(Shared by Anthony Bly, South Dakota State University Extension's iGrow service. Click here for the source website.)
Soybean harvest is near and fall fertilizer applications probably won’t be far behind. Some growers choose to use anhydrous ammonia (AA) as their source for nitrogen primarily because of cost and timeliness of application for their operation.
There are application considerations when using AA. Anhydrous is a liquid when stored and turns to a gas (ammonia) when applied to the soil. Some equipment maintains AA as a liquid through the application process and it isn’t until it reaches the soil that it turns to a gas, this is called cold flow injection.
With older applicators, the AA turns to a gas within the manifold and is solely gas when it leaves the applicator shank. Due to the properties of gasses, the possibility of ammonia loss can occur. The AA needs to come in contact with soil water to be converted to ammonium (NH4+).
The ammonium ion which is positively charged can be held on the soil cation exchange capacity (CEC) and will not leach with moving water. If the soil is above 50 degrees Fahrenheit, microbial activity is increased which can convert the ammonium ions to nitrate (NO3), which is leachable in water moving through the soil.
If the soil is dry, during AA application, there is more air filled pore space for AA gas to ‘leak out’ to the atmosphere. There is also less water for conversion to (NH4+), but unless the soil is extremely dry (wilting point or below) there is usually enough moisture for this conversion to take place. Dry soils also don’t seal well, therefore deeper application depths may be necessary to avoid gas loss. Gaseous AA loss can also occur when the soil is extremely wet, because the sides of the shank wall smear and don’t seal together.
There are indicators when too much AA is escaping as gas. After making passes around or through the field and the smell of ammonia is detected, adjustments should be made. Visible white water vapor in the air above applicator shanks also indicates ammonia escape. The white clouds are water vapor interacting with the colorless AA.
Lastly, if AA applications are made prior to soils cooling below 50 degrees Fahrenheit, additions of N-Serve (nitrapyrin) can be made to reduce the conversion of ammonium to nitrate. Fall N applications should be avoided on coarse textured soils and fields prone to spring flooding.
As always with AA, safety is foremost. AA gas can suffocate and burn, so precautions against leakage and accidents are paramount. Stay safe by understanding this product and handling it in a responsible manner. For more information about anhydrous ammonia and safe handling instructions view the Anhydrous Ammonia HOSTA Task Sheet.