The committee concluded that each year in American agriculture there are six to nine million metric tons more nitrogen in farm fields than can be used by the crops growing in those fields. In other words, one fifth to one third of the nitrogen in the environment is unneeded. If farmers converted freely available nitrogen into reductions in fertilizer use, they would reduce contamination of water supplies and save an estimated two to three billion dollars annually1. Accounting for All Sources of Nitrogen On a regional and farm level, farmers are ignoring major sources of nitrogen and overapplying billions of pounds of fertilizers each year. These calculations did not include additional nitrogen available to crops in these field from legumes and manure.
Animal manure is the most often overlooked source of nitrogen for crops.
Many growers apply manure to their fields, and nationally, economically recoverable nitrogen from animal manure accounts for approximately eight to nine percent of total nitrogen inputs, and even higher in some regions of the country. As a result, they then overapply nitrogen fertilizer. Another substantial source of nitrogen is that made available from nitrogen-fixing crops such as alfalfa or beans. When these crops are grown, they leave available nitrogen in the soil, and when the next crop is planted, farmers do not need to apply as much fertilizer. As with manure, however, most producers do not fully account for this crop-supplied nitrogen.