Mineralogical Transformation in Benchmark Soils of Prakasam District, Andhra Pradesh as Influenced by Weathering and Pedogenic processes

The mineralogical analysis of fine sand, silt and clay fraction at different physiographic division of soils of Prakasam district was undertaken to identify the dominant minerals present in the sand, silt and clay fractions and also for understanding the rate of weathering in different soil orders developed over quartzite, sandstone and shale landscapes. The area lie between 15° 30’ to 16° 00’ N latitudes and 78° 45’ to 79° 30’ E longitudes and represented by five major physiographic divisions. The soils of hill side slope and undulating pediments are shallow, developed over quartzite and sandstone and are classified as Typic Ustorthents. Soils of upper pediplains, lower pediplains and stream bank are moderately deep to very deep, developed over quartzite, sandstone and shale landscapes and are classified as Typic Haplustepts, Typic Natrustalfs and Typic Haplusterts. Mineralogical analysis of clay fraction indicates that soils developed over quartzite and sandstone are mostly dominated by mica and kaolinite group of minerals whereas soils developed over shale are dominated by smectite group of minerals. Weathering Index clearly indicates that the rate of weathering in Vertisols was higher than Entisols, Inceptisols and Alfisols. Interestingly, the rate of weathering in Alfisols was lower than Entisols and Inceptisols. It was due to presence of exchangeable cations as it reduces the rate of weathering. The sand and silt fractions among soils orders were mostly dominated by quartz, mica, K, Na and Ca-feldspars and trace amount of heavy minerals respectively.