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

ABSTRACT

A study was undertaken to determine whether the reduced nodulation of cowpeas [Vigna unguiculata (L.) Walp.] grown in certain acid, Al-rich soils resulted from the poor survival of the potentially infective rhizobia. Two strains of Rhizobium capable of nodulating cowpeas were used. The lowest pH for growth in defined liquid medium was 4.2 for one strain and 3.9 for the other. Only the latter was Al tolerant and could grow in a defined liquid medium containing 50 µM KAl(SO₄)₂. The survival of the bacteria and their ability to nodulate cowpeas in three soils were measured after the soils were amended with Ca or Al salts to give pH values ranging from 5.7 to 4.1 and extractable-Al concentrations from <0.1 to 3.7 cmol(p⁺)/kg of soil. Only small differences in survival in 7 or 8 weeks were noted between the two strains. Plants inoculated with the Al-sensitive strain bore significantly fewer nodules in the more acid, Al-rich soils than in the same soils with higher pH values and less extractable Al. No significant reduction in nodule number was evident for plants inoculated with the Al-tolerant strain and grown in the more acid, Al-rich soils compared to cowpeas grown in the same soils with higher pH values and less extractable Al. It is suggested that the Al content of soil is not a major factor in the survival of cowpea rhizobia but that it does have a significant effect on nodulation.

¹ Contribution from the Dep. of Agron., Cornell Univ., Ithaca, NY 14853. Supported by a grant from the United Nations Development Programme.

² Postdoctoral Associate, Graduate Research Assistant, and Professor of Soil Science, respectively.

Received for publication September 22, 1982. Accepted for publication January 19, 1983.