Applying NH3 this fall for 2012 corn crop
Some producers in Kansas would like to apply anhydrous ammonia this fall to ground intended for the 2012 corn crop. This practice has some appeal. For one thing, fall fertilizer application spreads out the workload so that there’s more time to focus on corn planting in the spring. Secondly, wet spring weather sometimes prevents producers from applying anhydrous ammonia in the spring ahead of corn planting, and forces them to apply more expensive sources after planting. Equally important for many growers have been issues with ammonia availability at times in the spring the past year or two.
Despite those advantages, a fall application of anhydrous ammonia involves some risks, and is not recommended as a general practice in Kansas. This is due to the potential for higher nitrogen (N) loss in the spring following fall application, as a result of nitrification of the ammonium during late winter and very early spring and subsequent leaching, or denitrification.
Reactions of nitrogen in the soil
When anhydrous ammonia is applied to the soil, it quickly reacts with water to form ammonium hydroxide, NH4OH. A large portion of the ammonia is converted to ammonium (NH4+), and can be bound to clay and organic matter particles within the soil, primarily through the cation exchange process. As long as the nitrogen remains in the ammonium form, it can be retained on the exchange capacity of the soil, and does not readily move in most soils, so leaching is not an issue. Also, in the ammonium form it is protected from denitrification should the soil become saturated. In some very low CEC sandy soils however, the retention of ammonia is weak and some leaching of the ammonium can occur. This can dilute the application band and speed up nitrification.
At soil temperatures above freezing, ammonium is converted by specific soil microbes into nitrate-N (NO3-). Since this conversion is a microbial reaction, it is very strongly influenced by soil temperatures. The higher the temperature, the quicker the conversion will occur. The reaction of ammonia with water immediately upon application creates a high pH, alkaline zone at the point of application. The high pH and toxic effects of the ammonia kills many of the organisms in the application band.
Thus, nitrification of the applied ammonia/ammonium is slowed initially and then begins to increase again on the outer edges of the band. Nitrification then gradually moves towards the center of the band as microbial populations rebound. This toxic effect of ammonia killing organisms in the application zone makes ammonia a self-inhibiting nitrogen product in terms of conversion to nitrate. Depending on soil temperature, pH, and soil moisture content, it can take 2-3 months or more to convert all the ammonia applied in late summer/early fall to nitrate. As nitrification proceeds, the pH of the soil in the application zone will drop. The pH can become so low in the nitrifying ammonia band that it can begin to lower the rate of nitrification of the remaining ammonium.