http://impact.nola.com/environment/print.html?entry=/2012/07/dead_zone_pollutant_nitrate_gr.html

Dead zone pollutant grows despite decades of work

“Farm fertilizer and livestock manure are the two biggest sources of total nitrogen in the Missouri River watershed, together responsible for 70 percent, according to 2011 USGS data. A 2008 study of the entire Mississippi River watershed had similar findings, with agriculture contributing 70 percent of the nitrogen and phosphorous that ended up in the Gulf. Scientists in 2009 also reported a direct correlation between intensive crop production, particularly corn, and nitrate-nitrogen levels in rivers.

“Nationally, consumption of nitrogen fertilizer has tripled since the 1960s, surging to 12.3 million tons in 2010, according to USDA data. The amount of nitrogen applied as farm fertilizer grew 18 percent between 1987 and 1997, according to a 2006 USGS study.”