When manure is applied to field with soil test P between 100 and 200 ppm on a WPDES permitted farm, the field must maintain an average P Index of less than 6 and a P2O5 balance that is 50% less than crop removal for a period of no more than 4 years. While the rotation used for soil loss calculations on these fields can still be up to 8 years in length, the rotation used for calculating average P Index and the P2O5 balance cannot be longer than 4 years and must start no earlier than the previous or current planning year. Whenever a CAFO field has a soil test of 100-200 ppm, an additional control for the CAFO P Rotation Settings will appear under the P2O5 and K2O balances. This control has two parts:
Start: This is used to set the start year for the P2O5 balance and P Index averaging period.
Year: To the right of the start year setting, the number of years in the CAFO P rotation is displayed. This number cannot be changed by the planner and is either 4 years or the number of years from the start year to the end of the general rotation, whichever is shorter.
The Avg P Index and P2O5 and K2O Removal and Balance results in the blue highlighted cells are for the period shown in the CAFO P Rotation Settings, rather than for the regular rotation. The P2O5 balance message displayed under the CAFO P Rotation Setting represents applications at 50% of crop removal, rather than the 75% of crop removal level used for meeting 590 guidelines for fields with soil test P greater than 100 ppm.
Example: The field shown in the Cropping screen show below has a soil test P of 150 ppm and manure is planned for 2014, so the CAFO P Rotation must be used for checking the P2O5 balance and the Avg P Index. The general Rotation Settings are for a 6-year rotation starting in 2013. The CAFO P Rotation is also set to start in 2013 and will extend for 4 years. In this example, the CAFO P Rotation Avg P Index is 4, which is less than 6, and the CAFO P Rotation P2O5 Balance is -202, which is less than -125. Therefore, the manure application planned for 2014 meets the guidelines.
Initially, the CAFO P Rotation start year will be set to the same year as the general rotation. If manure is planned in a year that is not within the years designated by the CAFO P Rotation Settings, then the application will flag orange on the Cropping screen and the Nutrient Application Planner will show a message indicating that the start year should be changed so that the Avg P Index and the P2O5 balance and P2O5 balance target message will be for a time period that includes the manure application (See Additional Restriction Flagging for WPDES Permitted Farms for more information.)