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United States Department of Agriculture

Agricultural Research Service

Title: Evaluation of the Phosphorus Source Component in the Phosphorus Index for Pastures.

Authors
item Delaune, Paul - UNIV OF ARKANSAS
item Moore, Philip
item Carmen, Dennis - USDA NRCS
item Sharpley, Andrew
item Haggard, Brian
item Daniel, Tommy - UNIV OF ARKANSAS

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 6, 2004
Publication Date: November 1, 2004
Citation: Delaune, P.B., Moore Jr, P.A., Carmen, D.K., Sharpley, A.N., Haggard, B.E., Daniel, T.C. 2004. Evaluation of the phosphorus source component in the phosphorus index for pastures. Journal of Environmental Quality. 33:2192-2200.

Interpretive Summary: Complex models that predict phosphorus (P) runoff are difficult to use due to the large number of required input parameters. As an alternative, a P index for pastures was developed to determine how much P can safely be applied to a given field. This study was done to determine how well the P index for pastures can estimate the amount of P in runoff water by using simulated rainfall on small plots. Poultry litter was applied to small plots based on either the amount of P in the soil (soil test P) or on the P index for pastures. The amount of P in runoff from two fields were also used to determine the effectiveness of the P index. Results showed that soil test P alone provided a poor estimate of the amount of P in runoff water. However, the P index for pastures gave a much better estimate of the amount of P in runoff water. Although litter application rates were higher based on the P index compared to soil test P, P in runoff water did not differ. Studies on pastures receiving natural rainfall indicated that the P index for pastures predicted P loads very accurately. This study indicated that the P index for pastures can be used to determine how much P can safely be applied to fields, much better than applying litter based on soil test P.

Technical Abstract: Models that predict phosphorus (P) runoff from a variety of landscapes and farming systems are not widely available and are often difficult to use due to the large number of required input parameters. Therefore, a P index for pastures was developed to write nutrient management plans that determine how much P can safely be applied to a given field. The objective of this study was to validate the P index for pastures by conducting rainfall simulation studies on six farms with contrasting soils under various management scenarios. Poultry litter was applied based on either an environmental threshold soil test P level or on the P index for pastures. The P index was also validated using two, 0.405 ha watersheds which had been fertilized annually with poultry litter since 1995. Results showed that soil test P alone was a poor predictor of P concentrations in runoff water (r = 0.06). However, the relationship between P index values and P concentrations in runoff water were highly significant (P<0.0001; r = 0.77). Although litter application rates were significantly higher based on the P index compared to threshold soil test P levels, P concentrations in runoff water were not significantly different. Validation studies on pastures receiving natural rainfall indicated that the P index for pastures predicted P loads accurately without calibration (y = 1.16x - 0.23, r = 0.91). These data indicate that the P index for pastures can accurately assess the risk of P loss under natural conditions and provide a more realistic risk assessment than threshold soil test P levels.

Last Modified: 8/21/2014
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