Anthropogenic soils in the Central Amazon: from categories to a continuum

Amazonian Dark Earths (ADE), one of the best-known examples of anthropogenic (man-made) soils, are the result of Amerindian settlements in the pre-Columbian period. ADE are highly variable in terms of their size, shape, depth and physical and chemical make-up. Scholars tend to divide ADE into two categories: terra preta and terra mulata. The former are dark and highly fertile soils replete with ceramic shards, indicating former areas of habitation, while the latter are lighter in colour, less fertile, lacking pottery and thought to be old agricultural fields. While a scientific consensus on the origins of terra preta has existed for several decades, the origins of terra mulata remain enigmatic and contested. We argue that owing to the overlapping and constantly changing boundaries of agricultural and habitational areas, it is unlikely that there exist two clear soil fertility classes. This article examines the hypothesis that rather than two distinct anthrosol categories, ADE sites should exhibit a highly fertile 'core area', which grades into more subtly modified soils, with a continuum of fertility between them. Using principal components analysis (PCA) and interpolations based on the geographic distribution of the soil samples, we show that ADE along the Middle Madeira, Brazilian Amazon are extremely diverse, but data support more of a gradient between areas of greater and lesser fertility rather than two distinct categories. We also assess local people's perceptions and classifications of anthropogenic and surrounding soils using ethnographic data. Rather than discarding the terra preta-terra mulata opposition however, we suggest abandoning only the idea that they are separate categories, and instead emphasise a continuum, the darker, bluff edge 'central' regions with abundant ceramics are consonant with published descriptions of terra preta, which grade into surrounding areas with lighter, less fertile soils that better fit terra mulata descriptions