The efficiency of fertilizer used for maize production in the major maize producing states (Illinois, Iowa, Indiana, Minnesota, and Nebraska) in the Mississippi River Basin increased between 1986 and 2000. Maize yields have increased about 20% from 1986 to 2000, while N fertilizer use has remained roughly constant. Between 1976 and 1986, an average of kgN of fertilizer was applied for each kg of maize harvested. Between 1996 and 2000, an average of kgN of fertilizer was applied per kg of maize harvested. If this improvement in fertilizer use efficiency had not occurred, nitrate flux in the Mississippi River in 1996–1998 would have been about 50% greater than the measured.
