Evaluation of Alabama phosphorus index using edge of field data

Authors: BHATTA, A - Auburn University; PRASAD, R - Auburn University; CHAKRABORTY, D - Auburn University; Watts, Dexter; Torbert Iii, Henry; Kleinman, Peter

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2026
Publication Date: 2/13/2026
Citation: Bhatta, A., Prasad, R., Chakraborty, D., Watts, D.B., Torbert III, H.A., Kleinman, P.J. 2026. Evaluation of Alabama phosphorus index using edge of field data. Journal of Environmental Quality. 55:e70152. https://doi.org/10.1002/jeq2.70152.
DOI: https://doi.org/10.1002/jeq2.70152

Interpretive Summary: The Phosphorus Index (PI) is a field-scale tool developed in the 1990s to assess field vulnerability to phosphorus (P) loss and guide P management decisions. Initially, PIs used an additive formulation, but continued research efforts over the years refined structural modifications to multiplicative and component-based frameworks. Alabama's PI, which was implemented in the early 2000, stayed additive and has not been tested for its performance. The objective of this study was to evaluate the Alabama PI using Edge-Of-Field Monitoring (EOFM) data and explore the applicability of multiplicative and component-based PIs from neighboring states. In addition, this study tested the performance of a modified version of Alabama PI. The results showed that there were substantial disparities in P loss risk categorization compared to observed P loads. This indicated that Alabama PI is not correct in both direction and magnitude. To address these limitations, a modified version of Alabama PI was proposed, replacing soil test P (STP) with the phosphorus saturation ratio (PSR) and substituting the underground outlet system with the timing of P application and slight adjustment of weighting factors. The modified PI demonstrated significant and satisfactory correlations with DRP, TPP, and TP loads. Additionally, the modified version was found to be directionally correct and can be a reliable interim tool for P loss risk assessment in Alabama.

Technical Abstract: The Phosphorus Index (PI) is a field-scale tool developed in the 1990s to assess field vulnerability to phosphorus (P) loss and guide P management decisions. Initially, PIs used an additive formulation, but continued research efforts over the years refined structural modifications to multiplicative and component-based frameworks. Alabama's PI, which was implemented in the early 2000, stayed additive and has not been tested for its performance. The objective of this study was to evaluate the Alabama PI using Edge-Of-Field Monitoring (EOFM) data and explore the applicability of multiplicative (Tennessee) and component-based (Georgia) PIs from neighboring states. In addition, this study tested the performance of a modified version of Alabama PI. The results showed that the relationship between the annual dissolved reactive P (DRP), total particulate P (TPP) and total P (TP) loads with Alabama PI score were not significant and poorly correlated. There were substantial disparities in P loss risk categorization compared to observed P loads. This indicated that Alabama PI is not correct in both direction and magnitude. Similarly, the multiplicative and component-based PI showed inconsistencies and are not directly applicable to Alabama farms. To address these limitations, a modified version of Alabama PI was proposed, replacing soil test P (STP) with the phosphorus saturation ratio (PSR) and substituting the underground outlet system with the timing of P application and slight adjustment of weighting factors. The modified PI demonstrated significant and satisfactory correlations with DRP, TPP, and TP loads. Additionally, the modified version was found to be directionally correct and can be a reliable interim tool for P loss risk assessment in Alabama. However, continued research and data collection from EOFM is essential for future improvements and the development of a quantitative P loss model, ensuring its accuracy and applicability for Alabama.