5TE Operator’s Manual
3. Theory
9
ε
p
can be calculated from soil temperature using:
ε
p
= 80.3 - 0.37 * (T
soil
- 20)
(2)
where
T
soil
is the soil temperature (C) measured by the 5TE.
ε
b
is also measured by the 5TE. Raw VWC counts can be con-
verted to bulk dielectric by the ECH
2
O-TE dielectric calibra-
tion:
Finally,
ε
σ
b
= 0 is an offset term loosely representing the
dielectric permittivity of the dry soil. Hilhorst (2000) recom-
mended that
ε
σ
b
= 4.1 be used as a generic offset. However,
our research in several agricultural soils, organic, and inorganic
growth media indicates that
ε
σ
b
= 6 results in more accurate
determinations of
σ
p
. Hilhorst (2000) offers a simple and easy
method for determining for individual soil types, which will
improve the accuracy of the calculation of
σ
p
in most cases.
Our testing indicates that the above method for calculating
σ
p
results in good accuracy (± 20%) in moist soils and other
growth media. In dry soils where VWC is less than about 0.10
m
3
/m
3
, the denominator of equation 1 becomes very small,
leading to large potential errors. We recommend that
σ
p
not
be calculated in soils with VWC < 0.10 m
3
/m
3
using this
method.
Pore Water vs. Solution EC
As noted in the previous section, pore water electrical conduc-
tivity can be calculated from bulk EC using the probe-mea-
50
Raw
b
ε
ε
=
(3)
