Potential acid sulfate soil impact on agriculture and environment Soils which become acid when drained due to oxidation of pyrite (FeS 2 ) WRB Potential acid sulfate soil contains sulfidic soil material that contains pyrite but has not oxidized to an extent that the soil-pH dropped to a value below 4.0
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Potential acid sulfate soilimpact on agriculture and environment
Soils which become acid when drained due to oxidation of pyrite (FeS2)
WRBPotential acid sulfate soil contains sulfidic
soil material that contains pyrite but has not oxidized to an extent that the soil-pH dropped to a value below 4.0
Formation of pyrite
Fe2O3 + 4SO42- + 8CH2O + 1/2O2 = 2FeS2 + 8HCO3
- + 4H2O
Iron must be presentSulfur must be presentAnaerobic condition must prevail to reduce SO4
2- & Fe3+
Organic matter as energy source for the microbes
The process increases pH
Location of pyrite in the landscape
In delta regions, marshes and laguneswhere sea water is meeting fresh water.
Inland wetland areas which are enriched with ferro iron and sulfate from higher parts of the landscape
Soil material with high content of pyrite is called sulfidic soil materials
Fluvisols and gleysols
Histosols
Oxidation of pyrite
If the soil is drained pyrite will be oxidized:
4FeS2 + 15O2 + H2O -> 2 Fe2(SO4)3 + 2H2SO4
pH drops significantly and not only ferro iron but also ferri iron will be mobile.
Soils which become very acid due to oxidation of pyrite are classified as actual acid sulfate soils
Oxidation of pyrite might forma thionic horizon
• Definition of thionic horizon
• A thionic horizon must:• have a soil-pH < 4.0 (in 1:1 water suspension); and • have
– yellow/orange jarosite [KFe3(SO4)2(OH)6] or yellowish-brown schwertmannite [Fe16O16(SO4)3(OH)10.10H2O] mottles; or
– concretions and/or mottles with a Munsell hue of 2.5Y or more and a chroma of 6 or more; or
– Direct superposition on sulfidic soil materials; or– 0.05 percent (by mass) or more of water-soluble sulphate; and