Annual species of Medicago differ greatly in their ability to nodulate on acid soils

It is possible that the use of annual species of Medicago may be broadened by the selection of genotypes more capable of achieving nodulation on acid soils when only low numbers of their microsymbiont, Rhizobium meliloti, are present. In this study 36 genotypes from 14 species of Medicago were examined for their ability to nodulate over a range of inoculum levels in a soil of pH 4.9 (1:5,0.01 M CaCl2). Variation both within and between species was evident. M. murex in particular, as well as M. polyrnorphu and M. soleirolii, were outstanding in their ability to achieve nodulation in the acid soils. This character should assume some priority in pasture selection programmes targeted for acid soils.