Fall application of nitrogen fertilizer to cropland that will be planted to corn in 2008 is about to begin. Ideally, nitrogen fertilizer applications should be made in the spring or sidedressed after corn emerges. However, fall nitrogen applications are sometimes necessary to cover the large amount of acres planted to corn in Iowa, considering the limited amount of application equipment available.

Fall applications are especially appealing to farmers who have a large acreage of corn to plant when time and labor are limited in the spring, says John Sawyer, Iowa State University Extension soil fertility specialist. If you are convinced you must apply nitrogen in the fall, then anhydrous ammonia is the only form of N you should use. Anhydrous ammonia runs less risk of nitrogen loss than liquid 32% nitrogen fertilizer or the dry forms of nitrogen fertilizer when applied in the fall.

"Application of anhydrous ammonia should not be done until soil temperatures are 50-degrees F or colder, with continued soil cooling in the weather forecast," says Sawyer. The colder the soil is, the better for limiting nitrification and allowing a fall ammonia application to behave like a spring preplant N application does.

Watch ISU's reports of soil temperatures

What about using a nitrification inhibitor? Using a nitrification inhibitor product with the nitrogen fertilizer can further slow the ammonia to nitrate conversion, thus increasing the amount of the N remaining as ammonium next spring.

Soil temperatures statewide are expected to reach and remain below the 50-degree F guideline during the first week of November in 2007.

Why is ammonia the best form of N to apply in the fall? "Ammonium ions attach to exchange sites in the soil, thereby limiting its movement," explains Sawyer. "Also, anhydrous ammonia initially limits its own conversion to nitrate in the soil and therefore is the only nitrogen fertilizer suggested if you want to apply N in the fall in Iowa. However, even if you apply now when the soil temperatures at the 4- inch depth are below 50-degrees, keep in mind that the weather and the soil can still warm up later on this fall. Warm soil temperatures increase microbial activity that converts the ammonium form of N to nitrate."

Delay fall N application until soil cools

Nitrate is mobile in the soil so it can both leach and also be lost through denitrification. "Nitrogen losses represent decreased economic return from the fertilizer investment and leaching increases the amount of nitrate reaching the water supply," notes Sawyer.

"Thus, you should delay fall nitrogen application until soil temperatures are 50 degrees F and cooling," he advises. To follow ISU's county soil temperature information each day, along with the forecast model for the future this fall, go to extension.agron.iastate.edu/NPKnowledge. 

In recent years, soil temperatures at the 4-inch depth have cooled to below 50 degrees F statewide on dates ranging from October 22 to November 21. Keep in mind that soil temperatures typically cool earlier in northern Iowa than in central or southern Iowa.