Amorphous materials in soils under different pedogenic development stages of sub-tropical humid region of Assam

Introduction The amorphous materials in clay refers to a class of materials that are non crystalline and do not have a regular or the repeating order, internal atomic structure that is detectable by X-ray diffraction analysis. The principal forms of amorphous materials in soils are oxides and hydroxides of iron, aluminium and silica and silicates of iron and aluminium, all in various combination of water (Wada and Harward 1974). These materials commonly occur as coatings or gel-hulls on larger minerals grains and as aggregates of discrete particles in the clay fraction. These materials though often regarded as minor soil constituents, may contribute significantly to the physical and chemical properties of soils as they may have high pH dependent cation and anion exchange capacity, large specific surface area and high reactivity with anion like phosphorus (Hodges and Zelazny 1980). The mineralogy of weathering is dominated by three elements Si, Fe, and Al and at the stage of weathering of igneous rocks represented by loss of primary structure, these three elements constitute 85% of the residual metals. The consequent crystallization of these elements present in noncrystalline state may lead to the formation of crystalline clay minerals and therefore amorphous materials are long been recognized as the precursors of crystalline clay minerals (Eggleton 1987). Amorphous or non-crystalline materials thus assume significance in pedogenic development of soils. Amorphous materials in soils derived from volcanic ash have been extensively studied, but comparatively little is known about the amorphous materials in soils derived from other parent materials. The presence of appreciable amount of amorphous materials in Indian soil under tropical and sub tropical environment was reported by few researchers (Krishna Murthy et al. 1976; Gupta and Tripathi 1983; Choudhury and Dhir 1983; Seshagirirao et. al. 1992; Mishra and Ghosh 1994; Karmakar and Borah 1996; Karmakar 2012). The characterization of amorphous material in India (Karmakar and Borah 1996; Karmakar 2012; Krishna Murthy et al. 1976) and in abroad (Shoji and Abstract: Amorphous material was studied in soils of different pedogenic development stages under sub humid environment of Assam, India. Soil profiles collected from four soil orders namely Entisols, Inceptisols, Alfisols and Ultisols representing different pedogenic development stages were taken for the purpose. The amorphous material in the clay fraction (<0.002mm) varied from 10.8 to 28.77 %. The highest average amount was observed in Ultisols (25.63%) and lowest was recorded in Entisols (13.15%). The amorphous materials were siliceous in nature with molar ratio ranging from 1.33 to 3.77 for SiO 2 /Al 2 O 3 and 1.16 to 2.95 for SiO 2 /R 2 O 3 . The highest content of SiO 2 , Al 2 O 3 and Fe 2 O 3 was observed in Ultisols (10.9, 8.03 and 2.02%, Weighted mean, respectively) followed by Alfisols, Inceptisols and Entisols. The major portion of the amorphous material was composed of tetrahedral Si 3 AlO 6 (OH) 4 component (47.49 to 78.23 %). The octahedral Al(OH) 2.5 and neutral FeOOH components constituted 15.38 to 42.18% and 6.00 to 13.24% respectively. The calculated hydroxyl water content of amorphous materials was found to increase with the advancement of soil development and enhanced by deposition organic matter and acidification of soil. Agropedology 2015, 25 (01), 79-86