Studies on Soil Structure Development in Pelosols (Vertic Cambisols) of South-West Germany

Abstract

Investigations were carried-out on nine Pelosols (Vertic Cambisols) profiles developed from Mesozoic sediments (four profiles on Dogger delta-zeta, carbonates containing; 5 profiles on Dogger alpha- Opalinuston, carbonates-free). Direct observations on soil structure were made macro- and microscopically, and indirectly by determining total pore volume and pore size distribution from the moisture retention data. Stability of aggregates was also measured. In thin sections, the edges of the platy structure (of parent rock) appeared softened due to the action of water, leading to coherent structure (of parent rock). The structure of the clay-rich sub-soil horizons came to expression through swelling-shrinking process, which is the typical characteristic of Pelosol. Waterlogging keeps the structure to remain massive. The plasmic fabric of parent rock, in case of Dogger delta-zeta formation was crystic-argillasepic-sepic-unistrial and that Opalinustion was argillasepic-unistrial (strong). The transformation of the plasmic fabric through weathering was clearly traceable. The parent rock’s plasmic fabric contributed to that of the middle horizons and different manifestations of the plasmic fabric were observed. Biotic factor was strongly expressed in A-horizons, however the preferred orientation of the plasma aggregates was unclear (weak sepic). Because of high clay content, the related distribution pattern was porphyroskelic, with the exception of A-horizons, where it was weak intertextic, due to biotic factor and the coarser texture. Illuviation clay cutans (clay migration) were observed in most of the profiles, also present as papules. Stress cutans were observed in carbonate-free middle horizons. The pore volume was, in general, a function of depth in Pelosols. These are fine pores which showed positive correlations with clay and humus contents. The parent rock lost its stability through weathering; the stability of top most horizons was high. A distinction may be made of the concept “schwere Boeden†(heavy soils) on the basis of stability factor and the clay content. Different classes of sepic fabric in the horizons from parent rock to A-horizon were broadly discussed in relation to measured soil physical properties. Change in porosity and water retention from parent rock (lower value) to A-horizon (higher value) showed a relationship with the plasmic fabric i.e., unistrial in parent rock to different variants of sepic fabric (clay-rich middle horizons) to weak sepic plasmic fabric in the A-horizons (activity of soil fauna). More micromorphological data need to be collected for developing possible correlations with soil properties and land use.