Agriculture Reference
In-Depth Information
near-infrared radiation
,
mainly volume-reflectance
no absorptance,
high transmittance
red radiation,
mainly surface-reflectance
high absorptance,
hardly any transmittance
reflectance and the LAI
60
50
40
irradiation
irradiation
30
20
10
red (670 nm)
0
1
357
leaf area index (LAI)
Fig. 9.32
The optical background of volume and surface reflection (The curves in the right part
are from Guyot
1998
)
At very early growth stages -
i.e.
below EC or BBCH 29 with small grains - the
nitrogen content of the leaves often is not yet a reliable indicator of the supply by
the soil. This is, because the seedlings get their initial supply from the seed and not
from the soil.
There may be exceptional cases that call for an early nitrogen dressing on a site-
specific basis. These cases might arise when local differences in field emergence
develop because of uneven fields or irregular sowing and an early site-specific
nitrogen dressing is targeted at compensating for this. The compensation might not
be based on the nitrogen content in the leaves, but instead rely on using a fluores-
cence technique for sensing the
plant density
per unit area (Thoren
2007
). An
alternative to this method could be a site-specific first dressing that is oriented at
the height of the plants (see next section).
Up to now, fluorescence sensing for controlling the nitrogen application is rarely
used in practical farming. Interesting applications for fluorescence sensing might
develop in combination with the detection of fungal diseases.
9.4.6
Sensing Nitrogen Based on Bending
Resistance or Height
These methods inherently omit the chlorophyll as a control factor for the nitrogen fertil-
ization. Instead, these methods rely on physical properties of crops for the control, either
the resistance of the canopy against bending or alternatively the height of the plants.
