The aim of the present paper was to determine the properties of organic matter of forest soil (especially humic acids) representing the natural ecosystem and to compare its properties with agriculturally used soil – located in the vicinity of the forest. To carry out the investigations forest soil (chernozem) was sampled from the nature reserve Veľký Báb (Slovakia) and from the agricultural field adjacent to it. It was shown that in forests soils a constant supply of fresh organic matter significantly modifies the properties of humic acids. Humic acids (HAs) of the humic horizon of the forest soil as compared with HAs of arable soil showed a lower degree of aromaticity, polydispersity, a lower share of carboxyl groups and hydrophilic fractions. It was found that down the profile the HAs molecules contain more carbon and less hydrogen, higher is the share of carboxyl groups and hydrophilic fractions, the lower is the share of aromatic lignin structures and hydrophobic fractions and the higher is the degree of polydispersity. Quality parameters which have been obtained for humic acids of the layer of 40–50 cm can suggest that the process of migration of organic carbon compounds introduced into soil takes places every year together with plant drop very inconsiderably. The highest effect of the plant drop on HAs properties is observed at the soil layer directly adjacent to the organic horizon.