The availability of soil phosphate. III.—The phosphate‐fixing capacity of soil

Abstract

A method for measuring the phosphate fixation of soil is described, in which the soil‐phosphate mixture is evaporated to dryness and the unfixed phosphate subsequently extracted, using the Morgan reagent. In long‐term fertilizer experiments, ammonium sulphate decidedly increased the phosphate‐fixing capacity of soil. The effect of different forms of nitrogen on fixation was observed in laboratory experiments, those decreasing the pH causing increase in the fixing capacity of soil. Superphosphate reduced the fixing capacity of soil, probably through its partial saturation of the positive residual valencies of the clay complex. Potassium sulphate did not alter the phosphate‐fixing capacity of soil. Lime precipitated the added phosphate in less soluble forms and thus reduced the fixing capacity of the soil. Dung caused a marked reduction in phosphate‐fixing capacity, this effect being enhanced by application of lime. The existence of an H2PO4−‐Ca‐humic acid‐micelle linkage would explain the phenomena. Addition of iron oxide (hydrated) greatly increased the phosphate‐fixing capacity of soil. Addition of acid increased it and that of alkali reduced it, both in the absence and presence of added iron oxide. By removing free iron oxide, the fixing capacity of soil was not reduced and in some cases was further increased, suggesting that free iron oxide in soil was not the main constituent responsible for the fixation. The relative effects of exchangeable bases on the fixation of phosphate were, in decreasing order, H, K, Na, Mg, Ca. The present work supported the theory of an H2PO4−‐Ca‐micelle in soil.