N application options
“In my opinion, the best options to sidedress N are to inject it into the soil or dribble it between rows,” Fernández says. If anhydrous ammonia is used, make sure the knife track gets properly sealed to avoid crop injury from free ammonia escaping to the atmosphere.
While most fields look very dry on the surface, adequate moisture is still present below the surface to retain the ammonia. According to Fernández, “If the application is done at least 6 in. below the surface in fine-textured soils or at least 8 in. below the surface in coarse-textured soils, there should not be problems with ammonia volatilization.”
The advantage of dribbling N between crop rows over broadcast applications is that dribbling reduces the potential for volatilization of urea-containing fertilizers (urea and UAN) and reduces fertilizer contact with the foliage, thus reducing foliar damage.
If injection or dribbling options are not available, Fernández recommends broadcast urea. This product has the least impact on leaf burn when compared to UAN, ammonium nitrate or ammonium sulfate. If the canopy is wet, it is best to wait until it dries to minimize dry fertilizer adhesion to the leaves.
If there is not a high likelihood of rain, Fernández suggests applying urea with a urease inhibitor (such as Agrotain) to minimize N volatilization. When urea is broadcast on the soil surface without a urease inhibitor, N losses start to increase three to four days after the application if there is no rain. After 10 days without rain, as much as 30% of the application can be lost. By contrast, ammonium sulfate and ammonium nitrate are not subject to volatilization losses if left on the soil surface.
The least desirable option is to broadcast a UAN solution because this application is most likely to injure the crop. If no other options are available, the application should be done as soon as possible because the smaller the plant, the less chance for fertilizer to make contact with it.
Some studies have shown that there is little damage if a UAN solution is broadcast when plants are about 6 in. tall. Similarly, for bigger plants (V4 stage), an application of up to 100 lbs. N/acre is unlikely to cause substantially reduced yield. This kind of application is best done a few hours before rain so the fertilizer can be washed off the leaves, although this is not advisable if a herbicide is combined with the UAN application (read the label to ensure this is allowable). Also, be aware that including a herbicide with UAN can intensify leaf burn damage.