Terra Preta

 

Today, I would like to introduce you to something rather mysterious and, arguably, rather magical. Terra preta. 

Also known as Amazonian dark earth, terra preta is one of the world's most fertile soils found in sporadic, unpredictable patches within the heart of the Amazonian rainforest. The fact this coveted soil is found in the Amazon is rather surprising, because tropical rainforest soil is notoriously, and counterintuitively, infertility. This is because so much life recycles nutrients so efficiently through the ecosystem that almost all the carbon and nutrients are locked up in the forest's living and decaying matter. They barely get the chance to reach the soil beneath, which remains thin and infertile.

This is why subsistance farmers in the Amazon are often forced to practice slash-and-burn agriculture. This destructive farming technique involves clearing and burning a section of forest, growing crops on the land for about three years after which time the soil's fertility is exhausted, and then moving on to slash-and-burn a new area.

So, where oh where do these strange fertile pockets of terra preta come from? The rich dark colour of this soil itself offers a clue. These prized sites all have a feature in common. The soil is chock full of charcoal and that charcoal was deposited by people. We know this because of the multitude of potsherds, ceramic pieces, found consistantly within them.

Yes, that's right. Amazing, isn't it? Archaeologists now know ancient indigenous peoples, lost through time, purposefully made this soil from a combination of charcoal, compost, manure, and bone. The magic is that this winning combination created what appears to be permanently fertile soil, at the very least, soil which has remained fertile for hundreds of years.

How is this possible you ask? Well, charcoal possesses some rather unusual properties. Charcoal itself is almost pure carbon, low in nutrients and incredibly stable. It has the ability to stay in the soil unchanged for over 500 years. The microscopic structure of charcoal is where the magic lies.

As you can see in the microscopic cross-section of plant material above, wood and consequently charcoal possesses a myriad of pours maximising its surface area. It is estimated a single gram of charcoal possesses a surface area of over 800 square metres. When I heard that fact, I was flabbergasted. 

This expansive surface of nooks and crannies just so happens to be a brilliant size for beneficial soil microbes. It provides them the perfect habitat. The microbes multiply, accumulating nutrients along the way, which in turn allows crops to thrive. They grow significantly more vigorously year after year on charcoal amended soil.

This amazing story has attracted such interest over the last twenty years or so that the boring name of charcoal has been replaced by the trendier biochar. I find this 'rebranding' rather amusing more than anything, but if it helps promote this beneficial soil amendment, I'm not complaining.

In a future post, I'll explain the techniques I'm playing around with in my own garden to make and use charcoal, or biochar if you prefer, to convert my soil into terra preta. So exciting.