Page Banner

United States Department of Agriculture

Agricultural Research Service

Title: EVALUATION OF THE PHOSPHORUS SOURCE COMPONENT IN THE PHOSPHORUS INDEX FOR PASTURES.)

Author
item Delaune, Paul
item Moore, Philip
item Carmen, Dennis
item Sharpley, Andrew
item Haggard, Brian
item Daniel, Tommy

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2004
Publication Date: 11/1/2004
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: 8/24/2016
Footer Content Back to Top of Page