Horizons and humus forms in beech forests of the Belgian Ardennes

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 obj ective bases for their classification, humus form horizons in 13 beech stan ds (Fagus sylvatica Ehrh,) were compared. Test sites were located in the Be lgian Ardennes (western Europe), which encompasses a wide range of ecologic al 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 h orizons, adding new criteria for their characterization, such as the root s ystem 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 va ries according to humus form. The vertical distribution of soil organisms a nd their vertical movements were considered the origin of discontinuous and continuous processes taking part in the transition from one horizon to ano ther. The observation of horizons under a dissecting microscope may help to find more reliable bases for their nomenclature, even without the use of c ostly soil sections.