Issue No. 4, Article 8/April 29, 2011
Myths and Truths about Anhydrous Ammonia Fertilizer
Anhydrous ammonia (AA) is the most widely used nitrogen (N) fertilizer source in Illinois. Some of the reasons for its importance include the fact that this source is by far the most concentrated, with 82% N (less weight of fertilizer per unit of N); it is readily available, because AA is used in the manufacture of many commercial N fertilizers; it can be applied long before the crop is planted, including in the fall, when other N sources are not acceptable because the potential for N loss is too high; and, most important, it normally represents a less expensive source of N. Some of the drawbacks of AA include the need for special facilities to store the gas as pressurized liquid and for special equipment to transport and apply it; the fact that application can be slower than withsome other N sources; and the potential risk to human health if AA gas is not handled properly. Every year as farmers start applying AA, I invariably receive similar questions.
What is a good soil-moisture condition for anhydrous ammonia application, and can I apply it deeper to compensate for less-than-ideal conditions? When AA is applied it quickly reacts with organic matter, clay particles, free hydrogen ions, and, most importantly, with soil water, preventing 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 about 2 to 5 inches in diameter.
AA losses to the atmosphere at the time of application depend mostly on soil moisture and depth of injection. Ideal soil conditions are around 15% to 20% moisture. Within this range a fine-textured soil, such as a silty clay loam, feels slightly moist. Pressed in the palm of your hand, it will form a weak ball with rough surfaces that crumbles under pressure and does not leave water stains on the hand.
If soil conditions are too dry, ammonia can escape because it travels too far in the soil to react with soil water. This is rarely a problem in our fine-textured soils, and certainly it will not be a problem this wet spring! If soils are too wet, the knife track may not seal properly, creating a direct conduit for ammonia to escape to the soil surface. This is definitely a concern given the wet soil conditions in Illinois this spring. When soil moisture is slightly above or below the ideal, increasing application depth can reduce the risk of ammonia loss. An adequate application depth under ideal moisture conditions is about 6 inches for fine-textured soil and 8 inches for coarser textured (sandy) soil. For wet soil, increasing the application depth is not always sufficient to minimize ammonia losses, and it is recommended that you use some type of device behind the knife to close the slot it creates. 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 loss is occurring.
Is applying anhydrous ammonia after every other row as effective as applying it after every row? Yes. Application after every second row requires less horsepower to pull equipment across the field and fewer knives to be maintained. And, as shown by research in Illinois, there is 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 preplant applications using RTK guidance to ensure that each corn row will have access to the applied N 15 inches away.
Another potential advantage to applying AA every other row is that the rate of N is effectively doubled in the application zone compared to an application every row. AA reacts with water and free-hydrogen ions and creates a temporary alkaline (high pH) zone that 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 is high for N loss due to leaching or denitrification.
How long do I have to wait after anhydrous ammonia application to plant corn? The length of the wait depends on several factors: AA rate, soil moisture, and application depth. The only risk of planting soon after 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 normal application rate (100 to 200 lb N/acre), there is typically little risk of seedling injury, even if seeds are planted on top of the application zone right after AA application. That said, I would not recommend intentionally 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 recommend applying AA at 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 3 to 5 days before planting is typically enough of a wait to reduce the risk of seedling injury.
How long after anhydrous ammonia application can I till the soil? The wait depends on depth of application and soil conditions. With 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 disturb the ammonia retention zone normally will cause no problems if done a few days after the application. Smelling ammonia after doing a tillage pass indicates that the conversion of ammonia to ammonium is not complete, and you should wait to till the soil.
Does anhydrous ammonia application compact soils? No. This myth has been circulating for many years, but research has shown that repeated application of AA caused no soil compaction (as measured by bulk density of the soil) in or below the plow layer. Similarly, AA applications have not been shown to reduce soil organic matter.
Does anhydrous ammonia application change chemical conditions in the soil and affect soil microbes? Although the conversion of ammonia to ammonium creates temporary alkaline conditions in the ammonia retention zone, 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; other N sources have similar effects on soil pH.
Applying AA is toxic to microorganisms in the ammonia retention zone and can drastically reduce bacterial and fungi populations 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 recolonized after several weeks.--Fabián G. Fernández