The study has been carried out in the experimental field of the Centre for Ecological Research of the Polish Academy of Sciences, PAN, located in Dziekanów Leoeny (north of Warsaw). Six hundred lysimeters were installed covering the experimental area. The surface of the experimental plots (both lysimeters and their vicinity) was sown with either one grass (Festuca rubra) on half of the area, or with a mixture of 8 grass species on the other half of the area. In the next year, geophagous earthworms A. caliginosa were introduced to half of the lysimeters. The content of C-org, N-total, and the fractional composition of soil humus, were first determined at the beginning; at the end of the experiment also pH and the capacity of sorption complex were identified. The empirical results were subject to statistical analyses. After two years of the study, in comparison to red fescue sodding, the grass mixture sodding caused an increase in the content of organic carbon, total nitrogen and carbon of the humus fraction. The differences between the mean values of both soddings were not statistically significant. In soils under grass mixture, A. caliginosa caused an increase in the contents of organic carbon and fulvic acids carbon in relation to the initial soil; the CHA/CFA ratio significantly decreased. A slight increase in the degree of organic matter humification was observed in both soddings in combination with earthworms.