Agriculture Reference
In-Depth Information
Reassessing the 'Midden Model' of Dark Earth
Formation
With the goal of obtaining information from the greatest variety of dark
earth contexts occurring in the lower Tapajós and Arapiuns River basins of
Brazilian Amazonia, we physically inspected a large number of dark earth
areas from uplands, river bluffs, beaches and plateau outliers. The sites
ranged in size from 0.5 ha to > 120 ha, and included both sand- and
clay-dominated soil matrices. We analysed samples collected from 20 of
these sites using the following procedures: (i) elemental extraction through
HNO
3
/HCl digestion with determination by inductively coupled plasma
emission spectrophotometer (ICP); (ii) organic carbon (OC) by loss-on-
ignition; and (iii) electrometric measurement of pH in 1 : 2 soil : water
mixture. Figure 3.10.1 is based on aggregate data from 75 samples taken
from 12 sites.
The results of the field and laboratory analysis showed that the dark
earths in the study area are generally not middens, or aggraded piles of
human garbage (Woods and McCann, 1999). While some of the sites did
indeed exhibit the classic indicators of human habitation - abundant
potsherds and high concentrations of P and Ca - the majority did not.
Furthermore, significant human-induced aggradation was not apparent
even at the chemically rich sites. On the other hand, natural aggradation
clearly played an important role in the formation of many ADE profiles,
most notably at riverine sites subject to high rates of deposition of wind-
and water-borne sand. At Vila Franca near the confluence of the Arapiuns
and Tapajós, for example, the dark zone extended beyond 2 m below
the surface. Deposition of garbage by humans was indeed important to
the formation of these soils, not as a bulk contributor of mass, but rather
for the indirect chemical changes that stimulated soil biota, enhanced
fertility and melanized the soil.
Fig. 3.10.1.
Two kinds of dark earth.
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