How to interpret the PI
The Snap-Plus program gives a P Index value for every crop year entered for each field. It also gives an average value for the rotation. This value can be compared among fields on a farm or in a watershed, and the fields can then be ranked according to their potential for contributing P to the nearest body of water. Below is a table showing ranges of P Index values and relative P delivery risk categories for Wisconsin agricultural fields.
| P Index Range | Potential for P delivery to nearby surface water |
| 0-2 | Low to medium |
| 2-4 | Medium to high |
| 4-6 | High to excessive |
| Over 6 | Excessive |
At present, the Natural Resource Conservation Service Wisconsin Nutrient Management Standard 590 (PDF) (September 5, 2005) is the only Wisconsin standard that gives a target value for the annual P Index. According this standard, a planner who follows a P Index strategy for phosphorus-based planning must keep the rotation-averaged P Index below 6 in order to continue to apply manure to a field.
