A method has been devised for reproducibly separating from soils minimally altered, clay‐size soil organic‐mineral particles (SO‐MPs) by water dispersion and density centrifugation. Active surfaces of SO‐MPs so obtained should represent more closely those actually interfacing with root systems and microbes than the chemically cleaned clay or humic materials conventionally obtained in laboratory prepartions. The method reduces heterogeneity within given size fractions of SO‐MPs, which is desirable for studying relationships between composition and properties of minimally altered soil “fines”.