Agriculture Reference
In-Depth Information
in the direction of travel behind the current electrode pair - independently sense
electrical conductivity. With increasing distance between the current pair of elec-
trodes and a respective voltage pair, signals from deeper soil are delivered. The
increased number of response curves allows a more accurate resolution of depth
signals. Hence principally the prerequisites for precise three-dimensional recording
and mapping of electrical conductivities via inversed signals are better.
Further enhancing of the depth resolution is possible by varying
electrical fre-
quencies
within the range of 1 mHz to 1 kHz and thus increasing the depth of sens-
ing by lower frequencies or
vice versa
. Four different frequencies can be sensed
simultaneously, which multiplied with five voltage electrode spacings allow to
record 20 response curves almost simultaneously on-the-go. And when operating at
the higher frequency ranges even more than 20 response curves can be obtained.
This is because current with higher frequencies is at least partly transferred by
induction via its magnetic field and not solely via conduction. And the transfer via
induction instead of conduction causes phase shifts in alternating current. Since
these phase shifts diminish the power that is transmitted, they too can be recorded
and allow to obtain additional response curves.
Operating modes such as travel speed, field capacity and number of local read-
ings per ha come up to those mentioned in the latter part of Sect.
5.2.1.1
.
This method is not yet used commercially and more experimental experience
might be necessary (Lueck et al.
2009
). And more response curves alone do not
alter the fact that there still is overlapping for all response curves near the soil sur-
face. This is because all response curves - even if these successively go deeper and
hence differ in shape - still originate at the soil surface similar to the gradients
shown in Fig.
5.6
from other sensing instruments. By elaborate post-processing that
involves
inversion
of all electrical conductivity measurements, mapping of rather
thin layers or horizons might be possible - as mentioned above (Gebbers et al.
2007
). In the future, powerful computers might even allow to do this on-the-go.
Another approach for a more detailed depth resolution and hence better sensing
of soil horizons is based on
varying the coil orientation
of the electromagnetic
induction system (Figs.
5.4
and
5.5
) continuously on-the-go between the vertical
and the horizontal position (Adamchuk et al.
2011
). Hence the vertical horizon is
scanned in modes that alternate between deep and shallow readings. This concept
allows to sense in varying depths with a very compact implement. Postprocessing of
the response curves via inversion procedures here too is necessary.
The prospects of thus sensing soils in three dimensions with a high resolution
deserve attention. It is not the layers or horizons of soils alone that are of interest.
Equally important is what happens to the water that is moving through the soil. The
water travels through the soil to water tables on top of the saturated soil zone and
from there into rivers and might carry nutrients from mineral- or organic fertilizers
as well as even pesticides with it (Schepers
2008
). Hence there is increasing concern
about the
preferential water flow
routes that bypass most of the soil and can be
regarded as traffic lanes for an unwanted transport of these components into the
environment. Soil electrical conductivity is related to the water content and thus
principally also to preferential water flow routes. However, there exist substantial
