Geoscience Reference
In-Depth Information
Dolomite, CaMg(CO
3
)
2
, is much more resistant to decomposition. The
effervescence is produced in the cold only on a rock sample that has
been powdered well.
The relation between pH and the proportion of calcium on the exchange
complex (base saturation) is given in Figure 7.2 (Legros
et al.
1987).
pH and base-saturation percentage
Percentage saturation by Ca
100
pH
4.0
4.5
5.0
5.5
6.0
6.5
Fig. 7.2
Relation between pH and base-saturation percentage (Legros
et al.
1987).
The base-saturation percentage can exceed 100 because the calcium
extracted by the traditional method (ammonium acetate) is not only
that held on the clay or on the organic matter. To some extent it is the
calcium extracted from lime nodules, if present. Thus, in a calcareous
soil, more 'exchangeable calcium' is found than the exchange sites
capable of retaining it. It is seen that the relation is approximately linear
between pH 4 and pH 6.5: the more the calcium the higher the pH.
7.2
DISTRIBUTION OF CALCAREOUS AND CA-SATURATED
SOILS
What follows applies to Western Europe, unless otherwise indicated.
7.2.1 Role of Climate
If the climate is humid, it makes decarbonatation possible. With time,
all soils on limestone should become Ca-saturated or acidified, in
accordance with the solubility of CaCO
3
. The soils remain calcareous in
particular cases. For example, in mountain country, the friction between
coarse elements restricts the decarbonatation of scree.
In dry climate, the Ca
++
is retained in place or barely displaced
(Fig. 7.3).
