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not only through the soil solution, but also through solid soil particles and via exchangeable cations
that exist at the solid-liquid interface of clay minerals. The shift away from extracts to the measure-
ment of EC a occurred 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. Historically, the utility of EC a has
been in identifying geological features in geophysical sciences and explorations (McNeill, 1980;
Zalasiewicz, et al., 1985) and in agricultural soil salinity surveys for diagnostics, leaching, and salt
loading (Corwin et al., 1996; Rhoades and Ingvalson, 1971; Rhoades et al., 1990). During the past
decade, there has been an increased interest in using EC a maps to infer the spatial variability of soil
properties important to crop production. In particular, there is an emerging interest in utilizing the
spatial variability in EC a for the purposes of guiding soil sampling (as opposed to systematic grid
sampling) and developing management zones to vary agricultural inputs.
The measurement of soil EC a is primarily through the use of the geophysical techniques of
ER, EMI, and TDR. Among the many advanced sensors recently introduced in precision agricul-
ture, EMI and ER EC a measuring devices provide the simplest and least expensive soil variability
measurement. Electrical resistivity introduces an electrical current into the soil through current
electrodes at the soil surface, and the difference in current flow potential is measured at potential
electrodes that are placed in the vicinity of the current flow. Generally, there are four electrodes
inserted in the soil in a straight line at the soil surface, with the two outer electrodes serving as
the current electrodes and the two inner electrodes serving as the potential electrodes (Figure 4.1).
A resistance meter is used to measure the potential gradient. For a homogeneous soil, the volume
of measurement with ER is roughly π a 3 , where a is the interelectrode spacing when the electrodes
are equally spaced. The most commonly used ER equipment is the Veris Soil EC Mapping System
(Veris Technologies, Salina, KS). The Veris 3100 unit has six coulter electrodes mounted on a plat-
form that can be pulled by a pickup truck. It uses a modified Wenner configuration to measure EC a
by inducing current in the soil through two coulter electrodes and measuring the voltage drop across
the two pairs of coulters that are spaced to measure EC a for the top 0.3 m (shallow) and 0.9 m (deep)
of soil (Lund et al., 2000). The shallow and deep EC a readings at each measurement point in the
field are useful in examining soil profile changes. Although soil compaction affects EC a due to the
reduced porosity and increased soil particle-to-particle contact, compaction is not easily identified
from a Veris EC a map, as the compacted layer represents only a small percentage of the domain of
EC a measurements.
The Veris unit interfaces with a differential Global Positioning System (GPS) and provides
simultaneous and geo-referenced readings of EC a . The Veris unit is designed to operate in tilled or
untilled conditions, where the coulters penetrate the soil 20 to 50 mm (more penetration for drier
Resistance Meter
Current
electrode
Current
electrode
Potential
electrodes
C 2
P 1
P 2
C 1
a
a
a
fIGURe 4.1 Electrical resistivity with a Wenner array electrode configuration where the interelectrode
spacing is equal between current and potential electrodes: C 1 and C 2 represent the current electrodes, P 1 and
P 2 represent the potential electrodes, and a represents the interelectrode spacing. (From Rhoades, J.D., and
Halvorson, A.D., Electrical conductivity methods for detecting and delineating saline seeps and measuring
salinity in Northern Great Plains soils, ARS W-42, USDA-ARS Western Region, Berkeley, CA, pp. 1-45,
1977. With permission.)
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