Humic substances, especially humic acids, are classified among the so-called soil auxiliary substances that positively affect the quality of the soil. As part of the diploma thesis, humic acids were isolated from these sources. Their influence on selected soil characteristics was investigated, particularly the carbon content in different size fractions and the number of water-resistant macroaggregates. The experiment was conducted in the form of pot vegetation experiments. It was found that the highest content of organic carbon contained leonardite. On the contrary, the smallest amount was found in the compost. The influence of humic acids on the formation of macroaggregates was demonstrated by the method of dry and wet sieving. The best ef...