Crop nutrient leaching is agriculture’s dirty laundry. The corn/soybean system on tile-drained soils is blamed for the Gulf of Mexico dead zone the size of Massachusetts. “This allows fertilizer and any mineralized soil nitrogen (N) to be lost when roots aren’t present, which is most of the time,” says Mark David, the University of Illinois biogeochemist who recently documented this connection.

A recent University of Illinois/Cornell University study blames the most heavily tile-drained Corn Belt regions for the Gulf of Mexico’s nitrate problem.

“Farmers use the same amount of N as they did 30 years ago and get much higher corn yields, but we have created a very leaky agricultural system allowing nitrate to move quickly from fields to the Gulf of Mexico,” says David.

Agronomic researchers have advice for ways to reduce nutrient leaching.

Cover crops and crop rotations containing perennial crops effectively anchor nitrates and sediment. And legumes added to corn-bean rotations contribute valuable N less likely to leach downstream.

University of Minnesota Soil Scientist Gyles Randall found 30-50 times more nitrate leaching from corn-soybean fields than from alfalfa. Nitrate concentrations in tile drainage water are related to crops grown, rate and timing of N application, he found.

The worst nitrate leaching is “on intensive corn and soybean rotations in heavily tile-drained areas,” says David, who’s studied the problem for 17 years.