Is biochar worth using in a garden?

Biochar attracts unusually strong claims in both directions. The reasonable position is that it is a genuine soil amendment with a real mechanism and a real evidence base, that the evidence is much stronger on degraded tropical soils than on temperate garden soils, and that it is expensive relative to the alternative, which is compost. It is a legitimate thing to use and a poor first thing to buy.

Step by step

1. Understand what it is.Biomass — wood, husks, straw — heated to several hundred degrees in the near-absence of oxygen, a process called pyrolysis. What is left is a highly porous, chemically stable carbon skeleton. It is not ash, and it is not barbecue charcoal, which is made differently and may contain binders and accelerants.
2. Understand the mechanism.Its enormous internal surface area holds water, holds nutrient ions against leaching, and provides habitat for soil bacteria and fungi. It is chemically stable, so it stays in the soil for a very long time rather than being consumed like compost. It supplies no nutrients of its own.
3. Charge it before use — this is not optional.Raw biochar's surface area is empty, and in the soil it fills up by adsorbing nutrients out of the soil solution. Applied uncharged, it makes nutrients less available for a season or more. Charge it by mixing it into a compost heap for a few months, soaking it in a comfrey or seaweed liquid, or mixing it thoroughly with manure or worm compost and leaving it for a few weeks.
4. Be honest about where the evidence is strong.The strongest results come from degraded, weathered, low-organic, acidic tropical soils, where biochar's water and nutrient retention makes a large difference. On temperate, reasonably fertile soils that already have organic matter, measured yield responses in field trials are small and inconsistent.
5. Match it to the soils where it does most.Very sandy soils that leach nutrients and dry out. Very low-organic soils being brought into cultivation. Container mixes, where its water and nutrient retention is genuinely useful. Acid soils, where it also has a mild liming effect. It contributes least on a good loam with an established compost programme.
6. Apply it at a modest rate and mix it in.Typical garden rates are a few per cent by volume of the soil it is mixed into. It has to be incorporated to be useful, because its effect is on the soil it is in contact with. Unlike almost everything else in this cluster, this one goes in rather than on.
7. Do not treat it as a fertiliser.It contains essentially no nitrogen, phosphorus or potassium in plant-available form. A garden fed biochar and nothing else will be underfed. It works by holding onto nutrients you supply, not by supplying them.
8. Do not make it at home casually.Producing biochar properly requires controlling temperature with restricted oxygen. Improvised burns produce a lot of smoke, are frequently a nuisance and in some areas a legal problem, and yield an inconsistent product. Burning garden waste in the open is restricted in many places and is a statutory nuisance if it affects neighbours.
9. Compare the money against compost.For the price of a bag of biochar you can buy several times the volume of green-waste compost, which will do more for a typical British garden soil. If your soil already gets an annual compost mulch, biochar is a refinement rather than a priority.

Tips

• The most reliable garden use is in container and potting mixes at around 5 to 10 per cent by volume, charged first. Its water and nutrient retention genuinely helps in a pot, where those are the limiting factors.
• Adding raw biochar to a compost heap is a good way to charge it and also reduces nitrogen loss from the heap. Two benefits from one step.
• Barbecue charcoal is not biochar. Briquettes contain binders and sometimes accelerants and should not go on a garden. Lump charcoal from untreated wood is closer but is still made for burning rather than for soil.
• Biochar's carbon is very stable, which is the basis of the carbon sequestration argument for it. That is a climate argument rather than a horticultural one and it is a legitimate reason to use it even where the yield effect is small.
• Terra preta, the anthropogenic dark earths of the Amazon basin, are the historical evidence that biochar plus organic matter can transform a soil permanently. Note the 'plus organic matter' — those soils contain charcoal, bone, pottery and organic waste together.

Questions people ask