Acid sulfate soils form in coastal areas that are inundated by seawater containing sulfates. When these soils are drained and aerated, the iron sulfides they contain oxidize, producing sulfuric acid that lowers the soil pH below 4. This makes the soil toxic to plants. Such soils are common in coastal regions of Australia, Southeast Asia, and parts of India like Kerala. Maintaining flooded conditions prevents oxidation, while liming and leaching can remediate acidified soils over many years. Precise management depends on the extent of acidification and thickness of sulfidic layers.

1. ACID SULPHATE SOIL

2. WHAT IS ACIDSULPHATESOIL
Definition:
Soils containing high sulphides (FeS2) that
become strongly acidic (pH < 4.0) when drained and
aerated enough for cultivation are called acid
sulphate soils . These soils are also called as “cat
clay”
(cat clay : from Dutch word Katteklei)

3. Occurrence
Acid sulphate soils are mostly develop in
Inundated lands (costal sediments) covered by salt
water (sea / ocean) that contains appreciable amount
of pyrites(FeS2).
They occur in many regions of the world , mostly along
costal areas where the land is inundated by sea water .
countries having extensive areas of acid sulphate soils
are:

4. World distribution and extent of AASS and PASS

5. World distribution and extent of AASS and PASS
Region Area (m ha)
World 17.1
Australia 3
Newzealand 1.8

6. Cont..
In India, acid sulphate soil are mostly
encountered in certain parts of west coast of India
(Kerala).
Murthy (2010) reported three types of these lands
namely ;
1. Kari soil :Occupying about 80,000 ha mostly in
kuttanad (the rice bowl of kerala)
2. Pokhali saline acid sulphate soil : 26,400 ha
3. Swamp acid sulphate soil : 2,500 ha in kerala.

7. Condition favoring formation of acid sulphate soils

8. Condition favouring formation of acid sulphate
soils
Formation of pyrite can take place during
sedimentation in a marine environment if the fallowing
conditions are met:
1. Iron must be present. most costal sediments
contain easily reducible iron oxides or hydroxides.
2. Sulfur must be present. Sea water and brackish
water contain sulfates.

9. Cont..
3. Anaerobic conditions must prevail to allow reduction
of sulfate and iron oxides. this condition met by
inundation / submergence
4. Iron and sulfate – reducing microbes must be
present ; these occur in all costal sediments
5.Organic matter is needed as a source of energy for
the microbes ; it is present in abundance where
there is lush pallustric vegetation (e.g a mangrove
forest, reeds or sedges)

10. Cont..
6. Tidal flushing must strong enough to remove the
alkalinity formed in the process of pyrite formation.
7. Sedimentation must be slow . Otherwise , the time
will be too short to form sufficient pyrite for
potentially acid sediments.

11. Mechanism of pyrite formation
The mechanism of pyrite formation is essentially
as follows:
o When the sea rise and inundation the land ,
sulfate in the sea water mixed with land
sediments containing iron oxide and organic
matter
o Under anaerobic condition , lithotropic bacteria
such as Desulforvibrio desulfuricans reduces ferric
(Fe3+) iron to ferrous (Fe2+) ions ,and sulfate(So42- )
to sulfide (S2
- )

12. Cont..
o Organic matter is decomposed in the process,
ultimately to bicarbonates.
o Thus the potential acid compound (pyrite) and
alkali compounds (bicarbonates) are formed in
an initially neutral system.
o Tidal flushing removes the alkalinity (HCO3
- )
and potentially acid pyrite remains behind.

13. Pyrite is formed in a number of steps , but over all
reaction equation reads
Fe2O3 + 4SO42- + 7 CH2O + ½ O2 → FeS2 + 8HCO3- + 4H2O
The drainage and aeration of soil rich in sulphides particularly
pyrites (FeS2) bring oxygen into these previously waterlogged
soil. The accumulated sulphides (FeS2) and oxidized to
sulphuric acid by a combination of chemical and bacterial
activity, which strongly acidify these soils.
sediment seawater Organic matter Oxygen pyrite bicarbonate water

14. Oxidation of pyrites / sulphides
The oxidation of mineral sulphides (FeS2) is slow and
mostly non bacterial until the soil pH reaches 4.0.But,
below pH 4.0, the bacteria Thiobacillus ferroxidans
are most active oxidizers and the acidity builds up
rapidly.

15. Cont..
I. Non – biological :
2Fes2 + 2H2O + 7 O2 2 FeSO4 + 2H2SO4
II. Biological(accelerated by bacteria at pH below 4.0)
4FeSO4 + 2H2SO4 + O2 2 Fe(SO4)3 + 2H2O
III. Non-biological (rapid in acid pH)
FeS2+ 7 Fe2(SO4)3 + 8H2O 15 FeSO4 + 8H2SO4
Iron sulphide Ferrous sulphate
Ferrous sulphate Ferric sulphate
Thiobacillus ferroxidans
Further oxidation
Ferrous sulphate

16. Properties of acid sulphate soil
Acid sulphate soil contain a sulphuric horizon
which has a pH below of 3.5 in 1:1 soil-water extract
and a high sulphide characterized by bright yellow or
Straw- coloured mottles of the mineral Jerosite.
KFe3(SO4)2(OH)6

17. Problems of AAS & Harmful effects on plant
growth
Acid sulphate soils pose Chemical, Physical, Biological
problems to crops
A. Chemical constraints
I) When submerged
1. Toxicity of high levels of sulphides (high levels of
sulphides ie., H2S Often causes damage to roots and
affects absorption by plants. This rice disease is called
Akiochi or Brusone)
2. toxicity of Fe, Al, Mn, and possibly H ions

18. II) When aerated
1. Low pH (less than 3.5 to 4)
2. Toxicity of Fe and Al
B) The physical problems
1. Inhibit the root development in the acid sulphate
horizon
2. The soils are poorly structured (very low base like
Ca)
3. The soil remain soft and unable to bear heavy loads
4. Field drains /pipes may block due to iron oxide
deposits

19. c)Biological constraints
Unsuitable condition for most microorganisms, impede
the release of nutrients from the organic matter

20. Management and reclamation of AAS
Management techniques are extremely variable and
depend on many specific factors viz,
a. The extent of acid formation.
b. The thickness of the sulphide layer.
c. Possibilities of leaching or draining the land etc.

21. Management
1.Keep the area flooded
2. Control water table
3. Liming and leaching
1. Keep the area flooded
i. Maintaining the reduced (anaerobic). Soil inhibits acid
development.
ii. use of the area to rice growing.

22. 2. Control water table
keep only the sulphuric layer under water
(anaerobic) is possible.
3. Liming and leaching
AAS soils may require from several metric tons per
hectare per year, up to even 224 MT per hectare
(100/acre) within a 10-year period or less . after
leaching has to be done

23. REFERENCE
 Biswas, C.R. and A.K. Bandyopathyay. 1987. Agronomy of
rice in coastal saline soils – a review. Journal of Indian Society
of Coastal Agricultural Research.
 Biswas, T.D and S.K. Mukherjee, 2005. Textbook of Soil
Science: 2nd edition. Tata McGraw-Hill Publishing Company
Limited, New Delhi.
 Sehgal, J, 2005. A textbook of Pedology: Concepts and
applications: 2nd edition. Kalyani Publishers, Ludhiana.

24. Classification of acid sulphate soil
Characters Potential acid sulfate soils
(PASS)
Actual acid sulfate soils
(AASS)
1)Condition Contain iron sulfides but not
oxidized by exposing air
When PASS exposed to oxygen AASS
are form
2)Oxidation of iron
sulfides
Contain un-oxidized iron
sulfides
Contain oxidized iron sulfides
3)Occurrence Found in water logged soil Found in aerated / disturbed soil
condition
4)pH Close to neutral 6.5 to 7.5 Less than 4
5)Soil texture usually soft, sticky and
saturated with water
vary in texture.
6)Presence of Jerosite Not contain Jerosite contain Jerosite
It like a source for AASS It is by product of PASS by oxidation
process