What is Terra Preta, how does it create so much fertility, and can I apply it on a home scale in my temperate-climate garden? These questions led me on a fascinating hunt through scientific journals, newspaper gardening columns, historical accounts, and biochar-making demonstrations from both American businesspeople and Brazilian Indigenous communities. Jump to the bibliography to check out these resources for yourself, or keep reading for my synthesis of the information I found.

What is Terra Preta?

Terra Preta, short for Terra Preta de Indio (“Dark earth of the Indian”), is the Portuguese name given to large deposits of black, fertile soil intentionally created by ancient Indigenous civilizations in the Amazon rainforest. The term refers specifically to these pre-Columbian soil deposits, so it isn’t something a modern-day grower can go out and buy at the garden centre. But we can learn from it and replicate some of its successes in our own soils. And we can make or buy biochar—more on that later.

The main ingredient in Terra Preta is wood that has been covered to keep oxygen out, charred thoroughly with the heat of a fire, and pulverized into small particles. Researchers suspect that the charring process was done by laying the wood in trenches or pits and covering it in damp plant matter before burning it. In fact, this process is used today by some Indigenous communities in Brazil.

Once the char was prepared, it was mixed with bone fragments, broken pottery, and manure. To understand the significance of what happened next, it’s important to know that a soil’s fertility is directly tied to how much carbon is stored in it. Despite the bad reputation carbon gets for wreaking havoc in the atmosphere, in the earth it is the building block of all living things. 

Soil and Carbon

The compost we add to garden soils is dominated by carbon, which is then consumed by the plants that grow in it. If the garden contains only annuals, which turn the compost into flowers and fruits that are then picked, the carbon has to be replaced every so often. If the garden is designed to use perennial plants and mulch instead, the carbon is recycled through the system for a long time. This is good, but Terra Preta took things a step further. When the mixture of char and other materials was added to the thin soil of the Amazon, it created a storehouse of carbon that is actually able to replenish itself when disturbed—even to this day.

In most environments, nature is extremely slow to replace topsoil that has been depleted. If there is too much disturbance, a feedback loop might even be triggered that degrades the soil around the disturbed location too. But Terra Preta can regenerate itself at a rate of 1 cm per year, which is very fast. One theory connects this to Terra Preta’s high levels of mycorrhizal fungi. These fungi help the members of a soil ecosystem exchange nutrients with each other, and it’s thought that they might play a role in quickly transforming organic matter into soil. A lot more research is needed in this area.

The fertility and depth of Terra Preta is extremely  impressive in a rainforest environment where the soil is actually poor and thin. It might be surprising that a rainforest has poor soil, but the huge abundance of life in the ecosystem means that most nutrients are immediately recycled and consumed by other plants and critters. Without the productive pockets of Terra Preta, it would have been impossible to grow the food needed to support the complex societies that thrived in this region before European contact. In fact, before the Terra Preta sites were documented by Europeans, many historians rejected stories of Amazonian civilizations out of hand. They were sure the environment couldn’t have supported the agriculture required. 

Origins of Terra Preta

For more than a hundred years, researchers debated whether Terra Preta was a human-made product or a happy natural accident. They speculated that forest fires or volcanoes could have left ash deposits to be buried over the course of millennia. However, investigation has shown that the material was added much too regularly for these explanations to be reasonable. Besides, the bone fragments of small rodents, turtles, and other food animals only make sense as deliberate additions by human hands. Most convincingly, analysts have proven that human urine and feces were significant ingredients. No natural disaster could have added those to the mix. It’s also important to remember that, as mentioned above, nutrients in a tropical rainforest are recycled quickly. If nature had laid a layer of ash over the ground in the Amazon, it would have been used up as soon as the forest regrew.

In recent years, deposits similar to the Brazilian Terra Preta have been found in other countries around the world, including Ecuador, Peru, Liberia, and South Africa. We know that fire was used as a land management tool by Indigenous peoples throughout history, so it may be that further deposits will be identified in the future. This seems particularly likely as this ancient practice receives new interest as a potential modern solution for soil fertility. 

Why is Terra Preta so effective for growing plants?

Present-day studies show that Terra Preta creates a great habitat for healthy soil ecosystems, improves plants’ ability to take nutrients from the soil, lowers soil acidity, and improves its ability to hold water. Here are some of the known reasons for its effectiveness:

• The process of charring turns wood into tiny particles that are studded with even smaller holes. This creates a huge amount of surface area that can be colonized by microbes and mycorrhiza (the networks of fungi that help plants exchange nutrients with the rest of the soil ecosystem). 
• The manure (including human waste), compost, and bones incorporated into the Terra Preta provide the nutrients plants need, including nitrogen, phosphorus, and potassium.
• The chemical structure of the charred wood is extremely stable, which means its molecules won’t easily break apart. This keeps the soil’s nutrients from leaching into the air or water.
• Charcoal is alkaline, so incorporating it into too-acidic soil creates a better environment for most plants.
• The light, porous nature of the char and the other organic matter in the soil creates tiny gaps where water can soak in and be retained, much like the holes in a sponge.

How can Terra Preta be created today?

Today, many scientists and companies are eagerly exploring the possible uses of biochar, a modern name for the charred wood described above. With current technology, we can improve the efficiency of the burning process (called “pyrolysis”) so that almost all of the wood’s carbon is retained in the biochar. Contrast this with traditional pit or trench techniques, which can turn up to half of the wood’s carbon into carbon dioxide that is lost into the atmosphere. Commercial operations create biochar in large facilities, but individuals can get good results by burning wood in a nested pair of steel drums with strategically drilled holes that control the flow of oxygen.

Charcoal on its own can be a useful soil amendment, but it does not give the same dramatic results and long-lasting soil improvement of Terra Preta. Scientists have determined that blending biochar with organic matter (compost, manure, etc.) is essential for recreating the conditions of the Amazonian soils, and plenty of gardeners have found the same is true in their own experiments. Many proponents recommend “charging” or “activating” the biochar with microbes before adding it to your soil. The simplest way to do this is to mix it with your compost pile and leave it alone for several months, but you can also speed up the process by using a microbe-rich material like worm castings. At the end of the charging process, add your activated biochar to your soil. Burying it will give the best results. 

Research is ongoing about whether we can and should recreate Terra Preta in order to help feed our fast-growing population and to help sequester carbon in the soil. Many experiments show promising results, but it’s not a universal cure-all. Some climates affect soil-building in unpredictable ways, and it may be that some regions don’t have the right conditions for Terra Preta. In some places, mixing charcoal with organic matter can actually speed up the process of converting humus to carbon dioxide, which is the opposite of what we want. More studies will be needed before Terra Preta-creating techniques can be adopted for large-scale agriculture around the world.

So will I use it in my garden?

Having learned all this from my research, I’d like to try working with biochar. Temperate-climate trials have had mixed results, so I won’t go all in straightaway. But an experimental bed will be well worth my while. If it works, I’ll have an incredible boost to my fertility. If it doesn’t, I’ll still have learned a lot about how humans can feed big populations while leaving the earth richer than they found it.

Bibliography

A. Schmalenberger, A. Fox, “Bacterial Mobilization of Nutrients from Biochar-Amended Soils” Advances in Applied Microbiology, 2016. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/terra-preta (accessed January 1, 2020).

Schmidt HP “Terra Preta – model of a cultural technique” the Biochar Journal, 2014. www.biochar-journal.org/en/ct/4 (Accessed January 9, 2020).

Glaser, Bruno, “Prehistorically modified soils of central Amazonia: a model for sustainable agriculture in the twenty-first century” 362 Phil. Trans. R. Soc. B, 2006. https://royalsocietypublishing.org/doi/10.1098/rstb.2006.1978 (accessed January 5, 2020). 

Barbano, Paul, “Add charcoal to create rich, quality garden soil” Cape Gazette, 2012. https://www.capegazette.com/article/add-charcoal-create-rich-quality-garden-soil/25068 (Accessed January 9, 2020).

US Biochar Initiative, “Soil and water benefits of biochar” USBI Building the Future from the Ground Up, 2019. http://biochar-us.org/soil-water-benefits-biochar (Accessed January 9, 2020). 

Gisser, Martin, “Terra Preta” The Azimuth Project 2015. https://www.azimuthproject.org/azimuth/show/Terra+preta (accessed January 9, 2020).  

Cornell University Department of Crop Sciences, “Terra Preta de Indio,” Soil Fertility Management and Soil Biogeochemistry, 

http://www.css.cornell.edu/faculty/lehmann/research/terra%20preta/terrapretamain.html (Accessed January 5, 2020).

SGEV Costa Rica. “How to make Biochar from locals in the Peruvian Amazon.” YouTube video. June 24, 2014. https://www.youtube.com/watch?v=KNFDOGWozKU

Living Web Farms. “Biochar workshop Part 1: How to Make Biochar.” YouTube video. November 21, 2013. https://www.youtube.com/watch?v=svNg5w7WY0k