Data from the 2012 census of agriculture was used to estimate livestock populations and production within each county. Available nutrients in manure were calculated by estimating average animal populations and multiplying this value by a manure production coefficient, a manure capture coefficient, the amount of nutrient expected to remain in the manure after storage, and finally the percent of those nutrients that would be crop available. These calculations were performed for both nitrogen and phosphorus.
The nutrient assimilative capacity of cropland was estimated by multiplying the amount a crop produced in 2011 times the nutrient content of that crop. This is a low estimate of actual crop need as it only considers nutrients exported with the grain (about 0.8 lb N are removed per bushel of corn harvested).Additionally, nitrogen requirements for production of soybean or alfalfa were not included. These plants are both legumes, meaning that can obtain some of their nitrogen from the air; however, research has shown that these plants tend to use nitrogen available in the soil first and in some cases may require supplemental nitrogen to achieve desired yields.