Geoscience Reference
In-Depth Information
Pathways of Electrical Conductance
Soil Cross Section
3
1
2
Solid
Liquid
Air
fIGURe 4.3
T
The three conductance pathways for the EC
a
measurement. (Modified from Rhoades, J.D.,
Manteghi, N.A., Shouse, P.J., and Alves, W.J.,
Soil Sci. Soc. Am. J.
, 53, 433-439, 1989. With permission.)
θθρ
w
=⋅
gb
(4.7)
(4.8)
θ
ws
=
0 639
.
θ
+
0 011
.
w
ρ
θ
=
b
2.65
(4.9)
ss
(4.10)
EC
ss
=
0 019
.
(
SP
)
−
0 434
.
=⋅
EC
⋅ ⋅
⋅
ρ
SP
SP
(4.11)
EC
=
e
b
EC
w
e
100
θ
θ
w
g
The reliability of Equation (4.6) through Equation (4.11) has been evaluated by Corwin and
Lesch (2003). These equations are reliable except under extremely dry soil conditions. However,
Lesch and Corwin (2003) developed a means of extending equations for extremely dry soil condi-
tions by dynamically adjusting the assumed water content function. Using the above theory, Fara-
hani et al. (2005) deduced the importance of various soil properties to explain EC
a
variability in
nonsaline soils of three Colorado fields. Their examination of the above theoretically based EC
a
model was useful in highlighting the general complexity of EC
a
, its major pathways in the soil, and
the concept as a whole, even though crude assumptions were made in approximating the parameters
in Equation (4.6) through Equation (4.11).
Equation (4.3) and Equation (4.6) through Equation (4.11) indicate that EC
a
is influenced by EC
e
,
SP, θ
g
, ρ
b
, and temperature. The SP and ρ
b
are both directly influenced by clay content and organic
matter (OM). Furthermore, the exchange surfaces on clays and OM provide a solid-liquid phase
pathway primarily via exchangeable cations; consequently, clay content and mineralogy, cation
exchange capacity (CEC), and OM are recognized as additional factors influencing EC
a
measure-
ments. Soil EC
a
is expected to increase with increasing clay and OM contents, because EC
ss
and θ
w
(or more correctly θ
ws
) increase with clay and OM. With EC
ss
regarded as a function of clay, CEC,
and OM (Rhoades et al., 1976; Shainberg et al., 1980), it is not surprising that most field observa-
tions of EC
a
versus soil properties have identified clay, CEC, and OM as main factors dominating
EC
a
variability in nonsaline soils. It is noted from Equation (4.6) that conductance through alter-
nating layers of soil particles and soil solution pathway (the first term) is complicated, and its con-
tribution to EC
a
is not obvious as the dynamic soil properties of EC
w
and θ
w
change. As soil water
content changes, EC
w
is expected to change. As soil water evaporates, EC
w
is expected to increase
