Agriculture Reference
In-Depth Information
spatial resolutions still imply laborious manual sampling procedures. Either online
and on-the-go procedures for proximal sensing or alternatively remote sensing
methods are needed, at least for site-specific farming on large areas.
The sensing concepts that are available for this rely almost exclusively on
elec-
tromagnetic radiation
or on
electric current
. The wide spectrum of electromag-
netic radiation provides for a huge variety of sensing alternatives. The interest is
focussed on using visible as well as infrared light, micro- or radarwaves and finally
also on electric current as indicators for soil properties. It is important to realize that
in all cases no direct indications of soil properties are possible. The indications
always are indirect phenomena. The respective soil properties can at best influence
details of the radiation or current such as amplitude, frequency, speed of propaga-
tion
etc
. Via changes in these details it is possible to make deductions or inferences
for soil properties. In short, the estimations of the soil properties are based on cor-
relations to physical details of electromagnetic radiation or electric current. These
correlations allow to use intermediates that can easily be sensed or recorded online
and hence applied for a control.
It is very important to realize for which part of the soil the sensing should take
place. Should the information about the respective property be obtained on a soil
volume basis
or on a soil
area basis
? The interest in most cases is focussed on a soil
volume basis. The crops rely on texture, organic matter and water for a soil depth
that approximately equals the vertical root length. Hence the respective properties
in the topsoil as well as in an adjacent part of the subsoil count. So if the maximal
vertical root length is about 1 m, it might be appropriate to get site-specific signal
averages that are related to this soil layer.
But there are exceptions for which such averaging procedures do not comply to the
needs. A special case for which such coarse averaging does not fit is when sensing
water in the seedbed zone
solely is required. In this special situation it might be
reasonable to sense at which depth a dry topsoil zone ends and a subjacent moist zone
precisely begins, where then the seeds should be placed. Hence in this special case the
objective would be more a method of water sensing on a two dimensional basis. Or if
the sensor scans along a
vertical cross section
within the soil, it could be defined as
sensing for a water line
. In addition, there may be reasons for controlling the applica-
tion of soil-herbicides according to the organic matter content of the soil just at its
surface, hence also for sensing this soil property on an area basis. There may be addi-
tional cases where sensing on an area basis or on a line basis conform to the needs.
If sensing properties on a volume basis is the objective, getting the signals from
the surface would suffice only if the soil constituents were uniformly distributed
within the volume. This usually is the case for the topsoil that is cultivated and thus
mixed. But this does not hold for the subsoil. So if information about soil properties
is needed within the layer that the roots of the crops penetrate, signals that are
obtained from the soil surface alone do not suffice. One- or two dimensional sensing
of soil water or soil organic matter can be reasonable for the special cases of con-
trolling the sowing depth or the application of soil herbicides as explained above.
This does not alter the general need for sensing texture, organic matter and water on
a three-dimensional or on a volume basis.
