Agriculture Reference
In-Depth Information
might vary somewhat within the range indicated above. Wavelength pairs that have
been proposed for this
red edge ratio index
are
e.g
.:
R
R
730
780
(Reusch
2005
) or also
R
760
730
(Jasper et al.
2009
) or identical to this
R
R
R
760
730
(Erdle et al.
2011
) and
R
R
740
780
(Müller et al.
2008
).
These proposals are based on experiments with winter-wheat (Reusch
2005
;
Jasper et al.
2009
as well as Erdle et al.
2011
) and winter-oilseed rape (Müller
et al.
2008
). The small spectral differences between these indices probably
hardly matter and inverse ratios of the wavelengths indicated can be used as
well. Jasper et al. (
2009
) and Erdle et al. (
2011
) found that the red edge ratio
index was largely unaffected by different varieties, varying seed densities and
growth stages.
How do red edge ratio indices compare with the standard indices? And can nitro-
gen sensing still be improved by using more sophisticated indices,
e.g.
normalized
difference indices with wavelengths that come exclusively from the red edge range?
The criterion of a normalized difference spectral index is that it relates the differ-
ence of two wavelengths to the sum of the same wavelengths (Table
9.3
). Would a
normalized difference red edge index that relies solely on wavelengths from the red
edge still improve the results?
The coefficients of determination (r
2
) for the prediction of nitrogen uptake by
reflectance indices in Table
9.5
refer to winter-oilseed rape. Among the standard
indices, the best results again are obtained with the near-infrared to green ratio and
especially with the red edge inflection point. Yet still better predictions were sup-
plied by the new indices that rely exclusively on wavelengths from the
red edge
range
. Whether
red edge ratio indices
or alternatively
normalized difference
indices
from the
red edge range
are used seems to be unimportant (Table
9.5
).
These good results with various indices from the red edge range are in line with
findings that support the significance of this spectral region for sensing of chloro-
phyll (Fig.
6.8
), which is closely related to the nitrogen supply.
The relation between red edge ratio indices and the supply or uptake of nitrogen
is nearly linear in most cases (Fig.
9.28
). Similar regressions have been obtained
with barley, oats, oilseed rape and maize and at different growth stages (Reusch
et al.
2010
). The sensor readings depend on the growth stages of crops, as does the
nitrogen uptake of crops.
There is some influence of various
fungal infections
on the reflectance, as might
be expected. Reusch
1997
made trials with barley that was infected by leaf blotch
(
Rhynchosporium secalis
), by leaf rust (
Puccina hordei
) and by powdery mildew
(
Erysiphe graminis
) and that was either sprayed to remove the diseases or not. The
result of the infections on the red edge inflection point index were rather small when
compared to nitrogen effects. Yet it must be realized that the severities of fungal
infections will affect such results.
