The symbiotic properties of 42 isolates of Rhizobium meliloti collected from widely separated sites in the upper basin of the Darling River were tested in bacteriologically controlled culture with 18 species and varieties of Medicago, three species of Melilotus, and one species of Trigonella. Seven strains of Rh. Meliloti of commercial interest were included in the experiments for comparative purposes. Nodule formation occurred on all plants in 889 out of 974 symbiotic combinations examined. Only Medicago laciniata (L.) Mill, among the hosts and the isolate from M. laciniata among the rhizobia exhibited distinctive nodule formation behaviour. Nitrogen fixation occurred in 637 combinations. On the basis of nitrogen fixation, eight groups of Rh. Meliloti were recognized; there were nine host groups. The groups could be arranged in an interlocking pattern of increasing host and bacterial specificity. These results, which modified to a small degree the bacterial and host groupings previously reported, strongly suggested that the pattern will be widely applicable to Rh. meliloti of diverse origin. In general, the isolates examined were promiscuous and 24 were capable of effective symbioses in association with 10 or more of the hosts. M. laciniata, Trigonella suavissima Lindl., and Medicago rugosa Desr. were the hosts most highly strain-specific in their requirements for nitrogen fixation, and Medicago sativa L. was the most promiscuous. A system for classifying host species into compatibility groups is proposed.

Sixteen field experiments were conducted to assess whether inoculant applied as a liquid or in solid form separately from the seed but into the seed bed could be used as a substitute for conventional methods of legume seed inoculation. The experiments were done over a period of 8 years, on several soil types, with both crop and pasture legumes. Criteria used to measure response included success of the applied strain of rhizobia in forming nodules, quality of nodulation, seedling establishment, foliage dry matter production, and seed yield. Under favourable conditions for sowing, solid and liquid inoculants were generally as good as seed inoculation at similar total rates of application in promoting root nodulation and plant growth. However, when conditions were unfavourable for survival of rhizobia, e.g. when seed was dusted with toxic fungicide or when germination was delayed owing to environmental circumstances, solid or liquid inoculant gave better nodulation and very often better plant growth and seed yield than seed inoculation. The implications of these findings on alternatives to conventional seed inoculation and how they might be applied to agricultural practice are discussed.

Twenty-three strains of rhizobia were isolated from vigorous well-nodulated Lotus hispidus Desf. sown in the field at Milton, N.S.W. The isolates bore no symbiotic resemblance to the strains used as seed inoculant nor to rhizobia isolated from other members of the tribe Loteae. It was concluded that the effective field nodulation of L. hispidus at Milton resulted from infection by rhizobia present at very low levels in the local microflora. L. hispidus rhizobia possessed the unusual characteristic of fixing nitrogen in association with both Lotus corniculatus L. and Lotus pedunculatus Cav. (formerly known as Lotus uliginosus Schk.). Their reaction with L. pedunculatus was good enough to indicate their potential value in commercial inoculants.

In an investigation into the cause of poor nodulation of subterranean clover on a yellow podzolic soil the following results were obtained: (1) Failure of seed inoculation was due to microbiological antagonism which prevented normal colonization of the rhizosphere by the bacteria in the applied inoculum. (2) The antagonism was shown to be confined to the clover rhizosphere. (3) Greater rhizosphere populations were obtained by the use of increased rates of inoculation or with normal rates on the sites of log fires. This latter effect was attributed to the presence of charcoal. (4) The technique used for the determination of numbers of nodule bacteria in the clover rhizosphere was found to give an early and reliable evaluation of the success of inoculation.

(i) Tr'ifolium amb'iguum M. Bieb. was differentiated into a polyploid series of 2n, 4n, and 6n r~ces, which were examined in symbiosis with a number of st,rains of nodule bacteria.