The long-lasting, eco-friendly, carbon-storing wonder stuff

There is a popular saying among organic gardeners: “feed the soil not the plants”. For the past 80 years, organic gardeners have fed their soils with a wide range of composts, from the home-made, high-fertility wormy stuff to nutritionally-balanced nursery-bought bags based on peat.

Biochar producer Craig Sams

But recently, an additive has been discovered, or rather rediscovered, prompting excitement among gardeners. Available alone or now as pre-mixed, peat-free compost, the wonder material is biochar, a charcoal-like substance produced through the oxygen-free, slow burning of woody biomass garden cuttings, grasses, crop waste.

Biochar, according to its fans, not only dramatically increases carbon content in the soil, it may also capture excess carbon, acting as a possible prescription against climate change. Michael Hayes, director of the Carbolea Research Group at the University of Limerick, says: “Garden soil that’s continually cultivated loses its organic matter as carbon dioxide. This is the reason gardeners put compost in the soil: to increase organic matter content. But of course, as every gardener knows, compost decomposes. The slow transformations in biochar mean that it retains carbon and can last in the soil for millennia.”

One of the producers of biochar-based garden products is Craig Sams. He founded the first macrobiotic restaurant in London in the 1960s and then started the first UK-based organic food exporter, Whole Earth Foods. He also co-developed the world’s first organic chocolate company, Green and Black’s. Sams, 66, believes biochar will play a role in revitalising soil without gardeners having to resort to peat, a finite resource. “Using peat releases large amounts of CO2 in the atmosphere. Most gardeners don’t realise they are polluters.”

He first became aware of soil quality on his family farm in Nebraska, where the fields were once famous for their striking “black earth”. Yet over-farming took its toll, often degrading the soil: “The first settlers in the 1880s started out with rich soil that was 45 tons of carbon per hectare: now it’s five. All that carbon was ploughed out. And then driven out by chemical fertilisers.” In 1991, Sams came across peanut farmers in Togo who grew organic cocoa beans. Sams, then treasurer and later chairman of the UK Soil Association, started to pay closer attention to the make-up of compost. “We had to think not just about what went into the gut but what went into the compost; how you got the balance between fungi, bacteria, nitrogen and other nutrients.” Charles Mann’s book 1491 introduced Sams to terra preta, the Amazonian “black earth”, often 2m deep that, say researchers, turned out to be man-made, essentially biochar, produced hundreds of years earlier in an apparent systematic bid to fertilise poor soils.

Produced in covered pits, in the charring process called pyrolysis, biochar had another plus, says Sams: it took advantage of soils as natural carbon sinks: “Two-thirds of the carbon within the smouldered biomass stays in the biochar, feeding the soil. Biochar also provides a matrix for decomposition.” Climate change advocates have proposed wide-scale projects to turn a range of wastes into biochar and take advantage of the carbon-storing properties of soil.

As a global cure-all, however, biochar has some strong sceptics, including Daniel Schrag, director of the Harvard Centre for the Environment: “I think the jury is still out. I just did a review and while there are some provocative results, there is not yet good evidence to determine what fraction of biochar produced will stay in the soil for hundreds of years. And biochar is not one material but a range of materials depending on the temperature at which you make it and the feedstock you use: is it gardening material? Wood? Animal waste? And burning fields to increase nutrient loads of the soil: that’s a long-standing practice.”

Sams counters that unlike the traditional “slash-and-burn” methods of farmers, the ancient “slash-and-char” practice captures carbon that would be lost to the atmosphere, heating it up, and returns it to the soil, dramatically increasing plant yields. He adds that growers have already observed progress in trials in Belize and the UK. “We need more trial studies,” concedes Cecile Girardin, an Oxford-based researcher also selling biochar. “You have to do localised research.” Professor Hayes says that there are already abundant data on the benefits of biochar: for instance, it encourages the growth of good soil fungi.

Hayes calls for regulation of biochar to give consumers more information. “At the moment, there is no regulation over biochar,” says Girardin. While “biochar is a potentially great contribution in mitigating soil depletion and CO2 emissions,” says Hayes, the big challenge will be to get the biochar made right: “We have to look at composition, what it’s made from and how. We’ve found that biochar, made with some types of pig waste and animal sludge can be mutagenic, or carcinogenic and potentially harmful to human health.” However, Hayes says biochar made from non-animal sources/vegetation is safe.