State Nitrogen Availability Estimates for Manure and Their Implications for Potential Phosphorus Loading of Soils.

Extensive research has documented fertilizer value of manure nutrients for crops. It has been long recognized that manure nitrogen (N) excreted by animals is not 100% available to crops. Surveys indicate failure to credit or under crediting manure nutrient value to a crop by farmers continues to be an issue.  Our goal was to assess the current state of manure nutrient availability recommendations and requirements in the US. We surveyed state recommendations for state nutrient availability calculations for four sources of manure: finish hog slurry, dairy cow slurry, solid cattle manure and broiler litter. The top 12 states for production of each associated commodity were determined using inventory data from the 2012 Agricultural Census. For each state and each manure type surveyed we attempted to identify nitrogen availability calculation recommendations from three sources: the State Land Grant University, the state USDA Natural Resource Conservation Service (NRCS) standards and supporting documents, and the state regulatory documentation for operations with a National Pollution Discharge and Elimination System (NPDES) permit. Availability estimates among states were more variable for strategies known to be more variable (e.g. surface application of liquid manure).  Our work emphasizes the varied approach to N availability calculations as we cross state borders.  The resulting differences can have substantial impact on land need for manure application and phosphorus loading for nitrogen-based manure applications.