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Fig. 9.6 Technology for sensing of soil nitrate in slurry samples on-the-go by ion-selective elec-
trodes. The soil sampler is in a lifted position (From Sibley et al. 2010 , altered)
the use of slurries nor of liquid extractions from soil samples are state of the art in
site-specific farming. The reason for this is probably the more complicated sensing
procedure compared to direct sensing of naturally moist soil. However, prototypes
that can be used to sense nitrate by ion-selective electrodes in soil slurries in an on-
the-go mode have been developed (Fig. 9.6 ).
The soil samples are collected by means of a hydraulic-powered wood saw blade.
This device cuts an about 15 cm deep slot into the soil and throws a spray of fine soil
onto a belt conveyor. The belt of this conveyor is provided with pockets of an oblong
shape. These pockets in the belt thus receive samples from the soil. A scraper above
the belt levels all soil samples to the same height. When the belt passes around the tail
end roller of the conveyor, the pockets are stretched lengthwise. This stretching of the
pockets facilitates their complete emptying and the precise delivery of the samples
into a small container, that houses an electrochemical cell , the nitrate sensing unit.
Just prior to the discharge of the soil sample, water for the extraction of the
nitrate ions is pumped into this container, where the ion-selective electrodes are
located. A stirrer within this container is activated and creates a soil slurry . And
consequently, the nitrate in the soil is rapidly extracted into the watery fraction of
the slurry. The ion-selective electrodes sense the activity of the nitrate ions via the
voltage, and the georeferenced result is recorded. This design provides an unchang-
ing ratio of naturally moist soil to water (Sibley et al. 2008 ), which is important for
accurate measurements of nitrate ions (see Fig. 9.3 ). In gravimetric terms, about
four times as much water as naturally moist soil is in the slurries (Sibley et al. 2009 ).
Because of the intermittent operation, a sensing signal is obtained every 10 s in a
similar way as with soil water pH sensing.
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