ARS | Publication request: A critical evaluation of the Kentucky phosphorus index

Submitted to: Journal of the Kentucky Academy of Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 12, 2011
Publication Date: August 25, 2011
Repository URL: http://handle.nal.usda.gov/10113/55055
Citation: Bolster, C.H. 2011. A critical evaluation of the Kentucky phosphorus index. Journal of the Kentucky Academy of Science. 72(1):46–58. 2011.

Interpretive Summary: The transport of land-applied phosphorus (P) to surface freshwater bodies can adversely affect water quality by accelerating eutrophication. Eutrophication from excess P loading is widespread among U.S. water bodies with a substantial portion of the P loading originating from agricultural fields. In response to water-quality concerns over P export from agricultural fields to surface waters, the P Index was developed as an assessment tool to identify fields which are most vulnerable to P loss by accounting for the major source and transport factors controlling P movement in the environment. The Kentucky P Index is used to assign risk of P loss based on a field’s runoff potential, soil erosion potential, soil test P (STP) concentration, distance to water body, location, P application method, and width of vegetative buffer. The value of the P index is then used to determine whether P application needs to be restricted. Because the KY P index has not been modified since its initial formulation, nor has it been critically evaluated, the objective of this study was to critically evaluate the KY P Index by comparing the output with simulated P loss data obtained from a validated P-loss model to identify areas where the Index may need revising. Moreover, the general formulation of the KY P Index is evaluated against our current understanding of the processes controlling P transport in the environment. Results of this research demonstrate limitations in the KY P Index and indicate that the Index should be revised.

Technical Abstract: The U.S. Department of Agriculture’s Natural Resource Conservation Service (USDA-NRCS) is currently revising its 590 standard. As part of this revision, USDA-NRCS is considering requiring that states test the accuracy of their phosphorus (P) index using either measured P loss data or simulated P-loss data generated from process-based models. With this in mind, the objective of this study was to critically evaluate the Kentucky P Index by comparing Index output with simulated P loss data obtained from a validated P-loss model to identify areas where the Index may need revising. Results suggest that in some areas the Index does a good job in assigning P loss risk, however, this analysis also showed some important deficiencies in the Index, primarily the neglect of important factors known to affect P loss (e.g., soil erosion and P application rates) and how the different factors in the Index are weighted. To reduce the amount of P that is exported from agricultural fields to waterways within the Commonwealth of Kentucky, resources should be devoted to revising the KY P Index to address these limitations as well as developing long-term monitoring sites where the Index and more process-based models can be evaluated against measured P loss data.