The purpose of this report is to estimate annual phosphorus delivery from each field to the nearest surface water with current and alternative management practices. It uses the same equations as the P Index calculations but there are some differences in the information that goes into the calculations and the way the results are reported. It is designed to for use in agricultural runoff phosphorus reduction projects rather in than for nutrient management planning.
How the report is calculated:
This report is calculated with the P Index equations but there are two differences in how RUSLE2 is used. First, the soil used for the P Trade calculations is the predominant soil, the one that makes up the majority, of the field, rather than the dominant critical soil. Second, instead of being run as a rotation, soil loss is calculated for each field in a sequence of years. This means that changing data in later years will not change the calculated annual values for earlier years, as happens when soil loss is calculated in a rotation. To be able to calculate soil loss accurately with a sequence of years, two "set-up" years are required. That is why this report requires at least two crop years of information before the start year for each field.
How the P Trade pounds P (PTP) are displayed:
The P Trade report can be run for a farm, subfarm , or group. The report contains a row for each field with the field name, the soil series used for the RUSLE2 parts of the P Index equations (predominant), and the field acres. Then there is a column for every year in the calculation record with the total estimated P lb for the field for that year (calculated P loss per acre x acres). The P loss for each field will differ from year to year depending on crops, tillage, and fertilizer and manure applications.
How the P Trade report is used:
Planners can compare the P Trade report for a farm or subfarm under suites of alternative managements to see determine how much they might reduce P losses field-by-field and farm wide. The P Trade Report was developed to facilitate phosphorus water quality trading in Wisconsin and more information on how to use it can be found in P trade guidance from DNR.