UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY 4, CALIFORNIA REVISION OF THE SOIL-RATING CHART' R. EARL STORIE 2 This method of soil rating, known as the Storie Index, is based on soil characteristics that govern the land's potential utilization and productive capacity. It is independent of other physical or economic factors that might deter- mine the desirability of growing certain plants in a given location. Essentially the present revision sets up a new factor C to evaluate slope; the original factor C is now designated as factor X. Percentage values are assigned to the char- acteristics of the soil itself, including the soil profile (factor A); the texture of the surface soil (factor B) ; the slope (factor C) ; and conditions of the soil exclusive of profile, surface texture, and slope — for example, drain- age, alkali content, nutrient level, erosion, and microrelief (factor X) . The most favorable or ideal conditions with respect to each factor are rated at 100 per cent. The percentage val- ues or ratings for the four factors are then multiplied, the result being the Storie Index rating of the soil. The characteristics of the soil profile (factor A) are essentially the features of the subsurface layers. For California purposes the soils have been divided into nine profile groups. 3 For example, soils that are deep and readily pervious to roots and water (listed in profile group I in the soil-rating chart) are rated at 100 per cent. Profiles with dense clay subsoils (listed in profile group IV on the soil- rating chart) are rated lower. Primary or re- sidual soils (listed in profile groups VII, VIII, and IX) are rated in accordance with the depth to bedrock. Next, the soils are rated on the basis of the texture of the surface soils (designated as fac- tor B) . Medium-textured soils, such as the loams and the silt loams, are rated highest; the extremes in texture, such as sands and clays, lower. This is a revision of the soil-rating chart published originally by the author in Bulletin 556, An Index for Rating the Agricultural Value of Soils , 1933. and later in the revised edition of 1937. g Lecturer in Soil Technology and Associate Soil Technologist in the Experiment Station. 3 Storie, R. Earl, and Walter W. Weir. Key to the soil series of California. Published by Associated Students Store, Berkeley, Calif. 1941 UNIVERSITY OF CALIFORNIA LIBRARY COLLEGE OF AGRICULTURE DAVIS Rating of the slope of the land is considered in factor C. Nearly level or gently sloping land is rated at 100 per cent. As the slope in- creases, the rating for this factor decreases. As shown in the soil-rating chart, single letters are used to indicate simple slopes, and double letters to indicate compound slopes. The per- cent slope expresses the number of feet rise or fall for 100 feet horizontal distance. Conditions exclusive of profile, soil texture, and slope are considered in factor X on the soil-rating chart. These conditions consist of drainage, alkali or salt content, general nutri- ent level, acidity, erosion, and microrelief (surface regularity). If two or more conditions exist that are listed under factor X, the rat- ings for each are treated independently; that is, they are multiplied in order to secure the factor X rating. Soil Rating for a Tract of Land The index for each soil type in the tract is calculated separately, and then a rating for the entire tract is obtained by weighing each soil index according to the proportion of the acreage of that soil in the tract. As an example, using the accompanying soil map the rating of the tract is determined as follows: 1. Index for the area Yl-A (Yolo loam, nearly level): This is a recent alluvial soil, deep, smooth, well drained. Rating in per cent Factor A: Yolo series, profile group I . . . 100 Factor B: loam texture 100 Factor C: slope A, nearly level 100 Factor X: no other modifying factors .... 100 Index rating = 100% x 100% x 100% x 100%= 100% 2. Index for Ac-BB -(Antioch clay loam, undu- lating) : This is a claypan terrace soil with undulating topography. Rating in per cent Factor A: Antioch series, profile group IV . 60 Factor B: clay loam texture 85 Factor C: undulating topography 95 Factor X: no other modifying factors .... 100 Index rating = 60% x 85% x 95% x 100% = 48% (Continued on page 1+) [1] (Storie Soil Index rating SOIL-RATING CHART = factor A x factor B x factor C x factor X) FACTOR A--Rating on character of physical profile FACTOR B — Rating on basis of surface texture I. Soils on recent alluvial fans, flood plains, or other secondary deposits having undeveloped profiles 100 i- shallow phases (on consolidated material) , 2 feet deep 50-60 x-shallow phases (on consolidated material) , 3 feet deep 70 g-extremely gravelly subsoils .... 80-95 s-stratified clay subsoils 80-95 II. Soils on young alluvial fans, flood plains, or other secondary deposits having slightly developed profiles . . 95-100 x-shallow phases (on consolidated material) , 2 feet deep 50-60 x-shallow phases (on consolidated material), 3 feet deep 70 g-extremely gravelly subsoils .... 80-95 s-stratified clay subsoils 80-95 III. Soils on older alluvial fans, allu- vial plains, or terraces having moder- ately developed profiles (moderately dense subsoils) 80-95 x-shallow phases (on consolidated material) , 2 feet deep 40-60 x-shallow phases (on consolidated material) , 3 feet deep ....... 60-70 g-extremely gravelly subsoils .... 60-90 IV. Soils on older plains or terraces having strongly developed profiles (dense clay subsoils) 40-80 V. Soils on older plains or terraces hav- ing hardpan subsoil layers at less than 1 foot 5-20 at 1 to 2 feet 20-30 at 2 to 3 feet 30-40 at 3 to 4 feet 40-50 at 4 to 6 feet 50-80 VI. Soils on older terraces and upland areas having dense clay subsoils rest- ing on moderately consolidated or con- solidated material 40-80 VII. Soils on upland areas underlain by hard igneous bedrock at less than 1 foot 10-30 at 1 to 2 feet 30-50 at 2 to 3 feet 50-70 at 3 to 4 feet 70-80 at 4 to 6 feet 80-100 at more than 6 feet 100 VIII. Soils on upland areas underlain by consolidated sedimentary rocks at less than 1 foot 10-30 at 1 to 2 feet 30-50 at 2 to 3 feet 50-70 at 3 to 4 feet 70-80 at 4 to 6 feet 80-100 at more than 6 feet 100 IX. Soils on upland areas underlain by softly consolidated material at less than 1 foot 20-40 at 1 to 2 feet 40-60 at 2 to 3 feet 60-80 at 3' to 4 feet 80-90 at 4, to 6 feet 90-100 at more than 6 feet 100 Medium-textured : very fine sandy loam , . . fine sandy loam . . . . i loam silt loam sandy loam loamy fine sand silty clay loam clay loam Heavy-textured : silty clay ........ clay Light- or coarse-textured: coarse sandy loam . . . . loamy sand very fine sand fine sand sand coarse sand Gravelly: gravelly fine sandy loam . gravelly loam gravelly silt loam . . . . gravelly sandy loam . . . gravelly clay loam . . . . gravelly clay gravelly sand Stony: stony fine sandy loam . . stony loam stony silt loam stony sandy loam stony clay loam stony clay stony sand per cent 100 100 100 100 95 90 90 85 60-70 50-60 70-90 80 80 65 60 30-60 70-80 60-80 60-80 50-70 60-80 40-70 20-30 70-80 60-80 60-80 50-70 50-80 40-70 10-40 [2] FACTOR C— Rating on basis of slope per cent A - Nearly level (0 to 2%) 100 AA - Gently undulating (0 to 2%) .... 95-100 B - Gently sloping (3 to 8%) 95-100 BB - Undulating (3 to 8%) 85-100 C - Moderately sloping (9 to 15$). . . . 80-95 CC - Rolling (9 to 15%) 80-95 D - Strongly sloping (16 to 30%) .... 70-80 DD - Hilly (16 to 30%) 70-80 E - Steep (30 to 45%) 30-50 F - Very steep (45% and over) 5-30 FACTOR X--Rating of conditions other than those in factors A, B, and C ^ . per cent Drainage: well-drained 100 fairly well drained 80-90 moderately waterlogged 40-80 badly waterlogged 10-40 subject to overflow variable Alkali : alkali-free 100 slightly affected 60-95 moderately affected 30-60 moderately to strongly affected . . 15-30 strongly affected 5-15 Nutrient (fertility) level: high 100 fair 95-100 poor 80-95 very poor 60-80 Acidity: according to degree 80-95 Erosion: none to slight 100 detrimental deposition 75-95 moderate sheet erosion 80-95 occasional shallow gullies 70-90 moderate sheet erosion with shallow gullies 60-80 deep gullies 10-70 moderate sheet erosion with deep gullies 10-60 severe sheet erosion 50-80 severe sheet erosion with shallow gullies 40-50 severe sheet erosion with deep gullies 10-40 very severe erosion 10-40 moderate wind erosion 80-95 severe wind erosion 30-80 Microrelief : smooth . 100 channels '. 60-95 hogwallows 60-95 low hummocks 80-95 high hummocks 20-60 dunes 10-40 [3] 3. Index for Acl-CC (Altamont clay loam, roll- ing) : This is a brown upland soil from shale parent material; bedrock at a depth of 3 feet. Rolling topography, moderate sheet erosion, with occasional gullies. Rating in per cent Factor A: Altamont series, profile group VIII 70 Factor B: clay loam texture 85 Factor C: rolling topography 90 Factor I: moderate sheet erosion with shallow gullies 70 Index rating = 70% x 85% x 90% x 70% = 37%. 4. The index for the entire tract shown on the map may then be calculated according to the acreage of each soil, as follows: Index Acreage Yolo loam 100 x ' 10 = 1,000 Antioch clay loam ... 48 x 5 = 240 Altamont clay loam. . . 37 x 5_ 185 20 1,425 Index rating for the tract = ^ — = 71%. Soil Grading For simplification, six soil grades have been set up in California by combining soils having ranges in index rating as follows: Grade 1 (excellent) : Soils that rate between 80 and 100 per cent and which are suitable for a wide range of crops, including alfalfa, or- chard, truck, and field crops. Grade 2 (good) : Soils that rate between 60 and 79 per cent and which are suitable for most crops. Yields are generally good to excellent. Grade 3 (fair) : Soils that rate between 40 and 59 per cent and which are generally of fair quality, with less wide range of suitability than grades 1 and 2. Soils in this grade may give good results with certain specialized crops. Grade 4 (poor) : Soils that rate between 20 and 39 per cent and which have a narrow range in their agricultural possibilities. For example, a few soils in this grade may be good for rice, but not good for many other uses. Grade 5 (very poor) : Soils that rate between 10 and 19 per cent are of very limited use ex- cept for pasture, because of adverse conditions such as shallowness, roughness, and alkali con- tent. Grade 6 (nonagri cultural) : Soils that rate less than 10 per cent include, for example, tidelands, riverwash, soils of high alkali content, and steep broken land. SOIL MAP 330 J60 SCALE IN FEET • • • Acl-CC • • • • 27 • • • Ac- -BB * Yl -A \ • » • • • MAP SYMBOL SOILS |Yl-A J YOLO LOAM |ac-BB| ANTIOCH CLAY LOAM ACREAGE INDEX 10 IOO 27 \ ALTAMONT CLAY LOAM 48 37 [4] 5m- July, '44(1126)