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How Much Nitrogen Does Corn Need?

Fred Below Fred Below
Department of Crop Sciences
(217) 333-9745
  Patricia Brandau Patricia Brandau
Senior Research Specialist
Department of Crop Sciences
(217) 244-5953

Recent surveys and one-on-one interviews have shown that many Illinois farmers tend to err towards excess applications of fertilizer N. Although the reasons for overapplication are many, the most commonly cited are the desire to produce high yields, uncertainty about the true N requirements of corn, and a lack of research pertaining to their specific area and production practice.

Nitrogen fertilizer overapplication has been viewed by many as a cheap form of insurance to assure against the possibility of N losses and to make certain that sufficient N is available in case the environment is supportive of high yields. However, skyrocketing costs for N fertilizers and increasing concerns about water quality are compelling reasons for farmers to justify and/or improve their management of N.

Specifically, the goal of our current research has been to verify how well the Illinois fertilizer N recommendation rate of 1.2 lbs of N (minus any applicable credits) per bushel of grain produced applies to modern high-yielding corn hybrids. Documentation that this recommendation is adequate (or too high) for modern hybrids would serve as justification against fertilizer overapplication, which would save farmers money and improve the quality of Illinois waters.

N-deficient corn plants
Figure 1. N-deficient corn plants
showing characteristic leaf
yellowing and firing.
corn yield response

Figure 2. Representation of
a corn yield response to the
rate of fertilizer-applied N.

A series of on-farm and experiment-station trials conducted over a wide range of cultural practices and production conditions were used to evaluate fertilizer N needs in Illinois. In addition to unfertilized plots to assess the soil's capacity to supply the crop with N, we varied the rate of fertilizer-applied N (six to seven rates in 30 to 40 lb increments) in order to cover the range in amounts needed to optimize yield. All other management practices at the individual locations were in accordance with local recommendations considered conducive to high yields.

Of approximately 40 trials that we have conducted over the past three years, in no instance was more than 1.2 lbs of N per bushel of grain needed to optimize productivity, and in most cases, considerably less was required. Averaged over locations and years, our calculated economic optimum N rates average slightly less than 1.0 lbs per bushel (with a range of almost none to 1.2 lbs). Somewhat surprising were the relatively high yields produced without any supplemental N (generally over 100 bushels per acre), and the fact that the highest N requirements were typically associated with the lowest-yielding environments. Conversely, locations with the highest yields tended to have lower per-bushel N requirements, presumably because the soil supplied the N in these productive environments.

Our data refute the common reasons given for overapplication of fertilizer N and support using no more than current University of Illinois recommendations.

This research was supported by the Illinois Council on Food and Agricultural Research (C-FAR) and the Potash & Phosphate Institute Foundation for Agronomic Research (FAR).

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