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Assess Nitrogen Fertilizer in Corn after Heavy Rains

Assess Nitrogen Fertilizer in Corn after Heavy Rains

Test to determine if you need to side-dress N after the big downpour.

Nitrogen, important for corn growth, has been a recent concern.

The first concern was with the poor tillage conditions last fall. At that time, drought was the weather condition on everyone's mind. Now with the record rainfalls, there are concerns that nitrogen has been lost to leaching or denitrification.

Assessing leaching or denitrification problems

GONE, GONE, GONE? That anhydrous you put on earlier this year may have been washed away. Testing now can help determine sidedress needs.

If your field is tile drained, one of the first things to look at is if there is water draining from the tile. If not, then it is more than likely the soil was dry enough before the rain to store the water. The nitrate in the soil profile may have been moved deeper in depth but it will still be available for plant use. There is not enough water to cause the anaerobic conditions needed for denitrification to occur. If the tile line is draining water, then there is a chance that the soil is waterlogged. There may be some chance of denitrification.

However, if water is standing and soil temperatures are greater than 50 degrees, then denitrification can and will occur.

Assessing the amount of available nitrogen

There are really only two tools left at this time of the growing season to determine whether to apply more nitrogen to a growing corn crop under non-irrigated conditions.

The first is the pre-side dress nitrate-nitrogen test. This soil test was developed at Iowa State University in the 1990s. The soil test was for a sample taken to a depth of one foot. The researchers in Iowa were able to calibrate it to an amount of nitrogen to apply.  It does not work well under Minnesota conditions. 

The only interpretation from many Minnesota studies on the pre-sidedress nitrogen test is that if the nitrate-nitrogen concentration is greater than 20 ppm then you do not need to apply extra nitrogen to the crop. This tool cannot be used to determine the amount of nitrogen fertilizer to apply.

The second tool is University of Minnesota Extension’s Supplemental NitrogenWorksheet for Corn, which can be found at .

This simple worksheet was developed in 1992 and has been modified and tested over the years as a means of helping people decide if supplemental nitrogen is needed. This decision aid is for situations when all of the nitrogen fertilizer was applied pre-plant, either in the fall or spring. It was not developed for determining nitrogen rates in a split-nitrogen program.

Keep in mind that good judgment is still important when using this decision aid. The worksheet should be used in June while you have side-dress application options available. The worksheet outcome is based on the answers to three questions. Each answer is weighted on how it affected nitrogen in the soil. The tool provides options based on your score.

This year, the state of Minnesota has run the whole gamut of soil moisture conditions. The effect of these conditions on the nitrogen available for corn growth will very across the state. Extension’s Supplemental Nitrogen Worksheet for Corn is a useful tool for determining if there is a need for additional nitrogen application to corn. If additional nitrogen is needed, 40 to 50 lbs. per acre will do the job.

To read a more detailed version of this article, visit Extension’s Crop News newsletter at

-By John Lamb and Dan Kaiser, soil fertility specialists, University of Minnesota Extension.

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