Extension of Laboratory‐measured Soil Spectra to Field Conditions

Abstract

Spectral responses of two 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. An Exotech Model 20C spectroradiometer was used to obtain spectral data in the laboratory under artificial illumination. Reflectance measurements ranged from 0.52‐to 2.32‐µm in 0.01µm increments. Asbestos tension tables were used to maintain a 0.10‐bar moisture equilibrium following saturation of crushed, sieved soil samples. 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. Results indicate that laboratory‐measured spectra of moist soil are directly proportional to the spectral response of that same field‐measured moist bare soil over the 0.52‐ to 1.75‐µm wavelength range. The magnitudes of difference in spectral response between identically treated Chalmers and Fincastle soils are 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.