The aim of study was to assess the influence of cultivated plant species (potato and rye) in monoculture and in crop rotation as well as the impact of the applied fertilizers on the content of soil organic carbon which was extracted by a 0.004 M CaCl₂ solution (EOC). Soil samples were taken from a long-term static fertilization experiment carried out in Skierniewice, Poland. The content of TOC in the soil of the plots fertilized with manure was higher than in objects with mineral fertilization. In monoculture, the higher mean content of EOC was found in soil organic matter under the rye and in crop rotation system in soil under potato. Regardless to cultivated plant species as well as regardless to type of fertilization higher contribution of EOC was determined in crop rotation system than in monoculture. Regardless to cultivated plant species, EOC contribution in organic carbon (TOC) in both cultivation systems depended on fertilizer and was highest in the soil fertilized with NPK.