Liming Three Plots With Different Soil pH
A farmer in Idukki has three plots and measures the soil pH with a school pH meter: Plot A 4.8 under cardamom, Plot B 8.3 after heavy liming last season, and Plot C 6.4 under pepper. The farmer has three materials available: lime (CaCO3), slaked lime (Ca(OH)2) and gypsum (CaSO4·2H2O). A local supplier advises applying lime to all three plots because ‘calcium is always good for soil’. The farmer also reads that acid rain from a nearby industrial area has pH 4.5 and may be affecting Plot A. He wants to know which material to use where, and whether more lime is always better. He plans to grow a crop that needs available phosphorus and micronutrients such as iron and zinc.
1. Recommend a material for each plot and justify your choice using the pH values.
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Plot A (pH 4.8, acidic): apply lime (CaCO3) or slaked lime (Ca(OH)2). Both neutralise acidity: CaCO3 + 2H+ → Ca2+ + H2O + CO2; Ca(OH)2 + 2H+ → Ca2+ + 2H2O. Lime is safer and slower; slaked lime is faster but easy to over-apply. Plot B (pH 8.3, alkaline): do not add lime. Gypsum (CaSO4·2H2O) is suitable if calcium is needed, because it is only slightly soluble and does not raise pH much; it also supplies calcium and sulfur without worsening alkalinity. Plot C (pH 6.4, near neutral): no liming needed; adding lime could push pH above the optimum and reduce micronutrient availability. The supplier's blanket advice is wrong.
2. Explain why over-liming can reduce crop yield even though calcium is a plant nutrient.
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Over-liming raises pH above about 7.5. At high pH, iron, manganese, zinc and copper become less soluble, so plants cannot absorb enough of them; phosphorus is also fixed as insoluble calcium phosphates. Soil structure may deteriorate and microbial balance may shift. Calcium is needed, but it is a secondary nutrient and the amount required is modest; excess Ca2+ is not beneficial. The farmer's aim is to bring pH to the crop's optimum (roughly 5.5–6.5 for many Kerala crops), not to maximise calcium. Plot B at pH 8.3 already shows the risk, so more lime would be harmful.
3. Acid rain of pH 4.5 falls on Plot A. How does this change your recommendation, and what is the limitation of lime as a remedy?
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Acid rain adds H+ and also sulfate and nitrate, so it will continue to acidify Plot A and may leach base cations. Lime is still appropriate to neutralise the existing acidity, but it is a treatment, not a cure: if acid deposition continues, repeated liming may be needed and may cause imbalances. The farmer should also reduce the source of acid rain if possible, add organic matter to buffer pH, and monitor pH rather than adding lime blindly. Lime does not remove sulfate or nitrate pollution; it only raises pH and supplies calcium. The recommendation for Plot A becomes ‘apply lime based on a soil test, then recheck pH’, not ‘add lime once and forget’.