3.3. Correlation between reflectance data and soil properties
Absorption of radiation at molecular vibrational frequencies in the visible-infrared region forms the basis of this spectroscopic technique and a plot of correlation coefficient in the Vis-NIR and MIR region (Fig. 4 a & b) was used to establish the most sensitive spectral region for predicting of electrical conductivity. In the Vis-NIR region, peaks were observed in the visible region at 460-500 nm (with low correlation coefficient) and NIR region at 1890-1906 nm (correlation coefficient= +0.16) followed by a broad shoulder. Gaikwad (2020) reported conspicuous absorption in the region close to wavelength 427, 487 and 1917 nm and weak absorption features near 950, 1414, 2206, 2380 and 2460 nm; whereas Margate et al., (2001) reported most sensitive spectral regions for determination of EC in soils of south Spain being 390-400, 615-625, 685-695, 800-810, 950-960, 1410-1420, 1935-1945, and 2350-2360 nm. The absorption features close to 1400 and 1900 nm represent the stretching of oxygen (O)-hydrogen (H) and bending of H-O-H of the free water and its overtones (Nawar et al., 2014). In the MIR region, five peaks had correlation coefficient > 2, three of them in the positive quadrant (5275-5280, 6660-6670, 7305-7310) and two in the negative quadrant (4200-4310, 8290-8300) and could be important spectral regions for predicting EC using models.