Considerations for applying N in the fall

Considerations for applying N in the fall

ISU soil fertility specialist explains what you need to know about fall nitrogen fertilizer application.

Much of the primary fertilizer nitrogen (N) for corn is applied in the spring as preplant or sidedress, where efficiency of N use should be greatest. Fall N fertilizer application success can be enhanced by following several suggestions. John Sawyer, Iowa State University Extension soil fertility specialist, provides the following information and observations and guidelines to help farmers and fertilizer dealers make prudent nitrogen management decisions.

GET A GOOD SEAL: When injecting anhydrous ammonia, pay attention to soil conditions and coverage. Make sure the knife track is getting a good seal of ammonia into the soil. Adjustments may be needed.

First, you should only use anhydrous ammonia for fall applying N. Second, apply it in late fall after soils cool to 50 degrees F (at the 4-inch depth) and are trending cooler. The colder the better, and most years in Iowa this is sometime in November. Consider using a nitrification inhibitor to further slow nitrification to nitrate. And avoid fall application to soils that are more prone to wetness or leaching (poorly or excessively drained soils).

Fall-applied ammonia can work, says Sawyer. However, wet springtime conditions will reduce the efficiency and success in achieving corn yield potential, he adds. Other N fertilizers, like urea and UAN (urea-ammonium nitrate solution), nitrify too quickly which increases chance for loss and therefore should not be fall applied. For example, research across many years at the ISU Northern Research Farm at Kanawha has documented lower corn yield with fall incorporated urea compared to spring incorporated urea. The co-nitrogen application in DAP and MAP can be at risk with fall application due to rapid nitrification (N in DAP and MAP is in the ammonium form), and with the often early fall application.

Where to find soil temperature information
Soil temperatures can be found at several websites. One site gives the 3-day, 4-inch depth soil temperature estimates for each county, and the six- to 10-day weather forecast. That site can also be accessed through the ISU Agronomy Extension and Outreach Soil Fertility website, either from the weather page or nitrogen topic page. The 4-inch soil temperatures 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.

What if soils are dry? Should you go ahead and apply?
Probably not a widespread concern this fall, but in some areas soils may be or may become quite dry. 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 in addition compacted soils, can sometimes be an issue for retention of anhydrous ammonia at application time. Following is a discussion of what happens when anhydrous ammonia is injected into soil.


Can anhydrous ammonia be applied to dry soils?
Yes. Dry soil can hold ammonia. Even air dried soil contains some moisture, although 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 ("goes into solution") 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. 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. 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. Reducing the application rate or narrowing the knife spacing reduces concentration of ammonia in each injection band.

What happens 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. 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 to 4 inches in radius in most soils. The shape and size of the ammonia retention zone vary greatly depending upon the rate of application, knife spacing, the soil type, and soil conditions at injection (soil texture, soil structure, organic matter and moisture status).

Why you need to pay attention to soil conditions
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. 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 (also less soil moisture to retain free ammonia in solution with drying soils). A similar movement within the 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 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 over time
1) NH3 + H2O = NH4+ + OH–. This initial chemical reaction of ammonia (NH3) with water and 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 and not ammonium that can be lost from soil at application and is damaging to microorganisms and plant seedlings. As pH goes above 7.3, the equilibrium between ammonium and ammonia results in increased free 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 are the steps in the biological nitrification process that occurs with ammonium in soil, and ultimately results in a lowering of pH back to the original pH or lower. Nitrification occurs first at the outer edges of an ammonia band, and progresses inward as the initial effects of ammonia injection decrease and the soil conditions become more conducive to microbial growth.

Is there a maximum ammonia application rate soils can hold?
The rate of anhydrous ammonia that can be held in soil is not a direct relationship with CEC. Soil properties affect the size of the injection zone, but ultimately several other factors are more important, such as moisture content, depth of injection, and soil coverage, especially with dry soil or coarse textured soil. Within agronomic rates of application, there is no real limit or maximum application rate. Anhydrous ammonia has been successfully injected into sandy soils at rates over 200 pounds N per acre. In research conducted with alternate row injection (example 60-inch spacing in 30-inch row corn), 200 pounds N/per acre has been successfully applied -- which is an equivalent of more than 400 pounds N per acre per injection knife. It's injection depth, soil coverage, and overall soil conditions that determine potential volatile loss, not simply CEC.

The bottom line: be careful if soil conditions aren't favorable
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. If the soil is breaking into clods, 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 condensed water, not ammonia which is colorless), 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.

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