Sensing of Natural Soil Properties - Precision in Crop Farming - page 93

barriers that exist outside atmospheric windows (Fig. 3.3 ) can affect near- and
mid-infrared radiation. In addition, clouds can block the transmission of all visible
and all infrared waves. And regarding crop- or plant sensing, the unique effect of
visible light on the photosynthetic process must be considered.

5.3.3 Concepts and Results for Surface Sensing in Fields

The ultimate goal is site-specific control of field operations by means of local soil
properties. Proximal on-the-go sensing from farm machines lends itself for doing
this either by simultaneous online control in real-time or by subsequent control via
mapping. When remote sensing from satellites or from aerial platforms is used, up
to now online control in real-time is not possible, so in these cases subsequent con-
trol via mapping is the choice.
Soil moisture hardly is suitable for mapping because of its transient feature. Its
use for online control of the sowing depth is dealt with in Sect. 8.3.1.3 . Other natu-
ral soil properties that can be recorded by reflectance such as texture, organic matter
and cation-exchange-capacity are rather constant over time and hence well suited
for control via mapping.
Sensing soils in fields occurs under much less controlled conditions than in
laboratories, where dried and sieved samples in an accurately fixed position are
subjected to the radiation. In fields, the soil moisture changes, and the soil particles
vary from dust to crumbs, clods or residue pieces. Furthermore, on-the-go sensing
excludes any fixed position. Hence a lower accuracy must be expected.
All concepts that have been used so far for proximal sensing by field machinery
sense from a flattened soil surface . This allows to keep the distance to the soil
rather constant. The flattened surface is obtained by sensing the soil at the bottom
of a cultivator sweep (Fig. 5.25 ). Hence the information about the soil properties
does not originate precisely from the top surface but instead from an area a few cm
below this depending on the depth adjustment of the cultivator shank. This system
of sensing underneath a cultivator sweep sometimes simply operates in the
space between the upper flanks of the sweep and the soil below it (see Sect. 8.3.1.3 ) .
However, the concept that is outlined in Fig. 5.25 uses a closed bottom of the
sweep. The radiation passes a sapphire window that is mounted along the bottom
of the sweep. Contact between the passing soil and this window is supposed to cre-
ate a self-cleaning effect and to prevent contamination of the optical path by dust
or mud (Christy 2008 ).
Such soil property sensing by means of reflectance lends itself for combining
with suitable field operations. It seems reasonable to group the sensing according
to the time spans for which the respective soil properties can be used. Properties like
e.g. conductivity, organic matter, clay and cation-exchange-capacity of soils are
valid over a long time. Hence it is sensible to record these properties with simultane-
ous georeferencing in the same map or map-series and thus to combine the respec-
tive sensors into one machine (Fig. 5.26 ).