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Nitrogen Management:
Impact On Yield and Water Quality

Robert Hoeft Robert Hoeft
Department of Crop Sciences
(217) 333-4424; rhoeft@illinois.edu
Jeffery Warren

Jeffery Warren
Senior Research Specialist
Department of Crop Sciences
(217) 244-0153; j-warren@illinois.edu

Not pictured: Liza Gonzini, Senior Research Specialist, Department of Crop Sciences, (217) 244-0153, l-gay@illinois.edu

The warm fall and early winter, followed by a cooler and wetter than normal spring, has raised a lot of questions about nitrogen management for the 2002 crop. The full answer to those questions will be known only this fall, when the corn crop has been able to integrate the impact of these conditions into its final yield.

Even though farmers waited until November to fall-apply nitrogen, the soil temperatures resulted in a higher rate of conversion of ammonium to nitrate. Based on previous research and actual soil temperatures for this year, the amount of ammonium that had been nitrified by mid May varied from 100 percent in southern Illinois when ammonia was fall-applied without a nitrification inhibitor to less than 20 percent when applied with an inhibitor in early December in northern Illinois (Table 1). This is at least double the rate of conversion that would be expected in a typical year.

Ammonia App. Date N. IL C. IL S. IL N. IL C. IL S. IL
Ammonia w/o inhibitor Ammonia w/ inhibitor
% ammonia nitrified by May 13
Nov. 1 63 77 100 26 31 88
Dec. 1 46 59 100 19 23 70
Mar. 15 37 38 100 15 17 53
Apr. 1 33 38 48 14 16 48

Table 1. Effect of date of application of ammonia, use of a nitrification inhibitor,
and location in Illinois on the rate of ammonium conversion between application date and mid May.

Conversion of ammonium to nitrate in itself does not cause nitrogen loss; rather, it allows the nitrogen to be in the form (nitrate) that is susceptible to loss by the process of denitrification and/or leaching when soils become excessively wet. In 2002, the high rate of conversion of ammonium to nitrate immediately preceded an extended rainy period that left soils saturated for several days. Under typical May soil temperatures, denitrification would have been the major loss mechanism on medium- and finer-textured soils. However, the cooler than normal soil temperatures in late April and early May reduced the potential for this loss mechanism significantly (Table 2).

Soil Temperature % NO3-N Lost/
day of saturation
> 65 °F 4-5 %
55-65 °F 2-3 %
< 55 °F 1-2 %

Table 2. Relationship between soil temperature and rate of denitrification.

Combining the information in Tables 1 and 2 and assuming a person had applied 180 lb N/acre on Nov. 1 and had soils saturated for nine days in mid May when temperatures were between 55 and 65 degrees would indicate a nitrogen loss of about 40 pounds per acre from denitrification.

A study was initiated in central Illinois to evaluate the effect of time and rate of N application on the amount of N lost via tile lines. Preliminary results have shown that rate of application was the most important factor affecting the amount of N lost (Table 3).

Sample Period Time of N application
Fall Spring
Fertilizer N, lb/acre
0 140 210 210
N loss, lb/acre
Nov 2.0 1.2 1.7 1.7
Dec 3.1 5.5 6.4 4.0
Jan 1.3 2.3 6.0 5.7
Feb 0.6 0.6 2.2 1.2
Mar 3.4 7.6 4.0 4.9
Apr 9.0 18.1 14.7 12.8
May 16 10.0 13.2 21.0 18.3
Total 29.4 48.5 56.9 48.6

Table 3. Effect of time and rate of ammonia application on
the amount of N lost from tile lines from early November through mid May.

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