Effects of solution calcium concentration and calcium sink size on the dissolution of Gafsa phosphate rock in soils

SUMMARY The dissolution of a reactive phosphate rock (Gafsa, GPR) was evaluated in a range of UK soils which varied considerably in calcium (Ca) status and in a soil that had been amended with different amounts of a cation exchange resin (CER) to provide a Ca sink of varying size. As expected, but not apparently previously reported, increasing solution Ca concentration caused a decrease in the dissolution of GPR, consistent with the solubility product principle. At 40 d, the extent of GPR dissolution was correlated more closely with soil solution Ca concentration (r=−0.91) than with either the number of unoccupied exchange sites for Ca or the Ca-exchange capacity (CaEC) of the soil (r=0.61 and r=−0.35, respectively). Unoccupied exchange sites for Ca were only effective in influencing GPR dissolution when solution Ca concentration was sufficiently low (< 1 mmol dm−3) for the sites to be operating on the well-buffered portion of the Ca-exchange isotherm. The combined effects of an initial low solution Ca concentration and the presence of unoccupied Ca exchange sites caused the relationship between GPR dissolution and solution Ca concentration to become curvilinear, whereby the proportion of GPR dissolved at 40 d could be related to the initial soil solution Ca concentration by an exponential equation. A similar interaction was obtained for the relationship between GPR dissolution and CaEC (operating at solution Ca concentrations ≥0.6 mmol dm−3) following the addition of CER to the soil. The effectiveness of the additional sites for Ca in maintaining the product of ionic concentrations below the solubility product of GPR was such that the amount of GPR dissolved at 40 d correlated linearly and strongly (r= 1.00) with the CaEC of the soil-CER mixtures.