The use of synthetic fertilizer increased seven to eight times over the last forty years (Figure 2). The major effects of fertilizer come from nitrogen (N) and phosphorus (P). Nitrogen pollution is now the single greatest source of water pollution in lakes, rivers, and bays. About half of the nitrogen and phosphorus applied is not taken up by crops; it is left in the soil or leaches into ground and surface water. In industrialized (First World) countries, about 70 percent of crops are fed to livestock, which are typically raised in confined conditions such as feedlots and "factory farms." The animal waste is concentrated in a small area and becomes a further source of nitrogen pollution.
Excess fertilizers lead to "eutrophication." As fertilizers enter water systems, they cause an explosion of growth by algae and aquatic plants. When those organisms die they are decomposed by microbes that quickly deplete the oxygen in the water, thus killing animals such as fish and shellfish.
Fertilizer runoff from industrial agriculture has created forty large, oxygen-starved "dead zones" around the world. A dead zone the size of New Jersey forms at times where the Mississippi River drains into the Gulf of Mexico. This zone receives fertilizer from a tremendous agricultural area, including Kansas, the Dakotas, Iowa, and Illinois.
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