High Temperature Effects on Corn, Soybeans

How is soybean nodulation affected by high temperatures?

Soybean nodulation is influenced by temperature. Greatest nodule weight and nitrogen (N) fixation (the conversion of gaseous nitrogen to plant-available N) has been found to occur when soil temperature is 75° F. When soil temperature exceeds 86° F, nodule initiation and growth decreases. According to the OARDC Weather System, maximum soil temperatures ranging from 79° to 89°F in the top four inches of soil have been recorded for July. Under a soybean canopy, soil temperatures may be lower. However, in fields where canopy closure has not yet occurred, high soil temperatures may result in decreased nodulation and nitrogen fixation. Applying nitrogen fertilizer to nitrogen-stressed soybeans can increase yield in some cases, but this practice is not without risks (see this source (pdf)). Applying urea forms of N to warm, damp soils results in N loss as a gas (volatilization). Additionally, if dry weather persists, the N may not be moved down into the soil and be accessible to the soybean roots.