Mineralogy Class of Calcareous Zeolitised Vertisols

There is an incompatibility between marked shrink-swell characteristics and mineralogical classification of Vertisols in U.S. Soil Taxonomy. It was stated that it is the smectite mineral alone which governs the vertic characteristics of soils and therefore mineralogy class of Vertisols should only be smectitic to the tune of minimum 20% in the clay (<2 μm) fractions. Besides smectitic mineralogy class, U.S. Soil Taxonomy provides a carbonatic mineralogy class for Vertisols, if the content of CaCO₃ plus gypsum exceeds 40% in the <20 μm size fraction. Soil Taxonomy, however, does not offer explanations to justify this (carbonatic) mineralogy class except a mention of CaCO₃ by weight. The present study provides pedo-edaphic justification of retaining smectitic mineralogy class in the light of natural soil degradation in terms of formation of CaCO₃ in these soils vis-à-vis crop performance, ensured by the presence of natural soil modifiers like zeolites.