A simple model for predicting the rates of dissolution of sparingly soluble calcium phosphates in soil

SUMMARY A simple model is developed for predicting the rates of dissolution of particles of sparingly soluble calcium phosphates in acid soils. The model comprises analytical solutions to equations describing the steady‐state diffusion of phosphate and hydrogen ions in soil. The concentrations of the ions in solution at the boundary between soil and mineral are found from the solubility product of the calcium phosphate and by equating the fluxes of the ions across the boundary. The diminution of the mineral particles as they dissolve, and the effect of neighbouring particles on each other, are allowed for. The model's predictions for dicalcium phosphate agreed very well with those of the more complex model, tested experimentally (Kirk & Nye, 1986a), but only one‐thousandth of the computer time was necessary to simulate equivalent conditions. he extension of the model to predict the rates of dissolution of the apatites in rock phosphates is described, and examples of the predicted rates of dissolution are given.