"It doesn't take much moisture for ammonia (NH3) to react with H+ ions from water to convert to ammonium (NH4+)," said Fabian Fernandez, University of Illinois Extension specialist in soil fertility and plant nutrition. "The biggest concern should not be whether there is enough moisture in the soil to react with ammonia, but rather how moisture conditions impact the sealing of the ammonia knife track."
When soils are dry, big clods of soil may form and leave large channels for ammonia to move quickly and escape to the atmosphere. In those situations, ammonia loss can be substantial. If soil conditions are such that the knife track is sealed with loose soil, Fernandez said that one can expect ammonia to be retained in the soil.
In dry soils, anhydrous ammonia will move in the soil a little further than when there is good moisture content. In very dry soils, anhydrous ammonia can move 5 to 6 inches from the point of application. Applying anhydrous ammonia at a depth of 8 inches in such conditions would be advisable to ensure that ammonia does not diffuse all the way to the soil surface in search of water, Fernandez said.
"I would caution that deep applications can mean longer time for corn roots to reach the nitrogen band next year," he said. "I would not suggest applying deeper than 8 inches. If applications are done deeper than 8 inches, then it would be to one's advantage to reduce the fall nitrogen rate and apply that nitrogen near the soil surface in the spring so it is close to the newly developing root system."
Fernandez said it's possible to combine shallow tillage such as field cultivation or disking with ammonia application in fine-textured soil if the soil has adequate moisture and ammonia is applied behind the tillage operation at sufficient depth to ensure that ammonia diffusion does not reach the soil surface. If deeper tillage is needed after the application, it is important to wait at least five to eight days to allow sufficient time for the ammonia to react with soil water and form ammonium.
"The easiest way to test whether your process is adequate is to go back to the field after an application path and smell for ammonia," he said. "While the human nose is very sensitive to this smell, it is impossible to quantify how much is being lost. However, if one can still smell ammonia a few hours later, the best thing to do would be to wait for better application conditions. If there is no smell, there is a good chance that nitrogen loss from the application is minimized."
Many people worry that they are losing large amounts of ammonia in the white puffs they see during the application. Fernandez said ammonia gas is colorless and these white puffs are simply clouds of water vapor. As long as ammonia smells do not persist after application, these white clouds should not be a major concern.
"Minimizing nitrogen loss during or soon after application is important," he said. "But volatilization losses are not the only concern with fall nitrogen applications. Because of the long time between application and when the corn plant uses nitrogen, fall applications are typically riskier in terms of nitrogen lose than pre-plant or side-dress applications."
For more information, read The Bulletin online at bulletin.ipm.illinois.edu/.