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Occurrence and pedogenesis of acid sulfate soils in northeastern Brazil

Abstract:

Coastal floodplain soils are characterized by high organic matter content, a strong marine influence and poor drainage. Under these conditions, the soils experience anoxic conditions, which lead to the formation of hypersulfidic materials (mainly pyrite). These soils, called potential acid sulfate soils (PASS), can develop high acidity when drained, transforming themselves into acid sulfate soils (ASS). This transformation usually causes serious environmental impacts. In this context, soil physical, chemical and mineralogical characterization was performed, including specific analyses such as total sulfur, incubation pH, soluble anions and electrical conductivity of the saturated paste extract, in the Camocim River floodplain (Paraíba State, Northeast Region, Brazil). The objective of the study was to identify the occurrence and properties of acid sulfate soils in order to guide management strategies towards transforming this area into a larger area of sugarcane cultivation or preserving it under natural conditions. The soils were classified as Dystric Gleysol (Loamic) in profile P1, Thionic Histosol (Fluvic) in profile P2 and Fluvic Thionic Gleysol (Clayic) in profiles P3 and P4. A thionic horizon was identified in the upper horizons of the poorly drained profiles of the floodplain. The ASS showed high electrical conductivity due to the formation of sulfuric acid from the oxidation of pyrite (high H+ and SO42− concentrations) but no actual marine influence (low Cl− concentration). The diagnosis of high acidity, high sulfur content, thionic horizons in the profiles, and the presence of jarosite suggest that the soils are in an advanced stage of oxidation of the sulfidic material, which must have special management to allow the cultivation of sugarcane. Regulating and raising the water level to maintain soil saturation and calculating the limestone required to neutralize the acidification associated with drainage and the subsequent oxidation of iron sulfides are some of the special management measures that must be adopted for these areas.