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Anhydrous Ammonia Applications

Fabian Fernandez, Nutrient management specialist

Anhydrous ammonia (AA) is one of the most widely used nitrogen (N) fertilizer source in Minnesota and the Midwest. Some of the reasons for its importance include the fact that this source is by far the most concentrated N fertilizer with 82% N (less weight of fertilizer per unit of N); it is readily available since AA is used in the manufacture of many commercial N fertilizers; it can be applied several weeks before planting with less N loss potential than other N sources; and most importantly AA normally represents a less expensive source of N. Some of the drawbacks of AA include the need for special facilities to store this gas as pressurized liquid, and special equipment to transport and applied this fertilizer; the application of AA can be slower than that of some other N sources; and because AA is released as a gas, it can pose a risk to human health if not handled properly. Every year as farmers start applying AA, invariably I get asked similar questions which I will try to address today.

What is a good soil moisture condition for anhydrous ammonia application, and can I apply it deeper to compensate for less-than-ideal soil conditions?
When AA is applied it quickly reacts with organic matter, clay particles, free hydrogen ions, and most importantly with soil water that prevent volatilization of ammonia. Other factors that can influence the size and shape of the retention zone include the rate of application and the equipment or method of application, soil texture, and soil structure. Typically, AA stays in an oval-shaped retention zone of about 2 to 5 inches in diameter.

Anhydrous ammonia losses to the atmosphere at the time of application are mostly dependent on soil moisture and depth of injection. Ideal soil conditions are around 15 to 20% moisture. Within these moisture levels a fine-textured soil, such as a silty clay loam soil, feels slightly moist. If pressed in the palm of your hands it will form a weak ball with rough surfaces that crumbles under pressure and will not leave water stains on your hands.

Moisture conditions across the state vary significantly this spring. If soil conditions are too dry, ammonia can escape because it will travels too much distance in the soil to react with soil water. This is rarely a problem in fine-textured soils. This year some fields look dry, but there is good amount of moisture just below the soil surface.

If soils are too wet, the knife track might not sealed properly creating a direct conduit for ammonia to escape to the soil surface. This is definitely a concern given the wet soil conditions in the state this spring. When soils are slightly above or below the ideal moisture conditions, increasing application depth can reduce the risk of ammonia loss. An adequate application depth under ideal moisture conditions is approximately 6 inches for fine-textured soil and 8 inches for coarser textured soils (sandy soils). For wet soils, sometimes increasing the application depth is not sufficient to minimize ammonia losses, and it is recommended to also use some type of device behind the knife to close the slot created by the knife. Always the best test to determine if a proper seal is obtained is to go back to the application zone and smell. If ammonia can be smelled for a while after the application, that’s clear evidence that ammonia losses are occurring. Also, when considering AA applications in wet conditions, beware that likely you will be creating compaction and smearing of soil surfaces. That’s not a trivial issue especially if it turns dry later in the season and crop roots were not able to grow well because of compaction created at this time.

Is anhydrous ammonia application between every-other-row as effective as application between every row?
The answer to this question is, yes. This application requires less horsepower to pull the application equipment across the field, less knives to be maintained, and research has demonstrated no yield difference compared to the same rate of N applied between every row in standard 30-inch row spacing. Every-other-row applications are most effective at sidedress time where the location of the rows is known or for pre-plant applications using RTK guidance to ensure each corn row will have access to the applied N.  

Another potential advantage to applying AA in every-other-row is that effectively, the rate of N is doubled in the application zone compared to an application in every row. As explained earlier AA reacts with water and free-hydrogen ions and creates a temporary alkaline (high pH) zone which inhibits bacterial transformation of ammonium to nitrate. The higher the rate of application in a localized zone, the longer the inhibition effect remains. Retaining N longer in the ammonium form can be especially important for early N applications or when the potential for N loss due to leaching or denitrification is high.

How long do I have to wait after anhydrous ammonia application to plant corn?
The only risk of planting soon after AA application is if seeds fall within the ammonia retention zone. To avoid seedling injury separation in time or space can be important. Under ideal soil moisture conditions and proper application depth of a typical agronomic rate normally there is little risk of seedling injury even if planted on top of the application zone right after AA application. That said, this can be risky and I would not recommend planting on top of the AA row. If you have RTK guidance it is very easy to apply AA between the future corn rows. If RTK guidance is not an option, I would recommend applying AA on an angle to the direction of planting to minimize the potential for planting on top of the AA band. If application conditions are less than ideal and you have no RTK guidance to ensure a safe distance from the AA band, then waiting 3 to 5 days before planting is typically enough time to reduce the risk of seedling injury.

How long after anhydrous ammonia application can I till the soil?
The answer to this question is similar to the previous question in the sense that it depends on depth of application and soil conditions. Under ideal soil conditions and typical application depth, shallow tillage can be done immediately after application. Under some conditions shallow tillage may also help seal the knife tracks. The reaction of ammonia to ammonium is very rapid in the soil and deep tillage that would disturbed the ammonia retention zone normally will cause no problems if done a few days after the application. As mentioned earlier, if you smell ammonia after doing a tillage pass it would indicate that the conversion of ammonia to ammonium is not complete and you should wait to till the soil.   

Does anhydrous ammonia application compacts soils?
The answer to this question is, no. This is a myth that has been circulating for many years. Research has shown that repeated application of AA caused no soil compaction (as measured by bulk density of the soil) in the plow layer or below the plow layer. Similarly, AA applications have not shown to reduce soil organic matter.   

Does anhydrous ammonia application change chemical conditions in the soil and affect soil microbes?
As explained earlier, the conversion of ammonia to ammonium creates temporary alkaline conditions in the ammonia retention zone. However, the process of nitrification (conversion of ammonium to nitrate) is an acid forming reaction and the overall effect of AA applications is a reduction in soil pH. It is important to recognize, though, that this acidification is not unique to AA, but other N sources have similar effects on soil pH.

Application of AA is toxic to microorganisms in the ammonia retention zone and can drastically reduce populations of bacteria and fungi at the time of application. Studies have shown, however, that the effect is highly localized (within a few inches of the release point) and temporary as the retention zone becomes re-colonized after several weeks.

For more information, visit the Nutrient management website.
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