Geoscience Reference
In-Depth Information
losses are not intuitively perceptible. They could represent 80 or 90
per cent of the original mass, though! We will understand nothing of
pedogenesis by reasoning without taking them into account.
It happens that elements lost from the A horizons of soils precipitate just
below, that is in the B horizons. The principal causes of insolubilization,
often working in association, are the following:
Change in pH
. In many soils, the extraction of bases is higher in
the upper part of the profile because of its greater exposure to
atmospheric agents. This causes an acidification that diminishes
downwards. For example, in most of the granite landscape under
forest in France, the lower horizons often have a pH one unit
higher than the surface layer. This is quite sufficient to provoke
flocculation of certain colloids.
Change in water content
. Roots pumping water from the intermediate
soil horizons can cause a local drying, resulting in insolubiliza-
tion at that level.
Change in carbon dioxide content
. Biological activity is often more
intense in the organic horizons. Indeed, when the CO
2
content
in the underlying horizons drops, the pH rises (see § 3.3.5).
In some cases, the translocations do not feed the underlying B
horizons, but the downslope portions of toposequences.
Absolute concentration
Neoformation takes place in closed systems, for example in reclaimed
marshes and also in the micropores of B horizons in which water
circulates poorly. There are different paths to crystallization:
Neoformation
Direct precipitation of salts from a solution (e.g., formation of
sodium chloride, gypsum or dolomite).
Appearance of a structured mineral starting from volcanic
glasses or more correctly from minerals without very marked
crystalline structure (allophanes). This is a very classic case.
Gibbsite is formed in hot climate, and halloysite or kaolinite in
cold climate.
Appearance of a new mineral at the interface with another
mineral that could provide its chemical constituents but cannot
transmit its structure because it is different (e.g., kaolinitization
of a feldspar and thus passage from a framework structure to a
