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).