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Oh Where Oh Where did my Nitrogen Go?

by Daniel Kaiser and Jeffrey Coulter, University of Minnesota Extension Specialists

With all of the flooded soils and wet fields there likely are questions on denitrification and whether side-dress nitrogen (N) should be applied. The fact is that it can be difficult to predict the amount of N lost. However, two things should be considered when dealing with denitrification:

  1. Denitrification only occurs when nitrogen is in the nitrate form. For fall applied N as long as application occurred after the soils cooled down to appropriate levels then the risk of N loss over the winter and in early spring should be low, especially since soil temperatures have been fairly low for most of the spring.
  2. Denitrification is due to microbial processes. Therefore, as soil temperatures decrease microbial activity decreases and the risk for denitrification decreases.

Nitrogen deficiency in corn
Photo 1. Nitrogen deficiency symptoms in lower canopy of corn.
At this time of the season there is always going to be some risk for N loss, but as temperatures remain relatively cool we should have a smaller risk for denitrification than if temperatures were warmer. The amount of N lost can be a small percentage to nearly the entire amount of N applied. Should we be worried about the worst case scenario at this time? Maybe not and the best thing to do is to wait and see what type of visual symptoms corn is showing around V4 or V5. The worksheet listed below was developed to aid producers in determining whether supplemental N should be applied to fields with all N applied either in Fall or Spring. U of M Supplemental Nitrogen Worksheet for Corn

N deficiency symptoms on corn leaf
Photo 2. Nitrogen deficiency symptoms on corn leaf.
Fields that should be scouted first are those that had N applied the earliest in the fall, especially if soil temperatures had not fully stabilized below 50 degrees by the time of N application. Also, keep an eye on fields where N may have been applied where soils were wet enough to lead to less than optimal incorporation.

Side-dressing N is a viable option if it is determined that some N loss has occurred. Generally, the earlier N can be applied when side-dressing, the better. Although N uptake occurs throughout the growing season, peak demand generally starts at the V5 growth stage and progresses to tasselling, at which time about 60% of the total N is taken up by.

There is no exact recommendation for the amount of supplemental N to apply after crop emergence. Use sound judgment in making rate decisions. The supplemental N worksheet suggests a range of 40 to 70 lb N/acre if corn meets certain criteria. According to Dr. Gyles Randall, a supplement of 30 to 40 lb N/acre is sufficient for corn following soybean on most soils or where corn follows corn on silt loams or sands. However, higher rates (60 to 70 lb N/acre) may be needed for corn following corn on heavy soils.

UAN injection
Photo 3. UAN side-dress injection
About any source of N can be used to side-dress, even dry sources such as urea. Remember that there is some potential for N loss from surfaced applied urea or UAN solutions; therefore, application before a rain event can help to incorporate the urea to lessen the risk. In addition, inhibitors such as Agrotain for dry urea sources may be beneficial to lessen the risk of N loss. Coulter injection or incorporation is always preferable. Remember that nitrate is mobile and therefore moves with water in the soil. Even with poor root development, nitrate can get to the roots. With side-dress applications, it is generally best to apply early so that the N is applied before the crop gets too tall and that the N has time to move to the root zone. If our pattern of weather does not change and the window to side-dress is short, get to fields that had greatest risk of N loss first.  For other mobile nutrients such as sulfur there is a risk for movement but if sulfur was applied pre-plant or with the planter there is no reason to apply sulfur in a side-dress application. 

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