World Reference Base for Soil ResourcesMineral Soils conditioned by Parent MaterialMineral Soils conditioned by TopographyMineral Soils conditioned by a wet (sub) Tropical Climate

Thionic Fluvisols (acid sulfate soils)

Table of contents

  1. Introduction
  2. Parent material
  3. Regional Distribution
  4. Definition
  5. Genesis
    a. Formation of pyrite
    b. Pyrite oxidation: formation of active acid sulfate soils
    c. Pyrite oxidation in the presence of carbonate
  6. Spatial distribution of actual and potential acid sulfate soils
  7. Chemical characteristics of thionic Fluvisols
  8. Management and Use of thionic Fluvisols
1. Introduction

Fig.1 Active acid sulfate soil landscape
( Source: FAO, 2001.)

  • Thionic Fluvisols belong to the Reference Soil Group # 4, the Fluvisols. Further information on Fluvisols see ( chapter fluvisols) .
  • Other names for thionic Fluvisols are acid sulfate soils (ASS)
    a. Potential acid sulfate soils (PASS)
    b. Actual or active acid sulfate soils (AASS), sometimes named cat clays
2. Parent material and environment
  • The only difference between the parent material of thionic Fluvisols and that of other Fluvisols is the presence of pyrite (FeS2) in the former.
  • Thionic Fluvisols are commonly situated in coastal lowlands and are influenced by the following conditions:
    • Sea water: contains sulfur (sulfate)
    • Sediments: contain Fe-oxides
    • Organic material: comes from mangroves
    • Anaerobic conditions and sulfate reducing bacteria
3. Regional distribution

Fig.2 Fluvisols worldwide
( Source: FAO, 2001.)

  • Worldwide about 24 Mio. ha (~ 0.2 %) are found, mostly in SE Asia; often influenced by tide.
  • Thionic Fluvisols are found in the coastal lowlands of:
    a. SE Asien (Vietnam, Indonesia, Thailand)
    b. W-Africa: Senegal, Gambia, Sierra Leone
    c. NE-coast of S-America (Venezuela, Guyana)
4. Definition (see also Annex 3, WRB)

  • Having within 100 cm a sulfuric horizon or sulfidic soil material.
  • Signs: occurrence of light-yellow material (jarosite).

    Fig.3 Sulphuric horizon
    ( Source: FAO, 2001.)

5. Genesis
  • The majority of acid sulfate soils (ASS) were formed during the last 10.000 years (Holocene) by sea level rising.
  • A few ASS developed in old marine stones some million years ago.

5a. Formation of pyrite

Fig.4 Pyrite

  • The formation of pyrite (FeS2) can take place during sedimentation in a marine environment if the following conditions are met:
  1. Presence of Fe: most coastal sediments contain easily reducible iron oxides or hydroxides.
  2. Presence of S: seawater and brackish water contain sulfates.
  3. Anaerobic conditions : to allow reduction of sulfate and Fe-oxides -> is met in fresh coastal sediments.
  4. Fe and S-reducing microbes: occur in all coastal sediments.
  5. Organic matter: energy source for microbes and comes from mangroves, reeds or sedges.
  6. Tidal flushing: to remove the alkalinity formed in the process of pyrite formation.
  7. Slow sedimentation: to form sufficient pyrite for potentially acid sediment.

Mechanism of pyrite accumulation:

Fig.5 Diagrammatic representation of the process of pyrite formation (after Berner, 1983)

  1. Microorganisms reduce sulfate (SO42-) to sulfide (S2-) or H2S (hydrogen sulfide) under anaerobic conditions.
  2. During this process organic carbon (OC) is decomposed to form bicarbonate (HCO3-) ultimately (-> the electrons are needed for reduction)
  3. Microorganisms reduce Fe3+ (ferric iron) to Fe2+ (ferrous iron) and H2S reacts with Fe (via FeS) to form pyrite (FeS2).

    Fig.6 Reaction of Pyrite
    ( Source: FAO, 2001.)

  4. Bicarbonates (HCO3-, alkaline) are removed with tidal flushing, and therefore a potentially acid compound – pyrite – remains behind.


  • When no oxidizing conditions occur: Fe and S remain in their reduced form and the pH remains neutral -> these soils are named potential acid sulfate soils.
  • They are brown to dark grey, soft and have a spongy consistence.

    Fig.7 Potential acid sulfate soils
    ( Source: clarencecatchment/issue1.htm)

5b. Pyrite oxidation or transformation of potential acid sulfate soils (PASS) to actual acid sulfate soils (AASS)

  • When pyritic sediment falls dry, O2 penetrates and pyrite is oxidized to sulfuric acid (H2SO4) and mobile Fe (ferric hydroxide [Fe(OH3)]) -> actual acid sulfate soils are formed.

Fig.8 Oxidation of pyrite to sulfuric acid
( Source: Brady and Weil, 2002.)

  • Oxidation process occurs either chemically (slow process) or by microbial intervention through Thiobacillus ferrooxidans (accelerated process).
  • Within days a drop in pH below pH 3 occurs.
  • Per each ton of pyrite that is oxidized, 1.64 tons of sulfuric acid are produced.
  • Indication for the presence of potential acid sulfate soils:
    a. Aerobic incubation of the soil for some days leads to a drastic drop in pH if the soils contain pyrite.
    b. Presence of jarosite [KFe3(SO4)2(OH)6 ]6 indicates pyrite oxidation. Jarosite is an intermediate product (partially oxidized) in the process of pyrite oxidation and is formed under strongly oxidizing conditions at pH < 3.7. It has a typical straw-yellow color and a malodor of cat excrements (-> cat clays).

    Fig.9 Details of jarosite lining the pores in a sulphuric horizon
    ( Source: FAO, 2001.)

    Fig.10 Here: jarosite formation in in root channels where oxygen is present
    ( Source: FAO, 2001.)

5c. Pyrite oxidation in the presence of carbonate

  • In the presence of carbonates, for example shells in mangroves,

    Fig.11 Neutralising carbonate potential of oyster shells in mangroves
    ( Source: FAO, 2001.)

    no drop of pH occurs although sulfuric acid is produced.

    Fig.12 Buffering of acid in the presence of carbonates

6. Spatial distribution of actual (AASS) and potential acid sulfate soils (PASS)

[example: Mekong delta, Vietnam]

  • In the Mekong delta about 1 Mio ha of AASS and PASS are found in close vicinity to each other depending on conditions during formation and also management practices.
  • A sketch map of the Mekong delta shows that

    Fig.13 Map of the mekong delta, Vietnam
    ( Source: FAO, 2001.)

  • unit 4 contains thionic Fluvisols with pyritic sediments (lies in a depression), whereas in
  • unit 3 and 5 Fluvisols are found. In unit 3, due to fresh water deposits no S was present during sedimentation. In unit 5 the sedimentation rate was too high for the formation of pyrite.
  • The high micro-variability in the spatial distribution of AASS has a tremendous impact on agricultural management and rice production.
  • Often AASS and PASS change within 10-20 m due to micro-relief conditions and concomitant water level: while AASS occur on upper positions above 85 cm PASS are found at lower terrain positions (< 75 cm above sea level).

    Fig.14 Spatial occurrence of actual and potential acid sulfate soils of the Mekong Delta, Vietnam
    ( Source: Dent, Pons (1995).)

7. Chemical characteristics
  • A distinction must be made between PASS which are not yet oxidized but contain pyrite in the soil material and AASS. Here we talk about the unfavorable properties of AASS:
    1. Low pH
    2. Al-toxicity

      Fig.15 Al-toxicity
      ( Source: FAO, 2001.)

    3. P-deficiency: high Al causes precipitation of Al-Phosphate complexes
    4. Fe-toxicity: e.g. in flooded rice fields non-soluble Fe3+ is oxidized to soluble Fe2+ -> Fe toxicity may set in

      Fig.16 Iron toxicity in rice fields
      ( Source: FAO, 2001..)

    5. H2S-toxicity: when rice soils are flooded for long periods, sulfate is reduced to H2S -> very toxic to plants.
    6. Acidification of surface water in flooded rice: the oxidation of Fe2+ to amorphous Fe-hydroxide [Fe(OH)3] release of H-ions

      Fig.17 Acidification of surface water in flooded rice
      ( Source: FAO, 2001.)

    7. N-deficiency: the mineralization of soil organic matter is slow in wet, cold actual acid sulfate soils.
    8. Engineering problems through acid drainage water: sulfuric acid dissolves lime out of the concrete.

      Fig.18 The large quantity of acid released corrodes metal and concrete structures
      ( Source: FAO, 2001.)

      Fig.19 Acid drainage water


8. Land use and management
  • A problem in the management of PASS is that FeS or FeS2 gives the soil a black color. This often led to the erroneous assumption that abundant SOM is present.

    Fig.21 Black pyrite - containing sulphuric layer in the subsoil

    Often, drainage of the land was initiated for agricultural land use.

    ( Source: FAO, 2001.)

    However, the drainage of soils produced aerobic conditions: oxidation process set in and AASS were formed.
  • Similar problems occurred through reclamation and cultivation of peat soils: drainage of water led to oxidation of pyrite and release of sulfuric acid.

    Fig.23 Development of acid sulfate soils through reclamation of peat soils for a managment
    ( Source: Dent, Pons (1995).)

Reclamation and agricultural use of potential acid sulfate soils: possibilities.

Fig.24 Drainage of potential acid sulfate soils
( Source: FAO, 2001.)

  1. Drainage and complete oxidation of the soil -> flushing of the sulfuric acid with water. Solves the problem forever. This strategy was followed with some success in the Philippines and Sierra Leone but was disastrous in the Netherlands and Senegal (not enough water). Disadvantages of this method:
    a. Expensive
    b. Production of acid drainage water
    c. Nutrient (bases) leaching
  2. Gradual drainage produces less sulfuric acid. The produced acid can be:
    a. Washed out
    b. Buffered by soils
    c. Removed by tide (tidal flushing)
    • However, the acid production of drained floodplains can yield 100 – 500 kg sulfuric acid per hectare and year.
    • The gradual drainage is often performed in combination with raised beds and furrow systems.

      Fig.25 Pineapple plants on raised beds on potential acid sulfate soils
      ( Source: FAO, 2001.)

    • Important: fluctuating water levels through tide needs to be considered.
  3. A widely followed strategy: maintaining a high groundwater table, i.e. just above the sulfuric horizon . This, however, implies investment in water management.

    Fig.26 Raised beds on potential acid sulfate soils
    ( Source: FAO, 2001.)

  4. Another strategy is: rice production and shrimp breeding on a rotational basis. Rice is produced during the rainy season when soils are flooded with fresh water. During the dry season brackish water is let in for shrimp breeding so that sulfuric layer remains under water (anaerobic conditions).