Many questions are crossing grower's minds regarding nitrogen application. However, with this season's variability in precipitation, Fabian Fernandez, University of Illinois Extension specialist in soil fertility and plant nutrition, says those are difficult questions to answer generally.
The most practical approach to determining whether additional N is needed is to perform strip N applications in a field to see if there is a response in growth or level of greenness due to N.
If additional N is needed, Fernandez offers a few reminders for growers to consider.
1. When should I apply N?
Little N is needed by the crop during early vegetative stages to about the fifth leaf development stage. The largest portion of the total N taken up by corn occurs during the eighth leaf to VT (tasseling) development stages. Nitrogen uptake is mostly done shortly after pollination. Thus, applying N before the V8 development stage is best. Research has shown that if applications are done around V6, it is very rare to see yield loss due to N stress. This is because most soils in Illinois can provide sufficient N to satisfy the demands of young corn plants.
Of course, if a portion of the total N was applied pre-plant or at planting, a delay in application of supplemental N is not likely going to cause plant N stress. In cases where no N was applied, or the N supply is very low, make it a priority to apply early (preferably before V6) to avoid loss of yield potential.
2. What are the best options for sidedressing N?
Injection into the soil or dribbling the N fertilizer between rows are the best ways to sidedress because this application can reduce volatilization of urea and protect the crop from foliar damage. If ammonia is used for the application, it is important to watch soil conditions to ensure that the knife track closes properly to avoid foliage damage by free ammonia escaping to the atmosphere.
When injecting or dribbling are not viable options, broadcast applications of N would be the next alternative. Urea granules will have the least impact on leaf burn compared to UAN or dry products such as ammonium nitrate or ammonium sulfate. To minimize adhesion of dry products to the leaves, it is best to apply when the foliage is dry. Remember, urea is subject to volatilization if rain does not fall within 3 to 4 days after application. As much as 30% of broadcast urea can volatilize if there is no rainfall within approximately 10 days after the application.
If UAN solution is broadcast over corn when plants are small (about 6 inches), it's likely the damage will not result in yield loss. Even when plants are bigger (V4), the foliage damage caused by a rate as high as 90 to 100 pounds N/acre typically does not cause significant yield reduction.
One way to reduce damage from UAN is to apply in advance of rain. If rain falls within a few hours after application, it will wash the fertilizer off the foliage and will also reduce the potential for volatilization of urea. If a broadcast application of UAN is the only option available, do it as soon as possible because the smaller the plant, the lesser the potential for foliar damage. If you plan to include herbicide with your UAN application, be sure you read the herbicide label first to make sure such application is allowed.
Additionally, be aware that including herbicide with the UAN solution can intensify leaf burn. In Minnesota, addition of 2 pounds atrazine/acre at a rate of more than 90 pounds N/acre at V3 caused severe leaf burning. Applying 2 pounds atrazine/acre at 60 pounds N/acre causes similar leaf burning as applying 120 pounds N/acre with UAN alone.
3. If my crop is too tall already, can I apply N "over the top"?
In some fields, crops are getting tall and some still need additional N. Application of dry products, such as ammonium nitrate and urea, "over the top" can result in foliar damage. Typically, this damage is an aesthetic concern and rarely translates into yield reduction.
"Over the top applications" of UAN are the least desirable way of applying N. However, if this is the only alternative and the plant needs more N, the yield benefit from the additional N will likely outweigh the leaf burn caused by the application. Research has shown yield reduction when a rate of more than 60 pounds N/acre was applied at V8. To avoid extensive foliage damage, when N applications are needed later than V8, it's very important to fit the high clearance equipment with drop hoses so UAN is applied directly on the soil surface without touching the crop canopy.
"If additional N is needed, sidedress earlier rather than later in crop development if at all possible," Fernandez said. "If you have an option on how to apply N at sidedress, choose first injection applications or dribble UAN solution between rows; second, broadcast of solid ammonium-containing fertilizers; and third, broadcast UAN solution."