While soil temperatures were warmer than normal last winter, the dry soil conditions have resulted in very little nitrogen (N) loss this spring. According to Assistant Professor of Crop Sciences Fabián Fernández, soils in the state were dry at the beginning of autumn 2011, with above-average precipitation levels only during November and December.
“In March, I predicted that the risk of N loss would increase only if the spring became too wet,” says Fernández. Precipitation thus far this year is, overall, below average in the state. Thus, so far, the likelihood of N loss this year is very low
Taking a look at soil N also provides evidence that applied N remains in the soil.
“Nitrification has proceeded quickly this spring,” Fernández says. In central Illinois, a field with anhydrous ammonia plus NServe applied in November 2011 contained 80 ppm ammonium in the top 12 in. of soil at the end of February. Now, in mid-May, the concentration is 27 ppm. During the same time interval, nitrate concentrations have increased from 10 to 31 ppm. Although ammonium has transformed to nitrate, there is still a considerable amount of ammonium in the soil, probably because not all of it has been nitrified and also because organic nitrogen in the soil is being mineralized to ammonium.
Last week, samples were also collected for the 12-24-in. depth. As expected, there has not been sufficient water to move nitrate down the profile and N concentrations were low: soil nitrate was only 9 ppm and soil ammonium was 3 ppm.
At this time, most cornis growing rapidly and starting to take up N. “If all the N has already been applied, I do not anticipate a need to apply additional N for this crop,” Fernández says. “If no N or only a portion of the N was applied, now is the time to start applying the balance of the application.”
The N needs of the corn plant are low from the early vegetative development stages until about V5 (fifth-leaf stage). Most N is taken up during the V8 to VT (tassel) stages. Soon after pollination, N uptake is essentially completed.
Because the potential for N loss by leaching or denitrification at this time of the growing season is very low and corn plants will soon enter a rapid N uptake phase, Fernández recommends not delaying the application. However, if farmers are not quite ready, research has shown that the chance for yield loss due to N stress is very low, even when applications are done as late as the V6 development stage, because in most soils in Illinois, mineralization of soil organic N provides ample N for early crop demands. Moreover, if part of the total N was already applied, a delay in applying the remaining N is not likely to cause plant N stress.
In fields with low N-supplying power (soils with low organic matter) or where no N has been applied, early application (before the V6 development stage) is recommended to avoid loss of yield potential. Another reason not to delay application, especially because of the dry conditions this year, is that the sooner N is applied, the greater chance it will have to be moved into the root zone by rain.
“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.