If you’re topdressing corn acres this spring with dry fertilizer, keep in mind how that product is managed in a high-residue system will determine whether the fertilizer feeds your crop or disappears into thin air.
Ken Ferrie notes that farmers in his area, central Illinois, commonly use ammonium sulfate, urea and potash for topdressing. He says every hour untreated urea sits on the field surface is a chance for the nitrogen (N) in the fertilizer to gas off and disappear.
“The ammonium sulfate is stable, but the urea has potential to get away when it breaks down,” explains Ferrie, Farm Journal Field Agronomist.
That “getting away” is nitrogen loss caused through volatilization—when N escapes as ammonia gas instead of being captured in the soil as ammonium. In a corn-on-corn rotation, with a lot of stalks and leaves on the field surface for instance, the risk for volatilization is even higher.
Residue Can Supercharge Urease
The problem starts with a naturally occurring soil enzyme called urease. It’s what kicks off the breakdown of urea into ammonia and then ammonium. In a corn-on-corn field with lots of residue, the urease is supercharged.
“The thing about urease enzyme here in the surface with all this residue, it is 10 times higher than it would be in the soil,” he says.
The enzyme goes to work quickly, converting urea to ammonia at the soil–air interface, and that ammonia can simply drift off into the atmosphere. The more time it spends on the surface, the higher the odds of loss.
That’s why timing and management of dry fertilizer applications are critical.
“We sometimes say you need to keep the pin in the grenade – keep the urease enzyme at bay until we can get it worked in or rained in,” Ferrie says.
Evaluate Your Risk Potential
If tillage is in the plan, your solution to prevent volatilization is simple. Apply the fertilizer, then work it into the soil as soon as field conditions allow. When urea is incorporated, even lightly, any ammonia that forms is far more likely to be captured in the soil and converted to ammonium, where the crop can use it.
“There’s probably not a lot of worry in that scenario,” he says. “You’re going to incorporate this urea, and when it gasses, it’ll be in the soil, it’ll be captured.”
But not every system or scenario involves immediate tillage. In many no-till or strip-till fields, or when soil conditions are too wet for equipment, growers end up spreading fertilizer and then waiting on the weather to do the incorporation work. In those situations,
Ferrie warns, the risk of volatilization can increase quickly.
“If it’s going to lay out here and depend on rain [for incorporation], depending on how long that’s going to be, we’re going to need a urease inhibitor to give us time to get it rained in,” he says.
Urease inhibitors can temporarily slow or stop enzyme activity, giving farmers a bigger window before significant nitrogen loss occurs. For fields with a lot of residue, that extra time can make a big difference—especially when the forecast is uncertain.
Alongside conventional urea plus a urease inhibitor, Ferrie points to another option – using ESN, a polymer-coated, encapsulated urea.
“The ESN basically keeps the urea protected,” he says. “In that situation, if we lay it on the surface, you’re going to have about 60 days of protection. If you incorporate it, in our studies, [it] would show about 30 days of protection.”
ESN uses a physical coating to regulate how quickly water gets in and dissolves the urea. For growers who want extended protection or are looking to match nitrogen release more closely with crop uptake, that can be a useful tool. Still, Ferrie’s quick to point out that this isn’t a one-size-fits-all solution.
“It’s quite a bit more expensive,” he notes, underscoring the need to weigh costs against potential risks. For some high-yield, intensively managed corn-on-corn systems, the extra investment might pencil out. For others, a urease inhibitor on regular urea, combined with smart timing and placement, might be the more economical choice.
In addition, farmers need to think through when and how the urea in a fertilizer blend will get treated, Ferrie says If a urease inhibitor is added after everything is mixed together, you end up paying to “treat” nutrients that don’t actually need it.
“Treat the urea before you add the ammonium sulfate and the potash, or you’re going to end up treating all of the product, otherwise,” he cautions.
