Fig. 3. (a) Principal component similarity maps determined for visible and near infrared spectra of the single-field dataset, showing 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), and (b) spectral patterns SP1 and SP2 corresponding to the principal components PC1 and PC2, respectively.