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

Five soils developed on lower piedmont plain (P1: 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 SiO₂/Al₂O₃ ratio of 1.13.2and SiO₂/R₂O₃ ratio of 1.0–2.7. 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 Si₃AlO₆(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.