The terra preta display high levels of soil organic matter (SOM) and nutrients such as
nitrogen, phosphorus, potassium and calcium (Table 4). These characteristics and their high
fertility is attributed in part to a high char content (Glaser et al., 2001), which is the main
reason why the terra preta tend to be much darker in colour than adjacent soils (Figure 8).
The source of the large amounts of char is considered to have been incompletely combusted
biomass carbon, such as wood from kitchen fires or in-field burning, conducted by relatively
large and settled communities in the distant past.