Effects of pH and Al³⁺ activity on survival of Rhizobium leguminosarum bv. trifolii in a simple solution and on nodulation of red clover in acid soils

Abstract

Effects of pH and Al concentrations on growth of rhizobia have been reported. However, the effects of Al³⁺ activity, considered to be the most phytotoxic of the monomeric Al species, have not been shown. Thirty-seven Rhizobium leguminosarum bv. trifolii strains from various geographic locations were tested for tolerance to pH ranging from 3.5 to 5.5 by assessing survival in a CaCl₂-MgSO₄ (0.3 mM of each) solution after 48 h. Differences in strain survival occurred at pH 4.O. Twenty strains survived in numbers ⩾ the inoculant population. At pH 3.5 five strains survived (⩾30% of the inoculated population). Fifteen strains survived 10 μM Al³⁺ activity at pH 4.5 and five survived (⩾30% of the inoculated population) in the presence of the same Al³⁺ activity at pH 4.0. Six pH- and Al-tolerant strains and one pH-sensitive strain were used to determine the percent of red clover (Trifolium pratense L.) plants nodulated over a 10-day period in three Appalachian soils which had different soil solution pH and estimated Al³⁺ activities. Two did not nodulate plants even at day-0, one nodulated plants only at day-0, one nodulated plants at day 0 and day 2 (the pH-sensitive strain), and three nodulated plants over the 10-day period. It appeared that the Rhizobium strain was more important than soil factors in controlling nodulation over the 10-day test period.