What happens when anhydrous ammonia is injected into soil?
Several physical and chemical reactions take place following anhydrous ammonia injection: dissolution in water, reaction with soil organic matter and clay, and attachment of ammonium ions on the soil cation exchange complex. These reactions all tend to limit the movement of ammonia, with water having the greatest initial effect.
The highest concentration of ammonia is at/near the point of injection, with a tapering of the concentration toward the outer edge of the retention zone. Usually the greatest ammonia concentration is within the first inch or two of the injection point, with the overall retention zone being up to 3-4 inches in radius in most soils.
The specific size and shape of the ammonia retention zone vary greatly depending upon the rate of application, injection spacing, soil, and soil conditions at injection (soil texture, soil structure, organic matter, and moisture status).
Ammonia moves farther at injection in coarse-textured soils and soils low in moisture. Also, if the injection system causes sidewall smearing (occurs when soils are wet), then ammonia may preferentially move back up the injection slot. Movement toward the soil surface can occur for some time after application if the soil dries and the injection track “opens up” as the soil dries (also less soil moisture to retain free ammonia in solution with drying soils).
A similar movement within soil can occur if the soil breaks into clods at application and there are large air voids left in the soil. These conditions can result in greater ammonia concentration toward the soil surface, and greater potential for loss to the atmosphere at or after application.
When ammonia is injected into soil, the initial reaction at the point of release is violent. The ammonia reacts and binds with soil constituents such as organic matter and clays. It reacts with water to form ammonium (NH4+). These reactions help retain ammonia at the injection point. With the high affinity for water, soil moisture is important for limiting the movement of ammonia, but water does not ultimately determine retention in soil.
After conversion to ammonium, which is a positively charged ion, it is held on the soil exchange complex and does not move with water. Only after conversion to nitrate (NO3–), via the nitrification process, can it be lost from soil by leaching or denitrification.
Chemical and biological reaction of anhydrous ammonia in soil
(1) NH3 + H2O = NH4+ + OH–
The reaction of ammonia with water (1) causes an initial alkaline pH in the ammonia retention zone (pH can temporarily rise above nine at the point of highest concentration). It is free ammonia (NH3) and not ammonium that can be lost from soil at application and is damaging to microorganisms and plant roots/seedlings. As pH goes above 7.3, the equilibrium between ammonium and ammonia results in increased ammonia (the percentage as ammonia would be 1% at pH 7.3, 10% at pH 8.3, and 50% at pH 9.3).
(2) 2NH4+ + 3O2 = 2NO2– + 2H2O + 4H+
(3) 2NO2– + O2 = 2 NO3–
These two reactions (2 and 3) are the steps in the biological nitrification process that occurs with ammonium in soil, and ultimately results in a lowering of soil pH back to the original pH or lower. Nitrification occurs first at the outer edges of an ammonia retention band, and progresses inward as the initial effects of ammonia injection decrease and the soil conditions become more conducive to microbial activity.
Can anhydrous ammonia be applied to dry soils?
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Dry soil can hold ammonia. Even air dried soil contains some moisture, although the moisture content is quite low. Ammonia dissolves readily in water, but it is held or retained in soil by clay and organic matter. The problem with dry soil and low moisture is that soil moisture is needed to temporarily hold (“go into solution”) the ammonia so it can become attached to clay or organic matter as ammonium.
If dry soils are cloddy and do not seal properly at application, free ammonia can be lost at injection, or seep through the large pores between clods after application. Therefore, proper depth of injection and good soil coverage are a must for application into dry soils. Also, reducing the application rate or narrowing the injection spacing reduces the concentration of ammonia in each injection band.
Closing disks can reduce ammonia loss by covering up the injection track with soil that traps the ammonia as it moves to the soil surface.