Many fields planted in early May made it up, but looked yellow for a week or two. Bob Nielsen says it's because they weren't getting photosynthesis underway very quickly due to cool conditions. He's the Purdue University Extension corn specialist.
Jim Camberato, Purdue Extension soil fertility specialist, says it's also because plants couldn't take up enough nitrogen. Nitrogen was being released slowly from the soil due to cool weather, and roots weren't reaching fertilizer bands due to slow growth. Once it warmed up and the sun stayed out for a few days, most of these fields greened up almost overnight. They also grew quickly at the same time.
Now some fields are yellow in spots due to heavy rains and ponding. That can again be tied to various factors, including lack of nitrogen. However, if corn outside of waterlogged spots is slow to green up and retains a yellowish cast, Camberato says sulfur may be deficient. Depending upon the hybrids, some plants may show striping with sulfur deficiency, but that's not always the case. Some just remain yellow. Often they grow out of it later on.
Until a couple decades ago, power plants put enough sulfur into the atmosphere through burning coal that sulfur was seldom deficient. Camberato says fields still receive several pounds per year in the atmosphere. However, in some cases sulfur shortages can develop.
The only way to know for sure is to pull tissue tests and have them sampled at a lab, he says. If there are green spots in the field, pull samples from both yellowish and green areas. The results that you receive back will indicate whether or not sulfur is low in the plants. It will also indicate nitrogen levels in the plants as well, Camberato says.
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