ABSTRACT
240The lateritic soils of Malabar region of Kerala have a strong geographical association with varying degree of chemical weathering in relation to rainfall and lithology. In the study, four representative ferruginous soils on lateritic landscape developed over Precambrian rocks in parts of Kottayam of Kerala were studied to understand the weathering pattern and genesis through geochemistry. These soils are strongly to moderately acidic, reddish brown with low Cation Exchange Capacity (CEC) and base saturation having SiO2 – 33 to 57%, Al2O3 – 16–31%, Fe2O3 – 8 to 15% and TiO2 – 0.7 to 1.4%. The Kanjirapalli (P3) and Athirampuzha (P4) soil series were more intensely weathered as compared to the Kinalur (P1) and Chingavanam series (P2) with silica to alumina-iron ratio less than 2 and had a significant negative relationship with CIA (R = –0.75**), Harnois index (R = –0.678**), Richie index (R = –0.953**) and Plagioclase Index of Weathering (R = –0.705**). The Si-Al-Fe ternary diagram showed strong kaolinization process associated with dark red subsoils indicating well drained conditions. The Kanjirapalli and Athirampuzha soils have silica to sesiquioxide ratio less than 2 indicating lateritic nature with low bases to Al ratio in control section. The trace elemental concentration ranges were above the values of world soils having an enrichment index more than 1 in Kanjirapalli series (P3) and Ni contamination in genetic horizons (Ni>200 μgg-1). The cluster analysis showed similar major oxide concentration pattern in Group – 1 and Group – 2 but varied in trace elemental pattern with Cr>Ba>Cu in Group – 1 and Cu>Cr>Ba in Group – 2 soils whereas Zr>Ni>Mn in Group – 3 to Ni>Mn>Zr in Group – 4 soils. The study further showed that differential rates of weathering in soils under tropical climate were further accelerated due to anthropogenic activities such as improper land use practices and deforestation on steep slopes.
