FIELD-EVALUATION IN ACID SOILS OF STRAINS OF RHIZOBIUM-LEGUMINOSARUM BV TRIFOLII SELECTED FOR THEIR TOLERANCE OR SENSITIVITY TO ACID SOIL FACTORS IN AGAR MEDIUM

We report field experiments which tested the performance of ten strain s of Rhizobium leguminosarum by. trifolii in acid soils. The strains h ad been classified as either tolerant or sensitive to mineral imbalanc es associated with acid soils and to low pH, on an agar medium. The so ils of the two sites were pH 4.0 and 4.2 (10 mm CaCl2) and all strains with one exception originated from acid soils at or near these sites. All strains colonized the rhizospheres of subterranean clover equally well but were more successful at the site where the naturalized popul ation of rhizobia was smaller. Nodulation by the inoculant strains in the first year was poor; they formed only 8 and 2.9% of the nodules at Sites 1 and 2 respectively. The two strains originating from Site 1 f ormed a mean of only 2% of the 173 nodules tested from plots where it had been introduced at Site 1. The naturalized strains formed a dispro portionate number of nodules relative to their number in the rhizosphe re. Higher rates of inoculation in the second year, although not marke dly affecting rhizosphere numbers, increased nodule initiation by the inoculant strains but not in proportion to their numbers. Unexpectedly , the most successful strains in each year were those classified as ac id-sensitive; they formed a higher proportion of the nodules than thos e classified as acid-tolerant. The results suggest that in soil, facto rs other than pH and inorganic nutrients are important or that a strai n's genetic background is of such importance that it can mask the impr oved nodulation in an acid soil expected from selecting for tolerance on an agar medium.