Recent above-normal rainfall across much of Iowa has left yellow and pale green corn plants in areas of many fields. Farmers with pale and yellow patches of corn are questioning if they have enough nitrogen left in the soil for their crop to yield well. They are wondering about making an additional application of nitrogen to try to turn that corn to dark green again.
John Sawyer, Iowa State University Extension soil fertility specialist, provides the following answers and insight regarding these questions.
What's the best way to apply additional N to a growing corn crop?
When conventional application equipment can be moved through the field (that is, when the soils are dry enough and the corn is still short enough), then injection of anhydrous ammonia or urea-ammonium nitrate (UAN) solutions would top the list of best options, says Sawyer.
Next would come UAN solution, surface dribbled between corn rows. Next on the list would be broadcast urea, ammonium sulfate, or ammonium nitrate. If there is a sulfur deficiency, and plants are small, then ammonium sulfate would also supply plant-available sulfate. If injecting or surface dribbling UAN, then addition of ammonium sulfate or ammonium thiosulfate would supply sulfur.
If only sulfur deficiency is a problem (no shortage of N), then broadcast calcium sulfate could supply plant available sulfate, he notes. Ammonium thiosulfate should not be broadcast onto plant tissue. Preplant application of sulfur products is preferred, but if the plant symptoms are caught early, rescue sulfur applications can increase corn yield, says Sawyer. Application is best when plants are still small; a sulfate containing product is needed for an immediate available sulfur form.
Avoid broadcasting UAN on large corn because of crop injury
Broadcasting a UAN solution should be avoided on corn larger than the V7 growth stage, says Sawyer. With tall corn, supplemental UAN will need to be applied with high-clearance equipment. Injection coulters or drop tubes between every other row or every row should work equally well.
Dry nitrogen materials can be broadcast with buggy or high clearance dry box spreaders if they can be driven between the corn rows, or this material can be aerially applied.
What about using a urease inhibitor? Should you use it in the nitrogen fertilizer you apply to corn as an additional nitrogen treatment? For broadcast urea, use of a urease inhibitor can help slow volatile nitrogen (N) loss from warm wet soils, answers Sawyer. A urease inhibitor would not be needed with injected UAN, and there is a low probability that it would pay to use the inhibitor if the UAN is dribbled on the soil surface between rows. That's because there is limited UAN contact with the surface soil since the fertilizer is dribbled on instead of spread.
With broadcast dry nitrogen or fertilizer products, some of the fertilizer material will fall into whorls of corn plants, but will cause only cosmetic damage to leaf tissue. "That will show as spots or streaks on the corn leaf margins, when the leaf eventually grows out of the whorl. Of course, to get the yield benefit from surface applied nitrogen or sulfur, it needs to be moved into the root zone with rainfall."