Using soil phosphorus behavior to identify environmental thresholds

Concern over the transport of phosphorus from agricultural soils to surface waters has focused attention on the role of soil phosphorus in environmental risk assessment. This study explores the existence of natural soil phosphorus thresholds as expressed by Quantity/Intensity relationships. Fifty-nine samples, collected from agricultural soils in New York's Delaware River Watershed, were analyzed for Morgan, Meblich III, and 0.01 M CaCl₂extractable P. Soil P sorption saturation was calculated as a function of oxalate extractable P, Fe, and Al. In addition, P sorption isotherms were determined for all soils. Thresholds in the relationships between CaCl₂P and Morgan P, Mehlich III P, and P sorption saturation were identified by segmented linear regression (change point analysis). Thresholds in the relationship between CaCl₂P and Morgan P, Mehlich III P, and P sorption saturation occurred at CaCl₂P concentrations of 0.9 mg kg^-1, suggesting a threshold for soil P that may have use in environmental risk assessment. A P sorption threshold was also identified by segmented, quadratic-linear regression of the sorption isotherms. Results described a fundamental property of soils: A nonlinear sorption of P in soils that exhibits a threshold, above which the potential for P release from soil to water increases. This threshold describes a critical point in the release of P and, therefore, may be of environmental importance in estimating the potential for soluble P loss from soil by runoff and leaching. (Soil Science 2000;165:943-950).