Soil algal bioavailable P (ABP) is the P fraction that contributes most directly to eutrophication in freshwaters affected by agricultural nonpoint-source pollution. There are uncertainties regarding the algal bioavailability of P sorbed to calcareous glacial till soils in the upper Midwest. The ABP of soil samples with a broad range of pH and calcium carbonate content from six sites across the Minnesota River Basin (MRB) was measured by algal bioassay, and relationships with various soil physical and chemical properties were studied. For soils of the MRB, a major agricultural watershed in the upper Midwest, ABP was significantly correlated (p < 0.001) with Bray P, Mehlich-III P, NaOH P, Oxalate P, and Fe-paper P. Among them, Fe-paper P approximated ABP best, particularly for calcareous soils. For acidic soils, amorphous Fe and Al apparently were the primary P retention agents in soil particles. Soil sorption data were well described by the linearized Langmuir sorption model. Although the P sorption maximum (Γ∞) could not be predicted from basic soil physical and chemical properties, the sorption energy constant (b) was highly correlated with soil pH, clay content, and organic matter (OM) content. A P saturation index (PSIs) that uses sorptivity (Γ∞ X b) as the measure of sorption capacity gave the best estimate of soil ABP among the predictors used in this study. Phosphorus saturated index itself can be approximated by the widely available Bray-P value for soils in the MRB.