Fall Anhydrous Ammonia Application to Dry Soil and N Loss

In the fall of 2011 many areas of the Midwest had dry soils during anhydrous ammonia application. Many of those areas continued to be very dry through the end of February 2012. In addition, some areas also experienced warm fall and winter weather conditions. This has led to questions about how much nitrogen (N) may have been lost from the fall application, and how to determine whether additional N should be applied to help produce a good corn crop in 2012.

The Chemisty of Anhydrous Ammonia Applied to Soil

Fall Anhydrous Ammonia Applications

Fall anhydrous ammonia applications are common in many areas largely due to time constraints in the spring and economic considerations. It helps to spread out the workload so there is more time to focus on corn planting in the spring. Wet spring weather can also prevent applying anhydrous ammonia ahead of corn planting, forcing producers to apply more expensive sources of N after planting. However, fall applications are considered riskier than preplant or sidedress applications in terms of N loss because of the long time between application and when the corn plant uses the N.1

Fall applications of anhydrous ammonia normally occur on soils that become cold enough to limit nitrification. The average soil temperature is recommended to be below 50°F before making applications in the fall.2 Freezing temperatures and snow will eventually stop nitrification from occurring. Application to moist soils helps provide a fast conversion of ammonia to ammonium, which binds to soil particles minimizing N loss through volatilization.

Dry Soil Conditions at Application

Anhydrous ammonia can be applied on dry soils with limited N loss as long as the ammonia is applied deep enough to get it in moisture and the soil is well sealed above the injection slot to prevent volatilization. Ammonia can move up to 6 inches in very dry soil, and with shallow placement it could diffuse to the soil surface and be lost to the atmosphere. Applying to a depth of 8 inches in dry soil is recommended so the ammonia does not diffuse all the way to the soil surface in search of water.3 If the soil is cloddy, channels can be present for the ammonia to move through the soil and escape as gas.

It is very difficult to determine how much ammonia can be lost through volatilization in dry soil. The best way to determine if loss occurs is by smell. If the ammonia smell was obvious and lingered for several hours or more after application, loss probably occurred.4

Warm Fall and Winter Weather Conditions

There is also the potential for higher N loss following fall anhydrous ammonia applications because of the warm fall and winter weather experienced in many areas. N loss can occur by leaching and denitrification any time after soil temperatures allow for nitrification of ammonium. At soil temperatures above freezing, ammonium-N can be converted to nitrate-N by soil microbes. Nitrification increases as the soil temperature increases. Warm weather accentuates one of the potential downsides of fall application of N in that nitrification can begin earlier and essentially be complete before the corn crop takes up much N. More N loss can occur with significant rainfall this spring, whereas less loss of N can occur with a dry spring. If a nitrification inhibitor was applied with anhydrous ammonia in the fall, the potential for N loss could have been reduced by slowing down the nitrification process.

Will Supplemental N be Necessary?

It is very difficult to determine how much N ends up being lost after a fall application of anhydrous ammonia. It depends on many factors including soil type/structure, soil moisture and condition at application, sustained fall and winter soil temperatures, spring temperatures, rainfall, days of soil saturation, and field drainage. Applications of supplemental N may be warranted if sufficient loss has occurred.

No soil test is ideal for providing information to determine how much N will be available and whether additional N is needed at sidedress time. N soil tests such as the pre-sidedress soil nitrate test (PSNT) can help by analyzing for both nitrate and ammonium levels.5 The accuracy of the test is highly dependent upon soil sampling and handling procedures. Although there are different opinions on the reliability of the PSNT, it is a tool that gives data when trying to make a difficult decision. If soil N measurements suggest insufficient N remains from the fall application, calculations can be made and recommendations followed to apply additional N.

 

Cornfield​Corn showing N loss from leaching and denitrification of nitrate-N under wet and saturated soil conditions.

 

Chlorophyll meters and aerial images can also be used to detect N deficiencies in corn. Another approach is to perform strip applications of additional N in a field to see if there is a response in corn growth or level of greenness.6 A drawback to these approaches is that N deficiency detection may not occur until it is too late for a timely application.

Management

Where deep applications of anhydrous ammonia were made last fall, at least 30 pounds of N for early season corn growth should be considered for application through the planter or as a preplant herbicide carrier. If significant N has been lost, then more N should probably be applied via sidedressing. UAN liquid solutions can be applied as a band on the surface with drops, even on fairly large corn. To help minimize volatilization and maximize effectiveness, rainfall or irrigation is needed to move UAN and urea into the soil. Up to 30% of the urea could be lost due to volatilization if no rainfall occurs within two weeks and temperatures are warm.

Assessing N loss and requirements is not an exact science, but it can help provide estimates that impact your bottomline. Growers should pay close attention to those fields that were fertilized with anhydrous ammonia last fall under dry conditions. Monitoring early season corn growth will be important to assess supplemental N application needs.

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