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October, 2018
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Test to Account for Available Nitrogen 


Much of the upper Midwest had above average rainfall this spring, but what does that do to nitrogen levels in the soil? It depends on several factors.

Useable nitrate and ammonium can be lost from the root-level soil through leaching and denitrification, especially during a wet spring. “When water passes through the soil, nitrate is moved below the root zone where it cannot be utilized by crops. This leaching often means a financial loss for the grower and raises environmental concerns,” states Dr. Jim Friedericks, AgSource Laboratories’ Outreach and Education Advisor.

Denitrification is also an issue during wet springs. Cool soil slows down the process, but in saturated soil nitrogen loss is a real concern.   “The same microbes that break down animal and plant waste to make usable nitrogen also need oxygen to live. This is normally not a problem, but when the soil is very wet these microbes strip the oxygen from nitrate in the soil, releasing gaseous nitrogen to the air in its pure, unusable form,” he said.   

The best way to know what is going on with nitrogen in your soil? Test it. “It a good idea to test and see where things are at. Even the fertilizer you applied this spring could have dissipated. Testing is the best way to know if you need more nitrogen to get higher yields.”

To get the whole picture of what is going on in your fields, Friedericks recommends testing for nitrates, ammonium and doing a Soil Health test.   

Spring nitrate testing is important for several reasons, including: 

  • Verifying that the current N fertilization program will supply adequate nitrogen to this year’s crop
  • Measuring fields for nitrate carryover after a drought, or monitoring fields after a wet spring
  • Checking fields that have a history of manure applications
  • Evaluating fields that have had previous legume crops in the rotation for nitrogen availability

An ammonium test provides:

  • A more complete measure of the nitrogen available to a crop
  • An evaluation of the conversion process from ammonium to nitrate
  • Potential savings on applied nitrogen which means reduced loss to runoff or leaching

Soil health testing measures:

  • The value of mineralizable nitrogen in the organic matter, which tells how much nitrate will become available as the crop grows
  • Basic health characteristics such as Soil Health Score, CO2 Respiration, C:N Ratio
  • Detailed soil health information including water soluble carbon, total nitrogen, water saturation percentage and more.

“If manure was applied in the fall or spring then testing the soil for nitrate or ammonium might not give the whole picture.  More nitrogen can become available through the season as the organic nitrogen in the manure mineralizes in warm soil.  A soil health test can help measure that nitrogen source and how much will be available to plants later in the growing season,” Friedericks said.

Many things affect the continuous natural nitrogen cycle, including moisture levels and soil temperature. When nitrogen gas in the air is converted into usable nitrate and ammonium, this process is called fixation. However, nitrate and ammonium may not remain in this usable state and can be changed back to pure nitrogen in a variety of ways. Thus, the nitrogen cycle is a continual process of recycling nitrogen from a pure state to a usable state back to a pure state again.

“When nitrogen is added to the soil as nitrate, ammonia or organic nitrogen in manure it is not stable. Hopefully most will be taken up by the crop but some will be lost to leaching or converting back to gaseous nitrogen. Rather than guessing what is going on test the soil so you can manage it,” he said.