The Farmer with pH 4.8 Soil and Three Bags of Lime
Anil, a farmer near a fertiliser factory in western Uttar Pradesh, tests his field before sowing wheat. The soil extract gives pH 4.8 on pH paper, while his tube-well water gives pH 7.8. Wheat grows best between pH 6.0 and 7.0. In his shed are three bags: quicklime (CaO), slaked lime (Ca(OH)2) and gypsum (CaSO4·2H2O). A neighbour advises spreading common salt, and a shopkeeper offers a bottle of dilute hydrochloric acid, saying 'acid neutralises acid'. Anil also plans to apply ammonium sulphate fertiliser on the same day. He has no gloves or boots, wears only slippers, and wants the cheapest treatment for his 0.5 hectare field; the lime label recommends 200 kg per hectare. He has one day before sowing and cannot afford a soil-testing laboratory.
1. Which of the three bags should Anil use, and why? Write the neutralisation reaction with the soil acid, state two precautions, and explain whether he should apply ammonium sulphate on the same day.
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He should use slaked lime, Ca(OH)2. It is a base, so it removes soil acidity, and it is far safer to handle than quicklime. Quicklime also neutralises acid, but only after reacting violently with water and releasing much heat (slaking), which can burn skin and eyes - a serious risk for a farmer in slippers with no gloves. Gypsum is a neutral salt; it supplies calcium and sulphur but does not raise pH.
Neutralisation: Ca(OH)2 + 2H+ → Ca2+ + 2H2O, where H+ represents soil acidity. If quicklime were used it would first form slaked lime: CaO + H2O → Ca(OH)2.
Precautions: (i) wear gloves, covered shoes and a mask, and keep lime away from eyes and skin; (ii) apply the calculated dose evenly and mix it into the soil, then re-test pH - do not heap it in one place. For 0.5 hectare at 200 kg/ha, about 100 kg is needed.
He should not apply ammonium sulphate the same day. Lime reacts with ammonium salts and releases ammonia: (NH4)2SO4 + Ca(OH)2 → CaSO4 + 2NH3↑ + 2H2O. The nitrogen escapes and the fertiliser is wasted. Lime first, wait several days, then apply the fertiliser.
2. Explain why dilute hydrochloric acid and common salt are both wrong choices. Why is excess lime also harmful even though it is a base?
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Dilute HCl is itself an acid. Adding it would push the pH below 4.8, making the soil more acidic and damaging germinating wheat. The shopkeeper's phrase is misleading: an acid neutralises a base, not another acid.
Common salt, NaCl, is a neutral salt. It supplies neither H+ to remove nor OH- to neutralise, so it cannot correct acidity at all. Worse, sodium ions disperse clay particles, make the soil hard and crusty, and reduce water movement; it is not a liming material.
Excess lime is harmful because the aim is pH 6.0-7.0, not the highest possible pH. Above about 7.5, iron, zinc and manganese become insoluble, so plants show yellowing (chlorosis); phosphate availability also falls and soil micro-organisms are disturbed. Lime must therefore be applied in measured amounts, mixed in, and followed by a pH re-test, not added until the soil 'looks treated'.
3. Anil says the soil is 'only 1.7 units below 6.5, so it is almost all right'. Using the pH scale, show why he is wrong. If the H+ concentration at pH 6.5 is x, what is it at pH 4.8?
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pH is a logarithmic scale: pH = -log10[H+]. A change of 1 pH unit means a tenfold change in hydrogen-ion concentration, not a change of 1 in a simple ruler sense.
The gap from 4.8 to 6.5 is 1.7 units, so the acid concentration at pH 4.8 is 10^1.7 ≈ 50 times that at pH 6.5.
If [H+] at pH 6.5 is x, then at pH 4.8 it is about 50x. The soil is therefore about fifty times more acidic than the target - far from 'almost all right'. This is why a pH reading must be taken seriously and why lime has to be applied in a calculated dose and the field re-tested, rather than judged by the size of the number alone.