Fig. 1. Spectral data acquisition device
1: PC; 2: Multifunctional testing platform; 3: Standard white plate; 4: Fiber probe; 5: NIR Quest; 6: QE Pro; 7: Soil samples; 8: Light source

Fig. 2. Outlier filtering results of MCCV method

Fig. 3. Spectral reflectance curves

Fig. 4. Results of different methods of screening characteristic variables
(a): Variables selected by CARS method; (b): The distribution of variables selected by CARS; (c): Variables selected by SPA method; (d): The distribution of variables selected by SPA; (e): Variables selected by CARS-SPA method; (f): The distribution of variables selected by CARS-SPA; (g): The distribution of variables selected by UVE; (h): The distribution of variables selected by VCPA

Fig. 5. Modeling results of PLSR using different variables
(a): All-PLSR; (b): CARS-PLSR; (c): SPA-PLSR; (d)CARS-SPA-PLSR; (e): UVE-PLSR; (f): VCPA-PLSR