Heat wood or agricultural wastes strongly in the absence of air and you will eventually get charcoal through the process known as pyrolysis. Charcoal is almost pure carbon. When ground up and then added to the soil as a means of improving fertility or reducing water use, it is known as ‘biochar’. An Oxford company, staffed with academic researchers who work in related fields, is sponsoring a country-wide experiment to see if biochar can help domestic gardeners improve their crops.

Because charcoal is highly stable, it stores carbon for hundreds of years. Scientists such as James Lovelock have suggested that biochar might be a very effective way of storing very large quantities of carbon in the soil that would otherwise have returned to the atmosphere in the form of carbon dioxide. At application rates of 10 tonnes an arable hectare per year – a typical dose on a tropical soil – the world’s entire greenhouse gas emissions would be neutralised by using biochar on less than 10% of the world’s arable land area..

On poor tropical soils, biochar adds to agricultural production, often making a huge difference to yields. It seems to work by encouraging the growth of beneficial micro-organisms and by helping retain moisture. Does biochar improve yields in temperate climates? The data is less convincing than for hot countries with naturally carbon-poor soils. Some researchers have demonstrated that biochar can have beneficial impact but the overall effect on yields is much less clear-cut than on degraded soils. But anecdotal evidence is sometimes very compelling. The photograph at the top of this article compares biochar-dosed lettuces on the left with those planted just in conventional composts on the right. (Source: www.thecharlady.com)

The Big Biochar experiment has been designed to produce more evidence like this. The lead researchers from Oxford University’s Environmental Change Institute are distributing 1.5kg bags of biochar to domestic gardeners and people with ‘allotments’, small plots of public land rented to householders on which to grow their fruit and vegetables. Across different soil types, growing varied crops and at different times of the year, we will get an idea whether biochar can help people who cultivate their own food improve their yields. If you want to participate, details are here. You’ll need to pay the postage costs and commit to a trial that compares plant growth on a square metre of biochar-loaded soil to equivalent plants on standard soil.

Cecile Girardin, one of the scientists leading the experiment, is an expert on the carbon cycle in the tropics. (The carbon cycle is the natural process by which carbon dioxide is extracted from the atmosphere by growing plants and eventually returned when the plant dies and rots). She told me that she has a hunch that the experiment will demonstrate that root crops such as carrots or celeriac should benefit most from the addition of biochar to the soil. At this time of year in the UK, plants such as this will not generally be growing. However bulbs such as garlic and onions can be planted now (early October 2011) and will grow slowly through the autumn and winter. I think garlic would be a particularly good crop to use in the experiment. If biochar works, the bulbs should result in stronger stalk growth over the next months. I have done something slightly different, planting pak choi seeds in small pots, half of which have 10% biochar added. I will be looking for differences in root growth and leaf formation after a couple of months.

For those who have become convinced of biochar's virtues, the next step may be to club together to buy a kiln for making biochar. Craig Sams's business Carbon Gold is selling a simple retort for large scale charcoal making. At a cost of £3,500 plus VAT, the kiln is not cheap but garden clubs and allotment associations may be able to afford the investment

Biochar is potentially very important. The evidence is growing that it can both increase yields on some soils, reduce the need for expensive artificial fertilisers and cut losses in drought. The more we experiment the better our knowledge will be and sceptical policymakers will see the advantage of sequestering large volumes of carbon in the world’s soils.