The manipulation of white clover ‘host preference’ for strains of Rhizobium trifolii in an upland soil

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SUMMARY The nodule occupancy of white clover cv. Milkanova three months after inoculation in an upland soil with a 1:1 mixture of an effective (CP3B) and an ineffective strain (R4) of R. trifolii, differed significantly depending on the lime level. The percentage of nodules occupied by the effective strain decreased from 64% to 29% with increasing lime level whilst the ineffective strain nodules increased from 4% to 24% with mixed nodule infections also increasing from 9% to 22% There was no such difference after 14 months with this cultivar nor with cv. S184 at either harvest. Inoculation with a higher ratio of the less preferred strain (R4) resulted in this strain dominating the nodules and significantly reducing the dry weight of the plants (first harvest) and clover cover of plots (second harvest). Where antibiotic resistant mutants of these strains were used, with one exception, there were no significant differences between the wild‐type strains and their mutant derivatives for any of the parameters measured. In a comparison of the two marker techniques used, fluorescent ELISA was less sensitive than the antibiotic resistance technique for the identification of the occupants of small ineffective nodules. The use of both techniques in large field experiments is discussed. Clover establishment was affected by lime level during the establishment phase but the beneficial effect of lime had disappeared by the second harvest year.