Phosphate sorption by Pacific Northwest calcareous soils. Soil Sci

Understanding phosphorus (P) sorption of calcareous soils is impor-tant 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 North-west. Sorption isotherms of 18 primarily calcareous soils ranging widely in soil physical and chemical properties were constructed by equilibrat-ing 4 g of soil with 40 mL of 0.01M CaC1 2 containing between 0 and 700 mg P 1.- 1 for 24 h. The P sorption isotherms at low to medium P con-centrations fit the Freundlich isotherm (r2 0.93). The slope of the isotherm generally increased abruptly at high P concentrations, suggest-ing Ca-P precipitation. The maximum P sorption prior to Ca-P precip-itation 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 precipi-tation 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 ulti-mately impact the plant availability and potential off-site transport of P from these soils. (Soil Science 2003;168:368-375