The amt. of exchangeable K in the surface soils of the Corn Belt is directly related to the ability of the soil to supply K to crops. This relationship can be expressed by the modified Mitscherlich equation as follows: Log(A-y) = Log A[long dash]c1b1 where A = yield in bushels when K is not deficient, y = yield obtained when b1 = amount of exchangeable K in surface soil and c1 = proportionality constant. The above relationship holds with approximately the same value for c1 where many physical and chem. soil properties vary within a rather wide range and where the ultimate yields under full treatment also vary considerably. The soil test for exchangeable K described in this paper measures the total exchangeable K with the accuracy necessary to establish the above relationship. Through the medium of soil-testing the above relationship can be applied in a practical way to each farm field. The values of c1 obtained for corn, legumes, soybeans, and wheat do not confirm Mitscherlich''s original idea of a constant c value for all crops.