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.
By delaying application until cold weather, most of the applied N can enter the winter as ammonium, and over-winter losses of the applied N will be minimal. This generally means that anhydrous ammonia applications should be delayed until the second week of November north of I-70.
Where is fall application an acceptable practice?
Traditionally, daily high soil temperatures of 50°F at a depth of 4 inches are considered the maximum for the application of ammonia in the fall. It's not that nitrification stops below 50 degrees, but rather that soils will soon become cold enough (in all likelihood) to limit the nitrification process. In many areas in Kansas, soils may stay warmer than 50 degrees well into late fall, and only freeze for short periods during the winter. As a general rule, applying ammonia in the fall for corn is probably not a good idea south of I-70 because soil temperatures are not normally cold enough on a consistent basis during the winter to prevent nitrification in the fall. North of I-70 applications can be made in soils which are, or soon will be, cold enough to limit nitrification. The use of a nitrification inhibitor such as N-Serve can help reduce N losses from fall N applications under specific conditions, particularly during periods when soil temperatures warm back up for a period after application.
One should also consider soil type when considering fall application. Fall applications of N for corn should not be made on sandy soils prone to leaching, particularly those over shallow, unprotected aquifers. Rather, fall N applications should be confined primarily to deep, medium- to heavy-textured soils where water movement is slower.
When is N lost?
More N is likely to be lost from a fall application of anhydrous ammonia during early spring than during the fall and winter. Loss of N during the fall and winter is not normally our problem in Kansas. The conversion of ammonium to nitrate during the fall and winter can be minimized by waiting to make applications until soils have cooled, and by using products such as nitrification inhibitors. The fact that essentially all the N may remain in the soil as ammonium all winter, coupled with our dry winters, means minimal N is likely to be lost over winter.
However, soils often warm up early in the spring and allow nitrification to get started well before corn planting. Generally, if the wheat is greening up, nitrification has begun. Thus one of the potential downsides of fall application of N is that nitrification can begin in late February and March, and essentially be complete before the corn crop takes up much N in late May and June. If N is applied closer to the time of corn planting, or after corn has been planted, a higher percentage of the N is likely to still be in the ammonium form during the wet periods of late spring. More of the N from fall applications than spring applications can be lost to heavy May and June rains through leaching and denitrification because of the higher portion of N present as nitrate with fall-applied N.
The bottom line is this: If anhydrous ammonia is to be applied in the fall, there are a number of factors that must be considered, including soils, temperature, and soil moisture. Consider the following guidelines:
* In general, in central and eastern Kansas, fall applications of anhydrous ammonia for corn should only be done north of I-70. In western Kansas, there are no good general guidelines. However, the drier conditions and cooler soils in western Kansas would suggest that the potential for N losses from fall-applied ammonia should not be as great as in central and eastern Kansas, even south of I-70.
* Do not apply anhydrous ammonia in the fall on sandy soils.
* On silt loam or heavier soils, wait to apply anhydrous ammonia until soil temperatures at 4 inch depth are below 50 degrees (records indicate in most years this will be in November).
* Use a nitrification inhibitor such as N-Serve with anhydrous ammonia to help reduce fall nitrification rates.
* To check the soil temperature in your area visit the K-State Research and Extension Weather Data Library at: http://www.ksre.ksu.edu/wdl/