Understanding phosphorus (P) sorption of calcareous soils is important for the development of successful fertilizer and manure management practices. This study was conducted to identify soil chemical properties controlling P sorption in semiarid calcareous soils of the Pacific Northwest. Sorption isotherms of 18 primarily calcareous soils ranging widely in soil physical and chemical properties were constructed by equilibrating 4 g of soil with 40 mL of 0.01MCaCl2containing between 0 and 700 mg P L−1for 24 h. The P sorption isotherms at low to medium P concentrations fit the Freundlich isotherm (r2≥ 0.93). The slope of the isotherm generally increased abruptly at high P concentrations, suggesting Ca-P precipitation. The maximum P sorption prior to Ca-P precipitation was closely related to organically complexed Fe and Mn (R2> 0.98), suggesting that such complexes may regulate P sorption in these soils. The equilibrium P concentration at the point where Ca-P precipitation begins to dominate was closely related to pH and organic carbon. A better understanding of the role of organically complexed metals in controlling P sorption in calcareous soils is needed to determine how P solubility is affected by organic matter additions and how this will ultimately impact the plant availability and potential off-site transport of P from these soils.