Odor and Nutrient Management Newsletter

Summer 2004

The phosphorus index and manure management plans for confinements

by Jeremy Klatt, Iowa Department of Natural Resources

The Iowa Environmental Protection Commission (EPC) adopted rules that include using the phosphorus (P) index to determine application rates for manure management plans (MMPs) at their June 21 meeting. The P index was developed by scientists from Iowa State University, the National Soil Tilth Laboratory and the Natural Resources Conservation Service (NRCS) to determine the risk of P loss from a field. The index is based on erosion, soil test, location of the field, P applications and other factors.

Soil sampling to determine soil test phosphorus levels for use in P-index calculations
Soil sampling to determine soil test phosphorus
levels for use in P-index calculations.

The Department of Natural Resources (DNR) developed rules for using the P index to determine manure application rates as required by state law. The rules apply to confinement feeding operations that are required to use MMPs and will be phased in over the next four years.

Depending on the P index results, manure application rates can be based on phosphorus or nitrogen (N). The EPC adopted the following application rates based on results of the P index.

Very Low (0-1). Nitrogen-based manure management.

Low (>1-2). Nitrogen-based manure management.

Medium (>2-5). Nitrogen-based manure management if practices will not raise the field to the “high” risk category. Manure applications must not result in application of P in excess of two times the P removal of the crop rotation.

High (>5-15). Until Dec. 31, 2008, fields with a P index rating between 5 and 10 may receive manure at a P-based rate if practices will be adopted to reduce the P index of the field to the “medium” risk category. After Dec. 31, 2008, no manure may be applied until practices are adopted that reduce the P index to the “medium” risk category.

Very High (>15). No manure application.

Here are some important points to consider about the P index and MMPs.

  • A very high soil test does not equal a very high P index. The soil test classification is not the same as the P index risk category. Soil P is only one input to the P index; the erosion rate is typically more important than the soil test value.
  • Practices can be implemented to reduce the P index. If a field is classified as “high” or “very high” risk, soil conservation practices can be implemented to reduce the P index. For instance, filter strips, grass waterways, increased residue cover, terraces, changing the crop rotation, contouring or any other practice that reduces erosion or sediment delivery from the field also reduces the P index. The P index spreadsheet, available on the Iowa NRCS Web site, is a good tool for determining the impact of specific practices.
  • Fields can be split to reduce the P index. Relatively small areas are typically responsible for most P loss from a field. Therefore another option if a field is “high” or “very high” risk is to isolate the area that is causing the P index to be high and run the P index separately for that area. This should result in a higher P index for that high-risk area and a lower P index for the rest of the field. Manure could then be applied to these two areas based on their respective P index results.
  • The P index will be phased in. If the rules are finalized as planned, original plans submitted on and after Oct. 25, 2004 will need the P index. Plans submitted before this date will be phased in (see table below).

implementation dates

For more information about the P index, including the P index calculator go to the Iowa NRCS Web site at http://www.ia.nrcs.usda.gov/.

Additional changes to manure management plans that were adopted by the EPC:

  • Changes to manure standard table values used for MMP calculations. These values were updated to reflect current Iowa State University publication values (Managing Manure Nutrients for Crop Production, PM 1811).
  • The number of years records need to be kept has increased from three to five years. This change was made to be consistent with U.S. Environmental Protection Agency requirements.
  • A copy of the manure management plan and records must be kept within thirty miles of the site.
  • Records of commercial N and P applications must be kept for fields receiving manure.

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Iowa Manure Matters: Odor and Nutrient Management is published by Iowa State University Extension, with funding support from the USDA Natural Resource Conservation Service through Cooperative Agreement No. 74-6114-8-22. To subscribe or change the address of a current subscription, write to Angela Rieck-Hinz, 2104 Agronomy Hall, Iowa State University, Ames, Iowa, 50011-1010 or call 515-294-9590, fax 515-294-9985 or email: amrieck@iastate.edu. Please indicate you are inquiring about the Odor and Nutrient Management Newsletter. The newsletter's coordinators are Angela Rieck-Hinz, extension program specialist, Department of Agronomy, Wendy Powers, environmental extension specialist, Department of Animal Science, and Robert Burns, Department of Agricultural and Biosystems Engineering; the editor is Jean McGuire, the subscription manager is Rachel Klein, the production designer is Beth Kroeschell, and the web page designer is Liisa Jarvinen.

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