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SOIL PHYSICS

Scanning Electron Microscopy–Energy Dispersive Scan Analyses and Rheological Investigations of South-Brazilian Soils

Institute for Plant Nutrition and Soil Science, Hermann-Rodewald-St. 2, Christian-Albrechts-Univ. to Kiel, 24118 Kiel, Germany

^* Corresponding author (w.markgraf{at}soils.uni-kiel.de).

Scanning electron microscopy, including energy dispersive scan analyses and amplitude sweep tests with controlled shear deformation, were conducted on four kaolinitic, Fe-oxide-rich Brazilian Hapludox soils and one montmorillonitic Calciudert soil to elucidate results obtained from a parallel-plate rheometer. In the Brazilian Hapludox soils, the occurrence of pseudosand (an aggregation of fine particles with a grain size of sand), due to Fe (hydr)oxide cementation, had a significant effect on aggregation and microstructural stability. We could observe the presence of hematite, goethite, and, in rare cases, halloysite, as the major stabilizing minerals in such soils. The influences of soil organic matter (SOM), Fe oxides, and clay minerals on micromechanical shear behavior under oscillation were tested under saturated and predrained (at –60 hPa) conditions. Collected data included G (shear modulus), G' (storage modulus), G'' (loss modulus), linear viscoelastic deformation range, deformation limit (_L), and yield stress (_y). From the data, the dissipation of elasticity was derived, and the values ranged from 200 to 1000 Pa in all untreated samples, both Hapludox and Calciudert soils, and to 50 to 500 Pa in SOM-leached Hapludox samples. Minimum _y values of 10 to 50 Pa occurred after a sodium dithionite treatment (Fe_d leached) of the four Hapludox samples. These findings show that Fe (hydr)oxides have a more stabilizing effect on the microstructural stability than SOM.

Abbreviations: CSD, controlled shear deformation • EDS, energy dispersive scan • Fe_d, sodium-dithionite-soluble Fe • _L, deformation limit • G', storage modulus • G'', loss modulus • HAC, high active clay • LAC, low active clay • LVE, linear viscoelastic • SEM, scanning electron microscopy • SOM, soil organic matter • _y, yield stress • , angular frequency