Biochar

In the FT this Saturday, fab environmental correspondent Fiona Harvey writes about biochar a source of increasing interest for agriculture to hold onto more carbon in the soil.

"In Brazil’s Amazon basin, farmers have long sought out a special form of fertiliser – a locally sourced compost-like substance prized for its amazing qualities of reviving poor or exhausted soils. They buy it in sacks or dig it out of the earth from patches that are sometimes as much as 6ft deep. Spread on fields, it retains its fertile qualities for long periods.

They call it the terra preta do indio – literally, “the dark earth of the Indians”. Dense, rich and loamy, this earth forms a stark contrast with the thin, poor soils of the region. (It seems a paradox, but rainforest soils have low fertility. This is why farmers who cut down the forest for agriculture have to keep on felling – after a few years of cropping, yields collapse and they have to move on.) Patches of terra preta extend for many hectares in some places but until recently, no one really knew what the mysterious dark earth was. Some guessed it was volcanic, or the sediment of old lakes, or the residue of some long-rotted vegetation. Few imagined that it was man-made.

Terra preta, modern analysis has proved, is one of the last remaining traces of pre-Columbian agriculture in the Amazon basin. It was made more than 2,500 – and perhaps as long as 6,000 – years ago by people living by the river. These cultures survived and supported complex agriculture, despite poor soil, by making their own earth. They used dung, fish, animal bones and plant waste – the usual suspects. But the key ingredient in terra preta, and what gives it its dark colour, is charcoal.

“It’s wonderful stuff,” says Simon Shackley, a social science lecturer at the University of Edinburgh. “We started to get to know about it when Dutch scientists began to look at it in the 1960s. They found these dark soils in this area of very poor soil, where it was being put on fields like compost. It’s really the product of slash-and-burn agriculture, and other organic waste, incorporated into the soils over hundreds or even thousands of years – and it does appear to be fertile indefinitely, which is really a very odd thing.”

This ancient product of the Amazon is now the subject of intense scrutiny by climate change scientists. The tenacity of the charcoal of terra preta – retaining its fertilising properties over centuries – has given them an idea. Charcoal is a form of carbon, the burnt remains of plant and animal material. If it can stay intact in the earth for so long, without being released as carbon dioxide gas, why not lock up more carbon in the earth in this manner?

Scientists have begun to refer to the charcoal made from plants for the purpose of storing carbon as “biochar”. The theory is that biomass – any plant or animal material – can be turned into charcoal by heating it in the absence of oxygen. By taking CO2 out of the atmosphere, the impact on climate change could be huge".