5TE Operator’s Manual
3. Theory
10
sured dielectric permittivity of the medium. However, pore
water EC is not the same as solution EC. Pore water EC is the
electrical conductivity of the water in the pore space of the
soil. One could measure this directly if the soil was squeezed
under high pressure to force water out of the soil matrix and
that water was collected and tested for EC. Solution EC is the
electrical conductivity of pore water removed from a saturated
paste. In this case, the soil is wetted with distilled water until
the soil saturates, then the soil is placed on filter paper in a
vacuum funnel and suction is applied. An electrical conductiv-
ity measurement on the water removed from the sample will
give the solution electrical conductivity. Theoretically, the two
are related by the bulk density. An example calculation will
illustrate this relationship: A soil is at 0.1 m
3
/m
3
VWC, has a
pore water EC of 0.7 dS/m, and a bulk density of 1.5 Mg/m
3
.
We can calculate the solution EC as follows.
In this example, ø is the porosity,
ρ
b
is bulk density,
ρ
s
is den-
sity of the minerals (assumed to be 2.65 Mg/m
3
), subscript
d
is distilled water, and
θ
is volumetric water content. We
assume that the EC of the distilled water is 0 dS/m. In prac-
tice, solution EC calculated from this method and solution EC
taken from a laboratory soil test may not agree well because
wetting soil to a saturated paste is very imprecise.
(
)
( )
dS/m
162
.
0
0.43
0
1
.
0
7
.
0
EC
Solution
43
.
0
65
.
2
5
.
1
1
1
=
+
=
−
+
=
=
−
=
−
=
φ
θ
φ
σ
θ
σ
ρ
ρ
φ
d
p
s
b
