Geoscience Reference
In-Depth Information
The determination of salinity through the measurement of electrical conductance has been well
established for decades (U.S. Salinity Laboratory Staff, 1954). It is known that the electrical con-
ductivity of water is a function of its chemical composition. McNeal et al. (1970) were among the
first to establish the relationship between electrical conductivity and molar concentrations of ions
in the soil solution. Soil salinity is quantified in terms of the total concentration of the soluble salts
as measured by the electrical conductivity (EC) of the solution in dS m
−1
. To determine EC, the soil
solution is placed between two electrodes of constant geometry and distance of separation (Bohn
et al., 1979). At constant potential, the current is inversely proportional to the solution's resistance.
The measured conductance is a consequence of the solution's salt concentration and the electrode
geometry whose effects are embodied in a cell constant. The electrical conductance is a reciprocal
of the resistance as shown in Equation (2.1):
EC
T
=
k
/R
T
(2.1)
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 at a rate of approximately 1.9 percent per degree centi-
grade increase in temperature. Customarily, EC is expressed at a reference temperature of 25°C for
purposes of comparison. The EC measured at a particular temperature
T
(°C), EC
T
, can be adjusted
to a reference EC at 25°C, EC
25
, using the below equations from Handbook 60 (U.S. Salinity Labo-
ratory staff, 1954):
EC
25
=
f
T
• EC
T
(2.2)
where
f
T
is a temperature conversion factor. Approximations for the temperature conversion factor
are available in polynomial form (Rhoades et al., 1999a; Stogryn, 1971; Wraith and Or, 1999) or
other equations can be used such as Equation (2.3) by Sheets and Hendrickx (1995):
−
T/
26 815
.
f
=
0 4470
.
+
1 4034
.
e
(2.3)
T
Customarily, soil salinity is defined in terms of laboratory measurements of the EC of the satu-
ration extract (EC
e
) because it is impractical for routine purposes to extract soil water from samples
at typical field water contents. Partitioning of solutes over the three soil phases (i.e., gas, liquid,
solid) is influenced by the soil:water ratio at which the extract is made, so the ratio must be standard-
ized to obtain results that can be applied and interpreted universally. Commonly used extract ratios
other than a saturated soil paste are 1:1, 1:2, and 1:5 soil:water mixtures.
Soil salinity can also be determined from the measurement of the EC of a soil solution (EC
w
).
Theoretically, EC
w
is the best index of soil salinity because this is the salinity actually experi-
enced by the plant root. Nevertheless, EC
w
has not been widely used to express soil salinity for
two reasons: (1) it varies over the irrigation cycle as θ changes, and (2) methods for obtaining soil
solution samples are too labor and cost intensive at typical field water contents to be practical for
field-scale applications (Rhoades et al., 1999a). For disturbed samples, soil solution can be obtained
in the laboratory by displacement, compaction, centrifugation, molecular adsorption, and vacuum-
or pressure-extraction methods. For undisturbed samples, EC
w
can be determined either in the
laboratory on a soil solution sample collected with a soil-solution extractor or directly in the field
using in situ, imbibing-type porous-matrix salinity sensors. Briggs and McCall (1904) devised the
first extractor system. Kohnke et al. (1940) provide a review of early extractor construction and
performance.
The ability of soil solution extractors and porous-matrix salinity sensors (also known as soil salin-
ity sensors) to provide representative soil water samples is doubtful (England, 1974; Raulund- Ras-
mussen, 1989; Smith et al., 1990). Because of their small sphere of measurement, neither extractors
