SOIL FORMATION - Soil Ecology - page 188

is reduced and a bleached E (eluvial) horizon is formed. This is the beginning of
the podzolisation process, which is completed by the translocation to depth of humic
material which accumulates in a Bh (rich in dispersible, amorphous humus) horizon
(Duchaufour, 1982). As vegetation changes, pedogenesis proceeds; the pioneer grassland
vegetation gradually evolves toward a deciduous forest which is, in turn, progressively
replaced by coniferous forest or heath, depending on the climate.
Where the parent material is calcareous, its dissolution rapidly leads to the formation
of a rendzina (rendoll, mollisol; Soil Survey Staff, 1999). These soils have thick
and active A horizons, in which earthworms are generally abundant and within which
casts act as stable closed-aggregate systems. As dissolution proceeds, carbonates are
progressively eliminated leaving a residuum of clay and silt. At a further stage, leaching
and eluviation translocate cations and clay minerals further down the profile to accumu-
late in a Bt horizon. The fully developed soil that forms under such conditions is a
' sol brun lessivé ' (leached brown soil or alfisol; Soil Survey Staff, 1999) characterised
by an intense biological activity in the A horizon due to the neutral to alkaline pH
(Figure II. 11).