Agriculture Reference
In-Depth Information
5.2.1
Methods for Sensing of Electrical Conductivities
Electrical conductivity is a measure of the ease with which an electric current flows
through a substance, in this case through soil. It is indicated in units of Siemens per
m (S/m). Occasionally the electrical resistivity - the reciprocal value - is used
instead of the conductivity.
The electric current is introduced into the soil either by direct galvanic contact
or by electromagnetic induction between the measuring instrument and the
ground. Hence the sensing occurs either in an intrusive or in a non-intrusive way. In
both cases the reactions of soils to electricity are sensed.
Soil is a very heterogeneous matter for an electric current since it consists of
solids, gases and liquids. All these components can vary immensely. The solids
include both mineral- and organic matter. If rocks and unbound organic matter are
excluded, they can be broken down by particle diameters into sand (2.00-0.05 mm),
silt (0.05-0.002 mm) and clay (less than 0.002 mm). Sand is primarily quartz and
- if dry - can be considered as an electrical insulator. The clay size fraction is made
up not only of clay minerals, but in addition of organic matter that is bonded to the
minerals. These clay-humus bonds contribute considerably to current flow in soils,
especially under wet conditions. Silt has an intermediate position. While the air in
the soil too is a good insulator, the liquids can be regarded as an electrolytic aqueous
solution with ions that are dissolved in it. The ions in the liquids as well as on the
surface of clay-humus bonds are mainly responsible for current flows in soils.
5.2.1.1
Methods Based on Galvanic Contact with the Soil
Theoretically, either direct current or alternating current up to a frequency of 1 kHz
can be used (Table 5.1 ). Ceteris paribus , direct current senses deeper. But the elec-
trodes that introduce the direct current into the soil can get polarized by ions and
thus can loose electrical contact. This problem is alleviated by employing low fre-
quency alternating current (Allred et al. 2008 ), which some geologists still denote
as direct current. The present commercial implements mostly run on alternating
current with frequencies between 150 and 220 Hz (Lueck et al. 2009 ).
The sensing process is a rather simple procedure. The current flow occurs
between a rolling coulter at the left and right side of the machine (Fig. 5.3 ). The
conductivity of the soil is sensed by one or more pairs of voltage coulters that roll
between the current coulters.
A principally still simpler procedure would be to use only two electrodes for sensing
both the current flow and the voltage between them. Although this configuration can be
used, it is more unstable (Corwin 2008 ). The method of using an outer pair of current
electrodes and at least one pair of separate voltage electrodes between them that pro-
vides the data about the soil properties goes back to Wenner ( 1915 ). It is consequently
denoted as a “ Wenner array ”. This array has proven to supply more reliable results.
The sensing implement can be pulled by a vehicle with speeds up to 15 km/ h.
The distance between measurement passes should be adapted to local soil varia-
tions, it usually ranges from 6 to 20 m. Consequently between 60 and 200 ha can be
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