Shrink‐Swell Potential of Montmorillonitic Soils in Udic Moisture Regimes

Thirty‐two montmorillonitic soil pedons representative of Vertisols, Inceptisols, and aquic, typic, or vertic subgroups of Alfisols and Ultisols in the udic moisture regime of Alabama were evaluated for their shrink‐swell properties. Soil shrink‐swell potential was evaluated on control section samples from coefficients of linear extensibility (COLE) measured by thermo‐mechanical analysis (TMA), percent volume change (PVC) of remolded samples measured by a micrometer, interlamellar shrinkage (IS) determined by x‐ray diffraction analysis, and from field expression of soil cracking. No significant differences in COLE, PVC, IS, smectite or clay content were encountered between soils with and without vertic characteristics. Discrepancies were found between laboratory measured and field observed shrinking potential of aquic/typic/vertic Ultisols primarily due to lack of wide cracks. The poor field expression of cracking in montmorillonitic Hapludults was better explained by a combination of physical and morphological characteristics such as texture and thickness of the surface layer, size and type of structural units, solum thickness and depth to Cr, soil moisture deficit, drainage and vegetation than by variation in physical, chemical, or mineralogical properties. At present, classification of montmorillonitic Hapludults in vertic subgroups is proscribed by lack of evidence of wide or deep cracking. Modifications in the current vertic subgroup definition are suggested to account for agronomic and/or engineering limitations of these soils.