For some corn fields in the Midwest, applying nitrogen to corn in July is needed as soon as possible, says Fabian Fernandez, University of Illinois Extension specialist in soil fertility and plant nutrition.

As soils dry and crops grow fast and rapidly accumulate growing degree days, it is crucial to make sure those areas that did not receive sufficient nitrogen or N, or lost some of the applied N, receive it now.

“It is not at all uncommon to see light-green corn next to dark-green corn that is further along in development,” he says. “If N has been applied, the light-green corn is typically in areas where ponding of water occurred. In areas where there is adequate N, often waterlogged soils induce N deficiency-like symptoms. These symptoms should have disappeared in most places by now.”

But if symptoms persist after about a week from the time soils dried, you can be fairly certain the crop needs more N. For farmers, the question is how to most efficiently fix this problem whether it represents a large number of acres or just spotty zones.

“It’s important to realize that this late in the season, crops showing N deficiency have already lost some yield potential and applying a full N rate is not going to recover the lost potential,” Fernandez says. “In other words, the corn crop will not be capable of using that full rate to make yield.”

Importantly, the sooner you apply N, the better response you can expect to see. In Illinois this year, corn crop development is quite varied, he says.

“While some fields are still at early vegetative stages, others are rapidly approaching reproductive stages,” Fernandez says. “If your field fits the latter, remember that it is very likely to obtain a yield response by applying N until tasselling. Studies have shown that even until silking, corn has a great capacity to use N and produce an increase in yield if the application is done in severely N-deficient fields.”

Second, keep in mind that areas needing N application at this time are most often patchy, so targeted applications, rather than broad applications across the field, are fundamental to minimize cost, increase return on investment and minimize potential N loss to the environment.

“One way to determine where the trouble spots are is by aerial photography, or by observation from an elevated area above the canopy,” Fernandez says. “In most fields, the corn crop is reaching heights that would make it too difficult to tell where the problem areas are by ‘walking the field.’ Aerial photographs can be converted into variable N rate maps to guide the application.”

Canopy-sensing technology can also help guide N application rates when the plants are bigger, provided the equipment is properly calibrated and high-clearance equipment is used to make the application.

For rescue N, use between-rows applications of dribble or injected UAN solutions or urea plus a urease inhibitor such as NBPT (Agrotain). Another option is to broadcast urea with a urease inhibitor.

“The urease inhibitor is important to reduce the potential for volatilization losses when the product sits on the soil surface until it is incorporated by water,” he says.

“Regardless of N source, any product that is surface-applied will require water to move it into the root system so the plant can use the applied N,” he says. “Because of this, applying before it rains is a good approach.”

For more information, read The Bulletin online at http://bulletin.ipm.illinois.edu/