Fig. 1. Average visible and near infrared spectra recorded on soil samples of the single-field dataset for Groups 1 (0.0–0.025 kg kg^–1), 5 (0.026–0.075 kg kg^–1), 10 (0.076–0.125 kg kg^–1), 15 (0.126–0.175 kg kg^–1), 20 (0.176–0.225 kg kg^–1), and 25 (0.226–0.275 kg kg^–1) (a) before normalization, and (b) after normalization (normalization adopted before principal component analysis).