Phosphorus transport during transient, unsaturated water flow in an acid sandy soil

Sorbent-sorbate interactions heavily retard the movement of P during water flow in most soils. The effect of soil/solution ratio on sorption kinetics and movement of P during unsteady unsaturated water how were investigated. A series of batch experiments with soil/solution ratios ranging from 0.1 to 6.4 Mg m(-3) were conducted to obtain sorption rate coefficients. Aqueous P solutions (100-800 g m(-3)) were applied at two constant fluxes (1.4 x 10(-6) and 6.9 x 10(-6) m s(-1)) to columns of an air-dry spodic soil. The experimental data were simulated with a parallel two-site nonlinear, nonequilibrium transport model during unsteady, unsaturated water flow. Phosphorous sorption followed Freundlich-type reversible kinetics with a very fast reaction occurring in Type I sites and a slow reaction occurring in Type II sites. The sorption reaction of P in batch experiments was satisfactorily described by the model but the rate coefficients varied with the soil/ solution ratio. Experimentally determined rate coefficients described P movement in column experiments at an influx rate of 6.9 x 10(-6) m s(-1), but not for the slower influx rate of 1.4 x 10(-6) m s(-1), hence calibration of rate coefficients was necessary for describing P movement during the slower water influx. We developed a technique for determining rate coefficients under water saturated-unsaturated conditions that provides a way to validate P transport models during transient, unsaturated water flow.