Agriculture Reference
In-Depth Information
high water + high N
0.4
0.3
h
igh w
ater +
low
N
low w
ater +
low
N
0.2
0.1
0
500
600
700
800
900
wavelength in nm
Fig. 9.39
Spectral reflectance of pepper (Capsicum annuum) with four treatments concerning the
nitrogen- and water supply (From Filella and Penuelas
1994
, altered)
If the whole field is too dry, any application of nitrogen might be useless, except in
cases, when a remedy by artificial irrigation or by natural rain soon can be antici-
pated. But this is reasoning about the water supply in the
future
.
Reasoning about the water supply of the crop in the
past
leads to other consider-
ations. Since the signals of the past nitrogen supply of the crop have been derived
from the plants appearance, it could be possible, that these signals were influenced
by the previous water supply. This is indeed an important point.
Penuelas et al. (
1996
) have shown that a lack of water as well as an insufficient
nitrogen supply have similar effects on the visual appearances of wheat. In both
cases, the plants tend to have
xeromorphic characteristics
: the cell elasticity is
reduced, the cell walls are thicker as well as more rigid and the cellulose content of
the leaves is higher. But how are the effects on the reflectance?
Starting with the not
decomposed reflectance curves
of the visible and near-
infrared region, the prospects of separating the effects of nitrogen from those result-
ing from the water supply do not look good. The reason for this is that the influences
of nitrogen deficiency and lack of water have similar effects on the general course
of the reflectance curves in the visible and near-infrared range. A rising water stress
results in an increase of the visible- and in a decrease of the near-infrared reflec-
tance. The same effect is caused by a lack of nitrogen (Fig.
9.39
).
Fortunately, modern reflectance sensing generally has moved away from signals
derived from the course of not decomposed curves, which extend over long ranges
of wavelengths. Reflectance sensing for site-specific plant production has gone
from red-green-blue (RGB) digital imaging to broad band recording and presently
to hyperspectral narrow band signals. And further decomposing of the reflectance
