Agriculture Reference
In-Depth Information
Fig. 9.36
Measuring the
transmittance of the youngest
wheat leaf for precise
calibrating of site-specific
nitrogen application (From
Agri Con GmbH, Jahna,
Germany)
considers the local situation of the crop and makes a decision. Small handheld
meters that can indicate the chlorophyll content of plant leaves at a few spots in the
field (Fig.
9.36
) and specific schemes of advisory services are often used to assist in
defining the proper rate. Yet the decision still relies largely on the knowledge and
experience of the farmer or consultant. And this decision is indispensable for a uni-
form application in the whole field as well as for a site-specific application. The
difference is just that for a uniform application the decision is about the constant
rate in the whole field, whereas for a site-specific application a minimum rate, a
maximum rate - hence the limiting rates - and the control line between these must
be defined.
9.4.9.2
Limiting Rates and Slope of the Control-Line
Entering the minimum- and maximum rate into the algorithm is easily done with
modern process-controllers. For the control-line that extends between these
limit-
ing rates
, its relation to the reflectance index must be defined. Presently, the domi-
nating practice is to assume a simple linear relation between the points along the
control-line and the reflectance index,
e.g.
the red edge inflection point or the red
edge ratio. This simplifying assumption may not be far off, though there are indica-
tions that a still more precise definition of the relation between the nitrogen needed
and the reflectance indices is possible (Holland and Schepers
2010
).
The
control-line
stands for the relation between the reflectance index and the
nitrogen rate (Fig.
9.37
). As a linear line, its course is defined by the slope and an
intercept. Its course can also be determined by the position of the minimum- and of
the maximum rate in the graph that has the reflectance index and the nitrogen rate
as ordinates.
Hence a logical method of defining the control line is to direct the field of view
of the reflectance sensor on one spot in the field that needs only the minimum rate
