Agriculture Reference
In-Depth Information
Classification of Amazonian Dark Earths
Are we dealing with a single type of dark earth, several different entities or
is each occurrence of ADE so unique as to defy classification based on
characteristics attributable to human activities? Based on our laboratory
and field analyses of diverse dark earth locations, we found that ADE is
indeed classifiable, but it is not a discrete, single entity. Rather, we found
that all of the dark earths fell into two clearly distinct categories. The first
group, which we call terra preta (TP), is darker, richer in P, Ca and other
elements, and contains abundant cultural artefacts (mostly ceramics, but
also some lithic material). The second, and more widespread type of
dark earth, is typically dark greyish brown, not black, corresponding to a
Munsell colour 10YR 4/2 or darker. It is often found surrounding smaller,
darker zones of TP. Ca and P levels are not significantly higher and cultural
artefacts are rare. Like TP, however, it has elevated concentrations of
organic carbon when compared with the background sands and clays (SC).
To distinguish it from terra preta , we call this brown soil terra mulata (TM),
following Sombroek (1966). In his reconnaissance of soils in the Tapajós
region, Sombroek also observed expanses of slightly lighter soils devoid of
ceramics encircling darker TP sites, almost certainly the same phenomenon
that we now describe. We will return to the question of how TP and TM
were formed and why their unique characteristics persist, but first we will
present the results of additional research we designed to test the idea that
two distinct types of ADE indeed exist.
Near Infrared Reflectance Spectroscopy (NIRS)
NIRS holds great promise for efficient sample grouping and organic
characterization - precisely those qualities needed in the dark earth study
where important questions revolve around: (i) the nature of the organics
that produce the dark coloration; and (ii) the degree of organic commonal-
ity between both types of the dark soils. Because laboratory infrared
techniques have not yet been used extensively in the study of soils (White
and Roth, 1986), we will provide some background on the technology, and
describe our specific methodology.
NIRS has become a commonly used tool for both the qualitative and
quantitative determination of structural information concerning organic
substances. Dramatic advances in computer technology during the last 20
years have led to great improvements in NIRS instrumentation, preparation
techniques and data processing capabilities. Common applications of NIRS
include assessing the nutritional quality of forages and feed grains and
identifying sewage sludge decomposition products. The technology is also
used to analyse pharmaceuticals, textiles, dairy products, petrochemicals
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