Nodulation and growth of pasture legumes with naturalised soil rhizobia.: 2.: Balansa clover (Trifolium michelianum Savi)

Balansa clover (Trifolium michelianum Savi) is grown widely across southern and eastern Australia. The presence of suitable rhizobia (Rhizobium leguminosarum bv. trifolii) for this species of clover, and their ability to form an effective symbiosis, was assessed in a survey of 43 soils collected from across Australia. The effectiveness of the soil rhizobia is compared with that of strains WU95 and WSM409. The study confirmed the widespread occurrence of clover rhizobia in Australian soils. No clover rhizobia were detected in 6 of the 43 soils and this was probably associated with a history of clover absence at these soil locations. Thirty of the soils contained more than 500 rhizobia per gram. These rhizobia varied from highly effective for soil T2 (128% of WSM409) through to ineffective for soil V9 (22% of WSM409). Few soil inoculants were more effective than strain WSM409. Commercial inoculant strain WSM409 was generally more effective than former inoculant strain WU95. The suboptimal performance of the rhizobia in many of the soils indicates that there is an opportunity to improve the level of nitrogen fixation by balansa clover in the field. Bearing in mind the difficulties associated with the introduction and persistence of applied inoculant strains, cultivar selection is considered as an approach to improve the symbiosis. However, this study indicated little variation in the performance of the 3 commercial cultivars, Frontier, Paradana and Bolta, in this regard.