Acid soils are defined by a high concentration of hydrogen ions (H⁺) in the soil solution and on the cation exchange sites, leading to a low pH and reduced base saturation. This chemical imbalance can significantly influence nutrient availability and crop growth. The table below indicates the typical pH ranges and corresponding levels of acidity:
| pH range | Nature of acidity |
|---|---|
| 3–4 | Very strong acidity |
| 4–5 | Strong acidity |
| 5–6 | Moderate acidity |
| 6–7 | Slight acidity |
Formation and Occurrence
In Acid Soil Regions (ASR), the annual rainfall typically exceeds evapotranspiration, causing intense leaching that removes basic cations such as calcium, magnesium, potassium, and sodium. Over time, prolonged weathering of minerals results in the accumulation of hydrogen and aluminum ions, making the subsoil—and in some cases the entire soil profile—acidic. Acid soils cover nearly 60% of the Earth's land surface, forming primarily in humid climates from non-calcareous parent materials across tropical, subtropical, and temperate regions.
- Global distribution: ~800 million ha
- India: ~100 million ha
- Tamil Nadu: ~2.6 million ha (20% of the state's geographical area)
Examples of concentrated acid soil areas in India include: 95% of Assam's soils, 30% of Jammu and Kashmir, 2.2 million ha in West Bengal, 0.33 million ha in Himachal Pradesh, 2 million ha in Bihar, and most hill soils in the former Uttar Pradesh. Odisha (80%), Kerala (88%), Karnataka (45%), and Maharashtra (20%) have large acidic zones. Tamil Nadu’s lateritic regions and 40,000 ha in Andhra Pradesh are also affected.
Causes of Soil Acidity
- Intense leaching due to high rainfall
- Acidic parent rocks and alumino-silicate minerals
- Continuous application of acid-forming fertilizers such as ammonium sulfate
- Accumulation of humus and organic acids from decomposition
- Carbon dioxide production and hydrous oxide activity
- Acid rain caused by industrial emissions
Constraints to Productivity
Acid soils often pose multiple challenges to crop growth:
- Toxic levels of aluminum (Al), manganese (Mn), and iron (Fe) can damage roots
- Deficiencies in calcium (Ca) and magnesium (Mg)
- Reduced availability of essential nutrients like phosphorus (P) and molybdenum (Mo)
- Suppressed microbial activity and slower organic matter decomposition
Management Strategies
The objective is to improve crop productivity by either chemically amending the soil or adapting agronomic practices to the prevailing environmental conditions.
Soil Amelioration
Lime application is the most effective chemical amendment for acid soils. It reduces the toxicity of Al, Fe, and Mn, raises base saturation, and increases the availability of P and Mo. Liming also stimulates microbial activity, improving nitrogen fixation and organic matter mineralization. Large-scale liming should be preceded by cost–benefit analysis.
Common Liming Materials
- Commercial and dolomitic limestone
- Agricultural lime (carbonates, oxides, hydroxides of Ca and Mg)
- Natural deposits: calcitic, dolomitic, and stromatolitic limestone
- Alternative sources: marl, oyster shells, steel mill slag, blast furnace slag, lime sludge, sugar mill pressmud, cement waste, precipitated calcium carbonate
- Efficiency comparison: basic slag (110%), dolomite (94%) relative to pure limestone (100%)
- Fly ash as a supplementary amendment for pH correction and nutrient supply
Lime Requirement Determination
The amount of lime needed depends on the current pH, buffering capacity, and target pH. The Shoemaker, McLean, and Pratt (SMP) buffer method (1961) is widely used to estimate lime requirement.
Crop Selection
In areas where liming is economically unfeasible, cultivating crops that are naturally tolerant to acidic conditions offers a practical alternative. Plant breeding programs focusing on acid-tolerant varieties can substantially improve yields. Crops should be matched with soil pH categories to optimize productivity.
Relative tolerance of crops to soil acidity
| Crops | Optimum pH range |
|---|---|
| Cereals | |
| Maize, sorghum, wheat, barley | 6.0-7.5 |
| Millets | 5.0-6.5 |
| Rice | 4.0-6.0 |
| Oats | 5.0-7.7 |
| Legumes | |
| Field beans, soybean, pea, lentil etc. | 5.5-7.0 |
| Groundnut | 5.3-6.6 |
| Others | |
| Sugarcane | 6.0-7.5 |
| Cotton | 5.0-6.5 |
| Potato | 5.0-5.5 |
| Tea | 4.0-6.0 |
Acid Sulphate Soils
Definition
Acid sulphate soils are unique coastal wetland soils that become extremely acidic (pH < 4) due to the oxidation of pyritic minerals, particularly iron sulfides like pyrite (FeS2). These soils are typically formed in low-lying, waterlogged environments such as estuaries, mangroves, and tidal flats. When waterlogged and deprived of oxygen, these soils remain in a reduced state and are referred to as potential acid sulphate soils (PASS), which are not initially acidic. However, when exposed to oxygen through drainage, excavation, or other disturbances, the oxidation process triggers significant acidification.
Types of Acid Sulphate Soils
-
Potential Acid Sulphate Soils (PASS):
These soils have not yet been exposed to oxygen and generally maintain a near-neutral pH ranging between 6.5 and 7.5. They are saturated with water, often soft and sticky, and may exhibit a gel-like texture. PASS can include a variety of textures from clays to wet sands and gravels. The presence of unoxidized iron sulfides makes them chemically stable under waterlogged conditions, but once drained or aerated, they can generate large amounts of acid, leading to environmental hazards.
-
Actual Acid Sulphate Soils (AASS):
These develop when PASS are exposed to air, resulting in the oxidation of iron sulfides. This produces sulfuric acid, sharply lowering soil pH to values often below 4. AASS typically exhibit distinctive yellow mottles or coatings due to jarosite, a mineral indicator of oxidation. Their texture can vary from fine clays to coarse sands, but their extreme acidity can be harmful to plant growth and aquatic life.
Occurrence in India
Acid sulphate soils, also known as cat clays in parts of Europe, are found in coastal lowlands where seawater influence is significant. In India, they are most prevalent in Kerala’s Kuttanad region, the coastal belts of Odisha, Andhra Pradesh, Tamil Nadu, and the Sundarbans region of West Bengal. These areas are prone to tidal inundation, leading to the accumulation of sulfides. Though initially appearing neutral or slightly alkaline, these soils rapidly acidify upon drainage and aeration.
Formation Process
The formation of acid sulphate soils begins with prolonged submergence in sulfate-rich waters such as seawater. Sulfate-reducing bacteria, active in anaerobic conditions, convert sulfates into sulfides, which accumulate in the soil profile. When soils are drained or disturbed, oxygen penetrates the profile, oxidizing sulfides into sulfates and releasing sulfuric acid (H2SO4). The severity of acidification depends on factors such as the concentration of sulfides, rate of oxidation, soil texture, and climate. Iron pyrite oxidation can also lead to the release of iron oxides, further influencing soil chemistry.
Characteristics
- Presence of a sulfuric horizon with a pH less than 3.5 (1:1 soil-water ratio)
- Yellowish mottling due to jarosite formation
- High toxicity from soluble aluminum, iron, manganese, and hydrogen sulfide (H2S) gas
- In paddy fields, H2S toxicity can cause akiochi disease, stunting plant growth and limiting nutrient uptake.
Management Practices
Successful management and reclamation of acid sulphate soils require a detailed understanding of their depth, severity, and hydrology.
-
Continuous Flooding:
Maintaining waterlogged conditions prevents sulfide oxidation, keeping the soil in a reduced state. This is a common approach in rice cultivation, although prolonged drought can rapidly reverse these benefits.
-
Controlled Water Table Management:
If a non-acidic top layer exists above the sulfuric horizon, partial drainage can be used to avoid exposing the acidic layer to oxygen.
-
Liming and Leaching:
Applying lime (e.g., agricultural limestone) can neutralize acidity. Early leaching after oxidation can reduce lime requirements by flushing out acidic by-products, though high water tables and low permeability often complicate this process.
-
Use of Seawater for Preliminary Leaching:
In certain cases, seawater can be applied strategically to displace acidic water and dissolve salts before switching to freshwater leaching.
By combining these techniques and carefully monitoring soil and water chemistry, it is possible to reclaim and sustainably manage acid sulphate soils for agriculture and environmental restoration.
Acid Sulphate Soils
Definition
Acid sulphate soils are unique coastal wetland soils that become extremely acidic (pH < 4) due to the oxidation of pyritic minerals, particularly iron sulfides like pyrite (FeS2). These soils are typically formed in low-lying, waterlogged environments such as estuaries, mangroves, and tidal flats. When waterlogged and deprived of oxygen, these soils remain in a reduced state and are referred to as potential acid sulphate soils (PASS), which are not initially acidic. However, when exposed to oxygen through drainage, excavation, or other disturbances, the oxidation process triggers significant acidification.
Types of Acid Sulphate Soils
-
Potential Acid Sulphate Soils (PASS):
These soils have not yet been exposed to oxygen and generally maintain a near-neutral pH ranging between 6.5 and 7.5. They are saturated with water, often soft and sticky, and may exhibit a gel-like texture. PASS can include a variety of textures from clays to wet sands and gravels. The presence of unoxidized iron sulfides makes them chemically stable under waterlogged conditions, but once drained or aerated, they can generate large amounts of acid, leading to environmental hazards.
-
Actual Acid Sulphate Soils (AASS):
These develop when PASS are exposed to air, resulting in the oxidation of iron sulfides. This produces sulfuric acid, sharply lowering soil pH to values often below 4. AASS typically exhibit distinctive yellow mottles or coatings due to jarosite, a mineral indicator of oxidation. Their texture can vary from fine clays to coarse sands, but their extreme acidity can be harmful to plant growth and aquatic life.
Occurrence in India
Acid sulphate soils, also known as cat clays in parts of Europe, are found in coastal lowlands where seawater influence is significant. In India, they are most prevalent in Kerala’s Kuttanad region, the coastal belts of Odisha, Andhra Pradesh, Tamil Nadu, and the Sundarbans region of West Bengal. These areas are prone to tidal inundation, leading to the accumulation of sulfides. Though initially appearing neutral or slightly alkaline, these soils rapidly acidify upon drainage and aeration.
Formation Process
The formation of acid sulphate soils begins with prolonged submergence in sulfate-rich waters such as seawater. Sulfate-reducing bacteria, active in anaerobic conditions, convert sulfates into sulfides, which accumulate in the soil profile. When soils are drained or disturbed, oxygen penetrates the profile, oxidizing sulfides into sulfates and releasing sulfuric acid (H2SO4). The severity of acidification depends on factors such as the concentration of sulfides, rate of oxidation, soil texture, and climate. Iron pyrite oxidation can also lead to the release of iron oxides, further influencing soil chemistry.
Characteristics
- Presence of a sulfuric horizon with a pH less than 3.5 (1:1 soil-water ratio)
- Yellowish mottling due to jarosite formation
- High toxicity from soluble aluminum, iron, manganese, and hydrogen sulfide (H2S) gas
- In paddy fields, H2S toxicity can cause akiochi disease, stunting plant growth and limiting nutrient uptake.
Management Practices
Successful management and reclamation of acid sulphate soils require a detailed understanding of their depth, severity, and hydrology.
-
Continuous Flooding:
Maintaining waterlogged conditions prevents sulfide oxidation, keeping the soil in a reduced state. This is a common approach in rice cultivation, although prolonged drought can rapidly reverse these benefits.
-
Controlled Water Table Management:
If a non-acidic top layer exists above the sulfuric horizon, partial drainage can be used to avoid exposing the acidic layer to oxygen.
-
Liming and Leaching:
Applying lime (e.g., agricultural limestone) can neutralize acidity. Early leaching after oxidation can reduce lime requirements by flushing out acidic by-products, though high water tables and low permeability often complicate this process.
-
Use of Seawater for Preliminary Leaching:
In certain cases, seawater can be applied strategically to displace acidic water and dissolve salts before switching to freshwater leaching.
By combining these techniques and carefully monitoring soil and water chemistry, it is possible to reclaim and sustainably manage acid sulphate soils for agriculture and environmental restoration.
