The results of this study indicate that Rhizobium leguminosarum bv. trifolii is widely distributed in slightly acid soils with pH_KCl 5.6–6.0. The average content of rhizobia was 540.0 · 10³ cfu g^–1 of soil. Less Rhizobium leguminosarum bv. viciae and significantly less Sinorhizobium meliloti and Rhizobium galegae were found. Rhizobium significantly declined in acid soils (pH_KCl 4.1–5.0).

Most of biological nitrogen was fixed at soil pH_KCl 6.1–7.0. In this case, Rhizobium galegae accumulated 196 to 289 kg N ha^–1 of nitrogen, whereas rhizobia of alfalfa and clover were less, and it depended on strain efficiency and soil pH. Soil liming had a positive effect on nitogenase activity in red clover. The soil liming (CaCO₃ rate 6.2 t ha^–1) in combination with inoculation have increased biological nitrogen fixation by red clover at 106 kg N ha^–1. Associative diazotrophes in non–legume rhizoplane have fixing the biological nitrogen too. The effective strains of Rhizobium spp., Agrobacter radiobacter and Arthrobacter mycorens have made up an active association with barley, timothy and spring rape and accumulated 11.0 to 20.4 kg N ha^–1 of biological nitrogen.

Keywords: Rhizobium, distribution in the soil, nitrogen fixation, associative diazotrophes