2.4 Spectroscopic measurements
The spectral reflectance measurement of soils at 350-2500 nm wavelength were recorded using Fieldspec®4 ASD Spectroradiometer (Analytic Spectral Devices, CO USA). The bandwidth or sampling intervals of the Spectroradiometer was 1.4 nm and 2 nm in the 350-1000 nm and 1000-2500 nm wavelength range, respectively. The measurements were acquired using a contact probe attached to Spectroradiometer with a fiber-optic cable having a built-in light source and a 20 mm viewing window. The Spectroradiometer was calibrated with the Spectralan® panel (100% spectral reflectance) before recording the spectral measurement of the soil samples. For each sample, 50 spectral observations were captured and averaged to get the final spectral signature to improve the signal-to-noise ratio. Only every 5th wavelength was retained between 400 to 2500 nm and this led to 420 wavebands for further analysis.