Geoscience Reference
In-Depth Information
The first is climate. For example, evolution in a dry environment is
particularly slow. In the state of Colorado, development of the B horizons
on loess has been linked to age (Birkeland
et al
. 2003). At the end of
100,000 years, the local Alfisol is still only very slightly differentiated
and decarbonatation of the profile remains very superficial. Table 8.3
presents the data obtained.
Table 8.3
Correspondence between age and development of the soils on loess
in Colorado (Birkeland
et al.
2003).
Age in thousands of years
10
140
640
2000
Clay content of the B in %
5
20
30
40
The other fundamental factor of evolution is the texture of the parent
materials that undergo weathering (Sauer
et al
. 2009). It is evident that
the formation of clay and its later translocation will be faster if the
process starts with feldspathic sand than if the material were originally
a moraine rich in large pebbles of very resistant granite.
The estimates presented below (Fig. 8.12) are, therefore, only orders
of magnitude.
10,000 years
0
100, 000 years
1 million years
Calcaric
Cambisol
Eutric
Cambisol
Luvisol
Æ
Albeluvisol
Planosol
Fig. 8.12
Durations of formation for different soils types in humid temperate climate
(logarithmic time-scale).
Beyond the large differences in the figures, differences we have
explained as being of climatic, petrologic or semantic origin, one thing
is certain: Luvisols have required for their development such periods of
time that modern man is responsible for neither their differentiation nor
their degradation. It is therefore necessary to consider as very unreliable
the schemes or models explaining that planting of coniferous trees in
place of broad-leaved trees hinders the biological recycling of bases
and starts the translocation of clay! The last is a natural phenomenon
that appeared on the Earth in regions of high rainfall in the middle
latitudes under forests of broad-leaved trees. It is an ancient and general
phenomenon!
But this does not mean to say that man does not have to take
precautions. On the contrary, if it takes between 150,000 and 200,000
years to make a very distinct Luvisol, just a few centuries of flawed
