Agriculture Reference
In-Depth Information
exchangeable ions are the carriers of electricity. But secondly, the ions need a moist
medium - sufficient water - to fulfill this function. The clay also supplies this pre-
requisite. This is obvious when the
field capacity
, which is the moisture held in soil
after excess water has drained away and the downward movement has stopped, is
compared. At field capacity, a pure sandy soil would have about 15 % volumetric
water, whereas the content for a clay soil can go up to 50 % (Lueck and Eisenreich
2000
). And a third factor contributes to the electrical conductivity of clay soils. In
most cases, the soil content of organic matter increases with its clay content. This
can be the long-term result of the higher water content of these soils, since this
decreases the decomposition of the organic matter. In addition, many clay constitu-
ents can form special bonds with decomposed products from organic matter. The
clay-humus bonds that thus are created still further enhance the water holding
capacity of soils. So in essence, a positive interaction between clay, water and
organic matter can further enhance the high electrical conductivity of the clay frac-
tion. The clay constituents - left alone - cause only part of this effect. And the
interaction between clay and water means that on a temporary basis neither static
properties nor dynamic properties completely dominate (see above). Yet this situa-
tion exists in reality for all texture classes as a result of varying moisture in the soil.
Excessive electrical conductivities exist in
saline soils
(Table
5.2
). These soils
are found in dry, arid regions when hardly water moves downward in soils, but
instead water is sucked to the surface where it evaporates. This water transports
dissolvable salts
towards the surface plus topsoil and leaves them there. The
resulting salinity limits water uptake by plants because it reduces the osmotic
potential for this.
The dominating effect of salinity explains why historically the sensing of soil
conductivities started in arid regions (Corwin
2008
) and from there later spread out
into humid areas as well. And it must be mentioned that Table
5.2
conceals the fact
that the electrical conductivity always is defined by interactions of several factors.
If the influence of temperature on the signals is eliminated by means of careful cali-
brations, adjustments or post-processing, there remain three important factors or
parameters in
arid regions
, namely the concentration of salt ions in the soil water,
soil texture and water content. In more
humid areas
where no accumulation of salts
near the soil surface has taken place, the dominating factors that affect the conduc-
tivities are just texture and water content of the soil. Less important factors such as
the bulk density of soil here can be left out.
5.2.2.1
Electric Conductivities and Soil Properties in Humid Areas
Figure
5.8
shows an example of the influence of texture classes on electrical soil
conductivity for humid conditions. The sensing was done in two fields in an EM 38
vertical mode, thus with a depth up to about 1.5 m (Fig.
5.6
).
For both fields, the effects of clay were in the same direction and similar, there-
fore, the signals were pooled. So the data from both fields appear on the same
regression, but they stand for different ranges of clay content.
