A recipe for fertility
The vast Amazon rainforest grows on surprisingly infertile and acidic soils, primarily reddish, clay-like oxisols. Yet, scattered throughout the basin are patches of terra preta do índio (Indian black earth), a dark, exceptionally fertile soil that is an artifact of pre-Columbian civilizations. These human-made soils, or anthrosols, were created between 450 BCE and 950 CE. They are found in plots averaging 20 hectares, though some sites can be much larger.
The main ingredient is charcoal, produced from burning organic matter at low temperatures in a process now called pyrolysis. This "biochar" gives the soil its characteristic black color and stable structure. Unlike the slash-and-burn agriculture common today, which releases most carbon into the atmosphere, this ancient "slash-and-char" technique converted up to 50% of the biomass carbon into a stable form. This charcoal was mixed with a dense combination of settlement waste: animal and fish bones, manure, compost, and large quantities of broken pottery. This mixture creates a soil with up to 70 times more charcoal than the surrounding earth. The porous nature of the charcoal provides a habitat for beneficial microorganisms and fungi, which help retain water and nutrients.