Agriculture Reference
In-Depth Information
spectro-
photometer
detection
cable
illumination cable
direction of travel
soil surface
soil surface
sensing level
Fig. 8.8 Principle of moisture sensing at various depth levels below the soil surface by infrared
reflectance (not to scale)
8.3.1.3
Control via Soil Moisture
In many cases, the moisture content rises with the vertical distance from the soil
surface. The logical consequence of this would be to increase the seeding depth until
the soil moisture needed is arrived at. This leads to a site-specific control of the depth
of the openers that is based on sensing the soil moisture. The moisture levels, which
are needed for various seed species for germination and emergence, are well known.
Such a depth control calls for a precise sensing of the water content in successive
horizontal soil layers. Up to now, no sensing technique that is simply directed at the
soil surface can do this. Yet scratching the soil surface by an opener so that the
planes of different soil layers are accessible allows to sense the situation (Fig. 8.8 ).
The sensing principles used for this rely either on the reflectance of near-infrared
radiation (Price and Gaultney 1993 ; Mouazen et al. 2005 ) or on electrical resistance
or -voltage (Carter and Chesson 1993 ; Bowers et al. 2006 ). Details on water sensing
principles are dealt with in Chap. 5 .
However, sensing the moisture at a predetermined level below the soil surface
alone does not suffice. A moisture seeking control algorithm is needed, which
automatically searches for the water content necessary by sensing at various depth
levels while preventing unneeded deep placement of the seeds. Price and Gaultney
( 1993 ) developed such a control algorithm for maize.
The need for such a control system in rainfed regions probably depends very
much on the climate. Its usefulness can be questioned for regions with a distinct
maritime climate . Rather frequent drizzling rains here are the rule. Even if at the
time of sowing the soil moisture at the normal seeding depth is not quite sufficient,
the probability of some rain within a short time span is rather high. This also
means that the concept of rising moisture with increasing depth below the soil
surface - which relies on rather long time periods of evaporation into the atmosphere
without rain - quite often is not valid here.
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