Two Plots, Two pH Problems
Two adjacent wheat plots on a farm near Begusarai give pH 4.6 (plot A) and pH 8.4 (plot B). The soil is clayey in both plots. Plot A shows stunted plants with pale yellow leaves; plot B has a white crust after irrigation and water ponds on the surface for hours. The farmer has 50 kg of quicklime (CaO), 50 kg of gypsum (CaSO4·2H2O), and 25 kg of ammonium sulphate. He plans to spread ammonium sulphate on both plots because it is the cheapest nitrogen source, and to add gypsum to plot A because 'gypsum is a soil conditioner'. A neighbour advises adding quicklime to plot B to 'kill the alkalinity'. The irrigation water is slightly alkaline. The farmer wants to correct both plots before the next sowing and has only one week before the field must be prepared.
1. Recommend which amendment to apply to each plot and justify with an equation or ion-exchange explanation. Which of the farmer's and neighbour's plans is wrong, and why?
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Plot A is strongly acidic (pH 4.6), so it needs a base. Quicklime is correct: CaO + H2O → Ca(OH)2, then Ca(OH)2 + 2H⁺ → Ca²⁺ + 2H2O. This neutralises soil acidity and raises pH. It should be spread and mixed in before sowing, not left in lumps.
Plot B is alkaline/sodic (pH 8.4) with a white crust and poor infiltration. Gypsum is correct: CaSO4·2H2O supplies Ca²⁺, which exchanges with Na⁺ bound on clay. The Na⁺ leaves as soluble sodium sulphate, the clay flocculates, and water infiltration improves.
The farmer's plan to add gypsum to plot A is wrong because gypsum does not neutralise strong acidity effectively; it is a calcium source for sodic soil. The neighbour's plan to add quicklime to plot B is also wrong: quicklime is alkaline and would raise the pH further.
2. Explain why ammonium sulphate is unsuitable for plot A even though it supplies nitrogen. What should be used instead?
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Ammonium sulphate is an acid-forming fertiliser. In the soil, ammonium ions are converted to nitrate by nitrifying bacteria, and this process releases H⁺ ions; the sulphate also leaves an acidic residue. On a plot already at pH 4.6, this worsens acidity and can increase aluminium toxicity, which stunts roots.
The pale yellow leaves on plot A may look like nitrogen deficiency, but adding more acidifying nitrogen will not correct the underlying pH problem. The farmer should first apply quicklime to raise the pH toward about 6.5, then use compost or farmyard manure, or a less acidifying nitrogen source after liming. Ammonium sulphate should not be spread on both plots indiscriminately.
3. Describe a simple field test using pH paper or litmus to check whether the treatments worked, and give one safety precaution for handling quicklime.
Read the worked answer
Take a small soil sample from each plot, mix it with distilled water in a 1:2 soil-to-water ratio, stir, and let the soil settle. Dip universal pH paper into the clear liquid and compare the colour with the chart. Before treatment, plot A should give an acidic colour (pH below 5) and plot B an alkaline colour (above 8). After treatment, plot A should move toward pH 6.5–7 and plot B should move down toward neutral.
Litmus can also be used: blue litmus turns red in the acidic plot A extract, and red litmus turns blue in the alkaline plot B extract. The change after treatment should be toward neutral.
Safety: quicklime reacts exothermically with water. Wear gloves and goggles, avoid inhaling the dust, and add lime slowly to water rather than adding water to lime. Keep it away from eyes and skin.