A modified molybdenum-blue method is described, with stannous chloride as the reducing agent, for determining phosphate in soil solutions and soil extracts; ferric iron in amounts up to 15 p.p.m. had no ill effect.

The influence of varying concentrations of sulphuric acid, ammonium molybdate and stannous chloride on the formation of phosphomolybdenum-blue complex and the interference due to silicate, arsenate and ferric iron were examined.

A study of the effects of various ions showed that ions commonly occurring in soil solutions and some soil extracts did not affect the accuracy of direct determinations of phosphate, but arsenate should be absent from the test.