PHOSPHATE SORPTION BY PACIFIC NORTHWEST CALCAREOUS SOILS

Abstract

Understanding phosphorus (P) sorption of calcareous soils is important\nfor the development of successful fertilizer and manure management\npractices. This study was conducted to identify soil chemical properties\ncontrolling P sorption in semiarid calcareous soils of the Pacific Northwest.\nSorption isotherms of 18 primarily calcareous soils ranging widely\nin soil physical and chemical properties were constructed by equilibrating\n4 g of soil with 40 mL of 0.01M CaC12 containing between 0 and 700\nmg P 1.- 1 for 24 h. The P sorption isotherms at low to medium P concentrations\nfit the Freundlich isotherm (r2 ? 0.93). The slope of the\nisotherm generally increased abruptly at high P concentrations, suggesting\nCa-P precipitation. The maximum P sorption prior to Ca-P precipitation\nwas closely related to organically complexed Fe and Mn (R2 >\n0.98), suggesting that such complexes may regulate P sorption in these\nsoils. The equilibrium P concentration at the point where Ca-P precipitation\nbegins to dominate was closely related to pH and organic carbon.\nA better understanding of the role of organically complexed metals in\ncontrolling P sorption in calcareous soils is needed to determine how P\nsolubility is affected by organic matter additions and how this will ultimately\nimpact the plant availability and potential off-site transport of P\nfrom these soils.