How to use wood ash on your shamba
Quick answer: wood ash makes acidic soil less acidic and adds potassium and calcium. Use it only where the soil is acidic: spread up to 100 g per square metre (a 1 kg bag covers about 10 m²), dig it in, and wait 2–3 weeks before planting. Once a year is enough. Wear gloves, keep it off seedlings, and don't apply it together with urea, CAN, DAP or fresh manure.
Wood ash is what's left when wood burns. Ours is purified the simple way: it comes from clean, untreated wood only, and it's sieved to a fine powder, with charcoal bits, stones and nails taken out.
What it does
Wood ash is strongly alkaline, so it works like a milder, faster-acting agricultural lime: it raises the pH of acidic soil. In less acidic soil, many crops can take up more of the nutrients that are already there. Ash also adds potassium and calcium, with a little magnesium and phosphorus.
It has almost no nitrogen, so it doesn't replace compost, manure or your usual fertiliser.
Check your soil first
Wood ash only helps acidic soil. On soil that is already neutral or alkaline, it does harm. You can't tell by looking: soil in much of Kenya's highlands is acidic, but not everywhere.
- A simple pH kit or meter gives a rough answer.
- A soil test from KALRO or a private soil lab gives the real answer, and tells you how much lime your soil needs.
- Most vegetables grow best between pH 6 and 7. Below 6, ash can help. At 7 or above, don't use it.
How much to use
Vegetable beds and kitchen gardens
Up to 100 g per square metre, once a year. A 1 kg bag covers about 10 m², for example a bed 1 metre wide and 10 metres long.
The compost heap
A thin sprinkle on each layer. Too much makes the heap lose nitrogen.
Very acidic soil or big areas
For soil below pH 5.5, or for a whole farm, agricultural lime from an agrovet costs less per square metre and lasts longer. Wood ash suits kitchen gardens and small plots: it works faster, and it adds potassium that lime doesn't.
These are safe starting points. Test your soil again every year or two before adding more: too much ash makes soil too alkaline, and that's hard to undo.
How to apply it
- Pick a dry day with no wind. Wear gloves, and keep your face away from the dust.
- Spread the ash evenly over the bare bed, close to the ground.
- Dig or rake it into the top 10–15 cm of soil.
- Water the bed if no rain comes, then wait 2–3 weeks before you plant or sow.
- Keep it off leaves, seeds, seedlings and stems, and never put it in the planting hole.
Don't use it with
- Urea, CAN, DAP, sulphate of ammonia or fresh manure. Together with ash they release ammonia, and the nitrogen is lost into the air. Leave 2–3 weeks between them.
- Crops that like acid soil: Irish potatoes (less acidic soil makes potato scab more likely), tea, pineapples and blueberries.
- Soil that is already neutral or alkaline (pH 7 or above), common in drier areas.
Safety and storage
- Ash is caustic, especially when wet, and can burn skin and eyes. Wear gloves, and wash your hands afterwards.
- If it gets in your eyes, rinse them with clean water for 15 minutes. See a doctor if they still hurt.
- Don't breathe the dust: spread it low, on a still day.
- Keep the bag dry and sealed. Damp ash goes hard, and rain washes out the potassium.
- Keep it away from children, animals and food. It's for soil only: not for cooking, eating or skin.
What we don't claim
We sell wood ash as a soil amendment for acidic soil, and we make no claims about pests or plant diseases. It isn't a complete fertiliser either. Results depend on your soil, your crop and the rain, and a soil test tells you more than any guide can.
Common questions
Is wood ash good for my garden?
Only if your soil is acidic. Then it raises the pH and adds potassium and calcium. On neutral or alkaline soil it does harm, so test your soil first.
How much wood ash per square metre?
Up to 100 g per square metre, once a year, dug into acidic soil 2–3 weeks before planting. A 1 kg bag covers about 10 m².
Can I mix wood ash with manure or fertiliser?
Not at the same time. With urea, CAN, DAP or fresh manure it releases ammonia, and the nitrogen is lost. Leave 2–3 weeks between them.
Can I use ash from my own jiko or fire?
Only ash from plain, untreated wood. Ash from charcoal briquettes, painted or treated timber, plastic or rubbish can carry chemicals you don't want in your soil. Ours is from clean wood only, sieved fine.