Organo-Mineral Complexes in Some Inceptisols of India: Physicochemical and Thermal Properties

Abstract

Under natural soil systems, organic matter and mineral constituents invariably exist as organo-mineral complexes of varying stability. For the characterization of such complexes, five Inceptisols varying in altitude, vegetation, and organic matter from Himachal Pradesh, India, were ultrasonically dispersed, and three size fractions of natural organo-mineral complexes (OMC) of clay (<2 μm), fine silt (2–5 μm), and coarse silt (5–20 μm) were isolated. A portion of OMC treated sequentially with H2O2, citrate-bicarbonate-dithionite (CBD), and ammonium oxalate (pH 3) to remove complex forming components (i.e., organic matter, sesquioxides and allophanes, respectively). The natural OMCs and after removal of each of the components were characterized for cation exchange capacity (CEC), specific surface area (SSA), K-fixation capacity and thermal properties (DTA and TGA). The CEC, SSA, K-fixation capacity, and thermogravimetric weight loss decreased with increasing size of the fraction. Peroxidation and CBD treatment in general decreased CEC, SSA, and thermogravimetric weight loss and increased K-fixation capacity. The CBD treatment in general was more effective than H2O2 treatment in changing CEC and K-fixation capacity and less effective in changing the SSA and thermogravimetric weight loss. The removal of allophanes in general increased CEC and K-fixation capacity but lowered SSA and thermogravimetric weight loss in OMCs. Differential thermograms of OMCs showed three-stage oxidation of complexed organic matter with exotherms around the temperatures of 340, 436, and 510°C attributable to organic matter adsorbed on clay mineral surface, complexed with sesquioxides and with allophanes, respectively.