B‐Fabric of some benchmark Vertisols of India in relation to their mineralogy

Abstract

SUMMARYAt present precise information on the b‐fabric of Vertisols is inadequate for gaining a comprehensive knowledge about such soils formed in alluvium of basic igneous and metamorphic rocks and spread in varying agro‐climatic zones of peninsular India. The aim of the present study, on five benchmark Vertisol series (Sarol, Aroli, Nimone, Bellary and Kovilpatti), was to assess the possible reasons for differences in the b‐fabric of the subsoils despite their common shrink‐swell properties, as evidenced by the presence of sphenoids and/or slickensides, and similar clay contents and amounts of fine clay smectite. In Sarol and Aroli soils from the sub‐humid region, the b‐fabric is porostriated, whereas in Nimone and Kovilpatti soils of the semi‐arid and Bellary soils of the arid regions the fabric is either mosaic‐speckled or granostriated, indicating weak plasma separation, a result of restricted swelling of clays. The related distribution pattern of the s‐matrix in all these soils was open porphyric.Recent literature has suggested that weak plasma separation is an effect of dissolution and crystallization of calcite. However, generally low concentrations of soluble calcium (c. 1 mmol dm−3) in soil solutions of Vertisols in arid and semi‐arid agroclimates suggest that the presence of calcite does not guarantee the presence of sufficient soluble calcium to impair swelling of clays by contracting the diffuse double layer. Weak plasma separation in Bellary and Kovilpatti soils compared with Sarol and Aroli soils is explained in terms of a decrease in the internal surface area of fine smectite, as evident from a very high degree of chloritization in smectite interlayers, and smaller proportions of coarse smectite. In Nimone soils the weak plasma separation is primarily due only to a decreased internal surface area of fine smectite.