Field portable soil spectroscopy: What’s beyond Vis-NIR?

The use of spectroscopy in soil properties measurement has been extensively investigated. To qualify and quantify soils with high speed and accuracy, field portable soil spectroscopy (FPSS) is a promising tool to carry out in situ soil surveys. These sensors can produce continuous spectra from different chemical principles, and soil properties can be either directly or indirectly inferred from the spectra. This review summarised the theories and applications of eight FPSS, including some already widely implemented and some still in an early stage of development. These FPSS are characterised by the information they provide, namely organic, mineralogical, elemental, and physical information about soils. Visible, near-infrared and mid-infrared spectroscopy methods are the primary vibrational spectroscopy method used to measure organic and mineralogical properties. Raman spectroscopy, another vibrational spectroscopy, is also promising, but requires more quantitative validation. X-ray fluorescence and laser-induced breakdown spectroscopy are effective in quantifying elemental concentrations and are commonly used as complementary sensors to the vibrational spectroscopy. Gamma-ray spectroscopy detects gamma radiation and is used to identify and quantify radioactive elements. Lastly, nuclear magnetic resonance and terahertz spectroscopy are two emerging sensors that have the potential to characterise organic matter and soil physical properties, respectively.