Forms and Distribution of Pedogenic Iron, Aluminium and Manganese in Some Benchmark Soils of West Bengal

Distribution of dithionite and oxalate extractable Fe, AI and Mn in some Benchmark soils of West Bengal occurring in Indo-Gangetic alluvial plain and red and laterite ecosystem were studied. The soils of both ecosystems showed variation in soil morphology and physico-chemical properties due to variations in physiographic position and composition of parent materials. The dithionite extractable iron (Fe_d) content in all the soils under red and lateritic ecosystem (Alfisols) increased usually with depth (0.64 to 1.75%) showing highest value in subsoil (Bt horizons). On the contrary, the amount of Fe_d in the soils of Gangetic alluvial plain representing Inceptisols and Entisols was low (0.26 to 0.93%) and did not show any definite trend with depth, whereas AI_d followed the increasing trend with the increase in depth. The content of Mn_d did not show any noticeable trend in their distribution in the soils under study. The oxalate extractable iron (Fe_o) was highest in surface layer of most soils while the Mn_o values showed a reverse trend increasing down the profile. Higher values of Al_o are observed in the clay enriched (Bt) horizon of soils of red and laterite ecosystem in comparison to the soils of Indo-Gangetic plains. The “active Fe ratio“ (Fe_o/Fe_d) in Alfisols occurring in red and laterite ecosystem decreased with depth indicating the presence of higher proportion of iron oxides as crystalline form in the lower horizons, while this was not true in Inceptisols and Entisols of Indo-Gangetic plain. The active AI and Mn ratio did not follow any regular trend in the soils of both the ecosystems.