Soil-Specific Calibration - Delta-T Devices PR2 User Manual

Appendix A

Soil-specific calibration

This note provides details of 3 techniques for generating soil-specific
calibrations:
1. Laboratory calibration for
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 (
which can be used in a linear
equation to convert probe readings into soil moisture:
ε
=
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 Soil-specific calibration
substrates*
a a
, )
0 1
θ
+ ×
a
0 1
and non-clay soils
Profile Probe User Manual 3.0a