Reflectance spectra and optical depth of some sandy soils

Soil surface materials often originate from different sources and are spectrally variable. Their presence will alter soil spectral features and mask the nature of the underlying soil surface horizon. The upper-most, thin, granular layer determines a soil sample's spectra. This study's objective was to characterize the optical depth of some sandy soils and their relationship to spectral reflectance from 0.35-2.50mm. The reflectance-optical depth relationships were determined for four, air-dried, granular, sieved samples, with particle sizes of 1.0-2.0, 0.5-1.0, 0.25-0.5, 0.125-0.25, 0.075-0.125, or <0.075mm. Each particle size separate has convergent reflectance spectra associated with an optical depth that ranged from 0.2 to 8.1mm. The optical depth was greater for larger sized particles than for smaller sized particles. Normalizing the sample depth by the mean particle diameter of each sieve fraction found the optical depth-spectral feature relationships were determined by a layer of granular materials that was 5-8 particles thick. Three non-sieved, well-graded composite soils were also evaluated and their optical depths ranged from 1.4 to 3.9mm. These non-sieved composite soils include a medium fused-silica sand, a medium calcareous sand, and a medium gypsum sand.