“Terra preta” means “black earth” in Portugese, and I first came across this phenomenon a few years ago when idly browsing the internet. It has remained an interest ever since, and every so often I dip back into the internet to see if there are any further developments. The following is a very brief explanation of Terra Preta.
It began over 2,000 years ago in the Amazonian basin by the indigenous population, who developed a form of soil-building as a means of resolving the poor tropical soil fertility. Large deposits of this black earth are still found today, with depths of up to two metres.
The main constituent is char-wood, or biochar, which is a type of charcoal produced by burning wood in a low-oxygen environment in a process called pyrolysis. Being a carbon source, biochar has extreme stability in soil. It is also porous, providing a stable surface for organo-mineral complexes to form – the chemical bonding of soil organic matter with soil minerals. It has high levels of phosphorous and nitrogen. Even in an environment of heavy tropical rainfall, the nutrients are not leached out of the soil.
In addition to biochar and high nutrients, analysis has found that arbuscular mycorrhizal fungi increases in biochar which provides an ideal habitat.
We are still not entirely sure how these ancient civilisations produced Terra Preta. It may be that they produced biochar then added organic nutrients in the form of bone or manure, then mixed it in the soil. It may be that it started as ash-fall from volcanic fires – or as sediment from lakes. It could even be a chance by-product from middens close to habitation.
However it started, it has some remarkable properties. Local farmers who remove it from in situ to sell as compost, report that it appears to regenerate itself. Its high nutrient levels, the presence of mycorrhizal fungi and its ability to lower pH levels slightly in the Amazonian acidic soil make it a highly attractive form of compost.
Unsurprisingly, in the last few years experiments and research have been undertaken to see if it is possible to replicate Terra Preta. This work has identified a number of features which prove helpful in a number of ways.
Low temperature charring of wood creates an internal environment within the biochar that encourages bacterial growth – researchers soaked new biochar in urine, plant tea etc to see (nice). The fungi thrived, which may explain how the soil regenerates – from a stable biochar base, the fungi will travel through the surrounding soil. Worms play their part too. Plain charcoal applied to the soil however acts like a sponge, absorbing nutrients and actually depleting the soil it is placed in.
The potential for Terra Preta to regenerate and enrich poor soil, provides hope for the growth of food crops for a burgeoning population. It also has another trick up its sleeve – it is an ideal way of sequestering carbon. Programmes have been initiated to try and change the practice of slash-and-burn in the forests, to slash-and-char and to subsequently bury the ash product in the soil.
It would seem Terra Preta has much to teach us and we have much to learn even after thousands of years.
Sue Bennett, 2015