UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA CITRUS CULTURE IN CENTRAL CALIFORNIA GORDON J. SURR and L. D. BATCHELOR BULLETIN 405 August, 1926 UNIVERSITY OF CALIFORNIA PRINTING OFFICE BERKELEY, CALIFORNIA 1926 Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://www.archive.org/details/citruscultureinc405surr CITRUS CULTURE IN CENTRAL CALIFORNIA* GORDON J. SURRt and L. D. BATCHELORJ INTRODUCTION Upon request of the citrus growers of Tulare County, California, cooperative field trials were conducted by the Citrus Experiment Station in Tulare County during the five years from 1919 to 1924. The citrus growers generally, through various fruit-shipping associa- tions, and the individual grove owners on whose property the trials were located, financially supported the investigation, and the senior author was continually in charge of the field work. 1 The following pages summarize the results of these experiments and of numerous observations in other citrus groves. Based upon these results a system of soil management is suggested which may possibly be applied beyond the confines of the area where the data were collected. Many of the recommendations and conclusions which follow are in harmony with and have been set forth as a result of other similar investigations elsewhere. In writing this account the published papers by Vaile and Surr, reporting on these trials, have been utilized. It was realized at the outset that the problems to be studied, such as low production and lack of vitality of citrus trees, were more in evidence on adobe than on other soils, hence the field trials were located on this soil type. Many observations were also made in citrus groves on other soils and numerous soil moisture determinations were made in connection with irrigation studies. Commencing with the spring of 1923, nitrate determinations were made of soil samples taken from the experimental plots, and this work was continued until the fall of 1924. HISTORY OF GROVES USED FOR THE EXPERIMENTS One of the experimental groves was located on the Ball-Emery property, about five miles southeast of Porterville. The trees, which are Washington Navels on sour stock, were planted in 1912, and * Paper No. 150, University of California, Graduate School of Tropical Agriculture and Citrus Experiment Station, Eiverside, California, t Field Assistant in Citrus Experiment Station. X Professor of Orchard Management, Horticulturist in Agricultural Experi- ment Station. 1 The authors wish to acknowledge the painstaking care with which Prof. R. S. Vaile planned the series of experiments herein reported, and the service he rendered in supervising them during the first three years of the cooperative arrangement. 4 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION although apparently healthy, they were small for their age, and had not usually borne satisfactory crops. It is believed that, in the past, inadequate water supply was one of the factors which had limited the yields in this grove. This area is now in an irrigation district and water will probably be more abundant in the future. In the block selected for the field trials seven plots of from seventy-two to seventy-five trees each were laid out. A second experimental grove was located on the property of the Kaweah Lemon Company at Lemon Cove. Here, the trees, which are Washington Navels on sweet stock, were planted in 1904. The irrigation water supply from the Kaweah River was generally abun- dant. Seven plots, of from forty-one to fifty-five trees each, were laid out in this grove. The trees are larger and older than those on the Porterville plots and many were in an unproductive condition when the experiments were started. The soil of the former grove is classified as Porterville Adobe, while that of the latter is classified as Olympic Adobe. The wilting point of the soil in the Lemon Cove grove is from sixteen to nearly twenty-two per cent. The soil shrinks, upon air-drying, to about half of its volume when wet. The following cultural treatments were included in both trials: CULTURAL TREATMENTS Plan of Treatments. — All the plots were fertilized in a similar way, with minor exceptions noted later. The seven differential cul- tural treatments were as follows: 1. Cover-cropped in summer, fallow in winter. 2. Straw mulch in summer, fallow in winter. 3. Cultivated in summer, straw plowed under in fall, fallow in winter. 4. Plowed several times during summer, with little or no other tillage, fallow in winter. 5. Intercropped with alfalfa in every other inter-space. Clean cultivation in alternate inter-spaces. 6. Clean cultivation in summer, legume cover-crop in winter. 7. Rotation of the above treatments except No. 5. Fertilization, — While adobe soils are commonly relatively rich, citrus trees on such soils generally respond to proper fertilization. It was decided, from past experience, to fertilize all the plots from the beginning. Essentially similar materials were used on both tracts but, as the trees were older, greater amounts were applied on the Lemon Cove than on the Porterville groye. BULL. 405] CITRUS CULTURE IN CENTRAL CALIFORNIA 5 Barnyard manure was applied each fall and some commercial nitrogenous fertilizer each spring. The applications of manure were at the rate of ten tons per acre each year on the Lemon Cove orchard, and about seven tons per acre on the Porterville orchard. The first year some of the manure was applied in furrows ; later it was always applied broadcast. At Lemon Cove the spring applications of commercial fertilizer, since 1920, varied from slightly over one pound to over two pounds of actual nitrogen per tree, while the spring applications at Porterville were from half a pound to a little over one pound of actual nitrogen per tree. The materials used were nitrate of lime, sulphate of ammonia, tankage reinforced with nitrate of ammonia, nitrate of soda, and in one year a so-called "complete fertilizer." Irrigation. — The irrigation practice on each orchard has been as nearly uniform as possible. In general, from two and a half to four acre inches of water per acre have been applied monthly during the summer season, the amount having been increased somewhat where cover-crops were growing. The water was allowed to run from twenty-four to a maximum of seventy-two hours during each irri- gation. Cultivation. — Except in the cases of the winter-cover-cropped plots the manure was plowed under in the late fall or early winter. The ground was left rough after plowing. Where winter cover-crops were grown, the manure was disked in before sowing the cover-crop seed, and the cover-crop was plowed under the following spring. The plots which were clean-cultivated received, as a rule, one cultivation after each irrigation. The cultivation given was generally shallow. It was soon found that, in these particular orchards, the cracking of the adobe soils, on drying out somewhat, allowed an excellent penetration of water. Consequently the ground was allowed to crack moderately between irrigations, and before each cultivation. Fre- quently the furrows for irrigation were made several days before irrigating in order to promote the cracking of the bottoms of the furrows. Changes in Treatment. — It is obvious that if radical changes are made after starting an experiment, it will be difficult or impossible to interpret the results. However, the plots with alfalfa growing in alternate middles soon began to show that this treatment was harmful to the trees. As the plots were kept well irrigated it was apparently not a case of competition for water between the trees and the alfalfa. The leaves of the trees became yellow and sparse, particularly on the Lemon Cove plot, and by June, 1921, the trees on this plot seemed b UNIVERSITY OF CALIFORNIA EXPERIMENT STATION to be in a critical condition. Consequently the alfalfa was plowed under on both plots in July, 1921, and, on the Lemon Cove plot, two pounds per tree of sulphate of ammonia was applied. Commencing with the spring of 1922 the original alfalfa plot at Lemon Cove was given heavier applications of commercial fertilizer than the other plots. YIELDS OF THE LEMON COVE PLOTS Table 1 gives the average yield per tree in field boxes of approxi- mately forty-two pounds each, on the Lemon Cove plots for the years 1919 to 1924, inclusive. The yields of every tree were recorded each year, but all diseased trees, replants or trees severely cut back were excluded. The table also shows the number of trees counted on each plot, and the average annual yield per tree for the five years of the experiment. TABLE 1 Average Annual Yields per Tree in Field Boxes on Lemon Cove Tract Yield in field boxes per tree Average Num- annual ber of Treatment During yield Before trial per tree during 1919 1920 1921 1922 1923 1924 trial 43 Cover-cropped in summer, fallow in winter 1.67 2.90 3.80 2.34 5.69 0.58 3.06 50 Straw mulch in summer, * 1.36 1.64 2.40 1.65 7.01 1.37 2.81 45 Cultivated in summer, straw plowed under in fall, fallow 1.28 1.54 2.52 1.26 5.77 0.97 2.41 55 Plowed several times during summer, little or no other cultivation, fallow in winter.. 1.38 1.51 2.14 1.73 6.43 1.39 2.64 49 Intercropped with alfalfa in every other middle, clean cultivation in alternate 1.34 2.55 3.13 1.93 6.78 1.30 3.14 41 Clean cultivation in summer, legume cover-crop in winter.. 1.62 2.14 3.54 3.70 7.26 1.40 3.61 45 Rotation plot receiving one of the above treatments each year, except growing alfalfa . 1.32 2 03 2.47 1.31 7.30 1.10 2.84 In the first place it will be noted that the crop harvested in 1919, which was picked in November, before the field trials were started, differed little on the various plots. The minimum average yield was 1.28 boxes per tree and the maximum 1.67 boxes, the greatest differ- ence having been therefore only a little over a third of a field box BULL. 405] CITRUS CULTURE IN CENTRAL CALIFORNIA 7 per tree. Expressed in per cent, using the highest yield as 100, the lowest-yielding plot was only twenty-three per cent below the maximum. Weather is apt to greatly influence any crop grown in the open air, and the Navel orange crop of 1922 in the Lemon Cove region was considerably smaller than the 1921 crop. The 1923 season was highly favorable to citrus production in Tulare County, probably largely owing to the direct and indirect influence upon "June drop" of the relatively cool spring and early summer. Whatever the causes, the fact remains that most citrus trees in Tulare County, unless obviously in poor condition, produced a good crop that year. The 1923 yields were good on all the Lemon Cove plots. There was an abrupt falling off in the 1924 yields on all the plots, apparently largely due to a heavy infestation of gray Citrus scale (Coccus citricola). Until ths infestation the trees had been kept commercially clean by spraying, but they were not sprayed in 1923. Moreover, the winter of 1923-24 was cold and dry. Irrigation water was not available for a long time in the winter and early spring. Throughout the Lemon Cove district as a whole the Navel orange crop of 1924 was considerably smaller than the 1923 crop. COMMENTS ON THE YIELDS THE TREATMENTS AND THE CONDITION OF THE TREES OF THE LEMON COVE TRACT It is plain that a moderate application of fertilizer has had a marked effect in increasing the yields of all the plots regardless of the cultural treatment. In average yield for the five years there is no striking difference between any of the plots. Considering the winter cover-crop plot, which produced the most fruit, as 100, the lowest-yielding plot was only 33 per cent less. Such a difference may be significant in this case, but it must also be realized that such differences are frequently well within the realm of chance variations in plot trials. The con- dition of the trees at the end of the experiment should also be considered as well as the yields as an indication of the effect of the several cultural treatments. The summer-cover-crop plot yielded more than the other plots the first year but was only slightly ahead the second year. In the fourth and fifth years the yields fell off decidedly in comparison with those of the other plots. In the fifth year, 1924, the yield was much below that of any other plot. 8 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The summer cover-crop was buckwheat and Melilotus alba sown together in 1920 and 1921, M . alba in 1922, and purple vetch in 1923 and 1924. Where summer cover-crops are regularly grown, weeds are apt to gain a foothold, and, for the three seasons, from 1922 to 1924, the summer cover-crops have been choked by a rank growth of "water-grass." At the close of 1924 most of the trees were in a very bad condition on this plot. Competition for water between the trees and the summer cover-crops apparently was not a factor but competition for available nitrogen probably was, for at times the nitrate nitrogen was very low on this plot. The plot with straw as a mulch in summer, and the plot ivith straw plowed under in the fall have, in general, been relatively low in yield. Among the objections to the use of a mulch of grain straw are the danger from fire and the growth of objectionable weeds, such as Bermuda grass, which seems particularly apt to get a good start while the ground is mulched. Further, after an irrigation or two, there is likely to be a rank growth of volunteer grain, making an undesirable summer cover-crop. The use of grain straw at Lemon Cove has not been worth while, whether directly plowed under or as a summer mulch. On these plots the trees have shown fairly good condition, and in certain other citrus orchards in Tulare County on adobe soils there has seemingly been no injury from the use of straw. Attention is called to this point as cases are well known elsewhere on other soil types, of definite injury to citrus trees, caused by applications of grain straw. A straw mulch in winter may be highly injurious to citrus trees even on adobe soil. For example, straw was plowed under on the Lemon Cove plot late in October, 1920. Some of the same straw was spread on an adjoining block not in the experiment with the intention of plowing it under there also, but rain came on and the straw remained all winter as a mulch. The leaves of the trees on this area turned yellow and three years later the ill effects were still apparent. The plot which was worked almost exclusively with a plow showed an improvement in both tree and soil condition sooner than any other plot. The yields were considerably below those of the winter-cover-crop plot for the first three years. In the third year of the experiment (1922) this plowed plot yielded less than half as much as the winter- cover-crop plot. In the last two years of the experiment, however the yields of the plowed plot closely approached those of the winter- cover-crop plot. BULL. 405] CITRUS CULTURE IN CENTRAL CALIFORNIA 9 The soil rapidly responded to the plowing, becoming a friable loam rather than an intractable adobe, and during the five years it was always in better physical condition than the soil of the other plots. As early as the fall of 1920, only a few months after the first plowing, the trees on this plot showed a definite response, with healthy, dark green foliage and a general appearance of vigor. For nearly two years the trees on this plowed plot looked decidedly better than those of any other plot. By December, 1921, however, the winter-cover-crop plot was equally as dark green and vigorous and, from this time to the close of 1924, the trees on these two plots, as a rule, appeared to be in better condition than those on any other plot. The plot with the growing alfalfa in every other inter-space during portions of 1920 and 1921 yielded slightly more fruit in 1920 than the average of all plots even though the trees were in a very devitalized condition. The larger amounts of commercial fertilizer used on this plot since the alfalfa was turned under have so far not been reflected in the yields. As already stated, the growing of alfalfa proved injurious to the trees, and this injury was apparently not due to competition for water. The trees showed a decided improvement soon after the alfalfa was turned under. The plot which was cultivated in summer and sown to a legume cover-crop in winter had a greater average annual yield during the five-year period than any other. The excess over some of the others was not great, but the crops were more regular and this was the only plot which gave an increased yield in 1922 compared with the previous year. The winter cover-crop in 1920 and 1921 was Melilotus indica sown in September, and in 1922 and 1923 purple vetch also sown in Sep- tember. Purple vetch proved very satisfactory as a winter cover-crop. Weather permitting, the cover-crops were turned under not later than the end of February. From the standpoints of both yield and tree condition the winter- cover-crop plot has been the most satisfactory of all the Lemon Cove plots. The rotation plot produced an average annual yield practically the same as the average of all plots during the five-year period. This plot had a summer cover-crop of Melilotus alba and buckwheat in 1920 a winter cover-crop of Melilotus indica in 1921, clean cultivation in 1922, a summer mulch of four and a half tons per acre of alfalfa hay in 1923, and clean cultivation during 1924. The trees became somewhat pale during the late summer of 1920, while the summer cover-crop was growing, and at the end of April, 1921, were still yellowish. 10 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION YIELDS OF THE PORTERVILLE PLOTS The treatments on the Porterville plots were essentially the same as at Lemon Cove. The lowest-yielding plot before the experiments began produced thirty-seven per cent less than the highest-yielding plot. TABLE 2 Average Annual, Yields per Tree in Field Boxes on Porterville, Tract Yield in field boxes per tree Treatment Average Num- ber of trees Before triil 1919 During period of trial (1920-1924) annual yield per tree 1920 1921 1922 1923 1924 during trial 74 Cover-cropped in summer, fallow in winter 81 0.83 63 0.75 2.19 1.23 1.13 75 Straw mulch in summer, fallow in winter 0.58 1.07 0.74 0.70 2.23 1.86 1.32 72 Cultivated in summer, straw plowed under in fall, fallow in winter 0.81 0.96 0.70 61 2.19 1.20 1.13 75 Plowed several times during summer, little or no other cultivation, fallow in winter.. 0.59 85 0.78 0.60 1.91 0.92 1.01 74 Intercropped with alfalfa in every other middle, clean cultivation in alternate middles 90 0.85 0.41 0.50 1 37 0.94 0.81 73 Clean cultivation in summer, legume cover-crop in winter.. 53 0.94 0.84 0.84 2.23 84 1.15 75 Rotation plot receiving one of the above treatments each year, except growing alfalfa . 57 1.09 0.81 96 2 05 1.27 1.24 COMMENTS ON THE YIELDS THE TREATMENTS AND THE CONDITION OF THE TREES OF THE PORTERVILLE PLOTS The effectiveness of the fertilizer treatments was less marked on the Porterville plots than heretofore noted on the Lemon Cove plots. It is believed that a shortage of irrigation water during the entire period of the experiment was such a limiting factor in production that the cultural treatment and the increased fertilizer applications were largely counteracted by this unfavorable condition. The yields showed a small upward trend during most of the period of experi- mentation. The plot with alfalfa growing in every other inter-space showed a notable falling off in yield compared with the other plots. It is doubtful if there is any other significant difference in the yields of the remaining plots. BULL. 405] CITRUS CULTURE IN CENTRAL CALIFORNIA 11 The summer -cover-crop plot was sown to Melilotus alba in 1920, Virginia soy beans in 1921, Whippoorwill cowpeas in 1922 and purple vetch in 1923 and 1924. The summer cover-crops were not notably rank and were always free from water-grass. The trees for the most part remained in good condition. The practicability of the cultural treatments was similar to those of the Lemon Cove plots heretofore discussed. Fig. 1 shows a view of the straw mulched plot, directly after mulching. This treatment proved impractical. FERTILZER PRACTICES FOR CITRUS ORCARDS IN CENTRAL CALIFORNIA The following suggestions are based not only upon the experiments just outlined, but also upon numerous observations in other orchards in central California. So far, in both central and southern California, organic matter and nitrogen have proved to be of paramount importance in fertilizing citrus trees. An excellent fertilizer program, judging from results, is to apply manure in the fall and commercial nitrogenous fertilizer in the spring, a practice which has been generally advised by Kelley 2 , Vaile and Surr 3 , Hodgson 4 and others. When manure is mixed with soil the nitrate or available nitrogen temporarily bceomes unavailable in the cells of organisms decomposing the manure. From this standpoint, and also for convenience, if winter cover-crops are sown in the autumn the manure is preferably applied before sowing the cover-crop. Citrus trees which are healthy and vigorous tend not only to set good crops but to hold a good proportion of the fruit in the June-drop period. Every effort should therefore be made to keep them in the best condition possible during the spring and early summer. Appli- cations of manure in the spring would tend to reduce the available nitrogen temporarily and thus lower the vitality of the trees at what seems to be a critical time. In mature citrus orchards ten tons of barnyard manure to the acre annually may be considered, as a rule, a profitable application. 2 Kelley, W. P. Our present knowledge concerning fertilization of citrus. Calif. Citrograph, 76: — . 1922. s Vaile, E. S., and Gordon Surr. Progress report of orchard management studies in Tulare County. Calif. Citrograph, 9i: — . 1923. 4 Hodgson, E. W. Fertilizing citrus trees in California. California Agr. Exp. Sta. Cir. 283: 1-22. 1925. 12 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The use of manure in reasonable quantities each year is recommended rather than occasional applications of much larger amounts. Adobe soils are commonly slow in drying out in the spring in central California and after a rain it may be a long time before they can be cultivated properly. If rain immediately follows a fertilizer application, the cover-crops or weeds may reap the benefit of the fertilizer rather than the trees. This has happened in the past, the rank growth of weeds plainly showing where the fertilizer had been applied. For this and for other reasons it is a good plan to disk a winter cover-crop before applying the fertilizer and to plow after the application. As nitrification is slow in cold wet soils, the nitrogen in the spring applications of commercial fertilizer to citrus trees on adobe soils should preferably be in part or wholly in the nitrate or immediately available form. The nitrates most commonly used include nitrate of lime, nitrate of soda and nitrate of ammonia, all of which have given excellent results on adobe soils, when used in conjunction with barnyard manure and cover-crops. Light soils, however, warm up readily and fertilizers may be used on them in which the nitrogen is not immediately available, such as dried blood or sulphate of ammonia. Where a mature grove receives ten tons of good manure to the acre annually, about one pound of commercial actual nitrogen to the tree should suffice in most cases for the spring application. The profitable amounts of commercial fertilizer which may be applied depend, however, upon various factors, such as the quantity and quality of the manure applied, the size and condition of the trees, whether good winter cover-crops are grown, and the prices prevailing for the fruit. There is a popular impression that manure, used alone, tends to make citrus fruits coarse. This, however, does not seem to be the case when manure is used in reasonable amounts. Manure is a complete fertilizer, and it contains the principal elements of plant food in proportions much nearer to those actually taken up by the trees and plants than do some complete high-phos- phorus, so-called ' ' balanced, ' ' commercial fertilizers. Nitrogen, unlike phosphorus and potash, tends to be leached away from the root zone, which is one reason why supplementing manure with commercial nitrogen is generally useful in citrus groves. The adobe soils planted to citrus in central California have largely been derived from gabbro and serpentine rocks that are relatively low in potash. However, potash is pretty firmly held by the clayey BULL. 405] CITRUS CULTURE IN CENTRAL CALIFORNIA 13 materials in soils, and both potash and phosphorus are of course added when manure is applied. Suffice it here to say that, from what has been learned to date, little or no benefit to citrus trees in central California can as yet be definitely ascribed to either potash or phosphorus applied in commercial fertilizers. The Use of Lime, Gypsum and Sulfur. — Vaile and Surr endeav- ored to show in 1921 that the use of lime was in general not worth while in citrus groves on adobe soils in Tulare County 5 , and the same seems to be the case with citrus groves on other soils in central California. The following analyses show the amount of lime carbonate in air-dried soil of the Lemon Cove plots : 1st foot, 0.65 per cent, which is equal to 10 tons per acre foot; 2nd foot, 1.41 per cent, equal to 21 tons per acre foot ; 3rd foot, 2.38 per cent, equal to 36 tons per acre foot; and 4th foot, 3.21 per cent, equal to 48 tons per acre foot. Soils in the Porterville region are reported which contain from 12 to 39 per cent lime carbonate in the 3rd foot 6 . The three applications of gypsum on the Lemon Cove plots, total- ing about thirteen and a half tons to the acre, have not shown any effect on tree growth or yield or upon the growth of cover-crops and weeds. At times the surface soil an inch or two in depth has had a better texture where the gypsum was applied. On the other hand, the soil on the plot which was worked almost wholly with a plow has been consistently in much better physical condition where no gypsum was used than the soil on the other plots to which gypsum was added. The same is true of the applications of gypsum to the Porterville plots. Sulfur was applied to orange trees in Tulare County at the rate of ten, twenty, and forty pounds to the tree in 1920, but no material benefit has followed the application of this material. Culls and Soil Acidity. — The question is frequently asked if cull oranges and lemons worked into the soil will cause soil acidity. There is no probability at all of any acidity arising from the use of culls, of which over twenty tons to the acre were applied one year to an orchard in Tulare County. The acids in citrus fruits are soon broken down by soil organisms, and it was found, in one laboratory experi- ment, that citric acid itself applied at the rate of ten tons to the acre did not increase the acidity of the soil. Citrus fruits are mostly s Vaile, E. S., and Gordon Surr. Use of various compounds of calcium on adobe soils of the foothill regions of Tulare County. Calif. Citrograph, Vol. 7i: 3, 24, 26, 27. 1921. 6 Hilgard, E. W. Marly subsoils and chlorosis or yellowing of citrus trees. California Agr. Exp. Sta. Circ. 27: 1-4. 1906. 14 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION water and at best are decidedly low in fertilizing value. A ton of fresh ripe oranges, for example, contains on an average only 3.66 pounds of nitrogen, 1.10 pounds of phosphoric acid and 4.22 pounds of potash. Fig. 1. — Straw mulch, five tons per acre, Ball-Emery plot. IRRIGATION PRACTICES It would be folly to attempt to lay down hard and fast rules for the irrigation of citrus groves, on account of the differing conditions. On adobe soils the aims should be to let the ground crack moderately between irrigations, and to run the water as quickly and for as short a time as possible without washing the soil. When soils, particularly heavy soils, are wet for long periods, not only do the roots of citrus trees cease to function but in many cases they actually die and rot. Figures 2 and 3 are photographs of Navel orange trees on adobe soil, just below and just above an irrigation pipe-line in the same grove. Both photographs were taken from the same spot. The ground just below the pipe-line has been wet for lengthy periods with disastrous consequences to the trees. In dealing with citrus orchards on open soils and subsoils it should be borne in mind that, when large amounts of irrigation water are applied, water and soluble plant food materials may be carried below the root-zone, in which event they are practically wasted. For the best results from every point of view, including economy in the use of water, the irrigation pipe-lines, in a citrus grove on any type of soil should not be much over three hundred feet apart. Bull. 405] CITRUS CULTURE IN CENTRAL CALIFORNIA 15 Fig. 2. — Navel orange trees with rotten roots on adobe soil just below the irrigation pipe-line. Fig. 3. — Healthy Navel orange trees in the same grove just above the pipe-line. Both photographs taken from the same spot. 16 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION In some orchards there are wide strips in the tree rows which get no water during the whole irrigation season. Yet the winter rains wet these areas and generally cause the roots therein to become active in the spring, which activity persists as long as moisture is available. One of the best ways at present known of reducing the ravages of June-drop is to keep citrus trees in the best possible condition, and it is obvious that if many roots cease to function, owing to the drying out of the soil in the tree rows, the vigor of the trees may be correspondingly impaired at what seems to be a critical time. It would appear desirable to change the positions of the irrigation furrows, instead of having them in exactly the same places at each irrigation, and it is a good plan to make the furrows a few days before applying the water on adobe soils, thus aiding the bottoms of the furrows to crack. Finally, it is well to remember that over-irrigation is apt to cause far more serious injury to citrus trees than under-irrigation. CULTIVATION PRACTICES Veihmeyer, and Beckett, of the University of California, found that, under field conditions, the differences in the losses of moisture, between cultivated and uncultivated soils were negligible, provided that weeds were not allowed to grow. 7 Much harm is often done by cultivating orchards when the ground is too wet. When it is fully realized that cultivation, in itself, is of little or no avail in conserving moisture, it is hoped that there will be less haste in working the soil after irrigation or rain. Cultivation is necessary to kill weeds and to turn under cover - crops and fertilizers, and certain soils do not take water well without some cultivation. Cultivation, furthermore, is apt to stimulate nitrifi- cation, particularly in heavy soils. Some citrus groves are plowed once a year, some several times a year, and some not at all, with equally good yields and tree condition. Plowing, however, indirectly much improves the texture of adobe soils and it will be of interest to briefly discuss, the reasons. Adobe soils swell on wetting and shrink on drying and when a slice of adobe soil, turned over with a plow, is soaked by water, expansion takes place to a marked degree as there is little or no side-pressure to interfere with the swelling. On drying out, the greatly expanded soil falls apart and crumbles into small particles. Repeated plowings may 7 Eeport of the College of Agriculture and the Agricultural Experiment Station of the University of California, July 1, 1921, to June 30, 1922, p. 105. BULL. 405] CITRUS CULTURE IN CENTRAL CALIFORNIA 17 have little effect, however, on the texture of soils which do not swell much on wetting. In this connection it is of interest to note that both the plots on adobe soil, which were frequently plowed, in the Tulare County trials, had a higher content of nitrate nitrogen than the plots receiving ordinary cultivation, including plowing once a year. Many orchards are cultivated more than is either necessary or desirable and great improvements in soil conditions have occurred after reducing the amount of cultivation, in fact there are productive citrus groves, with excellent soil conditions, which receive very little cultivation. When orchards have remained uncultivated all winter, with regular cultivation throughout the irrigation season, the earlier irrigations may be efficient but, in many cases, the penetration of the water becomes increasingly poor as the season advances. The more frequent the cultivation the more the soil may be packed with the formation of so-called "plow-sole," which condition, however, in a number of instances, appears to be due in part to irrigation. The system of cultivation employed is largely bound up with the way the ground takes irrigation water. In some groves irrigation is satisfactory after running the water several times in the same ftirrows, and in other orchards it may be advisable to make new furrows before each irrigation. Such problems should be solved by each grower himself, by means of trials, made preferably on a small scale at first. The proper depth of cultivation likewise depends upon circum- stances. The roots of citrus trees have occasionally been found at considerable depths, but the feeding root-zone is commonly shallow in heavy soils or where hardpan is near the surface. Consequently, whatever the real or supposed merits of deep cultivation, it should not be practiced in citrus groves on either heavy or shallow soils. COVER-CROP PRACTICES Organic matter is one of the great essentials of soil fertility and it is not too much to say that active organic matter is the life of a soil, the term active here signifying organic materials which decom- pose more or less readily under favorable conditions when incorporated with the soil. The necessity for the addition of organic matter has long been recognized in general farming, as shown by the common inclusion of a green-manure crop, such as clover, in the crop-rotation program. The growing of cover or green-manure crops in orchards has also 18 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION become a well established practice. In handling such crops, however, certain points must receive attention or more harm than good may result. In the first place the cover-crop in citrus or other orchards should be a legume of some kind, both purple vetch and Melilotus indica being largely used for winter cover-crops in citrus groves in Cali- fornia. Legumes can obtain nitrogen from the air but, if the soil is well supplied with available nitrogen, the growing legumes may draw more upon the soil than upon the air for their nitrogen. However, legumes are desirable in other ways as cover-crops on account of their manner of decomposition when turned under. Green legumes decompose readily under suitable conditions and soon furnish available nitrogen to the trees, unlike wild oats, water-grass and cereals in general which temporarily render nitrogen unavailable when incor- porated with soil. It is believed that, in many cases, citrus yields in central California have been considerably reduced by rank growths of wild oats in orchards. Not only do such cover-crops draw heavily upon the soil nitrogen but they also cause the nitrogen to be unavail- able for a long time after they are turned under. Again, legumes may have a decidedly favorable influence upon the texture of soils on account of the "humus" formed during their decomposition. For example, Sievers and Holtz state that, in eastern Washington : ' ' The farmer has been unable to influence the physical condition of his soil to any considerable extent through the application of straw or strawy manure in moderate amounts due to the fact that practically no humus was developed. In contrast to this the growing of a legume crop for only a few years will generally benefit the physical condition of the soil so decidedly that its effects are felt for several years." 8 The roots of cover-crops also have a favorable mechanical effect upon the texture of soils and subsoils, and instances have been seen in central California and elsewhere of marked improve- ment in the physical condition of soils and subsoils following the growing and turning under of cover-crops. Cover-crops also largely prevent the washing away of the surface soil by rains and the leaching below the root-zone of soluble plant food materials. Among the advantages of growing cover-crops in winter in Cali- fornia, are the relative dormancy of the trees and the utilization of rain instead of irrigation water. The mistake, however, is often made of sowing the winter cover-crop too late and turning it under too late s Sievers, F. J., and H. F. Holtz. The silt loam soils of eastern Washington and their management. Washington Agr. Exp. Sta. Bull. 166: 40-42. 1922, Bull. 405] CITRUS CULTURE IN CENTRAL CALIFORNIA 19 in the spring. Melilotus indica grows slowly in cold weather, but if sown early, say about September 1, it gets a good start while the weather is warm and by February has often made a very good growth. Purple vetch grows faster in cold weather than Melilotus indica and has made a very satisfactory winter cover-crop both in central and in southern California. Fig. 4. — Summer cover-crop of water-grass in Tulare County orange grove. Winter cover-crops in citrus groves in central California should be turned under not later than the end of February or early in March regardless of the tonnage of the cover-crops. If cover-crops are turned under late in the spring, citrus trees may lack available nitro- gen at what seems to be a critical time, with a consequent reduction in yield. One of the benefits of a winter cover-crop on adobe soils may be due to the drying out of the soil in the spring by the growing cover- crop, thus bringing about aeration, and encouraging earlier root activity of the trees. In young citrus groves winter cover-crops may furnish all the organic matter and nitrogen required, but such is not usually the case in the older orchards. For one thing, where the trees are large, it is often difficult to grow a heavy winter cover-crop on account of the shade. 20 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION In some citrus groves both winter and summer cover-crops are grown regularly, but many growers do not have sufficient water for the needs of both summer cover-crops and the trees. Instances have been seen where exceptionally small fruit was associated with the growing of summer cover-crops. This may have been in part an indirect effect of the poor penetration of the irrigation water in the old furrows. Tests were made in the Lemon Cove Laboratory on samples of soil from orchards in central California in which summer cover-crops were growing or had just been turned under. The nitrate or available nitrogen proved to be extremely low under rank growths of Melilotus alba, Sesbania, and water grass, and quite low where cowpeas or purple vetch were or had been growing. Weeds are apt to get a good start in cover-crops, particularly where summer cover- crops are grown regularly. The accompanying photograph (figure 4) shows a heavy growth of water grass (Echinochloa crus-galli) in a Tulare County orchard in which summer cover-crops of cowpeas have been sown for several seasons. In young citrus orchards both summer and winter cover-crops may possibly be grown with advantage, provided the water supply is sufficient. In this way the soil can be built up without competition between the trees and the growing crops. Later, when competition becomes a factor, the growing of summer cover-crops is of doubtful value. JUNE-DROP The Navel orange is especially susceptible to June-drop, which takes a heavy toll in some seasons. Various experiments were tried in central California in connection with June-drop. For example, in 1923, a number of trees were sprayed with lime-sulfur, oil and spreader, and with Bordeaux mixture. Some were sprayed when in full bloom, others ten days after blooming and others when the fruit was from one-half to three-quarters of an inch in diameter, and there were also combinations of these treatments. Still other trees were sprayed with whitewash in May, 1923. However, losses from June- drop were negligible in 1923 and the yields were not increased by any of these treatments. In the spring of 1920 six Navel orange trees were enclosed in tents, one purpose being to note the effects upon June-drop of the relatively moister air inside the tents. On account of the greater humidity the water evaporated through a porous porcelain bulb, BULL. 405] CITRUS CULTURE IN CENTRAL CALIFORNIA 21 inside one of the tents, was about one-third of the amount evaporated through a similar bulb in the open air. As thrips like dry air they caused no damage whatever to the fruit in any of these tents, although they were introduced into one of them. In spite of the moister air, June-drop was just as heavy in the tents as outside on adjacent trees. It was found that heavy summer cover-crops reduced the evapora- tion of water from porous porcelain bulbs as much as 17 per cent, compared with the evaporation from similar bulbs on clean-cultivated areas. In these particular trials, however, the rank-growing cover- crops were detrimental to the orange trees, which, in their weakened condition, suffered as much loss from June-drop as adjacent trees with clean cultivation. The effect of readily available nitrogenous fertilizers upon June- drop of Navel oranges was clearly shown by the fertilizer trials at Arlington. 9 Immediately after some extremely hot weather in June, 1917, most of the young fruit dropped on the unfertilized plots as well as on the plots receiving manure in the spring. On the other hand, the plots which received a readily available nitrogenous fertilizer in the spring retained a medium crop. One of the best ways at present known to produce good crops of Navels, in spite of June-drop, is to keep the trees in the best possible condition of vigor and productivity, particularly from early spring until after the June-drop period. With adequate fertilization and proper irrigation, combined with efficient pest control, June -drop is not apt to be a serious problem either in central or in southern California, ROTTEN ROOTS AND CITRUS ROOTSTOCKS The kind of stock has a decided bearing on the occurrence of root rot. When heavy soils have been wet for long periods, citrus trees on such soils often have diseased roots. In advanced stages of root rot the foliage becomes scanty and of light color, production ceases and the trees finally die. Whole groves, once highly productive, have been removed for this reason. Figure 5 is a photograph of an orange tree with rotten roots in adobe soil. In some cases, marked improvement has followed more careful irrigation, stopping leaks in pipe lines, and removing the soil from against the trunks down to 9 Vaile, E. S. Fertilizer experiments with citrus trees. California Exp. Sta. Bull. 345: 1-47. 1922. 22 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION the crown roots. Where these things have been done, the trees in some cases have grown new root systems, after severe cutting back of the tops. On account of the removal of once productive citrus orchards owing to root rot at ages when they should be in their prime, the question is often raised as to the future of the citrus groves in central California on adobe soils. Some of these defunct orchards were heavily irrigated, with water in the furrows for lengthy periods. . : Fig. 5. — Orange tree with rotten roots oi soil in Tulare County. There are many healthy and productive citrus orchards on adobe soils in this region, which have been irrigated with only moderate amounts of water, with no indication of root troubles. Some of the older citrus groves in central California were handi- capped from the start, in that many trees were originally of poor type. The buds were not properly selected, the rootstocks were of various kinds and strains, and some trees were essentially runts which had remained in nurseries until reaching salable size. As is well known, sour orange is more resistant to disease than other root stocks commonly used. In view of this fact, it is urgently advised in the light of our present knowledge, that only good sour stock be used in planting or replanting citrus trees on adobe or other heavy soils. BULL. 405] CITRUS CULTURE IN CENTRAL CALIFORNIA 23 It is true, however, that sweet stock often produces larger and more vigorous trees than sour stock, and of this there is at least one excellent example in Tulare County. The striking variation and prevalence of inferior strains in sour rootstocks, as shown by the studies of Webber 10 may account for the poor trees in certain groves of this stock. At present, however, we are mainly concerned with the proper management of old groves already established on stocks susceptible to disease. "Where the root injury is due primarily to water it is evident that the first step is to dry out the ground. Instances have been noted on the contrary where the ill-advised practice has been followed of making short furrows with hoes in order to give especially heavy irrigation to trees already suffering from rotten roots caused by excessive moisture. Where the diseased trees are near pipe lines, one irrigation furrow in the center of each middle may be used to get the water past the trees which are on water-logged soil, and the furrow may then be divided into two or more among the good trees farther from the pipe line. With a little practice, the flow of water can be kept evenly divided by using pipes or by embedding burlap in the ground at the junction of the furrows to prevent washing of the soil and consequent shifts in the flow of water. In other cases the use of one furrow in the center of each middle, for the full length, may be advisable, or the tree rows may be irrigated on one side only, and then on the other side, after the drying of the soil. Summer cover-crops soon dry out the soil but may be hard on unthrifty trees by competing for plant food materials. Lack of aeration appears to be the primary cause of root rot, consequently the cracking of adobe soils should by all means be encouraged where rotting roots are present. 10 Webber, H. J. Selection of stocks in citrus propagation. California Agr. Exp. Sta. Bull. 317: 1-32. 1920. 15m-8,'26