The importance of humus in the environment is related to its quantity and quality, and hence there is a constant search for parameters that define the properties of humus. One of the indicators determining sorption capacity and solubility of humic substances, and so their susceptibility to biodegradation, is the share of hydrophilic and hydrophobic fractions. The hydrophilic and hydrophobic properties were determined for humic acids formed in the process of post-harvest residue decomposition under soil conditions and outside the soil. It was demonstrated that the share of hydrophilic and hydrophobic fractions in the molecules of humic acids depends on the plant species of incubated plant residue and the incubation time. The changes in the share of hydrophilic, and thus also the hydrophobic fractions, result in a change in the value of the HIL/HOB ratio. This ratio can be used as an indicator of the degree of transformation of organic matter of soils.