Soil Moisture Determinations
Using Capacitance Probe Methodology
RONALD T. ATKINS, TIMOTHY PANGBURN,
ROY E. BATES, AND BRUCE E. BROCKETT
simple calibration procedure. Moreover, this mea-
INTRODUCTION
surement may be accomplished with the use of
This report provides the initial test results of a
electronic measurement equipment that is highly
method for determining soil moisture by the mea-
repeatable and that can therefore be expected to
surement of the related parameter of the dielec-
determine soil moisture reliably through the
tric constant of the soil. The concept of measuring
indirect measurement of the capacitance of a probe
soil moisture by measuring the capacitance or
placed in the soil. Since the composite soil
dielectric constant of the soil is not new; however,
waterair dielectric constant can be expected to
the emphasis on this technique has most recently
exhibit typical Debye spread, the calibration
been on the measurement of the dielectric con-
should be accomplished at the same frequency at
stant using time domain reflectometry (TDR). The
which soil moisture measurements are expected
method described here measures the capacitance
to be taken. For the tests described in this report, a
directly and was developed jointly by Dartmouth
10-MHz frequency was used. For more discus-
College and the U.S. Army Corps of Engineers,
sion on frequency effects, see Campbell (1988).
Civil Works Remote Sensing Research Program
(CRREL). Thus, the method will be referred to as
the Dartmouth/CRREL system throughout this
CAPACITANCE MEASUREMENTS
report.
The objectives of the tests described here were to
The capacitance probe and its accompanying
readout system as described here were devel-
1. Provide initial data that would demonstrate
oped cooperatively by Dartmouth College and
the proof of concept.
CRREL. The sensor itself is a coaxial probe con-
2. Provide initial data on the performance lim-
taining five heavy steel tines mounted on the
its of the electrical measurement equipment.
outer ring of a copper base as one plate of the
3. Report the results of a field measurement
capacitor and a central steel tine at the center of
program that used this technique to mea-
the probe assembly as the other (Fig. 1). The origi-
sure soil moisture throughout an entire win-
nal design requirements were for a probe that
ter freezethaw cycle.
would be strong enough to withstand repeated
This soil moisture measurement technique is
insertions and extractions from soils in a field
based on the concept that the dielectric constant
measurement program and yet be sensitive
of a dry material consisting of soil particles and
enough to determine soil moisture to within 5%
air is relatively small (1.5 to 4), whereas the
by volume. The readout electronics were designed
dielectric constant for water is very much larger
to measure the capacitance of the probe directly
(80 at room temperature). Therefore, even small
so that field measurements could be made using
amounts of water in a soil cause the dielectric
battery-operated equipment with data displayed
constant of the resultant soilwaterair mixture
directly on digital panel meters and so that the
to exhibit a composite dielectric constant that can
output could be interfaced to data logging equip-
be related to the soil moisture content through a
ment at a later date.
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