Phosphate sorption by Pacific Northwest calcareous soils

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 0.01 M CaCl2 containing between 0 and 700 mg P L-1 for 24 h. The P sorption isotherms at low to medium P concentrations fit the Freundlich isotherm (r(2) greater than or equal to 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 (R-2 > 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.