An examination of the use of ammonium fluride as a selective extractant for aluminium-bound phosphate in partially phosphated systems

Abstract

The release of phosphate from aluminium-bound, iron-bound, and calcium-bound phosphates by the solutions proposed by Chang and Jackson (1957) and by Fife (1959b) for the estimation of aluminium-bound phosphate in soils was examined. Release was studied in the presence and absence of iron and aluminium oxides. Ammonium fluoride solutions released most of the aluminium-bound phosphate and in addition variable amounts of the other two types of phosphate. The proportion of these phosphates extracted by ammonium fluoride was decreased by the presence of iron oxides and, in certain cases, by the presence of aluminium oxides. It is suggested that many reported estimates of aluminium-bound soil phosphate obtained using the procedures of Chang and Jackson and of Fife are in error. Examples of how these errors arise are discussed. The practice of equating ammonium-fluoride extractable soil phosphate to the aluminium-bound phosphate content of the soil does not appear valid.