Geoscience Reference
In-Depth Information
network). With 15 or 20 metres of development, these soils are the
deepest in France.
The complex terminology does not give an explanation for the most
important thing for understanding the continuity of evolution:
decarbonatation
Æ
argillization
Æ
rubefaction
Æ
derubefaction
9.1.2 Principal Features of the Red Horizons
The colour of these red horizons exactly matches 5YR or 2.5YR or even
10R in the Munsell colour chart (Bech
et al
. 1997). Detailed study and
analysis of the most typical red horizons (in the soils in the middle of
the sequence) reveals the following features:
Carbonates are absent but the clays are still rich in calcium
and other basic cations, the base saturation usually being high
(65% or more) and the pH close to neutrality (6.5-7.5); but on
acid rocks, desaturated red soils with base saturation of merely
20 to 25 per cent do exist, for example, in Portugal on schist
(Lamouroux 1983); the pH is then 4 to 5.
High clay content in the fine earth (clay or silty clay texture); but
there are sandy red soils too; the most typical feature, therefore,
is the constant absence of high silt content.
A complex clay-mineral suite with mixtures of inherited clay
minerals (illite, interstratified minerals, sometimes chlorites),
transformed minerals (smectites) and neoformed ones (kaolinite);
but inherited clay minerals are usually predominant and confer
on the clay a SiO
2
/A l
2
O
3
ratio > 2 or Si/Al ratio > 1.
High iron content; the (Fe
d
- Fe
o
)/Fe
t
ratio (see § 9.1.3) is high,
always greater than 0.5, frequently reaching 0.8; haematite and
goethite are both present.
Good structure with shiny faces, whence the name Nitisols (=
shiny soils) given to certain soils studied in this chapter. But
these are essentially
pressure cutans
linked to alternate wetting
and drying of the clay in a climate with contrasting seasons; they
are not always coatings made up of translocated clay (Fedoroff
1997).
In thin section, we see complex structures: matrix as well as
argillans
(clay skins),
ferriargillans
(red-clay skins), iron concre-
tions; all writers have remarked that the amount of clay in the
coatings present is insufficient to explain the high clay content
in the middle horizons of the profile.
