Composition arid properties of organo-mineral complexes of four cultivated soils(black soils,albic soils,brown earth and soda-solonchaks) were studied by ultrasonic dispersion method.Results showed that the content of <10μ varied greatly with organic matter content in the soils,e.g.,that in black soils was two times as much as that in brown earths and soda-solonchaks.