Relationships of spectral reflectance and color among surface and subsurface horizons of tropical soil profiles

Abstract

The reflectance spectra (400–2500 nm) and color attributes of samples from the major horizons of six soil classes with broad distribution in Brazil were analyzed for intrinsic relationships (principal components analysis) as well as for correlations with chemical components. The objective was to detect the spectral changes with depth that are related to the soil chemical properties, thus improving the pedological characterization of the soil profile and vertically extending the links between topsoil properties and remote sensing data. The albedo (first principal component) differentiated the studied soil profiles in all horizons and was inversely related to FeA (total iron), Ti0 2, and Al 20 3 contents. The spectral variability within each soil profile was associated with changes in the shape (slope) of the spectra (second principal component). Such changes produced ratio values between longer and shorter wavelengths that tended to decrease with depth because of the increasing quantities of clay components, and the interplay between iron and carbon, respectively. Thus, in each profile, the albedo was generally preserved, but the shape of the spectra was not, except for soils with very low albedo. Hematite-rich soils were distinguished from goethite-rich soils by the redness index, which was estimated from the three color parameters: dominant wavelength, purity, and luminance. The results suggested that the use of spectroradiometers to measure reflectance of samples from several horizons can expedite soil surveys, and also optimize laboratory analysis for soil chemical evaluation. Also, they can be important to characterize the relative distribution of some soil constituents in large unvegetated areas.