Assessment of the Phosphorus Index for Louisiana.

Measured loss of phosphorus (P) in runoff from small plots (1.5 m², simulated rain) was compared to P loss ratings calculated with the Louisiana P-index.  Study sites included pasture (hay field and grazed) and forest (timber and timber + straw harvest) on coastal plain soils that are typical for the poultry-producing region of the state.  Results showed poor correlation between P in runoff and P loss ratings, partly due to high losses despite low P loss ratings for grazed pasture.  Considerably higher P often appeared in runoff from forest soil than expected based on soil P desorption, and the source of this P may have been the forest litter layer.  Difficulty in predicting P loss based on mechanistic approaches supports use the P-index.  However, better understanding of mechanisms responsible for P loading into runoff should lead to improvements in the P-index and better practices for controlling P loss to surface water.