Abstract: Phosphorus (P) transported from agricultural fields can be a major contributor to eutrophication of aquatic systems. Alaquods and Paleudults with subsurface Bh and Bt horizons, respectively, occur extensively in the coastal plains of southeastern United States. Environmental risk of P loss from soil horizons can be evaluated using P saturation ratio ((PSR) molar ratio of P to (Al + Fe)) and soil P storage capacity (SPSC) concepts. The change point (zero SPSC) amounts to a threshold PSR value above which P runoff or leaching risk increases. The objective of this study was to compare P release behavior using PSR and SPSC and relate P release to the compositional differences among Alaquod and Paleudult subsurface horizons. Results indicate that most Bt horizons have high SPSC and minimal equilibrium P concentrations (EPC0). Phosphorus retention at environmentally relevant solution concentration was controlled mainly by noncrystalline metal oxides despite relative abundance of crystalline components. The Bt samples have high Langmuir adsorption maximum Smax caused by the relative abundance of crystalline components that contribute to P sorption at higher solution concentrations (above the threshold PSR); however, the risk of P loss is most closely tied to noncrystalline metal oxides, as observed in other sandy soils of Florida. Therefore, the PSR and SPSC concepts originally developed for sandy soils are likely applicable to loamy or clayey horizons of coastal plain soils. Furthermore, the relationship of PSR and SPSC to EPC0 suggests that these parameters can be used to predict EPC0 obtained through the generation of time-consuming Langmuir isotherms.