University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

November 16, 2007

The Phosphorus Index: New Developments and Issues

Assessment of the risk of phosphorus runoff from fields of concentrated animal feeding operations (CAFOs) is done with a P Index. The 2006 version of the P Index is still valid but some small problems were corrected and the new version is available at http://cnmp.unl.edu/cnmpsoftware2.html#Nebraska. The file name is NEPI0507.xls. The background and instruction manual, “The Nebraska Phosphorus Index (2005): Background and Users Guide” (EC195), also was revised and is available at http://www.ianrpubs.unl.edu/epublic/live/ec195/build/ec195.pdf.

One issue that arises is which soil to use in the assessment when there are several soils in the field or in a sub-field. Most runoff phosphorus loss originates from small parts of fields and it is important that the assessment is done for the areas of highest risk. These are often the areas with the greatest potential for erosion. Medium and low risk ratings do not indicate a need for changed 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. 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.

Charles Wortmann
Extension Nutrient Management Specialist

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