marigold, see Calendula and Tagetes Market, distance from, 31-32; expanding in California, 31-32; lor bedding plants, 30-31 Materials, sources, 302-6 Matthiola, see Stock Meadow nematode (Pratylenchus), 17, 139, 227, 229 Measures, table of, 301 Mechanical applicators of fumigants, 201. 20 1 -5, 200 [318] Mechanization in growing (Section 17), 271- 84; also frontispiece, l', 25-27, 32, 33, 266 adaptability of U. C. system to, 25, 90, 94, 270,271 adoption when moving, 31 advantages of sloping land for, 32, 274 applied to new nursery, 274-75; to old nursery, 274 bicycle-wheel cart, 281-82 disease control required, 26-27, 32-33 fertilizing, 78, 204, 283-84, 302 filling containers, 25, 26, 33, 166, 168, 272- 73,^ 279-80, 282, 302 flat-making machine, 33, 276 flow diagrams, 1, 272-73 germination in covered flats, 26-27, 264 glasshouse arrangement and design, 275 laird slope in mechanization, 32, 274 machine planting, 25-27, 264, 283, 302 mixing and screening soils, 25, 83-84, 97, 98, 187, 265, 272-73, 276-79 mobile bin and potting bench (type 3), 132, 148, 164, 166, 168-69, 273, 278, 280-81 paving the yard, 275 planning layout, 31, 274-84 planting, 25-27, 264, 283, 302 preparing soil mixes, 83-84, 105, 133, 276 processing and stockpiling materials, 275-77 requirements to make possible, 26-27, 32-33 seeding in place, 25-26, 264, 283, 302 segregation of operations, 21, 163, 166, 276 storage of components, 275-76; of soil mixes, 13, 25, 71-75, 276 transplanting, 25, 81, 264, 282-83 transporting containers, 281-82; soil, 25, 280, 282 watering operation, 27, 283-84 See also Chemical treatment of soil; Con- tainers, treatment of; Dry source of heat for soil treatment; Hot-water treatment of soil; and Steam treatment of soil Medicago, see Bur clover and Alfalfa Meloidogyne, see Root-knot nematode Mercuric chloride, treatment of planting stock, 20, 230, 232 Methyl bromide, 143, 200, 201-3, 206, 208-9, 264, 267, 269, 272-73 application, 16, 18, 124, 198, 201-2, 204 cost, 18, 124, 137, 197-98 effectiveness, determination of, 124; in crop refuse, 124, 203; in soil, 16, 124 ineffective against Verticillium, 16, 18, 22, 124, 203 residual toxicity to plants, 16, 124, 199 sources, 304 steam-methyl bromide for soil treatment, 185 temperatures for use, 124, 201-2 toxicity to crops, 10, 16, 17, 18, 124, 199, 208 treatment, of containers, 19, 212; of farm machinery, 201, 212; of soil in stacked containers, 18, 201-2, 272 use in "pinto tag" certification, 48, 129, 201 Micronutrients, 106-7; supply in U. C.-type mixes, 12, 89, 101, 106-7, 109, 110 Microorganisms, soil (Section 14), 237-54 abundance, 237-38 antagonistic, 4, 20, 21, 24-25, 35, 250-54 balanced population, 21, 25, 238-40, 250-54 beneficial, 238-40 buffering capacity, 299 carbon cycle, 246-47 cause of disease, 3, 4, 33-35, 138 competition, 21, 24, 238-40, 242, 300 concentration in rhizosphere, 240-41; in surface layer, 120, 238, 240 controlled colonization, see Controlled colo- nization crop-antagonistic, 22, 238-40 decomposition of organic matter, 89, 95, 115-19,237,240-44 dependence on green plants, 241 depth in soil, 6, 120, 238, 240 distribution, 240 dynamic equilibrium, 21, 25, 238-40, 250- 54 effect of fungicide dosage on, 249-50; of organic matter on, 25, 238, 240, 242-44; of oxygen on, 121, 240, 244, 298; of soil moisture on, 21, 25, 49-50, 240; of pH on, 120-21, 245-48; of soil treatment on, 19, 113-15, 115-19, 120, 204, 205, 248-50; of soil temperature on, 21, 25, 240, 244 fermentation, 138 harmful, 238-40 having the same name, 23, 255-62 included in this manual, 34 injurious to plants, 238-40 inoculum potential, 5, 42, 204, 260, 300 nitrogen cycle, 245-46 nutrient requirements, 242-44 release nutrients in soil, 89, 237, 244-48 retardants to pathogens, 4, 20, 21, 24-25, 35, 250-54 spread, see Spread of microorganisms survival in soil, 21, 238, 243, 261 types 1 and 2 in nitrogen conversion, 245, 254 variability, 255-59 Minerals supplied by soil, 10, 89, 138 Mix, soil, see Soil mixes and U. C.-type soil mixes Mixed infections, 261 Mixes, fertilizer, see Fertilizer mixes and Fer- tilizers Mixing, U. C.-type soil, 83-84, 97, 98, 105. 133, 276-79; uneven, effect on steaming, 134 Mobile bin for soil steaming (type 2), 16, 132, 148, 164, 166, 168, 193, 273, 280-81, 303 [319] Moisture soil, 29, 87-88, 120, 125, 300; deficiency, ac- cumulative effect of, 87-88; relation to damping-off, 5, 49-50; relation to salin- ity measurement, 61; see also Soil, mois- ture supplied by soil, 87 Molybdenum required by plants, 89, 107 Mono-ammonium phospbate, 14, 78 Monocalcium pbosphate, 70, 106 Morning-glory, Rhizoctonia on, 257 (fig. 124) Mosaic, anemone, 21, 233; carnation, 234; cel- ery, 265; freesia, 21, 233; geranium, 234; po- tato, 221; ranunculus, 21, 233; rose, 7, 21, 51, 232, 236; stock, 235; tobacco, 140 Mosses, 211 Mother block (nucleus block) propagation, 21, 31, 222, 235-36; maintaining horticul- tural quality, 235 Movable Thomas method for ground beds (type 18), 180 Moving rake method of soil steaming (type 23), 131, 148, 165, 166, 178, 182-84 Multiplicity of nursery soil mixes, disadvan- tages, 89-90, 93, 264-65 Multipurpose tank (type 7), 16, 131, 132, 148, 164, 166, 171-73, 176; for hot-water treat- ment of stock, 171-73; for removing salts from containers, 171-73; for soil and con- tainer treatment, 164, 166, 171-73 Muriate of potash (potassium chloride), 14, 76, 77, 78, 106, 305 Mushroom growers, 90 Mustard, host of stock mosaic, 235; seed heat resistant, 139 Mycelium, 38, 39, 44-45, 300 Myrothecium as a retardant, 25; effect of ad- ding cellulose to soil on, 251, 253; inhibits Rhizoctonia, 25, 251, 253; may stunt plants, 251, 253 Xabarn (Dithane D-14), 19, 207, 209; sources, 304 Narcissus, stem and bulb nematode on, 258 Nasturtium, 215; presoaking seed before heat treatment, 224; spotted wilt, 235 Necrosis of plant, caused by soil toxicity, 9, 91-95; bv damping-off, 35-37, 44-45; by sa- linity, 7-9, 55-56, 58-60 Nemagon, 206, 305 Nematorides, 18, 21, 198, 206, 208; chloropk- rin, 18, 208; I)D mixture, 18, 206, 209, 304; ethylene dibromide (EDB), 18, 198, 199, 206, 209, 804; may increase losses from fungi, 261; methyl bromide, 18, 208-9; Ne- magon, 206, 305; sodium selenate (P- 10). 232, 305; Vapam, 18, 204-5, 208; V-C 13, 206, 805 Nematodes, 55, 200, 206, 238, 800 control, 15, L8, 107, I OH, 202-3, 203-5 daggei (Xiphim ma), 229 foliar (Aphelenchoides), 20, 47, 139, 220, 232-33 fungi which trap, 240 longevity in soil, 261 meadow (Pratylenchns), 47, 139, 227, 229 potato root nematode (Heterodera), 139 root knot (Meloidogyne), see Root-knot ne- matode root lesion (Pratylenchns), 47, 139, 227, 229 spiral (Heiichotylenchns), 229 stem and bulb (Ditylenchus), 47, 139, 227, 258 sting (Belonolaiynus), 261 survival in soil, 261 symptoms, 47 variability, 258-59 Nemesia, 215 Nephthytis, hardening for heat treatment, 223-24; Rhizoctonia on, 220-21; U. C. sys- tem for, 267; water-mold root rot, 220-21 Nerium (oleander), 220 New Improved Ceresan, treatment of planting stock, 20, 230 New York soil heater (type 14), 142, 165, 166, 176-77, 179, 195, 303 Nicotiana (tobacco) 82, 140, 214, 215, 248, 258, 261 Nitrate nitrogen, 13, 95, 105, 109, 111, 115, 116-19, 122, 243, 245, 253-54; leachability from soil, 109; when to use as fertilizer, 14, 105 Nitrifiers, see Nitrifying bacteria Nitrifying activity in soil effect of soil depth on, 114-15, 238, 240; of low organic nitrogen on, 116-18; of pH on, 13, 120-21, 245-46; of temperature on, 13, 115-19, 121, 245-46 elimination by steaming, 13, 95, 113-15, 115-19, 120, 245-46, 254 reinoculation of soil by bacteria, 13, 25, 113-15, 116, 253-54; without ammoni- fiers, 13 Nitrifying bacteria, 13, 119, 239-40, 245; ef- fect of pH on, 13, 120, 245-46; effect of temperature on, 13, 115, 119, 121, 245-46; inoculation in treated soil, 13, 25, 113-15, 116; sensitivity to soil treatment, 13, 95, 113-15, 115-19, 120, 245-46, 254 Nitrite, toxicity from, in soil, 121, 245 Nitrogen (Section 7), 108-22; also 105, 108, 110, 237 content in organisms, 242-44 conversion, 13, 245-46 cycle, 245-46; in soil, 109, 245-46; in plant, 109 deficiency, 1 8 essential to plants, 89, 106 fixed from air, 89, 244-45 fixing bacteria, 2 15 loss in organic matter decomposition, 65-67, 99, 100-1, 105-7 [ °>20 1 relation to damping-off, 42-43 starter solutions, 13, 14, 111-12, 254 tied up in soil by organic matter, 242-44; by sugar, 243, 244 See also Ammonium; Fertilizer ingredients; Fertilizers; Nitrate nitrogen; and Organic nitrogen Nitrobacter, 245 Nitrosomonas, 245 Nucleus block (mother block), 21, 31, 222, 235-36 Nursery industry in California (Section 2), 28-33 amount of soil used, 3, 29 climatic relations, 29-30, 233 decreasing returns, 32, 196 distance from market, 31-32 expanding local market, 31-32 future developments, 27, 193, 254 kinds of plants grown, 29 labor cost increasing, 32, 96, 196, 271 location in state, 28, 29 mechanization, see Mechanization in grow- ing moving to rural areas, 31-32, 274 number of units, 28 population pressure increasing, 31-32, 271, 274 production cost increasing, 31-32 real-estate development, 31, 271 rising land values, 31 size, 28-29, 33 smog injury, 31, 32, 274 tax rates increasing, 31, 271, 274 unit containers for marketing, 25, 31, 127 year-round growing, 3, 7, 30-31, 49 zoning restrictions, 31 Nursery Sanitation Code, 22-23 Nutrients for plants, availability affected by microorganisms, 89, 237, 244-48; deficiency, accumulative effect, 88-89; excess may kill plants, 53, 54, 56-57, 64-65, 88-89 Nutrition research, only on healthy plants, 51 Oak leaves, 100 Oak-root fungus (Armillaria root rot), 135, 198, 220, 260-61 Objectives of manual, 4, 29, 33; microorgan- isms included, 34 Obtaining pathogen-free planting stock, 219- 33 aging of seed, 21, 232 aseptic culturing of growing point, 20, 221 chemical treatment of stock, 20, 41-42, 230, 232, 236 continued roguing of stock, 21, 232, 300 cultured-cutting technique, 20, 221-22, 236 environmental control, 220-21 few healthy plants, 20, 219, 236 grow up away from soil, 20, 219-21, 236 heat treatment of stock, see Hot-water treatment of planting material indexing for viruses, 232-33 new seedlings, 233, 231, 236 sanitary practices, see Sanitary practices select growing areas, 21, 233, 236 specialist propagator, 20, 31, 219, 233 tip cuttings, 20, 219-21 use of true seed, 21, 233, 234 Ocimum (sweet basil), 205 Odors from compost piles, 12, 90, 271; from organic nitrogen, 122 Ohio soil heater (type 15), 165, 166, 177, 179, 195 Oil, fuel for boilers, 16, 136, 162, 163, 164-65, 174, 176, 185, 190, 192, 194 Oil-drum, method of soil steaming (type 12), 131, 164, 176 Oleander, stem and leaf gall, 220 Onion, stem and bulb nematode on, 258 Oospores, function in fungus carryover, 44- 45; occurrence, 44; useful in disease diag- nosis, 44 Organic matter, 11, 104, 109, 120 carbon/nitrogen ratios, 242-44 decomposed by microorganisms, 89, 95, 115— 19, 237, 240-44 effect on aeration, 99; on microorganisms, 25, 238, 240, 242-44; on soil salinity, 15, 65-67 high-nitrogen materials, 243-44 low-nitrogen materials, 242-43 particle size, relation to soil leaching, 15, 65-67 rate of decomposition, 241-42 relation to aeration, 98-99, 120, 143, 149, 240; to steam penetration, 127-28 role in soil toxicitv, 95 shrinkage, 12, 85, 89-90, 100-1, 241 steps in decomposition, 241, 246-47 Organic nitrogen, 105, 108-9, 245 application rate, 13, 71-75 applied as top dressing, 13-14, 118-19, 121, 122 conversion in soil, 13, 53, 109, 112, 113, 245 effect of aeration on, 121; of chemical treat- ment on, 119; of microorganism popula- tion on, 108, 113, 120, 122; of moisture on, 121; of quantity applied, on conver- sion of, 117, 120; of steaming on, 94, 108- 9, 113-15, 115-19; of temperature on con- version of, 13, 53, 105, 112, 115-19, 120 insoluble in water, 108 minimal nitrogen level supplied, 94, 121, 122 relation to ammonium injury, 13, 14, 95, 112, 113; to salinity, 70, 76' relative rates of conversion. 13. 14. 118, 121 soil organisms converting, 13. 105, 245 unavailable to plants, 13-14, 108 [321] Origin of plant, relation to distribution, 86, 91, 109 Orobanche, 261 Orthocide 406 (captan), 19, 43, 207, 209, 304 Osmotic concentration of soil solution, 95, 300 Overhead watering, 27; relation to disease spread, 20, 23, 38, 39, 45; relation to salin- ity burn, 9, 15, 53-54, 64 Oyulinia azaleae, 21, 30, 232, 233, 234 Owgen, 87, 121, 244, 298 Oystershell lime, 106 P-40 (sodium selenate), 232, 305 Package boilers, see Steam-generating equip- ment Pallet for stacking flats, 22, 25, 26, 166, 171, 174, 211, 212, 272-73, 276-77, 279, 280-81 Palms, 29, 284 Panics in nursery business, 7, 51 Pansy, 42, 43, 111, 207, 215, 264, 269 Papaver (poppy), 111, 215, 233 Paraformaldehyde, 19, 212-13 Parasite, 299, 300 Parasitic organisms in soil, attacking patho- gens, 24, 238-40, 300; competition escape by infecting plant, 21, 238 Parasitism, specialization, 5, 21, 46-47 Parathion, sources, 305; treatment of plant- ing stock, 20, 232 Particle size, see Soil, compaction, and Soil, drainage Parzate (zineb), 267, 305 Pathogen, 127; definition, 300; depth in soil, 6, 120, 259; rate of increase, 218; soil-in- habiting, 20, 218-19; sources, 123, 217; vari- ability, 255-59 Pathogen-free planting stock (Section 13), 217-36; also frontispiece, 1, 6, 11, 19-20, 22, 24, 31, 34, 48, 111, 123; benefits from use, 49-51, 217, 218; importance, 7, 29, 31, 43, 217; maintaining, 21, 31, 226, 234-36; methods of obtaining, 20, 21, 31, 41-42, 219- 33, 236, 300; obligation of nursery to pro- duce, 6, 36, 262; report diseased stock to propagator, 20, 233 PCNB ( 1 erraclor), 19, 43, 205, 207, 208-9, 305 Pea, 43, 215; Fusarium wilt, 257 Peach, root-knot nematode on, 259 Peanut, root-knot nematode on, 259 Peat, black (sedge), 69, 98, 265; source of sa- linity, 15, 53, 98, 101-5; of soil toxicity, 95 Peat, hypnum, 69, 104 Peat, sphagnum, 12, 81, 91, 94, 97, 98, 99, 101, 104, 115-19, 112, 144, 228, 242, 268, 275, 276 buffering capacity, L06 cost, 69, 85, 100 cllcd on teachability of soil, 65-67 formation, 244 heal conductivity, 2!)!) ingredient <>f l . C. type soil mixes, 72-75, 77, 78,81, 83-84, 85,93, 211 micronutrients, 107 mixing, 83-84 nitrogen content, 116, 244 pH, 69, 106, 120 seed cover for suppressing damping-off, 43 types, 69, 104 water retention, 69, 100 weight, 69 Pelargonium (geranium), 20, 21, 29, 32, 35, 215-16,221,234,261 Pelleting of seed, 25, 283 Pellionia, hardening for heat treatment, 223- 24; hot-water treatment, 229; Rhizoctonia on, 220-21; U. C. system for, 267; water- mold root rot, 220-21 Penicillin, 255-56; as a retardant, 25, 251-52; effect of pH on, 251 Peperomia, Rhizoctonia on, 220; U. C. system for, 267; water-mold root rot, 220 Pepper, 25-26, 29, 111, 214-15, 251-54, 256-57, 260, 264-65; heat treatment of seed, 226; Phytophthora root rot, 258; Rhizoctonia in seed, 25, 41-42, 43, 226, 260; root-knot nema- tode on, 259; spotted wilt, 235 Perforated pipe method of soil steaming (type 20), 16, 128, 131, 132, 138, 148, 150, 152, 156, 165, 166, 178, 180-83, 184, 194 Perlite, 12, 97, 100, 101, 227 Permeability of soil, 64—65; affected by clay content, 143 Peronospora, 46-47 Persea (avocado), 45, 258 Persica (peach), 259 Petunia, 25, 207, 215, 264, 283; ammonium injury, 13, 111, 112, 113; chlorosis, 113; formaldehyde injury, 213; Rhizoctonia on, 43; Sclerotinia on, 46 pH, 95, 299, 300 desirable ranges for plants, 106 effect of peat on, 69, 106, 120 effect on ammonifiers, 120, 246; on con- trolled colonization, 251; on nitrifying activities, 13, 120-21, 245-46; on Peni- cillium, 251; on soil microorganisms, 120- 21, 245-48; on Streptomyces, 251; on Tri- chodcrma, 251 of U. C.-type soil mixes, 69, 70 relation to nutrient availability, 247 Phaseolus, see Bean and Lima bean Philodendron, bacterial stem rot, 220, 229; heat treatment of plants, 229, 231; Rhi- zoctonia on, 220-21, 229; root-knot nema- tode on; 259; II. C. system for, 267; water- mold root rot, 220-21 Phlox, 111, 113, 261 Phomopsis, 233 Phosphorus and phosphate, 12-13, 70, 95, 106, 108, 110, 115; essential to plants, 89; fixation, 77, 78; loss in organic matter de- composition, 100; mono-ammonium phos- phate, 11, 78; monocalcium phosphate, 70, [322] 106; rendered available bv microorgan- isms, 237, 247 Photosynthesis. 247 Phvcomvcetes. 301 Phyllosticta, 233 Phytophthora, 5, 7, 37, 44-46, 49, 82. 220: capsici. 258; cinnamomi, 45-46, 218-19. 258; parasitica f. nicotianae, 258, 261; richardiae. 229; variability in pathogenicity, 258 Pice a (spruce), 258 Pike's Peak plastic covering. 303 Pine. Pythium root rot, 258; Phytophthora root rot. 46; shavings, 65-67, 244 "Pinto tag" in certification. 48. 129. 201 Pipe spacing in relation to steam flow rate. 130-31, 153, 180, 182 Pipe, steam-distributing. 195-96 Pisum specifications, 102-3 unsuitability for U. C.-type mixes, 68, 97 use, in J. I. composts, 91; in propagation, 126, 146, 184; in sand culture, 68 Sand, fine, for U. C.-type soil mixes, 12, 71-71, 77, 83-84, 93, 97, 101, 110, 264, 268 aeration, 98 availability, 12, 275 bacteria sparse in deep source, 13, 113-15, 240 buffering capacity, 106 content of coarse sand and of silt and clay, 12, 103 cost, 85, 100 determination of particle size, 103-4 gravel and sand quarries, source, 85, 97, 103-4 porosity, 143 retention of minerals, 99 sources, 85, 97, 103-4, 275 specifications, 12, 69, 101-4 unique ingredient of mix, 68 Sand culture, 68 "Sand snake," explained, 202 Sanitary practices, frontispiece, 1, 6, 11, 22- 23, 111, 123, 129,236,250 containers on ground, 6, 22, 23, 24, 41 covering containers, 6, 23, 41 degree of cleanliness required, 24 dipping cuttings in water, 5, 22, 38 discard flats with diseased seedlings, 22 glasshouse cleanup program, 213, 215 isolation house, 21, 23, 24, 234-35, 261 keep hose nozzle off ground, 5, 23, 24, 38-40 microorganisms of the same name, 23, 255- 62 overhead watering, 20, 23, 27, 38, 39, 45 pathogen-free stock, see Pathogen-free planting stock plant clean stock only in treated soil, 22, 48 removal of diseased parts, 21, 232 scattering dust in handling, 23, 41 segregate propagation activities, 21, 22-23; treated from untreated containers, 22; nursery operations, 21, 163, 166, 276 spattering soil in watering, 23, 24, 220 taking of cuttings, 22 treating, containers, 23, 24; floor before dumping soil, 18, 22, 134, 166, 199; soil in containers, 22, 126-27; tools, 19, 22, 23, 24, 40, 48, 201, 226 treating soil in containers, 22, 126-27 unnecessary handling of soil, 23 use treated containers for treated soil, 22. 210,278 walking over planted flats, 6, 23, 24, 40 washing hands, 23 Sanitation Code, 22-23 Sansevieria, chlorosis, 107 [ 325 ] Saprophytes, 238-40, 300 Saprophytic organisms in soil, 238-40 Saturated-soil extract method for salinity measurement, 61-63; comparison with dilu- tion methods, 61; compensation for elec- trode, 63; converting reading to ppm, 61; description, 62-63; importance in salinity measurement, 61; readings on U. C.-type mixes, 65, 70, 266 "Saucepan" soil steamer, 176 Sawdust, 12, 69, 80, 81, 97, 98, 99, 100, 242-43, 244; cost, 85, 100; decomposition of, nitro- gen required, 99, 242-43 Sderotia, 38, 39, 46, 300 Sclerotinia camelliae, 21, 232, 233, 234 Sclerotinia cottony rot, white blight, 5; con- ditions favoring, 46; life history of causal fungi, 46; number of spores produced, 218; symptoms, 46-47 Sclerotinia minor, 5, 46 Sclerotinia sclerotiorum 5, 46; lethal tempera- tures, 139 Sclerotium crown rot, 139, 226 Sclerotium rolfsii, 226; lethal temperatures, 139 Screening soils, 276-79 Screw-type soil treaters, see Rotating screw Sedge peat, see Peat, black Seed, infection of by Rhizoctonia, 41-42 Seed decay, 4, 35-36 Seed germination, relation to salinity, 9, 42, 55; soil mixes for, 74 Seed transmission, 6, 40-42, 217, 249, 255, 260; Alternaria, 229, 233; bacteria, 229; Fusa- rixim, 47; Phytophthora, 45; Rhizoctonia, 41-42, 260; Sclerotinia sclerotiorum and minor, 46; Septoria, 226; viruses, 7, 21, 232- 34 Seed treatment chemical, 230; eradicative, 230; protective, 20, 21, 42, 230, 232; protective, relation to soil-inoculum potential, 42 heat, see Hot-water treatment of planting material Seed vitality and damping-off, 5, 43 Seeding, mechanical, 25-26, 264, 283, 302; com- mercial use, 25, 264, 282; seed size in rela- tion to, 25, 283 Semesan, 19, 207, 209; injury to roses, 207, 209; injury to seedlings, 207; sources, 305 Senecio (cineraria), 1 1 1 Separator strips lor stacking Hats, 201, 279 Septoria late blight of celery, 21, 51, 226, 232; number of spores produced, 21 H Septoria leal spot of chrysanthemum, 233 Sewage sludge, 15 Shading ol plains, effecl on damping-off, 5, 42-43; on salinity injury, 15, 55 Shalil peach, 259 Shavings, wood, HO, HI, 97, 98, 99, 100, 242-43, 211, 268; (ost, loo; died on teachability, 65-67; require nitrogen in decomposition, 99, 242-43 Shepherd's purse seed, heat resistant, 139 Shredder for peat moss, 98, 276 Shrinkage of organic matter in composting, 12, 80, 85, 89-90, 97, 100-1, 241 Shrubs, 29 Silt, 97, 98-99, 104; determination in fine sand, 103-4 Sisalkraft cover for soil treatments, 178, 303 Skip-load tractor, 25, 84, 168, 272-73, 276, 278- 79, 280 Slugs, lethal temperatures, 139 Smog, injury, 31, 32, 274 Snapdragon, 25, 111, 207, 215, 263; ammonium injury, 13, 113, 253; anthracnose, 233; chlorosis, 113; damping-off, 47; downy mil- dew, 46, 47; methyl bromide injury, 16, 18; Phyllosticta leaf spot, 233; root-knot nema- tode on, 259; rust, 218, 234 Snyder's plastic covering, for soil treatment, 303 Sodium, 64, 106, 109, 300; in water, related to quality, 64, 106, 109 Sodium acetate, 112 Sodium hypochlorite, 216, 221-22, 226; sources, 305 Sodium nitrate, 105 Sodium oxalate solution, preparation, 103-4, 305 Sodium selenate (P-40), sources, 305; treat- ment of planting stock, 232 Sodium sulfate, 65-67 Soft rot (bacterial), 21, 35, 226-27, 233, 235 Soil absorption of water, gases, and salts, 95 analysis, 84, 99 clods, avoidance by U. C.-type mixes, 98, 133, 161, 276; relation to steaming, 15, 127, 133, 134, 140, 149, 160-61, 190 compaction, relation to fumigant move- ment, 89, 142; to heat transmission, 130, 134, 142; to particle size, 97, 99, 103, 104, 142; to steam movement, 130, 134, 142, H8-49, 152, 155, 156 conditioners, 15, 18, 65-67 drainage, relation to aeration, 60, 99; to particle size, 97; to root rot, 44-45; to salinity, 15, 53-54, 57, 64; restriction by container boundary, 61, 87 drenches, see Spot treatment functions for" plants, 10, 86-89 grinding of clods, 98, 133, 161 handling, separating various operations, 21, 163, 166, 276 heal capacity, compared with water, 133, 160 heal conductivity, 299 moisture, 29, 87-88, 120, 125, 300; effecl on damping-off, 5, 33, .'50, 49-50; effect on permeability, 143; effect on pore size, 1 13; [326] effect on soil steaming, 15, 130, 133, 131, 149, 155, 160, 193; high levels reduce salinity injury, 9, 11, 15, 33, 49, 53, 55- 57, 64; relation to salinity measurement, 61; resulting from steaming, 135, 150-51; see also Moisture, soil nonspecificity for crops, 10, 86 particle size, in relation to concretion, 99, 103, 104, 142; see also Soil, compaction, and Soil, drainage, above permeability, 64-65, 143 porosity, effect of clay on, 99, 143; effect of moisture on, 143, 149; in various soils, 98- 99, 143-44; porosity and pore size com- pared, 143-44; relation to carbon dioxide diffusion, 143; relation to convection movement of steam and fumigants, 143- 44; relation to heat transmission, 141-44, 149 quantity used by California nurseries, 3, 29 relation to disease spread, 38, 45, 123, 217 relation to plant distribution, 86, 91 source of mineral nutrients, 89, 107 source of salinitv, 15, 30, 53-54 specific heat, 301 structure, 125 supplies most of plant requirements, 138 temperature, effect on ammonifiers, 121, 245-46; on damping-off, 5, 6, 43; on dis- ease, 50, 260; on organic matter decom- position, 53, 115-19 testing equipment, 303 top, cost, 85 weight, 135 See also Aeration of soil; Chemical treat- ment of soil; Dry source of heat; Equip- ment for soil handling; Hot-water treat- ment of soil; Mechanization in growing; Microorganisms, soil; Soil mixes, below; Steam treatment of soil; and Toxicitv after treatment of soil Soil- and refuse-borne viruses, 127, 140 Soil mixes (sections 5 and 6), 68-107 conventional types, 91 Einheitserde, 94, 96 historv of development, 90-94, 110 ideal, 97, 109-10 John Innes composts, 91, 93 mix A, 12, 69,71, 77, 78, 85 mix B, 12, 69, 72, 77, 78, 79, 80, 81, 85, 111, 112, 115, 117, 135 mix C, 12, 69, 73, 77, 78, 79, 80, 81, 85, 149, 150, 153, 154, 156 mix D, 12, 69, 74, 77, 78. 80, 81, 85 mixE, 12,69,75, 77, 78,81,85 multiple vs. single mixes, 86. 89-90, 91 philosophies behind, 91, 93-94 U. C. type, 69-76, 93-94, 110; see also U. C. type soil mixes Solanum, see Eggplant and Potato Soluble salts, see Salinity Solubridge for salinitv measurement, 9, 60- 63, 299, 300, 303 Sore-shin damping-off (wire-stem;, 5, 35-36, 43 Sources, of equipment, 302-4; of fungicides and chemicals, 304-6 Soybean, Rhizoctonia on, 257 Space in nurseries, reduced b\ U. C.-type soil mixes, 10, 12, 89-90, 100 Specialist propagator, 20, 31, 219, 233 Specific heat, 301; of soil, 135; of water, 135; of water vs. soil 15, 133, 131, 160, 196, 301 Spergon (chloranil), 230 Sphagnum peat moss, see Peat, sphagnum Spike method for soil steaming (type 21), 135, 148, 150, 165, 166, 178, 181, 183 Spiral nematode (Helicliotylenchus), 229 Spore, 5, 46, 250, 298, 299, 301 Spot treatment, fungicidal, 19, 43, 48, 207, 209; application, 19, 207; importance of cor- rect dosage, 207, 249-50; salvage technique, 19, 48, 207 Spotted-wilt virus, 21, 221, 235; endemic cen- ters of infection, 235 Sprays, use, 4, 34 Spread of microorganisms, b\ : air-borne spores, 5, 38, 46 cloth flat-covers, 6, 23, 41 containers, 5, 39-40, 45, 48 hormone solutions, 5, 22, 38 hose nozzle, 5, 23, 24, 38-40 placing container on ground, 6, 22, 23, 24, 41 planting material, 6, 19-20, 40-42, 43, 45-46, 48 seed, 6, 40-42, 45, 47, 217, 229, 233, 249, 255, 260 soil particles, 5, 38, 39, 48 tools, 6, 40-41, 45, 48, 217, 249 water, 5, 11,38,45,48 workers' hands or feet, 6, 23, 24, 40 Spruce, Pythium root rot, 258 Stacking of flats, for seed germination, 264; for soil treatment, 131, 173, 189, 201-4, 211 Starch, 241 Started plants, 29 Starter solutions (nitrogen), 13, 14, 111-12, 254 Steady state in heat transmission, 145-46 Steam box, for bulk soil (fixed front) (tvpe 4a), 16, 132, 148, 156, 164, 166, 167, 169-70, 176. 273, 303; for bulk soil and containers (re- movable front) (tvpe 4b), 16, 17, 131, 132, 148, 156, 164, 166, 169-70, 303 B.t.u. requirements per cu. ft., 16, 159-60 calculation of volume required for soil treat- ment, 132-33, 159-60 definition, 141 delivered, calculation, 301 differentiated from heat. 142 distribution, see Distribution system for steam [327] Steam, continued escape from treated soil, 128, 130, 131, 149- 50, 153, 155, 159-60 flow rate, relation to pipe spacing, 130-31, 153, 180, 182 forms, for soil treatment, 15-16, 129-30, 135, 149, 157-59, 164-65, 166, 196, 211, 300, 301; equipment for generating, 191-92; for specific types of soil-treating equipment, 167-74, 178-84, 187; with steam-air mix- tures, 149 free-flowing, see Free-flowing steam or Steam, forms of, above heat content in various types, 156-57 movement, see Steam movement through soil, below pressure, see Pressure steam for soil treat- ment or Steam, forms of, above pressureless, from boiler, see Free flowing steam; in soil, 129, 149, 158, 191 quantity of soil heated per pound, 16, 132- 33, 159-60 saturated, 300 specific heat, 301 superheated, see Steam, forms of, above tunnel, 211 volume required for soil treatment, 16, 132- 33, 159-60 Steam-air mixtures in soil treatment, 127, 139, 148, 184 Steam/air ratio, and condensation, 146-48, 149-51, 152; relation to width of condensa- tion zone, 146-49, 150, 152 Steam-chemical (type 26), 148, 165, 166, 184- 85,213 Steam-generating equipment, 191-95 boiler horsepower rating, 159-60, 193-94, 298, 300 boiler output, 298 built-in, 174-76, 184 cost, 135-37 distribution system, 129, 132, 136, 191, 194, 195-96, 299 flash steamers, 192, 302 fuel, 16, 135-36, 162, 163, 161-65, 174, 176, 177, 184, 185, 187, 190, 191, 192, 194-95 high vs. low pressure types, 191-92 modified from hot-water boiler, 192 package boilers, 302 portable unit, 168, 191 possible group-ownership, 193 regular boilers vs. flash steamers, 159, 192 size boiler required, I, 16, F32-37, 193-94 sources, 302 stationary <">. portable units, 163, 192, 193, 191 Buperheal vs. free-flowing vs. pressure types, 1 58-59 thermal efficiency, 191, 299 types ol boilers, 159, 192, 302 watei softeners, 192 Steam movement through soil, 149-51 atmospheric pressure of, 129, 149, 158, 191 condensation zone, 128, 146, 149, 150, 152 condensing capacity of soil, 149-52, 154-55 distance and flow rate are compensating, 130-31, 149-53, 156 effect of clods, 15, 127, 133, 134, 140, 149, 160-61, 190 expanding spheroid around outlet, 128, 131, 151, 152, 153-55 heat transferred by expelled air, 148, 150 movement through pores, 89, 149 relation to expanding volume of spheroid, 128, 153-55; to organic-matter content, 149; to soil compaction, 134, 142, 149, 155; to soil moisture, 134, 149, 152; to soil pores, 134, 148-49, 152 relative movement horizontally and later- ally, 128, 153 steam-air mixture, 146-48 with high flow rate of steam, 152-53; inter- mediate flow rate of steam, 152-53; low flow rate of steam, 149-52 Steam treatment of containers, 19, 40, 111, 133, 134,211-12 Steam treatment of soil (sections 8 and 9), 123- 61; also frontispiece, 1, 11, 22, 48, 111, 123, 252, 254, 264, 280 advantages over chemicals, 1, 16, 18, 123- 25, 210; over dry heat, 125-26, 146 application to soil surface, 131-32, 170-76, 178-81, 184, 187, 188 balanced steaming, 130, 150, 155, 159-60; methods of achieving, 159-60 batch methods, 133, 162, 166, 168, 191 benefits, 126 best done in containers, 15, 19, 48, 126-27, 164-65, 278 best treatment method, 15, 125, 197 "blow-out" from soil surface, 130-31, 149, 152-53, 155, 161 breakdown of urea-formaldehyde, 14, 118 bulk soil, 273 ceiling of 21 2°F, 125, 140 compaction of soil in relation to, 130, 134, 142, 148-49, 152, 155, 156 compared with chemicals, 1, 16, 18, 123-25, 210; with dry source of heat, 125-26, 146; with hot water, 126, 1 16-18, 152, 156-57, 18 1 condensation process in soil, 128, 1 16, 119, 150, 152, 154-55 condensed water in steam line. 133, 152, 178, 196 conditions precluding steaming, 125, 133- 34, 1 18-19, 160-61 container size for chamber steaming, 131 continuous-batch equipment, 16, 166, 169, 171, 171, 176 cooling soil after treatment, 16, 134 cosl of, and iis calculation, is, 121, 135-37 | 328 | depth of inputs in soil, 130-31 development, 138 effect on microorganisms, 113-15, 115-19, 120, 248-50; on soil structure, 126; on watering practices, 7, 126 effectiveness, 16, 124; determination of, 121, 139 efficiency, different levels of, 135-36, 152, 159; factors in, 130-32, 149-53, 164-65, 194; measurement, 299 equipment for, see Equipment for soil heat- ing escaping steam, from chamhers, 130, 131; from soil surface, 128, 149-50, 153, 155, 159-60 final temperature attained, 127, 128, 140, 163 flow rate, 130, 132-33, 149-55 impractical uses, 125, 133-34, 148-49, 160-61 in bulk, 162, 169; in containers, 162, 169, 272; in home vards, 36-37, 135, 193 in "pinto tag" certification, 48, 129 increased growth response, 126 injection into soil, 130-31, 153-56, 167-70. 180-81, 185-87 lumps of soil, effect, 15, 127, 133, 134, 140, 149, 160-61, 190 measurement of effectiveness, 125 moving soil mass, 16, 127, 140, 148, 162, 185- 91 objectives, 141 plow, 182 quantity of steam required, 132-33, 153-56, 159, 298 recommended time and temperature, mov- ing soil, 127, 140; stationary soil, 128, 140 safe to use, 125 salinity from, 53, 264 settling of soil around steam pipes, 156 size of boiler required, 159-60, 194 soil leveled for treatment, 130, 155 soil moisture after treatment, 135, 150-51 soil moisture required, 15, 130, 133, 134, 149, 155, 160, 193 soil structure, effect on, 125; required, 15, 133, 160-61, 193 spacing of inputs in soil, 130-31, 134, 153, 155-56, 160, 180, 182; of steam pipes, 130- 31 stacking flats, 131, 189 stationarv soil mass, 16, 128, 132-33, 140, 162, 163, 166-77 steam-air-chemical mixtures, 125, 148 steam-air mixtures, 127, 139, 148 steam flow rate, most efficient, 130, 149-55 temperature, and time required, 15, 16, 113- 15, 124, 127-29, 139-40, 159, 163; in mov- ing mass, 127; in stationary mass, 127; measuring, 156 time, per cu. yd., 159, 194 toxicity, see Toxicitv after treatment of soil "trickle finish," 128,130, 132, 155, 173 types of fuel, see Fuel for steam boilers uneven heating, factors in, 133-34, 160-61 used in closed areas, 125, 210 used near living plants, 124, 125, 135, 210 . watering practices in treated soil, 125 weed control, 15, 126, 139-40 Steaming soil, equipment for, see Equipment for soil heating Steel, specific heat, 301 Steer manure, see Manure Stellaria (chickweed), 235 Stem and bulb nematode [Ditylenchus), 47, 227; lethal temperatures, 139; variability in pathogenicitv, 258 Stem and leaf gall, oleander, 220 Stem rot, 4, 35, 36; see also Damping-off Stemphylium, 233 Stericover for soil treatments, 178, 303 Sterilite, for soil treatments, 303 Steriltex cover for soil treatments, 178, 303 Sting nematode (Belonolaimus), 261 Stock (Matthiola), 111, 215, 219, 263; am- monium injury, 112, 113; bacterial blight, 218, 229, 261; Botrytis blight, 8, 9, 55; hot- water treatment of seed, 229; mosaic, 235; Rhizoctonia foot rot, 42, 43, 207; salinitv injury, 7, 8, 9, 55, 58; Sclerotinia white blight, cottony rot, 46; Verticillium wilt, 256 Stoddard solvent, sources, 305; toxicity to plants, 215; used with copper naphthenate, 19,213 Stone-fruit viruses, 232 Storage of soil mixes, 13, 71-75, 275 Strains of a pathogen, determination, 6, 256- 59; differences between, 6, 43; effects on disease, 6, 256-59; relation to pathogen-free stock, 43 Strawberry, 220 Strelitzia, 229 S:reptom\ces as a retardant, 25, 251; effect of pHon,251 Streptomycin, 255-56 Stunting of plant, from salinity, 8, 42, 55; from soil toxicitv, 9, 79, 95, 111 Subirrigation, relation to salinitv, 63, 119, 183 Substrate, 301 Succulents, 20, 29, 37 Sugar, 241 Sid fate in water, 64 Sulfate of potash, see Potassium sulfate Sulfur, bacteria, 246-47; evele, 246-47; essen- tial to plants, 89, 106; rendered available by microorganisms, 237, 246-47; soil acidifi- cation, 246-47 Sulfuric acid, 247 Summary, general (Section 1), 3-27 Superheated steam, 16, 135. 211. 301; advan- tages, 130, 158-59, 166; equipment for gen- erating, 192; heat content, 130. 157-59, 166; thermal efficiency. 130, 157-59; see also Steam, forms of soil treatment [ 329 ] Supernatural cause of disease, 3, 34 Superphosphate, single and double, 14, 70-75, 76, 77, 91, 94, 106;\reble, see double, above; use finely ground product, 70 Support, as a soil function, 10, 87 Suppression of damping-off is not control, 6, 23, 36-37, 43-44, 45 Surface application of organic fertilizer, ad- vantages, 13-14, 119, 121, 122; disadvantages, 122 Surface methods of soil steaming, 131-32, 170— 76, 178-81, 184, 187, 188 Survival of microorganisms in soil, 21, 238, 243, 261 Susceptibility of host, to ammonium injury, 13, 96, 111, 112, 115; to damping-off, 5, 42; to disease, 5, 300; to nematodes, 258-59; to root rot, 256-58; to salinity, 7, 55, 61 Sweet alyssum (Lobularia), 13, 111, 112, 113, 264 Sweet basil, 205 Sweet pea, seed decay, 230 Sweet potato, 221 Syngonium, black cane rot, 229; hot-water treatment of plants, 227, 229-30; effect on dormancy, 229-30 Systox (Demeton), 232, 304 Tagetes, starter-solution tests, 111; Rhizoc- tonia on, 256 Tanks, hot- water- treating, 302 Tax rates increasing, effect on nurseries, 31, 271,274 Tear gas, see Chloropicrin Temperature, 29; lethal to microorganisms and weeds, 127, 139-40; recommended for soil steaming, 15, 16, 113-15, 124, 127-29, 139-40, 159, 163; relation to plant distribu- tion, 86; relation to variability of micro- organisms, 259; see also Tempil pellets and Thermometers Tempil pellets for temperature measurement, 129, 156, 303 Terraclor (PCNB), 19, 43, 205, 207, 208-9; ef- fectiveness, 205; sources, 305; specificity, 205 I ersan (thiram), 19, 43, 207, 209, 305 Thermometers, 128-29, 156, 302; expanded- scale type, 225, 302; precision type for check- ing those used, 224-25, 302; see also Tempil pellets Thielaviopsis basicola, variability of patho- genicity, 258 Thiram (Arasan, Tersan), 19, 43, 207, 209; sources, 305 I hornas method lor soil steaming, 163; effec- tive depth, 131, 178; equipment, 178-79; for beds or benches (surface) (type 18), Hi, 131, 132, 11H, 163, 165, 166, 171, 178-80; movable variant, 118, Kit;, 180; lor contain- ers (type ")), Hi, 131, 132, 118, 164, 166, 170- 71, 179,272 I hrips, 'J Tip cuttings to eliminate pathogens, 20, 219- 21 Tipburn, salinity, 8-9, 55, 58, 60 Tobacco, 82, 214, 215, 248; mosaic, 140; Phy- tophthora root rot, 258, 261; Thielaviopsis on, 258 Tomato, 25-26, 29, 111, 260, 264-65; broom- rape on, 261; crown gall, 258; Fusarium wilt, 257; Rhizoctonia on, 41, 42, 256; root-knot nematode on, 259; spotted wilt, 235; Verti- cillium wilt, 256 Tools, relation to spread of pathogens, 6, 40- 41, 45, 48, 217, 249; treatment, 19, 22, 23, 24,40,48, 201,226 Top rot of seedlings, 4, 35, 37, 38 Toxicity after treatment of soil, frontispiece, 1,9-11,90,93,98, 124, 140 caused by ammonium, 9, 13-14, 79, 95, 96, 98, 111-13, 115, 121, 122, 199, 253-54; by manganese, 9, 95, 98; by other agents, 95; by salinity, 9, 95, 98; by soluble organic matter, 9-10, 95, 98 effects on plants, 9, 94-95 factors affecting type and severity, 94 persistence, 95 reduction by aging soil, 10, 93, 95, 96; by immediate planting, 10, 95, 96; by leach- ing soil, 10, 95, 96; by steaming ingredi- ents separately, 93; by using U. C.-type mix, 9-10, 11, 12, 15, 90, 93, 96, 97-98, 124, 129, 140, 199, 270 residue of chemicals, see Chemical residue Transit mixers, see Concrete mixers Transpiration, 54—55, 56-57, 87 Transplanting, 25, 81, 264, 282-83 Transplants, soil mixes for, 71, 72, 74 Treated soil, see Chemical residue; Recon- tamination problem; and Toxicity after treatment of soil Treatment, see Chemical treatment of soil; Containers, treatment of; Dry source of heat for soil treatment; Hot-water treatment of soil; Obtaining pathogen-free planting stock; Steam treatment of soil; and Tools, treatment Trees, 29 Trichoderma as a retardant, 25, 240, 251-52; effect of pH on, 251 "Trickle finish" in soil steaming, 128, 130, 132, 155, 173 Trifolium (clover), 259 Triton B-1956, spreader, 232, 305 Tropaeolum (nasturtium), 215, 224, 235 "Trough" soil steamer, 176 Tub containers, 131 Tubular soil heater with electric elements, 177 l ufedge (over for soil treatments, 178, 303 lung, Rhizoctonia on, 257 linl, for soil mixes, 91, 93, 110; source of composl variability, 10. 89; unavailability, 12,90, 93 | 330 | I'.C.L.A. blend, 93; see also U. C.-type s:»ii mixes U. C. system (Section 1), 3-27; also frontis- piece,' 27, 123; advantages, 1, 3, 30-33, 49, 51, 270; aids in adopting, 1-2; explanation, 3, 4; grower experience, 263-70; mechani- zation, 271 U. C.-type soil mixes (sections 5, 6), 68-107; also frontispiece, 1, 10-13, 93-94, 123, 197 adoption, 1-2, 27, 79, 263-64 advantages, 1, 10-12, 89-90, 93-94, 96, 265, 270 aeration, 87, 270 application to bench and bed crops, 12, 69, 80-81, 268; to can plants, 69, 72, 80, 267; to cvmbidiums, 268-69; to flatted plants, 12, 69, 72, 75, 79, 264-65; to foliage plants, 20, 29, 32, 266-67; to home-yard planting, 81; to planter boxes and dish gardens, 81, 84; to pot plants, 12, 69, 73, 77, 79-80, 266-67; to research, 81-83; to vegetable plants, 265-66 base exchange, 99 centralized soil service, 193 components, see Ingredients for U. C.-type mixes composting eliminated, 89-90, 93, 270 conductance, 65, 70, 266 cost, 12, 69, 80, 84-85, 100 cultural practices modified by use, 27 development, 93-94, 110 dump soil, re-use, 84 enable scheduled production, 94 evaluation, 263-70 facilitate mechanization, 25, 90, 94, 270, 271 fertilizers included in mixes, 69-76; see also Fertilizer ingredients fertilizing, see Fertilizers, dry, and Fertili- zers, liquid formulas, 76-79 leachabilitv, 65-67 micronutrients, 12, 89, 101, 106-7, 109, 110 mixes A to E, 69-76; see also Soil mixes mixing, 83-84, 97, 98, 105, 133, 276-79 moisture retention, 12, 69, 87, 97, 100, 270 nontoxicity after treatment, 9-10, 11, 12, 15, 90, 93, 96, 97-98, 124, 129, 140, 199, 270 permeability, 64-65 pH, 69, 70 plant growth, 27, 265-66, 270 preparation, 83-84, 97, 98, 105, 133, 276-79 reduce labor, 10, 84, 89-90, 100, 270; space requirement, 10, 12, 89-90, 100; odor and fly problems, 12, 90,271 reliability, 68, 270 reproducible, 10, 89-90, 93 retention of nutrients, 12, 97, 99 root distribution in, 82-83 salinity problem reduced, 10, 15, 53, 64, 65- 67,90,211,265-66,270 shrinkage in storage, 12, 80 transplanting to clay soil, 81 types planted within a week, 71-75, 275, 278 types stored before use, 13, 71-75, 275 uniform initial fertility, 12, 98-100 uniform materials, 12, 97-98 uniform results, 12, 68, 89-90, 270 uses, 12, 69, 79-83, 264-70 variants, 69-76, 93 water content, 12, 69, 87, 97, 100, 270 watering, 27, 81; adjustment when first us- ing, 79, 84 weight, per cu. ft., 12, 69, 80 Ultron cover for soil treatment, 302 Uneven soil heating with steam, causes, 134 Unit containers for marketing, 25, 31, 127 Unsteady state in heat transmisison, 145-46 Urea, 13, 14, 78, 79, 105, 106, 108, 109, 118, 245 Urea-formaldehyde resins, 13, 14, 79; biuret content, 13, 14, 78, 79, 119; effect of steam- ing, 14, 118; use as fertilizers, 105, 115-19, 121 Vacuum-plate seed planters, 25-26, 264, 283, 302 Vapam, 18, 204-5, 208; application, 18, 204-5; effectiveness, 204; sources, 305 Variability of pathogens, 255-59 Vault for soil steaming (type 6), 16, 25, 131, 132, 148, 164, 166, 167, 171-72, 174, 272, 277, 280-81 Y-C 13, 206, 305 Vegetable plant production, 29, 129; U. C. system for, 111, 264-66 Velon Fumicover for soil treatments, 178, 180, 303 Venturi tube, in steam-air mixing, 148 Verbena, 86, 11 1, 1 13, 264 Vermiculite, 12, 94, 97, 100, 101 Vertical cabinet for soil steaming (type 8), 131. 132, 148, 164, 166, 173-75, 176; self-generat- ing type, electric (type 11), 131, 164, 174, 175-76, 195 Verticillium albo-atrum, 198, 220, 222, 239-40, 250; survival in soil, 261; variability of para- sitism, 256 Verticillium wilts, 7, 18, 31, 49, 201, 203, 219, 235; cultured-cutting technique against. 221-22; ineffectiveness of methyl bromide against, 16, 18, 22, 124, 203 Vigna (cowpea), 95 Yinca, 111 Viola, see Pansy and Violet Violet, 263 Virulence of pathogens, 5, 6, 260, 300; varia- bility in. 256-59 Viruses, 299; elimination by culturing grow- ing point, 221; elimination by use of true seed, 7, 21, 232-34; inactivation by soil heat- ing, 140; indexing, 232-33; survival in refuse, 140; survival in soil, 140 Viscosity, of gases, 143; of steam, 143 [331] Visqueen cover for soil treatments, 178, 201, 302, 303 Volatilization of chemicals for soil treatment, 148, 165, 166, 184-85,213 Volume of steam, 16, 132-33, 149-52, 159-60 Water application by porous hose or drip system, 81 calcium and magnesium content, 64 changes form at different temperatures, 156 conductance, 9, 14, 63 deficit, cumulative effect on plant, 87-88 deionized, 15, 63-64 heat conductivity, 299 leaching with, see Leaching to reduce sa- linity loss by transpiration, 54-55, 56-57, 87 necessary to use excess, 9, 14, 63 organisms spread in water supply, 5, 11, 38, 45,48 quality, in relation to salinity, 9, 11, 14, 30, 63-64; characteristics, 63 retention by U. C.-type soil mixes, 12, 69, 87, 97, 100, 270 salt deposit on leaves from, 9, 63-64 sodium content, 64, 106, 109 solvent for minerals, 87 source, of micronutrients, 107; of salinity, 9, 11, 30,53-54, 192 specific heat, 301 supplied by soil, 10 use in plant metabolism, 87 Water breakers on hoses, 84 Water-culture growing of plants, 87 Water-mold root rots, caused by Pythium and Phytophthora, 7, 35-36, 43, 44-46, 49- 50, 82, 135, 139, 220, 226-27, 229, 233, 235, 250, 261; favored by very wet soil, 36, 43, 44, 49-50; symptoms, 35-36, 44-45; see also Damping-off Water molds, 5, 22, 38, 49-50, 220-21, 238, 301; lethal temperatures, 139; life history, 44-45; retard root development, 45; sur- vival in soil, 261 Water requirements of plants, 51, 87 Water softeners, for boilers, 192; unsuitable lor plant use, 64 Water spotting, 9, 63-64 Water trap in steam lines, 133, 196 Watering of plants, in steamed soil, 126; in U. C.-type mixes, 27, 79, 81, 84; mechan- ized, 27, 283-84; relation to disease, 43, 49 Watermelon, root-knot nematode on, 259 Weed hosts of viruses, 235, 275 Weed killers, 18 Weeding in containers, cost of, 126, 264; elimination by soil treatment, 126, 264-65, 269-70 Weeds, 261, 264, 275; control by soil treat- ment, 15, 18, 124, 126, 138, 197, 198, 200, 202-3, 204, 208, 248; lethal temperatures of seed, 127, 139-40 Weigelia, hot-water treatment of plants, 229; root-knot nematode on, 229 Weights, table of, 301 Wheat straw, 100, 254 Wheatstone bridges, see Solubridge for sa- linity measurement White blight, see Sclerotinia cottony rot Wild-oat seed, heat resistant, 139 Wild radish, stock mosaic, 235 Wilting, plant, 87; root rot, 36; salinity in- jury, 9, 55 Wire-stem damping-off, 5, 35-36, 43 Wolman salts for wood preservation, 215, 216 Wood, specific heat, 301 W^ood shavings, see Shavings, wood Woody plants, can-grown, U. C. system for, 69, 72, 80, 267 Worms, lethal temperature, 139 Xiphinema (dagger nematode), 229 Year-round growing, 3, 7, 30-31, 49; under glass, 7 Zantedeschia, see Calla Zea (corn), 240-41 Zinc, 95, 237; essential to plants, 89, 106; role of microorganisms in deficiency of, 248 Zineb (Parzate), 267; sources, 305 Zinnia, 111; Alternaria disease, 218, 229, 233; hot-water treatment of seed, 229; Rhizoc- tonia in seed, 42, 229 Zoning restrictions against nurseries, 31 Zoosporangia, 44-45 Zoospores, 44-45 Cooperative Extension work in Agriculture anil Home Economics College .,( v n. uJiure, University of California, and United Statei Department of Agriculture co operating. Diatribuled In IuiiIhi.hu.- of tin- ,\< i <>i < . ..( May h, un.l Jon.- 30, 1914 George 1!. Alcorn, Director, California Agrii ultural Extei Service. lm •<::>!< B8163 ;.\lk E ON THE LAST DATE "D BELOW No matte lat kill , Ac THER BORRQ , tave . . . ... if you employ usual nursery pr C T1ER ONE Wfs, this manual shows you how to cut your costs and losses, and incre 'O The nursery program it offers can help you to cut— maybe even eliminate— losses from diseases and weeds, and at the same time cut the cost of fighting them! The program is the result of 16 years' research by the Department of Plant Pathology, University of California, Los Angeles. The methods have been thoroughly tested in commercial nurseries, not just in the laboratory. The U. C.-type soil mixes are basic to the program. Besides, they offer you worth-while savings in labor costs and storage space, not to mention surer and probably faster results. Mechanizing your operations can further cut labor costs. With an effective disease-prevention program and a U. C.- type soil mix, mechanizing has become a fact. The manual describes mechanization as it has developed in a number of California nurseries. IK Your nursery is unique. **" Your problems are somewhat different from anyone else's. |VA nursery business in California is a varied one, and no rules of thumb will applf'to all the hundreds of crops grown here. Neither will detailed directions cover all future developments in the industry. Therefore the manual presents a general program, explains the facts behind it. It gives many down-to-earth examples. It describes and illustrates the various kinds of equipment you might use. With this background and the knowledge of your prob- lems, the program can be adapted to your needs, now and in the future. For your convenience, nn references where you can find still further information on specific points, a glossary, some methods of computation, a table of weights and measures, and sources where you can get equipment and materials are given in an appendix. An index is provided for rapidly locating information. ,,*'■ ' - To obtain additional copies of this manual or a catalog listing other manuals arid free publica- tions, see your University of California Farm Advisor (offices located in most California counties), or write to: Agricultural Publications 22 Giannini Hall University of California Berkeley 4, California When ordering manuals, send orders and payment to the address above; make checks or money orders payable to The Regents of the University of California.