Well drained soils with a „degraded" Bt horizon in loess deposits in Belgium; relationship with paleoperiglacial processes

Abstract

In Belgium the degradation of the Bt horizon in soils developed in loess is generally attributed to the presence of an intensively exploited oligotrophic vegetation. Detailed investigations, made by the authors through the loess belt, show that this correlation is not so valid as might be thought from the literature. It is observed that all degraded soils – including the well drained ones – do have a compact fragipan horizon in the transition zone between the A2 and the Bt horizon. This fragipan is absent in the well drained non-degraded soils and has a somewhat different morphology from the one generally described for this horizon in loess deposits. It is also found that degradation of well drained soils coincides with the presence, below the loess, of a less permeable substratum. This correlation is still observed in plateau positions where the contact with the substratum is situated at several meters depth.

The presence of the fragipan helps to explain the association of the degradation of the soil and the permeability of the substratum. This is possible on basis of the hypothesis which links the fragipan development to ice segregation in permafrost aggrading in an environment with relatively high water supply from the subsoil. It is concluded that the·degree of degradation, and its distribution in the landscape, can be explained in a much more satisfactory way by these processes associated with a periglacial climate than by the presence of a particular forest vegetation. The local coincidence between the land use and the degradation most probably is the consequence of the knowledge and the intelligence of the early settlers. These people started to deforest and to plow first those areas which did not present a compact layer at shallow depth.