How Ancient Soil Could Save The Rainforest

We need to send less food to landfill sites, reduce our reliance on fertilisers, and keep our soil healthy so we don’t keep clearing more wild spaces for new farmland. Thousands of years ago, people in the Amazon found a surprising solution to these challenges.

Our food system today faces many challenges. We need to keep producing enough healthy calories for a growing population, and we can’t keep wasting so much food or converting so much wild space to farmland. 

Here are three key challenges that we need to solve:

1. The rainforest needs a break.

Modern industrial agriculture already covers about 38% of the global land surface. Clearing wild spaces to make more farmland is a leading cause of biodiversity loss worldwide and is linked to 70-80% of tropical deforestation.^1,2

2. We need to get food out of landfills.

When food breaks down in these conditions, without oxygen, it releases potent greenhouse gases like methane - 28 times more climate-warming than carbon dioxide over a hundred years.³ 

3. Fertilisers come with a cost.

We use vast amounts of synthetic fertilisers to provide crops with essential nutrients such as nitrogen. This helps us produce enough food for everyone on less land, saving up to 1.5 billion hectares of natural land from becoming farmland!⁴ But 65% of fertiliser we apply isn’t even absorbed by the plant.⁵ Instead, it ends up polluting our air and water, contributing to ocean dead zones and releasing nitrous oxide - a greenhouse gas 300 times more climate warming than carbon dioxide.⁶

But what if these problems were actually solutions in disguise? Could we get our unwanted food out of landfills and use it to make our soils healthier? This way, we could reduce our reliance on chemical fertilisers at the same time that we take pressure off natural spaces and reduce the climate-warming gases coming out of food waste. Thousands of years ago, people in the Amazon solved this puzzle.

People have lived in the Amazon for thousands of years

When you think of the Amazon, what do you picture? If you’ve been reading the news recently, you might think of illegal land grabs, slashing machetes, and burning trees to clear land for livestock. You might imagine the destruction of “pristine wilderness,’ previously untouched by people. It’s true that the Amazon is under a huge amount of pressure as people clear it to make more grazing land and fields for crops. But it’s not always true that these farmers are the first people to arrive. 

The Amazon Basin is part of South America drained by the Amazon River. Image via Getty. 

Several archaeological findings confirm that people have lived and thrived in the modern-day Amazon rainforest for thousands of years. And I’m not talking about a few small villages dotted around the basin. I’m talking about hundreds of thousands or potentially millions of people, with complex farming systems like large-scale fish farms that stretch for hundreds of kilometres. The fertile soils these civilisations created - known as Terra Preta, which literally means ‘dark earth’ - remain productive today.^7,8,9

Reports of huge Amazonian civilisations led by women were made back in 1541 by European explorers and colonisers.^{10, 11, 12} But it was assumed these reports were exaggerated when they weren’t found on later voyages. Today, we know that around 99% of the Indigenous Population might have died due to smallpox carried by colonisers - so these earlier reports are being backed up with more recent archaeological evidence.¹²

Watch our documentary about how livestock is impacting the Amazon today.

How could people grow food on “toxic land”?

Researchers still debate how many people could have survived in the Amazon, mostly because of the thin, aluminium-rich soils prone to erosion. These soils struggle to hold nutrients well and are toxic to soil bacteria needed for many staple crops to thrive.¹³ Even if the Amazon is home to a diverse rainforest today, some argue that the soil would have been too poor for food cultivation - so it couldn’t have produced enough calories for a large population.¹⁴

But archaeological findings now suggest that a large and organised society could have lived in the region. As well as fish farms, people have discovered what they think are settlement mounds, fields, and major roads.⁸ (Admittedly,  what we call a major road thousands of years ago doesn’t look much  like today’s motorways!)

Square and circular earthworks seen in a deforested part of the Amazon, indicating that ancient communities inhabited the area. Photo by Sanna Saunaluoma via Sadie L. Weber. 

To feed those people, the communities living here would have produced an abundance of food on soil considered unproductive. At first glance, that might mean ancient societies were constantly clearing the land of trees, using up the existing nutrients until the crops couldn’t grow anymore, and abandoning them. But clearing rainforests with stone hand axes would have taken a huge amount of energy. So, it’s more likely that early inhabitants found a way to make these challenging soils more fertile.⁸ The result was Terra Preta.

Helena Lima, researcher and curator at the Museu Paraense Emílio Goeldi in Belém, Pará, profiles an archaeological excavation site in Caxiuanã. The top, dark layer is terra preta, the white layer is crushed shell that was used as construction fill, and the rust-colored clay is the original land surface. Photo by Sadie L. Weber via ReVista. 

The western Amazon was a centre of plant domestication. Varieties of crops like peanuts, Brazilian broad beans, chilli peppers, rubber, tobacco, cacao, peach palms, and cassava were probably domesticated in the western Amazon.

What is Terra Preta, and how did people make it?

Terra preta do Indio, or Terra Preta, is a dark, fertile soil found in patches in the Amazon Basin, including in Brazil. The dark colour comes from charcoal incorporated into the soil thousands of years ago. This charcoal, rich in minerals and phosphorus, has many pores, which can hold onto nutrients, improve soil structure, and provide a habitat to a wide range of plant-friendly microbes—one reason why charcoal remains a popular soil amendment today. 

There’s still some debate about exactly when and exactly how Terra Preta was created, but it was probably first made by humans around 7000 to 9000 years ago.¹⁵ Some researchers would say it’s even older.¹⁶ But what’s amazing about Terra Preta is that even today, thousands of years after its creation, local cash crops such as papaya and mango can grow three times faster on Terra Preta than on surrounding soils.¹⁷

A modern manioc and banana plot is planted over a terra preta archaeological site near Carrazedo. Photo by Sadie L. Weber via ReVista.

Despite attempts to recreate the crop-boosting Terra Preta soil, we don’t know exactly how the Indigenous people of the Amazon made it. But we do have one clue.

Archaeologists have found shards of pottery inside Terra Preta, which suggests that the soil may have come from an ancient waste disposal method. One theory is that Indigenous civilisations would have burned organic waste like food scraps in low-oxygen conditions, along with broken pots, which created the dark and fertile soil. It’s also possible that charcoal was intentionally added to ancient “landfill sites” of food waste to suppress odour, leading to the accidental creation of Terra Preta.¹⁵

Where does Terra Preta fit into modern agriculture? 

The ancient Terra Preta soils can offer us two key lessons. One, let's make the most of our food leftovers, as they can be a valuable resource to enrich our soil. And two, growing abundant crops on difficult soil is possible, even without synthetic fertilisers, so we don’t have to keep expanding our agricultural land.

Even if we haven’t quite cracked the recipe for the original Terra Preta, some promising solutions are popping up around the world as we speak. In the UK, one project is turning livestock slurry into charcoal. This charcoal captures carbon, preventing greenhouse emissions getting into our atmosphere and when applied to fields, it works like a natural fertiliser to make our soils healthier and support food production on our farms.¹⁶ Meanwhile in Cameroon, a group of young entrepreneurs have been turning food waste into smokeless charcoal so local people can stop cutting down trees to cook their food.¹⁷ Smokeless charcoal is also significantly better for people's health as a cooking fuel, because it prevents toxins from getting into food and the air.

These projects are both repurposing food waste to create a healthier food future. But like with many of the best ideas - someone might have already thought of it,  thousands of years ago!