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

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

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

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 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.

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

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.

Normal stream

Ochre from drains

Ochre poluted streams

Sampling area

Camp site and equipment for mapping potential acid sulfate soils

Sampling area

Travelling 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 and

pH 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