Acidic conditions with damaging levels of available aluminium (Al3 ) currently limit lucerne (Medicago sativa) production on soils in the New Zealand high country and in large areas of Australia. Increased lucerne nodulation could be achieved by using an Al3 -tolerant strain of Sinorhizobium meliloti to inoculate an Al3 -tolerant lucerne line. The Al3 tolerance of the current commercial Australasian inoculant strain for lucerne, S. meliloti RRI128, was compared with strain SRDI736, selected in Australia for tolerance to low pH. Four Al3 levels (0, 2, 4 and 8 µm) were created at pH 5.1 in a hydroponic system. The rhizobial strains were evaluated on SARDI AT7, a lucerne line selected for improved growth and nodulation in acidic solution culture with Al3 , and on Stamina 5, a commercial cultivar commonly grown in Australasia. SARDI AT7 when inoculated with strain SRDI736 produced more nodules per plant (3.6 vs 2.4) and had higher nodulation percentage (>80% vs <50%) at all Al3 levels than when inoculated with RRI128. The percentage of nodulated Stamina 5 plants after inoculation with the commercial strain was lower than after inoculation with strain SRDI736 (10–16% vs 25–70%) at all Al3 levels.
The potential of S. meliloti strains SRDI736, SRDI672 and RRI128 and rates of lime to increase lucerne nodulation and dry matter production in soils of low pH (<5.5, in water) and high Al3 (>3 mg kg–1 soil) was also investigated in a pot trial. Lime had a more consistent effect than inoculant strain for increasing nodulation. At 0.5 and 2 t lime ha–1, plants inoculated with strains SRDI672 and SRDI736 had more nodules per plant than plants inoculated with the commercial strain. At 4 t lime ha–1, the number of nodules per plant was highest for all three inoculants, and there were no differences among them. This confirms the importance of lime to increase lucerne nodulation in low-pH, high-Al3 soils. However, where liming is uneconomic or impractical, the results show that it was possible to select rhizobial strains to increase lucerne nodulation in acidic soils with damaging levels of available Al3 .
