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Particle-Size Distributions

Comparing Texture Systems, Adding Rock, and Predicting Soil Properties

^a Western Ecology Division, NHEERL, U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, OR 97333
^b Dept. of Crop and Soil Science, ALS 3017, Oregon State Univ., Corvallis, OR 97331-7306
^c OAO Corporation, 200 Sw 35th Street, Corvallis, OR 97333

View larger version (17K): [in a new window]   Plate 1. The (A) USDA and (B) ISSS texture class tetrahedrons show: triangles at 0% rock; projected triangles at 50% rock; apex (a) at 100% rock; and texture class labels: s = sand, ls = loamy sand, sl = sandy loam, l = loam, sil = silt loam, si = silt, scl = sandy clay loam, cl = clay loam, sicl = silt clay loam, sc = sandy clay, sic = silt clay, and c = clay. (C) Soil separates for clay, silt, and sand for the USDA and ISSS systems are compared

View larger version (24K): [in a new window]   Plate 2. Transformed soil texture triangles for the (A) USDA and (B) ISSS systems show relationships of particle-size distribution statistics. (C) The two systems are superimposed for comparisons. Round and triangular symbols show the location of the centroid for each texture class polygon

View larger version (31K): [in a new window]   Fig. 1. Transformed soil texture tetrahedrons for (A) the USDA and (B) the ISSS systems show relationships of projected planes at 25, 50, and 75% rock with the base plane at 0% rock. The sides of the tetrahedrons are labeled as in Plate 1. The dashed line (c) in (A) is the trajectory that connects the centroids of the clay texture class polygons from 0 to 100% rock

View larger version (29K): [in a new window]   Fig. 2. Centroid trajectories of texture classes for (A) the USDA and (B) the ISSS systems with triangular markers at projected texture class centroids at 20% rock intervals

View larger version (19K): [in a new window]   Plate 3. (A) Five general texture classes (USDA5), labeled at the base of tetrahedron, were created from 12 classes. The USDA5 and corresponding USDA12 classes are: cr = coarse = s + ls, mocr = moderately coarse = sl, mecr = medium coarse = l + sil + si, mofn = moderately fine = cl + scl + sicl, and fn = fine = sc + sic + c. (B) Coordinates of the USDA5 class trajectories (lines connecting centroids from 0 to 100% rock) in (A) were transformed to WSPSD statistics coordinates (curved lines). The broken lines connect texture class centroids at constant 10% rock intervals

View larger version (39K): [in a new window]   Plate 4. Conversion nomograms for percentage sand (A) from the ISSS to USDA and (B) from the USDA to ISSS systems. For each texture triangle, the dashed lines are the percentage sand being converted, and the solid lines represent values after conversion. Conversion of percentage sand from the ISSS to USDA system is independent of the clay content of a sample, thus the dashed lines in (A) are linear. Because conversion of percentage sand from the USDA to ISSS system is a function of clay content, the dashed lines in (B) are curved and concentrated together at low percentage clay values

View larger version (13K): [in a new window]   Fig. 3. Frequency distribution curves show the number of soil layers in the STATSGO data base relative to percentage sand in a sample for the USDA12 (solid line) and the ISSS (dashed line) systems. Due to its conversion from the USDA system, the ISSS frequency curve reflects higher sand separates

View larger version (30K): [in a new window]   Plate 5. Relationship of particle-size distribution statistics and soil cation-exchange capacity (CEC) in (A) the USDA12 and (B) the ISSS systems. Note that (A) and (B) display only the fine earths with 10, 30, 50, and 70% clay intervals and sand from 0 to 90%. (C) The relationship of CEC with soil samples that contain rock separates are presented in the USDA5 trajectory system