Strains of Sinorhizobium meliloti were compared for their saprophytic competence (ability to survive and colonise) in mildly acidic (pH_Ca 4.8–5.4) soils, using a ‘cross-row’ technique at 3 field locations in the south-east of South Australia. Some strains of rhizobia had greater saprophytic competence than others. Strain WSM879 performed consistently well, nodulating 36% of lucerne seedlings (mean of 3 sites and 4 sampling regions) compared with former inoculant strain WSM826 which nodulated 27% of lucerne seedlings. At one site, strain WSM879 was compared with the former and current Australian inoculant strains (WSM826 and RRI128, respectively). Here, all 3 strains nodulated a similar percentage of lucerne seedlings. However, the addition of 5 t/ha of lime to the soil at this site increased the percentage of lucerne plants nodulated from 23 to 43%. This increase was due to a combination of better strain survival and colonisation and indicates there remains some potential to further improve these aspects of strain performance.

The growth of 4 of the rhizobial strains from the field trials was measured on acidified agar media (between pH 4.0 and 7.5). There was virtually no colony growth (<10% of growth at pH 7.0) by strains WSM826, RRI128 and WSM879, at or below pH 6.0. Although strain MSUR52a was still able to grow (40% of potential) at pH 6.0 (in the absence of aluminium) this was not always reflected in better nodulation of lucerne seedlings by this strain in the field. Inclusion of aluminium in the media increased the sensitivity of the strains to acidity.

The ability of 6 selected S. meliloti strains to form effective symbioses with 15 plant hosts (from Medicago sativa, Medicago littoralis and Medicago tornata) was compared. All S. meliloti strains formed effective symbioses with all plant hosts. Overall, strain RRI128 was the most effective strain with both the lucernes and the annual medics, resulting in shoot weights similar to those of plants supplied with mineral nitrogen.