Geoscience Reference
In-Depth Information
nature of soil. There is some truth to this notion, because electrical conductance through soil is
complex due to the complex nature of soil. As a result, without a scientific understanding and expla-
nation for what was being measured, geospatial EC
a
readings and their correlations with soil prop-
erties are easily misinterpreted and misused to spatially characterize properties that are only loosely
related or completely unrelated to actual properties being measured. This fact became evident in the
early application of spatial EC
a
measurements to site-specific crop management when correlations
between EC
a
and crop yields were found that were positive, negative, and unrelated. If, in fact, EC
a
was measuring just water content and texture, or just salinity, then why were correlations with crop
yield so erratic? The answer lies in the principles and theory behind the EC
a
measurement.
4.2 bASIC pRInCIpleS And theoRy of eC
a
Electrical conduction through soil is due to the presence of free salts in the soil solution and
exchangeable ions at the surfaces of solid particles. The soil equivalent resistance model (Sauer
et al., 1955) provides the basis for the formulation of a mechanistic soil EC
a
model applicable to
the entire range of soil solution concentrations (Rhoades et al., 1989; Shainberg et al., 1980). Three
pathways of current flow contribute to the EC
a
of a soil: (1) a conductance pathway traveling through
alternating layers of soil particles and soil solution, (2) a conductance pathway traveling through the
continuous soil solution, and (3) a conductance pathway traveling through or along the surface of
soil particles in direct and continuous contact (Rhoades et al., 1989, 1999a). These three pathways
of current flow are illustrated in Figure 4.3. Conceptually, the first pathway can be thought of as a
solid-liquid, series-coupled element, and the second and third pathways represent continuous liquid
and solid elements, respectively.
Rhoades et al. (1989) introduced a model for EC
a
describing the three separate current-flow
pathways acting in parallel:
2
(
θ θ
θ
+⋅ ⋅
⋅
)
EC
EC
+⋅
EC
=
ss
ws
ws
ss
(
θ
C) (
+⋅
θ
C)
(4.5)
a
sc
sc
wc
wc
EC
+⋅
θ
EC
ss
ws
ws
ss
where θ
ws
and θ
wc
are the volumetric soil water contents in the series-coupled soil-water pathway
(cm
3
cm
−3
) and in the continuous liquid pathway (cm
3
cm
−3
), respectively; θ
ss
and θ
sc
are the volumet-
ric contents of the surface-conductance (cm
3
cm
−3
) and indurated solid phases of the soil (cm
3
cm
−3
),
respectively; EC
ws
and EC
wc
are the specific electrical conductivities of the series-coupled soil-
water pathway (dS m
−1
) and continuous-liquid pathway (dS m
−1
); and EC
ss
and EC
sc
are the electrical
conductivities of the surface-conductance (dS m
−1
) and indurated solid phases (dS m
−1
), respectively.
Equation (4.5) was reformulated by Rhoades et al. (1989) into Equation (4.6):
2
(
θ θ
θ
+⋅ ⋅
⋅
)
EC
EC
+−⋅
EC
=
ss
ws
w
ss
(
θ θ
) EC
(4.6)
a
w
s
w
(
EC
)
+⋅
(
θ
EC
)
ss
ws
ws
ss
where θ
w
= θ
ws
+ θ
wc
= total volumetric water content (cm
3
cm
−3
), θ
sc
· EC
sc
was assumed to be neg-
ligible, and solution conductivity equilibrium was assumed (i.e., EC
w
= EC
ws
= EC
wc
, where EC
w
is
average electrical conductivity of the soil water assuming equilibrium). According to Equation (4.6),
EC
a
is determined by the following five parameters: θ
w
, θ
ws
, θ
ss
, EC
ss
, and EC
w
. Using the following
empirical approximations, Rhoades et al. (1989) showed that these five parameters are related to
four measurable soil properties, which include soil salinity (EC
e
; dS m
−1
), saturation percentage (SP;
SP is the gravimetric soil water content at saturation), bulk density (ρ
b
; Mg m
−3
), and gravimetric
water content (θ
g
; kg kg
−1
):
