Response of Crops to Soil Acidity

General Effect of Soil Acidity

• Acid soils = pH < 6.5.
• About 49 million ha (≈15% of India’s soil) are acid soils – largest problem soil area in India.
• Found mainly in high rainfall zones: NE states, Odisha, Jharkhand, Chhattisgarh, Kerala, Karnataka, West Bengal.
• Acidity affects crops mainly due to:
• Al³⁺ and Mn²⁺ toxicity.
• Ca, Mg, Mo deficiency.
• P fixation.
• Poor microbial activity (esp. N-fixing bacteria).

Crop Tolerance to Acidity; Crops show different levels of tolerance depending on their physiology and nutrient needs.

• a) Tolerant to Strong Acidity (pH 4.0–5.0) These crops can survive even in strongly acidic soils.

• Rice (especially under waterlogged conditions, because flooding reduces acidity).
• Tea (Camellia sinensis).
• Coffee (Coffea arabica).
• Pineapple.
• Sweet potato.
• Cassava (tapioca).
• Some fodder grasses.

b) Moderately Tolerant to Acidity (pH 5.0–6.0); These crops grow well in mildly acidic soils but not in strong acidity.

• Maize.
• Potato.
• Groundnut.
• Soybean.
• Tobacco.
• Millets (finger millet, foxtail millet).

c) Sensitive to Acidity (Require Near Neutral pH, 6.0–7.5); These crops cannot tolerate acidity → yields drop sharply if pH < 6.0.

• Wheat.
• Barley.
• Sugarcane.
• Cotton.
• Lucerne (Alfalfa).
• Pea, Lentil, Chickpea.
• Vegetables like cabbage, cauliflower, onion.

Legumes & Acidity; Legumes are very sensitive to acidity because: Rhizobium bacteria do not survive well in acidic soils. Nodulation and N fixation is hampered at pH < 6.0. Exception: Some pulses like pigeon pea and cowpea show moderate tolerance.

Special Notes

• Tea, coffee, pineapple → called acid-loving crops.
• Rice can tolerate acidic soils better because standing water reduces Al³⁺ solubility.
• Barley and alfalfa → among the most sensitive crops to acidity.
• Soil microorganisms (like nitrifiers) are suppressed in acidic soils → low availability of N.

Summary Table: Crop Response to Acidity

Crop Response

• Tea → most acid-loving crop (best at pH 4.5–5.5).
• Rice → tolerates acidity well because flooding lowers Al³⁺ toxicity.
• Legumes (pea, lentil, chickpea, lucerne) → highly sensitive; nodulation reduced at pH < 6.0.
• Pineapple, sweet potato, cassava, coffee → thrive in strongly acidic soils (pH 4.5–5.5).
• Wheat, barley, sugarcane, cotton → highly sensitive, need near neutral pH (6.5–7.5).
• Millets, potato, groundnut, soybean, tobacco → moderately tolerant (pH 5.0–6.0).

Microbiology & Nutrients

• Rhizobium activity drastically decreases below pH 6 → reason legumes fail in acid soils.
• Nitrification slows down at low pH → ammonium accumulates.
• Aluminium toxicity → major growth-limiting factor below pH 5.0 (root stunting).
• Manganese toxicity also occurs in acidic soils.
• Molybdenum deficiency is typical in acid soils → important for legumes.

Exam Catchy Points

• Acid-tolerant crops: rice, tea, coffee, pineapple.
• Acid-moderately tolerant crops: maize, potato, groundnut, soybean.
• Acid-sensitive crops: wheat, barley, sugarcane, cotton, legumes.
• Tea estates are always located in acidic soils → reason why NE India & South India are tea belts.
• Flooding of acid soils (as in rice) increases soil pH slightly, reducing acidity effect.

Important Points

• “Which crop is most tolerant to soil acidity?” → Tea.
• “Why are legumes sensitive to acidic soils?” → Poor nodulation due to Rhizobium sensitivity.
• “Rice grows well in acid soils because?” → Flooding reduces Al toxicity.