The object of the studies conducted in the years 2000–2002 on a field of 3 years’ monoculture of soybean was rhizosphere soil of soybean cultivated after tansy phacelia, winter wheat, white mustard, rye, agrimony and soybean as previous crops. The purpose of the studies was to determine the effect of cultivating the above listed previous crop plants on the formation of microorganism communities in the rhizosphere soil of soybean. The lowest total number of fungal colonies was found in the rhizosphere soil of soybean cultivated after rye and winter wheat (21.09 × 10³ and 22.58 × 10³ c. f. u., respectively), while the highest number was found in soil after soybean (36.95 × 10³ c. f. u.). The highest total number of bacteria was found in 1 g of dr yweight of the rhizosphere soil of so ybean cultivated after agrimony, and the lowest after soybean (5.80 × 10⁶ and 4.09 × 10⁶ c. f. u., respectively). The largest proportion of pathogenic fungi was characteristic of the rhizosphere soil of soybean cultivated after soybean, and the smallest – of the rhizosphere soil of soybean after agrimony as a previous crop. The dominating species among pathogenic fungi in all experimental objects was Fusarium oxysporum. The rhizosphere soil of soybean cultivated after soybean was the poorest in saprophytic fungi (35.2% of all isolations). On the other hand, the highest number of saprophytes, including antagonistic ones, was found in the rhizosphere soil of soybean after agrimony and winter wheat.

The purpose of the presented studies was to determine the species composition of the fungi occurring on the underground parts of potato at an thesis and to establish the qualitative and quantitative composition ofmicroorganisms living in the soil environment of this plant. Besides, the studies aimed at finding antagonistic microorganisms inhibiting potato infection by soil-borne phytopathogens. The mycological analysis showed that the symptoms of necrosis on the roots and the stem base of potato were caused by Alternaria alternata, Fusarium spp., Colletotrichum coccodes and Rhizoctonia so/ani. The microbiological analysis of the potato rhizosphere gave twice as many bacteria and fungi as from the non-rhizosphere soil. The dominating pathogenic fungi in the examined soil samples were Fusarium spp., A. alternata and R. so/ani. Three times as many antagonistic bacteria Pseudomonas spp. and Bacillus spp. and more than twice as many antagonistic fungi (Penicillium spp., Trichoderma spp., Gliocladium spp.) were obtained from the potato rhizosphere as compared to the non-rhizosphere soil.