By Jennifer Shike, University of Illinois
Applying nitrogen (N) in July may not be in a grower's plan, but for some corn fields throughout the Midwest, it is needed as soon as possible, said Fabian Fernandez, University of Illinois Extension specialist in soil fertility and plant nutrition.
With soils drying and crops growing very fast as they rapidly accumulate growing degree days, Fernandez said it's a priority to make sure those areas that did not receive sufficient 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 said. "If N has been applied, the light-green crop is typically in areas where ponding of water occurred. In areas where there is adequate N, often waterlogged soils induce N deficiency-like symptoms. Those symptoms should have disappeared in most places by now."
If the symptoms persist after about a week from the time soils dried, it is pretty certain that the crop needs more N, he said. Whether N-deficient areas represent a large number of acres or just spotty zones, the question on farmers' minds is how to fix the problem and do it the most efficiently.
"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 said. "In other words, the corn crop will not be capable of using that full rate to make yield."
Another important point is that the sooner you apply N, the better response you are likely to see. This year across Illinois, corn is "all over" in terms of development stages, he said.
"While some fields are still at early vegetative stages, others are rapidly approaching reproductive stages," he said. "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, it is important to keep in mind that areas needing N application at this time are most often patchy, so targeted applications, rather than even applications across the field, are fundamental to minimize cost, increase return on the 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," he said. "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."
Another alternative is to use canopy-sensing technology to guide N application rates. Research has shown that canopy sensing is most useful when the plants are bigger (around V10 stage) and the plants are obviously N deficient, Fernandez said. Often, when plants are smaller, the sensor captures too much soil area relative to leaf area and can result in an overestimation of how much N is needed.
"If you are using canopy-sensing technologies, make sure the equipment is properly calibrated to an N-sufficient zone of the field," he said. "High-clearance equipment is likely the only way you would be able to put in the rescue N application in some fields. Fortunately, high-clearance equipment is becoming increasingly more available."
For rescue N, Fernandez suggests 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 said.
Fernandez does not recommend broadcast application of UAN due to the high probability of canopy injury. Also, the use of slow-release products (polymer-coated urea) is not a good practice at this point because N needs to be available to the crop immediately. While the coating can protect urea from volatilization (just like a urease inhibitor does), it will take time for the coating to breakdown and release N, resulting in a further delay in N availability to the crop.
"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 said. "Because of this, applying before it rains is a good approach."
He discourages application of foliar products because the amount of N that can be applied with such products is often very low and the cost-per-acre too high to make it profitable.