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, measured P loads 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 (DRP): 0.03-4.51 kg ha^-1; total P (TP): 0.03-6.88 kg ha^-1). Both measured annual DRP (R²=0.36; p<0.001) and TP (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 field values were used (DRP: R²=0.71; TP: R²=0.73), indicating that the Ohio PI should be utilized to evaluate average risk of P loss rather than be used 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 to 1) incorporate new input factors (i.e., P application timing, leaching potential) into the Ohio PI; and 2) calibrate and validate the Ohio PI to provide better P risk assessments and management recommendations.