A Comparison of Feature-Based MLR and PLS Regression Techniques for the Prediction of Three Soil Constituents in a Degraded South African Ecosystem

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 type	Wavelength	Assignment	Reference
𝐶org	(i) 1730 nm (AF); CR^1: 1600–1815 nm; 𝜆^2max: not defined	1650/1669 nm	2υ ^3 of aromatic CH stretch	[5, 7]
		≈1700 nm	2υ of CH stretch	[34]
		1706/1754 nm	2υ of CH, alkyl doublet	[7]
		1706 nm	4υ of aliphatic CH stretch	[7]
		1726 nm	2υ of aliphatic CH stretch	[5]
		1761/1769 nm	2υ of aliphatic CH stretch	[5]
		1730–1852 nm	4υ of methyl CH	[7]
	(ii) 2330 nm (AF); CR: 2240–2410 nm; 𝜆max: 2330 nm	2275/2279 nm	3υ of CH2, CH3	[5, 7]
		≈2300 nm	υ + υ ^4 of CH stretch	[34]
		2307–2469 nm	3υ of methyl CH stretch	[7]
		2309 nm	3υ of aliphatic CH stretch	[5]
		2331 nm	3υ of CH2, COO	[5]
		2337/2386 nm	3υ of COO, CH3	[5]
		2347 nm	3υ of aliphatic CH stretch	[5]
		2381 nm	3υ of CO stretch of carbohydrates	[7]
	(iii) 450–740 nm (HF)	Decrease in reflectance in the visible range		(e.g., [4–6, 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; 𝜆max: 550 nm	≈490 nm	ET^5 band of Fe^3+	[7, 36]
		503 nm	Goethite	[37]
		≈510 nm	ET band of Fe^2+	[36, 38]
		529 nm	ET band of hematite	[7]
		535 nm	Hematite	[37]
		≈550 nm	ET band of Fe^2+, hematite	(e.g., [36, 38, 39])
	(ii) 700 nm (AF); CR: 580–800 nm; 𝜆max: 700 nm	650 nm	ET band of hematite and goethite	[7, 39]
		665 nm	ET band of goethite	[37]
		700 nm	4υ of O–H	[7]
		≈700 nm	ET band of Fe^3+	(e.g., [35, 36, 38])
	(iii) 900 nm (AF); CR: 750–1300 nm; 𝜆max: 870 nm	≈850–870 nm	ET band of hematite	[39, 40]
		860 nm	ET band of Fe^2+ and Fe^3+	[34]
		868 nm	Hematite	[37]
		≈870 nm	ET band of Fe^3+	[35, 36, 38]
		884 nm	ET band of hematite	[7]
		≈900 nm	Transition bands of Fe^2+ and Fe^3+	(e.g., [34, 38])
		900–930 nm	ET band of goethite	[39]
		920 nm	ET band of goethite	[7]
		≈930 nm	3υ of O–H stretch, Goethite	[7, 37, 40]
		940 nm	υ + υ of O–H stretch	[7]
		1000–1100 nm	ET band of Fe^2+	(e.g., [35, 36])
		1025 nm	ET band of Fe^3+	[8]
		1075 nm	ET band of Fe^3+	[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; 𝜆max: 2206 nm	2160/216 + 2208/2209 nm	δ ^6 of AlOH bend of kaolinite doublet and υ + υ of OH stretch	[7, 37, 50]
		≈2200 nm	δ of AlOH bend and υ + υ of OH stretch of montmorillonite and illite	(e.g., [35, 40, 50, 51])
		2200/2204 nm	Montmorillonite	[37]
		2206 nm	υ of OH stretch of montmorillonite and illite	[7]
		2208 nm	δ of AlOH bend of kaolinite doublet	[7]
		2216 nm	Illite	[37]
		2230 nm	δ of AlOH bend of smectites	[7]
	(ii) 2340 nm (AF); CR: 2270–2410 nm; 𝜆max: 2340 nm	2308/2312 nm	Kaolinite	[37]
		2336 nm	Illite	[37]
		2340 nm	υ of OH stretch of illite	(e.g., [7, 22, 50])
		2372/2376 nm	Kaolinite	[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; ²𝜆_max: 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.