Geoscience Reference
In-Depth Information
Acid-sulphatation
is a process of intense acidification that may be
considered as a very special case of salinization. It leads to accumulation
of different sulphates of iron, aluminium, sodium, etc. We have discussed
this in Chapter 12 (mangrove swamps).
The exchange complex is in equilibrium with the soil solution, and
salinization is not contradictory to sodization. Let us take an example.
In Algeria, 1848 pedological horizons were sampled during mapping of
agricultural regions (Djili
et al
. 2000). Salinity was determined (threshold
kept at EC > 4 dS m
-1
) as well as ESP (threshold 15% of cation exchange
capacity). The results are given in Fig. 13.3.
Interactions
27% of the horizons examined have salinity problems
with the following relative percentage distribution
Saline horizons (62.5%)
Sodic (25%)
Saline-sodic horizons (12.5%)
Fig. 13.3
Salinity and sodicity of soils of the cultivated zone of Algeria (Djili
et al
. 2000).
The most saline horizons were also the most sodic (ESP = 81,
EC + 5.7). But the correlation coefficient
r
was only 0.52.
Besides, sodization is not contradictory to alkalization either; on the
contrary. Theoretically, carbonates in excess could be in equilibrium with
various cations. In reality, as calcium carbonate is not very soluble and
as magnesium carbonate is rarely abundant, it is evidently sodium that
is present in the solutions of alkalized soils. Thus it passes on to the
exchange complex.
13.2
ORIGIN OF SOIL SALINITY
13.2.1 Salinization By Seawater
Salinization by seawater is possible by the mechanisms outlined in
Figure 13.4. But it affects only the perimeter of the continents. In other
words, this type of salinization covers only small areas although certain
populations obviously suffer from the phenomenon.
The characteristics of seawater are the following: 36 g salt per litre
or 600 meq/l and electrical conductivity of about 50 mmhos/cm.
