EVALUATION OF THE SYMBIOTIC PROPERTIES OF RHIZOBIUM-FREDII IN EUROPEAN SOILS

The symbiotic properties of different Rhizobium fredii strains were evaluated in a controlled environment growth chamber, greenhouse, and field conditions. R. fredii strains HH 102-2 and HH 103 showed to be as good soybean inoculants as Bradyrhizobium japonicum USDA 110 in three years (1987, 1988 and 1992) of field experiments in Spanish soils. Thus, it is concluded that these two R. fredii strains are valid soybean inoculants in Spanish soils which, as all European soils, are devoid of indigenous B. japonicum and R. fredii strains. Competition experiments in field conditions (1987) showed that R. fredii strains HH 102-2 and HH 103-2 were more competitive than B. japonicum USDA 110-1 to nodulate soybean cv. Williams. Results of competition studies in greenhouse conditions using acid (pH 4.9), neutral (pH 6.6) and alkaline (pH 8.1) soils showed that in the acid soil B. japonicum strains USDA 110 and 3-15-b3 totally outcompeted R. fredii strain HH 103 to nodulate on cv. Williams. Conversely, in the alkaline soil the fast-growing strain occupied more than 80% of the nodules. These results indicate that the soil pH could be an important factor in determining the competitive success of R. fredii and B. japonicum for nodulating soybeans.