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Potential acid sulfate soil

Potential acid sulfate soil. 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 3.5. Formation of pyrite.

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Potential acid sulfate soil

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  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

  5. Histosols

  6. 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 actualacid sulfate soils

  7. Oxidation of pyrite might forma sulfuric horizon • Definition of sulfuric horizon • A sulfuric horizon must: • have a soil-pH < 3.5 (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 • 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

  12. Ochre poluted streams

  13. Sampling area

  14. Camp site and equipment for mapping potential acid sulfate soils

  15. Sampling area

  16. Travelling to sampling site

  17. Augering in wetland

  18. Samples

  19. Soil description scheme

  20. Determination of colour and pH

  21. 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

  22. Potential aciditylime containing samplesPotential acid sulfate if: %pyrite x 34 meq/100g > (Ca + Mg) meq/100g

  23. 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

  24. Map showing potential acid sulfate soils

  25. Red 50%-100%Yellow 20%-50%Green 20%-2%Blue: <2%Potential acidsulfate soil

  26. Area statistics

  27. Ochre investigation areasif the farmer wants to drain

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