Genesis and Taxonomy of Black Soils from Basalt and Basaltic Alluvium in Rajasthan

Six typical pedons representing black soils, two (P1 & P2) developedfrom basalts and the other four (p3 to P6) developed from basaltic alluvium in Rajasthan were selectedfor this purpose. Soil colour was found to be the function of free iron oxide to clay ratio while vertic properties were found to be governed by free iron to total iron ratio. The irregular distribution of organic carbon and regular depthwise function of CaCO₃ indicate differential pedoturbation with respect to these constituents. Fe₂O₃ and MgO content of the soils from basalts were more than the soils from basaltic alluvium while SiO₂, Al₂O₃ and K₂O content showed a reverse trend. The weathering indices, like lower Al₂/Fe₂O₃, CEC, SiO₂/R₂O₃ and K₂O/Na₂O molar ratios in clays and CEC of clay/clay ratios, reflected advanced stage of weathering in basaltic soils compared with their basaltic alluvial counterpart. CEC/clay ratio was found to decrease with the increase in Fe₂O₃ content of clays. Taxonomicallythese soils were classified under Chromic Haplusterts and Typic Haplusterts subgroups of Vertisol soil order.