Development of a phosphorus index for pastures

Phosphorus (P) runoff from watersheds receiving animal manure applications has received recent critical attention due to eutrophication of nearby water bodies. Based on the assumption that high P concentrations in runoff are due to high soil test P levels, some agencies are proposing environmental threshold soil test P levels above which animal manures can no longer be applied. However, recent studies have shown that other factors may play a more important role than soil test P on P runoff. The objective of this research was to develop and validate a P index for pastures by evaluating several variables affecting P runoff. Variables evaluated included: (1) effect of soil test P, (2) effect of soluble P in poultry litter, (3) effect of P in poultry diets, (4) effect of fertilizer type, (5) effect of poultry litter application rate, and (6) effect of time to the first runoff event after poultry litter application. Results from initial rainfall simulation studies showed that P runoff was most affected by the amount of soluble P in the fertilizer source, P application rates, and time to the first runoff event. Prior to manure applications, soil test P was directly related to soluble P concentrations in runoff water. However, results showed that soil test P had little effect on P runoff when animal manure was applied. Regression analysis showed that the amount of soluble P applied was the most important factor affecting P runoff, while soil test P was found to be insignificant. However, soil test P was included in the P index to account for background P losses and predict P losses from unfertilized fields. Transport factors and their respective weighting included in the P index were determined by consultation among several state and federal agencies within Arkansas. Poultry litter applications based on the P index were no less restrictive than those based on the environmental threshold when soil test P levels were below 300 lb P A−1. Validation studies on real pastures receiving natural rainfall indicated that the P Index for pastures predicted P loads very well (y = 1.16x − 0.23, r2 = 0.83). These data indicate that the P index for pastures can accurately assess the risk of P loss under real conditions and provide a better risk assessment than threshold soil test P levels. (Abstract shortened by UMI.)...