Sensing of Natural Soil Properties - Precision in Crop Farming - page 57

Fig. 5.3 Online and on-the-go sensing of soil electrical conductivity by a contact method, System
Veris. The right current electrode is concealed by the wheel (Photo from Lorenz, Lufa Nord-West,
Oldenburg, Germany, altered and supplemented)

sensed, georeferenced, logged and mapped per day. About 120 readings are obtained
per ha, so the spatial resolution is very much better than with conventional soil test-
ing methods. Yet soils that are frozen, very dry, stony or covered with much residues
can prevent the application.
The volume of soil that is sensed with the Wenner array can be adjusted. It
includes all the soil between the respective pair of voltage electrodes from the soil
surface to a depth that equals approximately the horizontal distance between the
voltage electrodes. Thus, taking the signals from the outer pair of electrodes instead
of the inner pair (Fig. 5.3 ) allows increasing the depth of sensing.

5.2.1.2 Methods Based on Electromagnetic Induction

Electromagnetic induction occurs when a magnetic field crosses a conductor or vice
versa . In this case, the soil is the conductor. The implement that generates the primary
magnetic field just is moved at a defined distance above the soil. Travel speed and area
capacity can be about the same as with contact methods (see previous section).
However, whereas the contact methods might use current that can be quasi or
almost direct current (see above), electromagnetic induction methods rely on alter-
nating current with a frequency well in the kilohertz range (Table 5.1 ). This is
because the induction process needs alternating current.