CHEMISTRY OF THE PHOSPHATE ION IN SOIL SYSTEMS

Abstract

The general occurrence of Ca ++ in soil solns., the prevalence of potentially soluble Ca compounds and the low solubility of Ca phosphate suggest that aggregates of these ions are the prime sources of phosphate to plants where the above conditions obtain. Other types of soil in which PO4[long dash] - is held in the adsorbed condition are also of frequent occurrence; they usually manifest a need for phosphate fertilization. The type to which a given soil belongs can be detd. by titration with acid and alkali according to a technique described. In the Ca-dominated type, the data obtained are inconclusive of fertilizer requirement because of secondary reactions with (non-phosphated) Ca complexes. In such so'' s good correlations between solubility and fertilizer neerds were obtained from water extracts with a wide ratio of water to soil. Better still, water displacements showed that the solns. obtained, after all of the soil soln. was removed, increased substantially in all soils which do not require phosphate fertilization, whereas such increases were negligible or lacking in physiologically deficient soils. These increases of phosphate are shown to be inversely related to concurrent changes in Ca + + cone. These correlations appear to indicate that the release of phosphate from the solid phase of Ca-dominated soils is primarily due to fluctuations in the proportions of Ca to phosphate in the soil-plant interphase, induced by the plant''s absorption of those ions. Peat soils comprise a 3d category from the standpoint of phosphate nutrition. Data from a "fen" type of peat indicated that release of phosphate from the solid phase must be due to a different mechanism than that discussed above, inasmuch as the acid-soluble phosphate is inconsiderable and there is so much Ca + + (and Mg + +) that the plant''s absorption could not substantially change the relation of these ions to the PO4 present in the liquid phase.