Geoscience Reference
In-Depth Information
transmission line is used to determine the velocity of propagation through soil, which is controlled
by the relative dielectric permittivity or dielectric constant. By measuring the resistive load imped-
ance across the probe, EC
a
can be determined. Although TDR has been demonstrated to compare
closely with other accepted methods of EC
a
measurement (Heimovaara et al., 1995; Mallants et al.,
1996; Reece, 1998; Spaans and Baker, 1993), it has not been widely used for geospatial measure-
ments of EC
a
at field and larger spatial extents. Only ER and EMI are commonly used at these
spatial extents (Rhoades et al., 1999a, 1999b).
Field measurements of EC
a
to determine soil salinity began in the early 1970s with the use of
ER (Halvorson and Rhoades, 1976; Rhoades and Halvorson, 1977; Rhoades and Ingvalson, 1971).
However, geospatial measurements of EC
a
in the field did not occur in earnest until the 1980s,
primarily with the use of EMI, which had definite advantages over ER because it was noninvasive.
Observational research through the 1980s and early 1990s largely correlated EC
a
measurements to
soil properties in an effort to sort out what soil properties were measured by EC
a
(Table 4.1). From
the late 1990s to the present, the complex spatial relationship between EC
a
, edaphic properties, and
within-field variations in crop yield for site-specific crop management has increasingly become
the focus of EC
a
research. However, over the past three decades and even today, measurements of
EC
a
are often misunderstood and misinterpreted. The misconceptions regarding EC
a
are the conse-
quence of incomplete knowledge of the basic principles and theory of the EC
a
measurement.
4.1.2 M
i s c o n c e P t i o n s
s
u R R o u n d i n g
t h e
a
P P a R e n t
s
of i l
e
l e c t R i c a l
c
o n d u c t i v i t y
(ec
a
) M
e a s u R e M e n t
When scientists began to take EC
a
measurements in the field and correlate them to soil proper-
ties, there were preconceived notions about what was being measured. Those scientists in the arid
southwestern United States felt that salinity was being measured, and those in the Midwest felt
water content and texture were being measured. In reality, both were correct, but each failed to
acknowledge that EC
a
is a complex physicochemical measurement influenced by any soil property
that influences electrical conductance pathways in soil. Additional research produced correlations
between EC
a
and soil properties that were not directly measured but indirectly related to properties
that were measured by EC
a
(Table 4.1).
With a few exceptions, the EC
a
related observations suggest salinity, soil water content, clay
content, exchange cations, temperature, and organic matter content are the dominating soil prop-
erties affecting EC
a
, but the strength of the reported correlations varies widely with coefficients
ranging from below 0.4 to above 0.8. Contrasting findings are evident in literature; for example, Dal-
gaard et al. (2001) reported a higher correlation between EC
a
and clay at higher water contents, and
Banton et al. (1997), in a detailed study on an experimental farm near Quebec City, found texture
parameters to have a stronger correlation with EC
a
for dry than wet soil conditions. Johnson et al.
(2001), on the other hand, found no strong correlations between EC
a
and a host of soil properties in
a 250 ha dryland no-till field in eastern Colorado, concluding that EC
a
delineations were useful in
identifying overall soil variability but not in producing specific maps of any individual soil prop-
erty. In nonsaline fields in Missouri, depth to claypans (a sublayer with 50 to 60 percent in clay and
varying in depth from 0.1 to 1 m) was found highly correlated to EC
a
(Doolittle et al., 1994). As EC
a
increased, depth to claypan decreased. A more extensive study on the Missouri claypan, however,
produced EC
a
maps that exhibited little resemblance to the maps of measured depth to claypans
(Sudduth et al., 1995), concluding that the EC
a
data were strongly influenced by the crop and farm-
ing system, soil water, and crop biomass at the time of EC
a
measurements. Additional difficulties
with interpreting literature are that most of the identified soil properties that dominate EC
a
vari-
ability exhibit significant codependency and thus provide overlapping (or redundant), but confusing,
information about EC
a
. Generally speaking, the degree of EC
a
association with a given soil property
