Agriculture Reference
In-Depth Information
coeff. of determ. between
N-uptake and wavel. ratio
0.8
1000
r
2
900
0.7
0.6
800
0.5
0.4
700
0.3
600
0.2
0.1
500
0.0
400
400
500
600
700
800
900
1000
wavelength of the numerator in nm
Fig. 9.26
Matrix showing coefficients of determinations (r
2
) of simple reflectance ratios for sens-
ing of N in wheat with a growth-stage in EC or BBCH of 31 (From Reusch
2005
, altered)
the light penetrates better into the canopy and thus can indicate its biomass. For
sensing nitrogen, sensing the biomass of a crop or it surrogate - the leaf-area-index
- is at least as important as sensing the chlorophyll concentration within the leaves
well. This context explains why among the standard indices, the ratio of near-infrared
to green and especially the red edge inflection points provided the best results for
nitrogen sensing (Table
9.3
).
However, none of the standard indices was developed especially for sensing of
nitrogen. Even the best standard index for nitrogen sensing - the red edge inflection
point - is essentially a chlorophyll index. Its potential for nitrogen sensing is derived
from the fact that chlorophyll is an important carrier of plant nitrogen (Lamb et al.
2002
), but it is not the only one.
These considerations led to
systematic searching
in steps of 10 nm within the
wavelength range from 400 to 1,000 nm (Reusch
2003
,
2005
). The searches were
confined to simple ratio indices with one wavelength in the numerator and the other
one in the denominator. Figures
9.26
and
9.27
show results that were obtained with
oblique viewing of winter-wheat canopies and with artificial illumination. The col-
ors stand for coefficients of determination between the nitrogen uptake and the
respective wavelength ratios. The locations within the squares of the matrices rep-
resent the coordinates of the wavelength ratios.
