sealPurdue News

December 1999

Alternative approaches to Gulf pollution

WEST LAFAYETTE, Ind. -- Modifying tile drainage systems and crop rotations in farmer's fields are the best methods to prevent nitrates from fertilizer from leaking off farms and into nearby streams, say two Purdue University agronomists.

Ron Turco, director of Purdue's Environmental Sciences and Engineering Institute, and Sylvie Brouder, assistant professor of agronomy, say that these alternatives are better than a "one-size-fits-all" approach of limiting nitrogen fertilizer use. Simply limiting fertilizer use probably won't solve the problem and will unfairly penalize some farmers, the agronomists say.

Excess nitrogen moving down the Mississippi River is thought to create a hypoxic zone, also known as a "dead zone," in the Gulf of Mexico. Some experts suspect that a large portion -- but not all -- of the nitrogen that ends up in the Mississippi River comes from fertilizer applications by farmers in the Corn Belt.

"I'm sure that you could travel through the Midwest and find some farmers who are applying more fertilizer than they need," Brouder says. "But that's not everybody. Most farmers are trying to use what they need."

A first step to reduce the amount of nitrate runoff would be to improve tile drainage-water management on farms, Turco says. "I maintain that a good chunk of water that is discharged into the Mississippi River and the Gulf of Mexico starts as tile water from agricultural drainage systems," he says. "I'm more convinced of this every day."

The Gulf of Mexico hypoxic zone is an area that has too little oxygen in the water to sustain fish and other marine creatures. The size of the hypoxic zone fluctuates from year to year, but the cigar-shaped zone can be as large as the state of New Jersey (7,000 square miles), stretching from the mouth of the Mississippi River to the Texas coastline.

The hypoxic zone doesn't kill large fish and sea mammals, because they are able to swim away to oxygenated water. Less mobile sea life, such as starfish, sea anemones and crustaceans, are unable to escape and die.

The zone is thought to exist because excess nitrogen flows down the Mississippi River each summer. Nitrogen is an important plant nutrient, and with so much of it in the gulf, small aquatic plants such as algae grow so much that they cause a lack of oxygen in the sea water.

The White House Office of Science and Technology Policy has embarked on a scientific assessment of the causes and consequences of hypoxia in the Gulf of Mexico. Otto Doering, professor of agricultural economics at Purdue, is one of six team leaders for the assessment. Doering is responsible for determining the economic consequences of proposed changes.

"The White House committee on the environment and natural resources has posted the reports from the six teams on the National Oceanic and Atmospheric Administrat Web," Doering says. "However, none of these reports calculated the effects of tile drain systems that are common throughout the Midwest. We just do not have the information on these to make scientific sense out of their impact.".

One of the many recommendations of the committees was that farmers in the Mississippi River basin reduce their nitrogen application rates by 20 percent.

According to Turco, the amount of nutrient moving into surface water varies greatly from farm to farm. "If you have a guy who is farming in a field that's not tiled, and that field isn't next to a river and it's not on sandy soil, then that farmer's not adding much to the problem," he says. "Asking him to cut his fertilizer use by 20 percent isn't fair."

More than half of the row crop farms in the Midwest use tile drainage systems to remove water so that the farmers can get into the fields for spring planting. Turco says that this drainage system is possibly a conduit between Midwestern fields and the Mississippi River. "With tile drainage you possibly have a direct avenue for drain water to enter surface water."

Turco points out that although tile drainage systems are a great benefit to farmers in the spring, they can actually hurt the farmer's crops later in the growing season. "You need tile drainage for three to six weeks in the spring because it allows an aerobic root zone for seed development. Beyond that its effectiveness is limited," he says. "When it rains during the dry summer months, the tile systems don't know that it's July and that water is needed. So the water that the crops need may move out of the root zone."

Turco suggests that farmers consider modifying their systems so that they trap water and hold it. "This would both conserve water in the months when it is needed and prevent the nitrates from entering the surface water by creating an anaerobic zone, which would remove nitrates as nitrogen gas," he says. "If you could store water in the tile system when you wanted to, you could effectively irrigate your field from below ground."

A second key factor in nitrate leaching is bare ground, according to Brouder.

"Nitrogen exists in the soil in several different forms," she says. "Nitrogen that comes from organic matter, such as decomposing plant residue, or nitrogen in the form of ammonium fertilizer does not move out of the soil. However, what happens is, if it is warm enough, microbes in the soil convert these forms of nitrogen into nitrate, which can easily be moved through drain tiles and into surface water."

Brouder says that as long as crops are planted on the ground, the crops will take up the nitrates as nutrition. However, when the ground is bare, nitrates produced by the microbes are carried away with heavy rainfalls.

"The length of time that the crops are in the ground is a key factor in whether excess nitrate is produced," Brouder says. "Short crop rotations that don't have plants growing late in the fall or early in the spring are leakier. Our data says that it's not just the applied fertilizer that is causing the nitrate leaching into surface water. It's also the type and length of the crop rotation.

"Thirty or 40 years ago, most farms in the Midwest were in a longer crop rotation system, which often included planting winter wheat and alfalfa. Now most farms are in a corn-soybean rotation, which is much shorter, in terms of ground cover during the year."

According to Brouder, one way to prevent nitrate runoff is by using cover crops, which are planted in the fall after the corn or soybeans are harvested and then are plowed under in the spring. "With a cover crop you have a place for the nitrogen to go," she says. "With bare ground, nitrification is going on and the nitrate is getting away. With plants, they take it up and hold on to it. This has the potential to be a no-net-cost solution to the farmer and perhaps eventually a net financial benefit to the farmer, because it could reduce the cost of his inputs." (Inputs is the general agricultural term for items such as seed, pesticides and fertilizer, including nitrogen, that a farmer must provide in order to produce a crop.)

Because the cover crops are plowed back into the soil, the nitrogen they have taken up is returned to the soil, and farmers don't have to apply -- or pay for -- as much nitrogen fertilizer.

Turco says that more research is needed on preventing nitrates from leaching into the tile drainage lines. "Our solutions take into account the variability of the farms," he says. "These are lower-cost ideas that have the potential to increase yields and profits."

Sources: Sylvie Brouder, (765) 494-4773;

Ron Turco, (765) 494-8077;

Otto Doering, (765) 494-4226;

Writer: Steve Tally, (765) 494-9809;

Purdue News Service: (765) 494-2096;

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