Soil genesis on the soft slopes of ancient coastal plains, southeastern Brazil

Abstract

Coastal plains are characterized by low-elevation and small altitude variation and occupy an area of 57 million km2 worldwide. Ancient coastal plains that occur in the State of Rio de Janeiro (RJ), Brazil, have their pedological diversity neglected due to the generalization of some soil classes, underestimating the diversity of soils in these regions, hence limiting the evaluation for use and management. The aim of this study was to characterize the diversity of soils in a toposequence along an ancient coastal plain and to understand the pedogenetic processes involved. For this, soil pits were opened in the lower third (P1), middle third (P2), upper third (P3) and at the top (P4) of a toposequence. Soil profiles were characterized for morphological, physical, chemical, mineralogical and micromorphological properties. Profile P1 was classified as Planosol, being formed from colluvial sediments. In this profile, a lithological discontinuity was identified based on the irregularity of the values of silt/clay and coarse sand/fine sand ratios in subsurface. In this profile, limited drainage conditions were also observed. The main formation processes identified were ferrolysis and clay eluviation/illuviation, which promoted the increase of the textural gradient and thickening of the E horizon. Profiles P2 and P3 were classified as Ferralsols, were kaolinitic, with no redox conditions, and with predominance of eluviation/illuviation processes associated with pedoturbation, with a small increment of clay in the subsurface in both profiles. In profile P2, more so than in P3, there was still a strong influence of colluvial sediments, identified by the presence of a lithological discontinuity. For profile P4, a Plinthosol, located at the top of the toposequence, its formation was related to the action of weathering on the underlying acidic granite-gneiss rock. Despite the presence of plinthite, which denoted the occurrence of plinthization, the micromorphological description made it possible to state that it was a relictual condition whose formation occurred under past climatic conditions with higher water table level and which is currently being destroyed.