Agriculture Reference
In-Depth Information
Fig. 9.31
Simultaneous sensing of the fluorescence ratio (F680/F735) and of estimates of the
plant density per unit area. The plant density is sensed by the number of signals per unit of area
above a noise level. The inclination of both sensors perpendicular to the direction of travel oscil-
lates with a frequency of 1-2 Hz while driving through the field. Hence the field surface is scanned
for signals (From Thoren
2007
, supplemented)
(Fig.
9.31
). By varying the inclination of this oblique view continuously while
driving though the field, the canopy surface was scanned. In this way, signals related
to the chlorophyll concentration in the leaf area as well about the vegetated area in
the field were recorded.
Yet despite this there are still limits with the present technology of nitrogen sensing
by fluorescence. These limits have to do with the irradiation that induces the fluores-
cence in the photosynthetic apparatus. The irradiance that causes the emission of fluo-
rescence has wavelengths below the near-infrared. In most cases, red radiation is used
for the induction. Contrary to this, the indices for nitrogen sensing by reflectance
sensing include near-infrared radiation. And there are fundamental differences in the
ability of near-infrared or red irradiance to record the development of a canopy.
The near-infrared irradiation is not absorbed, but highly transmitted. The red
irradiation is mainly absorbed and barely transmitted. Consequently, the near-
infrared irradiance induces mainly
volume-reflectance.
. Red radiation causes, for
the most part,
surface-reflectance
(Fig.
9.32
) or
surface-fluorescence.
.
This explains, why near-infrared reflectance responds much better to an increas-
ing
leaf-area-index
of a canopy than red reflectance does (Fig.
9.32
, right). Above
a leaf area index of 2, the red radiation does not deliver reliable signals. In principle,
the same applies to fluorescence that is induced by the red radiation.
In short, nitrogen sensing by fluorescence instead of reflectance is better at early
growth stages. But at later development stages, the situation is
vice versa
. Intermediate
growth phases can be sensed by either method. So the problem boils down to the
question, at which developmental stage of crops, site-specific nitrogen sensing is
reasonable or needed.
