Potential of Mid- and Near-Infrared Diffuse Reflectance Spectroscopy for Determining Major- and Trace-Element Concentrations in Soils from a Geochemical Survey of North America Based on a Continental Scale Transect

The objective was to examine diffuse reflectance Fourier Transform mid- and near-infrared spectroscopy for analysis of soils obtained from a geochemical survey of North America. Samples were collected from two transects (East-West across the USA, and North-South from Manitoba, Canada to El Paso, Texas USA, n=453 and 267, respectively, representing 31 lands uses and 123 covers. The samples represent a combination of depth-based sampling (0-5 cm) and horizon-based sampling (O, A, C) with 123 different depths, and were analyzed conventionally for 48 analytes (Total-C, carbonate-C, and 46 major and trace elements). Spectra were obtained (dried, ground samples) using a Digilab FTS-7000 in the mid- (4000 to 400 cm-1) and near-infrared (10,000 to 4000 cm-1) at 4 cm-1 resolution by diffuse reflectance. Partial least squares calibrations were develop using: 1. All samples as a calibration set; 2. Samples evenly divided into calibration and validation based on spectral diversity; and 3. Samples divided to have matching analyte contents in calibration and validation sets. In general, results supported the conclusion that neither spectral range would be useful in reducing the need for conventional analysis of soils from a continental-scale survey with poor calibrations even for total- and carbonate-C for which excellent calibrations have been obtained for diverse soils obtained from national collections. It is highly likely that the problem was due to differences induced by their use. While results indicated potential for DRIFTS and NIRS to differentiate soils containing high and low (> 100 vs. 0-50 ppm) levels of some metals (Co, Cr, Ni), due to the few number of high level samples, it is possible that differentiation was based on factors other than metal concentration. In essence, great variations in chemical and physical properties seen in these soils resulted in each sample being virtually unique, and thus not suitable for spectroscopic calibrations.