The goal of the research was to determine the nitrogen-fixing ability of perennial leguminous grasses and their use of biological nitrogen to form the crop of fodder in different environmental conditions of Western Siberia. The nitrogen-fixation coefficient (Cf) was determined by the method of comparison with a non-leguminous plant. It has been established that in the southern forest-steppe on meadow-chernozem soil biological nitrogen is used more efficiently using Medicago varia, Galega orientalis, Onobrychis arenaria и Astragalus galegiformis. Plants of these species fix up to 233–379 kg/ha of air nitrogen during the growing season, Cf is equal to 0.56–0.78. The accumulation of symbiotic nitrogen by leguminous grasses is 1.1–4.6 times less in the northern forest-steppe in solonetz fine than in meadow-chernozem soil. Medicago varia at solonetz forms due to biological nitrogen to 58% of yield and Astragalus galegiformis only 10-22%.