Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/1998
Publication Date: N/A
Interpretive Summary: The loss of phosphorus in agricultural runoff is of increasing concern in several areas of the U.S., primarily where the production of phosphorus in manure from confined animal operations exceeds local crop requirements of P. Some crop production systems are forced to continually use manures as fertilizers because of the lack of economically viable alternatives for manure utilization. These systems almost always build soil phosphorus levels well beyond the ranges considered optimum for most agronomic crops, because manures are normally applied at rates designed to meet crop nitrogen requirements, while avoiding groundwater quality problems created by leaching of excess manure nitrogen. Several states have identified soil phosphorus levels, above which fertilizer and manure applications would be reduced or eliminated to minimize the potential for phosphorus loss in surface runoff. Establishing these soil phosphorus levels is often a highly controversial process. Due to a lack of field data, these levels have been based more on intuition than fact and their validity can be contested. Thus, we conducted field studies to establish critical soil phosphorus values above which the increased potential for phosphorus enrichment of surface runoff may necessitate a change in management of fertilizer and manure inputs.
Technical Abstract: Soils that contain high P levels can become a primary source of dissolved reactive P (DRP) in surface runoff, and thus contribute to accelerated eutrophication of streams and lakes. In a previous study, we compared results from several soil test P (STP) methods on a single soil and found they all gave results that were significantly correlated to DRP levels in runoff, but distilled H2O and NH4-oxalate methods gave the best correlations. We hypothesized that results might differ on other soils, so runoff studies were conducted on three additional Ultisols to determine which STP method accounts for DRP levels in runoff most consistently across soil series, and what effect site hydrology has on the correlation between STP and runoff DRP concentrations. Surface soil (0-2 cm depth) of pasture plots (7% slope) was analyzed by Mehlich III, Olsen, Morgan, Bray-Kurtz P1, NH4-oxalate, and distilled H2O methods. P saturation of each soil was also odetermined by three different methods. Simulated rain was applied at 75 m h-1 to produce 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 used, with most STP methods giving high correlations (r>0.90) on all three soils. For a given level of H2O-extractable STP, low runoff volumes coincided with low DRP concentrations. Therefore, when each DRP concentration was normalized for (divided by) the volume of plot runoff, correlations to H2O-extractable STP had the same (p<0.05) regression line for each soil. This suggests the importance of site hydrology in determining P loss in runoff, and may provide a means of developing a single relationship for a range of soil series.