Continuous application of phosphorus (P) and its subsequent loss to waterways accelerated eutrophication of surface waters. Loss of P via surface and subsurface movement is therefore an environmental concern. Vertical movement of P through the soil profile results in its contact with Bh horizons of Alaquods and Bt horizons of Paleudults which occur extensively in the coastal plain of SE United States. Compositional differences between Bh and Bt horizons result in different risk of P loss with excess P application. Environmental risk of P loss from soil horizons can be evaluated from phosphorus saturation ratio (PSR; molar ratio of P to [Al+Fe]) and soil phosphorus storage capacity (SPSC). The change point amounts to a threshold PSR value above which P runoff or leaching risk increases precipitously. SPSC is a calculation of how much P can be added to a soil before reaching this critical PSR threshold. The objectives of this study were to i) evaluate the environmental risk of P loss from Bh and Bt horizons using PSR and SPSC; and ii) determine the predictive capability of reduction in SPSC of loamy or clayey Bt samples (dominated by crystalline Fe oxides and kaolinite) using the known-P-additions approach. Results indicate water soluble P and equilibrium P concentration (EPC₀) are minimum when SPSC is positive (PSR is below the change point) for Bh and Bt samples and begin to increase when SPSC becomes zero. Bh samples are more susceptible to P loss in comparison to Bt under comparable P loadings. Loamy or clayey Bt horizons have high P retentive capacity due to greater abundance of inorganic metal oxides along with crystalline Fe and phyllosilicates. However, the components responsible for tenaciously binding P and maintaining low P solution concentrations are noncrystalline metal oxides.