Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

settlements for several thousand years, but intensive habitation produced
it. Faced with trying to make a living from nutrient-poor soils, Amazonians
improved their soil through intensive composting and soil husbandry.
Found on low hills overlooking rivers, terra preta is full of broken ceramics
and organic debris with a high charcoal content and evidence of concentrated
nutrient recycling from excrement, organic waste, fish, and animal
bones. Abundant burial urns suggest that the human population
recycled itself too. The oldest deposits are more than two thousand years
old. Practices that built terra preta soils spread upriver over a span of about
a thousand years and worked well enough for sedentary people to prosper
in an environment that had previously supported a sparse, highly mobile
population.
Typically one to two feet thick, deposits of terra preta can reach more
than six feet deep. In contrast to the typical slash-and-burn agriculture of
tropical regions, Amazonians stirred charcoal into the soil and then used
their fields as composting grounds. With almost twice the organic matter
as adjacent soils, terra preta better retains nutrients and has more soil
microorganisms. Some soil ecologists believe that Amazonians added soil
rich in microorganisms to initiate the composting process, as a baker adds
yeast to make bread.
Radiocarbon dating of terra preta at Açutuba near the confluence of the
Amazon and the Rio Negro showed that the site was occupied for almost
two thousand years, from when the black soils began forming about 360
bc until at least ad 1440. When Francisco de Orellana traveled up the
Amazon River in 1542 he found large settlements no more than “a crossbow
shot” from each other. His conquistadors fled from the throngs of
people that swarmed the river at a large site near the mouth of the Tapajós
River where terra preta covering several square miles could have supported
several hundred thousand people.
Geographer William Denevan argues that slash-and-burn agriculture in
which farmers move their plots every two to four years is a relatively recent
development in the Amazon. He asserts that the difficulty of clearing huge
hardwood trees with stone tools rendered frequent clearing of new fields
impractical. Instead, he believes that Amazonians practiced intensive agroforestry
that included understory and tree crops that together protected
the fields from erosion, allowing rich black earth to build up through time.
Much like a villagewide compost heap, terra preta soils are thought to
build up from mixing ash from fires and decomposing garbage into the
soil. Similar darkening and enrichment of the soil has been noted around
w estward hoe 143