Modifications of the Fiske-Subbarow method are described by which susceptibility to interference by silicate, arsenate, and certain acids and salts was reduced. In view of the possibility that some soils may contain considerable amounts of extractable arsenate and ferric iron, a method for eliminating interference by these two ions, by reduction with metabisulphite, is described. By this procedure, arsenate and ferric iron up to 20 p.p.m. As2O5 and 80 p.p.m. Fe had no effects on the determination.

The applicability of the modified technique to the determination of phosphate in soil extracts and in plant ashes is examined.