Source: Douglas Beegle, Soil Fertility Specialist and Del Voight, Lebanon County Extension Educator, Pennsylvania State Crop Management Extension Group
As you inspect corn fields for nitrogen needs it is extremely important to account for all sources of N but also to factor in the timing of the N application. Here are some tips.
Soil physical properties will dictate the fate of N. In soils that have poor drainage and excess moisture, denitrification is the primary way for N to be lost in with wet conditions. There are also areas of coarse-textured soils in many parts of Pennsylvania with great drainage and high percolation rates. This allows for the movement of nitrate through soil by way of leaching. With the recent three-inch rains in some areas, N physically moved into waterways and away from the field. The lighter soils are more prone to denitrification. As rains fall, the nitrates applied (or converted) (UAN, Urea, MAP, manure nitrate) could already be lost. This is why split spring preplant or at planting N and sidedress application is an important management practice for those soils. That management can be helpful for poorly drained soils as well. Since the corn is small at this point, it is difficult to tell the losses that may have occurred this spring. Growers should wait to see as the roots expand and gain access to nitrate deeper in the soil to determine N levels with a Chlorophyl meter. On lighter shale-based soils it is critical to make this assessment and determine additional N needs due to the movement of Nitrate offsite. If growers relied completely on commercial N sources, the splitting of N applications is of high importance.
Dr. Beegle provides the following comments on Corn Nitrogen Management: One of the most important principles of nitrogen (N) management is to apply N as close to the time of crop need as is practical. Nitrogen is very dynamic and the longer it is in the soil before the crop uses it, the more it will be lost. The greatest losses will occur in very well drained soils and in very poorly drained soils during times of high rainfall. Delaying most or all of the N application as long as practical, especially in these soil and weather situations will reduce N loss. Applying a majority or all of the N as a sidedressing is usually the most efficient method. Also, delaying N applications allows the grower to better assess the crop yield potential 4 to 6 weeks into the season and select a more appropriate N rate than is possible preseason.
In situations where there is no manure in the system, one-third to one-half of the N should have been applied around planting time and the balance sidedressed. In systems with manure, any additional N that is required beyond what the manure is supplying and a small amount in the starter, can be applied as a sidedressing. In these situations delaying the application also allows use of the Pre-sidedress Soil Nitrate Test (PSNT) (Agronomy Facts 17) or Chlorophyll Meter Tests (Agronomy Facts 53) to evaluate whether the manure is supplying adequate N to meet the needs of the crop and thus reduce the uncertainty that goes along with deciding if additional N is needed in manured systems. Both tests require testing around V6, when the corn is about 12" tall and if additional N is recommended, it must be sidedressed. It is very important to remember that the Chlorophyll Meter Test cannot be used if more than 15 lb fertilizer N/A was applied at or near planting.
There are a number of ways that corn can be sidedressed. The most common is to dribble UAN solution on the soil surface between the corn rows using drop tubes or hoses over the nozzles. Dribbling reduces plant injury and also reduces volatilization losses compared to broadcast applications. Dribbling between every row is preferred, but N can be dribbled between every other row if it is precisely placed in the middle between the rows. Sprayers with 20" nozzle pacing with drop tubes can also be used but expect a little more injury because sometimes the tube will be on the row. The other common method that can be effective is to broadcast urea over the top of the corn. There is a greater potential for injury with this method, however the injury that does occur rarely hurts yield. The best advice with this method is to do it and then don't look at the field for a week or so, after that it will look fine. Also, there is usually greater volatilization loss with broadcast urea. These problems are minimized if it rains soon after the application. It also helps to minimize volatilization if the urea is broadcast on a dry soil surface (hopefully just before it rains). Losses are greatest if the urea is broadcast on a moist soil surface after a rain when evaporation is high. A urease inhibitor can also help reduce volatilization losses from surface applied urea.
Cultivation or knifing the N between the rows minimizes the volatilization losses of N. However, the benefits compared to dribbling are often not enough to justify the extra expense of cultivation or injection based on the N alone. Obviously, there may be other benefits to cultivation in certain systems that may make this more economical. Thus, if you are cultivating anyway put the N on right before cultivation.
Finally, don't wait too long. Make sure that you have time to apply the N before the corn is too tall. Start early enough to allow for wet weather that may limit the days that you or your custom applicator has to do the sidedressing.