Potential acid sulfate soil

1
Potential acid sulfate soil

• Soils which become acid when drained due to
  oxidation of pyrite (FeS2)
• 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 3.5

2
Formation of pyrite

• Fe2O3 4SO42- 8CH2O 1/2O2 2FeS2 8HCO3-
  4H2O
• Iron must be present
• Sulfur must be present
• Anaerobic condition must prevail to reduce SO42-
  Fe3
• Organic matter as energy source for the microbes
• The process increases pH

3
Location of pyrite in the landscape

• In delta regions and lagunes where 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

4
Fluvisols and gleysols
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Histosols
6
Oxidation of pyrite

• If the soil is drained pyrite will be oxidized
• 4FeS2 15O2 H2O -gt 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

7
Oxidation of pyrite might forma sulfuric horizon

• Definition of sulfuric horizon
• A sulfuric horizon must
• have a soil-pH lt 3.5 (in 11 water suspension)
  and
• have
• yellow/orange jarosite KFe3(SO4)2(OH)6 or
  yellowish-brown schwertmannite Fe16O16(SO4)3(OH)1
  0.10H2O mottles or
• concretions and/or mottles with a Munsell hue of
  2.5Y or more and a chroma of 6 or more or
• underlying sulfidic soil materials or
• 0.05 percent (by weight) or more of water-soluble
  sulphate and
• have a thickness of 15 cm or more.

8
Agriculture problemsactual acid sufate soils

• Low soil pH
• Aluminium toxidity
• Salinity (from sea water)
• Phosphorous deficiency (precipitation of
  aluminiumphosphates)
• H2S toxidity if flooded
• N-deficiency due to slow microbial activity
• Ingeneering problems as soil acidity attacks
  steel and concrete structures

9
Environmental problemsOchre polution of Danish
watercourses

• Severe ochre polution of Danish streams has
  frequently occured due to drainage of farmland.
• The ochre polution was believed to be due to
  oxidation of pyrite.
• In order to prevent ochre polution 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 polutions of the streams.

10
Normal stream
11
Ochre from drains
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Ochre poluted streams
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Sampling area
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Camp site and equipment for mapping potential
acid sulfate soils
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Sampling area
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Travelling to sampling site
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Augering in wetland
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Samples
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Soil description scheme
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Determination of colour and pH
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Potential acidityanalytical results for lime
free samples

• A sample is potential acid sulfate if
• pH drop below 3.0 within 16 weeks of oxidation
  and
• pH drops more than one unit within that period

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Potential aciditylime containing
samplesPotential acid sulfate if pyrite x
34 meq/100g gt (Ca Mg) meq/100g
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Potential acid sulfate soil classes

• Class 1 gt 50 acid sulfate soil profiles
• Class 2 20-50 acid sulfate soil profiles
• Class 3 2-20 acid sulfate soil profiles
• Class 4 lt2 acid sulfate soil profiles
• An acid sulfate soil profile is a profile
  containing at least one acid sulfate soil sample

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Map showing potential acid sulfate soils
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Red 50-100Yellow 20-50Green 20-2Blue
lt2Potential acidsulfate soil
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Area statistics
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Ochre investigation areasif the farmer wants to
drain