Three Plots, Three pH Readings
A farmer in a hilly taluk grows beans and chillies on three adjacent plots, each about 0.4 hectare. She tests each soil with pH paper: Plot A reads pH 5.0 and the bean seedlings are stunted with yellow leaves; Plot B reads pH 6.8 and the crop is healthy; Plot C reads pH 8.3 and shows a white crust after irrigation, with poor germination. The local store stocks quicklime (CaO), slaked lime (Ca(OH)₂), gypsum (CaSO₄·2H₂O), ammonium sulphate and compost. The farmer can afford to treat only the two problem plots this season. Upstream, a small dyeing unit discharges wastewater at pH 3 into the stream she uses for irrigation, and the panchayat asks the unit to neutralise it before release.
1. Match each plot with the treatment you would choose from the store and justify each choice with the chemistry involved. Include the equation for the reaction of slaked lime with soil acid and explain why ammonium sulphate should not be used on Plot A.
Read the worked answer
Plot A is too acidic, so use slaked lime. The acid soil supplies H⁺ ions: Ca(OH)₂ + 2H⁺ → Ca²⁺ + 2H₂O. The H⁺ is removed, the pH rises towards 6.5, and calcium is added. Slaked lime is preferred to quicklime because CaO reacts violently with soil moisture, releasing heat that can scorch seedlings; Ca(OH)₂ acts more gently and spreads evenly.
Plot C is alkaline and shows a white crust, so use gypsum with compost. Gypsum is neutral calcium sulphate; its Ca²⁺ displaces Na⁺ held on clay particles, improving the structure and allowing sodium to be leached away. It does not raise pH further. Plot B is near neutral, so only compost is needed to maintain it.
Ammonium sulphate must not go on Plot A: ammonium ions are nitrified in the soil, releasing H⁺, so this fertiliser makes soil more acidic and would worsen the problem.
2. Explain why the farmer must not simply add a large excess of lime to Plot A, and describe how she could check that the treatment has worked.
Read the worked answer
Lime is a base, so adding much more than the soil can neutralise pushes the pH above 8. At that pH iron, zinc and phosphorus form insoluble compounds that roots cannot absorb, so the crop becomes deficient even though the nutrient is present. Over-limed soil also sets hard, and fresh quicklime is caustic and can burn roots, worms and the farmer's hands.
The safe approach is to add the calculated quantity in two or three split doses, mix it into the topsoil and water it in. After two to three weeks she should test again: mix a sample of soil with distilled water, let the soil settle, and test the clear liquid with a pH meter or pH paper. A reading near 6.5 means the treatment has worked; if it is still below about 5.8, a small second dose can be given. The check must be delayed because lime reacts slowly, so a reading taken on the same day would be misleading.
3. For the dyeing unit's effluent at pH 3, recommend a neutralising agent, describe how the neutralisation should be carried out and monitored, and explain why releasing pH 3 water into the stream is harmful even though the stream is large.
Read the worked answer
Slaked lime slurry is the suitable agent: it is a cheap base, and its reaction with acid gives calcium salts and water, for example Ca(OH)₂ + 2H⁺ → Ca²⁺ + 2H₂O. It should be added slowly, in small increments, with constant stirring, and the pH should be measured after each addition with a pH meter or pH paper. Addition stops when the reading reaches about 7; the usual safe discharge range is roughly 6.5 to 8.5. Adding excess lime would make the effluent alkaline, which is also harmful, so the pH must be checked before release rather than judged by the colour of the water.
pH 3 water is a thousand times more acidic than pH 6 water. It attacks fish gills and their protective mucus, releases toxic aluminium from the stream bed, and kills fish eggs and fry. A large stream dilutes the waste only after mixing; close to the outfall the plume remains acidic, so dilution cannot be treated as neutralisation.