Pedogenesis and the study of lithological discontinuity in a Planosol in Southern Brazil

ABSTRACT Pedogenetic processes or phenomena not directly related to pedogenesis can strongly influence soil formation. This study aimed to identify the origin of the abrupt textural difference and the deep, dark horizon in a soil profile located in southern Brazil by examining pedogenetic processes and sedimentology. The research was conducted in the geomorphological province of the Coastal Plain of Rio Grande do Sul, characterized by marine and eolian sedimentary deposits from the Quaternary (Pleistocene) period, formed during alternating humid and arid climates. In this region, rapid observations of soil profiles were conducted, and one soil profile was selected for detailed study. The studied soil is strongly acidic, pH(H 2 O) ≤4.7, with low cation exchange capacity (CEC ≤5 cmol c kg -1 ) and base saturation, attributed to the high quartz sand content. Micromorphological analysis revealed sub-rounded to sub-angular quartz grains, suggesting that the sediments forming the soil were transported over long to medium distances. In terms of indicators used to assess lithological discontinuity, a difference of 0.20 or greater was observed in the fine sand/total sand ratio (FS/TS), and a uniformity value (UV) greater than |0.60| was found between horizons Ap, 2E, and 3Ab1. This same trend appeared in the cumulative curve and mean sand diameter across these adjacent horizons, indicating an allochthonous origin of the soil and that sedimentary processes buried the deep dark horizon. The high aluminum oxalate/soil organic carbon ratio (Alo/SOC) in 3Btg (1.6/2.0) than in 3Ab1 (1.6/14.3) indicated another source of Alo besides complexation with SOC, which is associated with the formation of low-crystallinity aluminosilicates rather than an Al-humus complex. The significant presence of low-crystallinity kaolinites confirms this finding. Interstratified smectite-kaolinite was identified by the asymmetry of the primary and secondary reflections of kaolinite, indicating that, despite the low base saturation, drainage impediment favors an intermediate weathering condition in the soil. Sediment deposition of sandy parent material was the main factor responsible of the abrupt textural change at greater depth. Deep dark horizons 3Ab1 and 3Ab2 were not formed through the pedogenetic process of podzolization but were instead buried by depositional processes that formed the Coastal Plain of Rio Grande do Sul.