Geoscience Reference
In-Depth Information
Slightly ca
lc
areous or n
on
calcareous
horizon
16
15
10
5
Calcareous horizon
Na
+
/CEC, %
0
10
20
30
40
Fig. 13.5
Relationship between dispersion and relative content of sodium on the exchange
complex (Pons and Gerbaud 2005). In each case, the range is represented by dotted lines.
This is explained as follows. According to theory, flocculation is
more difficult in presence of a monovalent cation than a divalent one
(Calvet 2003). For example, a sodic illite dispersed in water will precipitate
at a concentration 340 times higher than that of its homologue saturated
with Ca
2+
. For montmorillonite, the ratio is about 60 times. All this arises
from the hydration of sodium, which results in inter-particle swelling
so much so that the coulombic repulsive forces between the particles
prevail over the van der Waals attractive forces. We also know that for
dispersing a fine soil for particle-size analysis it is enough if the ions it
contains are exchanged by sodium (use of Na hexametaphosphate).
13.3.2 Decrease of Permeability
Servant (1970) conducted the following simple but spectacular experiment.
He percolated saline water through a sample of clay soil placed in a tube
open at both ends. The volume of water that passed through is constant
per unit time, and thus increases with time (Fig. 13.6). The permeability
is higher than if distilled water is used (control). Then after about an
hour, the saline water was replaced by distilled water. The sample
became so impermeable that water scarcely passed through it. The curve
became nearly horizontal.
The following mechanisms are involved:
1.
Direct role of salt concentration.
The higher the salt concentration
the more the clay tends to flocculate (compression of the diffuse
double layer). The permeability, therefore, is greater than with
