Morphological and Physicochemical Properties of Some Alluvial Soils of Assam

An examination of morphological features and physicochemical properties of five profiles of alluvial soils of Brahmaputra and Surma valleys of Assam has shown that upland levelled soils are devoid of mottlings and concretions and are moderately developed. Soils (Nowgong and Gauhati) affected by high water-table resulting in the formation of iron concretions and mottlings are weakly developed. Profiles of Brahmaputra valley indicate development of a weak argillic horizon. Surma valley (Cachar region) soils, which remain submerged for most part of the year, do not have iron concretions and show minimum horizon differentiation.

All the soils are acidic and are rich in organic carbon. Cation exchange capacity of Cachar soils is not high in proportion to their clay and organic matter contents probably due to inactivation of exchange sites by clay-humus complex and adsorption of organic matter. Illite is the dominant clay mineral with chlorite and/or kaolinite as a minor phase. Mineralogical composition of fine sand fractions does not show much variation due to similar composition of parent rock, which seems to be mica-schists.