Don't look for an easy solution to the hypoxia, or oxygen deficiency, problem in the Gulf of Mexico.
But do look for one, soil scientists urge farmers.
"Agriculture must do all it can to reduce the hypoxia problem," says Gyles Randall, University of Minnesota soil scientist. "Otherwise, there could be pressure from the public for regulations."
So what is the problem? Abundant nitrogen (N) flowing down the Mississippi River from its tributaries. Nitrogen stimulates the growth of oxygen-depleting algae and phytoplankton, which kill fish and other oxygen-loving aquatic life.
Called the "dead zone," the hypoxic area lies in a region that provides the U.S. with more than 40% of its commercial fisheries. Fishermen and shippers are blaming the hypoxic zone for declines in their catch.
Growing steadily over the past 20 years, the zone is estimated to now cover nearly 7,000 square miles.
Its size varies - it peaks in June, July and August, coinciding with spring and early summer N use in the Midwest. (It takes about a month for flow from the Upper Mississippi to reach the zone.)
Fertilizer, livestock manure and nitrogen fixed by legumes account for a relatively high percentage of the errant N, estimates University of Illinois soil scientist Bob Hoeft.
According to the U.S. Geological Survey, 56% of the N comes from fertilizer runoff, with the Upper Midwest as the biggest contributor. Another 25% of Mississippi N is from animal manure. Municipal and domestic wastes, in contrast, account for only 6%.
"Correcting the hypoxia prob-lem is going to be important to the public," points out Randall. "But we have a huge, complex biological system, involving some 33 states, feeding into the Mississippi River, and solutions will not come quickly. Nevertheless, agriculture will need to step to the plate and show others that we can improve our management and keep more nitrogen in the soil."
Farmers need to know how much N their fields hold and how much more is needed, points out Randall.
"I speak to many farmer groups," says Iowa State soils researcher Alfred Blackmer. "I know that they, along with the fertilizer industry, are aware of the need to improve nitrogen management. But there are no easy methods to quickly reduce the hypoxic areas."
Part of the problem is that some factors aren't controllable.
"One such factor is precipitation," says Randall.
During the drought of 1988, little hypoxia was found in the Gulf. But in wetter years, nitrate nitrogen in the soil is mobile, and some leaches into tile lines.
Soil organic matter, too, is a problem.
Southern Minnesota, Iowa and Illinois all have high-organic-matter soils and a lot of tiled ground. Water movement in the soil carries N from organic matter through tile lines and into surface waters.
"We need to find ways to minimize those losses - perhaps through strategically placed holding ponds where some of the nitrate can be biologically removed," says Randall.
Rotating corn and soybeans allows more subsurface drainage than alfalfa and pasture, he points out.