John Lamb, Fabian Fernandez, and Daniel Kaiser
University of Minnesota Nutrient Management Team
Nitrogen is important for corn growth, and has been a recent concern. This year similar to many years has not had normal weather. Planting has been delayed by moist conditions and cold temperature. Now with the record rainfalls last weekend (May 30 through June 1, 2014), there are concerns that nitrogen has been lost to leaching or denitrification.
Some of the small grains and corn looked yellow after emergence. This yellow color could have been caused by several conditions and the lack of nitrogen is one of them. First, nitrogen fertilizer applied last fall was lost due to soil water movement from the spring thaw. If the nitrogen fertilizer had time to convert from an ammonium form to nitrate-N it is possible for movement to occur. This is the reason nitrogen BMP's discourage fall application and use of a nitrate-N source of fertilizer before planting. If the tile lines were running with a large amount of water this spring, movement is a prime suspect. Second, the soil temperatures in May were colder than normal. This is a good news, bad news situation. The good news is that ammonium was not converted to nitrate-N (as well as sulfate-sulfur) very fast. The bad news is the cold temperatures slowed the mineralization of N from the soil organic matter. Because the soils were cold this spring, denitrification probably was not a big problem at this time.
Late May-Early June concerns
As mentioned in the introduction, most of the state of Minnesota received large amounts of rain at the end of May. The soil temperatures have finally warmed up. Now is the time to be concerned about leaching and denitrification.
Assessing leaching or denitrification problems
If your field is tile drained, one of the first things to look at is if there is water draining from the tile. If the tile line is not draining water, then it is more than likely the soil was dry enough before the rain to store the water. The nitrate in the soil may have been moved deeper in the soil profile but it will still be available for plant use. There is not enough water to cause the anaerobic conditions needed for denitrification to occur. However, if the tile line is draining water, it is very likely that the water is carrying nitrogen in the form of nitrate. Also, if there is water draining there is a chance that the soil is waterlogged. There may be some chance of denitrification, but 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 only two tools at this time of the growing season that have been demonstrated 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 and is based on samples 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 Nitrogen Worksheet for Corn, which can be found at
With an online calculator version:
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 sidedress 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. 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.
So you need to apply N, what are your options?
The following bring up three different scenarios.
First: I did not get my nitrogen on last fall or this spring: You need to start sidedressing nitrogen fertilizer as soon as possible. Your options include the use of 28 % as a carrier for the herbicide as part of a weed and feed program and a sidedress application of N fertilizer by itself. The soil needs to be dry enough to handle the traffic. Side dressing with anhydrous ammonia, urea, or UAN 28 % are good options. Anhydrous ammonia and UAN 28 % injected between the rows is best.
Second: If you applied N in your starter or surface band at planting, you have a little more time. Research conducted by Jeff Vetsch and Gyles Randall from the Southern Research and Outreach Center indicates that side dress applications are successful up to V6 to V8 corn growth stage. Waiting to after V8 growth stage in Minnesota increases the chances of grain yield loss.
Third: If you applied your fertilizer preplant in either the Fall or Spring, then we suggest you use the Supplement Nitrogen Worksheet for Corn to evaluate the N needs.
It seems that each year in Minnesota we have non-normal weather conditions. This is something we can not predict or fully manage around. Hopefully, we can move on from the current conditions and raise a good crop.
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