Soil information systems, proceedings of the meeting of the ISSS working group on soil information systems: S.W. Bie (Editor). Centre for Agricultural Publishing Documentation, Wageningen, 1975, 93 pp., Dfl. 15.00, ISBN 90-220-0592-5

Abstract

Soil mid-infrared (MIR) spectra contain absorption features related to soil physical, chemical, and biological properties. In this study, we explored the use of soil MIR spectra (7500 to 600 cm−1) to characterize and classify soil horizons and soil orders and identified the important features in the MIR spectra. The dataset consisted of 1167 soil samples collected from soil horizons of 270 soil profiles encompassing eight soil orders across 12 National Ecological Observatory Network (NEON) domains in the USA. The spectral features of the soil samples were qualitatively explored on the five master horizons (O, A, E, B, and C) and five B horizons (Bhs, Bs, Bk, Bt, and Bw). Random forest models were developed to investigate the predictability of soil MIR spectra on the soil master horizons and B horizons. Organic soils had different absorption features in the MIR spectra compared to mineral soils. Many absorption features in the MIR spectra were caused by organic functional groups, clay minerals, quartz, and carbonates in soils. The random forest models had an overall accuracy of 0.74 and 0.72 in classifying the five master horizons and the five B horizons for the validation, respectively. Hierarchical clustering analysis was applied on the concatenated topsoil (averaged from O and A horizons) and subsoil (averaged from E, B, and C horizons) MIR spectra of the 270 soil profiles to investigate the similarity of soil profiles in the spectral space. The MIR spectra of Spodosols had strong features of O horizons and Bhs and Bs horizons and thus Spodosols were well distinguished from other soil orders. Mollisols, Ultisols, Aridisols, and Inceptisols were not easily differentiated using MIR spectra. The random forest model to classify eight soil orders had an overall accuracy of 0.72 in the validation. The Alfisols, Aridisols, and Spodosols were fairly well classified by the MIR spectra, whereas the MIR spectra could not be used to differentiate Entisols, Inceptisols, Mollisols, and Ultisols. Soil MIR spectra can be used for characterizing and classifying soil horizons and soil orders when there are distinct spectral features in the soils. Soils with similar features cannot be easily distinguished using soil MIR spectra.