Amazonian terra preta black-earth soils have existed for up to 2500 years. They were man-made by ancient charcoal making communities. Old settlements have been rediscovered buried under ancient rainforest and confirm the observation of early 16 century explorers that there was at that time widespread human amazonian habitation. It would seem that ancient potters mixed their broken shards, charcoal, plant and animal waste in ancient middens and for uncounted generations added the fertile residues onto their soil. These terra preta soils even now, are up to two metres deep, fabulously fertile and high yielding.
Charcoal provides within it’s deep pores a wonderful substrate for arbuscular fungi. These are the fungi that provide rich mycorrhizal associations with plants. The mycelium of these fungi is rich in glomalin, the recently discovered long lasting organic matter that comprises a significant component of the world’s soils.
Charcoal in the soil has superb agricultural properties and holds plant nutrients, water and air. It reduces soil emissions of greenhouse gases such as methane and nitrous oxides. It is generally kind to all manner of soil life. Plants love it. In South America a native species of worm mixes it intimately with the soil. Best of all, carbon can be stable for millenia when sequestered in the ground!
In recent years there has been intensive research into the agricultural uses of charcoal. They call it biochar and is made by a slow burning process known as pyrolysis. There is huge potential for farm wastes to be charred and returned to the ground. It’s a win-win situation. It can lead to huge increases in soil fertility and at the same time sequester a significant amount of carbon.
|Traditional Oxfordshire charcoal production
Using charcoal at home
For six years now I have been burning my bonfires to make charcoal. I do it by the simple expedient of dousing my fire with water and not letting the embers burn through. I have an annual fire to burn my bulky prunings at both cemetery gardens and on the village plot as well at at home and generate about six barrow loads of char each year.
Char varies considerably depending on it’s source and method and temperature of its production. I cannot make any special claim for the black stuff made by my unconventional method but that has not stopped me having fun.
|Embers immediately turn black
Best practice is to shallowly cultivate biochar into the ground. As a none digger I have limited myself to applying it as a mulch on my vegetable garden, in the knowledge that future sowing and planting operations will work it in. I cannot, of course, claim any valid conclusions from my play. I have recently tentatively started using char as an ingredient in potting compost and as a soil amendment when planting ferns and other special plants that normally do not thrive in my soil.
Fresh charcoal, like wood ash, often contains lime. It strongly absorbs nutrients and can initially deplete the soil. This can be corrected by mixing in multi-nutrient fertilizer into an outdoor weathering pile of char. I also suggest that char be tried as a constituent of a compost heap and that the gardener recreates the ancient midden to charge the charcoal with fertility. Such compost will have some permanent bulk and will not, like normal compost, all eventually decay away.
With regard to my own efforts at composting I frequently have difficult choices of how to recycle the copious amounts of herbaceous tops that my garden generates each winter. Especially so, for plants with coarse leaves like miscanthus and other tall grasses with fibrous foliage.
Fifteen months ago I prepared a heap composed of succeeding layers of fresh charcoal and large amounts of this strawy debris. It was supplemented with a few handfuls of a nitrogen-rich general fertilizer and a little dolomitic lime. I also added liberal amounts of aquatic vegetation, skimmed out when cleaning my pond. Still not completely decayed I have recently applied the product of my heap as a thick and fairly coarse mulch over my asparagus, rhubarb and soft fruit.
|The bottom of the heap will be a great place to establish my marrows and butternut squashes.
|The soil surface of my vegetable plot after six years. With an old brick I have inadvertently added my version of shards of native indian pottery!
More about terra preta from wikipedia
I have been writing more about char, glomalin and humus here