Waiting to fertilize fields until soil temperatures drop to below 50 degrees can help reduce nitrogen loss and is better for the environment, according to a statement released this week by Iowa Agriculture Secretary Bill Northey.
"With farmers finishing harvest earlier than normal, it is important that they still wait for cooler soil temps to apply anhydrous ammonia fertilizers so that there is a better chance the fertilizer stays put and will be available to the crop next spring," Northey said. "Soil temperatures, like air temperatures, can change quickly so it is important that we wait with applications until soils are likely to remain below 50 degrees."
Elwynn Taylor, climatologist at Iowa State University Extension and Outreach, said his data shows soil temperatures at a 4-inch depth "historically" cool below 50 degrees in the northern third of the state during the first week of November.
He said the temperatures in central and southern Iowa, soil temperatures normally cool below 50 degrees during the second week and third weeks of November.
A statewide real-time soil temperature data map to assist ag retailers and farmers in determining when fall N applications are appropriate can be found on the ISU Extension and Outreach website: extension.agron.iastate.edu/NPKnowledge.
Northey said farmers should also be mindful to pay special attention when applying anhydrous ammonia to very dry soil.
Dry soil can hold ammonia, but if it is cloddy and does not seal properly, the ammonia can be lost at injection or seep through the large pores between clods after application.
Northey also advised that farmers and applicators should assure proper depth of injection and good soil coverage when applying into dry soils.
If, following a round of application in the field, the ammonia can still be smelled, the applicator should make adjustments or wait for better conditions.
Northey added that farmers with questions about timing of fertilizer applications may also obtain information from their local ISU Extension staff or ag retailers.