I thought that the sequence of reactions that takes place when anhydrous ammonia is added to the soil was understood by crop producers and those who advise them.

equations for anhdrous ammonia

However, judging by the telephone calls and e-mail messages that surfaced last fall, there is still some confusion about what happens when this popular source of N is added to the soil. In order to clear up some of the confusion,it might be helpful to describe and discuss the chemical reactions that occur when anhydrous ammonia is applied.

The chemistry is not complicated. From the standpoint of crop production, there are three reactions that occur. These reactions are listed below.

Anhydrous ammonia leaves the orifice in the shank of the applicator as a gas (NH3). The gas is quickly absorbed by water in the soil (see equation #1). This is a standard reaction governed by the rules of chemistry. Microoganisms are not involved. If there is adequare soil moisture, there is no loss of NH3.

If soil moisture is short or very limited as in the fall 0f 2011, there’s nothing stopping the movement of NH3 back into the atmosphere as a gas. The amount of N lost by this volatilization process is nearly impossible to predict. There are several factors that affect the amount of NH3 lost in this way. 

Also, there’s no general agreement about the magnitude of this loss. Some research at Kansas State University suggests that loss of N from volatilization losses can be substantial when soils are dry at the time of application. Loss was diminished as depth of application increased. Judging by soil conditions in the fall of 2011, anhydrous ammonia was probably applied at a shallow depth again increasing the potential for loss.

There’s no question. Volatilization of NH3 was a major concern in the fall of 2011. Soil was very dry. In addition, tillage was very difficult. Use of a chisel plow produced large clods and soils were not mellow after this tillage operation. Because of these two factors, loss of NH3 applied last fall could have taken place without detection. So, growers should pay close attention to early corn growth in these fields during the early part of the growing season.

After absorption by water, the NH3 is converted to ammonium (NH4+) and OH-(hydroxyl ion) as shown in equation #1. The OH- causes a temporary rise in soil pH which drops back to the original value in a short period of time. Bacteria, including those involved in the nitrification reaction do not grow and reproduce normally at the high pH values. Therefore, anhydrous ammonia can act as it’s own nitrification inhibitor. 

After N is in the ammonium (NH4) form, the nitrification reaction takes place (see equations 2 and 3). In reaction #2, the ammonium-N is converted to nitrite-N (NO2-N).  This reaction cannot take place without the presence of the Nitrosomonas bacteria. Reaction #3 is the second step in the nitrification process. With oxygen, the nitrate-nitrogen is converted to nitrate-nitrogen (NO3-N). The Nitrobacter bacteria are necessary for completion of this reaction.

We know that we can reduce the potential for loss of nitrogen due to leaching or denitrification by delaying the conversion of ammonium-nitrogen to nitrate-nitrogen. Since the products N-Serve and Instinct reduce the population of the Nitrosomonas bacteria, they slow the nitrification reaction. But, can we use these products to reduce the loss by volatilization? Good question.

In a perfect world, it would be nice to have such a product that can be used when soils are very dry (fall 2011). Unfortunately, there are no products that can be used for this purpose.  

Nitrogen loss by volatilization is not affected by the use of either N-Serve or Instinct. The product, Agrotain, affects the conversion of urea-N to ammonium-N and does not affect the volatilization loss of NH3. The product, Nutrisphere, has no effect on any of the reactions discussed. Therefore, there is no justification for the use of this product when soil are dry.

I’m not sure that we’ve ever experienced soil conditions like those present in the fall of 2011. Hopefully,many crop producers decided to delay N applications to the 2012 growing season. If not, play close attention to those fields that were fertilized with anhydrous ammonia last fall.