Geoscience Reference
In-Depth Information
Fig. 7.18
Leptosol (Humic, Eutric; WRB) on moraine with large limestone boulders, in
Courchevel, French Alps, 1800 m. The soil is covered with the calcifuge plant
Rhododendron
ferrugineum
, but separated from the limestone by organic matter and decarbonatation resi-
due.
Photo
: author.
Gypsum, CaSO
4
.2H
2
O, can form by crystallization in depressions in dry
environments. The soil profiles there are often poorly differentiated, but
they can also contain crusts (petrogypsic horizon). This applies to the
Mediterranean zone with low rainfall, northern Sahara, Arabia, southern
part of the former Soviet Union, the American deserts… Gypsum is
often associated with attapulgite and with sodium (Chap. 13).
Gypseous massifs of geological origin are found (deposits of the
Upper Triassic) in humid environment, in the Alps for example. Sodium,
if it had been present, has been leached away. In unstable regions, that
is, in active screes and in eroding zones where the rock has outcropped,
the soil is shallow and rich in gypsum fragments. Coniferous forests
develop here, apparently with no problem. It is true that such a salt of
a strong acid
H
2
SO
4
and a strong base
Ca
(
OH
)
2
has neutral reaction.
But, once the environment is stable, in the same Alpine regions, the
gypsum is dissolved and survives only in the rock below the soil. This
results in a non-gysiferous but generally calcareous weathering residue.
In fact, the dissolution of gypsum saturates the soil solutions with Ca
++
and calcium carbonate is not attacked. The regions with bedrock of
gypsum form a rare instance where calcareous soils are found in humid
high mountains. Their pH is then that of calcareous soils (7.4-8.4; see
Boyadgiev and Verheye 1996).
Soils on gypsum (Gypsisols)
