Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/1998
Publication Date: N/A
Interpretive Summary: All plants need phosphorus (P), and when available P is low, plant growth can be increased by supplying additional P. This response is beneficial when P increases the yield of agricultural crops, but it becomes a concern when excess P in runoff water causes excessive growth of algae in streams and lakes. Soils with high P levels can be a major source of P in runoff water. Reliable and quick soil testing methods for predicting P levels in runoff are still needed. In a previous study, we compared results from several soil test P (STP) methods on a Captina soil and found they all were useful in predicting P levels in runoff. However, distilled water and ammonium oxalate methods were the most precise on that soil. Because the results might differ on other soils, runoff studies were conducted on three additional soils to determine which STP methods are most consistent for predicting P levels in runoff. All of the tested STP methods were useful for predicting P levels in runoff from all soils tested, but the prediction equation was different for each soil. The distilled water method gave the most consistent STP results for predicting P levels in runoff from all soils tested in this study and the previous. Also, for this method, differences between the prediction equations were caused only by the amount of runoff. This may provide a way to use the distilled water STP method to develop a single equation for predicting P levels in runoff from a range of soils. Such a method would help researchers and watershed managers find better ways to keep P as a valuable nutrient on cropland and reduce excessive P losses in runoff.
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 methods 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 determined by three different methods. Simulated rain was applied at 75 mm/hr 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.