Comparing Southern P Indices to Runoff Data.

The Southern NIFA-Water Project Nutrient Management team compared the 12 phosphorus (P) Indices developed for in this region (AL, AR, FL, GA, KY, LA, MS, NC, OK, SC, TN, and TX). Different data sets, which varied based on erosion rates, cropping systems, rate of poultry application and buffer presence, were used in assessing similarities and differences in the tools. This comparison indicated a large degree of variation in the ratings between Southern P Indices. We then compared collected runoff data from pasture runoff plots in Georgia and small watersheds in North Carolina to which poultry litter was applied to ratings from the 12 Southern P Indices. Measured P losses from plots, ranged from approximately 3 to 32 kg ha-1, depending on the location. As in the original analysis, there was a high degree of variability in the rating factors derived from the Southern P Indices; the ratings ranged from Low to Very High as a function of the individual P Index and the site conditions. Thus, the strength of the relationship between P losses and P Index ratings varied as a function of the P index.