Geoscience Reference
In-Depth Information
principles and theories encompassing EC
a
measurements, which led to presentations and even pub-
lications with misinterpretations. To ground researchers in the basic theories and principles of EC
a
,
guidelines for EC
a
surveys and their interpretation were developed by Corwin and Lesch (2003).
After the research associating EC
a
to soil properties and to crop yield, the direction of research
gradually shifted to mapping within-field variation of EC
a
as a means of directing soil sampling
to characterize the spatial distribution and variability of properties that statistically correlate with
EC
a
. The early observational studies compiled in Table 2.1 served as a precursor to the mapping of
edaphic (e.g., salinity, clay content, organic matter, etc.) and anthropogenic (e.g., leaching fraction,
compaction, etc.) properties using EC
a
-directed soil sampling.
Soil sampling directed by geospatial EC
a
measurements is the current trend and direction
for characterizing spatial variability. The use of EC
a
-directed sampling has significantly reduced
intensive grid sampling from tens of samples or even a hundred or more samples to eight to twelve
sample locations for the characterization of spatial variability in a given field. The earliest work in
the soil science literature for the application of geospatial EC
a
measurements to direct soil sampling
for the purpose of characterizing the spatial variability of a soil property (i.e., salinity) was by Lesch
et al. (1992).
2.3 CURRent StAte-of-the-SCIenCe of ec
a
ApplICAtIonS
In AGRICUltURe—the pReSent
The current status of geophysical techniques in agriculture is reflected in ongoing research of
the U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) laboratories at
Ames, IA; Columbia, MO (Kitchen, Lund, and Sudduth); Columbus, OH (Allred); Fort Collins, CO
(Buchleiter and Farahani); and Riverside; CA (Corwin and Lesch). Researchers at these facilities
have been instrumental in organizing and contributing to symposia and special issues of journals
that demonstrate the current role of geophysical techniques, particularly the measurement of EC
a
, in
agriculture: Soil Electrical Conductivity in Precision Agriculture Symposium at the 2000 American
Society of Agronomy-Crop Science Society of America-Soil Science Society of America Annual
Meetings, Applications of Geophysical Methods in Agriculture Symposium at the 2003 Annual
American Society of Agricultural Engineers International Meeting, special symposium issue of
Agronomy Journal
(2003, vol. 95, number 3) on Soil Electrical Conductivity in Precision Agricul-
ture, and special issue of
Computers and Electronics in Agriculture
(Corwin and Plant, 2005) on
Applications of Apparent Soil Electrical Conductivity in Precision Agriculture. The most up-to-
date review of EC
a
measurements in agriculture is provided by Corwin and Lesch (2005a).
2.3.1 f
a c t o R s
d
R i v i n g
ec
a
-d
i R e c t e d
s
of i l
s
a M P l i n g
Three essential factors have driven the development of EC
a
-directed soil sampling as a tool to char-
acterize the spatial variability of soil properties: (1) the mobilization of EC
a
measurement equip-
ment, (2) the commercialization and widespread availability of a Global Positioning System (GPS),
and (3) the development or adaptation of a statistical sampling approach to select sample sites from
spatial EC
a
data. All of these came to fruition in the 1990s.
The development of mobile EC
a
measurement equipment coupled to a GPS (Cannon et al., 1994;
Carter et al., 1993; Freeland et al., 2002; Jaynes et al., 1993; Kitchen et al., 1996; McNeill, 1992;
Rhoades, 1993) has made it possible to produce EC
a
maps with measurements taken every few
meters. Mobile EC
a
measurement equipment has been developed for both ER and EMI geophysi-
cal approaches. In the case of ER, by mounting the electrodes to “fix” their spacing, considerable
time for a measurement is saved. Veris Technologies* developed a commercial mobile system for
* Veris Technologies, Salinas, KS. Product identification is provided solely for the benefit of the reader and does not imply
the endorsement of the USDA.
