Agriculture Reference
In-Depth Information
40
35
30
25
20
15
10
5
0
400
700
1000
1300
1600
wavelength in nm
Fig. 9.12
Average reflectance of 30 spectra of soil samples for three levels of plant available
phosphorus, defined by the Olsen extractant. Each sample contains diverse types of soil textures
and different soil moisture contents (From Maleki et al.
2006
, altered)
phosphorus, the lower the reflectance is. The courses of the spectra resemble those
of water sensing by reflectance (see Sect.
5.3.2
). More soil moisture also decreases
reflectance in a similar way. Hence the question of autocorrelation between avail-
able phosphorus and water in soils deserves attention (Maleki et al.
2006
).
Nevertheless, the
coefficients of determination
(r
2
) for the relation between the
plant available phosphorus based on Olsen extractants and the
full spectra reflec-
tance
from 400 to 1,660 nm were 0.73 and 0.75 and hence rather good. The small
difference depended on the data smoothing technique that was used.
This method of sensing available phosphorus by visible and near-infrared spectra
has been transferred from laboratory to field application for a site-specific on-the-go
operation (Fig.
9.13
). The spectral illumination is transferred by an optical fiber
cable to the flattened soil underneath a cultivator sweep. A second optical fiber
cable leads the reflectance back from this soil to the spectrometer. The cultivator
sweep is adjusted for this flat surface sensing along its open bottom plane with a
depth of about 15 cm from the soil surface.
For precise results, this
flat surface sensing
needs a very accurate guidance of
the illuminating probe in order to get a high and rather constant part of the diffuse
reflected light to the spectrometer. For this, the bottom edges of the subsoiler sweep
must be held exactly parallel to the soil surface and the illuminating optical probe
must be continuously sliding in a slanted position over the measured soil surface. In
case the bottom edges of the sweep are not precisely parallel to the soil surface, the
illuminating probe is guided somewhere above the sensed soil surface. But any
increase in the distance between the illuminating probe and the soil surface reduces
