An experimental investigation was made to determine the mechanical radiation impedance of a rigid piston on the surface of clay soil. The force on and the velocity of the piston were measured to determine both the real and imaginary parts of the radiation impedance as a function of excitation frequency of the piston. Measurements were made for six different combinations of moisture and density of the soil, and results of these measurements are presented in graphical form. In the frequency range 50–200 cps, the load presented to the piston by the soil is essentially the same as would be presented by a mechanical compliance, the value of which varies with both moisture and density of the soil. The value of this compliance was computed for six different combinations of moisture and density. Second‐order dependence of compliance on moisture and density was assumed, and coefficients in a second‐order equation relating compliance to moisture and density were computed. The three‐dimensional surface that represents this equation is presented in graphical form. Errors in the results are discussed.