National-scale spatial variations of soil magnetic susceptibility in Brazil

Abstract

Brazil presents high soil variability, however, the soil magnetic susceptibility (χ) distribution on national scale is yet to be investigated. Thus, soil data (218 samples from the literature and 36 original samples) were utilized to estimate soil low frequency χ (χlf) across Brazil through machine learning algorithms (Generalized Additive Model Using Splines, Generalized Linear Model, Random Forest and Support Vector Machine) in order to assess the applicability of such methodology on a nationwide scale. Two variable groups were tested: texture and Fe2O3 contents – Group 1; and texture, Fe2O3, Al2O3, SiO2 contents – Group 2. The best performing model was applied to a database containing 3817 and 3973 georeferenced soil samples from superficial and subsuperficial horizons, respectively, obtained from soil surveys conducted by the Brazilian Agricultural Research Corporation. Polynomial Support Vector Machine using Group 2 variables presented the best performance (R2 = 0.91). Variable importance measurements showed that χlf is chiefly related to Fe2O3 and clay content. The spatial distribution of χlf on national scale varied from very high (>150 × 10−6 m3 kg−1) to very low (≤5 × 10−6 m3 kg−1), considering the interpretation bands that we propose for Brazilian soils. Extensive areas of magnetic soils were associated with their formation upon mafic rocks or iron-rich formations. Despite few exceptions, the observed tendency shows that χlf increases parallel to soil red color, drainage and weathering degree, and hence Rhodic Nitisol, Umbric Nitisol, Rhodic Ferralsol and Umbric Ferralsol presented the highest χlf values.