Terra Preta on a cold December day.

Although I am going to go through what is known about these black earths, I think that it cannot be decided how useful this will be for amateur gardeners until we have tried it for ourselves.  This is what several of us are doing at allotments.uk.

Could the research on Amazonian black earths be applied to British soils to increase their fertility?  Research shows large areas of fertile black earths in otherwise very poor rain forest soils.  The fertility of these soils seems to be associated with charcoal added to the soil over thousands of years by indigenous South American civilizations.  This could have been done purposefully or coincidentally when adding midden contents to fertilize fields. Regardless of their motivation, it seems that this has produced a remarkable soil with some strange characteristics. 
It seems that regardless of its use it remains fertile for generations even when these fields were reclaimed by the rain forest.

Ignoring the weeds, this end of the pea rows did not get any inoculated charcoal. 

 This is the end that got the inoculated charcoal. Not the best peas in the world because they were on relatively infertile soil.

However, when scientists add charcoal to soil there is a noticeable reduction in fertility. This is cited as a reason for not developing a slow acting soil amendment from a charcoal base.  How could such a relatively inert substance, which does not degrade substantially over hundreds of years, reduce fertility?

There is some suggestion that charcoal adsorbs or absorbs nutrients in the soil.  It certainly has the capacity to do this having an intricate structure of many small channels giving it an extremely large surface area.

So nutrients may well have a tendency to adhere to the large surface area.  As there is a depletion of nutrients in surrounding areas of soil when charcoal is added to the soil, maybe adding nutrients to charcoal before putting it on soil might mitigate any adsorption or absorption problems that adding "neat" charcoal might produce. 

This could produce a slow nutrient release system.  Affinity of the charcoal for nutrients depends on the condition and type of soil it is in, but I would suggest that this would be a buffering mechanism.  It would adsorb nutrients when they were in surplus but as concentration decreases the charcoal will lose nutrient by a diffusion process maintaining equilibrium with the surrounding soil.
What kind of nutrients could amateur gardeners inoculate the charcoal with to give the maximum long lasting effect on the fertility of their soil?
Several different methods and recipes have been suggested; however I will only give you the method and nutrients that I use personally.  My charcoal, which is just barbecue lump charcoal, is marinated in:

·         Chamomile tea

·         Nettle tea

·         Sweet Cicely tea

·         Worm tea

·         Sugar

·         Blood, fish and bone meal.

The mixture is housed in an old plastic dustbin and left for at least two months.  After this time the charcoal is removed, allowed to dry and then crushed with a bull hammer into pieces no more than 1 cm³.  This gives the charcoal a larger surface area and makes it easier to add to the soil.  I have used bigger lumps but they are cast to the top of the soil continuously by the frost rather than being incorporated within the top 15 cm of the soil. 

There are large numbers of micro pores formed from the xylem and phloem structures within the charred wood and both soil bacteria and fungi could find a habitat in these pores free of predators.

The importance of the micro fauna and flora of the soil cannot be overestimated especially when considering the number of symbiotic or mutually beneficial, relationships there are between micro organisms and crop plants.   There are some tentative indications that mychorrhizal fungi associations with crop plants are encouraged by the addition of charcoal to the soil. Mychorrhizal associations with plants enable them to obtain nutrients from a wider volume of soil than otherwise would be the case, the plant giving the fungi sugars in return. 

My contention is that cultivation disrupts this association and kills off mychorrhiza fungi through cropping, digging and forking.  Adding these fungi to the charcoal may help to replace them in the soil.  A number of scientists suggest that the soil is abundant in spores of these fungi and that adding more is superfluous.  They seem to be basing their assumptions on research into natural ecosystems rather than cultivated environments and I wonder if this still hold true.  

·         So I add mychorrhizal fungi to the charcoal as well.  But only after crushing the charcoal. 

There is some evidence that adding charcoal to compost heaps achieves the same adsorption of nutrients but I have not tried this. 

How much charcoal to add to the soil is another unanswered question and one that we are trying to assess.  There seems to be two ways of adding it.  

·         Spread it broadcast over the soil.

·         Just add it to planting areas in planting holes or along rows of seed drills.  

At the moment I am just adding charcoal to planting areas. 

A further advantage of adding charcoal seems to be its ability to help the soil to retain water while also allowing better drainage. 

 Charcoal is relatively inert in the soil so there is a great opportunity for sequestration of carbon.  The more charcoal that can be added to the soil the larger and better our crop plants will become. All these different advantages of adding charcoal to the soil seem to be both long lasting and cumulative.