Geoscience Reference
In-Depth Information
The data obtained on more than 1100 profiles in Algeria show that
the CaCO
3
content in A and B horizons, the
maximum
of which can reach
or exceed 80 per cent below 450-mm annual rainfall, decreases when
the rainfall increases, and approaches zero when rainfall reaches 1000
mm y
-1
(Djili
et al
. 1999).
Humid climate:
Ca is
removed
Mediterranean climate:
Ca is slowly
displaced
Subdesertic climate:
Ca
++
++
++
is retained in
place
Ca-saturated
A and B
Calcareous
A and B
Dystric
A and B
Bca
or
Cca
R
R
R
Fig. 7.3
Role of climate in the distribution of calcium carbonate in soils on carbonate-bear-
ing rock.
7.2.2 Role of Time
Nearly all the soils that cover the Northern Alps, on calcareous materials,
are well decarbonated in the fine-earth fractions (sand, silt and clay)
because the climate is humid and because they had started their
development at the end of the last glaciation, ten or twelve thousand
years ago. On hard calcareous rocks, however, the clay particles remain
saturated by Ca
++
because of the presence in their close environment of
coarse calcareous fragments (pebbles and boulders) protected by their
large size. They release Ca
++
. On the contrary, on calcareous schists
that mainly give silty soils without coarse fragments, there is strong
acidification.
The soils of recent tills, in the high-altitude valleys, are calcareous
if the constituent material is calcareous. The old tills are all acidified in
the surface layer.
7.2.3 Role of the Parent Material
Several parameters have to be taken into account. Let us review them.
The solubility products presented in Table 7.1 are approximate and are
only given to establish concepts. All kinds of phenomena intervene to
modify the effective solubility: temperature, presence of foreign ions…
Type of material
