Sombric-like horizon and xanthization in polychrome subtropical soils from Southern Brazil: implications for soil classification

Abstract

The occurrence of dark subsurface horizons rich in organic matter (OM) associated with polychrome in the B horizon (yellowish over reddish hue) is common in soils from Southern Brazil. The formation of these horizons and the combination with such morphological attributes has not been properly documented, and neither has the cause effect relationship. Four soil profiles with such sombric-like horizons with a yellowish color at the upper part of the B horizon over red subsoil were studied in Southern Brazil. Results from micromorphology, extractable sesquioxide minerals, clay mineralogy and isomorphic substitution of Fe by Al in iron minerals showed that melanization, xanthization, bioturbation, moderate shrinking/swelling and moderate ferralitization were the most evident pedogenetic processes in role. Xanthization is closely related to the sombric-like horizon formation. In the studied area the findings demonstrated that no clay and OM illuviation had taken place. Therefore, the classification of these soils was revisited, so as to take into account the processes that underlie their genesis with emphasis on xanthization, clay illuviation and soil aggregation. The results suggest that the sombric horizon may need redefinition, unless profiles can be found in which illuviation of clay and/or OM can be proven.

Long and short-term pedogenetic processes in ferralsols from magmatic province of Paraná, Brazil

“…In addition, the weight of soil column, the increase of kaolinite content and the heritage of altered rock structure in the deeper horizons with few swell-shrinking cycles in alternating dry and humid periods, contribute to the formation of larger and denser structures, because kaolinite tends to form crystallite packaging in inter and intra-aggregates (Moniz, 1980). The lower activity of fauna and roots in these deeper soil horizons may also contribute to the development of a denser structure (Macedo et al, 2017;Chiapini et al, 2021). Associated with the increase in structure and density beyond the control section, waxy and shiny peds were observed in the field.…”