Setting the stage
Nitrogen is the most yield-limiting nutrient, but it's also the most mobile in the soil.
"Right now, growers really care about efficiency," says Alex Park, a division agronomist for Helena. "Margins are getting tighter and tighter, so when we think about any place where we're losing nitrogen and it's not going into the crop, we're losing money."
To achieve 230 bushel-per-acre (bpa) corn, the crop needs 256 pounds per acre of nitrogen, according to University of Illinois research. The most rapid uptake happens between V12 and R1, when corn takes up 150 pounds per acre to develop its stalks and leaves.
However, nitrogen loss, due to leaching and denitrification, happens far earlier. The greatest potential for loss is between April and July, when rainfall is most likely.
This risk is greatest if you're using urea, Park says. "That nitrogen has to last all winter long into the spring, and through that leaching and runoff potential," he points out.
Why fall apply?
Despite the risk, Greg Goodwin, director of the precision conservation management program at Illinois Corn, estimates 40% to 45% of farmers apply nitrogen in fall.
Compared with the urea used in spring, it's often cheaper to buy and apply anhydrous ammonia, the only real viable option for fall applications.
Also, timing applications in spring can be difficult.
"Farmers often have concerns about being able to get nitrogen put on across all their acres in the spring," Goodwin says. "We sometimes have a limited window, as far as when the weather is right, to be able to be in the field in the spring."
Make the most of fall applications
If fall application is the only sensible option, there are options to minimize risk. For example, apply nitrogen when soil temperatures are 50°F or lower. That's because as soils cool, the biology that converts ammonium nitrogen to nitrate "really slows down, almost to a halt," says Matt Fryer, a technical agronomist for Koch Agronomic Services. "Then," he adds, "when soils freeze, everything stops. There's no conversion happening in frozen soil."
Fryer says Koch's data shows it always pays to treat your nitrogen with a stabilizer when loss conditions are present. Nitrogen stabilizers slow conversion rates, keeping the nutrient in the soil until crops can use it.
"If you can't afford everything, reduce your nitrogen rate just a little to pay for the stabilizer," he recommends. "You'll be way better off." He adds that in many cases, a stabilizer-treated fall application outperforms an untreated preplant application.
Switch to spring?
The data also shows it pays to apply nitrogen in-season. An Illinois Corn cost analysis found that preplant and sidedress applications were more profitable than fall applications. Preplant applications are the most profitable, boasting a $19 per acre greater return than fall applications. Sidedress applications offer an $11 per acre greater return.
On average, farmers making fall applications use a higher rate of nitrogen and stabilizer but do not see a great enough yield increase to cover those costs.
"We see that when you move to doing in-season applications, you can oftentimes lower your nitrogen rate to what we call the 'maximum return to nitrogen level,'" Goodwin says. "That's a university-recommended approach to estimating nitrogen rates based on the price of corn and the price of fertilizer."
Evaluating efficiency
Regardless of when you apply nitrogen, check strips are a low-cost option to evaluate a fertilizer program's success. To implement a check strip without seeing yield drags, overapply nitrogen in a strip across the field. If the check strip outperforms the rest of the field, apply more nitrogen.
"Nitrogen can make up to 50% of a farmer's fertility budget in some areas," Park says. "With corn prices the way they are, efficiency is key."
