There are many soil additives on the market, all offering gardeners the appealing promise that they will make your plants bigger or healthier. Often there is little, if any, way of knowing whether they will work - the claims made by the products may simply not have been adequately tested, and it's hard to tell what's in them.
Biochar is touted as having many useful benefits – but is it worth buying biochar? We asked ecologist Ken Thompson for his advice.
What is biochar?
Biochar is basically charcoal and can be made by burning almost any kind of organic material. The result is as variable as the material you started with.
What is biochar used for?
There are countless claims relating to biochar's beneficial qualities and its effects on, say, soil microbes or heavy metals are huge research fields on their own. It is also said to have beneficial effects on plant growth and moisture retention when you add it to soil. So, to keep things simple, let’s focus on its effects on plant growth; first, because that’s probably what gardeners are most interested in, and second, because those marketing biochar to gardeners clearly see better plant growth as its main selling point.
Does biochar work?
There has been a great deal of research on biochar. First, the good news. Across all studies, biochar increases crop yield a little, on average.
But closer examination reveals that all the studies reporting improved yield are in the tropics. In temperate climates, it doesn’t do anything. Not only that, even in the tropics the effect of biochar is very variable.
Tropical soils are typically both acidic and low in nutrients, and biochar can potentially fix these problems, as long as the biochar itself is both nutrient-rich and alkaline, which is sometimes the case. In other words, biochar can sometimes have a beneficial liming and fertilising effect, in soils that need lime or fertiliser (although it would be simpler and cheaper just to add lime or fertiliser).
But low-nutrient biochar (made from wood, for example) doesn’t do anything, and acid biochar is positively harmful. There’s also some evidence that biochar might help to retain water on coarse, free-draining soils that would otherwise be at risk of suffering from drought. But this is just one of the symptoms of a lack of organic matter, and garden soils are usually adequate in this respect already.
Should I buy biochar?
You would probably do more good by just adding the original organic matter as a mulch, rather than wasting time and energy burning it and turning it into biochar. Biochar is sometimes also suggested as a good way of permanently trapping carbon in soil, but the carbon footprint of making and transporting it means the numbers don't really add up. Once again, it's probably better (and cheaper) just to mulch with the original organic matter.