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Competency Area 6: Nutrient Management and Planning

PO 49. Distinguish P-based from N-based manure application and describe implications.

Most manures contain approximately three times as much N as P.  Thus, applying manure to meet crop N needs will result in overapplication of P and K.  Unused P can be lost due to leaching, erosion, and runoff, and can contaminate groundwater and cause eutrophication.  Meanwhile, applying manure to meet crop P needs runs the risk of underapplying N, resulting in deficiencies for the growing crop and a poor yield.  It requires more land, as smaller manure applications will be utilized on each field, and requires supplemental N in some form, which can be expensive.

For example, compare nutrient content of poultry and dairy manure to crop needs:

When applied to meet N needs (left), both poultry and dairy manure provide much more P and K than needed.

When applied to meet P needs (right), both poultry and dairy manure fail to provide adequate N.