Agriculture Reference
In-Depth Information
spectral sensor
or GPS antenna
drop-point
for fertilizer
center of
spreading cone
machine-offset
offset of
spreading cone
Fig. 9.34
Offset distance between locations of signal reception and distribution of fertilizer, con-
sisting of a machine-offset plus a spreading cone-offset (From Griepentrog and Persson
2001
,
altered)
method of determining the heterogeneity of small cereal crops is this: is the sample
area of 1.5 m
2
that was used (Table
9.6
, legend) adequate? This thinking reduces the
question about the resolution in site-specific distributing to a problem of selecting
the suitable sample area. And the latter will certainly depend on the crop species and
on the nitrogen flux within a soil.
9.4.8
Distance- and Time Lag in Site-Specific Application
The signals for site-specific fertilizing normally are received by a process-controller
that is located within the tractor-cabin. These signals are supplied either by the
spectral sensor or - in case of mapping - by the GPS system that correlates the
mapped data to the geographic location within the field. The fertilizer, however, is
distributed some distance away from the tractor. This
offset distance
is composed
of a machine offset plus an offset of the fertilizer position in the field that is due to
the spreading (Fig.
9.34
). Accurate site-specific fertilizer placement is not possible
without compensating for this offset distance.
In addition to this offset distance, a
time-lag
in the control of site-specific spread-
ing or spraying must be considered. This time-lag is the result of response-delays in
the control adjustments for the spreading or spraying. With mounted centrifugal
spreaders that feed the spinning discs by gravity, this time-lag is approximately 2 s.
When the feeding of the spinning discs is done by conveyor belts, the time lag
increases to about 3.8 s.
And finally with mounted pneumatic spreaders, the time-lag is between 4 and 5 s
(Griepentrog and Persson
2001
). In most cases, sprayers for liquid applications have
rather low time-lags that are between 1 and 2 s (Bennur and Taylor
2010
).
