Wavelength Identification and Diffuse Reflectance Estimation for Surface and Profile Soil Properties

Abstract

Optical diffuse reflectance spectroscopy (DRS) has been used to estimate soil physical and chemical properties, but much of the previous work has been limited to surface soils or to samples obtained from a restricted geographic area. Our objectives in this research were: (1) to assess the accuracy of DRS for estimating variation in several important surface and profile soil properties across a wide range of soils from the U.S. Corn Belt, and (2) to determine the wavelength ranges and/or specific wavelengths that should be included in a DRS soil property sensor. Soil cores were obtained to a 120 cm depth from ten fields, two each in Missouri, Illinois, Michigan, South Dakota, and Iowa. Cores were segmented by pedogenic horizon and samples (n = 165 for the surface soil horizon, n = 697 for all soil horizons in the profile) were analyzed for texture fractions, cations (calcium, magnesium, and potassium) and cation exchange capacity (CEC), pH, total and organic carbon, and total nitrogen using standard laboratory procedures. Spectra were obtained on sieved, air-dried soils from 350 to 2500 nm using a commercial three-detector spectrometer. Reflectance data were related to soil properties using partial least squares (PLS) regression and stepwise multiple linear regression (SMLR). Calibration accuracies varied among the different soil properties, but for a given soil property, similar accuracies were generally obtained with PLS and SMLR. The most accurate estimates, with R2 values above 0.8, were obtained for organic carbon, clay, CEC, and calcium. When data from each of the three spectrometer detector ranges were analyzed separately with PLS, the third detector range (1770 to 2500 nm) provided results similar to those obtained using the complete spectral range. Discrete wavelength models that described 90% or more of the variance described by a full model were obtained using eight or fewer wavelengths for the profile dataset and six or fewer wavelengths for the surface dataset. Several wavelengths and wavelength ranges common to models for multiple soil properties were identified: 2070 nm, 1870 to 1915 nm, and 2220 to 2410 nm. Because additional wavelengths important for individual soil properties were dispersed across the 1770 to 2500 nm spectral range, a full-spectrum sensing approach is recommended for simultaneous estimation of multiple soil properties. A discrete-waveband sensor could be practical for estimating one or two individual soil properties.