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Phosphorus Transport during Transient, Unsaturated Water Flow in an Acid Sandy Soil

Dynamac Corp., 3601 Oakridge Blvd., Ada, OK 74820

R. S. Mansell and P. Nkedi-Kizza

Soil and Water Science Dep., Univ. of Florida, Gainesville, FL 32611

B. A. Burgoa

Univ. of Tennessee, Knoxville, TN

*Corresponding author (chen{at}soilwater.agr.okstate.edu)

ABSTRACT

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 flow 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.

Florida Agric. Exp. Stn. Journal Series no. R-04647.

Received for publication July 28, 1994.

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