Using the soil dilution – plant infection method, populations of several species of root-nodule bacteria (rhizobia) were enumerated in 46 soil samples collected throughout Poland. To exclude effects of host-plant cultivation on the interactions between soil physico-chemical properties and populations of the rhizobia, only soils not planted with legumes during at least the last 10 years were analysed. Rhizobium leguminosarum bv. viciae (symbionts of pea, faba bean, vetch) and R. leguminosarum bv. trifolii (symbionts of clover) were detected in 44 and 42 soils, respectively. Most of these soils contained moderate and high numbers of these species rhizobia. Symbionts of beans, R. leguminosarum bv. phaseoli, and symbionts of lupine, Bradyrhizobium sp. (Lupinus) were less frequent in the examined soils, of which 11 contained non-detectable populations of these bacteria. Sinorhizobium meliloti, rhizobia nodulating alfalfa, were sparse in the test soils, with 32 soil samples containing no detectable numbers of S. meliloti and only 3 samples harbouring moderate populations of this species. The estimated numbers of rhizobia were also related to the following properties of the soils: organic C and total N contents, pH in water and in KCl, and the content of soil mechanical fractions <0.02 mm and <0.002 mm. Results of this study have shown that soil texture (particularly clay content) and soil reaction have the greatest influence on populations rhizobia in soils.

Keywords: rhizobia, population, soil, leguminous plants