Geoscience Reference
In-Depth Information
nor salt sensors adequately integrate spatial variability (Amoozegar-Fard et al., 1982; Haines et al.,
1982; Hart and Lowery, 1997); consequently, Biggar and Nielsen (1976) suggested that soil solu-
tion samples are qualitative point-sample measurements of soil solutions that are not representative
quantitative measurements because of the effect of local-scale variability on small sample volumes.
Furthermore, salinity sensors demonstrate a lag in response time that is dependent upon the diffu-
sion of ions between the soil solution and solution in the porous ceramic, which is affected by (1) the
thickness of the ceramic conductivity cell, (2) the diffusion coefficients in soil and ceramic, and
(3) the fraction of the ceramic surface in contact with soil (Wesseling and Oster, 1973). The salin-
ity sensor is generally considered the least desirable method for measuring EC
w
because of its low
sample volume, unstable calibration over time, and slow response time (Corwin, 2002).
Developments in the measurement of soil EC to determine soil salinity shifted away from
extractions to the measurement of EC
a
because the time and cost of obtaining soil solution extracts
prohibited their practical use at field scales, and the high local-scale variability of soil rendered
salinity sensors and small volume soil core samples of limited quantitative value. Rhoades and
colleagues at the U.S. Salinity Laboratory led the shift in the early 1970s to the use of EC
a
as a
measure of soil salinity (Rhoades and Ingvalson, 1971). The use of EC
a
to measure salinity has
the advantage of increased volume of measurement and quickness of measurement, but suffers
from the complexity of measuring EC for the bulk soil rather than restricted to the solution phase.
Furthermore, EC
a
measurement techniques, such as ER and EMI, are easily mobilized and are well
suited for field-scale applications because of the ease and low cost of measurement with a volume
of measurement that is sufficiently large (>1 m
3
) to reduce the influence of local-scale variability.
Developments in agricultural applications of ER and EMI have occurred along parallel paths with
each filling a needed niche based upon inherent strengths and limitations.
2.2.1.1
electrical Resistivity
Electrical resistivity was developed in the second decade of the 1900s by Conrad Schlumberger in
France and Frank Wenner in the United States for the evaluation of ground ER (Telford et al., 1990;
Burger, 1992). The earliest application of ER in agriculture was to measure θ (Edlefsen and Ander-
son, 1941; Kirkham and Taylor, 1950). This adaptation was later eclipsed by the use of ER to mea-
sure soil salinity (Rhoades and Ingvalson, 1971). Electrical resistivity has been most widely used in
agriculture as a means of measuring soil salinity. A review of this early body of salinity research
can be found in Rhoades et al. (1999). Arguably, the early salinity research with ER provided the
initial momentum to the subdiscipline of agricultural geophysics.
Electrical resistivity methods involve the measurement of the resistance to current flow across
four electrodes inserted in a line on the soil surface at a specified distance between the electrodes
(Figure 2.1). The resistance to current flow is measured between a pair of inner electrodes while
electrical current is caused to flow through the soil between a pair of outer electrodes. Although
two electrodes (i.e., a single current electrode and a single potential electrode) can also be used,
this configuration is highly unstable, and the introduction of four electrodes helped to stabilize the
resistance measurement. According to Ohm's Law, the measured resistance is directly proportional
to the voltage (
V
) and inversely proportional to the electrical current (
i
):
V
=
R
(2.4)
where resistance (
R
) is defined as one ohm (ω) of resistance that allows a current of one ampere
to flow when one volt of electromotive force is applied. The resistance of a given volume of soil
depends on its length (
l
, m), its cross-sectional area (
a
, m
2
), and a fundamental soil property called
resistivity (ρ, ω m
−1
):
