The Solubilities of Carbonates and Phosphates in Calcareous Soil Suspensions

Previously reported discrepancies between measured soil carbonate and phosphate activity products and the solubility products of pure crystalline minerals may, in part, be attributable to the incomplete characterization of ion‐pairs in solution. Up to 50% of the soluble PO 4 and significant amounts of Ca, Mg, and SO 4 were present as ion‐pairs. Ignoring ion‐pairs resulted in significant differences in calculated activity products for both carbonates and phosphates. One soil was undersaturated and two were supersaturated with respect to calcite. Calcite was the only carbonate mineral identified by X‐ray diffraction measurements. Calcium carbonate activity products were highly correlated to Mg activities. Magnesium‐calcite apparently controlled the solubility of soil carbonate. The soil phosphate activity products fell between the solubility products of octacalcium phosphate and hydroxyapatite. The phosphate solubility data were compatible with, but not proof of, the presence of calcium phosphate minerals in soils.