Fall Nitrogen Application

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• Before applying anhydrous ammonia in the fall check that the soil temperature at a 4” depth is at or below 50F and that the weather forecast is trending cooler.
• Use N-Serve or Centuro as a nitrification inhibitor to preserve nitrogen in the ammonium form that attaches to the Cation Exchange Capacity (CEC).
• Avoid tillage 2-3 days post anhydrous ammonia application to give this nitrogen product time to react with the CEC.

Fall nitrogen applications will be next on the list for many individuals now that harvest has begun in many areas. In order to optimize ROI, there are several items that must be taken into account. Some of the most important items to keep in mind are discussed in this article.

When should N be applied?

Anhydrous ammonia should only be knifed-in when the environment will allow. Improper sealing of soil behind applicator knives due to wet soil conditions can result in gaseous loss of ammonia to the atmosphere. Proper soil temperature combined with the forecast trending lower assure the best chance of preserving this investment. 

Where should I place my Nitrogen?

Placing a source of N into the soil profile at a depth of 5 -6” will allow for N to be available to plant roots that are deeper within the soil profile. Plant nutrients are taken up by the plant with soil water and mass flow.  Placing N at a depth where water will be available in the next growing season helps minimize the impact of a normal, seasonal dry weather period, providing a source of N when plant demand is high.  If all the N is placed in the surface (upper 3 -4”) and the soil is dry (pre-tassel to post pollination), most of the N will be unavailable for plant uptake until rains resume, regardless of the quantity of N applied.

What source of N should I use?

In the fall, anhydrous ammonia should be the Primary N Source used. The use of N-Serve or Centuro with fall applications of anhydrous ammonia at the recommended rate is necessary to minimize nitrification and protect against early spring rains. Start fall applications when the soil temperature at at 4” depth is at 50oF or less and the forecast is trending lower before winter.  Avoid fall applications of N where the soils remain warm late into the fall, where the soil has a low CEC, or on coarse-textured soils.  Applications of anhydrous ammonia to these soils will likely result in excessive loss of N and lower harvest yield.  
 
The Reaction of Anhydrous Ammonia in the Soil

Following injection into the soil, anhydrous ammonia is not lost by leaching or volatilization without conversion to nitrate-N.  The conversion process from ammonium-N to nitrate-N is microbially driven by two groups of soil bacteria, Nitrosomonas, and Nitrobacter (Figure 1).  Cool weather does not provide a good environment for soil microbial activity.  In soils that are below 50o F, most of the N applied as anhydrous ammonia will likely remain as ammonium-N until soil temperatures increase the following spring.  Ammonium-N is considered a stable source of N that is held by the soil’s CEC. Unless the soil has a low CEC and/or is coarse textured, essentially all of the N applied should still be close to where it was applied as ammonium-N. Within 10 to 14 days after the soils warm up above 50°F, nitrification will be well on the way to converting the ammonium-N to nitrate-N. Here is the process by which ammonium is converted to nitrate: 

Anhydrous ammonia does work as its own N stabilizer due to free ammonia being toxic to most living organisms.  Microbial populations, including nitrifying bacteria, are greatly reduced in the area within the anhydrous ammonia band, which provides a temporary delay in nitrification until the nitrifying bacteria repopulate the area. Although there may be a slight inhibition of nitrification from the application of anhydrous ammonia, it is not considered a replacement for the fall use of nitrification inhibitors, such as N-Serve, or Centuro when applying anhydrous ammonia.

Should I work my soil after applying fall Nitrogen?

As long as there is a waiting period of two or three days, a field can be chisel-plowed after an anhydrous ammonia application. Once the free ammonia reacts with soil water to form ammonium-N, there should be little immediate risk of N loss. Ammonium-N is not likely to be lost by leaching, denitrification, or volatilization even after tillage. However, there is a risk of compromising the value of a nitrification inhibitor when chisel-plowing after an anhydrous ammonia application. Nitrification inhibitors create a protective zone around the applied anhydrous ammonia-N. This zone greatly reduces the number of nitrifying bacteria (Nitrosomonas), delaying the process of nitrification until the microbial populations rebuild and convert the ammonium-N to nitrate-N. The repopulating of the microbial population, and microbial activity needed to drive nitrification, may take several weeks, so the N source is kept in the stable ammonium-N form for this time period. Disturbing the nitrification inhibitor-treated soil with tillage exposes the ammonium-N to a population of healthy bacteria. This can shorten the period of microbial inhibition, thereby shortening the time N is held in the ammonium form.