Amorphous ferri-aluminosilicates in soils developed on different landforms in lower Brahmaputra valley of Assam

摘要

Five soils developed on lower piedmont plain (PI: Ruptic-Ultic Dystrudepts), alluvial fan plain (P2: Aquic Udipsamments), alluvial plain (P3: Umbric Dystrudepts), flood plain (P4: Oxyaquic Udifluvents) and monadnock (P5: Typic Hapludults) in lower Brahmaputra valley zone of Assam were studied for amorphous ferri-aluminosilicates (AFAS). The amount of AFAS in clay fraction of soils varied from 10.1-38.1 per cent with SiO_2/ Al_2O_3 ratio of 1.1-3.2 and SiO_2/ R_2O_3 ratio of1.0-2.7. The highest amount of AFAS (30.7-38.2%) was found in soils developed on monadnock (P5) and the lowest amount (10.1-12.4%) in soils developed on piedmont plain (P1). The Bt horizons contained the highest amount of amorphous material within the profile. The major portion of AFAS was composed of tetrahedral Si_3AlO_6(OH)_4 component followed by octahedral Al(OH)_(2.5) and FeOOH components. The hydroxyl water content of the amorphous material lies in the range of 16.4 to 21.1 per cent with an average of 18.5 per cent. The variation of hydroxyl water content was very narrow (18.1-18.4%) in more developed soils (Typic Hapludults) on monadnock.