Tree seedling production in forest nurseries, as a rule, takes place on light and very light soils. For this reason, attempts are made to find ways of improving properties of these soils using for this purpose, among others, composts produced by foresters themselves. These composts are usually manufactured using pine bark as a base and adding mineral nitrogen or its other sources. In the performed experiments, a mixture of crop plants (seradella, field pea, vetch and buckwheat) collected at the beginning of flowering was added to pine bark. The paper presents results concerning composting of bark alone (A), bark with urea (B), bark with a mixture of plants (C) and bark with a mixture of plants and with effective microorganisms (D). It was found that the dynamics of changes for nitrogen was higher than for carbon. Taking into consideration composts alone, the performed investigation failed to demonstrate that these changes were determined principally by temperature and they were the highest in bark composts with the mixture of plants. Effective microorganisms were not found to increase this dynamics, as indicated also by the quantities of soluble forms of nitrogen. The obtained research results revealed considerable, practical usefulness of the applied crop plant mixture in the process of bark composting, even without participation of effective microorganisms.