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Location: Soil Drainage Research

Title: Edge-of-field evaluation of the Ohio phosphorus risk index

item Williams, Mark
item King, Kevin
item Labarge, Greg - The Ohio State University
item Confessor, Rem - Heidelberg University, Ohio
item Fausey, Norman - Norm

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/2/2016
Publication Date: 9/15/2016
Citation: Williams, M.R., King, K.W., Labarge, G., Confessor, R., Fausey, N.R. 2016. Edge-of-field evaluation of the Ohio phosphorus risk index. Journal of Environmental Quality. doi:10.2134/jeq2016.05.0198.

Interpretive Summary: The Ohio Phosphorus (P) Index was developed in the mid-1990s to help producers manage P and minimize the environmental impacts associated with P loss. However, the Ohio P Index has yet to be evaluated using edge-of-field data. 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 test and evaluate the Ohio P Index. Results of the study suggest that the Ohio P Index could adequately predict the risk of annual P loss from monitored fields. The Ohio P Index, however, was much better at predicting the average risk (2-5 years; 1-2 crop rotations) of P loss compared to annual values. To improve the Ohio P Index, the risk categories (low, medium, high, and very high) and management recommendations need to be revised. In addition, new factors, such as tile drainage and P application timing, should be added to the Ohio P Index to increase its ability to predict the risk of P loss from a field.

Technical Abstract: 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.