Mid-infrared (MIR) spectroscopy provides fast, reliable estimates of key soil properties.
What is it?
Mid-infrared (MIR) spectroscopy is a technique used to analyze the molecular composition of substances by measuring the absorption of infrared light in the MIR range, from 2,500 to 25,000 nanometers. The resulting spectrum is a measured response (reflectance, absorbance, conductivity, etc.) as a function of some systemic portion of a continuum (wavelength, frequency). The organic material and mineral components in the soil affect the MIR spectrum.
How does it work?
MIR provides fast, reliable estimates of key soil properties that can be used for taxonomic classification, mapping, map unit development, correlation, interpretations, and monitoring. For a regional soil type or group of types, MIR spectra of soil samples from the Kellogg Soil Survey Laboratory (KSSL) spectral library are calibrated to the conventionally measured soil properties. NRCS staff then use the local calibrations to estimate those same properties from MIR spectra of newly collected soil samples from that local area. A few examples of soil properties that may be estimated include:
- sand, silt, and clay percent
- organic carbon content
- water content of a soil at which plants begin to wilt (1,500 kiloPascal water)
- acid-neutralizing capacity of an agricultural liming material (calcium carbonate equivalence)
- and measure of the soil's ability to supply important plant nutrients (cation-exchange capacity).
How do we use MIR in soil survey and soil conservation?
As of 2024, spectrometers are located inside the field office laboratories of 21 Soil and Plant Science Division major land resource area soil survey offices and 3 NRCS state or area offices. NRCS staff scan each dried and finely ground subsample to collect the spectrum. These offices have collected MIR spectra from hundreds of local samples. Staff analyze locally obtained spectra by applying calibrations built from the KSSL spectra or reference data to reliably and rapidly estimate properties.
NRCS staff use MIR spectroscopy in the following areas of soil survey and soil conservation:
- Initial soil survey
- Dynamic soil property projects
- Urban soil projects
- Update projects
- Collaboration with National Cooperative Soil Survey cooperators
Success Stories
Additional Information
Infrared Spectroscopy Program
Chemist Rich Ferguson has led the Kellogg Soil Survey Laboratory MIR data collection, participated in diverse collaborative research activities on soil IR spectroscopy, and trained many scientists inside and outside NRCS to understand and use soil IR spectroscopy.
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