Fig 1.
A soil probe developed to collect reflectance spectra of soils in Yujiang County of Jiangxi Province, China: (a) ASD contact probe, (b) ASD FieldSpec 4 spectroradiometer attached by a fiberoptic cable to the probe, and (c) soil sample.
Table 1.
Statistical characteristics of the organic matter content of soil samples developed from different parent materials in Yujiang County of Jiangxi Province, China.
Fig 2.
Average spectral curves of (a) the raw reflectance, (b) their corresponding standard deviation values (shaded regions), and (c) the first derivative of reflectance for the soil samples developed from different parent materials in Yujiang County of Jiangxi Province, China.
Table 2.
Statistical characteristics of soil physicochemical variables for soil samples developed from different parent materials in Yujiang County of Jiangxi Province, China.
Fig 3.
Correlation of the organic matter content of the soil samples in Yujiang County of Jiangxi Province, China with the original reflectance and the first derivative of the original reflectance (n = 232) at different wavelengths.
Fig 4.
Plots of the measured versus predicted SOM content in the calibration sets obtained via the local PLSR models for soils in Yujiang County of Jiangxi Province, China: (a) the original spectra and (b) the first derivative spectra, and the global PLSR models for (c) the red sandstone, (d) the shale, (e) the Quaternary red clay, and (f) the river alluvium. The 1:1 line is indicated on each figure.
Fig 5.
Plots of the measured versus predicted SOM content in the validation sets obtained via the local PLSR models for soils in Yujiang County of Jiangxi Province, China: (a) the original spectra and (b) the first derivative spectra, and the global PLSR models for (c) the red sandstone, (d) the shale, (e) the Quaternary red clay, and (f) the river alluvium. The 1:1 line is indicated on each figure.
Fig 6.
Plots of variable importance projection (VIP) scores (solid curves) and b-coefficients (dashed curves) associated with the PLSR cross-validation model for organic matter content of the soil samples in Yujiang County of Jiangxi Province, China.
The threshold for the VIP was set to 1 (solid horizontal line) and the thresholds for the b-coefficients were based on their standard deviation (σ = 52.4) (dashed horizontal lines).