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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
• have a thickness of 15 cm or more.
Jarosite in a soilfrom a marshland
Agriculture problemsactual acid sulfate soils
• Low soil pH• Aluminium toxidity• Salinity (from sea water)• Phosphorous deficiency (precipitation of
aluminium phosphates)• H2S toxidity if flooded• N-deficiency due to slow microbial activity• Engineering problems as soil acidity attacks
steel and concrete structures
Environmental problemsOchre pollution of watercourses
Severe ochre pollution of Danish streams has frequently occurred due to drainage of farmland.
The ochre pollution breaks down the ecosystems
The ochre pollution was believed to be due to oxidation of pyrite.
Normal stream
Ochre from drains
Composition and pH of ochrebased on 21 samples
• Fe2O3 30-70%• Al2O3 <10%• CaO <5%• MgO <2%• Mn <1% except few samples >10%• SO4 <5%• Organic C 5-20% • C/N 10-30 • Loss of ignition 30-60%• pH 3.5-7.0
Mapping of potentially acidsulphate soils
In order to prevent ochre pollution of the streams a mapping of potential acid soils was conducted
The mapping should be done within a 3 years period
Based on the mapping a legislation should be made to stop the ochre pollutions of the streams.
Sampling area
Camp site and equipment for mappingpotential acid sulfate soils
Sampling area
Traveling to sampling site
Augering in wetland
Samples
Soil description scheme
Determination of colour and pH
Potential acidityanalytical results for lime free samples
A sample is potential acid sulfate if:pH drop below 3.0 within 16 weeks of oxidation andpH drops more than one unit within that period
Potential aciditylime containing samples
Potential acid sulfate if: %pyrite x 34 meq/100g > (Ca + Mg) meq/100g
Potential acid sulfate soil classes
• Class 1: > 50% acid sulfate soil profiles• Class 2: 20-50% acid sulfate soil profiles• Class 3: 2-20% acid sulfate soil profiles• Class 4: <2% acid sulfate soil profiles
• An acid sulfate soil profile is a profile containing at least one acid sulfate soil sample
Map showing potential acid sulfate soils
Red 50%-100%Yellow 20%-50%
Green 20%-2%Blue: <2%
Potential acidsulfate soil
Area statistics
Ochre investigation areasif the farmer wants to drain
What to do?