Biochar is charcoal made from plant material — wood, straw, husks or prunings — burned in a low-oxygen fire so that it becomes a stable, porous carbon skeleton instead of ash. Gardeners value it because that skeleton lasts in the soil for years, even decades, holding water and nutrients in its tiny pores and giving soil organisms a place to live. It is not a fertiliser: fresh biochar contains almost no nutrients, and used wrongly it can temporarily make soil poorer, not richer. Used properly — charged with nutrients first, added in modest amounts — it is a long-term soil investment, especially in sandy or degraded ground. This guide explains what biochar is, how to charge it, how much to use, and the safety rules that matter.
Biochar is not new. The fertile dark earths of the Amazon, known as terra preta, were built over centuries with charcoal, pottery shards and organic waste, and modern biochar practice is an attempt to learn from that tradition. But terra preta was built with charcoal plus large amounts of organic matter and nutrients together — and that combination is the lesson: biochar on its own is not a complete soil fix.
How biochar works in soil
A piece of biochar is mostly empty space. Its internal pores are microscopic, and together they give biochar a huge internal surface area — a single gram can have a surface area of hundreds of square metres. In soil, those pores hold water, trap nutrients such as ammonium and potassium on their surfaces, and shelter bacteria and fungi from drying out and from predators. Over time the surfaces oxidise and become better at holding nutrients. Because biochar is mostly stable carbon, it breaks down very slowly, so its effects persist for years rather than the single season of a compost application.
That persistence cuts both ways. Fresh, uncharged biochar has empty pores that will absorb nutrients and water from the surrounding soil until they are full — which means it can temporarily starve the plants growing next to it. This is the single most important thing to understand about biochar, and it is why charging it before use is not optional. Biochar also tends to raise soil pH slightly, because it contains ash and is usually mildly alkaline, so it suits acidic and neutral soils better than already-alkaline ones. And its benefits are largest in poor, sandy or degraded soils, where water and nutrient holding are the limiting factors; in already-rich loam, the improvement is modest.
How to charge biochar before use
Charging means soaking biochar in a nutrient-rich liquid so its pores fill with nutrients before it meets your soil. Without this step, the biochar fills its pores from your soil instead, and plants go hungry for weeks. To charge it:
1. Soak it. Put the biochar in a bucket and cover it with compost tea, diluted liquid seaweed or fish emulsion, or a bucket of water mixed with a few litres of finished compost. Use roughly one part biochar to two parts liquid by volume.
2. Wait. Leave it for at least 24 hours, and ideally for one to two weeks, stirring occasionally. Longer soaking charges it more thoroughly.
3. Drain and use. Drain off the liquid — it can be poured on the garden — and use the damp charged biochar straight away. Do not let it dry out completely before use, or it becomes dusty again.
If you have no liquid feed, mix the biochar into your compost heap and leave it there for a few weeks: the composting process charges it naturally. This is the easiest method for most gardeners, and it means the biochar is ready when the compost is. Never add large amounts of dry, uncharged biochar directly to a bed of growing plants.
How much biochar to use
Biochar should be a modest fraction of your soil, not a main ingredient. Recommended rates are roughly 5–10 per cent by volume in the top 20 cm of soil, which works out at about 1–2 kg per square metre for a typical garden bed, depending on the biochar's density. In containers, use no more than about 10 per cent of the potting mix by volume. More is not better: above about 10–15 per cent by volume, biochar can make soil too alkaline, too dry on the surface, and can displace the organic matter and minerals plants actually need. Apply it once, dug into the top 20 cm, and let it stay there for years — there is no need to reapply annually the way you do with compost. For raised beds, work the charged biochar into the mix before planting; our raised bed soil mix guide gives the base recipe, and the raised bed soil calculator works out volumes.
Biochar suits sandy soils best, where its water and nutrient holding make the biggest difference, and degraded or compacted soils that need structure. It also works well in potting mixes for houseplants such as monstera deliciosa, where a small amount improves drainage and aeration. It is less useful on rich loam, and it should be used cautiously on alkaline soils because it can raise pH further — test your pH first; see our soil pH testing guide.
Making and buying biochar
Most gardeners buy biochar rather than making it, because making it well requires a controlled low-oxygen burn, and an open bonfire produces mostly ash, not biochar. If you buy it, look for biochar made from untreated wood or plant waste, with no added chemicals, and check whether it is sold pre-charged or raw — raw biochar must be charged before use, as described above. Avoid charcoal briquettes made for barbecues: they often contain binders, fillers and chemicals that do not belong in soil. Horticultural biochar is usually sold as small pieces or granules, 5–20 mm across; crush larger pieces before use so they mix evenly into the soil. Never invent or trust brand claims about guaranteed results — biochar's effects vary with soil type, and honest sellers will say so.
If you do make biochar at home, use only untreated wood and prunings, burn in a proper kiln or retort design rather than an open fire, let it cool completely before handling, and never leave a burn unattended. Check local rules on open burning first — many areas restrict it — and check your local extension service for advice on safe home production.
Common mistakes
The most common mistake is adding uncharged biochar to growing beds, which starves plants of nitrogen for weeks. Always charge it first. The second is using too much: more than about 10 per cent by volume brings diminishing returns and can raise pH too far. The third is treating biochar as a fertiliser — it holds nutrients, but it does not supply them, so you still need compost and feeding. The fourth is using barbecue briquettes or treated wood, which can contaminate soil. The fifth is expecting quick results: biochar's benefits build over months as its surfaces oxidise and organisms colonise it, so judge it after a season, not a week.
Safety notes
Biochar dust is the main hazard. Fine biochar particles irritate the eyes, skin and lungs, so wear gloves, eye protection and a dust mask when handling dry biochar, crushing it, or spreading it on a windy day. Wet it down before handling where possible — damp biochar is far less dusty. Keep biochar away from open flames and store it dry; it is flammable as a dust. Do not use biochar made from treated timber, painted wood or waste that may contain chemicals or heavy metals. If you are unsure about a source, check your local extension service for advice.
Frequently asked questions
Does biochar replace compost?
No. Biochar holds nutrients and water, but it does not supply nutrients. Compost supplies nutrients, feeds soil life and builds structure. The two work together — compost provides the nutrition, biochar provides the long-term holding capacity. Use both.
How long does biochar last in soil?
Biochar is mostly stable carbon and breaks down very slowly, so a single application can last for years or even decades in the soil. That is why it is applied once rather than annually, and why charging it properly before that single application matters so much.
Can I use biochar in potting mix?
Yes, at up to about 10 per cent by volume, charged first. It improves drainage and aeration and holds moisture in the mix. Mix it into a potting mix recipe rather than using it on its own, and use the potting mix calculator to work out quantities.
Will biochar change my soil pH?
Usually it raises pH slightly, because biochar is mildly alkaline. The effect is small at recommended rates, but it matters on already-alkaline soils — test your pH before and after, and use biochar cautiously if your soil is above pH 7.5.
Is biochar safe for vegetables?
Yes, when it is made from untreated plant material and charged before use. Do not use charcoal from treated wood, painted timber or barbecue briquettes, and wash all produce before eating as usual. If your soil is on a former industrial site, test for contaminants first.