Example: Estimating Manure Phosphorus Application Rates
P-Based Application Rate = Corn Phosphorus Need (2); Manure P Content (11)
- Corn Phosphorus Need was calculated previously in equation 2.
- Manure P Content is representative concentration of manure P from manure analysis.
The Nebraska P-Index developed by the University of Nebraska (2006) will be used for analysis. The P Index risk value is the sum of the erosion and runoff components. The interpretation of risk and recommended manure application risk fall into one of four levels:
Low (0-2). Current practices keep water quality impairment low due to agricultural P pollution. Manure can be applied at rates sufficient to meet crop P needs.
Medium (2-5). Delivery of agricultural P many cause some water quality impairment and consideration should be given to alternative conservation and P management practices. Manure can be applied at rates sufficient to meet crop N needs.
High (5-15). Phosphorus loss from the field causes much water quality impairment. Remedial action, such as alternative conservation measures or P management practices, is required. Manure can be applied, but applied P should not exceed crop P removal. Crop P removal can be the sum of single year crop P removal over 5 years with no manure application during the next four years to this same field.
Very High (> 15). Impairment of water quality is extreme and remedial action is urgently required. Phosphorus application should be discontinued. Improved conservation measures should be implemented.
Requirements for application of a phosphorus-based manure rate to a field include:
- No single manure application shall exceed the nitrogen-based rate of the planned crop receiving the particular manure application.
- Phosphorus in manure should be considered 100 percent available unless soil phosphorus concentrations are below optimum levels for crop production. In that case, 70% availability for first year will be assumed.
- If the actual crop schedule differs from the planned crop schedule, then any surplus or deficit of phosphorus shall be accounted for in the subsequent manure application.
Nebraska P-Index Results (Assessment completed in 2007)
|Field||P-Index Value||Application Rate|
|Field 1-Pivot||0.7||N-based Rate|
|Field 2-Feedlot Quarter||1.2||N-based Rate|
|Field 3-Dry Quarter (West)||10.3||P-based Rate; 4-year P supply for single application|
|Field 3-Dry Quarter (East)||6.6||P-based Rate; 4-year P supply for single application|