This paper describes the influence of the annual application of natural and mineral fertilizers for a period of many years on organic carbon content and its fractions in the arable layer of soil. In experiment I, cow manure and slurry were applied to lessive soil, while experiment II involved the application of pig manure and slurry to brown soil. The influence of the applied fertilizers on the amount and quality of organic matter was assessed by determining C-org. and its fractions using KMnO₄ solutions of various concentrations, as well as humic compounds extracted using Na₄P₂O₇ and NaOH. The average increase of organic carbon as a result of fertilization in relation to the control group was 1.94 g·kg^-1 in lessive soil and 1.41 g·kg^-1 in brown soil. Among the applied fertilizers, manure had the most beneficial effect on C-org. content. When compared to manure, the effect of slurry and NPK applied in doses balanced with manure using nitrogen was found to be 81.9% in lessive soil and 82.7% in brown soil, whereas in the case of mineral fertilizers it was 78.0% and 75.8% for the two soils, respectively. The effect of slurry at a dose balanced with manure using organic carbon was 87.6% and 94.2%, respectively. Fertilization led to an increase in the total content of oxidized KMnO₄ fractions in lessive soil from 0.29 g·kg^-1 in the plot of land with NPK to 1.16 g·kg^-1 in that with manure. In the case of brown soil subjected to the same kinds of fertilization, these changes were smaller, amounting to 0.09 g·kg^-1 and 0.51 g·kg^-1 soil, respectively. The studies also confirmed that 1.33 times more of the fraction dissolved in Na₄P₂O₇ solution, 2.34 times more fulvic acids, 2.23 times more humic acids, and 2.18 times more of the non-hydrolyzable fraction in lessive than brown soil. The percentage share of humic acids in C-org. was highest for both types of soils when manure was applied as fertilizer.