Andic properties in soils with histic horizon “O” in the highlands of Southern Brazil

ABSTRACT

Soils with andic properties are characterized by a low apparent density, variable charges, large amounts of allophanes, imogolite, ferrihydrite, and/or organo-metallic complexes with Al, and present high phosphate retention. Soils derived from non-pyroclastic materials rich in silicates, formed under a cold and humid climate, a large amount of organic carbon, acid weathering, andic properties can manifest when a large amount of Al is present in the form of organo-metallic complexes. This study aimed to evaluate the characteristics and geographical expression of soils with such properties in areas of altitude in the extreme south of Brazil, on the escarpment edges of the Serra Geral Formation, under a cold and humid climate. The sampling points were selected based on environmental characteristics such as geomorphology, geology, the coloration of the superficial horizon of the soil, and position in the landscape, covering a linear distance of approximately 185 km at the escarpment edge between the states of Santa Catarina and Rio Grande do Sul. Soil samples were described and collected from ten soils with histic horizon O, three soils with humic horizon, and one soil with histic horizon H. The undisturbed soil samples were collected using volumetric metallic cylinder to determine the soil bulk density. Organic matter, pH(H₂O), P retention, and selective dissolutions of Al, Fe, and Si analyses were performed. The Cambissolos Hísticos and Organossolos Fólicos showed andic properties, while the Cambissolos Húmicos and Organossolo Háplico did not meet one or more criteria, as required by the Brazilian Soil Classification System and the World Reference Base for Soil Resources. The horizons with andic properties were classified as aluandic, based on the predominance of Al associated with organic complexes. The cold climate and high cloudiness of the highest altitude areas in the extreme south of Brazil, occurring in a narrow strip of the escarpment of the Serra Geral Formation in the states of Santa Catarina and Rio Grande do Sul, allow the formation of a constantly humid environment. This environment favors the acid weathering of the source material, accumulation of organic matter in the soil, and its stabilization by the formation of organo-metallic complexes, mainly Al-humus. The combination of these factors gives the soils with histic horizons O a low bulk density, high phosphate retention, and Al_o + ½Fe_o ≥2 % values, meeting the criteria required for andic properties.

pedology; pedogenesis; organic matter; organo-metallic complexes