Don't Rush the Nitrogen Season

Watch Soil Temps When Applying Fall Anhydrous

Matt Wilde
By  Matthew Wilde , Progressive Farmer Crops Editor
Agronomists and fertilizer industry experts urge farmers to be patient and wait until soil temperatures are right before applying anhydrous ammonia. (DTN photo by Matthew Wilde)

ANKENY, Iowa (DTN) -- More than a few farmers are getting antsy to put the 2019 season in the books. When it comes to applying fall anhydrous ammonia, though, patience is more than a virtue. Agronomists and fertilizer industry experts say it helps the environment and public perception.

Reports that some anhydrous ammonia application has begun in fields where soil temperatures are still above 50 degrees Fahrenheit makes agronomists nervous. Doing so makes the popular corn nitrogen (N) fertilizer more vulnerable to leaching.

Lowell Gentry, a University of Illinois agronomist conducting nitrogen and drainage tile research, has worried that some producers may jump the gun given widespread harvest delays.

The temptation to use any available window to work comes on the heels of a wet, late 2018 harvest that left farmers and retailers scrambling to get fertilizer on this spring. Anhydrous ammonia typically costs more in the spring than the fall, which further adds to the frustration.

"Last year left a bad taste in everyone's mouth ... they (farmers) haven't forgotten," Gentry said. "But I would definitely wait until soil temperatures are trending below 50 degrees, and that generally means after Nov. 1 in Illinois. Water quality is my concern.

"People in the know will notice anhydrous tanks moving," he continued. "It's not a good optic right now with all the attention on water quality."

Anhydrous ammonia binds to soil particles as ammonium (NH4+) and is fairly stable, resisting leaching and denitrification when soil temperatures (top 4 to 6 inches) are below 50 degrees. Higher soil temperatures favor nitrification, a bacteria-driven process wherein ammonium converts to nitrites (NO2-) and then nitrates (NO3-), which are highly mobile in the soil and vulnerable to leaching.

Nitrates are a major source of water pollution, causing farm fertilizer practices to be regulated in some states. The Des Moines Water Works in Iowa's largest city unsuccessfully sued three counties, alleging they weren't doing enough to prevent nitrate pollution in source waters used for drinking.

That hasn't been the case in Illinois, though. "We've been very proactive with stewardship and research on nutrient management," said Jean Payne, president of the Illinois Fertilizer and Chemical Association (IFCA). "Right now, there's no animosity, regulation or litigation relative to agricultural nutrient use in Illinois."

She would like to keep it that way. Payne said that means all farmers and applicators need to recognize the importance of continuing to use best management practices when applying fertilizer. The vast majority do, she added.


Payne said some farmers started applying anhydrous ammonia despite soil temperatures in the low- to mid-50s statewide this week. Iowa State University Extension soil temperature readings showed only four of Iowa's 99 counties were below 50 degrees at the 4-inch depth. The same goes for counties in the southern third of Minnesota.

Several states, universities and the National Corn Growers Association track soil temperatures. Here are a few examples:





IFCA's advice to farmers and nutrient retailers: Wait until the soil temperature is 50 degrees and trending lower to apply anhydrous ammonia. This will be helpful to ensure nutrient application timing is voluntary rather than enforced.

"We've been coaching people to hold off a little longer ... and the vast majority have done that," Payne said. "The more people that stay true to this agronomic and stewardship recommendation, the better off we will be. Public perception of how we manage nitrogen is as critical as ever."

Some agronomists recommend farmers apply all their N in the spring and during the growing season to increase fertilizer efficiency, profit potential and reduce chances of N loss. Check out a DTN-Progressive Farmer story on the topic here:….


Gentry will wait to apply fall anhydrous ammonia until soil temperatures are below 50 degrees on farm ground that's part of a multi-year tile drainage study. He's monitoring nitrogen loss and nitrate discharge from 36 tiles in central Illinois, split between corn and soybeans. Replicated studies include 160 pounds per acre of anhydrous ammonia applied in the fall to future corn fields and 160 pounds per acre of anhydrous ammonia applied in the spring before planting.

The last three years of his research shows, on average, fields with 100% fall anhydrous ammonia application lose about 8 pounds per acre more of N per year than spring preplant application. Yield losses are negligible as a result of the nitrogen loss, Gentry said. However, the nitrogen that escapes equates to a little more than 30% of the tile nitrate load that enters waterways.

"You can see the dilemma farmer have between yields and water quality," Gentry said. "From a yield perspective, farmers aren't wrong to put N on in the fall. If just looking at water quality, you wouldn't put it on."

A few anhydrous tanks have been seen in Minnesota fields already, according to Jeff Vetsch, University of Minnesota soil fertility and nitrogen management specialist. Soil temperatures are low enough to apply N in the northern two-thirds of the state, but he recommends farmers hold off for another week in the southern third. Wet conditions will likely dictate that anyway, he said.

"The next time fields are fit, there's no reason to wait, even if fields are hovering just above 50 degrees now," Vetsch said. "At this time of the year with less daylight and sun angle decreasing, it's hard to get soil temps back up."


Soil temperature is only one caveat when applying fall N. Agronomists recommend the following:

-- Add a labeled nitrification inhibitor such as N-Serve or Centuro to lessen the risk of nitrification between application and springtime crop uptake.

-- Fields selected for fall anhydrous ammonia should have a pH level below 6.9 and a cation exchange capacity above 13 meq/100 g (milliequivalent per 100 grams of soil).

-- Avoid application in loess soils and karst topography due to leaching risk.

-- Use in-season soil and tissue tests or canopy sensor technology to determine if additional N is needed in-season.

For more information on the economics of applying fall fertilizer:….

Matthew Wilde can be reached at

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Matt Wilde