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Abstract "Heft 1"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des
Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: F. Hädrich

Heft 1

Winfried E. Blum

Sedimentogene und pedogene Entwicklungsmerkmale
von Böden auf Karbonatgestein - am Beispiel des südlichen Oberrheins

Freiburg im Breisgau 1968

ISSN 0344-2691


The goal of the present work was to determine the processes and their rates of soil genesis on carbonate rock by comparison of sedimentological and pedological criteria of different stages of soil development under defined conditions.
This was done investigating weathering of clay minerals and oxides of the non-carbonate fraction of the sedimentary rocks, during soil genesis.Moreover it was analysed how far the evolutionary conditions of the humic substances occurred concomi-tantly with the weathering of the mineral components, and finally if there exists a correlation between the actual humus stage and the weathering of the non-carbonate minerals.
With special regard to the factors of soil genesis 42 soil profiles from different sedimentary rocks were analysed. For investigations of soil genesis attention was paid to keep constant experimental conditions.The only parameters varied were soil parent material and the geomorphological location in order to be able to interpret safely the results with a minimum of variables.
In order to determine sedimentological criteria the parent material especially the non-carbonate fraction was analysed using X-ray diffraction, differential thermal and calorimetric analysis, optical, chemical and physico-chemical methods.
The degree of weathering of sedimentological minerals and the genesis of pedological minerals was analysed in the total soil, fine earth and in various size fractions. - Furthermore, the biological activity was determined in spring and autumn.
In addition to texture, pH, carbonates, Ct, N t and cation ex change characteristics, there were also determined the ex-traetabilities of K, Ca, Mg, Fe, Al, Mn, Si and P in various extracts from fine earth. - The weathering of primary clay minerals and oxides was analysed by optical, chemical and physico-chemical methods, X-ray diffraction, differential thermal and calorimetric analysis.
In humus analysis special attention was paid to the fact that humus is present as a organo-mineral complex.Therefore humic substances were extracted by a dispersive method using Na4P2O7 and NaOH at 3 different degrees of pH. C and Fe were determined in the acid soluble and insoluble fractions using conduc-tivimetric and colorimetric methods.Moreover the acid-insoluble fraction of humic acids was analysed by infrared spectro-photometrie. Further analysis on acid-insoluble humic substances was preformed using paper electrophoresis. The fractions thus separated were individually tested for their optical properties in the infrared range and also for their Fe-content.
The processes of soil formation are discussed with special reference to 9 selected soil profiles from lime and dolomite rocks.
The following results are of major importance:

-    Weathering of clay minerals is strongest where soil genesis has only weekly advanced.The weathering of micas (generated from 2M-muscovite in parent material) is of particular diagnostic importance.This alteration proceeds via successive stages (vermiculite, montmorillonite and mixed layer minerals) to give secondary chlorites in the upper soil horizons. There is some evidence that besides Al also Pe is transferred into the interlayers in the form of orga-no-metallic complexes.
These results obtained by X-ray diffraction could "be confirmed by differential thermal and calorimetric analysis as well as by chemical and physico-chemical methods. One of the main causes for the weathering of primary clay minerals could be due to the deprivation of potassium by the vegetation because micas are the only source of potassium for plant nutrition.
This view is supported by quantitative determinations of the potassium content. - Other reasons for the weathering of primary minerals are discussed on the bases of additional data.

-    Parallel to the stronger weathering of clay minerals in poorly developed soils,there is also a stronger decomposition of the opaque minerals (mainly Fe-oxides) in the heavy metal fraction. These results obtained by optical methods were confirmed by the extractability of Fe, Al, Mn and Si in various extracts from fine earth. The mobility of these various elements in the different profiles increases from the bottom to the top horizons, whereby the degree of mobility increases at a different rate for each of the elements.
This could "be demonstrated by a comparison between different profiles on the one side in relation to the content of elements in the non-carbonate fraction of the parent material on the other side thus characterizing the different degree of weathering of the primary oxides.

-    The differential weathering of primary minerals can be explained on the basis of the results obtained in the analysis of humic substances. It could be shown that the dissolution rate of carbonates is not in direct relation to the weathering tendencies as discussed above.

-    The humus content of the fine earth shows a negative correlation to the degree of soil development.This applies also to the total of extractable humic substances. In contrast to this, the degree of humification decreases with a concomitant increase in low molecular,acid-insoluble humic substances.

-    At the same time the percentage of Pe complexed with humus substances in the fine earth is increased.It is highest in the soils of a low degree of development. This points to a parallel increase in the weathering intensity of primary oxides.

-    Parallel to the intensity of weathering in poorly developed soils,there is an increase of stability of Fe-humus complex which indicates a more pronounced formation of organic-mineral complexes. These results were confirmed by paper electrophoresis and infrared spectroscopy. One can conclude from the nature of organo-metallic bonds that low molecular humic acids participate in the weathering of primary minerals.
The pedological soil characteristics are more pronounced in poorly developed soils,.In contrast to this,in well developed soils the sedimentological characteristics are much better preserved.

The different intensity of weathering and transformation of non-carbonates which is closely correlated to the metabolism of humic substances allows one to recognize the nature and rate of soil genesis on the basis of comparing sedimentological and pedological soil criteria provided experimental conditions are comparable.
Prom the results pointed out above,the conclusion was drawn that the different soils had developed at different' rates. This conclusion is based on the fact that the rate of weathering of non-carbonates is higher in soils with a low rate of development and also on the convergent humus metabolism which intensifies the tendency of this process.
These results based on the above described experiments were confirmed by field observations.
This showed that in the area investigated the state of brown calcareous soils could have evolved during the last ten thousand years in the plain under natural plant growth.

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