Spectral responses of two humid mesic region glaciated soils, Chalmers silty clay loam and Fincastle silt loam, formed under prairie grass and forest vegetation, respectively, were measured both in the laboratory under controlled moisture equilibria, and in the field under various moisture and crop residue conditions. The Exotech Model 20C spectroradiometer obtained spectral data in the 0.52 to 2.32 µm wavelength range in 0.1 µm increments while used in an indoor configuration with a bidirectional reflectance factor reflectometer providing an artificial illumination source consisting of a 1000 watt tungsten iodine coiled filament lamp with transfer optics. Asbestos tension tables were used to maintain a pF 2 (approximately one-tenth bar) moisture equilibrium following saturation of crushed, sieved soil samples held in 10-cm diam x 2 cm rings with 50 mesh wire bases. The same spectroradiometer was used outdoors under solar illumination to obtain spectral response from dry and moistened field plots with and without corn residue cover, representing the two different soils. Pressed BaSO₄ served as the reflectance standard indoors while a 1.2 m square painted BaSO₄ panel (which in turn was compared to pressed BaSO₄) served as the calibration standard in the field. Detector height above the indoor samples was 2.44 m using the 3/4° field of view mode, while measurements in the field were made at a 6.1 m height using the 15' field of view mode. Results indicate that laboratory-measured spectra of moist soil are directly proportional to the spectral response of that same moist bare soil in the field over the 0.52 to 1.75 µm wavelength range. The magnitude of differences in spectral response between identically treated Chalmers and Fincastle soils is greatest in the 0.6 to 0.8 µm transition region between the visible and near infrared, regardless of field condition or laboratory preparation studied.
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