As I write this article, (May 4th), very little field work has been accomplished due to one of the wettest spring's on record. Assuming the rain does eventually stop, and we get our corn planted, you should now be scouting your corn fields weekly. Unfortunately, the long range forecast is calling for wet periods into June and July as well. Excessive rainfall will not only affect herbicide programs and stimulate weed growth but can have negative consequences for our fertility programs as well.
Weather and soil physical properties dictate the fate of N in the soil. Poorly drained and flooded soils will experience denitrification. This process converts various forms of Nitrogen to N gas, which is then lost to the atmosphere. There are also areas of coarse-textured soils in many parts of Pennsylvania with high drainage and percolation rates. This allows for the movement of nitrate nitrogen through soil and out of the rooting zone by way of leaching.
Nitrogen is the most mobile nutrient--especially Nitrate (NO3)--which all inorganic N (commercial fertilizer) forms are converted into in warm summer soils. This process can occur rapidly and makes much of our commercial N vulnerable to loss. However, organic (manure) nitrogen must first be decomposed by soil bacteria, mineralized and then converted to nitrate; therefore, it is a slower release form of N. If you rely on manure for most of your N, you may not have lost a significant amount and as organic decomposition continues the N status of the soil often recovers; whereas, with commercial N it does not.
So, if you are relying on commercial N for most of your fertility--and these wet conditions have continued--you need to assess the fertility of your fields. The best way to combat this N loss is to prevent it by applying only 1/3-½ of your total N at planting and then side dressing the remainder. This nitrogen has then remained safely protected in the tank rather than out in the environment. Also if you do side dress a portion of your N needs, you can assess the status of the crop and the weather and increase or decrease fertility as needed.
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 side dress. In these situations delaying the application also allows use of the Pre-sidedress Soil Nitrate Test (PSNT)() or Chlorophyll Meter Tests() 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 manure systems. Both tests require testing around V6, when the corn is about 12" tall and if additional N is recommended, it must be side dressed. 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 side dressed. 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. Another 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.
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.
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 side dressing.