Data were analyzed on the elemental composition of humic and fulvic acids isolated from organic and mineral horizons of different-aged podzols developed on sand quarry dumps in Leningrad oblast. It was shown that the humic substances from the young podzols significantly differed from those isolated from the mature soils. The humic acids (HAs) of the young podzols composed a more homogeneous group of natural compounds compared to the fulvic acids (FAs). The HAs isolated from the organic, podzolic, and illuvial horizons of the embryopodzols significantly differed in their structure. The litter HAs were characterized by the minimum aromaticity and the maximum content of aliphatic components. The HAs from the illuvial horizons contained the maximum portion of aromatic components; a significant portion of carboxyls, ethers, polysaccharides, carbohydrates, and amino acids; and the minimum portion of aliphatic components. The HAs from the podzolic horizons occupied an intermediate position between the humic substances of the organic and illuvial horizons. It was shown that Mendeleev's formula was more suitable than Aliev's formula for calculating the calorific values of the HAs because of their structural features. Based on the particular features of the humic substances studied, they were classified as prohumic substances.