Keep Nitrates Where They Make You Money Pointers for Keeping Your Nitrogen Where You Benefit Most

By Susan Winsor, Corn and Soybean Digest
9 December 2010

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.  

How to reduce nitrate loss 

Here are ways to reduce nitrate loss from tile-drained fields from University of Minnesota soil scientistGyles Randall, Iowa State Agronomist John Sawyer, Kansas State University Agronomy Professor David Mengel, University of Illinois Biogeochemist Mark David and other land-grant university agronomists.

  • Limit average tile depth to 3 ft. instead of 3 1/2-4 ft. This will reduce nitrate outflow by 10-25% but still provide ideal field conditions for field activities, says Gyles Randall, University of Minnesota soil scientist.
  • Use plant-canopy sensing to determine the adequacy of optimal N rates and whether N is required in years with above-normal rainfall, Sawyer says. “This can add confidence for growers to use optimal N rates and not over-apply N to address potential losses that may not occur.”
  • Cover crops have been shown to reduce nitrate losses, especially where tile lines flow in February and March, Randall says. Cover crops capture nitrates remaining after corn harvest.
  • Cover crops “have to be planted early enough to get good growth prior to spring thaw and the start of the spring tile run, when most of the residual N remaining from the previous crop will have passed out of the system,” says Kansas State University Agronomy Professor David Mengel. (See related story p. 18.) “They can also pick up some of the early mineralized N in the spring if left to grow through March and April.”
  • Use targeted wetlands, controlled drainage and end-of-tile bioreactors where appropriate to remove nitrates from tile flow, Sawyer says. “Use buffer strips where appropriate to remove nitrate from subsurface groundwater flow to surface waters.”
  • Apply the right amount of N in the spring rather than the fall (or sidedress), establishing a more complex cropping system incorporating cover crops or even biofuel crops when there are markets. Install end-of-drainpipe controlled drainage, bioreactors or wetlands, advises University of Illinois Biogeochemist Mark David.
  • Rotate corn with other crops not fertilized with N, as the N application rate is typically less for the corn crop and the application rate is lower than for corn following corn, Sawyer says.
  • Apply fertilizer N products and manure at optimal timing for the product to increase supply to corn with lower chance of loss, Sawyer says.
  • Use controlled-release N fertilizers where appropriate to limit exposure of applied N during very wet springs and in early spring during periods of limited or no crop uptake, Sawyer says.
  • Consider “a scenario where you only apply 75-80% of the normal N early, preplant or sidedress, and then use sensors to determine if additional N is needed around tasseling, Mengel says. “If N loss was lower than normal, or mineralization higher than normal, no additional N would be needed. If N loss was higher than normal or mineralization lower, you would need more, but the crop would respond and no yield would be lost.”
  • Improve soil-N-testing methods to determine the availability of mineralizable N and previous crop carryover N, especially following dry years, legumes or past manure applications.
  • Know manure’s nutrient content and application rate, spread it uniformly and incorporate it immediately.