Geoscience Reference
In-Depth Information
total concentration of salts; too much salt increases the suction
the plant must exert for extraction of soil water to feed itself; the
potential of the water is reduced.
types of ion, because some of them (Cl
-
) are toxic; others result
in unfavourable physical properties (Na
+
), some lead to a large
increase in pH (HCO
3
-
, CO
3
2-
).
13.1.1
Determination of Quantity of Salts
The electrical conductivity is represented by
EC
. It depends on the
quantity of dissolved salts, type of ion (monovalent or divalent…)
and also on the temperature and water content. Therefore, it must
be determined under standard conditions, that is, at 25° C and at a
given water content. It is generally referred to a
saturation extract
(
EC
sat
)
obtained by saturation with water followed by centrifugation, because it
correctly reflects what happens in the wet soil. But often we are satisfied
with a determination on a
1:5 soil/water extract
(
EC
1:5
) that is easier to
prepare. The two determinations are correlated, of course (Visconti
et
al
. 2010).
Conductance has long been expressed in mhos. This is the word ohm
in reverse. Actually conductivity is the reciprocal of the resistivity.
Resistivity
(ohm cm) = 1/[
conductivity
] (mho cm
-1
)
The thousandth of a mho is a millimho. Nowadays the unit siemens and
its divisions are preferred (millisiemens = mS; microsiemens =
µ
S).
We have:
1 mmho cm
-1
= 0.1 S m
-1
= 1 dS m
-1
in SI = 1 mS cm
-1
Below 0.75 mmhos cm
-1
, the water is of very good quality. Above
2 mmhos cm
-1
in the soil, certain fruit and market-garden crops have
difficulty in sprouting. From 4 mmhos cm
-1
on we talk of saline
conditions in the soil. One can at times cultivate up to 8 mmhos cm
-1
:
resistant varieties, acceptable reduction in yield, adapted irrigation
technology (see later).
Electrical conductivity in a soil sample
Two pairs of electrodes are used. One pair serves to send a current
through the soil; the other pair is used to measure the potential
difference induced (Fig. 13.1).
Resistivity in the field
