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How to Measure N Levels after Rain

June 4, 2012

 

By Daniel Kaiser and John Lamb, University of Minnesota Soil Fertility Specialists
 
 
Nitrogen is important for corn growth. This has been a concern on growers' minds since March. First concern was with the poor tillage conditions last fall. Did the nitrogen applied stay in the soil. We attempted to answer that question in a March 18 E-news. At the time of that E-news, drought was the weather condition on everyone's mind. Now with the record rainfalls, there are concerns if nitrogen has been lost to leaching or denitrification.
 

How Do You Assess if Leaching or Denitrification can be a problem?

One of the first things to look at if your field is tile drained, 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 water logged. 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.
 

How do I assess the amount of N that is available to the corn plant?

There are really only two tools left at this time of the growing season to determine whether to apply more N to a growing corn crop under non-irrigated conditions. The first is the pre-side dress nitrate-N test. This soil test was developed at Iowa State University in the 1990's.
 
The soil test was for a sample taken to a depth of one foot. In Iowa, the researchers were able to calibrate it to an amount of N to apply. Similar research was conducted in Minnesota on many sites. A good calibration could not be developed in Minnesota. The only interpretation in Minnesota from the pre-side dress N test is if the nitrate-N concentration is greater than 20 ppm then you do not need to apply extra N to the crop. This tool can not be used to determine the amount of N fertilizer to apply!
 
The second tool is the supplemental N decision tool. 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, or extra, N is needed. This decision aid is for situations when all of the N fertilizer was applied pre-plant, either in the fall or spring. It was not developed for determining N rates in a split N 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.
 
  • Question 1. When was the N applied? The more points the greater the chances of N fertilizer loss. Nitrogen fertilizer applied in the fall when soil temperatures were higher than 50 degrees has a greater chance for loss than a spring application of N.
  • Question 2. What was the predominant spring (May) soil condition? The wetter the soil conditions are the greater the score. It takes into account if the soil is dry, moist, or if water has been standing. The more water in the soil the greater the score.
  • Question 3. How does the crop look? The more stress the crop is showing the greater the score. The stress is evaluated by the color of the corn and height.

 

How to Interpret

With a score of 7 points or less, your current nitrogen program is doing fine. With a score of 10 or more, supplemental fertilizer is recommended at a rate of 40 to 70 lbs of N per acre, depending on the situation. In most cases 40 to 50 lbs N per acre is plenty. A score of 8 or 9 falls into a gray area and it is recommended that you recalculate the worksheet in a week - the corn height/color will most likely change. The "re-evaluation" option is only viable as long as you have side-dressing options.
 
This year the state of Minnesota has run the whole gamut of soil moisture conditions. The effect of these conditions on the N available for corn growth will very across the state. The use of the U of MN Supplemental Nitrogen Worksheet for Corn is a useful tool to determine if there is a need for addition N application to corn. If addition N is needed, 40 to 50 lb N per acre will do the job.
 

 


 

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RELATED TOPICS: Corn, Agronomy, Crops

 

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