Characterization and Classification of Soils of Tipukjan Watershed of the Upper Brahmaputra Valley of Assam

Abstract

Nine soil profiles collected from low hill (P1), piedmont plain (P2, P3, P4 and P5) and alluvial plain (P6, P7, P8 and P9) of Tipukjan watershed of Assam were studied for characterization and pedogenic development. The soils varied considerably in morphological, physical and chemical properties. The dominant hue of soil colour was 10YR, the value varied from 2 to 7 and chroma varied from 1 to 8. The soil texture ranged from loam to silty clay loam in the surface and loamy sand to clay in the sub-surface horizons. The structure of the soils was subangular blocky except in the surface horizons of rice cultivated soils, in C horizons and in all horizons of P7 where the structure was massive. Red (2.5YR 5/6) to yellowish brown (10YR 5/6) mottles were observed in all the soils except P2 and P6. Thin patchy clay cutans and few Fe-Mn glaebules were observed in the sub-surface horizons of soils except P2, P7 and P9. The organic carbon (OC) content in the soils varied from 1.5 to 17.1 g kg-1. The soils were acidic in nature (pH 4.1 to 5.4). Exchangeable Ca2+ was the dominant basic cation followed by exchangeable Mg2+, Na+ and K+. The cation exchange capacity (CEC) of soils ranged from 7.11 to 16.15 cmol(p+)kg-1 and the base saturation varied from 20.7 to 57.9%. The soils were classified as Ultic Hapludalfs (P1), Typic Dystrudepts (P2), Oxyaquic Hapludults (P3), Typic Endoaquults (P4), Aeric Endoaqualfs (P5), Oxyaquic Hapludalfs (P6 and P8), Typic Endoaquents (P7) and Ruptic-Alfic Dystrudepts (P9). The mean relative horizon distinctness (RHD) value was minimum in P7 and maximum in P9. Relative profile development (RPD) values of a soil profile increased in the order: Entisols (RPD 12) < Inceptisols (RPD 19-40) < Ultisols (RPD 31-35) < Alfisols (31-54). The profile weighted mean of exchangeable Al3+, exchangeable acidity and Al-saturation increased with increasing soil development.