Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2004
Publication Date: 11/1/2004
Citation: Delaune, P.B., Moore Jr, P.A., Carman, D.K., Sharpley, A.N., Haggard, B.E., Daniel, T.C. 2004. Development of a phosphorus index for pastures fertilized with poultry litter - factors affecting phosphorus runoff. Journal of Environmental Quality. 33:2183-2191. Interpretive Summary: Phosphorus (P) concentrations in runoff water from land fertilized with animal manure is an environmental concern. Several state and federal agencies limit animal manure applications based on the amount of phosphorus (P) in the soil, or soil test P. Recent studies show that other factors are more indicative of P concentrations in runoff. This study was done to determine the most important factors affecting the amount of P in runoff water and to develop a P index for pastures to manage fertilizer applications. Several simulated runoff events were conducted to determine the effect of 1) soil test P, 2) soluble P in poultry litter, 3) P in poultry diets, 4) fertilizer type, 5) poultry litter application rate, and 6) time to the first runoff event after poultry litter application. The amount of soluble P applied was found to be the most important factor determining the amount of P in runoff water. Soil test P was important in predicting the amount of P in runoff water only when no P was applied. The P index for pastures was developed based in the results of rainfall simulation studies. The final P index value provides an estimate of the amount of P lost in runoff water. Because the P index integrates many factors affect P transport, the P index for pastures should provide a better risk assessment tool than soil test P alone.
Technical Abstract: Currently, several state and federal agencies are proposing upper limits on soil test phosphorus (P), above which animal manures can not be applied, based on the assumption that high P concentrations in runoff are due to high soil test P. Recent studies show that other factors are more indicative of P concentrations in runoff from areas where manure is being applied. The original P index was developed as an alternative P management tool incorporating factors affecting both the source and transport of P. The objective of this research was to evaluate the effects of multiple variables on P concentrations in runoff water and to construct a P index for pastures that incorporates these effects. The evaluated variables were: 1) soil test P, 2) soluble P in poultry litter, 3) P in poultry diets, 4) fertilizer type, 5) poultry litter application rate, and 6) time to the first runoff event after poultry litter application. Field studies with simulated rainfall showed P runoff was affected by the amount of soluble P applied in the fertilizer source. Prior to manure applications, soil test P was directly related to soluble P concentrations in runoff water. However, soil test P had little effect on P runoff after animal manure was applied. Unlike most other P indices, the P index for pastures provides an estimate of annual P loading in runoff at the edge-of-field. Because the P index integrates P transport potentials that are relevant for grasslands with that fraction of P most likely to move from grassland soil surfaces, the P index for pastures should provide a better risk assessment tool than soil test P alone.