Interactions between Organic and Inorganic Components in Particle‐Size Fractions Separated from Four Soils

Little information is available in the literature on the mechanism(s) of interaction of organic matter (OM) and inorganic crystalline and noncrystalline components in particle‐size fractions. To obtain a better understanding of this subject, five particle‐size fractions, ranging from coarse silts to fine clays, were separated from two soils from eastern Canada (Armadale and Bainsville) and from two prairie soils (Melfort and Regina). Each size fraction was characterized by chemical and mineralogical methods. Humic acids (HAs), fulvic acids (FAs), and humins were found to accumulate in size fractions ranging from medium silts to coarse clays, while aliphatics were concentrated in fractions from medium silts to fine clays. Amino acids tended to be enriched in clay fractions of the two prairie soils, but accumulated in the silt fractions separated from the two soils from eastern Canada. Except for one soil, NH 3 ‐N accumulated in the fine silt and finer fractions, which were rich in expandable phyllosilicates. In general, amounts of OM, aliphatics, and NH 3 ‐N tended to accumulate in fractions rich in noncrystalline inorganics. Noncrystalline inorganics separated from the two prairie soils were rich in Si, which appeared to contribute to the preferential accumulation of humin, HA, and neutral amino acids. On the other hand, noncrystalline components from the two soils from eastern Canada were rich in Al, which may have been associated with the accumulation of NH 3 ‐N and acidic amino acids in these soils. The pH‐dependent charges of noncrystalline inorganics may also have contributed to the accumulation of OM.