The secret of an ancient civilization that once thrived along the Amazon lay in their ability to transform the poor soils of the region into a magical substance known as the terra preta, or black earth. However much gold the Spaniards came away with, they missed this, the real treasure of El Dorado. Civilisations may rise with hoards of gold, but they fall when they lose touch with the earth. Today, wherever it is discovered, terra preta is prized by local farmers. Legendary for its stability and fertility, it is even credited with self-regenerative properties. Analysis has shown that a key ingredient in this soil is charcoal.

What is special about charcoal? If you can imagine folding a football pitch into the size of a golf ball (the trick is to make it very thin), then you would get an approximate idea of how much surface area there is wrapped up in a golf ball sized chunk of charcoal.


Charcoal is an intricate matrix of minute empty spaces – pores, holes and channels divided by thin structural walls of pure carbon. It is produced when any plant-based material (usually wood but it could equally be corn husks, nut shells, plant stems…) undergoes pyrolysis, a burning process in an oxygen-starved environment that drives off all the volatile compounds. What is left behind is the pure carbon framework of the plant, charcoal.

Charcoal, like diamond, is a very stable form of carbon, and can last in the soil for hundreds or thousands of years. This gives it a certain unique status. Charcoal is negatively charged and attracts positively charged soil nutrients. Technically speaking this gives it a high ‘CEC’ or ‘cation exchange capacity’ (apparently a very positive feature). Being a long lasting form of carbon, it holds onto these nutrients for a long time, giving stability to soil structure, and fertility. It is also in this carbon labyrinth that beneficial soil microorganisms that process these nutrients find a safe haven.

What are these tiny microorganisms, what do they do, and why do they need a safe haven?

Agriculture covers 38% of our terrestrial planet. That means soil. But the hungry corporate machine that drives modern agriculture is not friendly to soil biology, and is destroying it. If the soil biology were to completely vanish we would be left with the sort of barren movie landscape reserved for the latest Mad Max film. Yet in some parts of the world such as the Loess Plateau in China, this has already happened. Although remedial work is already underway there, much needs to be done worldwide to rescue our soil. Biochar could play a significant role in this.

All healthy soils contain purposeful populations of living organisms (with some exotic names); bacteria, fungi, nematodes, microarthropods, worms and many more. Together they form an integral ‘soil food web’ and, apart from eating one another, accomplish amazing tasks. They transform the minerals locked up in sand, silt and clay into vital plant nutrients, convert fallen trees, leaves and other organic material into humus, regulate soil moisture content, transport nutrients to where they are needed and create a rich, intelligent, cohesively-structured growing medium for plants.


Soil microorganisms also form mutually beneficial (symbiotic) relationships with plants, which exude specific sugars through their roots to attract them. These microorganisms in return deliver the right nutrients in the right quantity at the right time to the plant roots where they are needed.

Although much remains to be known about the precise mechanisms of biochar, as a soil amendment it has proven to be extremely helpful. But what makes it different from an ordinary piece of barbecue charcoal? Essentially it is the same thing, except in its preparation. The type of material used, and the temperatures and timings of the process, will produce some variations. Once made, the charcoal is broken into small pieces and ‘inoculated’ or primed with a compost (biologically rich) solution. Charcoal on its own would otherwise initially draw nutrients from the soil into itself, so the benefits for the first year would be delayed. The priming process gets the necessary biology started beforehand. At this point it becomes the ‘bio-char’ of the terra preta, ready to bring enlightenment to your soil.

In my next bog we will look a little more into the effects of agriculture and the soil, and the overall implications of widespread biochar use, and some very interesting projects. Are we going to burn all the trees to make biochar? Certainly not.

Useful info:
Youtube:  ‘The Secret of El Dorado – Biochar. Horizon
Youtube:  Dr Elaine Ingham ‘Soil not Dirt’.