Up to 80% of fine roots can be contained in the forest floor (FF); therefore, the composition and pools of organogenic horizons largely determine the productivity of the trees. The stages of litter transformation in FF are poorly studied. We tested the hypothesis of the influence of R2O3 on the quality as of the whole FF as humic substances in FF. The soils of four key plots of dry pine lichen-green moss forests of the Leningrad Region differed in terms of the R2O3 content in soil-forming sands (1.7–32.0 mass %) and in forest floor (4.1–17.1 mass%). The age of the main generation of pine was from 100 to 130 years. The FF thickness varied from 7 to 10 cm. Sampling was carried out in fivefold repetitions of L, F, H sub-horizons. The general physicochemical characteristics of the soil and the total elements were determined. Humic substances (HS) isolated from fermented and humified sub-horizons (F and H) have been characterized for their elemental composition (C, N, H). Changes in molar H:C, O:C, C:N and ratio of aromatic and aliphatic fragments have been revealed. Solid-state 13C nuclear magnetic resonance spectroscopy was applied to identify the structure of HS. HS of the FF showed low aromaticity and absence of correlation with R2O3. The ratio of gross C/N of the F and H sub-horizons of forest floor were from 28-32 to 37-43. The gross C/N and the total Fe2O3 content were not correlated. A weak correlation was found with the Al2O3content. More clear dependences were found for C/N of HS with Fe2O3 and Al2O3 1) y1 = -2,7124x + 35,182, R² = 0,698 and 2) y2 = -0,728x + 35,431 R² = 0,719 correspondingly. Conclusion. The effect of R2O3 was revealed only in relation to the nitrogenous part of the HS.