Lime Dose for an Acidic Kitchen Garden
During a school garden project, a student tests two plots with universal indicator paper. Plot A reads pH 5.0; Plot B reads pH 6.5. The beans they plan to sow grow best between pH 6.0 and 7.0, so Plot A needs about a 1.0-unit increase. The plot is 4 m² and the top 15 cm will be treated. The garden shed contains garden lime (calcium carbonate), whose label says 200 g per m² raises pH by about 0.5, and hydrated lime (calcium hydroxide), whose label says 100 g per m² raises pH by about 0.5 but warns that it is caustic and fast-acting. There is also a bag of wood ash, pH about 10, with no application rate. Sowing is in three weeks. The student has a balance, a bucket, a spade, and pH paper, but no pH meter. The teacher asks for a safe, measured plan rather than 'adding some lime until it looks right'.
1. Calculate the mass of garden lime needed to raise Plot A from pH 5.0 to 6.0 and justify choosing it over hydrated lime.
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The label rate is 200 g/m² for a 0.5-unit rise. A 1.0-unit rise therefore needs two such increments: 2 × 200 g/m² = 400 g/m². For 4 m²: 400 g/m² × 4 m² = 1600 g = 1.6 kg of garden lime.
Garden lime is the better choice here because it is calcium carbonate: it reacts more slowly and is much less caustic than hydrated lime. Hydrated lime would need only about 800 g by the label rate (2 × 100 g/m² × 4 m²), but it can burn skin and eyes and can raise pH sharply in patches. With three weeks before sowing, the slower, safer material is appropriate. The wood ash has no stated application rate and variable composition, so it cannot be dosed reliably.
2. Three weeks later the pH paper reads 6.4. The student wants to add another 800 g of garden lime before sowing. Evaluate this decision and give a better next step.
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Adding another 800 g is not justified. The target range is 6.0–7.0, and 6.4 is already inside it. Lime does not all react immediately, mixing may have been uneven, and pH paper has limited precision, so 6.4 may not mean the soil is still 0.4 units below target. Adding more lime could push the pH above 7.0 and reduce availability of some nutrients.
A better next step is to mix and water the plot, wait about two weeks, and retest several spots with pH paper rather than one sample. Only if the average is still clearly below 6.0 should a smaller, calculated amount be considered. The decision should be based on a retest, not on wanting the exact number 6.0.
3. Explain why pH paper is adequate here, and state one limitation compared with a calibrated pH meter.
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The decision is a soil-treatment judgement with a broad target range (6.0–7.0), not a precise chemical analysis. pH paper is cheap, needs no calibration or batteries, and can be used in the field. It can show whether the soil is still clearly acidic, near target, or too alkaline, which is enough to decide whether to add lime or stop.
Its limitation is resolution and subjectivity: colour matching may distinguish only about 0.5 pH unit, and colour perception varies between people. A calibrated pH meter can give a numerical reading to about 0.1 unit and is better for tracking small changes. However, a meter is only better if it is calibrated and the soil sample is prepared consistently; an uncalibrated meter can be more misleading than pH paper.