Agricultural scientists at the University of Illinois found that after a poor growing season in 1995, 100 pounds per acre of nitrogen remained in a test field. Field-tile monitoring in the subsequent winter and spring turned up some of the highest losses of nitrate during the six-year study. About 40 pounds per acre of nitrate was found in water flowing from the field tiles.

Nitrate contamination, which adversely affects the oxygen-carrying capacity of blood, most commonly occurs in ruminants, but also poses a risk to human infants.

The study -- detailed in March in Agricultural Ecosystems and Environment -- showed the relationship between the amount of nitrate remaining after harvest and that which leached in the next winter and spring. There also was a link between rainfall and leaching. The work was funded by the Illinois Groundwater Consortium and the Illinois Council on Food and Agricultural Research.

"The more rain that fell on the field, the greater the amount of nitrate found in the tiles," said Lowell Gentry, a research specialist in the U. of I. department of natural resources and environmental sciences. "Again, after the bad growing season of 1995, higher rainfall in 1996 increased the flow of water from the field tiles by 75 percent, but nitrate in tile flow increased by 125 percent."

Some of the nitrate in tile flow can be attributed to the rain, but some was due to the increased amounts left in the field after the bad growing season, he said. Forty-acres of tile-drained farmland can transport between 800 and 1,900 pounds of nitrate annually. Nitrate samples taken from the Embarras River adjacent to the test field closely matched the pattern in tile drainage and showed that an average of 3.7 million pounds of nitrate per year went downriver at Camargo, Ill.

Gentry has monitored field-tile flow for several years, noting that 95 percent of the nitrate flow occurred in winter and spring. This suggests that using nitrogen fertilizer in the spring is the best way to prevent leaching. Highest nitrate measurements came after a winter application of ammonium fertilizer. The next May, after heavy rains, 50 parts per million of nitrate-nitrogen were found in water flowing from the fields. "Whenever we see nitrate in tile water, it is some combination of fertilizer nitrogen and mineralized soil nitrogen," he said. "But when you see 50 parts per million after fertilization, it's pretty clear to me that the fertilizer was moving."

The push for earlier planting of corn, and fall price discounts for nitrogen fertilizer have encouraged fall application. To reduce nitrate flow into rivers, however, Gentry does not recommend fertilizing in the fall. The U. of I. Agronomy Handbook recommends waiting until soil temperatures drop below 50 degrees Fahrenheit at the 4-inch level before applying nitrogen fertilizer in the fall and also suggests the use of a nitrification inhibitor.

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