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Growth and Survival of Cowpea Rhizobia in Acid, Aluminum-Rich Soils¹

ABSTRACT

A study was undertaken to determine whether Al-sensitive cowpea Rhizobium survives in acid, Al-rich soils. The lower pH limit for growth of 20 strains in a defined liquid medium varied from pH 4.2 to less than pH 3.6. The mean lower limit for growth was pH 3.9. Several of the strains clumped in this medium at pH 4.5. Of 11 strains that were tested for tolerance to high levels of Al in a defined liquid medium at pH 4.5, nine tolerated 75 µM Al, and the other two were sensitive to levels above 15 µM. Three strains, one Al-tolerant, one Al-sensitive, and one Al-tolerant or Al-sensitive depending on the presence of vitamins in the medium, were selected for studies in Al-rich sterile and nonsterile soils. These rhizobia did not survive in soils of less than pH 4.7 sterilized by ⁶⁰Co irradiation. When inoculated into sterile soil at pH 4.7, the consistently sensitive strain initially failed to proliferate and then grew slowly, but populations of the other two rhizobia increased rapidly. No consistent relationship was found between the Al tolerance of these three rhizobia and their growth and survival in four acid, Al-rich soils. The data suggest that Al is of minor importance to growth and survival of cowpea Rhizobium strains in acid soils.

¹ Contribution from the Dep. of Agron., New York State College of Agriculture and Life Sciences, Cornell Univ., Ithaca, NY 14853. Supported by a grant from the United Nations Development Programme and by contract AID/csd2834 from the U.S. Agency for International Development.

² Postdoctoral Associate and Professor of Soil Science, respectively.

Received for publication May 11, 1982. Accepted for publication January 11, 1983.