Agriculture Reference
In-Depth Information
moves, this primary magnetic field induces an eddy current within the soil. This
eddy current in turn causes a
secondary magnetic field.
. Finally, both magnetic
fields induce currents in the second wire coil. This current varies with the soil
properties. The measurement units are the same as with conductivity methods that
rely on direct contact, namely Siemens per m (S/m). So actually, conductivity units
are sensed, but the conductivity is created by electromagnetic induction.
Because of the indirect measuring approach, the induction sensing is more difficult
to calibrate than contact methods that intrude the soil. The respective adjustments in the
field need more time (Sudduth et al.
2003
). On the other hand, the soil penetrating
methods rely on good electrical contact between the coulters and the soil, which can be
a problem on dry or stony soils. This problem does not exist with induction methods.
5.2.1.3
Depth of Sensing and Soil Layers
The sensing depth should fit to the maximal vertical soil penetration by the roots.
For many crops, this vertical penetration by the roots is in the range of 70-150 cm.
The penetration of the conductivity sensing can in a simplification be perceived as
a vertical cross section that starts at the soil surface. It is not precisely known
whether this
vertical cross section
perpendicular to the direction of travel should be
oriented at a rectangle - whose vertical side equals the root penetration depth - or
whether such a schematic approach is too fussy. Since the root density with many
crops decreases beyond a medium depth, it might be reasonable to aim for a sensing
density that too gradually tapers off with depth. The present sensing methods
actually follow this approach.
Principally, several possibilities exist for increasing the depth of sensing. For a
contact method (Fig.
5.3
) this can be done by
•
extending the distance between the voltage electrodes (Sect.
5.2.1.1
)
•
using lower frequencies.
With electromagnetic induction, an increase in sensing depth can be obtained by
•
changing the coil orientation from a horizontal mode to a vertical mode
•
lowering the height of the sensing implement above the soil
•
increasing the lateral distance between the coils
•
using lower frequencies.
Most sensing implements that are used commercially at present allow only for
one or two of these adjustments. The contact sensing method of Fig.
5.3
is presently
widely applied in the USA. It permits only to choose or alternate between two
different distances of the voltage electrodes. The electromagnetic induction method
of Fig.
5.4
is dominating in Europe and Canada. It allows for changing the coil
orientations and for adjusting the height of the implement above the soil.
The measured reading for a soil layer of a given conductivity depends on the dis-
tance from this layer to the instrument. An important point is how in detail the vertical
distances to the soil affect the results.
Response curves
that compare the presently
used techniques under
ceteris paribus
conditions show the depth effects (Fig.
5.6
).
