Crop sensors are a good option for making decisions about where to apply and where not to apply more N, but sensors aren’t always necessary, Scharf says. “The different options for applying N during the season are with urea from an airplane, with urea from a dry spinner or boom machine, UAN with pivot irrigation, or UAN with a high-clearance sprayer equipped with drop nozzles or injection arms,” he points out.

From the ground, farmers can detect N deficiency symptoms in their crop by looking for a “V-shaped brown or yellow burn up the midrib of lower leaves,” Scharf says. “From an aerial image, if your corn looks yellow or light green, you need to apply more N.”

During a wet spring, when a rescue N situation is necessary, farmers “don’t want to get too picky on how to apply the N,” he says. “I don’t recommend broadcasting liquid N, but everything else is fair game to use in order to get N on the crop in time.”

Still, the application window for applying a rescue N treatment is wide. “It’s best if you can get it done by shoulder-high corn,” Scharf says. “However, it’s still worth doing up to tasseling if you have a medium-N-stress area, and up to two weeks after tasseling with a high-N-stress area.”

Scharf says farmers can succeed or fail with any N form, depending on timing and weather, but some N applications practices should never occur. “If you are surface-applying urea and not working it into the soil, you must use Agrotain to prevent N loss,” he says. “Another practice to avoid is broadcasting liquid N solutions onto a high-residue cover. If you do, the N will get tied up in the residue and not go to the crop. Instead, either dribble it between the rows or use a coulter system to apply it.”

Problems with phosphorus (P) and potassium (K) applications are much less frequent compared to N application slip-ups, Scharf says. “Most methods to apply them are pretty reliable no matter when or how they are applied — unless you have sandy soils,” he says. “In that case, we recommend no fall K applications.”

Fertilizer application decisions typically revolve around the weather and its effects on soil conditions, Britt notes. The challenges from unpredictable weather aren’t likely to go away any time soon, but the newer application technology is a good start to overcoming them, he adds.