Pedogenesis in Some Subaqueous Soils of Brahmaputra Valley of Assam

Soils developed in the low lying areas of floodplain (bils) beyond natural levees were examined near Nimatighat and Kakilamukh of Brahmaputra valley in Assam to infer pedogenesis with respect to landscape position. The bil environs in the valley have channel bars in the midst of Brahmaputra river, swamps adjoining to the southern bank and flood plains over bils. These soils exhibit light grey to light brownish grey matrix, yellowish brown to reddish distinct mottles with high chroma (≥4) and abrupt variations in texture. These soils are slightly acid to slightly alkaline with irregular distribution of organic carbon, calcium carbonate, total and pedogenic iron. The mass balance of pedogenic iron showed considerable gain in the soils of floodplain but losses in soils of bils. The dominant pedogenic processes identified include sedimentary laminations, ferrolysis, development of acidity and weak structural development in B horizons. The soil morphology exhibits intensely expressed site-specific redoximorphic features related to degree of saturation. The pattern of iron distribution expressed through diagnostic redoximorphic features has been found to be an ideal tool to delineate wetlands. These soils were classified upto subgroup level as Typic Fluvaquents (Kakilamukhghat, Barchala bil and Baraligaon), Aeric Fluvaquents (Gohaingaon) and Typic Endoaquepts (Dangari bil and Rangadaijan).