This study had as objective to characterize the distribution of humic substances (free fulvic acids, humic acids, fulvic acids, and humin) in soil profiles from different regions of Brazil, that had been object of studies of correlation meetings and soils surveys carried through by Embrapa and other institutions. The soil samples were selected and fractionated following concepts recognized by the IHSSS. The humin fraction presented the highest values of C in all the profiles, followed by the free fulvic acid fraction. In the profiles located on high altitude the climate (Cwa, Cfb, Cwb) favored the formation of free fulvic acid. As for the chemical properties, both horizons showed positive correlation coefficients between hydrogen and organic carbon content, with r=0.58* and r=0.83** for A and B horizons, respectively. Amongst the humic substances the humin showed the best correlation with the organic carbon content, with r=0.89** and r=0.91**) for A and B horizons, respectively. A positive correlation was observed between the coarse sand fraction and the C-FAF, C-HAF, C-HUM and the relation C-EA:C-HUM in the A horizons, indicating that the coarse texture of some surface horizons favors the formation of these humic substances. The organic C content in the surface and subsurface horizons and the distribution of humic substances varied with pedogenesis and altitude conditions.
Soil organic matter; granulometry; soil classification
