Geoscience Reference
In-Depth Information
dams or weirs across narrow valleys or ephemeral brooks to collect sediments for
cultivation, using erosion to build up new agricultural land.
41
Terrace building
entailed a large investment of labor and material, and created long-lasting changes
to landscapes and their soils. Which kind of changes terracing brings to the soil
depends on many factors such as terrace and soil type, cultivation regime and
fertilization techniques. Positive and negative effects on soil quality have been
described in a worldwide survey of terraces by Jonathan Sandor.
42
Terracing in the
Americas is an old practice. For Puerto Rico, terraces can be dated to at least 1300
BP (ca. 650 CE). Human induced or enhanced erosion is at least of the same
antiquity in the region. Evidence of soil erosion appears in the sediment record
around the same time on Hispaniola.
3.3.2.3 Amazonia
The lush vegetation of tropical rainforests mislead European conquerors to assume
that the soils underneath the tropical paradise must be very fertile. It soon became
clear that rainforest soils can only be cultivated sustainably by long fallow swidden
techniques, as almost all nutrients are contained in the biomass. The soils are very
poor and prone to degradation and accelerated erosion once the forest is cleared. So
Amazonia became to be considered a vast tract of infertile land, until scientists at
the end of the 19th century discovered patches of dark earth underneath the forests.
Terra Preta de Indio or Amazonian Dark Earth is a local name for the soil of these
patches.
43
Nowadays it is known that the dark earths occur in several countries in South
America and probably beyond. They were most likely created by pre-Columbian
Indians between 1000 years BCE and 1500 years CE and abandoned after the
invasion of Europeans. Many questions about their origin, distribution, and prop-
erties remain unanswered, but it is clear that they are a product of indigenous soil
use. Whether they were intentionally created for soil improvement or whether they
are a by-product of habitation is not clear at present. As the notion comprises earths
of varied features throughout the Amazon Basin (terra preta and terra mulata being
frequently distinguished); they might have different histories. Terra mulata is
thought to be the product of intentional anthropogenic activities, based on intensive
swiddens or patch cultivation, with long-lasting agricultural activity, involving
recurrent clearing of vegetation and incomplete combustion of organic material.
Amazonian Dark Earths contain about
ve times as much carbon as the sur-
rounding soils, and the enriched horizons are twice to three times as thick as the
surrounding soil, most often about half a meter. Carbon is life
s building block, and
usually circulates relatively quickly through soils. But the organic matter in the dark
'
41
Whitmore and Turner (
2001
).
42
Sandor (
2006
).
43
Lehmann et al. (
2003
).
