Choosing a lime treatment for an acidic cocoa block
A cocoa farmer has a 0.2 hectare block near Buka. A soil test reports pH 4.8 in water and high exchangeable aluminium. The laboratory recommends raising pH to about 5.8, which for this soil needs 2 tonnes of agricultural lime per hectare. The local supplier sells 50 kg bags of lime at K45 each. The farmer has K300 saved and can get unlimited wood ash from the village for free. The farmer wants to spread ash heavily on the whole block and skip the lime. A neighbour says lime is 'just calcium' and will not change aluminium. The farmer has no pH meter and no money for a second soil test. The block is on a slope, and heavy rain is expected in three weeks. Cocoa yields have fallen from 800 kg to 500 kg over three seasons.
1. Calculate how many 50 kg bags of lime are needed for the whole block and the total cost. Decide what the farmer can realistically do with K300. Show your working.
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Calculation:
- Recommended rate = 2 t/ha = 2,000 kg/ha.
- Block = 0.2 ha, so lime needed = 2,000 × 0.2 = 400 kg.
- Bags needed = 400 ÷ 50 = 8 bags.
- Cost = 8 × K45 = K360.
The farmer has K300, so he is K60 short for the full block. With K300 he can buy 6 bags (K270), which is 300 kg, or about 0.15 ha at the recommended rate. Spreading those 6 bags over the whole 0.2 ha would under-dose and may not raise pH enough. A better judgement is to treat the most acidic or most productive part of the block properly and keep the rest for the next income, rather than spreading too little everywhere. Wood ash could be tested on a small area, but it is not a guaranteed substitute.
2. Explain chemically why wood ash can raise soil pH, and give one risk of using it heavily without testing.
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Wood ash contains calcium carbonate, calcium oxide and calcium hydroxide, plus potassium and magnesium compounds. These are alkaline: they react with hydrogen ions in the soil solution and on exchange sites, neutralising acidity and raising pH. Ash also adds potassium, which cocoa needs, so it can be useful.
The risk is that ash composition varies greatly depending on the wood and how hot it burned. Heavy application can raise pH too fast, add excess potassium and soluble salts, and upset the balance of micronutrients such as zinc or boron. Without a pH meter or soil test, the farmer cannot tell whether the pH has overshot 5.8. A small test patch, applied before the rains and checked by observing leaf colour and growth, is safer than spreading ash over the whole block.
3. Predict two effects on cocoa nutrition of raising pH from 4.8 to 5.8. Refer specifically to aluminium and phosphorus.
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At pH 4.8, aluminium is soluble as Al³⁺ and is toxic to cocoa roots; it damages root tips and reduces water and nutrient uptake. Raising pH toward 5.8 causes aluminium to precipitate as less soluble aluminium hydroxide, so aluminium toxicity falls and roots can grow better.
Phosphorus is strongly fixed by aluminium and iron oxides at low pH, so little is available to plants even if phosphate is present in the soil. As pH rises toward 5.8, phosphorus fixation decreases and more phosphate remains in solution for uptake. However, raising pH too far can reduce the availability of zinc, manganese and iron, so the target should be about 5.8, not 7.0.