UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA AN INDEX FOR RATING THE AGRICULTURAL VALUE OF SOILS R. EARL STORIE BULLETIN 556 SEPTEMBER, 1933 UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA CONTENTS PAGE Introduction 3 Method of rating 5 Factor A: character of soil profile 5 Group I, unweathered or only slightly weathered secondary soils (recent or young soils) 6 Group II, moderately weathered secondary soils (immature secondary soils) 13 Group III, strongly weathered secondary soils with dense clay subsoils (semimature secondary soils) 14 Group IV, maturely weathered secondary soils with hardpan 15 Group V, strongly weathered soils with dense clay subsoils resting on con- solidated materials 17 Group VI, soils developed on bedrock (primary or residual soils) ... 19 Factor B : texture of surface soil . • 21 Medium-textured soils 24 Medium-heavy-textured soils 25 Heavy-textured soils 25 Light-textured soils 26 Gravelly or cobbly soils 27 Stony soils 27 Factor C: soil-modifying conditions 28 Drainage 28 Alkali 28 Acidity 30 Infertility 30 Stratified subsoils (gravelly layers or stratified clay layers) 31 Shallow phases of alluvial soils 31 Eroded soils 31 Steep phases 32 Nonagricultural materials 32 Calculation of the index 34 Soil rating for a tract of land 35 Summary 39 Acknowledgments 39 List of selected references 40 Appendix: soil survey data in California 41 AN INDEX FOR RATING THE AGRICULTURAL VALUE OF SOILS 1 E. EARL STORIE 2 INTRODUCTION The growth and production of plants is dependent very largely upon the soil, and particularly the degree to which it presents conditions favorable for the extension and development of plant roots. On soil that is deep, pervious, relatively uniform in character, and that has fair water-retaining capacity, a very wide range of plants may grow vigor- ously. On soil with subsoil characteristics that retard the extension of plant roots, growth and development of the plants may likewise be retarded. If the subsoil is only moderately dense and root development only slightly hindered, the growth and production may be good, but if the density of the subsoil reaches that of a claypan, root penetration may be decidedly limited, and if the subsoil is cemented into a true hardpan, or underlaid by hard bedrock, penetration is definitely pre- vented. These variations in the character of the soil are directly related to its origin, its mode of formation, and the age or stage of weathering. Recent alluvial deposits are generally deep and pervious, and the profile, or vertical section through the soil mass, shows no variations such as are developed with age. Older soils, which have been exposed to the weather- ing agents for very long periods of time, have subsoils that have become more dense, until finally they may reach the claypan or hardpan stage. Such soils, which are considered maturely weathered, have approached a condition of equilibrium with the factors that have determined the particular characteristics of their profiles. The growth of plants on the soil is so closely related to these char- acteristics of the profile, and the chemical composition of the several horizons of the soil is likewise so closely related to the profile, that it appears possible to rate the soils with the profile characteristics as a base. i Received for publication April 25, 1933. 2 Assistant Soil Technologist in the Experiment Station. [3] 4 University of California — Experiment Station The index for rating soils as used herein is a numerical expression of the degree to which a particular soil presents conditions favorable for plant growth and crop production under good environmental condi- tions. In arriving at the relative index of soils three general factors are considered. These are (A) the character of the soil profile; (B) soil tex- ture; and (C) other modifying factors, such as drainage, alkali, and other miscellaneous conditions. Each of these three factors is evaluated on the basis of 100 per cent for the most favorable or ideal condition, with limiting maximum and minimum ratings ascribed to conditions that are less favorable for plant growth. The information set forth in the various soil-survey publications has been used in developing the index. For a discussion of the soil survey as a source of information, the reader is referred to the Appendix. In studying, mapping, and classifying soils, the character and degree of development of the soil profile, the reaction of the soil mass, the color, the physical composition of the various horizons, the mode of formation and composition of the parent material, the drainage and surface relief, the alkali content, natural cover, etc., are considered in detail. In arriving at the ratings the experience and judgment of the men engaged in soil-survey work in California has been called upon, and the ratings express the results of their collective studies of the relations of these several conditions of the soil to the growth and production of plants. Obviously, such ratings cannot be final and infallible and may be changed as experience with the use of the soil index indicates. The index is based on soil conditions alone independent of other physical or economic factors which might determine the desirability of growing certain plants in a given location ; hence it cannot be regarded as by itself an index for land evaluation. In the latter, other factors, such as variations in climate, availability of water for irrigation, facili- ties for transportation and markets, social conditions, and the like, must be included. Although the soil is only one of the many factors that determine the value of any given area of land surface, it is one that does not readily change and that cannot be materially modified without the expenditure of much effort. Its quality or value is one of the more stable elements that determine land value, and an index for rating soils should serve to simplify the work of land classification and evaluation. Other factors, such as the climate, or the quality, quantity, and availability of water for irrigation are likewise natural factors, more or less stable in char- acter, that may be rated in a similar manner, but some physical, eco- nomic, and social factors may be too variable or too readily changed to permit of such treatment. Studies of these other factors that influence Bul. 556] Index for Rating Value of Soils 5 or modify land values may show the possibilities of developing indexes for groups of these factors, and by integrating these several indexes, a positive numerical expression of the relative value of any portion of the land surface may result. METHOD OF RATING In most endeavors to rate soils on a comparative basis the "score card" method has been used, and the rating determined by adding the points credited to each of the soil characteristics or modifying factors. In the index method here presented the rating is obtained by multiplying the three factors, A times B times C, thus permitting any one of these to dominate or control the final rating. As an example, a soil may have an excellent profile condition warranting a rating of 100 per cent for factor A, excellent surface-soil conditions giving 100 per cent for factor B, but a bad alkali accumulation that would give a rating of 10 per cent for factor C. Multiplying these three percentage ratings — A x B x C — gives 10 per cent as the index for this soil. The severe alkali accumulation would dominate the quality of this soil, rendering it wholly unproduc- tive for plants and would justify the index of 10 for that soil. On the score-card basis, with possibly 20 or 25 points ascribed to alkali or simi- lar conditions, its worst rating could be only 75 or 80. The characteristics or conditions included under each of these factors is described in the following chapter, and their bearing on the quality of the soil is discussed. FACTOR A : CHARACTER OF SOIL PROFILE Factor A evaluates all the characteristics of the soil profile except the texture of the surface soils. These characteristics, which are determined by the kind of parent material, its mode of formation or accumulation, and the age or degree of modification of the soil material by the weather- ing agencies of climate, supplemented by vegetation and opposed by erosion and deposition, serve to separate the soils into groups of related individuals that are classed as soil series. The suitability of soils for plant growth is dependent to a very large extent on the profile characteristics that differentiate the soil series. A series may be defined as "a group of soils having the same character of profile ; that is, the same range in color, structure, and general sequence of soil and subsoil horizons, the same general types of relief and drain- age, and a common or similar origin and mode of formation." 3 The 3 Shaw, C. F. A glossary of soil terms. American Soil Survey Association Bul. 9:28-58. 1927. 6 University of California — Experiment Station name of the series implies all the characteristics used to define the soil type except that of surface texture. For factor A of this index, the soil series of California have been placed in six general groups and a rating given soils of these groups. In each group are series having similar modes of formation, similar sequence of soil and subsoil horizons (i. e., layers), a comparable age, and the same types of relief. The soil series of California officially rec- ognized by the Bureau of Chemistry and Soils of the United States Department of Agriculture and the California Agricultural Experi- ment Station are listed alphabetically in table 1 with their position in the six groups on the basis of factor A and with certain of their profile characteristics indicated. The profile characteristics and inclusive ratings for each group under factor A are given in table 2. Secondary 4 soils are placed in groups I, II, III, and IV; strongly weathered secondary and primary 5 soils having dense clay subsoils de- veloped on consolidated material in group V; and primary soils in group VI. In the following sections characteristics of the soils in each group are described and the profile rating of each explained. Where soil surveys have been made, the necessary information for this grouping can be obtained from the reports and maps, but if a survey has not been made, then a field examination of the area to be rated is needed in order to determine the profile characteristics. GROUP I, UNWEATHERED OR ONLY SLIGHTLY WEATHERED SECONDARY SOILS (RECENT OR YOUNG SOILS) When they have been transported by water, the unweathered second- ary soils are commonly referred to as the alluvial, stream-bottom, flood- plain, alluvial-fan, or recent, transported soils. Group I also includes soils deposited or reworked by the action of winds, commonly classed as aeolian or wind-modified soils. Soils of this group are characteristically more than 6 feet deep and consist of a mass of soil material which has not been modified to any extent by the action of weathering forces, which ultimately bring about a change in the general character of the soil profile. It is because of this that they are spoken of as unweathered soils, in contrast to the moder- ately weathered soils of group II. 4 Secondary soils are those formed by the accumulation and weathering of transported materials, originating from previously existing soils and from rock debris, and are often referred to as alluvial, aeolian, or glacial soils, according to the agency which transported them. s Primary soils are those formed by the disintegration and decomposition of rocks in place and the weathering of the resultant debris to true soil. Bul. 556] Index for Rating Value of Soils TABLE 1 Soil Series of California Listed Alphabetically Showing Their Positions in the Six Groups on the Basis of Factor A, and also Showing the Color* of the Surface Soil and the Soil Beaction Series Adelanto Agate Agueda Aiken Alamo Aliso Altamont Alviso Anderson Anita Antelope Antioch Arbuckle Arnold Arroyo Seco Ataseadero Ayar Ballard Barron Bayside Baywood Bear Belle vista Bieber Bishop Botella Bunting ville ... Butte Cachuma Cajon Canby Capay Carlsbad Carrizo Carson Cayucos Centerville Chamise Chino Chualar Churchill Clear Lake Climax Coachella Cole Columbia Commatti Conejo Contra Costa... Group Color of surface soil Grayish brown Brown Dark brownish gray Red Dark gray Reddish brown Brown Dark gray Yellowish brown Dark brown Dark gray Dark brown Grayish brown Brownish gray Light brown Dark brownish gray Brown Brown Grayish brown Grayish brown Dark brown Reddish brown Light gray Dark brown Dark brownish gray Dark brownish gray Dark grayish brown Brownish gray Reddish brown Light brownish gray Light gray Dark grayish brown Brown Light gray Dark gray Dark gray to black Chocolate brown Dark grayish brown Dark gray Dark brown Grayish brown Dark gray to black Dark gray to black Gray Dark grayish brown Light grayish brown Gray Dark gray to black Reddish brown General reaction III V I VI IV III VI I I II V III I VI I V VI II II I I I IV V I I II VI III I V III VI I I VI III V II II IV I VI I II I II I VI Calcareous subsoils Calcareous substratum Calcareous throughout Moderately acid Neutral Calcareous subsoils Calcareous subsoils Usually saline Neutral Neutral Neutral Calcareous subsoils Neutral Moderately acid Neutral Slightly acid Calcareous throughout Neutral Neutral Moderately acid Moderately acid Neutral Calcareous throughout Neutral Calcareous throughout Neutral Calcareous subsoils Neutral Neutral Calcareous throughout Calcareous subsoils Calcareous subsoils Slightly acid Calcareous throughout Calcareous subsoils Moderately acid.. Calcareous subsoils Moderately acid Calcareous subsoils Neutral Calcareous throughout Calcareous subsoils Neutral Calcareous throughout Slightly acid Neutral Calcareous subsoils Neutral Neutral * Color, one of the most obvious of the many soil characteristics, is so closely correlated with the condi- tions under which the soil has been formed that it often gives considerable information concerning the other characteristics of the soil, such as the organic matter content. University of California — Experiment Station TABLE 1— (Continued) Series Group Color of surface soil General reaction Coquille Corning Corralitos Cuyama Danville Daulton Delaney Delano Delhi Denverton Diablo Diamond Springs Docas Domino Dublin Ducor Dunnigan Egbert Elder Elkhorn Elna Empire Encina Escondido Esparto Exeter Fallbrook Fancher Farwell Feather Ferndale Foster Fresno Fullerton Galveston Gleason Garey Gazelle Gila Gloria Goldridge Gould Greenfield Gridley Hames Hanford Hartley Hesperia Holccmb Holland Holtville Honcut Hovey Huerhuero Hugo Imperial Indio I III II II I VI I II I III VI VI II IV I II III I I II IV III VI VI II III VI IV I I I I rv ii i VI ii IV i IV VI V I III III I III I III VI I I II III VI I I Grayish brown, mottled.. Light red Brown Brownish gray Dark brown Reddish brown Light brownish gray Light reddish brown Light brown Dark brown Dark gray to black Grayish yellow Brownish gray Light brownish gray Dark gray to black Dark chocolate brown Brownish gray Dark gray Dark brownish gray Brown Light gray Reddish brown Dark brown Yellowish brown Light brown Reddish brown Reddish brown Reddish brown Light chocolate brown Brown Brownish gray Dark brownish gray Brownish gray Brown Dark gray Dark brown '. Light reddish brown Dark brownish gray Light purplish brown Brownish red Yellowish gray Reddish brown Brown Brown Brown Light brown Light brownish red Light brown Light brown Brown Light purplish gray Reddish brown Dark gray to black Light grayish brown Yellowish brown Light purplish gray Brownish gray Acid; saline Neutral Slightly acid Calcareous throughout Neutral Neutral Neutral Calcareous subsoils Neutral Calcareous subsoils Calcareous subsoils Slightly acid Calcareous throughout Calcareous throughout Neutral Calcareous subsoils Calcareous subsoils Moderately acid Calcareous subsoils Slightly acid Calcareous throughout Strongly acid Slightly acid Neutral Slightly acid Neutral Neutral Neutral Calcareous subsoils Neutral Moderately acid Calcareous throughout Calcareous; alkaline Neutral Usually saline Neutral Moderately acid Calcareous throughout Calcareous throughout Neutral Moderately acid Neutral Neutral Neutral Slightly acid Neutral Neutral Calcareous subsoils Moderately acid Slightly acid Calcareous throughout Neutral Calcareous subsoils Calcareous subsoils Moderately acid Calcareous, alkaline Calcareous throughout Bul. 556 Index for Rating Value of Soils TABLE 1— (Continued) Series Group Color of surface soil General reaction Johnsonville Reefers Rettleman Rimball Rirkwood Rlamath Rneeland Ronokti Laguna Lahontan Landlow Las Flores Las Posas Lewis Lindsey Linne Liver more Lockwood Los Angeles Los Osos Lynndyl Madera Manzanita . . Marcuse Maricopa Marina Mariposa Marvin Marys ville . . May wood McClusky Media Melbourne . Meloland Merced Merriam Metz Mocho Modoc Mojave Mono Monserate. .. Montague Monterey Montezuma Moro Cojo Nacimiento Niland Nord Norman Oakdale Oakley Ojai Olcott Olivenhain.... Olympic II III VI III III III V VI I II V V VI VI III II VI II III VI VI II IV III III II I VI II IV I III VI VI I III III I I II III IV V rv VI in VI VI II I III I I III III V VI Brown Brownish red Gray Brownish red Dark gray Dark gray Dark grayish brown Brown Light brownish gray ... Light gray Brown to dark brown Light gray Brownish red Dark brown Brownish gray.... Dark gray Dark brownish gray Dark grayish brown Grayish brown Light brown Dark brown Light grayish brown... Brown Brownish red Dark gray Reddish brown Brown Brownish yellow Light brown Reddish brown Yellowish brown Dark grayish brown Brown Brown Light brownish gray.... Dark gray to black Light reddish brown. . Light brown Brown Dark brown Reddish brown Light brownish gray... Brown Dark grayish brown Reddish brown Dark gray to black Reddish brown Brownish gray Light brownish gray... Grayish brown Dark brown Grayish brown Light brown Pale yellow Brown Light brown Dark brown Calcareous subsoils Slightly acid Calcareous throughout Neutral Neutral Neutral Moderately acid Slightly acid Neutral Calcareous; alkaline Calcareous subsoils Slightly acid Neutral Calcareous subsoils Usually alkaline Calcareous subsoils Calcareous throughout Neutral Neutral Calcareous subsoils Moderately acid Calcareous throughout Neutral Moderately acid Usually alkaline Neutral Slightly acid Moderately acid Calcareous subsoils Neutral Neutral Moderately acid Neutral Moderately acid Calcareous throughout Calcareous subsoils Calcareous subsoils Calcareous throughout Calcareous throughout Neutral Calcareous throughout Calcareous throughout Calcareous subsoils Calcareous subsoils Moderately acid Calcareous subsoils Moderately acid Calcareous throughout Calcareous; alkaline Calcareous throughout Often alkaline Neutral Slightly acid Neutral Neutral Slightly acid Moderately acid 10 University of California — Experiment Station TABLE 1— (Continued) Series Orland Oxnard Pajaro Panoche Pentz Pinole Pit Placentia Pleasanton Pond Poplar Porterville Preston Ramada Ramona Redding Rhonerville Rincon Rocklyn Rosamond Rositas Sacramento Salinas Salsipuedes San Gabriel San Joaquin San Marcos San Ysidro Santa Cruz Santa Lucia Santa Rita Santa Ynez Santiago Shasta Shedd Sheridan Sierra Siskiyou Sites Solano Soledad Sorrento Stacy Standish Stockpen Stockton Sunol Sunrise Superstition Surprise Sutter Tangair Tassajero Tehama Tierra Tijeras Tujunga Group II I I I VI III I III II II II II I I II IV II II IV II I II II II I IV II II VI VI II III II II VI VI VI VI VI III II I I V III V II IV I I I IV I II V V I Color of surface soil Light grayish brown Brown Brown Brownish gray Dark grayish brown Yellowish brown Dark gray to black Brownish red Brown Brownish gray Light brown Chocolate brown Light grayish brown Light yellowish brown Reddish brown Brownish red Dark brownish gray Brown Brownish red Light grayish brown Light brownish gray Dark gray to black Dark brownish gray Dark brown Light brown Brownish red Dark brownish gray Brownish gray Reddish brown Dark grayish brown Grayish brown Dark grayish brown Dark gray Light brownish gray Gray Dark gray Brownish red Brownish gray Brownish red Dark brownish gray Brown Light brown Light brown Grayish brown Light gray Dark gray to black Reddish brown Light reddish brown Gray Dark brown Light brownish gray Brownish gray Brown Light yellowish brown Dark grayish brown Light reddish brown Light brownish gray General reaction Neutral Calcareous subsoils Neutral Calcareous throughout Neutral Neutral Neutral Neutral Neutral Calcareous; alkaline Neutral Calcareous subsoils Slightly calcareous Neutral Neutral Strongly acid Strongly acid Neutral Slightly acid Calcareous throughout Calcareous throughout Slightly calcareous subsoils Calcareous subsoils Slightly acid Neutral Slightly acid Calcareous subsoils Neutral Moderately acid Neutral Calcareous throughout Neutral Calcareous subsoils Neutral Calcareous throughout Neutral Moderately acid Neutral Moderately acid Calcareous subsoils Neutral Calcareous subsoils Calcareous throughout Calcareous throughout Calcareous subsoils Calcareous subsoils Neutral Calcareous throughout Calcareous throughout Neutral Neutral Moderately acid Neutral Neutral Moderately acid Calcareous throughout Neutral Bul. 556] Index for Rating Value of Soils 11 TABLE 1— (Concluded) Series Group Color of surface soil General reaction Tulare II V V VI VI I VI I VI II VI II I I II I VI VI Vista Strongly acid Dark brownish gray wants Yolo Dark gray to black TABLE 2 Eating of Soils on the Basis of Profile Characteristics Group Group number Description Surface relief Profile rating (factor A) in per cent Unweathered or slightly weathered secondary I Loose and friable material 6 feet or more deep Flat or gently slop- ing 95-100 Moderately weathered sec- ondary soils II Deep but have compact sub- soils and slight to moderate accumulation of clay Terraces, benches or valley floors 80-95 Strongly weathered sec- ondary soils III Dense clay subsoils, loose un- consolidated parent mate- rial Terraces 40-80 Maturely weathered sec- ondary soils IV Hardpan soils, often have fairly dense clay subsoils Flat terraces, valley floors, often hog- wallow surface 5-60 Strongly weathered soils having dense clay sub- soils, developed on con- solidated material V Coastal plain soils High eroded ter- races 20-40 Primary soils (residual) VI Formed in place from decom- position and disintegration of underlying bedrock Hilly, rolling or mountainous 20-70 12 University of California — Experiment Station The most valuable agricultural soils of California fall in this group ; they occur on the flood plains and alluvial fans of many of the California streams. They are especially valuable for orchards, vegetables, and deep- rooted field crops. Yields on many of the medium-textured alluvial soils have remained consistently high for a long period without recourse to fertilization. The crop yields are consistently higher than on soils of the other groups. These soils, providing conditions favorable for the growth of almost any crop plants, have been given a factor-A rating of 95 to 100 per cent. TYPE }^^ -^^-^-^^tX C^^^y LOCATION _ £? l *_"7~~*: ^^^^^HL^i^f^.^t^Z. relief _ BPh^J^l _ elevation § -4^L drainage _>y>i£_ hyo^X, RAINFALL _ 18. i?«J*^L NATIVE VE6ETATI0N _ Q^-^a^^O- _ _ USE _ 'M r '=d^.' u £^. _ _ ORIGIN ^f^^^jk^^r^^ DEVELOPMENT Or PROFILE _ 1&-S*"*- jf»*£-^_ _ _ _ _ REMARKS f~rt-o-l£-C< /coJTj^va fZ ItrO . PROFILE COLOR TEXTURE STRUCTURE CONSISTENCE AND DENSITY REACTION MISCELLANEOUS ROOTS, STONES, CONCRETIONS, PERMEABILITY, ETC. -. *?Z$ t«ee. ELEVATION J^J^jjffti _ DRAINAGE _ J^>j43 4 nj ^A: RAINFALL JZ. «2»?*«± NATIVE VE6ETAT10N _Q^a^>~_- J>"±f^L_ USE _WjM~J^3- 1 ORIGIN'S*"*^!*^ -| ,% ^ > rir g^^ife DEVELOPMENT OF PROFILE J=*T^ i? M 5i-'~l. ^^t^*X^_ REMARKS U^Cj^u^o _a. p.a&*^_ f) 9Q. _ STRUCTURE CONSISTENCE AND DENSITY MISCELLANEOUS ROOTS, STONES, CONCRETIONS, PCRMEABIMTY, ETC. "EL "t«^wJ,*L^ -rvu<JL f^Lh^JL^Mx. TfH JLfrOJtrKS a^Loyr^iiiaH, n.o »»*>i-»~L-e<-£C«_ 7.6 - 73 ■+, J /C0tV»*v yofVOKt ***f 7.0 Fig. 2. — Profile description of Eamona sandy loam, San Diego County, California, illustrative of the general characteristics of the soils in group II (moderately weath- ered secondary soils), having compact subsoil layers with clay accumulations. This is evidenced by the somewhat slower penetration of irrigation water. The parent material below the subsoil is usually permeable to a depth of 6 feet or more. These soils usually occur on smooth terraces and mesas, lower benches, and in some cases on typical alluvial-fan sur- faces, usually being slightly higher in elevation than those of group I. Many of the soil series in this group occur on terraces which have been slightly eroded so that the topography is somewhat irregular, especially on the borders. These are good soils, although of less value than those of group I, since they are older, have a moderate subsoil compaction and accumulation of clay, which retards the penetration of moisture, and may have a more 14 University of California — Experiment Station irregular surface, which lessens their value for irrigated crops. They are given a factor- A rating of 80-95 per cent. Soils of the Ramona series are the best known in this group. A typical profile description of Ramona sandy loam is given in figure 2. GROUP III, STRONGLY WEATHERED SECONDARY SOILS WITH DENSE CLAY SUBSOILS (SEMIMATURE SECONDARY SOILS) The soils of group III are characterized by a very heavy, dense, plastic clay subsoil layer that breaks up when dry into very hard dense struc- tural aggregates. These are semimaturely weathered soils. They are of TYPE Jhy^L t> -^--4^L ■<**2fpi &*?"?— LOCATION _ Qs*^£u^^_y^Cl COLOR STRUCTURE CONSISTENCE AND DENSITY MISCELLANEOUS ROOTS, STONES, CONCRETIONS, PERMEABILITY, ETC. JLbrxsm, *¥4 except ianyhj LcrtMy &v*ch OAAM^Jl jLytrZA. ^RJs Orr&.icf' eM^&J /V^> ^-f' JLo-a^r^/ JUlLj d-oSAJU* Jt Fig. 3. — Profile description of Antioeh fine sandy loam from Solano County, Cali- fornia; claypan soil of group III. alluvial or marine origin, but have been materially changed in chemical and physical make-up since they were deposited. All have a leached surface usually of sandy texture, low in available plant food, and nor- mally moderately acid in reaction. There is usually an abrupt change from the sandier surface layers to the heavy, dense clay subsoils. The downward movement of water is markedly retarded and the soils be- come temporarily water-logged after heavy rains. The heavy subsoils are underlaid by pervious parent materials penetrable by roots and water. In some cases these parent materials are loose soil, while in others they are fairly compact, but in all cases they are penetrable to a depth of 6 feet or more. Bul. 55(5 J Index for Rating Value of Soils 15 These soils occupy terraces or benches which are often somewhat eroded. They are of common occurrence in most of the California valleys and along the coast line of central and southern California. Their value is limited by the heavy subsoil layer and often to some extent by the eroded surface, which makes them difficult to irrigate and cultivate. They are given a rating of 40-80 per cent. Within these limits the rating will depend on the depth to the clay layer and the general surface conditions. Soils occurring under higher rainfall, such as the Fig. 4. — Profile of San Joaquin sandy loam (hardpan soil, group IV), San Joaquin Valley, California. The hardpan layer is about 2 feet thick, with about 3 feet of soil material overlying the hardpan. (Photograph by C. F. Shaw.) Antioch series, have surface soils as much as 20 to 24 inches in thickness, while the southern California soils of similar character (such as the Huerhuero series), but located where the rainfall is much less, have much shallower surface soils. Figure 3 gives details of a profile of the Antioch fine sandy loam located near Fairfield. This soil is given a higher rating than the Huerhuero because of its depth of surface soil and the level surface of the terrace on which it occurs. GROUP IV, MATURELY WEATHERED SECONDARY SOILS WITH HARDPAN The soils with hardpan appear to have reached a mature age or stage of weathering, and are characterized by the cemented, rock-like subsoil horizon (fig. 4) that has been formed as a direct result of the soil- weathering process. True hardpans do not soften when saturated with 16 University of California — Experiment Station water, thus being distinguished from certain dense clay subsoils, which are often erroneously referred to as hardpan layers. True hardpan soils occur under arid or semiarid climatic conditions, and the depth to the hardpan layer is usually closely correlated with the depth of the annual penetration of rainfall. A dense clay layer nor- mally lies just above the hardpan. Where the hardpan is continuous and unbroken, drainage conditions are poor and the surface soil becomes saturated after heavy rains. They usually occur on sloping terraces or on valley floors and often have a "hogwallow" surface of small mounds and depressions (fig. 5). Some areas have suffered considerable erosion. Fig. 5. — Natural mounds or "hogwallow" surface of San Joaquin sandy loam, Kern County, California. (Photograph by C. F. Shaw.) The largest areas of hardpan soils lie in the Great Valley of Califor- nia, especially in the San Joaquin. They are also scattered throughout the valleys of southern California. They occupy a total of over three million acres in the state-. Considerable discussion has developed over the subject of the utiliza- tion of hardpan soils and the extent to which the undesirable features, such as the limited depth of the soil material overlying the hardpan and the thickness and hardness of the hardpan, may reduce the value of such soils. The rooting zone of plants is limited when the hardpan is close to the surface. The moisture capacity is limited by the shallow depth of soil above the hardpan; irrigation water must be applied frequently and in small amounts, or saturation and poor drainage will result ; and other difficulties arise owing to the limited mass of soil material. The organic content of hardpan soils is usually low, and the clay fractions of such character that the soils have a tendency to bake hard on drying, thus rendering them more difficult to handle. The undulating hogwal- low surface configuration of such lands also renders them difficult to Bul. 556] Index for Rating Value of Soils 17 prepare for irrigation. When they are leveled, it is noted that crops re- spond better in the filled places than they do on the areas where soil has been removed. The following ratings are based on the depth to hardpan : Rating, in per cent Hardpan less than 1 foot 5-10 1 to 2 feet 10-20 2 to 3 feet 20-30 3 to 4 feet 30-40 4 to 6 feet 40-60 These figures should be raised in certain instances where the hardpan is soft or broken or occurs in thin plates. A typical profile of San Joaquin sandy loam, a red "iron" hardpan soil, is given in figure 6. TYPE ^J^^^A^^<^lnJ^y^r^. LOCATION 7Wj^4*J^-i'^t*4A \/o£JIsm. _ nCUEr $k%fo&££Zt: ELEVATION 25P_|ifC _ DRAINAGE S^4j^^%*ctn£<&£ RAINFALL _//_^2^~^L _ NATIVE VE6ETATION JSvla^u _ _ _ USE _ T^Gi/UL — ORIGIN ^^r^4^±-4^I^^AS^JL DEVELOPMENT OF PROFILE ~yt\^^S^_ -^tuithtyuL _ _ — REMARKS _ "7t^_V\*>^-£^4jP4^-£*tJI _ — — _ — — — /ia/u*^, trL ZS". COLOR ?^f4 STRUCTURE GONS'STEKCE AND DENSITY MISCELLANEOUS ROOTS, STONES, COUCHETIONS, PERMSABIi-VTt, ETC. ">nff-o4_ , T\4s%JL AcU I VI. AjV^. pJL'\S^L ~#4 UOJL, jggk "MSjuXHaJ lic^ur^n^Ke- J^ 4l~k. 3E3? ~*- M Fig. 6. — Profile description of San Joaquin sandy loam, a typical hardpan soil of group IV. GROUP V, STRONGLY WEATHERED SOILS WITH DENSE CLAY SUBSOILS RESTING ON CONSOLIDATED MATERIALS Soils of group V are common along the coastal plain of California. Their value is relatively low because of the leached surface, the heavy- textured subsoil, and particularly because of the hardpan-like substra- tum which, although composed of sediments, is essentially rock-like in its nature (fig. 7). Surface soils are normally somewhat acid in reaction. There is usually an abrupt change from the surface layer to the heavy dense clay subsoil, and after heavy rains these soils become temporarily water-logged. 18 University of California — Experiment Station With two exceptions, the Stockton and Landlow, all the soil series so far mapped occur on high terraces which have been subjected to con- siderable erosion. The Stockton and Landlow soils occur on flat, poorly drained plains, and instead of being sandy have heavy-textured surface soils. In this group the surface soils overlying the clay subsoils are usually less than 15 inches in thickness, and the clay subsoil varies from 10 to 24 inches in thickness. Thus the consolidated substratum is found at from 15 to 48 inches below the surface. Fig. 7. — Profile of Olivenhain loamy fine sand from San Diego County, California. Note the sandy surface soil lacking structure, the heavy-textured, dense subsoil, which breaks up into columnar structure when dry, and the massive consolidated sub- stratum. This is a typical soil of group V. (From Bui. 552.) In rating the soils of this group for factor A the points considered are the depth to the clay layer, the depth to the substratum, the nature of the substratum, and the surface relief. The range in rating is from 20 to 40 per cent. Surface texture and other factors are handled under fac- tors B and C. Soils of the Olivenhain, Tierra, and Las Flores series, which are typ- ical of this group, have similar sequence of horizons, and differ mainly in color. A profile of Olivenhain loamy fine sand from the coastal plain of San Diego County is given in figure 8. The factor-A rating of this soil is 25 per cent. Bul. 556] Index for Rating Value of Soils 19 TYPE J^-^c^La^ji^^y-i £^Ajx» _.-£_*J_' RELIEF _^^-_ J __ : ? , _ ct ELEVATION _ _f_P-__ — DRAINAGE _^^^ a _ Y _$_- _*°_ : _ J _"_' RAINFALL _ i>_ IJ>1 _ _ NATIVE VEGETATION _Sho?£ ^*4 USE _ «_yt^r_: *f»*_ a, fp 4 '- ORIGIN /^cx*^J&W«L. _ C £ V ELOPMENT OF PROFILE _^_>~_>*_^_' t _ _____ REMARKS _ — _ _ — _ — _ — - Jh$M K-aMA\*t _jr. LZSL __ _ . PROFILE COLOR TEXTURE STRUCTURE CONSISTENCE AND DENSITY REACTION MISCELLANEOUS ROOTS, STONES, CONCRETIONS, PERMEABILITY, ETC. XUvL i&bmL iWo_- P HG.Z n • 0T T m ~3%tvcn*mj 12- " 'H Ordbuici 7 ? 24- 3f- E tnacAtAK. 48- — - 60- Fig. 8. — Profile description of Olivenhain loamy fine sand, typical of coastal plain soils having dense clay subsoils (group V). GROUP VI, SOILS DEVELOPED ON BEDROCK (PRIMARY OR RESIDUAL SOILS) Soils of group VI have been formed by the disintegration and decom- position of the underlying parent bedrock (fig. 9). Shaw 6 has termed them primary soils. They are also referred to in soil literature as resid- ual soils. They are the hill lands of California, occupying a topography that is rolling, hilly, or mountainous (fig. 13) . Their characteristics are determined to a large extent by the character of the parent bedrock and the rainfall of the region in which they occur. Often they are shallow and stony. These factors, together with the topog- raphy and surface configuration, determine their value. Because of their slope, they are more difficult to farm than the terrace and valley lands. The steep slopes have a tendency to erode badly, especially when culti- vated. The normal progress of erosion is the main factor that keeps most primary soils from reaching a mature stage of soil weathering. Most of the primary soils have a moderate accumulation of clay in the subsoils as a result of soil weathering, but not nearly so much as exists in soils of groups III, IV, or V. Usually the upper portion of the underlying parent bedrock has been softened by weathering action so that roots and mois- ture have an opportunity to work their way down into the bedrock through cracks and crevices for some distance. 6 Shaw, C. F. A definition of terms used in soil literature. Sci.Proc. 5:38-64. 1928. Intntl. Congr. Soil 20 University of California — Experiment Station . ;, . Fig. 9. — Profiles of shallow primary soil (group VI), Sierra Nevada foothill district. TYPE B*M*~4? ^^±. ^»2^y_ _ LOCATION SltstA^. Tl^oyJa. jW^."^lA«>S?l RELIEF. J^^h ELEVATION -T^L-Af^ _ DRAINAGE _ l^WL^^Li^JL RAINFALL 30_^>ycXsS- NATIVE VE6ETATION i?/Wj£t. QaA, _ USE J = ^r^o^^xxeLxi, __ ORIGIN _T2^o^_A«^J^^ 4*142^ J3EVELOPMENT OF PROFILE REMARKS _ ~F ktriU U Hafcyz 61 t>S STRUCTURE r (Mvdtan, CONSISTENCE AND DENSITY Fig. 10. — Profile description of Holland sandy loam in the Sierra Nevada foothills. This is a primary or residual soil of group VI. AU.JL * -yicJbJttJL MISCELLANEOUS ROOTS, STONES. CONCRETIONS, PERMEABILITY, ETC. TZVVmuC&J? -try fr&vt&. ~rzjLn*~*-*XLJl '9*1 K^&Zg. S \MeNoociNol I \ ClCl \ £?w V C0L \I l3 I iOLAljtt. KB! ^^C^f^; -p N vU& @l t& \ ^"santaVarbari 1 *\ io <4w> 25^ 4" J||j not ( g IIP Fig. 17. — Soil-survey areas in California shown by shading 1. Hanf ord 2. San Jose 3. Bakersfield 4. Sacramento 5. Stockton 6. Butte Valley 7. Colusa 8. Bedding 9. Modesto-Turlock 10. Pajaro Valley 11. Porterville 12. Marysville 13. Woodland 14. Livermore 15. Madera 16. Eed Bluff 17. Fresno 18. Merced 19. Ukiah 20. Healdsburg 21. Honey Lake 22. Pasadena 23. Eiverside 24. San Fernando 25. Anaheim 26. Los Angeles 27. Santa Maria 28. Ventura 29. El Centro 30. Grass Valley 31. Willits 32. Shasta Valley 33. Big Valley 34. Brawley 35. Eureka 36. Victorville 37. Lancaster 38. Palo Verde 39. Coachella Valley 40. Gilroy 41. Hollister 42. Auburn 43. Bishop 44. King City 45. Chico 46. Salinas 47. Oroville 48. Clear Lake 49. Placer ville 50. Santa Ynez 51. Paso Eobles 52. San Luis Obispo 53. Oceanside 54. Capistrano 55. ElCajon 56. Suisun 57. Dixon 58. Alturas 59. Lodi 60. Napa Bul. 556] Index for Rating Value of Soils 43 TABLE 5 Soil Surveys Available in Pamphlet Form, 1932* Number as shown on map (fig. 17) Area Anaheim Auburn Big Valley Bishop Brawley Chico Clear Lake Coachella Valley El Centra Eureka Gilroy Grass Valley Hollister Honey Lake King City Lancaster Los Angeles Lower San Joaquin Reconnois sance Middle San Joaquin Recon- noissance Date of survey 1916 1923 1920 1924 1920 1925 1927 1923 1918 1921 1923 1918 1923 1915 1924 1922 1916 1915 1916 Number as shown on map (fig. 17) 28 Area Oroville Palo Verde Valley Pasadena Placerville Riverside Salinas San Diego Reconnoissance San Fernando San Francisco Bay Reconnois sance Santa Ynez Santa Maria Shasta Valley Central-Southern California Reconnoissance Upper San Joaquin Reconnois sance Ventura Victorville Willits Woodland Date of survey 1926 1922 1915 1926 1915 1925 1915 1915 1914 1927 1916 1919 1917 1917 1917 1921 1918 * Can be secured from the Division of Soil Technology, University of California, Berkeley, California. TABLE 6 Soil Surveys No Longer Available for Free Distribution, 1932* Number as shown on map (fig. 17) Area Bakersfieldf Butte Valley Colusa Fresnot Fresnot Hanfordt Healdsburg Imperialt Imperial! Indiof Klamath Reclamation. Livermore Valleyt Los Angelest Lower Salinas Valley ... Maderat Marysville Mercedf Date of survey 1904 1907 1907 1900 1912 1901 1915 1901 1903 1903 1908 1910 1903 1901 1910 1909 1914 Number as shown on map (fig. 17) Area Modesto-Turlockf Pajaro Valley Portervillet Redding Red Bluff Sacramento Sacramento Valley Reconnois sance San Bernardino t San Gabriel t San Josef Santa Anaj Stocktont Ukiah Venturaf Yuma Date of survey 1908 1908 1908 1907 1910 1904 1913 1904 1901 1903 1900 1905 1914 1901 1904 * May be consulted at the principal public libraries in bound volumes of the Annual Field Operations of the Bureau of Soils. t Covered by more recent surveys. 44 University of California — Experiment Station TABLE 7 Soil Surveys Completed But Not Yet Published, 1932 Number as shown on map (fig. 17) Area Date of survey Number as shown on map (fig. 17) Area Date of survey 51 1928 1928 1929 1929 1929 56 57 58 59 60 Suisun 1930 52 Dixon 1931 53 1931 54 1932 55 El Cajon 1932 13m-9,'33