The root nodules of Alnus incana species were taken off in Nálepkovo village (Eastern Slovakia). The N2-fixing organisms isolated from root nodules have ability to enrich not only themselves, but the soil as well. By consecutive manipulation, the microbial cultures of Nitrogen-fixing microorganisms were achieved. The activity of Nitrogen fixation is about 45.516 μMC2H4.hour-1g-1 Cbio in average.