PHOSPHORUS TRANSPORT DURING TRANSIENT, UNSATURATED WATER-FLOW IN AN ACID SANDY SOIL

Sorbent-sorbate interactions heavily retard the movement of P during w ater 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 ra te 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 column s 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 Freund lich-type reversible kinetics with a very fast reaction occurring in T ype I sites and a slow reaction occurring in Type II sites. The sorpti on reaction of P in batch experiments was satisfactorily described by the model but the rate coefficients varied with the soil/ solution rat io. Experimentally determined rate coefficients described P movement i n column experiments at an influx rate of 6.9 x 10(-6) m s(-1), but no t for the slower influx rate of 1.4 x 10(-6) m s(-1), hence calibratio n 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 provid es a way to validate P transport models during transient, unsaturated water flow.