Gerhard Bechtold

  Consultant for National (Geo-)Information Systems and Database Setup, for Natural Resources Assessments   

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Summary of Thesis about Anthrohumox in Brazilian Lowland

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In the Amazon lowland, Oxisol developed in scattered areas to 'Indian Black Earth' (Portuguese: ' Terra Preta'). High content of C (more than 9%) and ceramics are characteristics. Furthermore, they show weak acidity (but pH not over 6.5), low solum weight and high cation exchange capacity. The very most soil parameters are correlating with the percentage of C.

Terra Preta soil patterns can be found in the Amazon lowland ( map, based on literature research). Visited TP spots in Belterra (close to Santarem) show characteristics about their extension (seldom more than 2 acre), their depth (till about 50 cm with black color of value 3 or lower, in traces till 2-3 m depth) and their transition to the surrounding Oxisols (some few km). Size is correlating with intensity (low value - large areas) and frequency of ceramics in most cases. Due to oxic horizon in the lower Terra Preta, great group 'Anthrohumox' is proposed.

There are various theories about their origin. It is proved indeed, that Terra Preta is man made (high phosphate content, ceramics, situation on former village sites). About 1500-2800 years ago the Indios of the area had a strong influence on the Oxisols by their garbage, kitchen, compost and/or mulch 'farming' for a couple of hundreds years on their dwelling sites. C14 tests of the samples showed ages of 1780 (+ 270) and of 2260 (+ 280) years, thus dating back to between 800 BC and 500 AD.

Intensive activity of organisms and bioturbation cause a breaking up, a lowering of the solum weight, the loss of some hundred kg soil material per square m and of fine particles ('only' 65-80 % clay). High sand content indicates high C content (not in terms relative to other areas). All those characteristics decrease with increasing soil depth.

Strong impact of humic acids react a strong weathering of Si, Al and Fe. Different leaching ratios are discussed in the paper. They show a stronger weathering in the Terra Preta than in the Oxisol (lower Si, higher Al and Fe content). Al occurs mainly as 1:1 mineral (the main parent material), Fe as hematite or -favored by organic material - as goethite (over 70 % Fe d:Fe t).

Strong weathering (Aw climate) causes some characteristics at the depth functions of Si, Al, Fe, but leaching and cristallization is retarded by strong chelation (humic acids complex with metal ions). M o:M d relations quote always high (especially in Al and Mn, less in Fe). Very pronounced peaks of Al o (and of Fe o) are difficult to explain. Illuviation and podzolation seem to be very active. Different chelating groups (M(OH)++ or M(OH)2+) with different stabilities are more reasonable.

Very strong is the influence of the organic material on the content of Mn, Ca, Mg in the upper layers existing a manifold content, compared to the lower horizons, more than 10 times at Ca, due to the exchange sites of the organic substance (high solubility and erosion). At Mg existing fixation in anorganic compounds, e.g. Mg-Al-hydroxydes, at K in mineral interlayers are responsible for the lower percentages of the exchangeable to the total.

The vertical transition to the Oxisol depends on the element: Terra Preta and Oxisol have an almost identical C content at a depth of 1.50 - 2.00 m, an identical solum weight at 2.30 m depth, identical texture only at depths of more than 4 m, identical acidity and Si content at 2 m, Mn and K 40 - 50 cm, Ca and Mg at 1.50 m.

An exceptional profile with nodular laterite concretions (plinthic, plateau laterite) on a slope was analysed. It was found to have high C content and plenty of ceramics (therefore, Terra Preta), but in acidity and exchange capacity (content of K, Ca, Mg) it is more similar to Oxisols. In a different TP profile, burned charcoal induced high C contents and other characteristics of the upper horizons of Terra Preta.

At many elements it is possible to recognize, beside the organic influence, a 'basic content' (very low at nutrients) of Ca with 200 - 300 ppm, Mg with 180 - 190 ppm, K with 200 - 300 ppm, K with 50 ppm etc. A melioration by mobilization of these reserves is impossible.

Melioration is imaginable by pH raising (of soil and vegetation), addition of organic substances without fast mineralization and higher bioturbation.

Further research about Terra Preta, surrounding conditions of their development and analytics are described in the paper.

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Last update: Mar 2007