Effect of high soil temperatures on Rhizobium spp.

Abstract

Following field observations of poor nodulation of legumes sown on hot days, the possible effects of high soil temperatures on legume bacteria were studied. Eighty-seven strains of legume bacteria on nutrient agar showed maximum temperatures for growth of 31-38.4 deg. C., 32-32.7 deg. C., 36.5-42.5 deg. C. and 30-42.0 deg C. for clover, pea, medic, and tropical legume strains of Rhizobium respectively. Studies of the decline of viable population in sterile wallum-heath sand at 40 deg. C. indicated the rapid death of pea and clover strains; the death of tropical legume strains within 10 hours; and an initial drop of numbers of lucerne organisms followed by a lesser death rate. Sowing inoculated seed into moist soil followed by holding at 40 deg. C. before planting showed that strain of legume bacterium, initial concentration of inoculum and time of subjection to high temperatures are significant factors operating in the survival of legume bacteria on the seed. Methods of overcoming deleterious effects of high soil temperatures are discussed.