In this work we have analyzed soil samples from Oxisols collected from two traditional communities, one formed by Guarany Indians at South of Brazil and other by African descendants on North of Rio de Janeiro State, Brazil. The content and fractional composition of humus was investigated and the isolated humic acids (HAs) were characterized by elemental composition, 13C solid-state nuclear magnetic resonance, and high-performance size exclusion chromatography. The bioactivity of HAs was evaluated considering the effects on root growth of maize seedlings. Chemical properties from adjacent soils under native forest were used as control samples. The local field sites matching the traditional cropping requirements, were characterized by higher soil chemical fertility and soil organic matter hydrophobicity, as compared to the land plots considered as inadequate by rural peasants. The HAs from cropped soils revealed significant differences in respect to content, hydrophobicity, biostimulation and molecular dimension. Although all humic extracts promoted, both, root growth and the stimulation of lateral root emergence over control, the HAs from preferential local sites, revealed a larger bioactivity response on root stimulation even at lower concentration. The assessment of soil quality issued by local farmers, showed a valuable fitting with bio-chemical fertility indicators and SOM hydrophobicity.