4 fall nitrogen strategies

Perks of fall nitrogen (N) applications include the ability to shift some spring workload to the fall and applying N in more favorable soil conditions.

However, fewer fall N applications occur now than in recent years, says Fabian Fernandez, University of Minnesota (U of M) Extension nutrient specialist.

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"One of the reasons we're seeing this is because the weather is getting warmer," he says. "Farmers are seeing fall applications aren't as effective as they used to be. Plus, the ability to sidedress and apply nitrogen in the spring is becoming more common than before."

Still, fall remains an option for some farmers to apply part of their N for 2023. Here are four ways to get the most bang out of your fall N dollar.

1. Stick with anhydrous ammonia accompanied by a nitrification inhibitor.

"The only source I'd consider for fall application is anhydrous ammonia," says Fernandez. Anhydrous ammonia is the N source that has the best potential to remain in the soil until spring when accompanied by a nitrification inhibitor, he adds.

Cautiously eye biological products that have been touted as enabling farmers to shave N rates, he says.

"The jury is still out on the effectiveness of biologicals, especially for fall applications," Fernandez says. "There is not much research available, so stick with anhydrous."

Economics favor anhydrous ammonia as a fall N source, as it's typically less expensive than other N sources. Fernandez cites a five-year U of M study conducted annually in several locations in which researchers compared fall anhydrous ammonia applications to urea. Anhydrous ammonia gleaned better corn yields 60% of the time by an average of 49 bushels per acre.

Part of this study solely examined urea. It found that even with 29 more pounds of N contained in urea, fall applications still yielded 9 bushels per acre less than spring applications.

This makes clear that urea is not a good fall application option, Fernandez says.

2. Apply fall N only where loss potential is low.

"If you're located where you know the possibility of nitrogen loss is high, don't consider a fall application," Fernandez adds. "Places that have warm winters, falls, and springs, and places that get a lot of rain are not good candidates for it." N applications in northern locations fare better because of colder temperatures. Meanwhile, nix N applications on coarse soil types or soils that have a lot of drainage.

"Remember, only consider a fall application if the potential for loss is low," he adds.

3. Apply fall N at the right time.

"We typically apply anhydrous ammonia when soil temperature at 4 inches below the surface is 50°F and getting colder," he says. "It's important to wait as long as you can to apply anhydrous in the fall. Nitrification continues until temperatures reach 32°. Fifty degrees is a good practical temperature because nitrification slows considerably and the soils can still be worked because they are not yet frozen.

"As the soil gets colder than 50°, the potential for nitrification goes down," Fernandez adds, "but it is not unusual to have warmups that reduce the effectiveness of the application."

Thus, watch weather forecasts even if soil temperatures reach 50°.

"If temperatures keep going down, that's a good sign," says Fernandez.

4. Consider soil moisture at application.

"When using anhydrous, you can lose nitrogen at the time of application if the soil is too wet or too dry," Fernandez says. "If it's too wet, your knife track might not close and ammonia will escape. If it's too dry, the anhydrous retention zone grows as ammonia moves through the soil to react with water. If the retention zone is larger than the depth of application, some ammonia can also escape to the atmosphere."