Clay minerals in two ferruginous soils of southern India.

Abstract

Two Benchmark ferruginous soils namely Jamkhandi and Palathurai have been taken for this study. in contrast to the general understanding, these ferruginous soils are calcareous and alkaline in nature. Layer silicates present in silt, coarse clay and fine clay fractions of these soils are also in sharp contrast to those of ferruginous soils reported earlier. Dominant presence of smectite in all the size fractions is very common alongwith mica. Kaolinite is present in Jamkhandi soils- Under the prevailing semi-arid environment neither such a huge amount of smectite nor kaolinite can be formed. These soils have suffered truncation as evident from their thin Ap horizon followed by Bt horizon of >30 per cent clay, exposing a relatively unweathered soil formed in pre-Pliocene tropical humid climate. Smectite in fine clay fractions of Jamkhandi soils have both high and low charge density, whereas smectite in silt and coarse clay fractions have high charge. Mica present in silt and coarse clay fractions consists of more biotite than muscovite and it survived tropical weathering. In the present semi-arid climate biotite has weathered to high charge smectite. Aridity also induced the precipitation of calcium carbonate. Therefore. high amount of weatherable minerals in ferruginous soils of southern India has distinct relevance in their water and K-management. It appears that detailed clay mineralogical information should form one of the criteria for soil-site evaluation for better management of ferruginous soils of semi-arid tropical regions of Southern India.