With most of Kentucky’s corn emerged and rapidly growing, some producers are seeing yellowing on the veins of leaves.
This yellowing is called interveinal chlorosis and is caused by a nitrogen deficiency. It is a common occurrence in many fields and may show up and disappear throughout the growing season. If the nitrogen deficiency becomes more severe before plants begin to allocate nitrogen to the ear, the lower leaves will begin to turn yellow, starting at the leaf tip and going up the mid-rib of the leaf.
"Interveinal chlorosis can be caused by the corn growing faster than it can take up nitrogen from the soil," said Chad Lee, Extension grain crops specialist in the University of Kentucky College of Agriculture. "This is a normal occurrence, even in fields with corn that has already received a high rate of nitrogen fertilizer. Just having that symptom doesn't necessarily mean the crop needs more nitrogen."
Whether an additional side-dress nitrogen application is needed depends on a grower's field elevation, soil saturation and severity of the deficiency.
Above normal rainfall this spring kept some fields with low-lying areas saturated or flooded for periods of several days. When fields are saturated for more than two days, losses of nitrogen in the nitrate form can occur at a rate of 3 percent to 4 percent per day, said Lloyd Murdock, UK Extension soil specialist. The amount lost depends on the nitrogen source used in preplant applications, the time between the application and the onset of waterlogging, and the number of days the soil is saturated.
Those who have fields with low-lying areas and who have nitrogen deficiencies on more than half of their leaves may want to side-dress with additional nitrogen using high clearance equipment or aircraft.
If growers plan to side-dress, it is best if it is done before the V8 growth stage (eight fully emerged leaves) but you can still get a good response up to tasseling, Murdock said. About half of the state's crop could reach the tasseling stage within a week.
"Even if there is a severe deficiency, growers can get a response from an additional side-dress application up to tasseling," he said. UK trials in 1993 showed a yield increase of 11 bushels per acre for corn side-dressed with nitrogen under these conditions.
UK publication ID 139, "A Comprehensive Guide to Corn Management in Kentucky," offers a chart to help growers determine the amount of nitrogen lost in their fields. It can be accessed on the UK College of Agriculture's Web site or from the local UK Cooperative Extension office.
Another way to determine deficiencies is through a nitrogen soil test. For this test, producers should take soil samples at 12 inches deep sampling both high and low areas of a field when the corn is less than 18 inches to the whorl. If test results show nitrate levels below 25 parts per million, a supplemental application of nitrogen is warranted. If the results are at or above 25 ppm, no additional nitrogen is needed.