Ratio of Clay Spectroscopic Indices and its approach on soil morphometry

Abstract

Textural differentiation between soil horizons occurs due to the soil formation processes and is one of the main characteristics used for soil classification and erosion risk analysis. The aim of this work was to develop a Ratio of Clay Spectroscopic Index (RCSI) and establish a degree of granulometric differentiation between soil horizons using VIS-NIR-SWIR (350–2500 nm) and MIR (2500–25,000 nm) data. We described morphologically and collected 150 soil profile samples of five representative soils from the southeastern and central-western Brazil. Three spectral models (VIS-NIR-SWIR, MIR and VIS-NIR-SWIR-MIR) were classified in terms of analytical organic matter contents from A and B horizons. We constructed 15 RCSI and considered reflectance difference for specific bands or band combinations, which were selected based on the qualitative and quantitative spectral signatures assessment. The soil horizons were differentiated through their spectral regions associated to iron oxides, clay minerals and quartz. The use of RCSI-15 resulted in a R2 of 0.79 and a RPD of 2.21. The indices reduce the dependence on advanced statistical methods and support the development of optical equipment to work with specific spectral bands.