Geoscience Reference
In-Depth Information
where
k
is the cell constant accounting for electrode geometry, λ is the molar limiting ion conductiv-
ity (S m
2
mol
−1
),
M
is the molar concentration (mol m
−3
),
v
is the absolute value of the ion charge, and
i
denotes the ion species in solution. Marion and Babcock (1976), among others, have confirmed the
existence of the relationship between EC and molar concentrations of ions in the soil solution.
To determine soil EC, the soil solution is placed between two electrodes of constant geom-
etry and distance of separation (Bohn et al., 1979). The measured conductance is a consequence
of the solution's salt concentration and the electrode geometry whose effects are embodied in a
cell constant. Electrical conductance was considered more suitable for salinity measurements than
resistance because it increases with salt content, which simplifies the interpretation of readings. At
constant potential, the electrical conductance is a reciprocal of the measured resistance as shown
in Equation (4.2):
EC
T
=
k
/R
T
(4.2)
where EC
T
is the electrical conductivity of the solution in dS m
−1
at temperature
T
(°C),
k
is the cell
constant, and R
T
is the measured resistance at temperature
T
. Electrolytic conductivity increases
approximately 1.9 percent per degree centigrade increase in
T
. Customarily, EC is adjusted to a
reference temperature of 25°C using Equation (4.3) from Handbook 60 (U.S. Salinity Laboratory
Staff, 1954):
EC
25
=
f
T
· EC
T
(4.3)
where
f
T
is a temperature conversion factor that has been approximated by a polynomial form
(Rhoades et al., 1999a; Stogryn, 1971; Wraith and Or, 1999) and by Equation (4.4) from Sheets and
Hendrickx (1995):
−
T
/.
26 815
f
=
0 4470
.
+
1 4034
.
e
(4.4)
T
Soil EC is determined for an aqueous extract of a soil sample. Ideally, the EC of an extract of the
soil solution (EC
w
) is the most desirable, because this is the water content to which plant roots are
exposed, but this is usually difficult and time consuming to obtain. The soil sample from which
the extract is taken can either be disturbed or undisturbed. For disturbed samples, soil solution can
be obtained in the laboratory by displacement, compaction, centrifugation, molecular adsorption,
and vacuum- or pressure-extraction methods. Because of the difficulty in extracting soil solution
from soil samples at typical field water contents, soil solution extracts are most commonly from
higher than normal water contents. The most common extract obtained is that from a saturated soil
paste (EC
e
), but other commonly used extract ratios include 1:1 (EC
1:1
), 1:2 (EC
1:2
), and 1:5 (EC
1:5
)
soil-to-water mixtures. Unfortunately, the partitioning of solutes over the three soil phases (i.e.,
gas, liquid, and solid) is influenced by the soil-to-water ratio at which the extract is made, which
confounds comparisons between ratios and interpretations; consequently, standardization is needed
for comparison of EC measurements. For undisturbed soil samples, EC
w
can be determined either
in the laboratory on a soil solution sample collected with a soil-solution extractor installed in the
field or directly in the field using in situ, imbibing-type, porous-matrix, salinity sensors. All of these
approaches for measuring soil EC are time and labor intensive; as a result, they are not practical for
the characterization of the spatial variability of soil salinity at field extents and larger.
Because of the time, labor, and cost of obtaining soil solution extracts, developments in soil salin-
ity measurement at field and landscape scales over the past 30 years have shifted to EC measurement
of the bulk soil, referred to as the apparent soil electrical conductivity (EC
a
). The measurement of
EC
a
is an indirect method for the determination of soil salinity because EC
a
measures conductance
