Spectral reflectance for the mineralogical evaluation of Brazilian low clay activity soils

Highly weathered tropical soils are very important in terms of agricultural production but there are only a few spectral studies that have evaluated them in detail. Measurements of electromagnetic radiation (EMR) interactions with soil samples can provide useful information regarding mineralogy, organic matter content, granulometry as well as other important soil parameters. The objective of this study was to evaluate the relationship between analytical and spectral parameters of six important classes of tropical Brazilian soils. Spectral data were obtained under laboratory conditions using an IRIS (400-2500 nm) spectroradiometer. Soil chemical analyses were conducted as performed for pedological surveys. Spectral curves provided useful pedological information. Diagnostic mineralogical features were found for "absorption bands" that are well-defined "valleys" centred in some wavelengths of the spectral curve. The main absorption bands were centred at 450 and 850 nm and were attributed to interactions between EMR and the content of iron oxide in the soil. Water and hydroxyl absorption bands at 1450, 1950 and 2200 nm were also enhanced, and allowed a correlation with the presence of either 2 : 1 or kaolinitic mineralogy. Important pedological characteristics such as granulometry, quartz and the presence of magnetite could also be inferred. Reflectance increased as the iron content decreased and the soil's texture changed from clay to sandy. Iron forms differentially influenced the spectral data: crystalline forms were responsible for concavities in the 450 and 1100 nm spectral range and amorphous forms reduced the intensity of reflectance, but did not alter the concavities. Rich quartz soils reflected more than soils rich in iron and magnetite. The 1 : 1 mineralogy presented spectral contours that differed from the 2 : 1 mineralogy when evaluated with the bands at 1950 and 2200 nm. In conclusion, laboratory EMR analysis of soil samples collected in the field can provide a great deal of pedological information and assist traditional methods of soil evaluation.