Nationwide, farmers are getting more grain out of their fertilizer, according to the National Corn Growers Association. Nitrogen use has decreased 38 percent in terms of pounds per bushel of corn, said Rod Snyder, director of public policy for the trade group.
Nevertheless, corn farmers are using as much fertilizer per acre as ever on their high-yielding crops, according to federal data. On average, farmers applied 58 pounds of nitrogen per acre to corn crops in 1964. By 1985, that number had grown to 140 pounds per acre, where it remained in 2010.
In addition, more acreage is being planted as corn prices boom, according to U.S. Department of Agriculture data. In 2012, farmers planted 96 million acres of corn – the highest level in nearly 70 years, and up nearly 22 percent from a decade earlier.
“The primary cause [of nitrate pollution] is row crop agriculture, and the primary culprit of that is corn. That’s being exacerbated by the fact that corn is expensive right now. People are taking areas out of conservation and putting them into corn production,” said Matt Rota, science and water policy director for the New Orleans-based Gulf Restoration Network.
Farmers use many methods to reduce runoff: planting cover crops, adjusting the amount of fertilizer and when it’s applied. Such conservation practices in the Upper Mississippi River basin reduce nitrogen loss by an average of 18 percent per acre, according to a USDA report.
But the same report found that only 14 percent of cropland consistently had good nitrogen management.
“What we are showing is how bad the problem would be if there were no conservation,” said Lee Norfleet, a USDA soil scientist. “If there is an uptick, it would be a lot worse if a lot of (conservation) practices had not occurred.
In addition, Norfleet said, it takes time for farmers to adapt to the changing genetics of corn, which needs less nitrogen now. Farmers who don’t adjust could end up over-applying nitrogen at the rate of 23 pounds per acre.
Other sources of nitrate also may have increased.
The Missouri River basin, which includes the cities of Denver, Omaha and Kansas City, grew by more than 1.3 million people between 1990 and 2000, now reaching more than 11 million people.
In addition to crops, “there’s also animal feeding operations, sewage treatment plants. Petrochemical industries discharge into water. And then urban runoff,” Rota said.
Home use of fertilizer for yards and gardens nearly doubled between 1987 and 1997, although it was just 1 percent of the total nitrogen, according to the USGS nationwide study. Nitrogen from manure increased almost 5 percent and nitrogen from the atmosphere, largely from the burning of fossil fuels, increased 13 percent.
Reliable data on another urban source of nitrate, sewage treatment plants, are not available. And while septic systems are responsible for about 12 percent of the nitrate in the Missouri River watershed, there is no data on whether it has increased.
None of this information provides definitive answers about what is driving the increased contamination of the river. Nitrogen applied to the ground may never reach the Gulf, depending on weather patterns, vegetation, waterway conditions and many other factors.
“There isn’t clear evidence to support the hypothesis that it’s ag or urban or wastewater treatment plants,” Sprague said. “We don’t have a good understanding of how these things have changed over time.”
A foot soldier in the war
The Missouri River has been monitored at Hermann since 1844, which makes it a good place to observe changes in the river.