Texture gradient is a prominent and ubiquitoud feature of Acrisol. The objective of this paper was to study the particle size distribution and mineralogy involved in the formation of the texture gradient of a subtropical soil in Rio Grande do Sul. Soil samples of a Red-Yellow Acrisol, formed on sedimentary parent material, were collected from trenches at two relief positions in an area of the Universidade Federal de Santa Maria, RS, Brazil. Soil samples were collected from profile 1, located on a slope and under natural pasture, in the horizons A1, A2, AB, and Bt and from profile 2, on a hilltop and under spontaneous vegetation, samples were collected in the horizons A1, A2, E, and Bt. Chemical attributes of the < 2 μm fraction were evaluated. The clay fraction (< 2 μm), after NaOH dispersion, was laser-diffracted, to discriminate the size distribution of the particles involved in the process. The < 0.2 μm fraction was separated by centrifugation, Ca2+ saturated and X ray diffracted after the following treatments: control, air-drying at room temperature (N); ethylene-glycol saturation (EG); and heating to 300 and 550 ºC. The raw X ray diffractograms were modelled using DecompRX. A texture gradient was observed in both soil profiles. Clay contents in the Bt horizons of profile 1 and 2 were 2.9 and 4.4 times higher than in the surface horizons. Laser diffraction diagrams of the Bt horizon revealed an enrichment in two modal diameter sizes (0.09 and 0.2 μm). The mineralogy of Bt horizons was composed of high kaolinite-smectite (K-S) amounts in detriment of Al-hydroxy interlayered vermiculite (HIV) and illite-smectite (I-S) clay minerals. It is suggested that the textural gradient in the soil profiles was due to migration of K-S particles with a modal diameter of 0.09 ìm and 0.2 μm. However, there are pieces of evidence pointing out for lithological discontinuity in profile 2, indicating that particle migration was not the only process of texture gradient formation.
interstratified kaolinite-smectite; clay translocation; laser diffraction; ferrolysis; bioturbation
