Agriculture Reference
In-Depth Information
can enhance his estimate about the nitrogen supply and hence the needed rate by
transmittance measurements from the youngest leaf of some plants (Fig.
9.36
). This
too supplies information about the chlorophyll content.
With all these site-specific calibration methods, the farmer does not know well at
the outset of the fertilizing operation how much nitrogen finally the whole field will
get. Because this depends on a sum of many small applications that are not known
in advance. Yet increasing the sample area that correlates reflectance and estimated
nitrogen rate allows to correct this at least partially. So instead of a spot or short strip
within the field, some farmers take a
full transect
in a typical region and relate its
mean reflectance index to an estimated nitrogen rate. But also in this case, the con-
trol line is based on a one point calibration concept.
Whenever default slopes from past experiences are not available, the two point
calibration method is indispensable. This method also is essential if more flexibility
in the site-specific application is desired. In case knowledge about a locally different
reaction of a crop on nitrogen fertilizer is available, this flexibility can be necessary.
But how to get information about this local reaction to nitrogen?
The use of
nitrogen-rich strips
within fields aims at getting information about
this. These strips are applied at the start of the growing season in one or several
small areas of the field with the objective to test the effect of the nitrogen. The term
“N-rich” indicates that the crop within this strip really has sufficient nitrogen. Since
it is known how much nitrogen was applied, the comparison of the strips with the
adjacent non-rich plants can inform about the slope that the control line should
have. If the reaction to more nitrogen is small, the slope to the horizontal should also
be small and
vice versa
.
But on what scale or resolution should this information from N-rich strips be
applied? The use can be oriented at
field scales
and hence assist in getting the right
slope of the control line for an individual field. Yet nitrogen-rich strips can also be
applied in such a way that the slope of the control line is continuously adjusted
while the tractor with the spreader or sprayer is moving through the field. The
implementation of such a continuously adjusting system for the slope of the control
line needs a thin enriched strip or
transect within each pass
of the fertilizing
machine. By sensing the reflectance precisely along this narrow strip and referenc-
ing it to a standard, the signals for adjusting the slope of the control line in an on-
the-go mode are obtained. Thus the N-rich concept is based on
site-specific cell
scales
. The result is that the site-specific control of nitrogen application occurs in a
combined
dual mode
. The control is based firstly on the nitrogen supply that the
plants have, but secondly also on the site-specific reaction to more nitrogen. Both
the nitrogen supply as well as the reaction to more nitrogen are sensed by reflec-
tance. For details to this concept of dual mode site-specific sensing of the nitrogen
supply as well as the response to nitrogen see Thiessen (
2001
,
2002
).
However, any calibration method - whether used on a field scale or on a site-
specific scale - represents a reaction that is based on the weather conditions of the
past. But the application control aims at reactions in the future. This temporal dif-
ference implies errors that can result from varying weather and its effect on chang-
ing soil or crop conditions (Roberts et al.
2011
).
