A Simple Model to Describe the Dissolution of Phosphate Rock in Soils

Dissolution of phosphate rock (PR) in contrasting soils was evaluated by extraction with 0.5 M NaOH following a prewash with 1 M NaCl to remove exchangeable Ca 2+ . This provides a simple and direct method for measuring the rate and extent of PR dissolution in soils. Dissolution of Sechura phosphate rock (SPR) in six soils was essentially complete at 90 d and the pattern of dissolution could be described by a modified Mitscherlich equation of the form y = A (1 − e ‐ex ), in which y = amount of SPR dissolved at time x ; A = asymptote, and c = curvature coefficient. Whereas A varied markedly across soils, c was independent of soil type. This exponential equation formed the basis of a simple model which describes and predicts the dissolution of SPR in soils. By establishing the relationship between A and a range of properties for 30 contrasting soils it was possible to identify those soil parameters that controlled PR dissolution in soil. Percent Ca‐saturation, P‐sorption capacity, and Ca‐exchange capacity of the soil were the three most important parameters influencing SPR dissolution in soils. When a model incorporating these three parameters was tested on soils not used to construct the model, the variance accounted for ranged from 66 to 76%, depending on the population of soils selected. These parameters determine the concentrations of Ca 2+ and H 2 PO ‐ 4 in the soil solution.