I am attempting to create a Terra preta type soil using lump barbecue charcoal. (Have a look at the links on the right to see the Terra preta film.)
This type of very fertile soil has been found in South America and has been made by people over thousands of years using charcoal as a soil amendment. The evidence that I originally read was that charcoal reduced the fertility of the soil when it was initially added to the soil. This discouraged me from doing experimenting on my allotment. However, after reading around the subject and talking to people on various websites such as Allotments.uk I developed the hypothesis that an "inoculated" charcoal may be able to be used as a beneficial soil amendment to produce a sustainably fertile soil.
Some people are still trying to develop an inoculated charcoal that is very similar to that found the South American Terra preta. My reasoning was that this seemed to be over complicated because if the charcoal could be amended by Amazonian nutrients then it should be able to be inoculated with nutrients originating in my local area.
I am experimenting with using comfrey, nettle and sweet cicely liquid fortified with worm bin liquid.
Inoculated charcoal is charcoal that has been marinaded in nutrients. Charcoal has some remarkable properties that allow it to absorb or adsorb chemicals within its labyrinthine structure. We can use this property to enable us to deliver nutrients to the soil in a form that might not readily leach away.
I would conjecture that the charcoal acts as a kind of buffering mechanism that allows nutrients to diffuse into the soil when there is a low concentration of them while adsorbing them when there is an excess.
The structure of charcoal also gives a relatively safe place for beneficial bacteria and fungi to live adding more nutrients to the soil.
Research has found that Terra preta soils also contain mychorrhizal fungi which form a symbiotic relationship with plants. The charcoal could give these fungi a source of nutrient that can be transferred to plants and also a protected environment for their spores.
Now all of this is complete conjecture because research in this area is in its infancy. We are trying to follow in the footsteps of ancient Amazonian peoples and it is difficult.
I have had impressive results this year after adding charcoal to the soil. Whether this is due to the charcoal or to some other factor this year, I don't know. We are attempting to replicate a soil that has developed in the Amazonian rain forests areas. There is no evidence that this can be replicated in places like England.
I would like to think that I have not wasted quite a lot of money buying charcoal and that it does have some beneficial effect on the fertility of the soil. What worries me is that there is not a rush to add charcoal to the soil by farmers and market gardeners. Is this because the data is inconclusive or for some other reason such as the cost effectiveness of charcoal.
Now as a warning, I need to emphasise that it is the genuine lump wood charcoal that should be used not the briquets. Charcoal briquets are made with cement and will alter the pH of the soil significantly. Charcoal itself is alkaline so any further raising of the pH will significantly change the character of the soil.
Is adding charcoal to the soil any better than just burying unburnt brush and logs? Hugelkultur raised bed systems, which the South Americans also developed, would suggest that there is little difference. Both are good ways in which to add large quantities of carbon to the soil. Is the quantity of carbon in the soil significant in raising its fertility?
Although plants need relatively large amounts of carbon dioxide and water to produce their food, the amount of other nutrients they require is very small.
Anything added to the soil that will help to retain moisture will be beneficial to plants. There is evidence that charcoal helps to retain moisture in the soil. Together with water retention there is also a benefit in that addition of long lasting carbon will help with drainage of the soil.
I would conjecture that there is an overall benefit of adding charcoal to the soil and this is similar to adding unburnt organic matter to the soil.