Applications of Anhydrous Ammonia
Specialist Gives Tips for Applying Anhydrous Ammonia to Wheat Fields in Dry Conditions
Manhattan, Kan. - As producers start thinking about anhydrous application for wheat this fall, extremely dry soils can be a concern, said Dave Mengel, K-State Research and Extension soil fertility specialist. The question often is, when the soil is dry, will it hold anhydrous ammonia or will some or most of the ammonia be lost shortly after application?
Three factors help determine whether ammonia might be lost after application under dry conditions, Mengel said.
The first is how quickly the ammonia gas is converted within the soil to a non-gaseous form that will stay in the soil.
"Ammonia gas needs to react with water shortly after application in order to convert into ammonium, which is the molecule that can attach to clay and organic matter in the soil, " Mengel said.
Converting from gaseous ammonia to the less-volatile ammonium ion takes a little time - it does not occur immediately upon contact with the soil, he explained.
"The higher the soil temperature and the wetter the soil, the more rapid the conversion occurs. If the ammonia does not react with water, it will remain as a gas that could escape from the soil. Also, a higher percentage of the ammonia will remain unconverted in the soil longer at higher application rates and at higher soil pH levels," he said.
The second factor to consider is how rough and open the dry soil is, Mengel said.
"Dry soils may be cloddy, with large air spaces where the soil has cracked. This can allow the gas to physically escape into the air before it has a chance to be converted into ammonium," the agronomist said. "Getting the soil sealed properly above the injection slot can also be a problem in dry soils."
The third factor is the amount of ammonia that might be lost, which depends on application depth, he said. The deeper the ammonia is applied, the more likely the ammonia will have moisture to react with, and the easier the sealing.
So, can anhydrous ammonia be applied to dry soils?
"Yes," the soil fertility specialist said, "as long as the ammonia is applied deep enough to get it in some moisture and the soil is well sealed above the injection slot. If the soil is dry and cloddy, there may be considerable losses of ammonia within just a few days of application if the soil is not well sealed above the injection slot or the injection point is too shallow."
Producers should be able to tell if anhydrous is escaping from the soil during application or if the ammonia isn't being applied deeply enough. If ammonia can be smelled, the producers should either change the equipment setup to get better sealing or deeper injection, or wait until the soil has better moisture conditions, he said.
Mengel said producers can minimize loss of ammonia when applied to dry soils by:
- Applying the anhydrous ammonia at the proper depth (at least 6 to 8 inches in 30 to 40 inch spacings);
- Using covering disks behind the knives or sealing wings ("beaver tails") on the knives; and
- Applying the anhydrous ammonia at least one to two weeks before planting. This waiting period should be even longer if soils are very dry.
Fall Applications of Anhydrous Ammonia
Some producers in Kansas would like to apply anhydrous ammonia this fall to ground intended for the next year's 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.
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. 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.
How anhydrous reacts in the soil and effects on nitrogen losses
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. As long as the nitrogen remains in the ammonium form, it 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.
At soil temperatures above freezing, ammonium is converted 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 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. 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.
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.
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 northwest Kansas. The conversion of ammonium to nitrate during the fall and winter can be minimized by waiting to make applications until soils have cooled. 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.
