Minimum threshold value of smectite for vertic properties

From a significant positive correlation between linear extensibility (LE) a nd the smectite content in the soil control section (SCS) of 8 soils (2 red soils, Alfisols, and 6 black soils, Vertisols, and their intergrades), the present study indicates an excellent compatibility between the marked shri nk-swell characteristics and the smectitic mineralogy. The initiation of ve rtic properties at LE of 6 in shrink-swell soils corresponded to a minimum threshold value of 20% smectite. In order to highlight the inherent relatio nship between vertic properties and the swelling minerals, the mineralogy c lass for shrink-swell soils in US Soil Taxonomy should be only smectitic.