Soil structural interfaces in some Texas Vertisols and their impact on solute transport

Structural surfaces and slickensides have been reported to be important features impacting sorption–desorption of chemicals and bypass flow of pollutants to shallow groundwater aquifers. This study provides visible, microscopic and submicroscopic characteristics of slickenside features in two Texas Vertisols of different geomorphological age. Transport of visible anionic dye and CaBr is studied as related to soil structural surfaces, root channels and slickenside planes. Macromorphic observations were coupled with thin section microscopy, scanning electron microscopy and electron microprobe analyses.Slickenside surfaces differed from subjacent micromass of the ped interiors by stronger preferred orientation of clay particles and reduction in macropores with corresponding increase in micropores. Such characteristics could make slickensides act as semi-permeable barriers to solute transport, but this phenomenon could not be confirmed from CaBr studies herein.The majority of the solute bypass flow in both soils was conducted through root channels and ped interfaces. Slickenside surfaces in these Vertisols conducted less than 1% of the flow. It was observed primarily when the dye solution was delivered to the surface of the slickenside via the adjacent root channel. In spite of the preferential orientation of clay particles along slickenside planes, the penetration of Br into the aggregate interior was not retarded like the larger organic anion dye. These results may reflect specific moisture conditions, which should be considered in future works.     

In-situ soil organic matter studies using scanning electron microscopy and low temperature ashing

The in-situ distribution and morphology of organic materials located on, or near, structural surfaces within some soils is examined on a submicroscopic scale by comparing scanning electron microscope images of the same areas of soil samples before and after low temperature ashing. Electron-translucent organic matter coatings up to 1/2 μm thick, and thinner, electron-opaque organic matter coatings were found on structural surfaces within these soils. Oribatid faecal pellets in one of the soils were found to contain aluminosilicate clay minerals. Fine-clay sized spheres of biogenic opaline silica were found to be contained within the epidermis of a decaying root. These studies show that the combined use of low temperature ashing and scanning electron microscopy will be a valuable technique for in-situ investigations of submicroscopic organic matter within soils.     

CLAY MINERALOGY OF SOME SEASONALLY FLOODED SOILS OF EAST PAKISTAN

The clay fractions of four seasonally flooded soils from East Pakistan have been studied using X-ray, DTA, and infra-red methods, total chemical analysis, surface area determination, and electron microscopy. The soils represent young alluvium (Borda), a juvenile groundwater laterite (Nunni), a groundwater lateritelatosol intergrade (Noadda), and a surface-water glei (Chhiata). Mica and kaolinite were present in all the soils, and vermiculite in all except possibly Noadda. Vermiculite, total magnesium content, and SiO₂/Al₂O₃ ratio decreased and kaolinite increased in the sequence Borda–Nunni–Noadda. Electron micrographs indicated that in the Borda soil, which is the youngest and subjected to the longest period of annual flooding, iron hydroxides are present as gels coating the surfaces of the clay mineral particles. Such coatings are less apparent in the Nunni clay, and in the Noadda clay most of the ‘free iron oxides’ appear as small granules. The Chhiata soil has about 12.5 per cent of its clay fraction in the form of an amorphous gel rich in silica, and much of the mica and vermiculite in a randomly interstratified complex.     

Interactions of Escherichia coli and Streptococcus bovis with soil clay surfaces as revealed by scanning electron microscopy

The attachment of Escherichia coli and Streptococcus bovis of bovine faecal origin to soil clay particles has been examined using scanning electron microscopy. Both organisms produce extracellular polymers through which the cells form bridges to clay surfaces. Differences in the extent of cell-to-particle bridging between E. coli-and S. bovis-soil complexes are explained in terms of the charge characteristics of the cells and the properties of the exopolymers.     

Strengths of a densipan,humus-pan and clay-pan in a Spodosol developed under kauri (Agathis australis) and the implications for soil classification

Three hard layers have formed in a Spodosol developed on aeolian coastal sands under the influence of litter from kauri (Agathis australis). A densipan in the albic horizon and a humus-pan in the spodic horizon slaked in water whereas a clay-pan in a buried soil only softened in 1 M HCl. Blocks sawn from these three pans showed a maximum of unconfined compressive strength in the humus-pan.Specimens made from remoulded pan materials with minimum compaction developed only a proportion of the original strength. Greater compaction enhanced the strength of all three materials. Raising the pH during remoulding caused a marked increase in strength, but above pH 10.5 all the pan materials lost strength when silicate was solubilised from surfaces of clay and quartz particles.Scanning electron micrographs showed that the densipan was a close-fit configuration of sand- and silt-sized quartz particles that attained a high density without cementation of particles. In the humus-pan organic matter bridged across a third of the surfaces of the quartz particles and prevented a close-fit configuration. The clay-pan below the spodic horizon had all its particles thickly coated with clay and humus and these were aggregated into an open-fit arrangement of lower density.According to Soil Taxonomy procedures, the Te Kopuru sand lies within the Aquod sub-group of Spodosols. In the absence of a non-slaking duripan it was the presence of a fragipan below the spodic horizon that directed the soil into being a Fragiaquod. It is the densipan in the albic horizon above the spodic horizon, however, that controls plant roots and cannot be ignored in soil classification. We suggest that a new great group, Densiaquod, be recognized for these soils.     

Effect of ionic strength and clay mineralogy on Na–Ca exchange and the SAR–ESP relationship

The relationship between sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) for all soils has traditionally been assumed to be similar to that developed by the United States Salinity Laboratory (USSL) in 1954. However, under certain conditions, this relationship has been shown not to be constant, but to vary with both ionic strength and clay mineralogy. We conducted a detailed experiment to determine the effect of ionic strength on the Na⁺–Ca²⁺ exchange of four clay minerals (kaolinite, illite, pyrophyllite, and montmorillonite), with results related to the diffuse double‐layer (DDL) model. Clays in which external exchange sites dominated (kaolinite and pyrophyllite) tended to show an overall preference for Na⁺, with the magnitude of this preference increasing with decreasing ESP. For these external surfaces, increases in ionic strength were found to increase preference for Na⁺. Although illite (2:1 non‐expanding mineral) was expected to be dominated by external surfaces, this clay displayed an overall preference for Ca²⁺, possibly indicating the opening of quasicrystals and the formation of internal exchange surfaces. For the expanding 2:1 clay, montmorillonite, Na⁺–Ca²⁺ exchange varied due to the formation of quasicrystals (and internal exchange surfaces) from individual clay platelets. At small ionic strength and large ESP, the clay platelets dispersed and were dominated by external exchange surfaces (displaying preference for Na⁺). However, as ionic strength increased and ESP decreased, quasicrystals (and internal exchange surfaces) formed, and preference for Ca²⁺ increased. Therefore, the relationship between SAR and ESP is not constant and should be determined directly for the soil of interest.     

Effects of Clay Surfaces on the Adsorption and Biological Decomposition of Proteinaceous Components of Fulvic Acid

Shaking of an aqueous fulvic acid (FA) solution with montmorillonite at pH 2.5 at room temperature, removed about 43 % of the FA fraction containing proteinaceous components. About 65 % of this fraction was adsorbed reversibly in interlayers, 20 % on external surfaces and 15 % was adsorbed irreversibly. The relative molar distribution of the amino acids (after hot acid hydrolysis) in the original FA, the FA fraction adsorbed in clay interlayers and in the FA fraction not adsorbed by montmorillonite was relatively uniform, indicating that clay interlayers were relatively inefficient for separating proteinaceous constituents from main structural FA components. External clay surfaces, however, appeared to adsorb peptides and/or proteins that differed qualitatively from those adsorbed on internal clay surfaces. Five successive treatments with fresh clay showed that only 61 % of the FA fraction containing proteinaceous components could be adsorbed. The original FA and all fractions separated from it by montmorillonite were able to support microbial growth without the addition of an external N source. The rate of biodegradation decreased when the FA was complexed with montmorillonite.     

四种农业土壤上生物炭-土壤的交互效应

Soils in south-western Australia are highly weathered and deficient in nutrients for agricultural production. Addition of biochar has been suggested as a mean of improving soil C storage, texture and nutrient retention of these soils.~Clay amendment in sandy soils in this region is a management practice used to improve soil conditions, including water repellence.~In this study a woody biochar (Simcoa biochar) was characterised using scanning electron microscopy before, and four weeks after, it was incorporated into each of four soils differing in clay content and organic matter. Scanning electron microscopy of Simcoa biochar after incubation in soil showed different degrees of attachment of soil particles to the biochar surfaces after 28 d. In addition, the effects of three biochars, Simcoa biochar, activated biochar and Wundowie biochar, on soil microbial biomass C and soil respiration were investigated in a short-term incubation experiment. It was hypothesised that all three biochars would have greater potential to increase soil microbial activity in the soil that had higher organic matter and clay. After 28-d incubation in soil, all three biochars had led to a higher microbial biomass C in the clayey soil, but prior to this time, less marked differences were observed in microbial biomass C among the four soils following biochar application.     

Interactions between H-bentonite and aluminous chlorite-like complexes

In synthetic chlorite-like complexes, amounts of amorphous and/or crystalline hydrous aluminum oxide were determined through use of H-bentonite that converts into Albentonite by reaction with Al-gel or gibbsite. In Al(OH)_x-complexes with less than 1,000 mequiv. of Al per 100 g of clay, hydrous aluminum oxide gel remains held on clay surfaces without crystallizing for a long time. The neutralization reaction between a one-symmetry-concentration of H-bentonite and chlorite-like complexes at 10°C is initially very fast and seems to affect exclusively the hydrous oxide in suspension or adsorbed on the external surfaces and edges of clay particles. After a few hours, the interlayer Al-gel begins to be neutralized and the reaction rate decreases according to first-order kinetics. In complexes of equal ages, more time is required to neutralize the hydrous oxide precipitated in the presence of clay than added to the clay, because of a different interlayering.The kinetics of reactions between complexes characterized by amorphous and crystalline aluminum hydroxide (more than 1,000 mequiv. aluminum per 100 g of clay) and H-bentonites were studied. H-bentonite reacts with gibbsite much more slowly than it does with amorphous forms, always according to first order kinetics.     

Submicroscopy and stable isotope geochemistry of carbonates and associated palygorskite in Iranian Aridisols

Pedogenic carbonates in arid and semi-arid regions of the world have a great significance as palaeoecological and palaeoclimatological indicators and form a major pool in the carbon cycle. We analysed the ultra-microfabric and the stable isotope composition of C and O in pedogenic carbonates in colluvial soils derived from limestone in an arid region of central Iran. Our objective was to determine the conditions for the formation of soft pedogenic carbonate nodules and their co-existence with palygorskite in the palaeo-argillic horizon. Scanning electron microscopy revealed that the calcite aggregates were matted with palygorskite. Ultra-microtome cuts, examined using transmission electron microscopy, provided more detailed information about the fundamental particle association of secondary carbonates and palygorskite. Although less abundant, other silicate clays were detected in both the acid-insoluble clay fractions and in ultra-cuts, mostly in fine clay size, suggesting the engulfing of palygorskite by growing calcite or illuviation of palygorskite during or after formation of the calcite. Coatings of illuvial clays on calcite crystals support the hypothesis that palygorskite was trapped by pedogenic carbonate when the climate was wetter than it is today to form an argillic horizon. However, electron microscopic evidence of the occurrence of fibres on the immediate pedogenic carbonate particle surfaces suggests the in situ formation of palygorskite. The δ¹³C and δ¹⁸O values of pedogenic carbonates suggest that these carbonates were formed in an environment with more available moisture and more C4 plants than now.     

Relationship between the characteristics of Mediterranean Red soils and the age of the geomorphological surfaces in central-western Spain

A study was conducted of the Mediterranean Red soils of a region in Central Western Spain with a goal of describing the relationship between their main characteristics and the age of the geomorphic surfaces over which they have developed. The main macromorphological characteristics of the soils were analyzed, with a statistical study of the analytical data on 70 samples of Mediterranean Red soils. The oldest surfaces contain Palexerults and Ultic Palexeralfs, Calcic Rhodoxeralfs, Calcic-vertic Palexeralfs, Typic Rhodoxeralfs, Typic Palexeralfs, Vertic Haploxeralfs and Typic Haploxeralfs have been observed in a chronosequence on terraces of the River Tormes. The soil typology changes with the age of the surfaces, from soils with a sharp textural contrast at the upper limit of the argillaceous horizon, with a dark red very clayey and very thick Bt horizon (on the oldest surface) to not very thick brown soils without a clear clay illuviation and with a lower content in clay (on the youngest surface).The effect of recent erosion on the morphology of the soils located on a single surface has caused new soils to form superimposed on pre-existing soils.     

Use of a three-plane model to describe charge properties of some iron oxides and soil clays

The pH-dependence of surface charge density of three iron oxides and the clay fraction of five horizons of a variable-charge soil has been studied by adsorption of potential-determining ions (p.d.i.) and by adsorption of electrolyte ions. The general trend of results for iron oxides can be satisfactorily explained by a model which considers a three-plane distribution of electric potential. When the model is applied to soil clays the outcome is less satisfactory and, although their charge properties seem to be largely determined by their high content of minerals with variable charge surfaces, the presence of minerals with a permanent charge (clay minerals) should be considered when explaining the particular data, even for those horizons where clay minerals are present in low proportions.     

Changes in surface reactivity and organic matter composition of clay subfractions with duration of fertilizer deprivation

Preservation of organic matter in soils depends on the chemical structure of organic compounds and on the surface properties of the mineral matrix. We tested the effect of mineral surface reactivity on organic matter decomposition by (i) investigating changes of organic matter composition in clay subfractions of an illitic Haplic Chernozem along a time series of fertilizer deprivation and (ii) simultaneously characterizing the reactivity of mineral surfaces. The soil was subjected to fertilizer deprivation for 18, 44 and 98 years, respectively. Mineral surface properties were characterized by selective dissolution of pedogenic oxides. The number of hydroxyls released after exposure to sodium fluoride was taken as an index for mineral surface reactivity. Organic soil constituents were determined by ¹³C cross‐polarization magic‐angle spinning nuclear magnetic resonance (¹³C CPMAS NMR). Clay subfractions had different mineral surface properties. The coarse fractions have more reactive surfaces and contain more organic carbon than the fine clay fractions. Mineral surface properties are constant over time and are not affected by fertilizer deprivation. Surface reactivity is a function of iron oxide density and controls carbon concentrations in the clay subfractions. Within the time frame of our investigation, alkyl C and aromatic C responded to the duration of fertilizer deprivation, but were indifferent to mineral surface reactivity. O–alkyl C seems to be protected by interactions with pedogenic oxides.     

Redox dependence of papain enzyme activity on charged kaolinite surfaces and in solution

The relative activity of an SH dependent enzyme, papain, is decreased by increasing the ratio of oxidizing disulfide to reducing thiol in solution. The same applies to papain adsorbed on negatively charged clay particles, but the effect is more pronounced if positively charged disulfides can be formed in the thiol-disulfide exchange equilibrium. Disulfides with a double positive charge are apparently more strongly attracted to clay than are singly charged, neutral, or negatively charged products of exchange. Preferential attraction of oxidizing disulfides to the particulate surfaces increases local, microenvironmental oxidizing potentials.     

THE EFFECT OF PROLONGED CROPPING ON THE EXCHANGE SURFACES OF THE CLAYS OF BROADBALK FIELD

Earlier studies have shown that the K-(Ca+Mg) exchange isotherms of field soils are not greatly changed by cropping, by additions of K-fertilizer, nor by moderate release of K from fixed forms. The present work shows that very prolonged K-enrichment or depletion in the Broadbalk trials has led to substantial differences between the exchange isotherms of soils from adjacent plots in proportion to the severity of their K-depletion. It is suggested that the changes result from the removal of inter-lamellar K and the consequent increases in the inter-lamellar spacings of parts of the clay crystals, which have increased the extent of the surface areas accessible to K-(Ca+Mg) exchange. It is possible that increases sufficient to cause substantial changes in the exchange isotherms may not be apparent in X-ray diffraction diagrams. Treatment of the K-depleted clays with m CaCl₂ appears to contract the newly expanded parts of the clay crystals, so that exchange on the occluded surfaces is once again too slow to be observed by the procedures used.     

The development of soil porosity in experimental sandy soils with clay admixtures as examined by quantimet 720 from besi and by other techniques

For this study thin sections have been prepared of natural and artificial sands mixed with 20% and 40% clay. These and other samples had already been examined using soil physical and soil mineralogical methods for loose samples. Initial thin section studies, with the help of the light microscope alone, have been enlarged upon in this investigation using backscattered electron scanning images and an image analyzer (Quantimet 720). This allowed quantitative information to be obtained on the development of different types of soil porosity patterns using various clay admixtures. Information was also obtained on the influence the form of mineral grains had on the shape of pores in the mixtures of sand and clay. It was also possible to obtain quantitative information on the amount of clays and larger mineral grains in the thin sections. Much of the present data cannot be obtained automatically but must be manually controlled by using the image editor of the Quantimet. This type of work, however, may provide basic data which can subsequently be used to help explain various processes which occur in soils.     

Effect of matrix composition on carbonate nodule crystallization

The mechanism of carbonate nodule formation in three soil profiles of different texture was elucidated by the application of several methods: petrographic microscopy, electron microprobe, and scanning electron microscope analysis. The profiles chosen for detailed investigation included: a loessial Serozem overlying a buried soil developed on calcareous sand in the semiarid climate of the northern Negev, Israel (EH); a polygenetic Husmas soil with secondary carbonate enrichment in a leached red mediterranean sandy clay loam from the southern Pleshet, Israel (GA); a brown Grumusol with carbonate nodules in the lower horizon from the Zebulon valley, Israel (GR).Three kinds of carbonate nodules were distinguished according to their morphology and origin: (1) orthic nodules, which have skeleton grains similar to the surrounding soil and a gradual transition to the soil matrix — these are formed in situ; (2) disorthic nodules, which on the basis of their sharp boundaries can be judged as having been subjected to some pedoturbation but have a fabric resembling the surrounding matrix; and (3) allothic nodules, which have a fabric that differs in composition from the soil in which they are incorporated and are thus judged to have been transported into the soil.This study indicates that the orthic nodules in the loessial Serozem and in the buried soil have been formed by gradual precipitation of carbonate in the microvoids of the matrix resulting in greater density and a partial expulsion of the non-carbonate clay to the fringes. X-ray spectroscopy traverses and cathode-ray distributions of the Al, Mg and Fe by microprobe indicate the presence of clay in the nodule and its gradual increase towards the still active fringes. In disorthic nodules no such increase toward the fringes was observed.Observations with the scanning electron microscope (SEM) indicate a diameter of 1.5–4 μ for the microcalcite in the Serozem and 4–7 μ for that in the buried sandy soil. Both are layered and built up from oval subhedral crystals 0.2–0.6 μ in size, a morphological type which has not been reported previously.From a detailed study of these profiles and from comparisons with some other soils, the authors conclude that the size and growth of the calcite crystals in the nodule is determined by the matrix composition, in particular by the presence of clay minerals. In a coarse-grained sandy matrix, microsparite and sparite precipitate, thus filling the voids. Similarly the vughs between the carbonate-free stable and compact peds of the Grumusol serve as good sites for the crystallization of a sparite fabric. On the other hand, the presence of dispersed clay minerals in the calcareous Serozem soil offers a large number of nucleation points for the formation of a micritic fabric, which then occludes part of the clay minerals. The presence of the clay retards and possibly even prevents a subsequent growth and recrystallization of the calcite crystallites.     

Effect of peroxidation on soil electrochemical properties

Treatment with hydrogen peroxide to destroy the organic matter shifts the surface charge towards positive values. This is in agreement with the suggestion that per oxidation produces artefacts in the soil residues, since metals released from organic matter precipitate as hydroxide coating and produce positively charged surfaces. In the presence of 0.1 M sodium pyrophosphate, treatment with hydrogen peroxide always shifts the surface charge towards negative values, notwithstanding the removal of the electronegative organic components. It is suggested that the bulk of electropositive iron and aluminium oxides in soil are associated with the organic matter and removed by the treatment, so that permanent negative charges of clay minerals dominate in soil residue.     

Soil formation on the Wallagaraugh Adamellite, southeastern N.S.W., Australia

Soil development on Wallagaraugh Adamellite has been studied within Yambulla State Forest, southeastern Australia. Twenty-five soil profiles have been described and soil chemical and physical properties determined for selected horizons. A geomorphological model is described for elucidating processes affecting soil development on this adamellite parent material. Three main geomorphic environments have been incorporated into this model: residual surfaces found on interfluves and broad hillcrests, transportational surfaces associated with ridges and hill slopes, and depositional surfaces associated with foot-slopes and valley floors. Residual surfaces have developed duplex primary profile forms (yellow podzolics) with structured B horizons of high clay content where exchangeable Al dominates the exchange complex. Transportational surfaces have soils with shallow, uniform to gradational profiles with minimal profile development, and highest, but most variable, concentrations of the exchangeable Ca, K, Na, and total P. Depositional surfaces are characterized by a thick accumulation of uniform, coarse-sandy colluvium-alluvium. Where this surface is stable and well drained, podzols have formed. In contrast, areas of impeded drainage on this depositional surface have humic gley profiles. Concentrations of total P, exchangeable Ca and Mg are low throughout these aggraded soils.The soil-landscape units derived from the geomorphological model are discriminated by multivariate analysis using soil chemical and physical properties. Surface soil chemical properties are shown to be superior in discriminating soil-landscape units to subsoil properties.The mineralogy, geochemistry and texture of the adamellite soil parent material are critical in determining the distinctive geomorphology which, in turn, affects the development of soils in this forest environment.     

A critical state soil mechanics model for agricultural soils

Abstract. The paper presents the experimentally derived state boundary surfaces of critical state theory for a sand, a loam and a clay soil. Orderly changes to these surfaces with moisture content and two soil micro-structural states have been identified. These findings are used as the basis for the formulation of a fairly simple universal model of the geometry of critical state space for unsaturated soils. Examples are given of how this model can be used to explain known soil behaviour in many practical situations. The indications are that this model can provide the theoretical framework for a fundamental comprehension of the many complex processes involved in soil loosening and compaction. There is, as yet, no simple experimental technique for measuring the critical state boundaries of field soils and this is a major impedement to the development of the model as a practical soil management tool.