Agriculture Reference
In-Depth Information
The results from different times within the growing season differ widely for the
same crops, and there is no uniform trend discernible. The same holds for differ-
ences in the results between the crop species. In short, there seem to be some factors
involved that are not listed.
From the averages of the upper limits of cell side length (Table 9.6 ), it can be
concluded that the working widths of most present day fertilizer spreaders probably
comply with the site-specific application needs for nitrogen. The lowest average
limit of cell side length is about 25 m. Although the spreading widths of modern
centrifugal spreaders exceed this range, the overlapping that occurs with this
method of distributing must be considered. The actual working widths of centrifu-
gal spreaders that are obtained after taking into account the overlapping are usually
not above this range.
For modern pneumatic spreaders , the working width can be above 25 m.
However, with many of these spreaders, it is technically feasible to practice sec-
tional control and thus to supply the right and left section with separate rates of
fertilizer. Hence with this method too it would be possible to stick to the needed
limits of cell side length.
Very high resolutions and thus small limits of cell side lengths can be real-
ized with sectional rate control for the outlets or nozzles of either pneumatic
spreaders or of sprayers. This method is state of the art for spot spraying of
herbicides in fallow fields with dryland farming (Fig. 6.5 ) . With proper reflec-
tance indices and variable control devices, it could be used for site-specific
nitrogen application as well and would allow for an upper limit of cell side
length that is as low as 0.5 m.
Hence it can be concluded that principal technical limits for nitrogen application
with resolutions that correspond to the needs of crops probably do not exist. The
overriding question is: what resolution is needed in order to correspond to the het-
erogeneity of the crop? Most present day machinery for site-specific nitrogen appli-
cation is designed for lateral cell side lengths that comply with the working widths
of conventional centrifugal spreaders (Figs. 9.24 and 9.25 ).
The really sensed widths or areas usually account only for a small fraction of the
fertilized width or area ( e.g. Fig. 9.22 ). The inaccuracy that might result from this is
partly offset by averaging of optical signals that are received in the direction of
travel. The finally obtained cell side lengths in the direction of travel are much
smaller than the working width in fertilizing.
So the presently realized resolution in distributing results in cell-shapes that cor-
respond to rectangles whose long side is oriented perpendicular to the direction of
travel. Theoretically it might be better to have squared cells. However, the present
rectangular cells with long sides perpendicular to the direction of travel result from
technical possibilities with conventional spreading machinery.
It must be mentioned that any results of heterogeneity sensing (Table 9.6 ) depend
on the sample areas that are recorded. This is because of the averaging effect that
automatically takes place when the sample area is increased. Thus the resolution
that is used for recording the heterogeneity of crops greatly influences the obtained
upper limit of cell side length (Solie et al. 1996 ). An interesting question to the
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