SOIL FORMATION - Soil Ecology - page 186

from the softer limestones, the amounts of active carbonate are small and the profiles
may be rich in sesquioxides (Duchaufour, 1982).

4.3.2.2 Acid rocks

Brunification is the dominant pedogenetic process occurring on all well-drained,
non-calcic parent materials although they must be capable of supplying adequate
clays and iron through weathering. This process also requires a mull humus type and a
pH in the range 5-6. It is characterised by the formation of silicate clay-ferric
ion-humus complexes. In the upper organic horizons, the ferric cations act to bridge
the humic acid molecules and clay minerals to form stable complexes that protect
the organic materials from decomposition and the clays from eluviation. In the pre-
dominantly mineral horizons, iron also forms films of hydrated amorphous oxides
surrounding the clay minerals (Duchaufour, 1982).
Chemical weathering rates in these parent materials are lower than in tropical conditions.
Active earthworm populations lead to the formation of biological closed-aggregate
structures through their casting activity and this protects minerals from the aggressive
hydrolytic activity of soluble organic acids. Consequently, such soils may contain
variable proportions of largely-unweathered minerals. Their clay contents are generally
low and minerals, mainly smectite and illite, are largely derived from neoformation
processes. Most of the elements released by the dominant carbonic acid weathering are
soluble and are progressively translocated down the profile leading to the formation of
leached brown soils (alfisols, Soil Survey Staff, 1999).
With more acid substrates, and particularly where biological activity is restricted by
low quality or toxic litter ( e.g., that of most conifers) and seasonal waterlogging,
biological aggressive leaching microstructures dominate. They accelerate the destruction
of existing clay minerals while neoformation rates are very slow. This leads to the
intense leaching and eluviation of clays, bases, sesquioxides and organic matter from
the surface horizons to form an E or eluvial horizon. The leached and eluviated materials
accumulate (podzolisation) in the B horizon to form the spodosols characteristic of,
but not restricted to, the areas where conifers dominate.

4.4 Chronosequences

Soils gradually pass through a series of intermediate stages during profile development.
Under some circumstances, the profile may be constantly rejuvenated by creep or erosion
and the formation of a fully-developed profile may be much delayed. Thus, under defined
conditions of climate, topography, parent material and the biota, Chronosequences ( i.e.,
age sequences) of soils occur that correspond to the successive stages of increased
weathering, translocation, and other transformations. However, Birkeland (1992) points
out that earth-surface processes have not been constant over time and that soils older
than the Holocene have been subject to variable soil forming influences due to changes
in the climate and therefore in other soil forming factors.