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Doctoral Degrees (Soil Science)

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Browsing Doctoral Degrees (Soil Science) by Author "Fey, Martin Venn."

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    Characteristics of sesquioxidic soils.
    (1974) Fey, Martin Venn.; Le Roux, J.
    Sesquioxidic soil clays from Oxisols in South Africa, Australia and Brazil, and two clays from Andosols in Japan and New Zealand, were investigated by XRD, OTA, IR, EM and quantitative mineralogical analysis. The volcanic-ash soil clays are dominated by allophane; clays from Natal are dominated by kaolin (30 - 45%) and free iron oxides (20 - 25%), with smaller amounts of gibbsite (0 - 12%) and pedogenic chlorite (less than 20%); Oxisol clays from Australia and Brazil contain free iron oxides (40 - 50%), gibbsite and kaolin (both about 25%). Acid ammonium oxalate (pH 3) was found to be superior to currently popular alkaline reagents for extracting amorphous aluminosilicates and alumina from these clays. Boiling 0,5- NaOH dissolved large amounts of finely-divided kaolinite and halloysite, while hot 5% Na[2]CO[3] reaction was too slow (partial dissolution of synthetic amorphous aluminosilicates with one extraction) and insufficiently selective (gibbsite and kaolin of poor crystallinity dissolve to a variable extent). On the other hand, synthetic gels (molar Si0[2]/A1[2]O[3] ranging from 0,91 to 2,55) dissolved completely after 2h shaking in the dark with 0,2tM acid ammonium oxalate (0,2 ml/mg). Specificity of oxalate for natural allophane was indicated by removal of similar quantities of silica and alumina using different clay: solution ratios. Oxalate extraction data indicated that allophane is absent in Oxisol clays, which are characterized by small quantities of amorphous, A1-rich sesquioxide (1,5 to 7%), some of which may originate in interlayers of 2: l phyllosilicate structures. Allophane was determined quantitatively in volcanic-ash soil clays by allocating hydroxyl water content to oxalate-soluble silica plus alumina on the basis of an ignition weight loss/chemical composition function for synthetic amorphous aluminosilicates. Both Si02/A1[2]O[3] ratios and quantities of allophane were found to be lower than those obtained using boiling 0,5N NaOH, in agreement with the interpretation that the latter treatment attacks crystalline aluminosilicates. Parameters of chemical reactivity and distribution of electric charges following various chemical pretreatments of allophane were found to correspond closely to those predicted on the basis of synthetic gel behaviour. Results for Oxisol clays suggested that the role of amorphous (oxalate-soluble) alumina in governing physicochemical properties is generally subdorninant to that of the poorly-crystalline, A1-substituted iron oxide component which is removed by deferration with citrate-dithionite-bicarbonate reagent. Hysteretic pH-dependent net negative exchange charge was shown to arise from hysteresis of positive exchange charge, while CEC is fully reversible by titration with strong acid. A mechanism is postulated to account for this observation. Levels of silica in the soil solution of Natal Oxisols are higher than those of more strongly-weathered soils from Australia and Brazil, and may be sufficiently high to exert a favourable effect on plant-available P following phosphate fertilization. Although soluble silica levels are also relatively high in volcanic-ash soils, a similar effect is not likely to manifest itself significantly owing to the very high P adsorption capacity of allophane. A study of soil solution equilibria indicated that in terms of silica and aluminium hydroxide potentials, kaolinite is the most stable mineral in all the soils. Allophane persists as a partial metastable equilibrium state in volcanicash soils while gibbsite formation in Oxisols is contingent upon periodic, nonequilibrium leaching conditions. The role of clay mineral suite in governing levels of exchangeable aluminium in acid soils is examined. A revised model system for allophane is proposed in which tetrahedral substitution of Al for Si may reach a maximum of 1 : 1 in an aluminosilicate phase. Additional alumina takes the form of discreet amorphous or crystalline material. The composition of allophane corresponding to maximum A1 for Si substitution will depend upon the availability of basic cations for charge balancing during neogenesis of the amorphous aluminosilicate.
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    Investigation of 2:1 layer silicate clays in selected southern African soils.
    (1986) Buhmann, Christl.; Fey, Martin Venn.
    As very little detailed X-ray diffraction investigations have been carried out in South Africa on 2:1 phyllosi1icates in soils, the aim of the present study was to contribute to the knowledge of soil genesis, as well as K-fixation and swelling, by investigation of the clay fraction of selected soils known to be rich in these minerals. X-ray diffraction analysis has been used almost exclusively as the investigative technique. In Chapter 1 a literature review is presented on the reasons for X-ray diffraction peak broadening and the problems encountered in the identification of swelling clay minerals. For interstratifications, the concept of an ABAB layer sequence, considered as having suggested an abab that the inter1ayer space, X-ray diffraction is questioned. data from which It is the ABAB arrangement is inferred can as well be explained in terms of an alternative AAAB layer sequence, having an aabb interlayer arrangement. Chapters 2, 3 and 4 deal with layer silicate formation/alteration in the course of soil development in dolerite and shale-derived profiles. Dolerite-derived pedons could· be characterized by one of the following layer silicate suites : suite i : discrete smectite (Fe-containing beidellite-montmorillonite) with or without traces of kaolinite and talc (Vertisol) suite ii : smectite-kaolinite interstratification (Vertisol) suite iii : 14 ft minerals (vermiculite, beidellite, montmorillonite, chlorite) and 7 ft minerals (halloysite, kaolinite ) in about equal proportions (Vertisol and Mollisol) suite iv : kaolinite with subordinate chlorite and traces of talc (Oxisol, Ultisol). Eccashale-derived Vertisols are dominated by mica-smectite interstratifications. The occurrence of an iron-rich pedogenic talc is discussed in Chapter 4. X-ray and chemical data suggest 30 - 50 mole percent substitutions of iron for magnesium. The mineralogical basis for K-fixation has been established in Chapter 5. Two K-fixing components could be identified : dioctahedral high-charge vermiculite as a discrete mineral and random mica-smectite interstratifications with 20 - 60% mica. In Chapter 6, some of the most expansive soils in South Africa have been investigated. They can be subdivided into two groups denoted by the swelling component as follows : (a) smectite-dominated (the smectite species involved being most probably beidellite with a heterogeneous charge distribution); (b) mica-smectite interstratification with random or ordered stacking arrangement.