Simulation of phosphate leaching in acid sandy soils

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Abstract
Prediction of changes in amounts of phosphate leaching from soils with changes in management practices requires a detailed knowledge of the adsorption and desorption characteristics of the soils for phosphate. Non-equilibrium soil distribution of inorganic phosphate was determined at various rates of flow from time courses of phosphate in the leachates of small columns of sandy soils sampled from the Swan Coastal Plain in south-west Western Australia. Equilibrium distribution isotherms were estimated by extrapolating to zero rate of flow. Time courses of phosphate concentrations in the leachates were also calculated with a numerical compartmental simulation model. First-order transport rate equations with time-dependent rate coefficients adequately described the experimental time courses of the leachate concentrations. Kinetic parameters were based on a consideration of the physical/chemical distribution of ortho-phosphate only. The error due to this approach was evaluated by measuring the (biochemical) fixation of phosphate in soil organic matter.