Over the last five years or so in Iowa, the major nitrogen (N) fertilizer concern has related largely to losses of N because of excessive rain. This year, nitrogen application questions are being raised for the opposite reason. It's been too dry. The lack of rain in many parts of the state creates this concern: Can plants access the N fertilizer that's been applied?
Work done in recent years by the Iowa Soybean Association's On-Farm Networkhas documented cases where N applied at sidedressing as UAN (such as liquid 28% N etc.) has affected crop growth differently when compared to that from fall applied anhydrous ammonia strips, where both strips received the same N rate. In the photo that accompanies this article, you can see a height difference of more than 8 inches in the corn.
"It was surprising to learn that the taller, better doing corn is where fall anhydrous was applied," notes Tracy Blackmer, director of research for the On-Farm Network. "Further exploration in the area showed that many of the fields that looked "pretty tough" were in fact the fields that were side-dressed. The most likely explanation is that the soil was so dry that roots couldn't get to the nitrogen, even though it was a liquid placed 3 to 4 inches deep in the center between the rows."
When correcting in-season nitrogen stress by making a sidedress application, it is important to recognize that the N you apply must first get into the soil, and then the roots of the corn plant need water in order to be able to access the nitrogen. Blackmer notes that many in-season applications focus on either dribbling the N on the ground (low pressure application) or injecting it into the soil by squirting the N behind the coulter (higher pressure). "The N needs to assimilate into moisture in the soil in order to be available to plants," he explains. "Without adequate soil moisture the roots cannot access or take up the N."
That's point number one, says Blackmer. Point number two, which follows logically, is that during drier years, a mid-season N application is not likely to increase yields in general. Prior studies done with the On-Farm Network have shown this to be true.
Having made those two points, is there any way to improve the odds of improving the yield by using crop sensors to guide in-season N applications this year?
"For this practice to pay, we must assume there is more to the strategy than just using the sensors to detect N stress," says Blackmer. "Crop sensors detect N levels in the plant and then control application of additional N where needed. It assumes the plant is short on N because there is too little N available in the soil. If instead, the crop is stressed because it can't take up the N from a too-dry soil, adding more N in this place won't solve the problem. The only way to rectify the N deficiency in this case is by adding soil moisture – something most Iowa growers can't do without a little help from Mother Nature."
Crop sensors can be beneficial in correcting N deficiency in corn in some situations
For prior On-Farm Network studies with crop sensors go here (pages 14-15) and here (pages 22-24. Dr. Peter Scharf, nutrient management specialist with University of Missouri-Extension, has done extensive research with crop sensors and shown how they can be beneficial in correcting N deficiency in some situations. For his presentation at the 2012 On-Farm Network Conference, click here.
In summary, it's not enough to recognize N stress in corn. "Before you go out and put on more N, you need to understand the reason for any N stress you're seeing," says Blackmer. "Granted, in most years, it's simply a matter of not enough N available, either because of losses or low application rates. But in dry conditions when you've applied sufficient N, it's also possibly due to insufficient soil moisture. Adding more N may not solve the problem, and will definitely add to your production costs."