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Fred Below Fred Below
Professor of Crop Physiology
Department of Crop Sciences
(217) 333-9745 fbelow@illinois.edu

Within Field Variation in Corn Response to Nitrogen

Yield monitors often reveal areas of differing productivity within individual fields that in many cases are not related to any obvious physical or topographical features. Intuitively, such variability should be manageable by altering the production inputs and/or the cultural practices, and a number of technologies have been developed to manage field variation. Variable rate applications of P, K, and lime are widely available, and similar technologies, are emerging, or are under development, for fertilizer N.

The prerequisites for successful use of variable rate nutrient applications are: 1) that areas of differing nutrient supply exist in the field; 2) that these areas can easily and readily distinguished; and, 3) that judicious application of the nutrient improves yield and/or reduces overall fertilizer use. These prerequisites are most completely met for fertilizer N use, where N deficiency is fairly obvious as paler green color and death of lower leaves, and where different field areas are likely to have varying levels of available N due to differences in the rate of N mineralization and/or the degree of N loss. Selecting the proper N rate is crucial for maximal corn productivity, as N rates lower than optimal can seriously decrease yields, while rates higher than necessary are economically costly and environmentally damaging. Thus, applying fertilizer N where it is most likely to achieve a positive yield response could simultaneously make the farmer more money and improve the quality of Illinois waters.

The objective of the research being reported today was to ascertain how much variability in responsiveness to fertilizer N exists within individual fields. Experiments were conducted in 2003 on four commercial-size fields in or around Beardstown, IL. In addition to unfertilized areas to assess the soil's capacity to supply the crop with N, we varied the rate of fertilizer applied N (rates of 100, 150, 200, and 300 lbs N/acre) in order to cover the range in amounts needed to optimize yield. All other management practices at the individual locations were in accordance with recommendations considered conducive to high yields. At maturity, we sampled ears in a manner that allowed for determination of both N treatment effects and field spatial variability.

Although all four fields exhibited a positive yield response to incremental increases in N supply, there were differences among fields in the type of response function, and in the degree of responsiveness to N. All four fields also had regions where the yield response to N was distinctly different from the field average; and all fields had areas of no, or minimal response, and areas of linear response to fertilizer N. An example of this variation within a 60 acre field is shown in the Figure. Each of the 10 regions depicted represents approximately 2.5 acres, and shows that all areas of this field did not respond the same to fertilizer N applications. Documentation of such variability is needed for devising N management strategies that utilize variable rate technologies for optimizing fertilizer N applications.

Research support provided by Cargill Crop Nutrition for this project is gratefully acknowledged.

Department of Crop Sciences
College of Agriculture, Consumer and Environmental Sciences
University of Illinois Extension
Copyright © 2004 University of Illinois
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