What are Acid Sulfate Soils and Why are They a Problem?

Acid sulfate soils are those soils in which sulfuric acid (H2SO4) may be produced, is being produced, or has been produced in amounts that have a lasting effect on the main soil characteristics (Prof. Leon Pons 1973).

Acid sulfate soils occur naturally in both coastal and inland settings where waterlogged conditions with sufficient organic matter, iron and sulfate is, or has previously been, present.

These conditions are suitable for the formation of reduced iron sulfides such as pyrite (FeS2) and iron monosulfide (FeS), the active "ingredient" in acid sulfate soils.

Left undisturbed, these soils are harmless, but when excavated or drained, the reduced iron sulfide minerals can oxidise and the soil can acidify (pH<4) due to the formation of sulfuric acid. Once acidified, acid sulfate soils pose a significant environmental risk, due to the low pH of the soil and leaching and transport of the acidity into groundwater or surface water systems.

The consequences following soil and water acidification are typically very severe and include:

  • Lethal and sub-lethal effects fish and other aquatic fauna
  • Release of dissolved metals and metalloids such as aluminium and arsenic impacting on water quality and water supplies
  • Plant death (loss of agricultural production, ecosystem damage)
  • Infrastructure damage (roads, bridges, houses)

Key biogeochemical reactions in acid sulfate soils are shown in the diagram below and are linked to drying (oxidation) and wetting (reduction) cycles. The redox boundary often determines the depth to which soil oxidation and acidification (to form sulfuric material) will occur with reduced, non-acidic soil (sulfidic material) below this boundary.

Secondary minerals such as Jarosite and Schwertmannite often form in the oxidised acidic layer, and in oxidised surface waters, and along with low pH are a useful visual indicator that acid sulfate soils are present.

  • Spelling of sulfate (not sulphate?)

    There has been some debate and protest about the spelling of the word ‘sulphur’ and the apparent trend to spell it as ‘sulfur’. The debate and protest is not confined to Australia and is going on in many countries.

    Much of the variation in spelling arises from scientific spelling as opposed to usage in day to day literature.  About 20 years ago, the International Union of Pure and Applied Chemistry (IUPAC), the equivalent of a ‘United Nations’ of world chemistry, and other scientific organisations adopted the ‘sulfur’ form.  For scientific correctness, most scientists have followed this form.  The Australian journals of science have also adopted this spelling.

    As outlined below in a segment quoted from Wikipedia, sulfur is a word of Latin origin and late Latin used the ‘f’ form of the word.  The ‘ph’ spelling to denote the ‘f’ sound is of Greek origin, hence the spelling of the Greek ‘phosphorus’.

    Really it boils down to choice, but it is likely that scientists in Australia will increasingly use ‘sulfur’ and this spelling will be taught in school science.   Non-scientists can probably use what spelling they like, but should accept that scientists will use the recommended form, if only because the journals they publish in follow the IUPAC decision.

    To quote from Wikipedia Sulfur Spelling and Etymology  :

    The element has traditionally been spelled sulphur in the United Kingdom, most of the Commonwealth including India, Malaysia, South Africa and Hong Kong, along with the rest of the Caribbean and Ireland, but sulfur in the United States, while both spellings are used in Australia, New Zealand and Canada. IUPAC adopted the spelling “sulfur” in 1990, as did the Royal Society of Chemistry Nomenclature Committee in 1992 and the Qualifications and Curriculum Authority for England and Wales recommended its use in 2000.

    In Latin, the word is variously written sulpur, sulphur, and sulfur (the Oxford Latin Dictionary lists the spellings in this order). It means brimstone. It is an original Latin name and not a Classical Greek loan, so the ph variant does not denote the Greek letter φ. Sulfur in Greek is thion (θείον), whence comes the prefix thio-. The simplification of the Latin word's p or ph to an f appears to have taken place towards the end of the classical period, with the f spelling becoming dominant in the medieval period.

    To quote from the Nature journal article 'so long sulphur':

    Language is our servant, not our master and it evolves to meet our needs. And in the case of sulfur, there seems to be no good reason to continue using the 'ph' form other than perhaps a mistaken sense of spelling jingoism.

  • Acid sulfate soil materials

    The following nomenclature and definitions used for acid sulfate soil materials is as defined by the 2nd edition of Australian Soil Classification (Isbell and National Committee on Soils & Terrain 2016):

    Sulfidic Material

    Hypersulfidic material: Sulfidic material that had a field pH of 4 or more and the pH dropped by at least 0.5 units to less than 4 when incubated at field capacity for at least 8 weeks.

    Hyposulfidic material: Sulfidic soil material that had a field pH of 4 or more and the pH dropped by at least 0.5 units to not less than 4 when incubated at field capacity for at least 8 weeks.

    Sulfuric Material

    Soil material that has a pH < 4 (1:1 by weight in water, or in a minimum of water to permit measurement) when measured as a result of the oxidation of sulfidic materials and evidence of sulfidic material such as underlying sulfidic material and/or the presence of yellow masses of jarosite along old root channels and faces of peds.


    Soil material containing ≥0.01% acid volatile sulfide.

    Isbell, R. F., National Committee on Soils and Terrain, 2016. The Australian Soil Classification, Second Edition. CSIRO Publishing, Clayton, Victoria, Australia