History of the Phosphorus Index
In the early 1990's, the USDA-Natural Resources Conservation Service decided a tool was needed to rank fields based on their potential to deliver P to surface water bodies to help manage manure phosphorus applications in agriculture. Lemunyon and Gilbert published the first approach in 1993. Their P Index ranked fields by risk levels in categories that included both P source (soil, manure, fertilizer) and transport (runoff and erosion) factors. Since then, most states in the U.S. - starting with Maryland - have developed their own P Indices.
In the late 1990's, that state of Iowa developed a second approach to ranking fields. Iowa's P Index calculates an estimate of annual per acre P delivery from a field to surface water. The approach organizes the best science into a form that can be applied widely and objectively, and that can be tested. It also is flexible enough to adapt to new data, new concepts, and site-specific information much more effectively than the original form of the P Index. The Wisconsin P Index uses Iowa's "quasi-modeling" approach with equations based on Wisconsin research.
Field-scale Tools for Reducing Nutrient Losses to Water Resources (PDF) contains a more detailed account of the history of P Indices.
