2
THEORY
GS3
them. Conductivity (referred to as “bulk electrical conductivity”)
is derived by multiplying the inverse of the resistance (conductance)
by the cell constant (the ratio of the distance between the electrodes
to their area). GS3 bulk EC measurements are normalized to EC at
25
◦
C We factory calibrate the bulk EC measurement to be accurate
within
±
10% from 0 to 10 dS/m. This range is adequate for most
greenhouse and nursery applications.
However, some special applications in highly saline substrates may
require measurements with bulk EC greater than the specified range.
The GS3 will measure up to 23 dS/m bulk EC, but user calibration
is required above 10 dS/m. Additionally, EC measurements above
10 dS/m are very sensitive to contamination of the electrodes by skin
oils, etc. Be sure to read the sensor cleaning section at the end of
the manual if you plan to measure the EC of salty soils.
2.4
Converting Bulk EC to Pore EC
For many applications, it is advantageous to know the electrical con-
ductivity of the solution contained in the soil pores (
σ
p
), which is
a good indicator of the solute concentration in the soil. Tradition-
ally,
σ
p
has been obtained by extracting pore water from the soil and
measuring
σ
p
directly. As one would expect, this is a time consuming
and labor intensive process.
The GS3 measures the electrical conductivity of the bulk soil sur-
rounding the sensors (
σ
b
). A considerable amount of research has
been conducted to determine the relationship between
σ
b
and
σ
p
.
Work by Hilhorst (2000), has taken advantage of the linear relation-
ship between the soil bulk dielectric permittivity (
ε
b
) and
σ
b
to allow
accurate conversion from
σ
b
to
σ
p
if the
ε
b
is known. The GS3 mea-
sures
ε
b
and
σ
b
nearly simultaneously in the same soil volume. It is
therefore well suited to this method.
The pore water conductivity can be determined from (see Hilhorst,
2000 for derivation):
σ
p
=
p
σ
p
b
−
σb
=0
(1)
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