Relationship between phosphorus levels in three ultisols and phosphorus concentrations in runoff

Soils that contain high P levels can become a primary source of dissolved r eactive P (DRP) in runoff, and thus contribute to accelerated eutrophicatio n of surface waters. In a previous study on Captina soil, several soil test P (STP) methods gave results that were significantly correlated to DRP lev els in runoff, but distilled N2O and NH4-oxalate methods gave the best corr elations. Because results might differ on other soils, runoff studies were conducted on three additional Ultisols to identify the most consistent STP method for predicting runoff DRP levels, and determine effects of site hydr ology on correlations between STP and runoff DRP concentrations. Surface so il (0-2 cm depth) of pasture plots was analyzed by Mehlich III, Olsen, Morg an, Bray-Kurtz P1, NH4-oxalate, and distilled H2O methods. Also, P saturati on of each soil was determined by three different methods. Simulated rain ( 75 mm h(-1)) produced 30 min of runoff from each plot. All correlations of STP to runoff DRP were significant (P < 0.01) regardless of soil series or STP method, with most STP methods giving high correlations (r > 0.90) on al l three soils. For a given level of H2O-extractable STP, low runoff volumes coincided with low DRP concentrations. Therefore, when each DRP concentrat ion was divided by volume of plot runoff, correlations to H2O-extractable S TP had the same (P < 0.05) regression line for every soil. This suggests th e importance of site hydrology in determining P loss in runoff, and may pro vide a means of developing a single relationship for a range of soil series .