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This article in SSSAJ

  1. Vol. 47 No. 1, p. 81-84
    Received: June 11, 1982



Growth of Rhizobium in Unamended Soil1

  1. J. J. Pena-Cabriales and
  2. Martin Alexander2



A study was conducted to determine the conditions in unamended soil that favored growth of Rhizobium. Moistening a dry soil resulted in growth of R. japonicum and a strain of Rhizobium nodulating cowpeas but not of R. meliloti. The extent of decline resulting from desiccation was usually greater than the increase following wetting. The extent of growth varied among soils. A freezing-thawing cycle did not promote growth of R. phaseoli and the cowpea Rhizobium. The latter bacterium was more tolerant to a freezing-thawing cycle than the former. Rhizobium japonicum and R. phaseoli grew readily in the presence of germinating seeds and developing root systems of soybeans, kidney beans, red clover, cowpeas, oats, wheat, and corn; the population size varied among the plant species, and legumes were no more stimulatory than nonlegumes. The numbers of R. phaseoli in the rhizosphere of several plants declined in 1 week, the abundance of R. japonicum fell after 6 weeks in the soybean rhizosphere, but the population size of the cowpea Rhizobium was high for at least 20 weeks. Counts of R. japonicum in soil increased after the soil was amended with soybean nodules. It is suggested that appreciable rhizobium growth in unamended soil occurs only in the presence of germinating seeds, growing roots, and decomposing nodules.

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