A procedure to infer complex refractive index and mean particle radius of soils from visible and near-infrared reflectance data

In a portion of the visible and near-infrared spectrum (0.40-0.85 mu m), high resolution (0.002 mu m) reflectances from 37 soil samples have been analysed to determine complex refractive indices and the mean radius of soil particles. The method uses an approximation to a radiative transfer model. The approximation assumes that measured spectral reflectances and the imaginary refractive index can be expressed as quadratic functions of wavelength. Using this approximation, three quantities, the real refractive index, the particle radius, and a constant relating to the imaginary index are derived from wavelength coefficients for measured reflectance spectra. The resulting values of particle radius generally increase with an increase in the median radius, which is measured in the laboratory. The complex refractive index is in general agreement with that estimated by using the previously developed analytic model. The spectral reflectance measured strongly depends on the product of the imaginary index and particle radius identified by the approximation, but does not depend on those separately. Based on results of a regression analysis, the imaginary index and particle radius are useful in explaining variations in soil components, which have a major influence on soil colour, such as humus and free Fe2O3.