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  • Blackwell Publishing Ltd  (283)
  • 1
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Surface crusting results from aggregate breakdown under raindrop impact. It reduces the infiltration rate and may induce erosion by increasing runoff. Soil crustability and erodibility generally increase as organic carbon content decreases. Samples of topsoil were collected from fields cropped continuously for maize after land clearance at various dates. Organic carbon content ranged 4–30 g kg-1. Aggregate stability was assessed by measuring fragment size distribution after different treatments. Samples were also subjected to simulated rainfall. The size of fragments forming the seal when the rain ceased and the infiltration rate during the rainfall were measured to characterize seal structure and hydraulic properties. Development of the seal and infiltration capacity were related to aggregate stability, which was itself a function of organic carbon content. The relations between aggregate breakdown, crusting and infiltration enabled us to predict soil physical behaviour from measurements of aggregate stability which are easier to make than direct measurements under rainfall. The proposed tests offer the opportunity to evaluate actual or potential soil physical degradation and erosion risks without extensive field measurements.
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  • 2
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: The relation between physical and mechanical responses and the structure of clay has been studied. Deformation in response to mechanical stress was measured on samples of Wyoming mont-morillonite and Saint Austell kaolinite in solutions of NaCl (0·005; 0·05; 0·5 and 5 M). Salt concentration was expressed in terms of osmotic potential of water and loading in terms of envelope pressure potential of water, and deformation was measured precisely under small mechanical stress. During shrinking and swelling, deformation was small and reversible, and consolidation was related to the largest mechanical stress applied previously in its history. The arrangement of the clay at equilibrium depends foremost on the type of clay mineral and physico-chemical properties of the liquid phase. For Na-montmorillonite suspensions, transition between gels with tactoids and rigid media with quasicrystals is abrupt and occurs at a potential of about −2000 J kg−1, whether the stress is osmotic or mechanical. Swelling indexes obtained under osmotic and mechanical stress are of the same order of magnitude. Deformation of smectite results from modifications of interassemblage and intra-assemblage organization.
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  • 3
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: A critical-state finite element model was used to simulate compaction under single and dual tyres and tracks. The compaction involved deformations at three different scales, from small tyres with a contact area of about 70 cm2 (single tyre) supporting a load of about 50 kg, to large tyres of about 1.2 m2 (dual tyres) supporting a load of about 4500 kg. The predictions were compared with measured values for several different quantities. These included: rut depths; vertical displacement and shear strain: vertical stresses; and, void ratios and precompression stress measured on sampled soil cores. In general, the predictions and measurements agreed reasonably well. However, the agreement between prediction and measurement depended on the precision of measurements, soil disturbance, and the volume of soil involved in a measurement relative to the volume of soil influenced by the tyre or track. This study shows that the critical-state finite element model is useful, offering insight into the compaction process, the dependence of compaction on soil strength and compressibility, and practical implications for soil management.
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Analytical modelling of the movement of water through porous solids consisting of more than one homogeneous layer is a problem in soil science and in construction engineering. In this paper equations based on the Green and Ampt model describe the absorption of water into a two-layer composite bar for the cases of absorption through a material of large sorptivity into one of smaller sorptivity and vice versa. These predictions are compared with experimental results. There is broad agreement between theory and experiment at all times. We conclude that the absorption of water into a two-layer composite is ultimately controlled by the second material in the absorbing sequence irrespective of whether this material has the larger or the smaller sorptivity.
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: We have developed a critical state, stress-strain analysis that predicts the entire sequence of states from start to the end of a constant cell volume triaxial test in p-q-v-ɛz, space, where p is the spherical stress, q is the deviator stress, v is the specific volume and ɛz is the axial strain. The analysis requires five soil properties to be specified, these being the critical state properties (M, λ and κ) and two elastic properties (any two of E, η, G and K—all four can be found from any two).In order to test the analysis, properties taken mostly from one series of constant cell volume triaxial tests are used to simulate the behaviour in a second test series. In both series the two soils (a sand loam and a clay loam) were tested at several different water contents. The first (property estimation) series of tests was performed using large samples, whereas the second (verification) series involved small samples. The behaviour is quite different in the two series. The elastic property E was not determined in the first series of tests but was estimated from the second series. Furthermore, for three very wet samples, λ had to be estimated by fitting the analysis to the data. The verification was not therefore fully independent of the input test data, particularly for the three wet samples. The stress strain analysis simulated the behaviour of both series of tests in all four dimensions of the p-q-v-ɛz space. The match in p-q-v space was good for all samples. On a q-ɛz plane, the value of q was under-estimated for several samples, but for most of the samples the match was good on this plane. The analysis was generally as good as, and sometimes better than, a previous analysis that deals only with the end point of the test in p-q-v space. The previous analysis did not take account of the elastic properties. The stress-strain analysis therefore seems to offer a useful framework for parameter estimation from constant cell volume triaxial tests. This extends the usefulness of the test itself, as the elastic properties may now be accounted for. The success of the analysis also strengthens the record of success of the critical state concept for unsaturated soils.
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Incorporation of labelled 15N and 14C amino acids and nucleic bases into soil humus fractions as well as humus turnover was investigated under field conditions. The dynamics of 15N and 14C incorporation into organic matter was characterized by the following main steps: rapid incorporation of the labelled substance prevailing for the first 1–3 weeks, and decomposition of included labelled fragments prevailing beyond one month after substance addition. The annual turnover rates of N and C in humus fractions due to incorporation of amino acids and nucleic bases were calculated. The turnover rate of N in humus is two to three times that of C. The contribution of amino acids to organic matter generation is about twice as great as that of nucleic bases and other N-containing organic substances. This indicates the important role of amino acids in the humification process and humus turnover. Turnovers of humic acids (0·002 year−1 for C and 0·02 for N) are the most rapid of humic fractions investigated, and humin is characterized by the slowest turnover (0·0002 year−1 for C and 0·007 for N). There are no significant differences in the turnover rates of fulvic acid fractions (0·0002 year−1 for C) with different molecular weight.
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: We simulated the distribution of water flowing over the soil surface in small plots using a lattice-gas approach. The numerical lattice-gas algorithm consists in simulating a fluid as a finite set of particles which move in a discrete lattice obeying deterministic rules. With this technique, complex boundary conditions and transient flow regimens are handled with ease. The amount of depressional storage and the onset of runoff were considered as functions of rainfall intensity, surface roughness, slope angle and infiltration dynamics. Rainfall was simulated by spherical raindrops of fixed diameter, which were generated by a space and time Poisson process of constant intensity. The effect of infiltration on surface flow was simulated by removing liquid particles at the solid surface with a chosen rate function. Depressional storage and runoff then result from the intrinsic hydrodynamic behaviour of the model under the imposed rainfall, infiltration rate and surface geometry. Results of numerical experiments are presented. They show that the lattice-gas approach is well-adapted for simulating the formation of excess water and the roles of roughness and slope in attenuating runoff for any arbitrary complex surface geometry.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Soil organic matter (SOM) dynamics are usually described by compartmental models. We have sought SOM separates that might be related to SOM dynamic compartments. The turnover of C in various separates from long–term field experiments with maize was measured using the natural 13C labelling technique. The Rothamsted carbon model gave a good prediction of the observed C turnover. Primary particle–size fractions coarser than 50 μm had short lives, and could be associated with the plant structural compartment of models. Water–extractable components are enriched in young C but cannot be associated with labile compartments. None of the chemical separates obtained by acid hydrolysis, wet oxidation, thermic oxidation, pyrolysis or alkaline extraction, were enriched either in young or old C. The results showed neither a sequential relation between fulvic acids and humic acids nor a resistance of nonhydrolysable material. The range of lifetimes of soil C seems to be determined more by physical position and protection than by the chemical nature of SOM.
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  • 10
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Copper sulphate (Bordeaux mixture) has been used as a fungicide against mildew in vineyards for more than a hundred years. This treatment has resulted in significant Cu accumulation in soils (from 100 to 1500 mg kg−1). It is desirable to determine the distribution of Cu in these soils to predict if this element is potentially toxic. Several speciation methods have been compared: sequential and single–step extractions, analytical transmission electron microscopy (ATEM, ASEM) and physical fractionation to study a profile of a vineyard acid soil in Beaujolais (France). Physical fractionation showed that copper is concentrated in the coarse organic fractions associated with plant residues and in the fine clay fraction. Analytical electron microscopy showed a great diversity of fixation sites (bacteria, amorphous organic and mineral compounds). Chemical sequential extractions showed that after the sequential extraction treatments, 60% of Cu was not extracted. Extraction data showed that in the case of an acid sandy soil, sequential chemical extraction did not seem to be sufficiently selective to speciate copper. In the single–step extractions study, the hydroxylamine treatment was the most selective. In the nonselective cases several phenomena may be responsible for the failure, the analytical electron microscopy study showed copper redistribution after certain single–step extractions were carried out.After comparing three different distribution methods we conclude that the distribution of this element in acid soils should be investigated by several analytical methods to establish the true speciation.
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