Soil Phosphate Solubility

Abstract

THE inorganic phosphate content of agricultural soils is very low (about 0.02 per cent phosphorus), and attempts to determine the nature of the phosphate compounds are beset with many difficulties. X-ray and optical methods cannot be used without first concentrating the phosphate fractions, and the process of concentration generally brings about changes of phosphate form. Methods based on chemical extractants effect a more or less complete redistribution of the soil phosphate compounds during the actual extraction and, in any event, do not act specifically on single phosphate components. Recent approaches to the problems have used solubility methods, in which attempts have been made to relate soil phosphate solubility of intact soil to the solubilities of known pure phosphate compounds. Based on experimental figures for the solubility products of pure phosphates, criteria have been proposed for assessing the existence of basic calcium phosphates, and iron and aluminium phosphates in soils¹. However, the application of these criteria to experimental determinations of soil phosphate solubility² has not been wholly successful³. Thus, while the results for some soils obey the relationships for pure compounds, others do not³, and the proposition that these can be attributed to substances of intermediate composition is not wholly satisfactory.