Technical Reference
⚫
51
Pore water conductivity
The electrical conductivity of the bulk soil,
b
, is a function of
both soil water content,
,
and the pore water conductivity
p
.
This is very similar to the relationship that has been found
between the electrical permittivity of the bulk soil,
b
, the
permittivity of the pore water,
p
, and
(e.g. Nyfors and
Vainikainen, 1989).
Malicki
et al
. (1994) found a high degree of linear correlation
between values of
b
and
b
for a broad range of soil types.
The following discussion proposes a theoretical basis for the
relationship between
b
and
b
, and explains how this is used
within the WET150 Sensor to derive readings of pore water
conductivity.
Bulk Soil Conductivity v. Pore Water
Conductivity
Consider the water that can be extracted from the pores of the
soil matrix. The permittivity and conductivity of the pore water
will be denoted by the subscript
p
. The imaginary part of the
complex permittivity of the pore water is
p
. In soil science it is
more practical to use the conductivity of the pore water,
p
,
which can be defined as:
+
=
=
0
0
0
ip
dp
p
p
[4.]
where
ip
represents the ionic conductivity of the extracted pore
water. Dielectric losses are frequency dependent and have a
maximum at the relaxation frequency. The relaxation frequency
of water is 17 GHz at 20°C (Kaatze and Uhlendorf, 1978). The
operating frequency of the WET150 Sensor is 100MHz, and at
that frequency
dp
is negligible, so Eq. [5] can be reduced to:
ip
p
=
[5.]
Usually
p
is referred to as the
EC
(Electrical Conductivity) of the
pore water.
Ionic conduction is a function of temperature. In the case of a
NaCl-water mixture, the conductivity increases by ~2.1 % per
°C. The values quoted for
p
are often corrected for temperature
dependence to a temperature of 20°C (or sometimes to 25°C).
This temperature correction depends on the ionic composition of
