Many amateur gardeners have already come across the term “Terra Preta”. Word is also slowly spreading that plant charcoal makes the soil more fertile, stores water and is climate-friendly to boot. But how exactly is all this connected? And what is the best way to improve the soil in your own garden or balcony with plant charcoal?
Who invented it?
Not so long ago, in the 1960s, researchers in the Amazon region discovered extremely fertile, deep, almost black soils. This was striking, because tropical soils are otherwise often light reddish in color and shallow. And they are only fertile as long as they are covered by an intact tropical rainforest, which continuously sheds leaves, fruits and branches. These rot quickly in the warm, humid climate and release the nutrients. If the forest is cut down to make way for farmland, this cycle is interrupted. As a result, the fertility of the soil decreases rapidly. As a result, high fertilizer inputs are needed to keep yields stable in the fields and plantations.
It is therefore not surprising that the researchers were surprised by their find and took a closer look at it. What they discovered were remains of pottery shards, fish bones, animal bones and human feces, as well as many very small charcoal particles. It is assumed that indigenous peoples rotted their organic waste with charcoal residues from their ovens in clay pots to form a kind of compost. This they stored for decades or centuries and/or actively spread. The charcoal in particular did not rot, but accumulated over time and turned the soil black. One gave these soils the name Terra Preta (black earth).
Vegetable carbon is a jack of all trades
What makes these soils so sustainably fertile? It is the special properties of plant charcoal. The charcoal particles have a porous structure and are permeated with cavities, much like a sponge. On the one hand, this “sponge” can store water well. On the other hand, the cavities provide suitable habitats for soil microorganisms. The excretions of these microorganisms are ideal nutrients for plants. The plant has to do nothing other than bring its root tips and root hairs directly up to the charcoal – and it’s in a land of milk and honey. If there is no root nearby, the nutrients are still not lost. The sponge-like structure of the charcoal means that the total surface area is immense. Nutrients can be easily “attached” or stored on this surface.
How charcoal works in the garden
The charcoal keeps the soil from leaching its nutrients. It buffers water during heavy rainfall and can store it for drier times. And it provides a place for tiny soil microorganisms to multiply and live out their positive effects on plants. This sounds like an ideal soil conditioner: soil that is plagued by heat waves, that can store water, and that no longer needs to be fertilized so frequently because the microorganisms are constantly supplying nutrients – what more could a gardener’s heart desire? In fact, many amateur gardeners now swear by plant charcoal. Be it in the vegetable patch, the flower border, in the raised bed or in the pot on the balcony. There are many ways to obtain and use it. The following is important to note:
Vegetable carbon should never be applied “fresh”, but only “activated”. This means that the plant charcoal has been enriched with organic nutrients and revitalized with microorganisms. There are various processes for this. If it were applied fresh and untreated, it would have the opposite effect in an initial period. It would extract nutrients from the soil and withhold them from the plant. The plant would then thrive worse or not at all.
The plant charcoal must come from a safe source. The term “biochar” (or English “biochar”) is also used to market charcoal that is not made from plant residues such as wood, wood waste or green waste. Rather, this was obtained from other organic materials such as sewage sludge and slaughterhouse waste. Here, contamination with pollutants cannot always be ruled out.
The plant charcoal should be applied as fine-grained as possible. The coarser the pieces, the more difficult it is for the plant roots to access the stored water and nutrients. Grain sizes between sand and very fine split gravel are optimal.
Do not confuse plant charcoal and ash. Vegetable carbon is black and shiny and can be broken into smaller and smaller pieces. Ash, on the other hand, is whitish-gray and dusty. It has a completely different effect than plant charcoal and can harm the soil in larger quantities.
Always activate homemade plant charcoal
If you want to make your own vegetable charcoal, you need a pyrolysis oven – a simple fireplace will not do. In pyrolysis, the wood is not completely burned to ash, but is pyrolyzed into charcoal – with almost no air supply and at temperatures above 400 °C. This means that almost all the other substances in the wood are removed. This means that almost all other substances still contained in the wood burn. Only the carbon skeleton remains.
In a second step, the coal obtained must be crushed and then activated. The coal can be activated in various ways. For example, by composting it together with horse or rabbit manure or nutrient-rich kitchen and garden waste.
In any case, the production is somewhat laborious and requires know-how and experience (which, by the way, numerous providers impart in courses). If you prefer to purchase ready-to-use plant charcoal, you will find various products on the market today. We recommend that you make sure when buying that it is a tested product that is free of harmful substances. In addition, the plant carbon should already be revitalized with microorganisms and enriched with nutrients. Thus, it shows a fertilizing effect from the beginning.