USING THE PHOSPHORUS ASSESSMENT-TOOL IN THE FIELD

The P index (PI) is a proposed tool for rating the potential for offsi te losses of P from a field or other management unit. The eight site c haracteristics comprising the PI were evaluated on several best manage ment practice (BMP) demonstration plots in the Dairy-McKay Hydrologic Unit Area (HUA) in western Oregon and on an entire subbasin in the Gra nger Drain HUA in eastern Washington. An estimate of site vulnerabilit y is obtained by adding the weighted values of the site characteristic s. Site vulnerability ranged from low to medium in the Dairy-McKay HUA . The medium values were due to P fertilizer application rates of > 66 lb P/acre and surface sidedress application. Site vulnerability ratin gs by soil type for fields in the Granger Drain HUA were concentrated in the medium and high categories. This was due to high values for sit e characteristics of irrigation erosion and surface runoff of irrigati on water, high soil test P levels, and application of organic P from a nimal wastes. This study demonstrates that the PI can be used to rank soils as to their potential for P loss between sites. Validations stud ies are needed to determine if high vulnerability sites are significan tly affecting surface waters.