Role of organic matter in obliterating the effects of iron on spectral reflectance and colour of Brazilian tropical soils

Abstract

The relations between reflectance and soil constituents of groups, stratified according to the organic matter (OM) content, were quantitatively analysed in the 400-2500 nm spectral region. In order to demonstrate the OM spectral influence, a group of soil samples was submitted to the removal of this constituent by H2O2. Despite the relatively poor correlations obtained with reflectance, OM seems to be the main component responsible for lowering the reflectance in the 550-700 nm range. The presence of more than 1.7 per cent of OM effaces the iron spectral features and induces a reduction greater than 40 per cent in the negative correlation between total iron (Fe2O3) content and reflectance. The greater the amount of OM, the stronger is its effect in subduing the iron features, and in reducing the correlations between Fe2O3 and colour attributes (dominant wavelength, purity, and luminance). In the short-wavelength infrared, iron- and titanium-bearing minerals and clay minerals are accountable for lowering the reflectance. After the removal of OM by chemical treatment, the soils show slightly reddish shades, which are more saturated and especially brighter than those observed in untreated samples, as indicated by increases in the dominant wavelength, purity, and luminance values, respectively. The treated samples present a substantial reflectance increase (more than 100 per cent) and conspicuousness of iron related features in the 600-900nm range. The general result of the OM removal is an increase in albedo and a change in the shape of the spectra. After the treatment, the increase in the values of the colour attributes and in the depth of the absorption band around 900nm is less accentuated in soils with Fe2O3 content greater than 10 per cent because of the presence of other opaques.