As air temperatures continue to fluctuate this fall, so do soil temperatures. This means it may still be too early to apply anhydrous ammonia (NH₃) to cropland without risking nitrogen losses through groundwater leaching.

DON’T GO UNTIL 50 OR BELOW: Farmers are reminded to wait until soil temperatures are below 50 degrees F before applying anhydrous ammonia fertilizer this fall. Soil temps have been slow to cool due to the unusually warm late October weather.

Anhydrous ammonia applied before soil temperatures, at a 4-inch depth, are 50 degrees F and trending lower will quickly convert to nitrate. While ammonium will mostly stay attached to the soil, nitrate will flow with water. The nitrate form of nitrogen can leach and be lost from the soil. “If we get more rain than the soil can absorb this fall or next spring, the nitrate is at risk to leach into groundwater, tile water and streams,” says Eric Hurley, nutrient management specialist with USDA’s Natural Resources Conservation Service (NRCS) in Des Moines.

Precaution should also be followed when applying manure
Since most of the nitrogen in liquid swine manure is in the ammonium form, hog manure should also be applied when the soil temperature is cold and the soil microorganisms which convert ammonium to nitrate are relatively inactive, says Hurley.

Nitrogen losses can also impact crop development, points out Barb Stewart, state agronomist for NRCS in Iowa. “We encourage farmers to wait until spring to apply fertilizers, especially nitrogen fertilizers,” says Stewart. “This not only prevents nitrogen losses throughout the fall and winter, but spring application ensures the nitrogen is available when the crop needs it the most.”

Check real-time soil temperature map to determine when to apply
As of Nov. 2, soil temperatures were slightly above 50 degrees in the northern third of the state, but were still in the mid to upper 50s in soils throughout the rest of Iowa. Farmers and fertilizer dealers are encouraged to check Iowa State University Extension’s Iowa Soil Temperature Map at extension.agron.iastate.edu/NPKnowledge to check the soil temperature for your area. The map shows trending soil temperatures throughout the state.

ISU maintains this statewide real-time soil temperature data map on its website. Fertilizer dealers and farmers use the information on this map to determine when fall N applications are appropriate. Anhydrous ammonia is the only form of N that ISU agronomist recommend for fall application.

Also, consider using a nitrification inhibitor with fall-applied ammonia
The reason for waiting to apply anhydrous ammonia until soils are cold is that nitrification, the process of biological conversion of ammonium to nitrate, occurs at a more rapid rate with warm soils,” explains John Sawyer, professor and ISU Extension specialist in soil fertility and nutrient management.

In addition to waiting for soils to cool below 50 degrees, use of a nitrification inhibitor with the anhydrous ammonia application should be considered to help further slow the conversion of ammonium to nitrate, he notes.

“Temperature is only one soil condition that farmers should consider when applying anhydrous ammonia,” says Sawyer. “Making sure the soil is not too dry, too hard or too wet, will reduce injection issues that allow ammonia to move to the soil surface and be lost to the air. If conditions are not suitable for application, then waiting for better conditions is suggested.”