Isotherm parameters (KL, KF, and KD) can be predicted for all major Spodosol and Ultisol horizons using the predictive equations originally developed from PSR and SPSC values obtained from soil test extractions of surface and sub-surface horizons of Ultisols. These predicted parameters can be used in subsurface P transport modeling, averting the need for time-consuming P adsorption isotherms and enabling more thorough site-specific evaluation of P loss risk. The fact that the Ultisol model worked also for Spodosols, a compositionally contrasting soil type, suggests that these equations might be applicable across a wide range of soils. The SPSC is itself a simple model that could predict P loss from each horizon and from a soil profile to a designated depth.