However, soil erosion can carry a considerable amount of particulate phosphate to streams. About 11 million tons of nitrogen and 2 million tons of phosphorus are applied annually in commercial fertilizer. Another 6.5 million tons of nitrogen and 2 million tons of phosphorus are applied in manure. Fertilizer applications are related to the intensity of agriculture and to practices such as tillage and irrigation. Animal manure also can be a significant source of nutrients.
Soil erosion is a major contributor of phosphorus to streams.
Annual sales of commercial nitrogen and phosphorus fertilizer in the United States. Between 1945 and 1985, the use of nitrogen fertilizer increased twentyfold. The use of phosphorus fertilizer increased about fourfold between 1945 and 1980. During 1989-93, the annual use of both fertilizers remained fairly constant. Estimated application of nitrogen in commercial fertilizer and manure during 1987.