Research

by Keith Reid, Soil Fertility Specialist, OMAF

It is popular in some circles to bash corn as the source of many environmental problems, including nitrate contamination of groundwater. After all, corn is grown on more than two million acres in Ontario, and it does use a lot of nitrogen. Before we condemn corn, however, we should back up and look at the big picture.

Nitrogen Use Efficiency
It is easy to point at the amount of nitrogen that is applied to a field of corn, but the other side of the equation is the amount that is removed in crop yield. From this perspective, a high yielding crop of corn can actually remove more nitrogen than is applied. A bushel of corn can contain anywhere from two-thirds to one pound of nitrogen, with an average N content of about eight-tenths of a pound. With this nitrogen content, a 150 bushel corn crop is removing about 120 pounds of nitrogen from each acre, which, incidentally, is close to the nitrogen recommendation for most areas that are producing yields in this range.

The harvested yield of a crop only represents part of the total amount the plants need to absorb during the growing season. Nitrogen is also required to grow the roots, stalks and leaves that, in turn, produce the grain. There is roughly 60% as much N in the aboveground part of the crop that is not harvested as there is in the grain, so the average corn crop will have 72 pounds of N in the stover, for a total uptake of 192 pounds of nitrogen per acre. Nitrogen accumulation in the roots is much harder to measure, but estimates range from the same amount as in the stover, up to the total amount in the aboveground part of the corn crop. Much of this additional nitrogen is cycled from the soil organic matter through the crop and back into the organic form, but it does represent a significant buffer for nitrogen applied above crop removal, particularly on soils with low organic matter contents.

Even with the best technology, the absorption of applied nitrogen is not perfect. Bacteria and fungi in the soil will use fertilizer nitrogen for their own growth, particularly if there is lots of organic carbon available. This nitrogen immobilization is usually temporary, but it can significantly impact the availability of nitrogen to the crop. More permanent is the loss of nitrogen to the air, either as volatilization of ammonia gas, or by denitrification.

What Do Nitrogen Leaching Measurements Show?
In the Partners in Nitrogen project, Dr. Gary Kachanoski compared the losses of nitrate nitrogen below the rooting zone with different rates of applied N. He determined that there was little difference in the nitrate concentration between the zero N plots and those plots receiving the optimum economic rate of nitrogen. Where the optimum rate was exceeded, however, the nitrogen losses increased pound for pound with the extra nitrogen applied. Similar results were found at Cornell University in New York, where the concentration of nitrate in drainage water was similar at zero and one hundred pounds of nitrogen per acre, but tripled in plots where two hundred pounds of nitrogen per acre were applied.

So Where’s the Problem?
The foregoing would indicate that growing corn, on its own, does not contribute to nitrate leaching. But there ARE situations where we should be concerned about the potential for nitrate leaching.

The first situation is where nitrogen is routinely applied above the rates required for optimum crop yield. This nitrogen application may be from fertilizer or manure or both. Unfortunately, this affects a lot of acres since the perceived financial risk of lost yield from underapplying nitrogen is greater than the cost of the nitrogen fertilizer. This will likely remain a problem until growers start using yield monitor technology to define the optimum nitrogen rates for their own farms.

The second risk situation is from manure applications in the late summer or early fall. Warm, dry soil conditions in this time period can result in high rates of mineralization and nitrification. With no growing crop to absorb the nitrate produced, there is a high risk of nitrate loss. This loss will be predominantly denitrification on heavy textured soils, and leaching on the light textured soils. For groundwater protection, fall application of manure should be restricted on sandy soils, and cover crops should be used to tie up nitrogen over the winter on any soil type.

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