Effect of Extractable Soil Surface Phosphorus on Runoff Water Quality

Abstract

Phosphorus (P) additions to surface water from agricultural nonpoint sources are of concern, because P often limits eutrophication of surface waters. Numerous sources of runoff P exist: indigenous soil and plant material, land-applied manure and sludge, and commercial fertilizer. In many soils receiving commercial fertilizer and manure, concentrations of P at the soil surface have been steadily rising due to either long-term or excessive applications of P. Critical levels of soil surface P may exist, above which runoff may promote eutrophication. Methods for rationally identifying these critical levels are needed to manage losses of P, which implies the need for accurate methods of relating soil surface P concentration (Ps) to runoff P concentration. A study was conducted on both pasture and tilled plots (with and without residue) to evaluate the relationship between Ps and dissolved reactive P in runoff (PR) using simulated rainfall. The data indicated that even for comparable storms, Ps alone was not a satisfactory estimator of PR. A model describing the kinetics of P release from surface soil to runoff was used to include additional variables in predicting PR. When used with uncalibrated parameters, the model explained a significant proportion of the variation in observed PR values for pasture plots (r2 = 0.43) but was less successful in predicting PR for tilled plots (with and without residue, r2 = 0.13). Calibration of (adjustments to) the extraction coefficients resulted in an overall coefficient of determination between observed and predicted PR values of 0.73. While the model was successful in describing how PR and the independent variables are related for the pasture plots, the extraction coefficients should be calibrated to obtain best estimates of PR. When used with calibrated extraction coefficients, the model provided realistic estimates of PR over the range of experimental conditions.