Edge‐Of‐Field Evaluation of the Ohio Phosphorus Risk Index

The Phosphorus Index (PI) has been the cornerstone for phosphorus (P)‐based management and planning over the past twenty years, yet field‐scale evaluation of many state PIs has been limited. In this study, P loads measured in surface runoff and tile discharge from 40 agricultural fields in Ohio with prevailing management practices were used to evaluate the Ohio PI. Annual P loads were highly variable among fields (dissolved reactive P: 0.03–4.51 kg ha−1, total P: 0.03–6.88 kg ha−1). Both measured annual dissolved reactive P (R2 = 0.36, p < 0.001) and total P (R2 = 0.25, p < 0.001) loads were significantly related to Ohio PI score. The relationship between measured load and PI score substantially improved when averaged annual field values were used (dissolved reactive P: R2 = 0.71, total P: R2 = 0.73), indicating that the Ohio PI should be utilized to evaluate average annual risk of P loss, rather than as an annual risk tool. Comparison between the Ohio PI and other established local and national metrics resulted in large differences in potential P management recommendations for the monitored fields. In the near term, revision of Ohio PI risk categories and management recommendations using local P loading thresholds is needed. To meet the minimum criteria for state PI tools, future research efforts should focus on using measured field data (i) to incorporate new input factors (i.e., P application timing and leaching potential) into the Ohio PI, and (ii) to calibrate and validate the Ohio PI to provide better P risk assessments and management recommendations. Core Ideas Measured P loads from 40 fields were used to evaluate the Ohio Phosphorus Index. Risk of P loss was linearly related to both measured annual and averaged P loads. Field P management recommendations varied among local, state, and national metrics. Field datasets should be used to improve and calibrate the Ohio Phosphorus Index.