Application Tips for Efficient Fall Anhydrous

Keep NH3 stable and available for plant uptake this spring

As combines are slowing, it’s time to consider fall fertilizer applications. Farmers who choose to apply nitrogen (N) in the fall should follow these guidelines to help reduce risk and reduce N loss.

Consider the Form

John Sawyer, Iowa State University Extension soil fertility specialist, recommends only using anhydrous ammonia. All other forms should be saved for spring-applied applications or sidedressing in Iowa. Other states may approve of banded or broadcast urea applications, but they may need to be applied at a later date. 

Watch the Temperature

Apply in late fall after soils cool to 50°F, says Sawyer. When checking the temperature, measure at a 4-inch depth. The temperatures should be trending cooler at the time of application. A temporary cold spell is usually too early for applications, says Sawyer. 

If anhydrous ammonia is applied before the temperature is 50°F., microbial activity can lead to the conversion of ammonium to nitrate. The more conversion of ammonium to nitrate, the better chance for nitrate loss, says Sawyer. 

Protect the N

"Consider a nitrification inhibitor to further slow nitrification to nitrate," says Sawyer. For farmers in Iowa, use of nitrogen fertilizer managers in the fall is on the approved list of practices for the voluntary nutrient reduction strategy.

Consider a Soil Sample

“Probably not a widespread concern this fall, but in some areas, soils may be or may become quite dry," says Sawyer. "Dry soil means there is room for moisture storage when rainfall occurs. That is important for potential loss of nitrate as leaching will be minimal until soil field moisture capacity is reached. However, dry surface soils, and compacted soils, can sometimes be an issue for retention of anhydrous ammonia at application time. 

"Dry soil can hold ammonia," says Sawyer. "Even air-dried soil contains some moisture, although it's quite low. Ammonia dissolves readily in water, but it is held or retained in soil by clay and organic matter. The problem with low soil-moisture conditions is that moisture is needed to temporarily hold the ammonia so it can become attached to clay or organic matter as ammonium. If dry soils are cloddy and do not seal properly during application, the ammonia can be lost at injection or seep through the large pores between clods after application."

This makes the proper depth of the injection and good soil coverage a must. Traditionally, depths have been 6 to 8 inches deep.  

When anhydrous ammonia is injected into soil, several physical and chemical reactions take place: ammonia dissolution in water, reaction with soil organic matter and clay, and attachment of ammonium ions on the soil cation exchange complex, he says.

"These reactions all tend to limit the movement of ammonia, with water having the greatest initial effect," says Sawyer. "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 to 4 inches in radius in most soils."

Shape and size of the ammonia retention zone can vary depending on application rate, knife spacing, soil type, and soil conditions at injection. Soil conditions can result in greater ammonia concentration toward the soil surface, which leads to a greater potential for loss.

"Movement toward the soil surface can also occur for some time after application if the soil dries and the knife track 'opens up' as the soil dries," says Sawyer.

"When ammonia is injected into soil, the initial reaction at the point of release is violent," says Sawyer. "The ammonia reacts and binds with soil constituents such as organic matter and clays. It reacts with water to form ammonium (NH4+)."

Those reactions help retain ammonia at the injection point, he says. "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.

"Be mindful of application date and soil temperature for fall anhydrous ammonia application, and what is happening at application, especially if soil conditions are not ideal," continues Sawyer. "If the soil is breaking into clods, there isn’t good coverage of the knife track with loose soil, and ammonia is escaping, 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."