URBANA, Ill. – April showers and cool temperatures hit Illinois with a vengeance recently, with more to come through the first week of May. Corn and soybean growers are nervously watching the forecast, wondering what the cool, wet weather means for planting and for new seedlings. Emerson Nafziger, professor in the crop sciences department at the University of Illinois, breaks it all down.
Emergence. Before the drop in temperature, corn was emerging within 7 or 8 days of planting. “Although not much corn has been planted since April 26, it will take at least 2 weeks for corn to emerge under current temperatures,” Nafziger says.
Chilling injury. Rain and cool temperatures have raised some concern about “imbibitional chilling injury” that can accompany such conditions. “We don’t think there should be much of this because seeds took up warmer water after planting,” Nafziger notes. “But growers should check seeds when 100 growing degree days (GDD) have accumulated after planting. They should look to see if corn seedlings show any of the corkscrew growth that often goes along with this injury. Soybeans may not show this, but may still fail to emerge.”
Flooding. Nafziger says a larger concern is how seeds and seedlings might be affected by heavy rainfall, especially where standing water has developed. “Seeds will usually not survive the low oxygen levels in saturated soils for more than a few days,” he warns. “They will survive longer in cool soil, because that slows growth and lowers oxygen demand, and also because cool water carries more oxygen into the soil.”
Emerged corn plants can survive flooded soils a little longer, especially with cool and cloudy conditions. But those submerged for more than 4 or 5 days may not survive. Soybean seeds and seedlings typically do not fare as well as corn in flooded soils, Nafziger says, adding that ponded areas in planted fields may need to be replanted.
Nitrogen. The fate of nitrogen fertilizer is another concern. Nitrogen from fall or early spring applications was still present before the rains came, but Nafziger explains that the longer it has been in the soil, the more ammonium-form fertilizer has converted to nitrate. “About 75 percent of fall-applied nitrogen was nitrate in samples taken in mid-April,” he says. Unlike ammonium, nitrate can move with water as it percolates down in the soil. Nitrate can also be lost to denitrification in saturated soils, but that process is slow when soils are cool.
Although growers are concerned about nitrogen leaving the rooting zone, Nafziger cautions it is too early to decide whether to apply extra nitrogen. “Water that runs off the field normally carries little nitrogen if the fertilizer was incorporated or injected, or if it moved into the soil with rainfall before runoff started. Water movement down through the soil in tile-drained fields is not very fast, and a return to drying conditions will further slow this movement.”
A return to warmer, drier conditions will also mean a resumption of mineralization, which will help provide nitrogen to emerging or growing seedlings. “Any ammonia or urea-based fertilizer nitrogen that was applied this spring should still be mostly in the ammonium form, which should remain in the soil after the heavy rain that fell,” Nafziger says.
Bottom line. Growers are itching to get the rest of their crop planted. According to USDA’s National Agricultural Statistics Service, 37 percent of the corn and 87 percent of the soybeans remain to be planted as of April 30. “Planting is ahead of normal, especially for corn, but progress will be slow this week. The sooner warmer temperatures arrive to dry things out to allow planting and to get the planted crop growing, the better,” Nafziger says.
For more information, read Nafziger’s Bulletin post on this topic.