Water‐dispersible particles from six epipedons and one Bh horizon representing six different soil orders were separated into sand, silt, and clay particles, and the clay‐organic particles were further fractionated by density. Organic C and N concentrations were lowest in sand, intermediate in silt, and highest in clay, but exceptions to this trend were observed and accounted for. Light clay‐size, organo‐mineral particles had (i) high levels of sorbed organic C; (ii) wide C/N ratios; and (iii) low amounts of alkali‐extractable C. The reverse was true for heavy clay‐size particles. A continuum of particle compositions existed between the density extremes which differ for different soils. The heterogeneity of clay‐size particles can be explained in terms of their position within the soil fabric: Light particles evolve at the surfaces of (micro‐) aggregates and are exposed to the soil biosphere, whereas heavy particles occupy intra‐aggregate space shielded from the soil biosphere.
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Young et al. (1979) studied this question.