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Use of a coating that is flexible permits measurement of bulk density of natural clods at more than one moisture content. Linear extensibility, expressed as a coefficient (COLE) or as change in horizon thickness, can be calculated from the increase in bulk density between the water content at ⅓‐bar tension and oven dryness. Extensibility data are given for a Marias soil (Entic Chromoxerert). Composition of the Marias soil remains similar with depth but the structure changes. COLE values are affected by the change in structure as are also PVC test results, but liquid and plastic limit values are affected little.

Soil erodibility is influenced by several soil properties including the extent to which the clay fraction will disperse in water. Because early methods for estimating soil erosion were empirical methods and did not utilize water‐dispersible clay as a parameter, few data have been collected. The recent development of the Water Erosion Prediction Project (WEPP) model, a process‐based model for predicting water erosion that uses water‐dispersible clay in the algorithm for computing interrill erodibility, resulted in an increased demand for these data. In order to accommodate this and similar models, a method for estimating the water‐dispersible clay content of soils based on existing information is needed. Data collected by the National Soil Survey Laboratory in support of the WEPP were used to identify soil properties that were significantly correlated with water‐dispersible clay and to develop equations to estimate the water‐dispersible clay content of soils based on those properties. The property most strongly correlated with water‐dispersible clay is total clay. Other properties significantly correlated with water‐dispersible clay are the water content at 1.5 MPa, dithionite‐citrate‐extractable Fe and Al, the coefficient of linear extensibility, Wischmeier's M, the very‐fine‐sand content, the ratio of cation‐exchange capacity (CEC) to total clay, Bouyoucos' clay ratio, and the CEC. A simple linear regression of water‐dispersible clay vs. total clay revealed that, for the soils included in this study, approximately one‐third of the total clay was water dispersible. However, the model only had an R2 of 0.604. When the ratio of the CEC corrected for organic carbon (CCEC) to total clay was included in the model, the R2 improved to 0.723. However, sorting the data by the ratio of CCEC to total clay instead of including it in the model improved the overall fit of the model and increased the R2 to 0.879.

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Although the clay in some moderately fine and fine‐textured Bt horizons of soils of arid and mediterranean climates of the southwestern United States is highly oriented, no distinct illuvial clay skins can be recognized. The distribution of clay skins is related to shrink‐swell potentials. Clay skins are absent in horizons having a shrink‐swell potential of more than 4% or a masepic or omnisepic plasmic fabric; they are present in equivalent horizons having low shrink‐swell potentials and an insepic or mosepic plasmic fabric. The clay content, mineralogy, and moisture regime of a Bt horizon in turn largely determine its potential to shrink and swell and hence determine its plasmic fabric.Evidence that clay illuviation has indeed taken place in these finer textured Bt horizons is based on four pairs of geographically associated soils with horizons of clay accumulation. Bt horizons of the coarser textured members of pairs have clay skins and the finer textured members do not. The distribution of biotite pseudomorphs in some of these pairs parallels the distribution of clay skins, suggesting that oriented bodies of clay can be destroyed. Clay orientation in one of the horizons was reformed experimentally to show that the highly oriented soil fabrics do not acquire their orientation by illuviation of clay.The studies further indicate that bodies of oriented clay in medium and fine‐textured B horizons have been erroneously described as clay skins.

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X‐ray diffraction (XRD) studies of about 2000 soil clays show that 1500 kPa water retention, in addition to Atterberg limits, cation exchange capacity, 33 kPa water retention, volume of water retained between 33 and 1500 kPa, and linear extensibility varies with kind of soil clay and the intensity of the XRD peak of the dominant clay mineral. Hence, knowledge of soil clay mineralogy is particularly useful for predicting the behavior of soils. In standard characterization work, however, determining the property of interest is usually more efficient than determining mineralogy and then estimating the property. Determining both is not necessary if their relationship is understood. In predicting the behavior of soils, identification of the dominant clay mineral is sufficient if that mineral is smectite. If the mineral is kaolinite or clay mica, somewhat greater accuracy of prediction of properties is possible if measurements of the relative intensities of the other clay minerals in the suite are also known.