Two plots, two pH readings, one bag of limestone
A Class 10 student spends a holiday at her uncle's two-acre vegetable plot near Balasore. A soil testing kit gives pH 5.1 for the upland plot where brinjal is grown, and pH 7.9 for a low-lying patch beside the pond where the plants are stunted and yellow between the veins. The uncle has a 50 kg bag of powdered limestone (calcium carbonate), a bag of gypsum (calcium sulphate) and a small tin of slaked lime. His plan is to spread limestone on both plots 'because calcium is calcium', and to put gypsum on the acidic plot because 'it is a calcium fertiliser too'. Heavy rain is forecast in ten days. The pond patch floods for a day or two after heavy rain and then dries to a hard, cracked crust. The uncle also wants to mix his ammonium sulphate with the limestone to save a trip across the field.
1. Which amendment should go on the pH 5.1 plot, and why are gypsum, slaked lime and a limestone–ammonium sulphate mixture each a poor choice there?
Read the worked answer
Limestone is the right choice. Calcium carbonate neutralises soil acidity slowly and safely:
CaCO3 + 2H⁺ → Ca²⁺ + H2O + CO2
It raises pH gradually and cannot easily push the soil past neutral, and it supplies calcium at the same time.
Gypsum cannot do this job. CaSO4 is the salt of a strong acid (H2SO4) and a strong base (Ca(OH)2), so it is essentially neutral in solution; it supplies calcium and sulphur but has no liming power. It is used on sodic and dispersive clay soils, not on acid soils.
Slaked lime, Ca(OH)2, does neutralise (Ca(OH)2 + 2H⁺ → Ca²⁺ + 2H2O) and acts fast, but it is strongly basic and caustic. A careless dose can swing pH past 7 in a week and burn roots; it is justified only when a quick correction is essential, and then only in small, calculated amounts.
Mixing with ammonium sulphate wastes nitrogen: the ammonium ion reacts with carbonate to release ammonia gas, so nitrogen is lost to the air and part of the lime is spent. Lime should go in 2–3 weeks before sowing, be mixed into the topsoil, and be kept out of the same band as nitrogen fertiliser.
2. The uncle wants to spread limestone on the pH 7.9 patch as well. Should he? Explain the yellowing there.
Read the worked answer
No. At pH 7.9 the soil is already slightly alkaline; adding lime pushes it towards 8.5, where iron, manganese, zinc, copper and boron become insoluble and phosphorus is fixed as calcium phosphate. The result is induced micronutrient deficiency - the opposite of what he wants.
The yellowing with green veins on young leaves is the classic picture of iron deficiency, but the cause is likely to be the waterlogging, not the soil pH. When the patch floods, air is excluded from the pore spaces; roots cannot respire aerobically, they produce alcohol and ethylene, and they rot. Damaged roots cannot absorb nutrients even when those nutrients are present. In saturated soil, nitrate is also lost as nitrogen gas by denitrification, so the plants starve for nitrogen as well. The hard crust after drying is a sign of poor structure and repeated slaking.
What the patch needs is drainage - shallow furrows leading to the pond, raised beds, and plenty of organic matter to build aggregates - plus a foliar spray of the deficient micronutrient if a test confirms it. Lime would make the problem worse. A proper soil test (pH, electrical conductivity, organic carbon) should come before any amendment.
3. How should the student measure whether the limestone actually worked, and what would make her conclusion unreliable?
Read the worked answer
Sample properly first: take cores about 15 cm deep from five or six spots across the plot in a W pattern, mix them in a clean bucket, quarter down to about 250 g, air-dry in shade, and test that composite rather than a single spot.
Measure pH in a fixed ratio suspension, for example one part soil to two parts distilled water, stirred and left to settle for about half an hour. Record a baseline before liming, then repeat at three weeks and at the end of the season. A pH meter is far more reliable than indicator paper, which is hard to read closer than about half a unit; if only paper is available, use narrow-range paper and read in good light against the colour chart.
Unreliable conclusions come from: testing right after liming, when the powder has not yet reacted; sampling only where the lime was heaped; using tap water or a dirty container; testing wet soil after rain; or judging by plant appearance alone, since yellowing can have several causes. The same soil measured in water and in potassium chloride solution gives different numbers, so the method must be kept identical between readings. She should also record how much lime was applied, so the result can guide the next application, and remember that pH should be rechecked every two to three years rather than every season.