A special section being published next month in the Journal of Environmental Quality addresses that question. The collection of papers grew out of a symposium at the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America 2011 Annual Meetings. The section acknowledges the problems that have been encountered with P Index development and implementation, such as inconsistencies between state indices, and also suggests ways in which the indices can be tested against data or models to improve risk assessment and shape future indices.This gets to some of the objections that farmers have against the 'Bay Diet' and the restrictions that it will bring to their operations, that the predictions of how much N and P are likely to leave a field and end up in Chesapeake Bay:
The P Index was proposed in a 1992 symposium after people became aware of the environmental impacts of P loss from fields. Many farmers were applying manure or other biosolids to their fields at rates that over-applied P. Researchers realized that assessing the risk of P loss from those products was important to protect water quality. The P Index tool was needed to connect various conditions because P loss is influenced by both site characteristics (e.g., soil test levels, connectivity to water) and the sources of P applied (e.g., inorganic fertilizer, organic sources). It was therefore a great improvement over the use of agronomic soil testing for P risk assessment.
"The P Index was meant to be something that could be easily computed with readily available data, so an NRCS agent would be able to obtain the necessary inputs," says Nelson. "But there are many different factors that influence P loss as you move from one physiographic region to the next. The differences in transport processes, soils, and landscapes in each state have led to 48 different versions of the P Index, and some of them are very different."And where the same data give different answers in different areas, there are economic consequences, also known as competitive advantages or disadvantages.
The inconsistencies of indices across states, along with a perceived lack of improvement in water quality in some regions, are now bringing the accuracy of the P Index into question. With different calculations in place, a set of factors may be categorized as low risk in one state and medium, or even high, risk in another. These discrepancies become especially obvious along state borders. Researchers understand the need to improve P indices and have made it a priority to base any changes on sound scientific data.So how soon?
Efforts to preserve, evaluate, and improve the P index led the NRCS to release a Request for Proposals within the Conservation Innovation Grant Program. Three regional efforts were funded to evaluate and improve the indices in the Heartland, the Southern State, and the Chesapeake Bay regions of the U.S. Additionally, a national coordination project and two other state-level efforts (Ohio and Wisconsin) were recently funded through the Conservation Innovation Program.
While the final suggestions for the next generation of the P Index are likely a few years off, the research is currently underway. Due to variations in regional characteristics and the problems previously encountered by state boundaries, it is likely that suggestions for improved indices will be based on regional distinctions, Nelson says. The objective is that the evaluations will lead to optimized P indices and better management tools that accurately incorporate site and source characteristics to predict the risk of P loss from fields.