Spectral reflectance properties of crusted soils under solar illumination

Reflectance of soil crusts has been studied under artificial light but not under solar illumination. Our objectives were to (i) compare reflectance from crusted and noncrusted soil surfaces under solar illumination, and (ii) explore the relationship between crust permeability and spectral signature. Two California soils were studied, Capay (fine, smectitic, thermic Typic Haploxererts) and Reiff (coarse-loamy, mixed, superactive, nonacid, thermic Mollic Xerofluvents), with samples for the latter taken from plots under organic (Reiff(OM)) and conventional (Reiff(CM)) management. A laboratory rainfall simulator was used to form crusts which were sampled at different stages of development and permeability. A portable spectroradiometer was used to collect spectral data in direct sunlight for wet and dry crusted samples. Baseline spectra (albedo) of the dry samples were, in most cases, higher than spectra from the corresponding moist samples. Crusted samples exhibited higher baseline spectra compared with the noncrusted samples. The absorption feature at approximate to1400 nm (related to lattice OH of montmorillonite) suggested accumulation of clay in the crust of the Reiff(CM), and clay depletion from the crusts of the Capay and Reiff(OM). An inverse linear relationship existed between reflectance at many wavelengths and crust permeability at different stages of crust development. The reflectance at 1700 nm provided highly significant correlations with infiltration rate (IR) for all three soils, and reflectance at 2130 nm also had a highly significant correlation for the Capay. Our data suggest that spectral measurements of soil surfaces under solar illumination can differentiate between crusted and noncrusted surfaces and assist in evaluating the degree of crust development.