🧫 Acid Soils

Acid soils impose major constraints on crop growth through nutrient imbalance and metal toxicities, especially under high-rainfall conditions. This lesson explains their characteristics, causes, and distribution patterns.

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Definition and Characteristics

Acid soils are soils with a pH below 5.5, where hydrogen (H+) and aluminum (Al3+) ions dominate the exchange complex. At pH values below 5.5, the solubility of aluminum and manganese increases sharply, reaching toxic concentrations for most crop plants.

Classification by Acidity Level

Category	pH Range
Slightly acid	6.0–6.5
Moderately acid	5.5–6.0
Strongly acid	4.5–5.5
Very strongly acid	< 4.5

Types of Soil Acidity

1. Active acidity — due to H+ ions in the soil solution; directly measured by pH meter
2. Exchange acidity — due to H+ and Al3+ ions adsorbed on the exchange complex; released when displaced by a neutral salt (KCl)
3. Residual acidity — acidity associated with non-exchangeable hydrogen bound to organic matter and clay lattice edges; the largest component of total acidity

Total acidity = Active acidity + Exchange acidity + Residual acidity

The exchange and residual acidity together constitute the reserve acidity, which buffers the soil against pH changes and is much larger than active acidity.

Causes of Soil Acidity

• High rainfall and leaching — basic cations (Ca2+, Mg2+, K+, Na+) are leached from the soil profile and replaced by H+ and Al3+ ions
• Parent material — siliceous and acidic rocks (granite, gneiss, sandstone) produce acid soils upon weathering
• Organic matter decomposition — produces organic acids (fulvic acid, oxalic acid, citric acid) that contribute to acidity
• Nitrification — oxidation of NH4+ to NO3- by soil bacteria releases H+ ions (each mole of NH4+ produces 2 moles of H+)
• Acid rain — deposition of sulfuric acid and nitric acid from atmospheric pollution
• Continuous use of acid-forming fertilizers — ammonium sulfate, urea, and DAP leave acidic residues in soil

Aluminum and Manganese Toxicity

Aluminum Toxicity

At pH below 5.5, aluminum dissolves from clay minerals and enters soil solution as Al3+, Al(OH)2+, and Al(OH)2+ species. Aluminum toxicity is the primary growth-limiting factor in acid soils worldwide.

Effects:

• Inhibits root elongation and causes stubby, thickened roots with reduced lateral branching
• Reduces calcium and magnesium uptake by competing for cation exchange sites on root surfaces
• Precipitates phosphorus in the rhizosphere, reducing P availability
• Interferes with cell division and DNA replication

Manganese Toxicity

At pH below 5.0, manganese solubility increases dramatically. Excess Mn2+ causes brown spots (necrotic lesions) on leaves, chlorosis, and reduced photosynthesis. Manganese toxicity is particularly severe in waterlogged acid soils where reducing conditions further increase Mn solubility.

Distribution in India

Acid soils cover approximately 49 million hectares (about 30% of cultivated area) in India:

• Northeast India — Assam, Meghalaya, Manipur, Mizoram, Nagaland, Tripura, Arunachal Pradesh (pH 4.0–5.5)
• Eastern India — Jharkhand, Chhattisgarh, parts of Odisha and West Bengal
• Western Ghats — Kerala, Karnataka, parts of Goa and Maharashtra (laterite soils)
• Sub-Himalayan region — parts of Uttarakhand, Himachal Pradesh, and Sikkim
• Andaman and Nicobar Islands

These regions typically receive high rainfall (>1500 mm/year), which drives the leaching of basic cations and progressive soil acidification.

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Summary Cheat Sheet

Key Recall Points

• Acid soils are defined by low pH and frequent Al or Mn toxicity risk.
• Leaching of base cations under high rainfall accelerates acidification.
• Site-specific liming and nutrient correction improve crop performance.

Exam Traps

• Not all low-yield acid soils are primarily phosphorus-deficient; toxicity may dominate.
• Acidity diagnosis must include pH, exchange acidity, and crop symptoms.
• Blanket liming rates can under- or over-correct different fields.

References

3 sources • [1] [2] [3]

[1]

ICAR and SAU Learning Materials on Acid Soils of India

Official

[2]

Soil Fertility References on Acidity and Metal Toxicity

Book

[3]

BSc Agriculture Notes on Acid Soil Distribution and Causes

Book