Several factors – including operation size, fertilizer prices, equipment availability, and the threat of wet springs – result in producer interest in fall application. If you’re one of those producers, following these four guidelines can help keep N available for your crop in the coming season.
1. WATCH THE TEMPERATURE
Dave Franzen, North Dakota State University Extension soil specialist, says it is key for you to pay attention to your state guidelines. “It’s important to look at the calendar and then the temperature,” he says. “We have a combination recommendation here.”
He encourages you to never apply any N fertilizer until October 1. After October 1, Franzen recommends watching the soil temperature. “Our guideline is, when it hits 50°F. between 6 a.m. and 8 a.m. at the 4-inch depth, it’s time to apply,” he says.
Due to microbial activity in warm soils, applying anhydrous ammonia before the temperature is 50°F. can lead to ammonium conversion to nitrite, explains John Sawyer, Iowa State University Extension fertility specialist. He also recommends waiting until the soil temperature is 50°F. and trending cooler before applying fall N.
“We know that the nitrification process doesn’t become really slow until the temperature gets to 32°F.,” says Franzen. However, waiting until the soil temperature is 32°F. is not practical. “With the losses we’ve had, farmers are much better about being patient than they were a few years ago,” he adds. “We woke people up when they found they were only growing 100-bushel corn on land that should be growing 200-bushel, and the problem was N loss,” Franzen says.
2. CONSIDER YOUR SOIL TYPE
“Soils that have a greater chance for N loss would be less preferred for fall applications,” says Sawyer. “Soils that are well drained, but not excessively well drained, would fare better with a fall application.”
Sawyer advises farmers to avoid fall N applications on historically wet fields. Water moves readily through sandy soils causing N loss through leaching. “Some of those (sandy) soils don’t even have enough clay in them to hold the ammonia itself,” adds Franzen.
"If there’s a low negative charge in the soil, then there’s nothing for the ammonia ions to stick to – even the ammonia will leach,” says Franzen. “Those are the soils that have K tests of 100 ppm or less.” He says N applications for those soils should be either spring applications or sidedress N applications instead.
For high clay soils, Franzen recommends that you apply half the N rate in the fall or preplant in the spring and then sidedress the other half in the spring when the corn reaches V5 to V8. “That way, if it does become really wet in May and early June, you only have half of your N at risk. You’re already planning to sidedress,” says Franzen.
3. CHECK YOUR FORM
Only certain types of N fertilizer, such as anhydrous ammonia and manure, should be fall-applied. “Some states don’t recommend putting on urea at all, but we haven’t found that to be an issue,” says Franzen. He does, however, recommend that you wait a week after the safe date for ammonia to put on banded urea and then another week for broadcast urea.
“When you apply anhydrous ammonia, there’s an aura of concentrated gas that kills almost everything biologically within a 2- to 3-inch radius around the point of ammonia escape. The ammonia itself acts as a nitrification inhibitor for a short time, maybe up to a week,” says Franzen.
A banded urea application produces a small aura of gas, but not nearly as much as anhydrous ammonia. Broadcast urea is essentially open to nitrification as soon as ammonia is produced by the urease enzyme. Franzen doesn’t recommend UAN for fall applications; it’s a spring and sidedress fertilizer, he says.
“A quarter of it is already nitrate. The whole idea of a good fall application is to go into the fall and freeze up with as little nitrate as possible,” he explains about the UAN fertilizer.
Sawyer only suggests anhydrous ammonia for fall applications. He recommends all other forms be spring-applied or sidedressed only.
4. STRIVE FOR EFFICIENT USE
When it’s time to apply fall N, remember that proper management can lead to increased efficiency and profitability. It also helps in keeping your valuable fertilizer out of the rivers and in the root zone