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MS Phosphorus Index: Can it adequately guide nutrient management plans in the Mississippi Delta?
Proceedings of the 2022 Mississippi Water Resources Conference

Year: 2022 Authors: Fernandez-Martinez F., Rodriguez L.S., Ramirez Avila J.J., Locke M., Achury S.O., Oldham L.

Agricultural nutrient runoff from fields in the Mississippi (MS) Delta contribute to surface and groundwater quality impairment in the region. The phosphorus (P) indices are tools widely used to assess the vulnerability of agricultural fields to P loss and support nutrient management strategies. The objective of this study was to identify the impact of each MS P-Index input on the assessment of P loss vulnerability for sites in the MS Delta. The probability distribution of the different input parameters (i.e. soil erosion rate, soil test P, P application rate, P fertilization source and method, and runoff class) was determined by using historical crop management data from Stoneville and Beasley Lake watershed, various USDA web tools, and GIS data for the region. Monte Carlo simulations (10,000 iterations) were generated to evaluate the existing MS P-Index and a version in which the model inputs are unweighted. Simulations showed that the distance to water, followed by the organic P2O5 application method were the parameters that most affected the P-Index score, while the inorganic P2O5 application rate was the least sensitive parameter. Stepwise regression showed that 87% of the variance of the P-Index scores was explained by five input parameters (distance to water, organic P2O5 application rate and method, inorganic P2O5 application method, and soil erosion), while all eight input parameters explained 89% of the variance. The mean P-Index scores derived from the simulations were 2.49 and 3.16 for the general and unweighted models, respectively, scoring a low P loss vulnerability in 94% and 83% of the cases, respectively. Since the outputs of this tool are not consistent with the reported issues related to nutrient leakage from agricultural watersheds in the region, results indicate the need for a more in-depth analysis of the MS P-Index and its ability to represent P-loss vulnerability of fields in the MS Delta. An adequate calibration and validation process with analyses of water quality are needed for further research and improvement of the MS P-index tool.

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