Assessment of the Phosphorus Index for Louisiana

• Main Author: Felicien, William Lucas
• Other Authors: Lewis A. Gaston
• Format: Others
• Language: en
• Published: LSU 2007
• Subjects: Agronomy & Environmental Management
• Online Access: http://etd.lsu.edu/docs/available/etd-07092007-182525/

A phosphorus index is a semi-quantitative to qualitative model for assessing the potential for loss of soil and fertilizer phosphorus (P) to ground and surface water, therefore,eutrophication risk. The concept of a P Index is recent and popular, with most states having developed a P Index that is either a simplification or extension the original 1993 concept. The Louisiana P Index assigns ratings for P loss potential depending on soil properties, topography and land use. These ratings are intended to guide P fertilizer (including animal waste materials) application so as to preserve water quality. This project has examined measured loss of P in runoff and compared it to P loss ratings calculated using the Louisiana P Index. This was done using small runoff plots and simulated rainfall, consistent with work done elsewhere in the country. Since loss of P from soils enriched in P from years of application of animal waste was the impetus for development of the concept, Louisiana sites included in this study were from the poultry-producing region of the state. Results showed poor correlation between P in runoff and P loss ratings, which was somewhat improved by omitting high P, low P loss ratings data for a grazed pasture. Further insight into P loading into runoff was gained by examining runoff and P concentrations as functions of time. Runoff could often be well-described using the Green-Ampt model for infiltration. For the forest soil plots examined, however, considerably higher P often appeared in runoff than expected based on soil P desorption, and the source of this P may have been the forest litter layer. Continued difficulties in predicting P loss to runoff based on more mechanistic approaches support use the simpler P Index. However, a better understanding of the mechanisms by which soil P is made subject to runoff loss should lead to improvements in the P Index and better practices for controlling the loss of P to surface water.