Table 1: Spectral features that are used for the delineation of soil organic carbon, iron oxides and clay content within approach A. References to studies previously describing these spectral characteristics are given.

Feature and typeWavelengthAssignmentReference

(i) 1730 nm (AF)
CR1: 1600–1815 nm
: not defined
1650/1669 nm2υ 3 of aromatic CH stretch[5, 7]
1700 nm2υ of CH stretch[34]
1706/1754 nm2υ of CH, alkyl doublet[7]
1706 nm4υ of aliphatic CH stretch[7]
1726 nm2υ of aliphatic CH stretch[5]
1761/1769 nm2υ of aliphatic CH stretch[5]
1730–1852 nm4υ of methyl CH[7]
(ii) 2330 nm (AF)
CR: 2240–2410 nm
: 2330 nm
2275/2279 nm3υ of CH2, CH3[5, 7]
2300 nmυ + υ 4 of CH stretch[34]
2307–2469 nm3υ of methyl CH stretch[7]
2309 nm3υ of aliphatic CH stretch[5]
2331 nm3υ of CH2, COO[5]
2337/2386 nm3υ of COO, CH3[5]
2347 nm3υ of aliphatic CH stretch[5]
2381 nm3υ of CO stretch of carbohydrates[7]
(iii) 450–740 nm (HF) Decrease in reflectance in the visible range(e.g., [46, 35])
(iv) 1460–1750 nm (HF) Decrease in reflectance in the near to shortwave infrared range(e.g., [4, 6, 35])

Iron oxides(i) 550 nm (AF)
CR: 450–680 nm
: 550 nm
490 nmET5 band of Fe3+[7, 36]
503 nmGoethite[37]
510 nmET band of Fe2+[36, 38]
529 nmET band of hematite[7]
535 nmHematite[37]
550 nmET band of Fe2+, hematite(e.g., [36, 38, 39])
(ii) 700 nm (AF)
CR: 580–800 nm
: 700 nm
650 nmET band of hematite and goethite[7, 39]
665 nmET band of goethite[37]
700 nm4υ of O–H [7]
700 nmET band of Fe3+(e.g., [35, 36, 38])
(iii) 900 nm (AF)
CR: 750–1300 nm
: 870 nm
850–870 nmET band of hematite[39, 40]
860 nmET band of Fe2+ and Fe3+ [34]
868 nmHematite[37]
870 nmET band of Fe3+[35, 36, 38]
884 nmET band of hematite[7]
900 nmTransition bands of Fe2+ and Fe3+(e.g., [34, 38])
900–930 nmET band of goethite[39]
920 nmET band of goethite[7]
930 nm3υ of O–H stretch, Goethite[7, 37, 40]
940 nmυ + υ of O–H stretch[7]
1000–1100 nmET band of Fe2+(e.g., [35, 36])
1025 nmET band of Fe3+[8]
1075 nmET band of Fe3+[8]
(iv) 550–590 nm (CF) Decrease in reflectance in the blue wavelength range
towards the ultraviolet light
(e.g., [34, 38, 39])
(v) 450–750 nm (HF) Decrease in reflectance in the visible range(e.g., [34, 38, 39])

Clay(i) 2200 nm (AF)
CR: 2100–2290 nm
: 2206 nm
2160/216 + 2208/2209 nmδ 6 of AlOH bend of kaolinite doublet and υ + υ of OH stretch[7, 37, 50]
2200 nmδ of AlOH bend and υ + υ of OH stretch of montmorillonite and illite(e.g., [35, 40, 50, 51])
2200/2204 nmMontmorillonite[37]
2206 nmυ of OH stretch of montmorillonite and illite[7]
2208 nmδ of AlOH bend of kaolinite doublet[7]
2216 nmIllite[37]
2230 nmδ of AlOH bend of smectites[7]
(ii) 2340 nm (AF)
CR: 2270–2410 nm
: 2340 nm
2308/2312 nmKaolinite[37]
2336 nmIllite[37]
2340 nmυ of OH stretch of illite(e.g., [7, 22, 50])
2372/2376 nmKaolinite[37]
(iii) 450–700 nm (HF) Increase in reflectance in the visible range (e.g., [34, 42])
(iv) 1460–1750 nm (HF) Increase in reflectance in the near to shortwave infrared range(e.g., [34, 42])

1CR: range where continuum removal is performed; 2 : wavelength of maximal absorption predominantly found in literature; 3υ: overtone absorption bands; 4υ + υ: combination bands of fundamental and overtone absorptions; 5ET: electronic transition bands; 6δ: fundamental absorption bands.