How to Keep 25% More of Your Fall Anhydrous in the Field

The most expensive mistake with fall anhydrous applications isn’t using the wrong rate or skipping the inhibitor — it’s applying too early.

University of Minnesota Extension specialists say farmers who apply anhydrous when soil temperatures hit 55°F instead of waiting for 50°F could watch up to 40% of their nitrogen convert to vulnerable nitrate before the ground freezes. Wait for proper conditions, and that number drops to 15% or less

“That (5-degree difference) doesn’t seem like it should be that big of a deal. But if you actually look at the rates of nitrification based on soil temperature, it kind of is,” says Brad Carlson, University of Minnesota Extension educator.

Why 50°F Is the Magic Number

When you apply anhydrous, it starts out as ammonium. But soil microbes are constantly working to convert that ammonium into nitrate, which is much easier to lose through leaching or denitrification.

The warmer your soil temperature is, the faster those microbes work.

“The cooler it is, the longer anhydrous is going to stay put,” Carlson says. “Think of it like your refrigerator — food lasts longer when it’s cold. Same concept with nitrogen in the soil.”

While the nitrification process doesn’t stop at 50°F, it does slow way down. It doesn’t fully stop until the ground freezes solid.

Carlson and Kaiser, Extension nutrient management specialist, laid out the math during their Advancing Nitrogen Smart Series. Their advice? Watch your soil temps, don’t let the calendar boss you around, and give your nitrification inhibitor time to work.

A Ten-Day Difference That Will Cost You

Carlson and Kaiser shared some scenarios to offer more perspective. Let’s say you apply when soils are still at 55°F, and it takes another 10 days for temps to drop to 50°F. Research shows about 25% of your nitrogen could convert to nitrate during just those 10 days.

They figure another 15% can convert between 50°F and freeze-up. That means if you apply early at 55°F, you could see 40% of your nitrogen turn into vulnerable nitrate before winter even arrives.

However, if you can wait to make applications until the temperature is 50°F and trending down, you’re going to lose 15% or less.

Those are ballpark estimates. Your actual numbers will vary depending on soil type, moisture and weather.

To check soil temps, Carlson recommends measuring at 6 inches. He says most National Weather Service stations report at 4 inches and 8 inches, so just average those two numbers.

And don’t get fooled by a quick cold snap where temperatures are likely to rebound. You need soil temperatures that are going to stay cool and keep dropping.

“We’re looking for it to be 50 and stable,” Carlson says. Check the seven-day forecast before you hook up.

Your Inhibitor Can’t Save You From Bad Timing

Some farmers do choose to apply early and rely on their nitrification inhibitor to compensate for the decision.

Products like N-Serve (nitrapyrin) definitely help slow down that ammonium-to-nitrate conversion. But the inhibitor itself breaks down over time, and warmer soil speeds up that breakdown.

Carlson’s numbers show that applying 10 days too early could burn through about 20% of your inhibitor before you even need it.

Here’s how it plays out: Apply at 50°F under typical southern Minnesota conditions, and your inhibitor could stay effective until around April 17 — right when you’re thinking about planting corn.

Apply 10 days early, and that protection might run out around March 28 — a couple of weeks, at least, before you’re ready to go to the field.

“You’re spending that additional cost for the nitrapyrin, so you want to make it work as much as possible,” Carlson says.

Kaiser adds a word of caution about product labels: The calculations they discussed focus on nitrapyrin. Other nitrification inhibitors like Centuro work differently, so don’t assume the same rules apply.

And urease inhibitors? Those are a totally different animal. They slow down the conversion of urea into ammonia— not the same thing as slowing ammonium-to-nitrate conversion. You can’t swap one for the other on fall anhydrous.

Spring Weather Can Make Early Applications Riskier

Even if you do everything right with your anhydrous and wait to apply until ideal timing, some of your nitrogen will still convert to nitrate before planting.

Heavy spring rains can leach it down through the soil profile. Wet, saturated soils can cause denitrification, where you literally lose nitrogen to the air.

Your soil type matters too. Kaiser points out that nitrapyrin works better in heavier clay soils than in some silt loams. That’s one reason Extension doesn’t recommend fall anhydrous in southeastern Minnesota and some other areas — the conditions just stack the deck against you.

You’ve Got More Time Than You Think

A lot of farmers rush fall anhydrous applications because they’re worried about running out of time before winter shuts them down.

But at University of Minnesota’s south-central research center in Waseca, there’s an average of 50 days between when soils hit 50°F and when they freeze. During that stretch, the area typically gets about 5.5 inches of rain, but Extension believes that still leaves plenty of working days to get in the field.

“We just don’t see the great urgency to have to be out there a couple weeks early,” Carlson says.

He and Kaiser encourage farmers to look at spring N application opportunities, as well. In south-central Minnesota, soils usually thaw around March 17 and reach 50°F by May 1. The main spring nitrogen window runs roughly March 20 through April 30.

While working days can be tight in early April, they say urea can provide options for growers then.

“You can spread it fast with a spinner or floater. With the right urease inhibitor, you can even apply it after planting. Just don’t try urea in the fall,” Kaiser advises, noting it’s too vulnerable to loss then.

“With urea, you tend to have more flexibility (in spring),” he adds. “So there are some options out there.”