When I decided to become a soil microbiologist, I knew that the soil could be the most diverse environment in the world, but there was minimal information regarding how extremely diverse Amazonian soils are. Today we know Amazonian soils contain more than 700 different insects and arachnids, including around 180 different species of ants and more than 80 different species of termites, five different families of earthworms and around 40% of the world’s diversity of a group of beneficial fungi denominated arbuscular mycorrhizal fungi. Most Amazonian soils are acidic with low fertility.
For many years researchers, colonists and explorers believed that human populations in the Amazon region were limited to small groups with simple social organizations that live in a simple manner with little or no technological developments. However, the Amazon region has been inhabited by many ancestral indigenous communities (an approximated 10 million people before the discovery of America by Europeans) who lived in complex and populated settlements with thousands of people. They needed to produce (through hunting, fishing and cultivation) enough food to sustain these settlements, much like the indigenous communities of today. But a common denominator between ancient and contemporary indigenous communities is that they do not perceive low-fertility Amazonian soils as a constraint for food production.
Most productive soils are rich in minerals that are nutrients for plants, and these minerals come from parental rocks that were also rich in these minerals. The parental material from which Amazonian soils originate are rich in kaolinite, which is a clay poor in nutrients, so the mineral portion of Amazonian soils does not supply nutrients to plants. Most of the phosphorus, which is essential for plants to elaborate proteins and energy, comes from organic matter and not from the mineral portion of soils. The acidity of Amazonian soils also limits the fixation of important ions and favours the fixation of aluminum which is not a plant nutrient and in high concentrations can be toxic for plants. If the mineral portion of the soil is not fertile, where do the nutrients for the plants come from? The answer is in the efficient transformation of organic matter by soil biological communities, who produce essential nutrients for the plants.
Traditional Shifting Agriculture of the Indigenous Amazonian People
I started studying the diversity of soil insects and microbial communities to understand why the traditional system of indigenous production, the shifting agriculture, is so efficient and had caused such little negative impact to the environment, although human beings had inhabited the Amazon region for thousands of years. The shifting agricultural system, known locally as “chagras,” is based on the temporal use of forested areas to convert them into agricultural plots. The forest is first cut and burned. Next, the different crops are planted and harvested for 2 to 3 years. After 3 years the soil cannot supply more nutrients to the plants and the plot is abandoned for more than 5 years (in some cases up to 40 years) for its natural restoration. After its natural restoration the same plot can be used again for agriculture.
Soils of the Amazon are characterized by a high diversity and abundance of ants and termites, which are known as “soil engineers” as they cut the organic matter of the forest into small pieces and move it into the soil, creating tunnels and funnels that also help to mobilize the water and oxygen into the soil. Within the soil, microorganisms act on the organic matter to decompose it to single molecules that the plants can access. In this production system, the insects (ants, termites and all the other soil engineers), earthworms and microorganisms such as bacteria and fungi that are affected by the initial soil burn of the chagra are reestablished by those communities coming from the surrounding forest once the plot is abandoned. In fact, there is evidence that the structure and diversity of biological communities are only affected temporally during this disturbing process. An important issue here is that the biological communities living in the forest are the most efficient ones for transforming organic matter into plant nutrients, and are also the same biological communities that maintain the fertility of agricultural plots for 3 years. However, when the supply of organic matter decreases due to no more litter being produced in the agricultural plots, there is less material to be transformed by microorganisms and the fertility of the soils decrease until the point that the production is not sufficient, so the plot is abandoned by indigenous farmers.
Cassava: From Staple Crop of the Amazon to Potential Global Staple Crop
Although agriculture needs fertile soils to be possible, the indigenous system of agriculture used in the Amazon region does not depend on high soil fertility. Indigenous people selected as their main staple food the crop cassava (Manihot esculenta), which can grow and produce well in soils with low-fertility. Cassava is always associated with a group of soil fungi denominated arbuscular mycorrhizal fungi and a single plant could associate with around 15 different fungal species. These fungi help the root system of cassava to access the limited nutrients in the soil. In this way, the cassava plant does not perceive a nutrient deficit, and produces as well as a plant cultivated in a fertile soil. Since around 70% of Amazonian soils are nutrient limited, indigenous people selected a plant that does not require fertile soils to produce, solving this natural constraint.
Indigenous people also diversified cassava. Having hundreds of cassava varieties of this single plant allowed them to prepare many different foods. Human selection of cassava varieties has also produced approximately 3,000 different varieties of cassava around the world, with cassava becoming a staple food in many tropical countries. What is surprising is that cassava contains cyanide in its roots, and the roots of some varieties are toxic for animal and human consumption. Instead of trying to select less toxic cassava varieties, indigenous people developed simple but efficient techniques to detoxify these roots. For this reason, the toxicity of cassava roots has never been a constraint for its consumption. More specifically, ancient indigenous peoples of the Amazon region learned how to detoxicate poisonous cassavas and how to conserve them by producing a kind of consumable flour denominated farinha that can be stored for more than two years without damage. This farinha is the base ingredient for many different traditional dishes, as well as a good food for long treks.
Cassava was one of the crops that Europeans took from America and brought to Africa, where the weather and soil conditions were similar to those in the Amazon region in some areas. When cassava arrived to Africa (around 400 years ago), there were multiple poisonings from the consumption of cassava roots with high cyanide content. But Africans have learned how to detoxify cassava roots and toxic cassava varieties are becoming preferred for some preparations over less toxic cassava roots.
Today cassava is more than a staple food. With climate change, the world will experience many challenges. One major global challenge is to increase food production by 60% to supply the food a growing population needs, while 30% of the world’s soils will become degraded and have limited production capacity. Furthermore, water is becoming more scarce and drought periods will be more prolonged than today. Due to the variability and characteristics of cassava starch, its tolerance to drought and its ability to be grown organically (without agrochemicals) in a range of low-fertility soils, cassava was proclaimed by the International organization for Food and Agriculture (FAO) as one of the few crops that can easily adapt to climate change. Today cassava not only feeds one billion people in 105 countries around the world but is also one of the most important crops to produce biofuels (as bioethanol in India) and plastic substitutes.
I believe that cassava is one of the most important contributions that indigenous people of the Amazon region have given to the modern world. It might be the crop to sustain future agriculture in an environment with water restrictions and low soil fertility. Cassava can produce an abundance of nutritious food compared to other crops in those limiting conditions, which is the reason cassava is already the main staple food for millions of people around the world. Due to these unique properties, cassava could be a very important global staple crop in the coming years of climate change.
Author Note: Clara P. Peña-Venegas is a Soil Microbiologist at Instituto Amazónico de Investigaciones Científicas Sinchi, Leticia, Amazonas, Colombia. cpena@sinchi.org.co.
Edited by Daniel Henryk Rasolt
