Watch the soil temperature at the 4-inch depth if you apply nitrogen in the fall. It can be below 50 degrees when you apply anhydrous ammonia, but if that soil warms up for awhile afterward, you'll lose nitrogen.
"Ideally, nitrogen fertilizer applications should be made in the spring or sidedressed after corn planting," says John Sawyer, an Iowa State University Extension soil fertility specialist. "However, fall nitrogen applications are sometimes necessary to cover the acreage planted to corn in Iowa with the available application equipment. Also, time and labor are limited in the spring if you wait to do all your fertilizing, field preparation and planting in spring."
If you are convinced you must apply N in the fall, the only type of nitrogen fertilizer ISU suggests for fall application in Iowa is anhydrous ammonia. "That's because the ammonium ion attaches to soil exchange sites, limiting movement in the soil," explains Sawyer. "However, with warm soil temperatures, microbial activity converts the ammonium to nitrate, which is mobile in the soil, can be leached and can be denitrified. Losses represent decreased economic return from the nitrogen investment and leaching increases nitrate in water systems."
You need to delay fall application
Application of anhydrous ammonia should not be done until soil temperatures are 50 degrees F or colder, with continued soil-cooling forecast. The colder the soil the better for slowing nitrification and allowing a fall ammonia application to behave like a spring preplant application does. Nitrification inhibitors can further slow the ammonia to nitrate conversion, thus increasing the fraction still in the ammonium form the next spring, says Sawyer.
Long story short: Delay fall nitrogen applications until soil temperatures are 50 degrees F and cooling. ISU's county soil temperature and forecast model is available at extension.agron.iastate.edu/NPKnowlege.
In the past five years, soil temperatures have cooled to below 50 degrees F statewide ranging from October 22 to November 21. Soil temperatures typically will cool a few days earlier in northern Iowa and a few days later in the south.