Organo-mineral complex status in soils. II. thermal nature of organo-mineral complex particles and their humic substances

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Abstract
Organo-mineral complex particles were isolated from podzols, brown and yellow-brown forest soils, red soils and volcanic ash soils according to their particle -size and density. They were studied mainly by thermal analysis (DTA and DTG). Organo-mineral complex particles of silt-size, black and brown in color, as well as of clay-size were found in the soils. The black particles were characterized by an exothermic reaction at about 400–450°C in the DTA, and the clay-size particles, by a reaction at about 300°C. These exothermic peak temperatures increased in response to the removal of the inorganic constituents in the particles by extraction with acid solutions. It is suggested that the aromatic parts of the humic substances as well as the peripheral functional groups play an important role in the organo-mineral combinations. The thermal nature of the HF-HC1 treated organo-mineral complex particles showed that the humic substances in the clay-size complex particles had both aromatic and aliphatic compounds in their molecules, while those in the black silt-size complex particles had fewer aliphatic compounds.