Anhydrous application and dry soils

Encyclopedia Article

It was just last fall when soils were very dry in many areas of Iowa, and this fall they may be even drier. So, questions being asked are, Can anhydrous ammonia be applied to dry soil? Will it be held in dry soil?

When anhydrous ammonia is injected into soil, several physical and chemical reactions take place: dissolution in water, reaction with soil organic matter and clay, and attachment of the resulting ammonium ions on the cation exchange complex. These reactions all tend to limit the movement and potential loss of ammonia. The ammonia retention zone has the highest concentration of ammonia near the point of injection, with a tapering of the concentration toward the outer edges. In many soils, the greatest ammonia concentration is within the first inch or two of the injection point, with the overall retention zone being approximately 4 inches in radius. The size of this zone, and shape, vary greatly depending upon the rate of application and knife spacing, the soil, and the soil conditions at injection (soil texture, soil structure, organic matter content, and moisture status). Because of the concentration and pH gradient, nitrification initially proceeds more rapidly at the edges of the injection zone.

Ammonia moves farther at injection in coarse-textured soils and soils low in moisture. Also, if the injection knife causes sidewall smearing (when soils are wet), then ammonia may preferentially move back up the knife slot. A similar movement occurs if the soil breaks into clods at application and there are large air voids left in the soil. Both of these conditions can result in greater ammonia concentration toward the soil surface, and greater potential losses at or after application (the same if the injection point is near the soil surface).

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 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.

So, can anhydrous ammonia be applied to dry soils? Yes. Despite what people may think, dry soil can hold ammonia. 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 the ammonia so it can become attached to clay or organic matter. If dry soils are cloddy and do not seal properly, the 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. Wing sealers immediately above the outlet port on the knife can help close the knife track, limit the size of the retention zone, and reduce vertical movement of ammonia. If soils are dry and in good physical condition, they hold more ammonia than soil that is moist.

What about damaging corn next spring? The potential is usually small for fall-applied ammonia to damage corn seed or seedlings. However, if the soil remains dry (and limits nitrification), the ammonia is injected shallow or there is poor soil structure (ammonia placed near the seed location), or the rate of application is high, then it is possible for ammonia damage to occur. A few cases were noted this past spring. The best cure is to inject deep enough with friable soil coverage to get adequate soil separation between the point of ammonia injection and the depth where corn seed will be planted. For example, if the injection point is 6 to 8 inches in depth, the outer edge of the ammonia retention zone (which would be low in ammonia concentration) is 4 inches from the point of injection, and seed is planted at a 2-inch depth directly over the ammonia track, then the seed would be outside the applied ammonia band. Shallower injection, greater movement upward from the injection point, wider knife spacing, or higher rates can lead to ammonia being placed in the seeding area at rates high enough to cause damage.

The bottom line is to be mindful of what is happening at application, especially if soil conditions are not ideal. If anhydrous ammonia cannot be injected deep enough, the soil is breaking into clods, the knife is smearing through the soil, there isn't good coverage of the knife track with loose soil, and ammonia is escaping (remember your nose tells you if ammonia is escaping; a white vapor is water vapor, not ammonia), then stop and either change the way the equipment is working or is set up, or wait until the soil has better structure or moisture.

This article originally appeared on pages 180-181 of the IC-484(23) -- October 23, 2000 issue.

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