Cool, Cloudy Conditions and the Soybean Crop

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Written August 28, 1997

During the past two weeks, Purdue soybean specialist Ellsworth Christmas has gotten inquiries from farmers concerned about how the recent below-normal temperatures might affect the soybean crop. He says most of his concern, however, lies with the lack of sunlight.

"My greatest concern over the past two weeks has been the persistent cloudy, overcast conditions. Poor quality sunlight has a greater potential of having a negative impact on soybean development and yield than the lower temperatures experienced to date," he says. "It has been and continues to be my opinion that the temperatures have not dropped to a level that will result in a significant negative impact on the soybean crop."

Christmas says there is very little data on the impact of temperatures on soybean pod and seed development. One study showed that when day and night temperatures stayed at 64F and 57F respectively, very few pods were set, however, plants flowered profusely.

"Optimum day/night temperature range for soybean seed ripening is reported to be 87 degrees [daytime] and 59 degrees [nighttime]," Christmas says. "The recent drop in temperatures experienced in Indiana, if only for a day or two, will probably have little impact on the soybean crop, assuming normal temperatures thereafter.

"The problem arises when the lower temperatures exist for an extended period of perhaps four days or longer or when temperatures at night drop below 50 degrees for several hours." Christmas adds that nitrogen fixation in soybeans peaks at about R 5.5 stage of growth and then drops rapidly as the plant sends nutrients to the rapidly developing seed.

"Soil temperatures below 68 degrees can slow nitrogen fixation. Cool nighttime air may also slow nitrogen fixation by slowing the translocation of sugar from the leaves to the root nodules," he says.

Full-season soybeans planted on a timely basis in Indiana have reached or are approaching R 5.5 now, Christmas says. Soil temperatures from Aug. 21-25 fell below 68 degrees in some areas.

Where soybeans have already reached R 5.5, little impact would be expected.

With regard to moisture, Christmas says soybeans in some areas of the state still need rain to permit the crop to continue rapid growth and development.