Because of their elevated charcoal content and the common presence of pottery remains, it is now widely accepted that these soils accreted near living quarters as residues from food preparation, cooking fires, animal and fish bones, broken pottery, etc., accumulated. The intentionality of the formation of terra preta has not been demonstrated, rather it is believed to have formed under kitchen middens. Areas used for growing crops around living areas are referred to as terra mulata. Terra mulata soils are more fertile than surrounding soils but less fertile than terra preta, and were most likely intentionally improved using charcoal.
What I am suggesting is that over the last centuries, it expanded on its own, as gardens and orchards do when left unattended
Instead of being pristine forests, barely inhabited by people, parts of the Amazon may have been home for centuries to large populations numbering well into the thousands and living in dozens of towns connected by road networks, explains the American writer Charles C. Mann.
In addition to parts of the Amazon being “much more thickly populated than previously thought,” Mr. Mann, the author of “1491,” a groundbreaking book about the Americas before the arrival of Columbus, said, “these people purposefully modified their environment in long-lasting ways.”
Sorry Bird but the unnatural histories programme not only shows it could have been but also how it was shaped by man at least and directly linked the 'black earth' to man. Black earth is the only fertile soil found in the rain forest
As Punkinworks says, the very fact that it IS so fertile argues against any human intervention at all.
This was not a primitive slash and burn culture according to the programme
Primitive human agriculture is very hard on the land. It's basically slash and burn, and after the land is used up, they move on. All nutrient value is gone and it takes awhile for the land to regain its fertility.
Again it was shown that although you are correct in our part of the world in the rain forest it does not work that way. The falling leaves/debris are quickly reabsorbed into the forest life that it does not have time to enrich the soil which would be a waste of time as the rain washes it away anyhow
Forests themselves have an extremely efficient method of turning land fertile because of their incredible biodiversity. As trees die and fall, they leave areas of sunlight were smaller trees arise and where the action of microbes, insects, and the vast diverse life works to enrich the soil.
This culture used a different model, they had little other choice as they cannot regulate the floods, have more than enough rain both of which strips away any attempt to enrich the soil. That is why our attempts to move that land into agriculture does not work. It appears they encouraged their environment to produce rather than made it produce
Humans and farming create monocultures. Insect, reptile, mammalian, life is tightly controlled (undesirables are killed or removed) and microbes are regulated (no sick plants or animals allowed.) We also regulate floods and rain events to manage topsoils and moisture.
To maintain that stance you need to explain the Geoglyphs, large settlements, road systems. Why every time you find black earth you find masses of pot shards. A high culture with fine pottery, and organised society that rivalled those in the West.
Human ability to create, manage, and regulate huge biodiversity is not easy even with modern tools.
So, no. It's natural and created over tens of millions of years.