Georges Stoops

Proposal for a micromorphological classification of soil materials. I. A classification of the related distributions of fine and coarse particles

Abstract As a first step towards a new micromorphological classification of soil materials, a classification is made of the mutual spatial distributions of the coarse and fine fabric elements of lowest order: the c/f-related distributions. The classification comprises five orthotypes: monic—only one size fraction is present; gefuric—bridges of finer material exist between coarser grains; chitonic—finer material coats the coarser grains; enaulic—the finer material occurs as small aggregates in the spaces between the coarser grains; and porphyric—the coarser grains are embedded in a dense mass of finer material. The relationship between the different types is visualized in a three-dimensional geometric figure: a trigonal bipyramid. An appropriate terminology makes it possible to express the different combinations of types, the size variations and the main composition of each size fraction.

Argillic horizon development in calcareous soils of arid and semiarid regions of southern Iran

Abstract Ten representative pedons with argillic horizons from southern Iran were selected for micromorphological and mineralogical study. They were classified as Haploxeralfs, Haplustalfs, Argixerolls, Argiustolls and Argigypsids with thermic, hyperthermic and mesic temperature regimes. Some of the argillic horizons were probably formed in a less arid previous climate, during which carbonate migrated through the profile, thus allowing clay to illuviate into deeper horizons. Thin sections showed few to no clear clay coatings in argillic horizons with greater clay content, where shrink/swell has disrupted them. Clay coatings superimposed on carbonate coatings, and in turn covered by calcite, indicate recalcification. Calcite depletion pedofeatures were the most striking features in these soils. Sodium may have played an important role in the dispersion of clay and formation of argillic horizons within two of the pedons studied. A micromorphological index of soil evolution in highly calcareous arid and semiarid conditions is proposed. It correlates with the content of free Fe2O3 (FeDCB in fine earth), the colour index, the clay illuviation index and the smectite/(chlorite+illite) ratio. The index can be useful for correlation of soils with argillic horizons formed on highly calcareous parent material in different landscape units. New subgroups of Haploxeralfs and Argiustolls are suggested for inclusion in Soil Taxonomy based on the properties of some of the studied profiles.

New trends in soil micromorphology

1. The role of soil micromorphology in the light of Euorpean thematic strategy for soil protection,- 2. Soil micromorphology and soil hydraulics,- 3. Micromorphology of a soil catena in Yucatan: pedogenesis and geomorphological processes in a tropical karst landscape,- 4. Soil evolution along a topsequence on glacial and periglacial materials in the Pyrenees range, Spain,- 5. A micromorphological study of Andosol genesis in Iceland,- 6. Improved paleopedological reconstruction of vertic paleosols at Novaya Etuliya, Moldova, via integration of soil micromorphology and environmental magnetism,- 7. Ferricretes in Tamil Nadu, Chennai, South Eastern India: from landscape to micromorphology, genesis and paleoenvrionmental significance,- 8. Palygorskite dominated vertisols of Southern Iran,- 9. Contribution of micromorphology to classification of aridic soils,- 10.Orientation and spacing of columnar peds in a sodic, tecture contrast soil in Australia,- 11. Physical fractionation or mormoder humus using cryo-coupes and micromorphological methods,- 12. Regional manifestation of the widespread disruption of soil-landscapes by the 4 kyr BP impact using pedo-sedimentary micro-fabrics,- 13. Clay illuviation in a Holocene palaeosol in the Chinese Loess Plateau,- 14. Soil microstructure and solution chemistry of a compacted forest soil in a sub-boreal spruce zone in Canada.

Weathering: Toward a fractal quantifying

Weathering occurs over a wide range of scales. To link features through these scales is a major challenge for interdisciplinary weathering studies. Fractal approach seems to be specially useful for this purpose. We introduce a multistep fractal weathering assessment scheme devoted to extract fractal weathering classifiers from texture analysis of the minerals image. Our scheme enables to quantitatively estimate the global and local information about the geometry of the weathering pattern. This information is basic to develop geometrical indices of weathering, which can significantly enrich the common qualitative and semiquantitative weathering assessment schemes. To justify the fractal approach, a strong statistical self-similarity has been documented for both the weathering and fresh features of two common silica minerals: quartz and biogenic A-opal (phytolith) over four orders of length scales. The procedure is fast, drastically reduces thresholding bias, promises to be universal, it is valid for genetically different minerals and rock types, scale independent, and specially useful for monitoring the changes in the minerals roughness during the alteration. Two of the proposed classifiers seem to be potentially useful for direct application in the field and be used by nonspecialist.

Pore space characteristics as indicators of soil behaviour in gypsiferous soils

Abstract In order to study the physical properties of gypsiferous soils (Gypsic Xerochrepts and Typic Xerofluvents) from an irrigated semi-arid area in Catalonia (NE Spain) and their relationships to their pore characteristics, twenty horizons with gypsum contents ranging between 0 and 90% were sampled for micromorphological and physical analyses. The micromorphological study consisted of thin section description and of image analysis of uncovered thin sections of samples impregnated with a UV-fluorescent dye. Eight fields of each horizon were studied. Two images were obtained from each field (2.1×3.1 cm 2 ): one illuminated with incident UV-light and the other with transmitted white light. Image processing with an IBAS-system allowed measurement of the geometric characteristics of three types of objects: actual pores (eq. diam.>100 μm), transparent objects infilled or not with gypsum and pores located in these transparent objects. The physical characteristics of the same horizons considered in the study were the water-characteristic curves and the saturated hydraulic conductivity. Although differences in total porosity (between 6 and 16%) are not related to other properties, an increase of gypsum content leads in the studied horizons to the replacement of large pores by smaller packing pores between gypsum crystals in infillings, which are interconnected and do not reduce saturated hydraulic conductivity. Available water is affected by the degree of mixing of the gypsum infillings with the groundmass, besides the crystal size distribution. The results stress the importance of pore clogging by gypsum in these materials as a major constraint for root development, as well as the need of describing the type of gypsum accumulation when assessing the suitability of these soils for plant growth.

Physical and biological surface crusts and seals

Publisher Summary The aim of this chapter is to provide an overview of the different micromorphological expressions of soil crust formation. Three types of surface soil crusts are distinguished: physical crusts, biological crusts, and chemical crusts or precipitates. Physical crusts are specific modifications of the topsoil caused by natural or other events, such as raindrop impact or sedimentation and subsequent drying, consisting of a hard thin surface layer with reduced porosity and increased bulk density; some authors prefer to use the term seal as long as no hardening has taken place. Biological crusts are living communities of lichen, cyanobacteria, algae or moss growing on the surface of the soil and binding it together. Chemical crusts or precipitates are also called salt crusts.

Regoliths and Soils on Volcanic Ash

Abstract Soils on volcanic ash have specific physical and chemical properties, as well as typical micromorphological characteristics. They are characterised by an extensive accumulation of short-range order Si-Al compounds, such as allophane and imogolite, formed during weathering. This results in a grouping at the highest taxonomic level as Andosols (WRB) or Andisols (USDA) and in the use of ‘andic’ designations at other levels. The microstructure evolves from coarse monic in fresh ashes over chitonic and enaulic to granular in Andosols. A weakly separated intrapedal granular microstructure may persist in Bw horizons with a blocky microstructure or with a lenticular (freeze-thaw) microstructure. In Vertisols on volcanic ash a blocky microstructure is observed. The coarse material generally includes large amounts of volcanic glass. Fresh ash can already be transformed by hydrothermal alteration, resulting in the presence of secondary products. In the first stages of pedogenic weathering, short range order Si-Al compounds are formed. Pedogenic weathering of basaltic glass can produce palagonite-like alteromorphs of allophane. Magnetite and ilmenite can persist even in strongly weathered soils. Phytoliths and fungal hyphae are generally very abundant. The b-fabric is undifferentiated as long as short range order clays and halloysite are predominant, and it becomes stipple- or mosaic-speckled when kaolinite or illite appears. Micromass coatings around coarse fragments, and internal hypocoatings in the case of pumice, develop during a fist stage of soil formation. Other textural pedofeatures, except for rare allophane coatings, are generally not present in Andosols, but appear in intergrades to Luvisols, often still composed of optically isotropic compounds. In certain duripans (Tepetate), clay coatings with opaline zones are observed. Gibbsite coatings and nodules occur in conditions with strong leaching. Opal nodules are common in many soils formed on volcanic ash.

Oxic and Related Materials

Publisher Summary This chapter discusses the oxic and related materials. The oxic horizon is a strongly weathered subsurface horizon with a clay fraction dominated by low-activity clays and iron and aluminium oxides, and with silt and sand fractions mainly composed of quartz and including little or no weatherable minerals. The oxic horizon essentially corresponds to the ferralic horizon of the World Reference Base for soil resources. Oxic and ferralic horizons are diagnostic horizons of Oxisols and Ferralsols, respectively. Micromorphological techniques have been often used in the study of oxic and related materials. Wide attention has been given to the study of the granular microstructure that is typical for these materials, but no consensus has been reached on its origin. Submicroscopic techniques were mainly used to study mineral weathering, mainly of quartz, and the formation of kaolinite, iron oxides, and gibbsite occurrences.

Properties, some criteria of classification and genesis of upland forest Podzols in Rwanda

Abstract In southwest Rwanda, upland forest soils developed on quartzites and micaceous sandstones along steep slopes have sometimes the macromorphological look of ‘true’ Podzols. An investigation of the micromorphological, mineralogical and chemical properties, however, reveals only weak indications of illuviation of amorphous organic complexes. This process of cheluviation seems to be secondary relative to biological activity, Fe precipitation and weathering. The concept that cheluviation is the dominant process in the formation of spodic horizons and spodic materials would exclude these soils for the class of Podzols. Definitions of spodic horizon and spodic materials proposed in international soil classification systems, should give preference to macro and micromorphological properties over chemical properties, because the former are directly related to different genetic processes (biological activity, oxido-reduction) which also could be responsible for their formation.

Precipitation and dissolution of gibbsite during weathering of andesitic boulders in humid tropical West Java, Indonesia

Abstract Several weathered andesite boulders, sampled in Typic Humitropepts from the Cigudeg area, West Java (Indonesia), were studied using micromorphological, mineralogical (XRD, TG, DTA, SEM, EDXRA) and chemical techniques. Because all data of the boulders are similar, only the results of the largest boulder are presented in this paper. Three zones are distinguished: the rim of the boulder core, a transition layer, and outer layers. In the rim of the core weathering attacks the glass matrix and the anorthite rich parts of the plagioclases. Even before sufficient iron is liberated from pyroxenes goethite is deposited in their cracks, as a first step to the formation of a boxwork. In the transition layer mono- and divalent cations have been lost, but Al shows an absolute accumulation as gibbsite in plagioclase pseudomorphs and pores. It is presumed that the Al originates from the more internal parts of the core. Towards the outer layer gibbsite gets completely removed, supposedly as the result of a lowering of pH, and the plagioclase pseudomorphs become filled with illuvial kaolinitic clay. Illuviation of clay, and the import of iron, from the surrounding soil material, put limits to the use of isovolumetric calculations. The goethite boxworks after pyroxenes reflect the original pattern of fractures, rather than cleavage patterns.