Assessing the risk of phosphorus runoff from fields where manure was applied from concentrated animal feeding operations is done with a P Index.
The 2006 version of the P Index remains valid, but some small problems were corrected, says Charles Wortmann, University of Nebraska Extension nutrient management specialist. The new version is available at cnmp.unl.edu/cnmpsoftware2.html#Nebraska. The file name is NEPI0507.xls. The instruction manual, "The Nebraska Phosphorus Index (2005): Background and Users Guide" (EC195), also has been revised and is available at www.ianrpubs.unl.edu/epublic/live/ec195/build/ec195.pdf.
The index is a partnership by UNL and the Natural Resources Conservation Service.
Nebraska Department of Environmental Quality regulations, as of Jan. 1, 2007, require that manure from CAFO operations must have a phosphorus assessment using the index.
One issue is which soil to use in the assessment when there are several soils in the field or in a sub-field. Most phosphorus runoff originates from small parts of fields and it is important that the assessment is done for the areas of highest risk, according to Wortmann. "These are often the areas with the greatest potential for erosion," he says. "Medium- and low-risk ratings do not indicate a need to change management practices for land application of manure."
If the highest risk part of the field rates medium or low, you may want to apply this rating to the whole field. If the most sensitive area rates high or very high, you can treat that area as a different management zone, while the lower risk parts of the field are assessed as one or more additional management zones, he says.
When the records of the CAFO are eventually inspected, it will be important that the inspector finds the phosphorus index assessment to be realistic for the field, Wortmann explains.