Richard Taylor, Extension Agronomist; rtaylor@udel.edu
I was asked recently for thoughts on sidedressing corn during the recent heat we’ve been experiencing. I thought I would share some arcane, although still relevant, information from my soil fertility and plant nutrition class at the University of Delaware. This concerns an old proven principle called (by me at least) the Sprengel-Liebig Law of the Minimum. Essentially what P. Carl Sprengel in 1828 and J. von Liebig, also, in the 1900s stated in various papers they published was that yield will be determined by whatever factor is most limiting to the crop.
For growers who irrigate their corn, chances are that the limiting factor will be something other than water availability. It may be a disease that impacts grain fill or it may be a lack of one of the essential nutrients or even a lack of enough sunlight during the growing season. We often see the latter in years where it’s cloudy or hazy for much of the summer. With respect to nitrogen (N) sidedressing, irrigated growers can easily apply a small amount of water to incorporate the N into the soil and therefore prevent ammonia volatilization losses from the urea component of the UAN (urea-ammonium nitrate) solution.
However for dryland corn producers who may feel the need to sidedress N on their rapidly growing corn crop, what likely is the most yield limiting factor their crop faces. Is it N availability? No in my opinion, the most limiting factor will always be water availability throughout the growing season. If it isn’t the total amount of N, it will be the distribution of the rainfall/moisture during the growing season. For these producers, the decision to sidedress N will come down to the speed of corn growth versus their ability to cover all the corn acreage before the corn runs out of starter N or becomes too tall to sidedress without causing crop damage.
Within the activity of sidedressing, there are some choices to minimize N loss as ammonia volatilization such as knifing in the UAN, using anhydrous ammonia rather than UAN, or using one of the urease inhibitors. Some of the old research suggests only about a 7 to 10 percent loss in N from dribbling the UAN solution on the soil surface. Even with this amount of loss, your most limiting yield factor will still be water availability throughout the growing season.