Greenhouse and field trials were done to test the likelihood that impairment of N fixation contributes to late summer decline in productivity of Medicago sativa L. In greenhouse solution cultures, acetylene reduction and N fixation by nodules were impaired by daily heating of the culture solutions to 32 C, and eliminated by repeated exposures to 36 C or a single exposure to 40 C. However, in a field trial conducted on a Yolo silt loam (Typic Xerorthent, fine silty mixed non‐acid thermic), alfalfa showed no signs of N deficiency and responded little to application of 500 kg NH4NO3/ha although soil temperatures above 32 C were recorded for 5 to 6 hours each day during a 6‐day hot spell following a late summer cut, with peak soil temperatures above 40 C at 2 cm depth. Lack of response in the field trial is explained by observations of temperature profiles and nodule distribution. Only the topmost 5 cm of soil heated above 30 C, and this contained less than 10% of the nodules. Most of the nodules were at depth 10 to 30 cm, and remained at nearly optimal temperatures of 22 C to 27 C. Heat inhibition of nitrogen fixation should not significantly limit growth of alfalfa in the field unless nodule‐formation is constrained to shallow depths.

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