Evaluation of Alabama phosphorus index using edge-of-field monitoring data

Phosphorus index (P-index) was developed to assess field vulnerability to phosphorus (P) loss and guide P management decisions. The original structure of the P-index was additive, and with continued refinement, multiplicative and component-based indices were developed. Alabama adopted the additive version in early 2000; however, the tool was never tested for its performance. The objectives of this study were to (i) evaluate the Alabama P-index using edge-of-field P loss data, (ii) test if multiplicative (Tennessee) and component-based (Georgia) P-indices perform better, and (iii) improve and test the performance of a modified Alabama P-index. We evaluated the performance by examining the strength and directional relationship between P-index scores and annual P loads. The Alabama P-index showed weak correlations (r < 0.50) between risk scores and measured dissolved reactive phosphorus (DRP), total particulate phosphorus (TPP), and total phosphorus (TP) loads. Additionally, directional inaccuracies were observed, indicating that the index misclassified the relative risk of P loss. Further, we evaluated multiplicative and component-based indices but found similar discrepancies between predicted risk scores and actual P loading. Subsequently, we modified the Alabama P-index by replacing soil test P with the phosphorus saturation ratio and substituting the underground outlet system factor with the timing of P application. Minor adjustments to weighting factors were made. The modified P-index demonstrated statistically significant correlations (r > 0.51) and directional alignment with DRP, TPP, and TP loads, suggesting it can serve as a reliable interim tool for assessing P losses. Future research should focus on restructuring and validating a component-based P-index tailored to Alabama's agricultural systems.