The Natural Resource Conservation Service Wisconsin Nutrient Management Standard 590 (PDF) (September 5, 2005) includes the P Index as a tool for reducing the risk of phosphorus (P) delivery to surface water when planning manure applications.
This standard allows producers to choose from two strategies for manure phosphorus management:
(1) Limits based on soil test phosphorus (Soil test P strategy)
(2) Limits based on the risk of runoff P delivery to surface water (P Index strategy).
A producer who follows the P Index strategy must keep the rotation-averaged P Index at or below a value of 6 in order to apply manure to a field. When the P Index is above 6, a field is deemed to have too high a potential for phosphorus losses for manure P applications to continue.
Fertilizer phosphorus applications are not subject to these P Index restrictions because the standard already limits them to crop need as identified by UW-Extension fertility recommendations.
A caution for nutrient management planners: Using the P Index standard, manure P may be applied to fields at rates that are in excess of crop P removal. Planners, however, should be aware that these excess P applications will cause soil P concentrations to increase over time. As soil P increases, the risk of runoff P losses, and consequently P Index values, also increase. If rotation average P Index values are at 5 or 6, building soil P concentrations can result in P Index values higher than 6 in future rotations. Altering field management to reduce runoff and erosion may be used to counteract the effect of rising soil P concentrations on runoff P loss risk, but under any management scheme that includes excess P applications, soil P concentrations can eventually rise to levels that will result in P Index values greater than 6.