More uniformity in methods of deriving fertilizer P recommendations from crop response data should improve accuracy and precision of fertilization rates. Experimental data that relate crop yields to soil test levels and describe the effect of fertilizer P on soil test levels provide the basis for determining fertilization rates for specific crop-soil situations. A modification of the Mitscherlich equation was used for the derivation of a new equation for calculating fertilizer P requirements as a function of soil test levels of P. The equation was applied to response data for 4 crops. Response curves and fertilizer requirements, as calculated for corn, soybeans, alfalfa, and clover grass, indicated that soybeans yielded relatively more than the other 3 crops at low soil test levels of P. Corn and alfalfa required higher soil test levels to reach 95% maximum yield and required higher rates of fertilizer P when initial test levels were low.