PEDOGÊNESE DE UMA SEQÜÊNCIA DE SOLOS DESENVOLVIDOS DE ARENITO NA REGIÃO

SUMMARY : PEDOGENESIS OF A SOIL SEQUENCE DEVELOPED ONSANDSTONE IN PIRACICABA (SP) The formation of soils with argillic horizons was investigated to identify the pedologicalprocesses involved in the formation of lamellae and the abrupt textural gradient as well astheir implication on the soil physical, chemical and mineralogical attributes. This studywas carried out on a sandstone-derived soil toposequence, with profile (P1) occupying thehigher part of the landscape (summit), profile (P2) on the backslope, and profile (P3) on thefootslope. Soil chemical, mineralogical, micromorphological and particle size distributionanalyses were performed. The profiles presented an A-E-Bt horizon sequence, being classifiedas Hapludults. The soils were acid, with clay mineralogy dominated by kaolinite. The analyzedmaterials evidenced uniformity in relation to particle size distribution, average diameter,roundness and sand fraction zircon/turmaline relation, as well as uniform morphologicalaspects, showing the autochthonous origin of these soils. The pedogenetic processes responsiblefor the abrupt textural gradient are clay degradation under oxi-reduction conditions andlessivage. The first process is evidenced by micro-domains at the E/Bt contact, where gleizationcolors can be seen, indicating temporary water saturation conditions. In this case, theformation of the albic horizon is due to the destruction of the top of the Bt horizon. Thelamellae would be an inheritance of the Bt horizon in the albic horizon. The presence ofilluvial clayskins, associated to argillans observed in the micromorphological studies,suggested a contribuion of the lessivage process to textural gradient formation.Index terms: argillic horizon, ferrolysis, illuviation, micromorphology, mineralogy, pedology.