Equilibrium mixtures containing water, synthetic hydroxyapatite and phosphate-free saturated single-base clays were prepared to determine the solubility of phosphate in absence and in presence of some sodium and calcium salts. The solubility is affected by the nature of exchangeable cations and increases in the series Ca < Mg < K < Na. The presence of sodium chloride, nitrate or sulphate increases the solubility with Ca-or Mg-clays, but decreases it slightly with K-or Na-clays. The corresponding calcium salts have a slight effect with Ca-or Mg-clays, but decrease the solubility considerably with the K-or Na-clays.Sodium carbonate decreases the solubility in all cases and calcium carbonate gives the lowest solubility.-4n explanation is proposed to account for the variation in phosphate concentration in the different systems.
IntroductionIt is well known that apatites are the most widespread reserves for soil phosphorus. Although a relatively large proportion of the applied soluble phosphate fertilisers revert ultimately to an apatite in neutral soils, they may continue to exhibit a beneficial effect. It is not surprising, therefore, that the practice of applying phosphatic fertilisers as insoluble phosphates is rapidly spreading. Many field experiments have been carried out to investigate the efficiency of phosphate rock as a fertiliser under various conditions. The availability of phosphorus was found to depend on pH of the soi1,l the lime level,2 the organic matter ~o n t e n t ,~ and the presence of some salts4 The interpretation of the results, under field conditions, is rather difficult because of the complexity of the soil system and the accompanying impurities in the natural phosphate rocks.The present study is an attempt to investigate, on a laboratory scale, the solubility of phosphate in the system hydroxyapatite-clay, with special reference to the effect of exchangeable cations and some sodium and calcium salts which are usually found in alkaline and calcareous soils, The information obtained from the results may be correlated with conditions under which phosphate rock is used as a phosphatic fertiliser in these soils.
