The U.S. Geological Survey’s National Water-Quality Assessment program requires nutrient input for analysis of the national and regional assessment of water quality. Detailed information on nutrient inputs to the environment are needed to understand and a
Gronberg, Jo Ann M.,Spahr, Norman E. nspahr@usgs.gov. County-level estimates of nitrogen and phosphorus from commercial fertilizer for the Conterminous United States, 1987???2006. 2012-01-01.