Pedogenesis and clay mineralogy of semi arid tropical soil in Madurai district, Tamil Nadu

Four representative soil profiles from Melur block of Madurai district in Tamil Nadu were assessed for the degree of chemical weathering, pedogenesis and clay mineralogy. Morphological, geochemical and mineralogical analysis were done following standard procedures. Soils are classified as follows: Pedon 1- Keeranoor was Fine loamy, non-acid, mixed, megathermic, Typic Hapluestepts, Pedon 2 - Arritapatti was Fine loamy, non-acid, mixed megathermic, Oxyaquic Hapluestepts Pedon 3 - Uranganpatti was Fine, mixed, non-acid, megathermic, fluventic Hapluestepts and Pedon 4 - Panangadi was Fine loamy, non-acid, mixed megathermic, Oxyaquic Hapluestepts. The soils generally exhibited a sandy texture with moderate weathering nature, comprising more than 50% of fine and medium sand fractions, little textural variation suggesting more uniform weathering. Soils are slightly alkaline, except for Pedon 2, having had a higher Exchangeable Sodium Percentage (ESP) (> 15%) and Cation Exchange Capacity (CEC) (15 cmol (p+ ) kg-1 ). Mineralogical investigation revealed dominance of kaolinite and illite mixed with smectites in pedons and increased silica activity with limited lessivage. The mean Chemical Index of Alteration values ranged from 80.75 to 93.44, indicating moderate soil weathering. The ternary A-CN-K diagram displayed preferential leaching of CaO and Na2O followed by K2O, as plagioclase was more susceptible to weathering. Bivariate plot results of SiO2 against (Al2O3+K2O + Na2O) indicated rapid weathering influenced by the climatic conditions.