Appendix A
Soil-specific calibration
This note provides details of 3 techniques for generating soil-specific
calibrations:
1. Laboratory calibration for substrates
*
and non-clay soils
2. Laboratory calibration for clay soils
3. Field
calibration
*
We use the term substrate to refer to any artificial growing medium.
Underlying principle
Soil moisture content (
θ
) is
proportional to the refractive index
of the soil (
√ε
) as measured by the
ThetaProbe and
Profile Probe
(see
Calibration
section).
The goal of calibration is to
generate two coefficients (
a
0
,
a
1
)
which can be used in a linear
equation to convert probe readings into soil moisture:
θ
ε
×
+
=
1
0
a
a
Using the ThetaProbe to calibrate the Profile Probe
Soil calibrations using the ThetaProbe and
Profile Probe
are very similar -
because they measure the same fundamental dielectric property (
√ε
) at the
same frequency (100MHz). However both their calibrations are influenced
by their slight sensitivity to conductivity - and they differ in how this
sensitivity changes with water content. The ThetaProbe (and methods 1. or
2. below) can be used effectively for creating soil-specific Profile Probe
calibrations at low water contents and/or low conductivities. At high
conductivity
and
high water content it is far better to generate
Profile Probe
calibrations using the field calibration technique (3.).
34
z
Soil-specific calibration
Profile Probe User Manual
3.0a
