Mineralogical drivers of fertility and pedogenetic processes in tropical mountain soils of southeastern Brazil

The mineralogical composition of the clay fraction plays a fundamental role in the chemical evolution and natural fertility of tropical soils, particularly in mountainous environments where intense weathering interacts with slope-driven lateral fluxes. This study investigated the mineralogical transformations and pedogenetic processes that govern soil fertility in residual soils developed from syenogranite in the Caparaó mountain region, southeastern Brazil. An integrated analytical approach was employed, including X-ray diffraction (Newmod II modeling), geochemical mass balance (τ), specific surface area, and micromorphological characterization. Although kaolinite is the dominant clay mineral across profiles, surface horizons contained appreciable proportions of interstratified 2:1 clay minerals, including illite–vermiculite and kaolinite–illite, indicating ongoing transformation of primary micas (biotite > > muscovite). The progressive alteration of biotite, mainly in B horizons, together with positive τ values for Si and exchangeable bases in A and C horizons, reflects the combined action of vertical differentiation and colluvial renewal. The latter, through selective supply of weatherable minerals from upslope lithological sources, sustains the formation of more reactive clay phases, enhances cation exchange capacity, and contributes to nutrient retention even under advanced weathering conditions. Micromorphological evidence of clay coatings, b-fabrics, Fe/Mn hypo- and quasi-coatings, and mangano-ferruginous nodules supports the co-occurrence of limited clay illuviation and redox-mediated Fe/Mn redistribution, particularly in Bt and Bw horizons. These pedofeatures indicate the interplay between vertically driven differentiation and slope-controlled hydrological and redox oscillations. Altogether, the results demonstrate that the natural fertility of these tropical mountain soils is sustained by the integrated operation of vertical and lateral processes, which together maintain dynamic mineralogical renewal, chemical resilience, and long-term fertility, a mechanism particularly relevant for low-input agricultural systems such as specialty coffee production in southeastern Brazil.