This paper presents results from the first 23 months of a field scale evaluation of the
predictive ability of a phosphorus (P) Index for pastures and crop fields in the Texas Blackland
Prairie. To perform this evaluation, water quality monitoring on pasture and cultivated
watersheds was initiated in 2000 prior to poultry litter application and continues to the present
under various poultry litter application rates. The P Index was able to accomplish its specified
purpose of estimating the relative susceptibility of fields to excessive phosphorus loss. When
results from both pasture and tilled fields were pooled, measured P loss tended to increase as P
Index values increased. P Index values also correlated well with both average and maximum
dissolved P concentrations in runoff. These initial results indicate that the Index might perform
well for cultivated fields, with only slight modifications, even though the P Index is not currently
set up to evaluate tilled systems. The P Index did not, however, perform as well when we
attempted to use it to quantitatively predict P losses. For all fields in the fallow year and for
control fields with no litter application, P Index values under-predicted P loss. Following litter
application, P loss from the pasture watersheds was drastically over-predicted. For the tilled
watersheds, P loss predictions did improve substantially following litter application, but relative
errors were still quite large.