Agriculture Reference
In-Depth Information
11.4.5
Sensors for Practice and Research
For an on-line use, the MiniVegN (Fritzmeier, Germany) or Laser-N-Detector
( Planto, Germany) are commercially available. These systems are able to measure
the fl uorescence emission ration F680/F730 of a small spot which is scanned beyond
the tramline. This principle is sensible to the chlorophyll content and therefore
mainly used for nitrogen application.
When using handheld commercial sensors, it is possible to measure almost
everything. Spectralfl uorimeters ( e . g. RF5001PC, Shimadzu, Japan) with ultravio-
let excitation can be applied in a laboratory or a fi eld to monitor the fl uorescence
emission. For full excitation spectra of plant fl uorescence, the Dualex and
Multiplex (Force-A, France) or the UV-PAM (Walz, Germany) are available. The
kinetics can be measured e.g. with the PAM ( Walz, Germany) or the fast kinetics
with the PEA (Hansatech, England).
11.5
Differentiation Between N Defi ciency
and Effects of Fungi
For any site-specifi c application of farm chemicals there is the problem that the
control signals of sensors may be ambiguous. Nitrogen defi ciencies in crops cause
increases of refl ectances in the visible range. The same effect can result from fungal
infections (Fig. 11.3 ). And there can be further examples of abiotic stress factors -
e.g. lack of water - that affect the signals of a sensor in a similar way as biotic stress
factors do. Because all signals of optical sensors are substitutes for the respective
site-specifi c crop properties. These substitutes can be infl uenced by other factors
than the one that the control should rely on.
The interaction which thus might arise between the effects of nitrogen and
fungi is especially disturbing. Because it is general experience that high nitrogen
rates promote the development of fungi. Hence if the control of site-specifi c nitro-
gen application results in increasing rates because of high refl ectances in the visible
range that are caused by fungal infections, the result on the development of the crop
might be disastrous.
To some extent, the present refl ectance indices for nitrogen sensing do prevent
this misinterpreting of the situation. Because these indices do not only include
details of the visible spectrum, but of the near-infrared radiation as well. This even
holds if the indices are extracted from the red edge range. In the near-infrared and
red edge range, the effects of fungi can deviate from the result of nitrogen (see Figs.
9.19 and 11.3 ).
Reusch ( 1997 ) designed a fi eld experiment with winter barley and strips that
were either sprayed against fungi or not. In addition, different nitrogen applica-
tion rates were included on all strips. The strips where no fungicides had been
applied and where higher nitrogen rates had been given were more severely
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