Attention was given to the following points: (a) adaptability of various strains of A. niger; (b) maintenance of stock cultures; (c) influence of reaction, various salts, acid radicals, and minor soil constituents and stimulants; (d) application of the method; and (e) comparison of results with other methods. Two strains of A. niger gave equally satisfactory results. The first strain was chosen because of its wide use by Niklas and others. Various Ca, Mg, and Na salts, acid radicals of K salts, and minor soil constituents and stimulants did not influence the growth of the mold sufficiently to interfere seriously with the results. The method can be applied to the quantitative determination of available K, and the results compare quite favorably with the chemical method, the Neubauer method, Mitscherlich''s method, and field tests. The test is simple, reliable, and may be used in a practical way.