Spanish conquistador Francisco del Orellana peered through the Amazon undergrowth. He saw thriving settlements and fertile agriculture along the river. This was 1542. Later visits by other explorers found only jungle and poor soils which could not support agriculture. The conquistador was dismissed as a nutcase who had spent too much time in the rainforests and become a teller of tall tales.

del Orellana has had the last laugh though: more recent investigations have proved he was telling the truth - the indigenous people of the Amazon did have a thriving agriculture, based on Terra Preta, "Black Earth", a mixture of ordinary soil, pottery shards, human and animal fertiliser, and the magic ingredient - charcoal. This mixture produces highly fertile, long-lasting soil. The native peoples had discovered and used a method of enriching their poor soils with easily-made charcoal to enable a source of food for a large population.

Recently scientists have got very excited about this for another reason too: charcoal is a stable form of carbon. It is made from vegetable matter, usually wood, but any form of excess material will do: dead plants, wheat stalks, grasses. This is "burnt" without oxygen, a process called pyrolysis, which has been known for 5000 years.

The resulting charcoal can be used as fuel, or it can be soaked in fertiliser to create "Biochar", which then can be mixed with soil to produce the characteristic black Terra Preta. Carbon which would have been returned to the atmosphere as a greenhouse gas when the plants rotted or were conventionally burnt, stays harmlessly in the ground, for probably hundreds, maybe thousands of years.

This is a big, important breakthrough. Famous climate activist Bill McKibben says, "If you could continually turn a lot of organic material into biochar, you could, over time, reverse the history of the last two hundred years... We can, literally, start sucking some of the carbon that our predecessors have poured into the atmosphere down through our weeds and stalks and stick it back in the ground. We can run the movie backward. We can unmine some of the coal, undrill some of the oil. We can take at least pieces of the Earth and – this is something we haven't done for quite a while – leave them Better Than We Found Them.”

The mysterious Amazon people left no records, and we do not know exactly the right recipe for creating the most potent soil improver. Is the pottery important? Did they mix in human or animal waste products in a certain ratio, or was it just that the pits used for charcoal production, were then - ecologically soundly - reused as middens (garbage dumps)? We don't know.

All over the world, scientists involved in the International Biochar Initiative, and other similar projects, are investigating the incredible properties of Biochar. If revitalising soil and capturing carbon weren't enough, the production of charcoal produces a fuel gas, called syngas, and also can be used to take noxious substances out of contaminated ground, thus returning it to use for growing food.

How do you make Biochar?

You can do it at home. If you take ordinary barbecue charcoal (not the easy-light charcoal that has been impregnated with flammable components to make it burn easier) then steep it in liquid fertilizer for a couple of weeks you have biochar. Most sources advise that the charcoal should be crushed into small pieces, which should be done when it is slightly wet to keep down the dust.

This should be mixed into the earth at a ratio of about 10% charcoal to 90% soil.

Recently commercial soil improvers containing biochar such as Carbon Gold, which is a mixture of biochar, seaweed, fungi and wormcasts, have come on the market, which means that ecologically-minded gardeners can simply add it to their flower or vegetable beds.

Biochar has been recommended as a carbon reduction process by scientists as respected as James Hansen of NASA and James Lovelock, originator of the GAIA hypothesis. It could be used to capture a considerable amount of carbon, and amazingly, doesn't need high technology - you can do it in your garden!