Acidification and Deacidification of Coastal Plain Soils as a Result of Periodic Flooding1
- N. van Breemen2
Periodic flooding of recent coastal plain soils causes either acidification or deacidification of the surface horizons. Acidification of sulfate-bearing “nonacid” soils (pH > 5) involves formation of FeS and partial loss of alkalinity (HCO3-) in the flood water during the wet season, followed by oxidation of FeS to ferric oxide and sulfuric acid in the dry season. Deacidification of acid soils (pH < 4.5) takes place during flooding. This involves reduction of ferric oxides to Fe2+, the simultaneous release of adsorbed SO42- or hydrolysis of basic sulfates (that function as proton donor during iron reduction at low pH), followed by oxidation of dissolved FeSO4 to ferric oxide and sulfuric acid at the soil-water interface, and removal of H2SO4 by lateral surface drainage. During soil reduction the actual pH increases and a shift from one process to the other takes place if the pH becomes high enough (4.5 to 5) for the weak acids CO2 and H2S to function as proton donors. Therefore, deacidification beyond pH 4.5 to 5 automatically invokes the release of alkalinity. Conversely, acidification to a pH below 4.5 to 5 during a dry season will be undone by deacidification in the following wet season. This explains why the surface horizons of periodically flooded acid sulfate soils and nonacid marine soils eventually attain the same pH under dry conditions. This pH is fundamentally related to the dissociation constant, the solubility and the partial pressure of CO2.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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