Geoscience Reference
In-Depth Information
the principal phenomena are:
geochemical wasting
and the appearance of
weathering fronts
. In the tropical zone, the corresponding manifestations are
spectacular and, therefore, relatively easy to observe. In the present topic,
we have attempted to generalize to the temperate environments where
the weathering fronts are nothing but the horizon boundaries.
The phenomena are certainly less distinct because the periods of
pedogenesis are shorter and weathering less drastic. But the laws of
physics and chemistry remain the same all over the Earth…
Soil is nothing but a residue. By being weathered, it slowly
dissolves, collapses into itself and is differentiated vertically at the cost
of considerable loss of material that disappears. This is the essential
fact as proved by the weathering balances. It is thus wrong to focus
pedogenetic studies on vertical transport of some substances (iron,
aluminium, carbon, clay, calcium carbonate and various salts) through
the skeleton grains (sands + silts). Of course all this does happen and
is clearly visible, but these are superimposed, accessory processes. It is
necessary to think of this when we wish to understand the genesis of
Luvisols and of Podzols. It is the same thing in Ferralsols in which only
what is left behind is seen, that is, at times less than 15 per cent of the
original material!
The progress achieved in the last decade in dating methods, and
also increase in the number of analyses done, leads us to revise the ages
earlier assigned to soils. In fact, many profiles were differentiated over
time periods too long to allow us consider as invariant their topographic,
hydrological and climatic environments. Lake Chad was enormous
6000 years ago. Therefore, the local Vertisols were submerged since
their age is 40,000 years! We can multiply examples. Many soils
are thus 'polyphasic' and have under gone a long history full of
tribulations and various vicissitudes. The pedogenetic mechanisms that
we have presented account for the differentiation of the most typical
soils but neglect the problems that have affected many others. We have
already said this in the introduction: we do not pretend to explain all
the cases.
Fewer than a dozen processes are sufficient to understand the
essentials of diversity of soils of the world: collapse of the profile on
itself, desilication, rubefaction, anoxia, salinization, etc. Pedogenesis
is easy! Of course, the discipline still suffers from a vocabulary
hard to comprehend. For example, the terms in Soil Taxonomy have
frightened many scientists and petrified in place more than one farmer.
