The P sorption capacity of soils must be evaluated at land application wastewater sites when water quality criteria control the design of the treatment system. The soil's capacity to react with P is often underestimated when equilibrium isotherms are used to describe P sorption. An analytical one-dimensional convective disperive solute transport model, assuming linear sorption and first-order irreversible precipitation, was evaluated to describe the movement of P in small laboratory columns. The model was calibrated to experimental data using nonlinear least squares analysis. The length of the column did not make a significant difference in the model coefficients. This finding suggests that the approximation of a linear sorption isotherm with first-order irreversible precipitation is adequate for making environmental assessments at land-treatment systems. Application of the model to the design of a land-treatment system would permit approximating a system life considering wastewater application rate, concentration of P in the applied water, and travel distance through the soil profile prior to discharge. Experimental data fit to a Langmuir equation using the same soils were found to seriously underestimate the capacity of soil to react with P. The same data fit to a Freundlich or other equilibrium isotherm would yield similar results. It was concluded that use of equilibrium isotherms without considering the kinetics of the reactions will lead to overly conservative designs of land application of wastewater treatment systems.