Applying nitrogen fertilizer is one of the most important decisions corn growers make. It’s a decision that impacts both profitability and the environment. In recent years some growers have switched to applying all their nitrogen in the spring before planting, or after planting as a sidedress application. The goal is to reduce risk of N loss from the soil by applying it closer to when corn plants can use it.

For other growers, fall application of N continues to be popular for several reasons. They include lower cost in the fall compared to spring, more time for application, equipment availability, often better soil conditions, and competing spring field activities.

Success with fall-applied N, compared to spring or sidedress timing, can be enhanced by following several long-standing suggestions, says Iowa State University Extension soil fertility agronomist John Sawyer:

Don’t go until 50 or below

Most years in Iowa, soils cool to below 50 degrees in late October and early November. Will soils be cold and stay cold early this fall? Soil temperatures in late October into the first few days of November this year have been near or below 50 degrees in much of Iowa, especially the northern two-thirds of the state. And the six- to 10-day forecasts are for below-normal temperatures, so the odds are looking like early and continued soil cooling.

The risk with early application, however, is temperatures could rebound and nitrification (conversion of ammonium form to nitrate form of N) will resume or continue for an extended period, albeit at a slow rate. On the other hand, a risk you must consider is the inability to get the fall application completed if you wait too long to apply. The ammonium form of N is more stable and less prone to loss than the nitrate form.

Use nitrification inhibitor

It’s always a good idea to include a nitrification inhibitor with fall anhydrous application, Sawyer says. Nitrification inhibitors work by slowing, not stopping, activity of nitrifying bacteria in the soil and hence slow the nitrification (ammonium conversion to nitrate) process. Nitrification inhibitors are more effective with cold temperatures, and thus delay nitrification longer when soils are cold. With warm temperatures, the inhibitors are “degraded” quickly and hence lose effectiveness.

Nitrification inhibitors can pay off, but not always, he notes. It all depends on ammonium conversion to nitrate, when its converted to nitrate, the inhibitor’s effectiveness, and if any major wet soil and loss conditions are “missed” because the applied N is still in the ammonium form, which is not subject to leaching or denitrification.

Most nitrate loss occurs in the spring, so having ammonium present during those months is important. Fall-applied N, to be more like a spring application, needs to have the applied N in an ammonium form — hence, late fall application, cold soils and consideration of a nitrification inhibitor.

Apply only anhydrous in fall

What about applying other forms of nitrogen fertilizer in the fall? Sawyer says other common N fertilizers, like granular urea and UAN (urea-ammonium nitrate solution), do not have the initial inhibitory effect on the nitrifying bacteria in the soil like anhydrous ammonia does, and therefore shouldn’t be fall-applied. For example, research across many years at ISU’s Northern Research Farm at Kanawha has documented lower corn yield with fall incorporated urea compared to spring incorporated urea.

The co-nitrogen application in diammonium phosphate (DAP) and monoammonium phosphate (MAP) can also be at risk with fall application due to rapid nitrification, he says, because the N in DAP and MAP is in the ammonium form, and these fertilizers tend to be applied in early fall. Triple superphosphate is becoming more available in the Midwest, and is a phosphorus fertilizer option that does not contain N.

Pay attention to soil temperature

To help decide when to apply anhydrous ammonia in the fall, soil temperatures can be found at several websites. One gives the three-day, 4-inch depth soil temperature estimates for each county, and six- to 10-day weather forecasts. That site can also be accessed through ISU’s agronomy Extension soil fertility site either from the weather page or Nitrogen Topic page. The 4-inch soil temps are estimated for each county based on interpolation of observed soil temperatures at multiple locations. The estimates are for soil temperatures on level, bare soil.

Maximum and minimum soil temperatures from ISU research farms are available from the same sources and are useful for watching the impact of warm air on soil temperatures. If you are curious about this fall’s past 4-inch soil temperatures (longer than three days ago), check the IEM Time Machine site.

Be mindful of application date and soil temperature for fall anhydrous ammonia application, soil conditions and what is happening at application if soil conditions aren’t ideal.

“If the soil is smearing, breaking into clods, or there isn’t good coverage of the knife track with loose soil, and ammonia is escaping, then stop, you should either change the way the application equipment is working or how it is set up, or wait until the soil has better structure or moisture before you resume applying,” Sawyer says. Remember, your nose tells you if ammonia is escaping; the white vapor you see is condensed water, not ammonia, which is colorless.”

How ammonia moves in soil

What happens when anhydrous ammonia is injected into soil? Several physical and chemical reactions take place, says ISU Extension soil fertility specialist John Sawyer. They are ammonia 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 or near the point of injection, with a tapering of the concentration toward the outer edge of the retention zone,” he explains. “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 most soils. The shape and size of the ammonia retention zone varies greatly depending on rate of application, knife spacing, soil type, and soil conditions at injection, such as soil texture, soil structure, organic matter and moisture status.”

Ammonia moves farther at injection in coarse-textured soils and soils low in moisture, Sawyer says. Also, if the injection knife causes sidewall smearing (when soils are wet), then ammonia may preferentially move back up the knife slot. Movement toward the soil surface can also occur for some time after application if soil dries and the knife track “opens up.” This is also due to less soil moisture to retain free ammonia in solution with drying soils.

“A similar movement within the soil can occur if soil breaks into clods at application, and there are large air voids left in the soil,” he adds. “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. 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,” Sawyer 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 negatively charged soil exchange complex and doesn’t move with water. Only after conversion to nitrate (NO₃^–), via the nitrification process, can it be lost from soil by leaching or denitrification.”