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Drought Stress in Soybean (07/29/21)


Due to the long stretch of dry conditions, many of the soybean fields are showing drought stress in the area. There also has been a long period of high temperatures along with the water stress. With temperatures in the high 80s and 90s, and with low relative humidity, the plant cannot extract water from the soil quick enough during the day, to keep up with transpiration and evaporation. As a result, the leaf stomata close preserving water and the underside of the leaf is turned up (Photo 1). The closed stomata stop gas exchange and no carbon dioxide can enter the leaves therefore reducing photosynthesis.

Drought stressed soybean plants are often short. Drought stress can also reduce or stop biological nitrogen fixation. Stress during the reproductive phase will result in low yield potential. Reduced availability of water in drought conditions may lead to the shedding of flowers and small pods. This plant stress often results in lower pod numbers per plant, fewer seeds per pod, and lighter seeds. If precipitation occurs after the R5 soybean (beginning of seed formation) growth stage, the plant may still be able to increase the seed size; however, there is a genetic limit on how large seeds can get. Normally, as the plant reaches the R6 (full-size seed) growth stage, pods will not be shed but the plant will preferentially fill the most mature pods to create viable seeds for the species to survive to the next generation. The grain filling period is reduced under prolonged drought stress, and the crop matures earlier than it normally would. For a detailed description of soybean growth stages, see extension publication A1174 Soybean growth and management.

Photo 1. Soybean field showing turned over leaves because of drought stress.



 Hans Kandel

Extension Agronomist Broadleaf Crops