σ
solution
. These extraction methods separate the solution from the solid then measures the
electrical conductivity of the extracted solution.
The following describes the relationship between solution and bulk electrical conductivity
(Rhoades et al., 1976)
σ
bulk
= σ
solution
θ
v
T + σ
solid
where σ
bulk
is the electrical conductivity of the bulk soil; σ
solution
, the soil solution; σ
solid
, the solid
constituents; θ
v
, the volumetric water content; and T, a soil-specific transmission coefficient
intended to account for the tortuosity of the flow path as water content changes. See Rhoades et
al., 1989 for a form of this equation which accounts for mobile and immobile water. This
publication also discusses soil properties related to CS650 operation such as clay content and
compaction. The above equation is presented here to show the relationship between soil
solution electrical conductivity and soil bulk electrical conductivity.
Most expressions of soil electrical conductivity are given in terms of solution conductivity or
electrical conductivity from extract since it is constant for a soil. Bulk electrical conductivity
increases with water content so comparison of the electrical conductivity of different soils must
be at the same water content.
The calibration equation in the CS650 operating system corrects the oscillation frequency for the
effects of σ
solution
up to 3 dS m
–1
for the CS650 and up to 10 dS m
–1
for the CS655. This is
equivalent to σ
bulk
values of approximately 0.8 dS m
–1
and 2.7 dS m
–1
respectively. If σ
bulk
exceeds these limits, the CS650 sensor returns 99999 for dielectric permittivity and volumetric
water content. The measured period average and voltage ratio values continue to report even if
the bulk EC is outside the operational range of the sensor.
8.3.3.2 Temperature correction of soil electrical conductivity
The EC value reported by the CS650 is bulk electrical conductivity. This value is temperature
dependent, changing by 2% per degree Celsius. To compensate for the effect of temperature,
convert EC readings to a standard temperature, such as 25 °C using the following equation:
EC
25
= EC
T
/ (1 + 0.02•(T
soil
–25)
where EC
25
is the σ
bulk
value at 25 °C and ECT is the σ
bulk
value at soil temperature T
soil
(°C).
8.3.4 Error sources in water content reflectometer
measurement
8.3.4.1 Sensor-to-sensor variability error
All manufactured CS650s/CS655s are checked in standard media to develop a sensor specific
span and offset value for electrical conductivity and dielectric permittivity measurements. These
CS650 and CS655 Water Content Reflectometers
24
