We don't know how the Amazonian people created the Terra Preta. These people have disappeared**. A very similar tradition with a long history still exists in West Africa, the African dark Earths. We have access to people who carry on this tradition. Their strategy is more or less cold composting. The integrated materials are: ashes, charcoal (from domestic fires), old thatch, bones, straw and the by-products from several activities such as soap making. Note that this technique is traditional in countries that have a fairly high rainfall: Guinea, Liberia, part of Ghana. The age of some settlments in these countries can be estimated by the depth of the black earths.
The Terra preta and the African dark earths are characterized by a black earth very rich in carbon, and with a thickness ranging from 50 cm to 2 meters. Structurally, this anthroposol is partly composed of charcoal grains which are nutrient traps for sme of the micro-nutrient needed by plants. The technique creates an aerated soil which is therefore warm, porous and which easily holds water thanks to the particles of charcoal and the increased level of organic matter.. It is therefore particularly conducive to good root development.
The incorporation of organic matter and charcoal increases the level of carbon in the soil and therefore helps reduce global atmospheric Co2 levels
The places where this method is found often have poor and relatively infertile soils. The technique allows the creation of soils conducive to cultivated plants. The places where I experimented with these black soils were not have very fertile: either made up of a lot of sand, a lot of silt or very heavy clay. I dug to create strips one metre wide, several metres long, to a depth of 15cm. I opened the bottom of the pits with a broadfork, also called a U-fork or grelinette. In these sunken strips I first ground up charcoal to make biochar (more about biochar below)*** which I spread over the pits, I threw in as much vegetable debris as I could find. Then I sowed green manures that I flattened down just as they flowered, this is when their root mass is the biggest. Then I sowed leguminous plants like peas, beans etc. I continued this process for a few years. Gradually, the depth and fertility of the bands increased.
** The Amazon wasn’t an untouched wilderness when Europeans invaded it. There were maybe as many as 8 to10 million people living there. The so called lost temples and cities that have been a subject for adventure films for some time, weren’t ‘lost’. They were in cleared area surrounded by farmland. This documentary explores some of the current thinking. https://www.bbc.co.uk/programmes/p00hm7jx
*** Biochar
Studies carried out on black soils show the importance of charcoal in these systems. Subsequently, other studies have shown that adding ground charcoal can help our vegetable gardens to:
increase the ability of soil to retain moisture
reduce the acidity of soil
detoxify soil ( toxins are less bioavailable in the presence of charcoal).
This is the time when I can best show the holistic approach of Permaculture. A criticism of biochar production is the emission of tars and a mixture of combustible gases. This is a valid criticism because if these by-products are not used then they are pollutants. A permaculture charcoal production system will use the tar and pyrolysis gases. The former preserves wood from rotting so for some peoples in the past charcoal was the by-product and the tar what they were looking for. Pyrolysis of softwood produces turpentine. This process produces a gas containing carbon monoxide and dioxide, methane and hydrogen, and we call this gaseous combination wood gas. It is still used in some countries as fuel for vehicles, here in Europe during and just after the 2nd World War.
I heat my vardo with a wood stove; when I empty it, I collect the ashes as well as the charcoal. I grind and sieve the latter until it is reduced to pieces of less than 2 mm. I spread this powder on the garden beds at the rate of one kilogram per square metre. When we open new area to cultivation we produce extra biochar using an old oil drum a a charcoal kiln.