Lime or Gypsum? Correcting an Acidic Tea-Garden Plot
A tea estate near Sonitpur sends three soil samples from a 2-hectare section to a school laboratory. The section has patches of poor bush growth. Results: soil pH 4.6; the stream at the foot of the garden pH 6.8; the estate store has 500 kg of slaked lime, Ca(OH)2, and 1,000 kg of gypsum, CaSO4·2H2O. The manager wants to raise soil pH to about 5.5 before the next flush. An extension officer recommends burnt lime (CaO) because 'it is stronger and cheaper per bag'. A worker suggests spreading all 500 kg of slaked lime in one afternoon. A second worker says gypsum is also a calcium source, so it will do the same job. The estate has no lime-requirement test kit, only a pH meter and universal indicator paper. The stream is used downstream for bathing.
1. Using the pH figures, classify the soil and stream, and explain why gypsum is a poor choice for correcting soil pH even though it supplies calcium.
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Classification: soil pH 4.6 is distinctly acidic; the stream at pH 6.8 is slightly acidic to near-neutral and is not the immediate problem.
Why gypsum is poor for pH: gypsum is calcium sulphate. The sulphate ion is the conjugate base of a strong acid (sulphuric acid), so it does not hydrolyse appreciably to produce hydroxide ions. Dissolving gypsum adds Ca²⁺ and SO₄²⁻ but removes very little H⁺, so the pH barely rises. Its calcium can displace aluminium on soil exchange sites and reduce aluminium toxicity in deeper layers, which is useful, but that is not the same as neutralising topsoil acidity. For raising pH, a base is needed: CaO forms Ca(OH)₂, and Ca(OH)₂ supplies OH⁻ that reacts with H⁺.
So the second worker has confused 'calcium supply' with 'liming'. Gypsum may still be justified for calcium nutrition or subsoil aluminium, but not as the main pH corrective here.
2. Compare 1 kg of CaO with 1 kg of Ca(OH)2 for neutralising capacity, and state one practical reason the estate might still prefer slaked lime.
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Per kilogram:
- CaO: molar mass 56 g/mol, so 1 kg = 1000/56 ≈ 17.9 mol. One CaO gives one Ca(OH)₂, which supplies 2 OH⁻ equivalents, so about 35.7 mol OH⁻ equivalents.
- Ca(OH)₂: molar mass 74 g/mol, so 1 kg = 1000/74 ≈ 13.5 mol, giving 2 × 13.5 = 27.0 mol OH⁻ equivalents.
CaO therefore gives roughly 35.7/27.0 ≈ 1.32 times the neutralising capacity per kilogram - about 32% more. That supports the extension officer's chemistry, but not necessarily the recommendation.
Practical reason to prefer slaked lime: CaO is caustic and reacts violently with water, generating heat that can burn skin, eyes and roots; it must be slaked carefully and stored dry. Ca(OH)₂ is already slaked, safer to handle and easier to spread evenly. Safety and even application can outweigh the higher mass efficiency of CaO, especially on a small estate without protective equipment.
3. The worker wants to spread 500 kg of slaked lime at once. Without a lime-requirement test, what can and cannot be concluded from pH 4.6 alone? Give a defensible recommendation.
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What pH 4.6 tells you: the soil is acidic and below the target of 5.5, so some liming is justified.
What it cannot tell you: the amount needed. Lime requirement depends on soil texture, clay and organic-matter content, cation-exchange capacity, the buffer capacity of the soil, and the depth of incorporation. pH measures intensity, not quantity. 500 kg over 2 ha is only 250 kg/ha, which could be too little for a heavy clay or too much for a sandy soil; the calculation cannot be made from pH alone. Applying all of it at once also risks local over-liming, raising pH above 6.5 and inducing iron, zinc or manganese deficiency.
Defensible recommendation: apply in split doses, incorporate into the topsoil, and retest pH after several weeks; use small trial strips first. Keep a vegetated buffer strip beside the stream and do not spread lime near the bank or before heavy rain, because soluble hydroxide can wash into the stream and raise its pH, harming fish. A proper lime-requirement test should be arranged before the full quantity is committed.