The only sources of nitrogen for fall application that are recommended by fertility specialists are anhydrous ammonia and ammonium sulfate. Fernandez says ammonia quickly converts to ammonium, and ammonium sulfate is already there. Ammonium quickly attaches to soil particles and is protected from leaching. But other forms of nitrogen which are in the nitrate form do not attach and can be leached away with soil moisture heading toward field tiles and waterways. Those include ammonium nitrate and urea ammonium nitrate (UAN). Urea converts to the desired NH4 overtime but has a greater potential for loss before it can be utilized by the spring crop. The same can be said for the polymer coated urea products, and they are lost before anhydrous ammonia is lost.
Fernandez says a desired benefit of anhydrous ammonia is that it kills nitrifying bacteria, preventing its conversion into a nitrate. To lengthen and enhance that process is the purpose of a nitrification inhibitor, such as N-serve, which is the most valuable when used while bacteria are active above 50 degrees F. Fernandez says, “The use of a nitrification inhibitor might not pay every year. For example, if the following spring is dry and cool, the inhibitor might not be as beneficial to enhancing ammonium recovery. However, the practice will overall ensure the greatest chance to protect your N investment and at the same time enhance environmental protection.”
He says ammonium sulfate is a good choice for no-till fields and is always best to apply it before soils freeze so it has the chance to dissolve and be absorbed into the soil. However, it will require a higher amount of lime because it is more acidifying than other nitrogen sources.
Since harvest was finished earlier than normal due to early planting and a smaller crop, Fernandez, many farmers may want to get fall tillage and fertility application completed early as well. That means nitrogen may be applied at warmer soil temperatures where nitrifying bacteria are lying in wait to convert your nitrogen into a nitrate and send it down the river. Consequently, either nitrogen applications have to wait or be combined with an inhibitor such as dicyandiamide (DCD) or N-Serve.
While bacterial activity slows below 50 degrees, they are still active above 32 degrees. He says apply nitrogen by the temperature and not the calendar. So, if the daily temperature does not exceed 50 degrees, is it acceptable to apply nitrogen? No, the soil temperature is the key, and Fernandez says, “Since soil temperatures can be influenced by multiple factors (including residue cover, soil color, and drainage), it is always best to monitor soil temperatures in individual fields prior to N application.”