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Potential acid sulfate soil Soils which become acid when drained due to oxidation of pyrite (FeS 2 ) WRB Potential acid sulfate soil contains sulfidic.

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Presentation on theme: "Potential acid sulfate soil Soils which become acid when drained due to oxidation of pyrite (FeS 2 ) WRB Potential acid sulfate soil contains sulfidic."— Presentation transcript:

1 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

2 Formation of pyrite Fe 2 O 3 + 4SO CH 2 O + 1/2O 2 = 2FeS 2 + 8HCO H 2 O Iron must be present Sulfur must be present Anaerobic condition must prevail to reduce SO 4 2- & Fe 3+ 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: 4FeS O 2 + H 2 O -> 2 Fe 2 (SO 4 ) 3 + 2H 2 SO 4 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 form a 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 [KFe 3 (SO 4 ) 2 (OH) 6 ] or yellowish-brown schwertmannite [Fe 16 O 16 (SO 4 ) 3 (OH) 10.10H 2 O] 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 problems actual acid sufate soils Low soil pH Aluminium toxidity Salinity (from sea water) Phosphorous deficiency (precipitation of aluminiumphosphates) H 2 S toxidity if flooded N-deficiency due to slow microbial activity Ingeneering problems as soil acidity attacks steel and concrete structures

9 Environmental problems Ochre 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 acidity analytical 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 acidity lime containing samples Potential 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 acid sulfate soil

26 Area statistics

27 Ochre investigation areas if the farmer wants to drain


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