ANKENY, Iowa (DTN) -- Farmers eager to apply nitrogen (N) this fall may need to wait longer than they would like. Agronomists say unusually warm soil temperatures across much of the Corn Belt are keeping conditions unfavorable for ammonia and manure applications.

Most fall N applications, primarily anhydrous ammonia and liquid manure, occur across the central and upper Midwest, according to Dan Castellano, an Iowa State University (ISU) agronomy professor. By mid-November, he said soil temperatures in those regions are typically at or below 50 degrees Fahrenheit and continuing to decline, a benchmark considered safe for applying ammonia-based fertilizers for the following year's corn crop.

John Baranick, DTN ag meteorologist, said the 50 degrees and falling threshold may not occur until well into December due to an unusually warm, wet fall driven by a Super El Nino -- an abnormally strong variant of the cyclical weather pattern caused by heightened warm surface waters in the eastern tropical Pacific Ocean, which is expected to peak next month.

Baranick and Castellano said farmers need to be patient and pay close attention to forecasts and soil temperatures to minimize N loss.

"All available forecasts have this weather pattern lasting through the end of November and into the winter season," Baranick said. "We really don't have anywhere in the country right now where soil temperatures are near or below 50 degrees, besides northern North Dakota and northern Minnesota. It's going to take time for temps to come down.

"Farmers will need to monitor soil temps closely and if they're close to 50 degrees, check the two-week forecast for the risk of temps rising again," he continued. "With how high fertilizer prices are, you don't want to waste it."

The latest DTN Fertilizer Index, a weekly compilation of fertilizer bids from agricultural retailers, pegs the average price of anhydrous ammonia at $1,007 per ton, its highest price since July.

Find retail fertilizer prices tracked by DTN for the final week of October 2026 at https://www.dtnpf.com/….

TEMPERATURE MATTERS

The most essential guideline to applying fall N is soil temperature, Castellano stressed. He said the 50-and-falling standard is a reliable and science-backed method agronomists have long recommended for minimizing nitrification, a stage of natural nutrient recycling that makes nitrogen fertilizer vulnerable to leaching into drainage tiles.

Castellano said when temperatures in the top 4 inches of soil are above that threshold, nitrogen-loss risk surges, reducing nitrogen availability for next year's crop and increasing the potential for water-quality issues.

"There's an exponential relationship between temperature and the transformation of ammonium to nitrate," Castellano told DTN. "When we don't follow best management practices, that leads to lower productivity, lower profitability and greater environmental impacts."

When ammonia contacts soil moisture, it becomes ammonium -- a nitrogen compound that clings to the soil because its positive electrical charge bonds with the negatively charged soil. When soil microbes consume ammonium, the nitrate created has a negative charge. The negative-on-negative clash repels nitrate from the soil, making it easy to wash away. Soil nitrate can also be lost as a gas through a different microbial process called denitrification.

When soil temperatures drop below 50 degrees, microbial activity that drives nitrification and denitrification drastically slows, making the risk of N loss minimal, Castellano said. The opposite occurs when soil temperatures are above 50 degrees.

For farmers tempted to start fall N applications as soil temperatures near 50 degrees, Castellano said they should think again.

"A couple of degrees can make a big difference in the rate ammonia transforms into nitrate," he said. "It's not worth the risk."

Find the latest soil temperatures:

Iowa: https://iowaweather.com/…

Minnesota: https://www.mda.state.mn.us/…

North Dakota: https://ndawn.info/…

South Dakota: https://climate.sdstate.edu/…

Nebraska: https://nemesonet.unl.edu/…

Wisconsin: https://wisconet.wisc.edu/…

Illinois: https://warm.isws.illinois.edu/…

CONSIDER NITRIFICATION INHIBITORS

For farmers planning fall nitrogen applications, Castellano recommended using a nitrification inhibitor. The additive temporarily slows the soil bacteria responsible for converting ammonium to nitrate, helping keep nitrogen in a more stable form and reducing the risk of loss.

"Farm survey data shows well over 80% of fall synthetic ammonia applications include the chemical because farmers recognize it works and provides a good return on investment," he said. "With the price of nitrogen, it pays."

Adding a nitrification inhibitor to swine manure, which accounts for 70% of Iowa manure fertilizer, isn't as common, he added.

"One Iowa State study showed adding an inhibitor to swine manure slows nitrification to a rate expected when soil temperatures are about 5 degrees lower," Dan Andersen, a biosystems engineering professor at ISU, said in a statement.

If manure pits need to be fully or partially emptied before soil temperatures are favorable, Andersen encouraged farmers to apply it on ground with cover crops.

"Having roots in the ground between harvest and planting provides restless nitrate a safe in-field destination," he said. "That's the No. 1 tool we have in that situation."

Learn more fall N tips at https://www.dtnpf.com/….

ISU recently released information about N application timing, placement and safety. Find the information at https://crops.extension.iastate.edu/….

PLAN FOR 45Z

The weather and soil temperatures aren't the only considerations before applying fertilizer this fall. Farmers may need to complete some paperwork first if they hope to collect premiums for grain going to biofuels under the federal 45Z Clean Fuel Production Tax Credit.

New federal guidelines on the 45Z require farmers to develop nutrient management budgets, document field practices and maintain records to verify their corn's carbon-intensity score. A key element here is that nutrient budgets must be done before fertilizer applications begin.

Learn more at https://www.dtnpf.com/….

Matthew Wilde can be reached at matt.wilde@dtn.com.

Follow him on social media platform X @wildeagwriter

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