A semi‐micro routine procedure for the partial fractionation of soil phosphorus

A semi‐micro procedure for the partial fractionation of soil phosphorus compounds is described, which has been developed for certain agricultural, forest and moor soils, containing down to 0.002 mg. of P per g. of soil for particular P fractions. The procedure uses a semi‐micro adaptation of McClean's acid attack for the extraction of total P, and Williams' extractants for the initial fractionation. All extracts receive similar simple preparative treatment, P then being estimated colorimetrically by reduction of phosphomolybdate with ‘ metol’. Critical steps in the procedure are discussed in brief experimental notes, and examples of fractionation data are included to demonstrate the necessity, when comparing different soil horizons, of expressing these data on a soil‐volume basis.