Garden Lime: Two Beds, One Bag, Different pH
At a school community garden, two raised beds are being prepared for silverbeet, which grows best at pH 6.0–7.0. A soil test gives bed A pH 5.2 and bed B pH 6.4. The students have a bag of garden lime (calcium carbonate) and a bag of sulphate of ammonia (an acid-forming fertiliser). A helper says, 'Just add two handfuls of lime to each bed-lime makes soil better.' The beds are 3 m² each. The students also plan to water with tap water at pH 7.6. They want to know which bed needs treatment, whether the same amount of lime is appropriate, and how they will know when to stop adding lime. The lime bag label says 100 g/m² raises pH by about 0.5 in sandy soil, but the garden soil is clay loam.
1. Which bed needs lime, and why is adding the same amount to both beds a poor plan? Include the pH range and the effect of over-liming.
Read the worked answer
- Silverbeet grows best at pH 6.0–7.0. Bed A at pH 5.2 is too acidic and is below the optimum range. Bed B at pH 6.4 is already inside the range.
- Lime is calcium carbonate, a base. It neutralises soil acidity by reacting with hydrogen ions, raising the pH. So bed A benefits, but bed B does not need lime.
- Adding the same amount to bed B could push its pH above 7.0, perhaps towards 7.5 or higher. At high pH, some nutrients such as iron, zinc and manganese become less available to plants, so over-liming can cause yellowing and poor growth even though the soil is less acidic.
- Judgement: treat bed A only, and monitor bed B rather than treating both. 'More lime is better' is false because pH has an optimum range.
2. The label says 100 g/m² raises pH by about 0.5 in sandy soil. Bed A is 3 m² and needs to rise from 5.2 to at least 6.0 (0.8 pH). Calculate a label-based amount, then explain why this is only a starting estimate for clay loam.
Read the worked answer
- Required pH rise = (6.0-5.2=0.8).
- Label rate for 0.5 pH rise is 100 g/m². For 0.8 rise:
((0.8/0.5)\times100=160\text{ g/m}^2) - For 3 m²:
(160\times3=480\text{ g}) - This is only a starting estimate because the label is for sandy soil. Clay loam contains more clay and organic matter, which buffer pH changes, so the soil may resist the pH change and need more lime than the label predicts. Lime also reacts slowly.
- Practical approach: apply in stages, water it in, retest after several weeks, and add more only if the pH is still below 6.0. Do not add a large excess at once.
3. Explain how the students should monitor the pH and why they should not also add sulphate of ammonia at the same time.
Read the worked answer
- Monitor with a pH probe or a soil test kit on a soil-water slurry. Sample several spots in each bed, record before and after values, and retest after watering or rain and at intervals of a few weeks.
- Sulphate of ammonia is an acid-forming fertiliser. Adding it at the same time as lime works against the lime by lowering pH, so the students would not know whether the lime had worked.
- There can also be a chemical problem: ammonium compounds can react with lime and release ammonia gas, losing nitrogen from the soil.
- Judgement: apply lime first, wait and retest, then use sulphate of ammonia later only if the plants need nitrogen. Tap water at pH 7.6 is slightly alkaline, so it will not acidify the soil, but it is not a precise pH treatment.