Geoscience Reference
In-Depth Information
Approximate thickness over which
dissolution operates during
pedogenesis, so that it is impossible
to see carbonate particles finer
than this size in the soil.
Original coarse particle
Present-day coarse particle
Fig. 7.9
Evolution of a coarse lime pebble in soil by dissolution.
These rounded carbonate pebbles maintain an
eutric
(high base
saturation) environment generally associated with a distinct rounded
fragmentary granular structure. This favours aeration. The colour is
brown (no
hydromorphy
or
redox features
synonymous with reduction
of iron).
In plains under temperate climate, CaCO
3
generally disappears from
the clay and fine silt fractions while being preserved in the sands
(Dupuis 1969).
In some calcareous soils, a sleeve of non-calcareous clay is seen around
the roots. Several hypotheses have been suggested in explanation. The
most credible is that of Jaillard (1985): the root acidifies the medium by
excreting H
+
-ions and organic acids; it thus makes room by dissolving
the CaCO
3
; the decalcified clays remain close by; they are oriented
parallel to the root under the influence of alternate wetting and drying
and perhaps because of the pressure exerted by plant growth. Thus, in
highly calcareous soils, some roots manage to protect themselves by
generating an acid environment.
Dissolution in the rhizosphere
The dissolution of calcite has been studied by means of the Atomic Force
Microscope (AFM). Split crystals with smooth, clean faces to begin with
are subjected to a dissolution process. The product obtained depends
on several factors (Callot
et al.
1980; Delmas
et al.
1987; Davis
et al.
2007; Harstad and Stipp 2007): (1) firstly, it is influenced by the pH of
the solution as affected by the dissolved CO
2
, (2) then the hydric flux
controls the way ions are extracted from the mineral surface, (3) other
ions such as Mn or Mg affect the solubility of calcite, (4) bacterial
colonies, according to circumstances, appear to protect the surfaces
Dissolution of calcite
