Farmer DVN shares his knowledge about using lime in agriculture, hoping to help coffee growers apply lime more effectively. In practice—and as discussed on the Y5Cafe forum—many farmers still misunderstand the basic role of lime and often use it incorrectly.
What Is Lime?
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In Vietnamese agriculture, “lime” refers to calcium hydroxide, Ca(OH)₂ (slaked lime or powdered lime). Chemically, “vôi” may also mean calcium oxide (CaO), but the form used on farmland is Ca(OH)₂.
Benefits of Lime
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Supplies calcium for crops. Calcium is an essential secondary nutrient. Besides lime, farmers can also use calcium nitrate [Ca(NO₃)₂] or fused phosphate. Avoid using ground limestone or seashell powder (CaCO₃) or gypsum (CaSO₄·2H₂O) as a calcium source; these are poorly soluble and may even harm crops.
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Neutralizes soil acidity. Acidic soils have a pH below 7. Most agricultural soils are acidic. For coffee, an ideal pH is around 5.5–6.5. When pH drops below the optimal range, lime is the cheapest and most effective material to correct acidity.
Potential Hazards of Lime
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Soil hardening. Excessive use of sulphate fertilizers can create surplus sulfuric acid (H₂SO₄), which acidifies soil. Farmers often add lime to neutralize it, producing gypsum (CaSO₄) through the reaction Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O. Over time, this can cause soil compaction and root restriction.
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Kills beneficial microbes. Lime can destroy many helpful soil organisms. You may notice earthworms dying immediately after lime application.
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Loss of nutrients.
– Lime reacts with nitrogen fertilizers, causing nitrogen loss.
– It can also bind with phosphate (P₂O₅), turning it into insoluble phosphate rock that plants cannot absorb. Most common inorganic fertilizers—urea, ammonium sulfate (SA), NPK blends, DAP, superphosphate—should not be mixed with lime.
– Organic fertilizers contain valuable humic acids (especially in manures and peat). These acids are water-soluble and highly beneficial, but when mixed with lime they form calcium humate, which is insoluble and unavailable to plants.
How to Use Lime Correctly?
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Apply lime only to counter soil acidity, not as a routine calcium supplement. Use other calcium sources (such as calcium nitrate or fused phosphate) when the goal is simply to add calcium.
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Apply separately from other fertilizers. Do not mix lime with any fertilizer. Apply lime at least 15 days after the final fertilizer application of the previous season and at least 15 days before the first fertilizer application of the new season (for coffee, a 30-day interval is even better).
These are the personal observations and practical recommendations of farmer DVN, offered for growers to consider when applying lime to coffee or other crops.