ROCKVILLE, Md. (DTN) -- The warm fall weather that allowed some farmers to speed through harvest has a downside: Soil temperatures across much of the Midwest are lingering well above 50 degrees.
That means no matter how tempting it is to get fall nitrogen applications out of the way, you need to wait, agronomists told DTN.
"A lot of guys harvested fairly quickly this year, so they're finished up and want to move on to the next thing," such as nitrogen applications, said Karen Corrigan, an agronomist with McGillicuddy Corrigan Agronomics based in central Illinois. Corrigan said she heard reports of farmers near Springfield, Illinois, applying anhydrous this week, despite soil temperatures for that region clocking in above 55 degrees.
Applying anhydrous before soil temperatures cool down sufficiently will increase the risk of nitrogen loss, reduce profits and may draw scrutiny from environmentalists concerned about nitrate pollution, Corrigan pointed out.
The premature nitrogen applications underway have provoked strong criticism from many farmers on Twitter in the past week. Dozens of tweeted comments from the Corn Belt have censured both the farmers applying nitrogen and retailers selling it to them. A number of growers have also expressed concerns that farmers applying nitrogen too early in the fall will give the EPA a good reason to start regulating these types of applications.
"We're not regulated by law on this, but we're trying to regulate within ourselves," so fall anhydrous isn't taken away as a nitrogen option, Corrigan said.
NITROGEN LOSS CONCERNS
A warm trend across the central U.S. has kept soil temperatures higher than normal for early November, DTN Senior Meteorologist Bryce Anderson noted. "This pattern is pretty well locked in through the first two weeks of November as well," he said.
When anhydrous ammonia is applied to soils, it binds to soil particles as ammonium (NH4+) and remains fairly stable, resisting leaching and denitrification, Corrigan noted. However, warm soil temperatures above 50 degrees 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. Leaching is of particular concern in coarse, sandy soils, or soils with a low cation exchange capacity (CEC), Corrigan said.
"The bacteria that work on taking NH4 to active NO3 operate best above 50 degrees," Corrigan explained. Once the temperature drops below that, this conversion is slowed, but doesn't stop completely until the soil freezes, she added.
"The goal is to keep ammonium as ammonium for as long as possible," added Iowa State agronomist John Sawyer. Applying anhydrous to warm soils leads to "a potential increase in nitrate movement to surface water through leaching, as well as the loss of nitrogen and an agronomic impact on corn yields the following year," he warned.
Many states have soil temperature maps posted online. See the University of Nebraska's here: http://bit.ly/… and Iowa State University's here: http://bit.ly/…. The Illinois Corn Growers Association maintains a map designed precisely to give guidance on when temperatures are suitable for anhydrous applications. See it here: http://bit.ly/….
DO IT RIGHT
In addition to waiting until soil temperatures (taken at 4-inches deep) dip below 50, Corrigan said she urges clients to follow a list of requirements for fall nitrogen applications.
She recommends adding a stabilizer such as N-Serve to lessen the risk of nitrification during the long stretch of time between fall applications and springtime crop uptake. The stabilizer should last until it turns cold, but it loses some effectiveness each time temperatures warm up over the course of the winter, she added.
Fields selected for anhydrous ammonia applications should also have a pH below 6.9, and a cation exchange capacity (CEC) above 13 meq/100 g (milliequivalent per 100 grams of soil), Corrigan said.
"The other thing we don't want farmers to do is put all their nitrogen on in the fall," she said. "Take the soil's CEC times 10 and then times two-thirds, and that is maximum amount we'll recommend applying."
For more information, see Sawyer's university press release on this topic, with a map of current Iowa soil temperatures: http://bit.ly/…, and his Iowa State University article on anhydrous ammonia rates: http://bit.ly/….
Emily Unglesbee can be reached at Emily.email@example.com
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