The movement and uptake of P in soils occur primarily in the soluble phase, so that the reliable simulation of P movement and uptake requires that the concentrations of soluble P forms be explicitly represented in mathematical models. To represent soluble P concentrations under dynamic boundary conditions, a convective‐dispersive model of P transport has been coupled to a model of P transformation in which adsorption‐desorption, precipitation‐dissolution, and ion pairing are explicitly represented as concurrent equilibrium reactions. This model is used to explain the temporal and spatial distribution of P among soluble and resin‐, NaHCO 3 ‐, NaOH‐, and HCl‐extractable fractions in soils following amendment with KH 2 PO 4 . Simulated reductions in soil pH following different P amendments caused solid‐phase P in the model to be recovered more from resin‐ and NaOH‐extractable forms and less from HCl‐extractable forms as solution P concentration increased. These changes were consistent with those observed experimentally using a P fractionation procedure on a Malmo silt loam (Typic Cryoborall) following its equilibration with 0 to 512 mg L ‐1 of KH 2 PO 4 and following its irrigation for 205 d with 50 mg L ‐1 of KH 2 PO 4 . Simulated displacement of cation coprecipitates from exchange sites allowed the model to reproduce the temporal and spatial patterns of water‐ and HCl‐extractable P in resin columns of different cation‐exchange capacities following a KH 2 PO 4 surface amendment. The results of model testing suggest that changes in soluble P concentrations following P amendments may be represented from concurrent equilibrium reactions for adsorption‐desorption, precipitation‐dissolution, and ion pairing. However, the rate at which these reactions proceed remains uncertain.
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Grant et al. (1997) studied this question.