Mid-Infrared Spectroscopy Method for Soil Survey Field Offices.

The Kellogg Soil Survey Laboratory (KSSL) has a large mid-infrared (MIR) spectra library from which models can be developed and used to predict soil properties. The MIR library was obtained using the Vertex-70 FTIR spectrometer (Bruker Optik). A procedure was developed for use in Soil Survey field offices, where soil samples are processed, and MIR spectra are obtained and run through models developed from the spectral library. In field offices, soil samples are broken up and dried at 35-37°C for 3 days. The air-dried soil is then passed through a 2-mm sieve, and dried again at 35-37°C for 1 day. A 5 cm³ scoop of the < 2-mm soil is loaded into a 25 mL milling cup (with 5 steel balls) and ground for 15 minutes (at 25 Hz) on a Retsch MM200 mixer mill (Certified Material Testing). The mill used for the field office was shown to produce the same grinds as the KSSL mill (80 mesh). Spectra of the fine grinds were > 99.3% similar between the two mills. Spectra are obtained using an Alpha FTIR spectrometer equipped with the front-reflectance module (Bruker Optik) and set to collect 128 scans (per sample). A fine ground sample (about 0.4 cm³) is loaded and lightly pressed (using press a rod) into the 6-mm diameter hole of the sample holder, to form a pressed pellet. A background scan is obtained using the Alpha reference, then the sample (pressed pellet in sample holder) is scanned. To obtain predictions, the OPUS Quant2 analysis software is used (Bruker Optik). Models developed from the KSSL spectra library (also using the OPUS Quant2 software) are loaded and spectra run through the models. A pilot project was set up in a field office to test the procedure and results against laboratory measured properties (OC, total C, carbonates, pH, CEC, total clay, 1500 kPa water) for Mollisols of the central US.