Iowa Phosphorus Index

The Iowa Phosphorus Index (IPI) has been used for a primary performance measure by four watershed councils.  The IPI was chosen as a performance measure because it connected closely with the phosphorus water quality issue.  Early development of recommendations suggesting the IPI might connected to performance-based incentives came out of a series of meetings hosted by the Winrock Foundation in the Maquoketa Headwaters watershed.

The IPI is a computer model used to assess the potential risk of P movement from fields to nearby water bodies. Increasing phosphorus concentrations in surface water results in increasing algae growth. The primary components of theIPI are soil loss (erosion), soil test P, rate and method of P application, field distance to water, and tile drainage. The IPI has been used as part of manure application regulations for confinement feeding operations since late 2004.  The components of the IPI are explained in a presentation by Iowa State University agronomist John Sawyer: Developing and Using the Iowa P-Index.  More information about the IPI and the IPI calculator can be accessed at the Iowa Natural Resource and Conservation Service Phosphorus Standard.

Variations of the following “per-farm” PHOSPHORUS INDEX incentives have been used in four watersheds.

  • $300 payment if the weighted whole farm P-index is less than a phosphorus loss risk of 3 (2-5 is medium risk).  All field scores weighted by the field size and risk of P loss from each field to attain a weighted average farm P-index.
  • $100 paid for annual data and P-index review after the first year.
  • $150 bonus – review P-index is less than 1 (very low) and for each 0.1 reduction in P-index.

To determine the the IPI on each farm, a technician works with the farmer to document management practices, phosphorus soil test levels and phosphorus applications.  This information is combined with soil and landscape characteristics within the IPI calculator.  A weighted average IPI is then calculated for each farm.  Field and farm results are provided to cooperating farmers.  The farmers also receive a listing of all the fields in the watershed – listed by highest IPI to lowest IPI.  This is done by having a farm ID number for each farm that allows for anonymity.  To review a real-world example, the 2012 Hewitt Creek watershed summary can be found here and includes watershed average values for six years.

No-till CornFarmers involved in performance-based incentive programs have found the no-till planting provides the most improvement in IPI scores.  Grassed waterways are another good way to improve IPI values.



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