Collection of soil spectral data in Vis-NIR range
A portable spectroradiometer (Model: FieldSpec3 FR; Analytical Spectral
Devices Inc., USA) equipped with a contact probe (10 mm spot size) was
used for spectral reflectance acquisition across the wavelength range of
350 to 2500 nm, covering the visible (VIS), near-infrared (NIR) and
shortwave-infrared (SWIR) regions. About 50 g of soil was placed in a
special container (10 cm diameter), and the soil surface was leveled
with a rubber cork used as a mallet (Mouazen et al., 2010). A
spectrum from each quadrant of the container was acquired by keeping the
contact probe at the respective positions so as to have four reflectance
spectra per soil sample. For each soil sample, a reference spectrum was
also collected using a 9.2-cm diameter Spectralon white reference panel
(Labsphere).