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A Simple Model to Describe the Dissolution of Phosphate Rock in Soils¹

ABSTRACT

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 Ca²⁺. 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 Ca²⁺ and H₂PO^-₄ in the soil solution.

¹ Research supported by the Fertilizer and Lime Research Centre and the Massey Univ. Agricultural Research Foundation, Massey Univ., Palmerston North, New Zealand.

² Former Postgraduate Research Assistant, now at Grasslands Div., Dep. of Scientific and Industrial Research, Private Bag, Palmerston North; Professor, Senior Research Officer, and Reader, respectively, Dep. of Soil Science, Massey Univ., Palmerston North, New Zealand.

Received for publication March 22, 1985.