International audience ; Forest soil organic horizons are named on the basis of visual observations made directly in the field, thus this is often subjective. To find more objective bases for their classification, humus form horizons in 13 beech stands (Fagus sylvatica Ehrh,) were compared. Test sites were located in the Belgian Ardennes (western Europe), which encompasses a wide range of ecological conditions. I used a semiquantitative micromorphological method for the description of horizons, and a multivariate method for data analysis. These methods helped to discern objective discontinuities among Oi, Oe, and Oa horizons, adding new criteria for their characterization, such as the root system of trees. Within these horizons, transitions between sub-horizons are gradual and thus do not lie on clear-cut criteria. The transition between Oa and A horizons was also gradual. The composition of Oa and A horizons varies according to humus form. The vertical distribution of soil organisms and their vertical movements were considered the origin of discontinuous and continuous processes taking part in the transition from one horizon to another. The observation of horizons under a dissecting microscope may help to find more reliable bases for their nomenclature, even without the use of costly soil sections.