Photo: Naturefund

It was used more than 2,000 years ago by indigenous peoples of South America. We explain why their use still makes sense today and how Terra Preta is good for the soil and the climate.

The Discovery of Terra Preta

For a long time scientists agreed that it is impossible that higher developed civilizations could develop in the rainforest areas of the Amazon. Developments of big towns have been unimaginable because the humus layer in primeval forests is too small to guarantee a suitable food supply for hundred thousands of people. In tropical primeval forests the soil type „Ferrasol“ rules predominantly, a coined word itself which derives from the words „Ferrum“ for iron, „Alumen“ for aluminum and „Sol“ for soil. These grounds were strongly weathered and acidified by persistent heat and humidity over a million years. This is why almost no unweathered minerals exist in the soil, from which nutrients could still be won for the ground. Concentrations of aluminum are already so high that they would be toxic for useful plants.

Nothing without humus

Nevertheless, there is no bare landscape on these grounds, but lush woods with a huge diversity of fauna and flora. The whole potential of these woods lies in a thin humus layer which releases nutrients and holds these nutrients within the system in close collaboration with the plants of the forest. It is self-explanatory that this complicated circulation of nutrients and water is extremely sensitive. If the primeval forest is cleared, the humus reduces in shortest time and the nutrients are rinsed out by the rain.

An innovation of the natives

Already in the 1960s, researchers discovered in the delta of the Amazon, Rio Negro and Madeira remainsof big precolumbian civilizations. This left behind a mystery for the researchers. How could big cultures guarantee food safety for themselves, on such an infertile ground? Years later the answer was given by a soil analysis. One came across “Terra Preta do Indio”. The mighty black top soils, rich in nutrients, which endured more than 2000 years in the areas and were still fertile, led to the survival of thousands of people. The up to two meter thick layers consisted of charcoal, bones, clay shards, traces of human fece, ash and fish bones. From this mix of organic materials the mighty humus layer had formed over time.

Biochar as an important ingredient

Terra Preta was discovered and with it the curiosity about its origin and the knowledge transfer. Publications, researches and attempts of imitations of the Terra Preta and the Biochar have steadily increased over the last two decades. One of the most important ingredients of Terra Preta is the Biochar (Plant coal). Its high content of carbon makes it very persistent and gives Terra Preta special characteristics. However, Biochar on its own doesn’t create fertile soil. To unfold the qualities of the coal completely, it must be charged first. This means that the coal first needs to be “fed” with micro-organisms. Typical procedures are here composting or fermentation.

How to: Biochar

However, which special qualities does the Biochar have and how can we get them? Biochar by definition is a product of pyrolytic procedures of carbonization with purely plant based materials, and distinguishes itself by its ecological and sustainable production. A pyrolysis exists when biomass carbonizes under a strong restriction of oxygen. The long-chain molecules of the herbal cells open up during this process. Slightly volatile components of the biomass are released, resulting in warmth, leaving the carboniferous structure of the biomass with its special qualities. Next to that mineral substances of the original biomass are bound in the pores and in the surface of the Biochar.

Like a sponge

Biochar stands out by its extremely porous structure. Resulting in a huge surface of partly more than 300 m2 per gramme. Because of this characteristic, Biochar is able to absorb five times as much of its own weight of water and its dissolved substances. This is called the adsorptive capacity of Biochar which may vary, depending on the temperature of the pyrolysis procedure and the chosen biomass. The adsorptive capacity of the Biochar gets the best results on a temperature between 450 and 700 °C during its production.

Ability to store important nutrients

Another advantage of Biochar is its great cation exchange capacity (CEC), which is due to its porosity. The CEC is a measure of soil science and describes the ability to store positively loaded ions on the surface and, as a result, make it available again for plants and micro-organisms.

A high CEC-Value secures that less mineral and also organic nutrients from the soil are washed out, leading to a higher availability of nutrients in the ground.

Reduce CO2 with Biochar?

Using Biochar for ground improvement has various positive qualities, which are of great importance. For example, it can lead to the fact that the water storage capability and the aeration of soil are clearly improved. Because of the better aeration of soil, at the same time a better activity of nitrogen bacteria is possible, reducing the escape of climate-damaging methane and nitrous oxide.

As already mentioned, Biochar provides a protected living space to soil bacteria because of its structure. This is why the nutrient conversion for plants is supported as well. The high adsorptive capacity also leads to a storage of toxic soil material (e.g. heavy metals) on the specific surface of the Biochar. These are no longer available for plants, protecting them and the ground water from contaminations.

Biochar and its contribution to climate protection

Biochar consists mainly of pure carbon which can only hardly or not at all be degraded by micro-organisms. If Biochar is now added into the soil, 80% of the carbon content will remain stable for more than 1000 years. As an example: the discovery of Terra Preta can be used, showing that after more than 2500 years the Biochar can still be found in the soil. This high persistence of the Biochar leads to the fact that the C02 taken away from the plant and the atmosphere in the long term, may be able to slow down the climate change.

The bond of CO2

Another potential of the carbon isolation lies within the process of pyrolysis itself. Biomass carbonizes to 40% pure Biochar and that means that the carbon of the biomass is not totally changed into CO2 and free to escape into the atmosphere. But a large part remains bonded within the Biochar. This can affect climate positively, too.

Bye Bye CO2

When the Biochar is added to the soil it remains stable for thousands of years and the carbon of the Biochar is removed from the carbon-cycle, because its not converting into CO2 or CH4 (methane) due to burning or putrefaction. 

Pflanzenkohle als Reinigungsschwamm

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Ilmo Kolehmainen von Carbons Finland Oy Ldt. aus Finnland stellt Ansätze zur Behandlung von Regenwasser in der Stadt mit Pflanzenkohle vor. Vortrag auf Englisch
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PD Dr. Gerhard Soja | "Pflanzenkohle zur Bodensanierung" | FVPK

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