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Is your fall-applied nitrogen escaping?

Last fall's fieldwork went smoothly for a lot of Corn Belt farmers. Harvest went quickly, then good field conditions and weather allowed plenty of opportunity to get this year's fertilizer applied before winter set in.

But now, unseasonably warm, dry weather this winter could mean last fall's efficiency in the field could come back to bite those farmers who got everything done.

It all lies in the fertilizer that's already in the ground. Watch the soil temperature: Once soil warms up to 50 degrees, things kick into gear. That's basically "biological zero," the point below which activity like nitrification stops. But, get above that point prematurely -- the chances of which have grown dramatically with the type of winter we've had in much of the nation's midsection -- and it could ultimately let that nitrogen you put down last fall escape from the soil before there's a crop there to nourish.

"Once it's in the nitrate form, that's when we start worrying," says University of Illinois soil fertility specialist Fabian Fernandez.

"It's much more mobile and more likely to not be in the environment when the plant needs it," adds Ag Spectrum Company technical director Cliff Ramsier. "Our concern is that people are aware that there's a growing risk that there's less nitrogen out there right now than what they put down last fall."

But, if you used a nitrification inhibitor when you applied that fall fertilizer, you should be protected from premature nitrification, right? Not quite, Ramsier says. Those products do prevent nitrification for a while, but the efficacy window isn't wide enough to keep it at bay for a long period of time when biological activity's taking place in the soil.

"Nitrification inhibitors are relatively short-term in their efficacy, and if you go back to the original application date, you have just about 21 to 28 days of efficacy, regardless of what the temperature is," Ramsier says. "Assuming temperatures stay below 50 degrees or 'biological zero,' you probably won't see nitrification happen. But between 50 and 62 degrees, you will see some activity, and above 62, you'll see really high levels of nitrification."

Usually by the time an inhibitor stops working, the soil's below 50 degrees, so those products do accomplish what they set out to do. That is, until Mother Nature decides to warm things up like she has this winter. While soil temperatures in much of the Midwest are still a ways from 50 degrees, once that trigger is pulled and that nitrogen gets on the move in the soil, your options are limited. Basically, it comes down to nailing down whether you've got a deficiency and, if so, plan on putting down more nitrogen this spring.

"Soil testing right now isn't very efficacious. It's just not a good indication. Find a way to monitor soil temperatures on a local basis and keep control of that information," Ramsier says.

When you do get around to soil testing as spring rolls around, though, don't be surprised if you find more nitrogen than you expected, not less, Fernandez says, especially if it's been dry since last summer. He recommends starting soil testing around late May.

"It seems like no matter what people did last year, they had nitrogen deficiencies, which are bound to happen in dry conditions," he says. "If you don't have water, there's no nitrogen take-up. I suspect there will be some leftover nitrogen. But, how much of that will be available this next year will depend on what conditions are like this spring."

And, though he agrees the chances of needing supplemental nitrogen is lower, Ramsier says based on today's corn market, it's better to be cautious and refine your nitrogen strategy to maximize uptake.

"At $6 corn, you can't afford to leave 5 to 15 bushels [per acre] out there by ignoring potential problems," he says. "Under high yield conditions, we'd like to see the crop out as long as we can. We apply nitrogen at preplant, then sidedress. Since nutrition is so valuable, we encourage inhibitors at sidedress."

On the other hand, despite their relative convenience compared to spring applications, Ramsier discourages fall nitrogen applications altogether.

"Fall applications are mechanisms for convenience, not optimal production," he says. "They shouldn't sell the concept short that what they're doing is extremely high-risk and they can be taken out of the game pretty shortly with the wrong weather patterns."

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