by John Sawyer, Department of Agronomy
When conventional application equipment can be moved through the field (i.e., the soils are dry enough and the corn is short enough), then injection of anhydrous ammonia or urea-ammonium nitrate (UAN) solutions would top the list of best options. Next would come UAN solution surface dribbled between corn rows, and then 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, then broadcast calcium sulfate could supply plant available sulfate. Ammonium thiosulfate should not be broadcast onto plant tissue. Preplant application of sulfur products is preferred, but if caught early, rescue sulfur applications can increase yield. Application is best when plants are still small; a sulfate containing product is needed for an immediate available sulfur form.
Broadcast UAN solution should be avoided on corn larger than the V7 growth stage. 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 corn rows, or aerially applied. For broadcast urea, use of a urease inhibitor can help slow volatile nitrogen (N) loss from warm wet soils. A urease inhibitor would not be needed with injected UAN, and low probability of need with surface dribbled UAN due to limited UAN surface contact. With broadcast dry products, some material will fall into plant whorls, but will cause only cosmetic damage to leaf tissue. That will show as spots or streaks on leaf margins when the leaf grows out of the whorl. Of course to get benefit from surface applied nitrogen or sulfur it needs to be moved into the root zone with rainfall.
John Sawyer is a professor of agronomy with research and extension responsibilities in soil fertility and nutrient management.