Soil Clay Mineralogy Research Articles

Relationship between soil clay mineralogy and carbon protection capacity as influenced by temperature and moisture

Environmental conditions like temperature and moisture could affect the carbon protection capacity of various clay types in soils. Using dominantly kaolinitic-illitic, smectitic and allophanic soils, we conducted systematic incubation experiments over 42 days at different temperatures (4, 22 and 37 °C) and moisture contents (30, 60 and 90% of water holding capacity (available water)). The basal respiration was monitored to study the relative effect of moisture contents and temperature on the carbon protection capacities and mechanisms of the three clay types. The results indicated that carbon decomposition increased with increasing moisture and temperature. A two-component quadratic equation could explain the carbon mineralisation process. The highest C respiration was observed at 37 °C with a 60% moisture level in each of the soil types. Under these conditions, the smectitic soil recorded the highest carbon decomposition followed by the kaolinitic-illitic and allophanic soils. The study of the priming effect using 14C labelled malic acid confirmed the trend of the bulk respiration results. The allophanic soil showed the lowest amount of carbon mineralisation under all experimental conditions. A strong inverse correlation (R2 = 0.90 at p < 0.05) was observed between CO2 emission rate and total sesquioxides (Fe and Al oxides) content. As evidenced by the pore size distribution, micromorphologies and thermogravimetric analyses, the microporous structure and microaggregate formation in the allophanic soil enhanced carbon sequestration. This study indicated that soil carbon stabilisation was related more to the sesquioxides content than to the clay types or their relative specific surface areas.

Role of soil texture, clay mineralogy, location, and temperature in coarse wood decomposition—a mesocosm experiment

AbstractOf all the major pools of terrestrial carbon (C), the dynamics of coarse woody debris (CWD) are the least understood. In contrast to soils and living vegetation, the study of CWD has rarely relied on ex situ methods for elaborating controls on decomposition rates. In this study, we report on a mesocosm incubation experiment examining how clay amount (8%, 16%, and 24% clay), clay type (soil reconstructed with kaolinite vs. montmorillonite), wood placement (on litter layer surface, at the litter layer–soil interface, buried in the mineral soil), and laboratory incubation temperature (10°, 20°, or 30°C) control decomposition rates of highly standardized stakes and blocks of coarse aspen wood. Clay type effect was pronounced, with wood decomposing more quickly in kaolinite‐ than in montmorillonite‐amended soils, perhaps due to a combined effect of moisture and microbial access to the substrate. Clay amount had only very limited effect on wood decomposition, which was a function of contact with the mineral soil (Surface &lt; Interface &lt; Mineral), perhaps due to greater contact with the decomposer community. Temperature effects were significant and dependent on interactions with clay type and wood placement. Effects of temperature on wood decomposition declined as the effects of soil variables increased, suggesting a hierarchy of controls on wood decomposition rates. Both water content and temperature had a strong effect on wood decomposition. Our results highlight that multiple interacting factors likely regulate wood decomposition processes. Multifactorial field experiments are needed to examine the physical, chemical, and biological factors controlling wood decomposition.

Physico‐geochemical and mineral composition of neem tree soils and relation to organic properties

The Neem tree, the oil of which has a long history of pesticide, fertilizer and medicinal use in India, has been studied extensively for its organic compounds. Here we present a physical, mineralogical and geochemical database resulting from the analyses of two Neem soil profiles (epipedons) in India. Neem tree derivatives are used in the manufacture of a variety of products, from anti‐bacterial drugs and insecticides to fertilizers and animal feeds. A preliminary geochemical and mineralogical analysis of Neem soils is made to explore the potential for chemical links between Neem tree derivatives and soils. Physical soil characteristics, including colour, texture and clay mineralogy, suggest the two pedons formed under different hydrological regimes, and hence, are products of different leaching environments, one well‐drained site, the other poorly drained. Geochemically, the two Neem soils exhibit similarities, with elevated concentrations of Th and rare earth elements. These elements are of interest because of their association with phosphates, especially monazite and apatite, and the potential link to fertilizer derivatives. Higher concentrations of trace elements in the soils may be linked to nutritional derivatives and to cell growth in the Neem tree.

Potassium forms in relation to clay mineralogy and other soil properties in different agro-ecological sub-regions of northern India

The distribution of potassium (K) forms, their inter-relationship and relation with soil properties and clay mineralogy was studied in soils under different agro-ecological sub-regions of Punjab. Surface (0–30 cm) soil samples were analyzed for physico-chemical properties, soil mineralogy and K forms. The XRD analyses showed that quartz was the dominant mineral in coarse fractions followed by feldspar and mica, and illite in clay fraction. Water soluble, exchangeable and non-exchangeable K contents were higher in soils of south-western alluvial plain region compared to other regions. Readily available (water soluble + exchangeable) and non-exchangeable K constituted 0.02–0.07 and 0.1–0.9% of total K in soils, respectively. Mineral K constituted about 92–97% of total K with content ranged between 27.6–54.5 cmol kg−1. High values of total K were mainly due to the presence of high amounts of K-bearing minerals. Positive correlations were observed between K forms indicating the existence of dynamic equilibrium between these forms.

Soil transference patterns on bras: Image processing and laboratory dragging experiments

In a recent Australian homicide, trace soil on the victim's clothing suggested she was initially attacked in her front yard and not the park where her body was buried. However the important issue that emerged during the trial was how soil was transferred to her clothing. This became the catalyst for designing a range of soil transference experiments (STEs) to study, recognise and classify soil patterns transferred onto fabric when a body is dragged across a soil surface.Soil deposits of interest in this murder were on the victim's bra and this paper reports the results of anthropogenic soil transfer to bra-cups and straps caused by dragging. Transfer patterns were recorded by digital photography and photomicroscopy.Eight soil transfer patterns on fabric, specific to dragging as the transfer method, appeared consistently throughout the STEs. The distinctive soil patterns were largely dependent on a wide range of soil features that were measured and identified for each soil tested using X-ray Diffraction and Non-Dispersive Infra-Red analysis.Digital photographs of soil transfer patterns on fabric were analysed using image processing software to provide a soil object-oriented classification of all soil objects with a diameter of 2 pixels and above transferred. Although soil transfer patterns were easily identifiable by naked-eye alone, image processing software provided objective numerical data to support this traditional (but subjective) interpretation.Image software soil colour analysis assigned a range of Munsell colours to identify and compare trace soil on fabric to other trace soil evidence from the same location; without requiring a spectrophotometer. Trace soil from the same location was identified by linking soils with similar dominant and sub-dominant Munsell colour peaks.Image processing numerical data on the quantity of soil transferred to fabric, enabled a relationship to be discovered between soil type, clay mineralogy (smectite), particle size and soil moisture content that would not have been possible otherwise. Soil type (e.g. Anthropogenic, gravelly sandy loam soil or Natural, organic-rich soil), clay mineralogy (smectite) and soil moisture content were the greatest influencing factors in all the dragging soil transference tests (both naked eye and measured properties) to explain the eight categories of soil transference patterns recorded.This study was intended to develop a method for dragging soil transference laboratory experiments and create a baseline of preliminary soil type/property knowledge. Results confirm the need to better understand soil behaviour and properties of clothing fabrics by further testing of a wider range of soil types and clay mineral properties.

Correlation of soil organic carbon and nutrients (NPK) to soil mineralogy, texture, aggregation, and land use pattern.

This work investigates the correlations existing among soil organic carbon (C), nitrogen (N), phosphorous (P), potassium (K), and physicochemical properties like clay mineralogy, textural components, soil aggregation, and land use pattern. Seven different locations were chosen in the tropical rainforest climate region of Assam, India, for the work. The soil texture classifications were clay, sandy clay loam, and sandy loam with mixed clay mineralogy consisting of tectosilicates and phylosilicates. Two distinct compositions of total Fe/Al oxides≥11.5 and <10.8% were observed along with two distinct groups of water stable soil aggregates of mean weight diameter≈6.42 and ≤3.26 mm. The soil clay and sand had positive and negative contributions respectively to the soil organic carbon (SOC) protection, which was observed to be dependent on lesser sand content, higher silt+clay content, and the presence of higher percentages of total Fe/Al oxides. Soil clay mineralogy suggested that the mineral, chlorite, favored retention of higher SOC content in a particular site. Under similar climatic and mineralogical conditions, both natural and anthropogenic soil disturbances destabilized SOC protection through SOM mineralization and soil aggregate destabilization as indicated by SOC protective capacity studies. Urbanization resulting in soil compaction contributed to enhanced SOC level through increased contact between the occluded organic carbon and the soil mineralogical constituents.

Influence of Olea europea L. and Ficus Carrica L. fine root activity on the K biodisponibility and clay mineralogy of the rhizosphere

The objective of this study was to compare the effect of fine root activity of Olea europea L. and Ficus carrica L. of soil in its immediate vicinity (in the so-called rhizosphere zone). The study was conducted on two stations in Northern Algeria: Guendoul and Bouira. Olea europea L. and Ficus carrica L. roots significantly modified some chemical properties of rhizosphere soil. Increases of soil carbon, KNH 4 + and KHNO 3 - were observed in the Olea europea L. and Ficus carrica L rhizosphere soil at both stations. Bulk and rhizosphere soil clay mineralogy was similar. Interstratified illite-smectite, smectite-illite and illite were predominant in the clay fraction. Chlorite and kaolinite were less represented. The decomposition of XRD diffractograms of two soil clay fractions using the Decomp program revealed that Olea europea L. roots promote nK + storage in interlayer position. Indeed, the lower abscissa position of the gravity center (cg) of the X-ray patterns, the peak displacement of clays populations PCI, I/S, S/I toward illite peak position indicates an increase of “illite-like” layer content in the vicinity of Olea europea L. roots. Olea europea L. roots appeared to have more influence on the rhizosphere soil than Ficus carrica L. roots probably because of its higher root biomass and the greater activity of the tree in winter (contrary to Ficus Carrica L., Olea europea L. keep their leaves in winter). The two species underground activity seems to be well reflected in their respective rhizosphere.

Analyzing the effect of various soil properties on the estimation of soil specific surface area by different methods

Depending on the method used, measuring the specific surface area (SSA) can be expensive and time consuming and limited numbers of studies have been conducted to predict SSA from soil properties. In this study, 127 soil sample data were gathered from the available literature. The data set included SSA values and some of the soil physical and chemical index properties. At the first step, linear regression, non-linear regression, regression trees, artificial neural networks, and a multi-objective group method of data handling were used to develop seven pedotransfer functions (PTFs) for the purpose of finding the best method in predicting SSA. Results showed that the artificial neural networks performed better than the other methods used in the development and validation of PTFs. At the second step, to find the best set of SSA for predicting input variables and to investigate the importance of the input parameters, the artificial neural networks were further used and 25 models were developed. The results showed that the PTF, containing the input variables of sand%, clay%, plastic limit, liquid limit, and free swelling index performed better than the other PTFs. This can be attributed to the close relation between the free swelling index and Atterberg limits with the soil clay mineralogy, which is one of the most important factors controlling SSA. The sensitivity analysis showed that the greatest sensitivity coefficients were found for the cation exchange capacity, clay content, liquid limit, and plasticity index in different models. Overall, the artificial neural networks method was proper to predict SSA from soil variables.

Characteristics and management options of crusting soils in a smallholder farming area of the Zambezi metamorphic belt in northern Zimbabwe

Crusting and surface-sealing soils present productivity challenges to smallholder farmers despite their high inherent fertility. Crop production on crusting soils is costly due to unfavourable conditions such as surface capping and compaction, which hinder crop emergence. A study was conducted at Bruton farm in Zimbabwe to characterise crusting soils and assess options for their management. Soil crusting was characterised using crust thickness, crusting susceptibility index, bulk density, infiltration rates, soil aggregate stability and clay mineralogy. Structured and semi-structured interviews were also conducted to capture methods used by farmers to control the crusting. The relationship between crust thickness and soil physical and chemical properties and management practices were assessed using stepwise regression analysis. Soil crusting was largely related to soil aggregation, infiltration, fine sand fraction, cotton monocropping and crop residue incorporation. Crop residue incorporation and crop rotation could be adopted to mitigate soil crusting at the scheme.

Dispersion and flocculation of Vertisols, Alfisols and Oxisols in Southern Brazil

Incomplete soil dispersion in particle-size analysis reduces clay content and increases silt and sand contents, whereas abrasion affects size distribution by large particles' breakdown into smaller size-fractions. We evaluated different dispersion techniques on particle-size distribution in ten soil horizons of seven soils from southern Brazil, with varying soil texture, clay mineralogy and organic matter. We tested two contact times of chemical dispersant (sodium hydroxide 1M): immediate shaking and shaking after 12h of contact; three types of spheres: glass, technyl and nylon; five horizontal reciprocating shaking times (30, 60, 120, 240 and 480min); and standard method (electric mixer) with chemical dispersant (sodium hydroxide 1M), 12h of contact time, plus 15min of physical stirring at 12,000rpm. Soils with high contents of clay and iron oxides or exchangeable aluminum were less dispersible, respectively, because of organo-mineral complexes and aluminum affinity to organic matter and clay minerals. Soils with kaolinite and quartz and with low organic matter content in the B horizon were easily dispersible. Nylon spheres are adequate for physical dispersion, either immediately or 12h after addition of chemical dispersant. Horizontal shaking for 60 or 120min is sufficient for adequate dispersion of kaolinitic soils, while for soils of difficult dispersion, such as oxidic soils, 240min of horizontal shaking is necessary. For water dispersible clay, horizontal shaking during 240min is suggested. These results allow for standardizing the contact time of chemical dispersant, horizontal shaking timing, and sphere types, for soils with varying texture, clay mineralogy and organic matter, in laboratories working in tropical environments, with improved dispersion and time saving.

Clay Mineralogy of Soils Developed from Granite-Gneiss of Kurnool District in Andhra Pradesh

The soils of Banaganapalle mandal in Kurnool district of Andhra Pradesh were developed from dolomite and granite-gneiss under semi-arid climate at an average elevation of 243 m above mean sea level (msl). These soils were investigated for clay mineralogy using X-ray diffraction technique and chemical analysis of clay. The clay films invariably exhibited characteristic peaks of smectite, kaolinite and illite. Semiquantification analysis of clay fractions based on relative areas under corresponding peaks indicated that pedons 1, 2, 3 and 5 were dominated by smectite followed by kaolinite, illite, feldspars and quartz while pedons 4 and 7 were dominated by kaolinite followed by smectite, illite, feldspars and quartz. The formation of smectite suggests that these soils were formed through a progressive landscape reduction process and appears to be under basic environment, impeded drainage and water logging conditions. The dominance of kaolinite may be due to their well-drained conditions. However, use and management of these soils for agricultural production require an understanding of their clay mineralogy.

Sorption and distribution of Zn in a sludge-amended soil: influence of the soil clay mineralogy

PurposeConventional pedological survey generally assesses soil metal pollution by comparing total metal concentrations in soil to geochemical baselines in parent rock. This global approach overlooks the sorption capacities of the clay minerals which form at micrometric scale in weathering microsystems and are essential for the behaviour of metals in soil. Therefore, our aim was to investigate the impact of these weathering microsystems and their clay mineralogy upon Zn sorption and distribution throughout a sludge-contaminated soil.Materials and methodsTwo soil profiles with the same diorite parent rock were sampled: (i) an amended soil profile (AS) that received for 10 years wet sewage sludges heavily loaded with Zn and (ii) a control soil profile (CS) free of sludge spreading. Soil samples were carefully collected as undisturbed blocks using plastic core samplers to prevent sample metal contamination. Each sample was further divided into two subsamples: the first was devoted to bulk chemical analyses, mineral grains separation and X-ray diffraction (XRD) analyses, whereas the second part was used for thin-section preparation and electron probe microanalyses (EPMA).Results and discussionZn in the control soil is inherited from the weathering parent rock, whereas it is supplied at the surface of the amended soil by sludges spreading. Each rock-forming mineral weathers into specific clay minerals: amphibole into saponite and montmorillonite and plagioclase into montmorillonite and kaolinite. Each clay mineral, with its specific sorption capacity, controls the Zn distribution within the soil: the smectites produced by the amphiboles have high sorption capacity and favour Zn retention in the upper horizons of the soil; the kaolinites produced by the plagioclases have lower sorption capacity, do not retain Zn in the surface horizons and allow it to migrate to deeper horizons where it is sorbed onto the montmorillonites.ConclusionsThe clay minerals appear to be important soil components controlling the mobility of Zn in the contaminated soils. The micrometric mineralogical approach proves to be relevant to describe the importance of the clay mineral species in the Zn fixation at the solid/solution interface. When applied to a wider range of heavy metals and clay minerals, it could be a useful improvement in the surface complexation modelling used to explain metal cation sorption in soils.

How Tree Encroachment and Soil Properties Affect Soil Aggregate Stability in an Eroded Grassland in South Africa

Gully erosion and woody plant encroachment are frequently observed in grasslands worldwide. Soil aggregate stability is one of the drivers of gully erosion and needs to be studied, especially in the context of both tree encroachment and gully erosion. This study compared the effects of tree encroachment [paperbark thorn; Vachellia sieberiana (DC.) Kyal. &amp; Boatwr. previously called Acacia sieberiana DC.] and soil surface horizon properties on the aggregate stability (as measured by mean weight diameter, MWD) in a degraded semihumid grassland in the KwaZulu‐Natal province of South Africa. Soil properties from under trees (UT) and open areas (OA) of both an Entisol within deep gullies (locally called dongas) and Alfisol from grasslands outside the gullies (grassland). The MWD values were higher in the grassland (3.06 ± 0.21 mm) than in the dongas (1.84 ± 1.03 mm). In the grassland, the MWD was significantly higher in the OA than UT, associated with more fine roots in the OA. In the donga, soil properties UT were not significantly different than those in OA. The soil aggregate stability showed strong local variability that depends on the age of the donga and the nature of the material that composes the pediments. Illite was associated with higher MWD values. The presence of interstratified illite–vermiculite located in active donga increases the shrink–swell potential of the aggregates, exacerbating their dispersion reflected in the low MWD values measured, especially when C and root contents were also low. These results suggest that the local soil clay mineralogy controls the orientation and extension of gullies rather than soil chemistry, topography, or the size of the drainage area.

Transformation of Kaolinite into Smectite and Iron-Illite in Brazilian Mangrove Soils

This study investigated soil clay mineralogy of mangrove forests along the Brazilian coast in eight regions of different environmental characteristics, with a focus on the crystallochemical features and genesis of 2:1 phyllosilicates. Samples from two different depths (0–30 and 60–90 cm) and two clay size fractions (<2 and <0.2 μm) were studied. The analytical tools used were X-ray diffraction, Fourier-transform infrared spectroscopy, and transmission electron microscopy–energy dispersive spectroscopy (TEM-EDS). A complex assemblage was found in both fractions, including high proportions of kaolinite, illite (Fe-illite/glauconite), and smectite (Fe-beidellite, Fe-montmorillonite), minor amounts of gibbsite and quartz, and traces of K-feldspar, halloysite, and amphibole crystals (TEM-EDS data). Evidence of illite–smectite and possibly kaolinite–smectite mixed-layer phases was found in the <0.2-μm fraction. An authigenic transformation process likely taking place, from kaolinite to Fe-illite/glauconite, through transitory kaolinite–smectite and illite–smectite phases. The reaction is triggered by high Fe activity in solution generated by dissolution of Fe oxides and pyrite present in the sediment, at approximately neutral pH and high salt content in the water. Smectite illitization takes place by substitution of Al3+ for Si4+ in the tetrahedral sheet, of Fe and Mg2+ for Al3+ in the octahedral sheet, and progressive Fe reduction, all of which produces an increase in the layer charge and K uptake. The sequential nature of the transformation (kaolinite–smectite–illite) suggests a solid-state transformation process. Two unusual kaolinite-rich mangroves were found, indicating low reactivity or high deposition rate of continental sediments of soils derived from the Barreiras Group, dominated by kaolinite.

The influence of the mound-building termite (Macrotermes bellicosus) on soil clay mineralogy

The influence of the mound-building termite (Macrotermes bellicosus) on soil clay mineralogy

Composition, properties and formation of Pseudogley on loess along a precipitation gradient in the Pannonian region of Croatia

Three Pseudogley profiles – Stagnic Cutanic Albeluvisol (Dystric, Siltic), Stagnic Cutanic Albeluvisol (Eutric, Siltic) and Luvic Stagnosol (Hypereutric, Siltic) – were studied along the 700–1000mm mean annual precipitation (MAP) gradient in the Pannonian region of Croatia. The aim was to compare their composition and properties and to determine the major processes of their formation, as well as to show whether investigations of Pseudogley climosequences in this part of Pannonian Basin are feasible. All three soil profiles were formed in situ from initially vertically homogeneous, previously cycled and post-depositionally modified loess deposits of same provenance (given their morphology, texture, geochemical ratios and mineralogy). Substantial morphological and physical (A horizons colors; Eg horizons thicknesses, colors, bulk densities and porosities; Btg horizons redoximorphic features) as well as chemical soil properties (pH, base saturation, topsoil organic C content) corresponded to the precipitation gradient. On the other hand, due to slight differences between their parent materials, clay content and CEC values of the investigated profiles did not increase with the increase of MAP. Nevertheless, processes of acidification and pseudogleying were found to be least pronounced at the location with the lowest MAP, and most pronounced at the location with the highest MAP. Intensity of lessivage, as one of major pedogenetic processes in all three investigated Pseudogleys, did not correspond to the precipitation gradient, since it was more a function of soil pH and clay mineralogy, than MAP. The three pedogenetic processes were recognized on the basis of obtained pH, base saturation, CIA (Chemical Index of Alteration) and Ti/Al values, as well as soil texture, morphology (presence and characteristics of clay coatings and redoximorphic features) and phyllosilicate dynamics (comminution of chlorite into the clay fraction, vermiculitization, smectite translocation) observed along the investigated profiles. Given the uneven environmental conditions of loess depositions across the Pannonian region of Croatia and the resulting incomplete uniformity of loess parent materials, investigations of Pseudogley climosequences in this part of the Pannonian Basin should be approached with caution.

Closure to “Quantitative Estimation of Clay Mineralogy in Fine-Grained Soils” by Bhaskar Chittoori and Anand J. Puppala

The authors’ contribution is a welcome addition to the literature. It brings out the importance of clay mineralogy in solving engineering problems connected with soil improvement techniques. The engineering behavior of fine-grained soils is controlled by the clay mineralogical composition of the soil. As discussed by the authors, soils of similar plasticity characteristics can contain different dominant clay minerals. Hence, it is correctly indicated by the authors that identical stabilization design or treatment cannot be given to soils with identical plasticity characteristics without the proper knowledge of the dominant clay minerals composing the soils. In fact, this is the major limitation of the plasticity chart used to classify finegrained soils. Hence, it is important to recognize the dominant clay mineral composition of soils in any project dealing with finegrained soils. The authors have proposed a methodology of quantifying the clay mineral composition of soils, which considers montmorillonite, illite, and kaolinite to be the major and stable clay mineral types, for which they deserve appreciation. The study of the paper under discussion reveals the following points: • The quantification of clay minerals in the soils with the available procedures such as X-ray diffraction, scanning electron microscopy, or infrared spectroscopy is not that simple to adopt. Sophisticated instrumentation and interpretation techniques are required. • The quantification of the dominant clay minerals in the soil by the method proposed by the authors requires information such as cation exchange capacity (CEC), specific surface area (SSA), and total potassium content (TP). The determination of these quantities, particularly SSA, is also not that simple. Hence, the proposed method is not user friendly. • There is no definite relationship between quantities such as CEC or SSA and the quantities of montmorillonite illite and kaolinite minerals predicted. In this context, the discussers feel that the exact percentage quantification of the clay minerals present in the soil is not required to judge the clay soil behavior. They opine that it is sufficient enough to classify the soils in to different groups based on their degree of expansivity, which is an indirect representation of the dominant clay mineralogy of soils. Sridharan and Prakash (2000) conducted a detailed analysis of the swelling capabilities of a number of soils of varying plasticity characteristics and varying clay mineralogical composition obtained from oedometer swell tests on soils, which give the most useful and reliable assessment of swell capacity of soils (Chen 1975; Winterkorn and Fang 1986) from air dry to saturated conditions under a surcharge of 7 kPa, in the compacted condition, and from other soil parameters such as liquid limit, plasticity index, and activity. Their analysis indicated that liquid limit, plasticity index, and activity cannot satisfactorily predict soil expansivity, because they do not consider the effect of soil clay mineralogy. On the basis of that analysis, they proposed a very simple and user-friendly methodology for the prediction of degree of soil expansivity and dominant clay mineral type present in the soil based on the free swell ratio (FSR), which compares exceedingly well with those from the oedometer swell tests. The FSR is defined as the ratio of the equilibrium sediment volume of a 10-g oven-dried soil sample passing through a 425mm sieve placed in a 100-mL graduated measuring jar containing distilled water (Vd) to that in carbon tetra chloride/kerosene (Vk), thoroughly mixed, and left for the sediment formation, after an equilibration period of a minimum of 24 h

Monitoring of biophysicochemical changes in a silty clay soil contaminated with LNAPLs

The interdependence between time domain induced polarization measurements at uncontaminated and artificially contaminated soil samples with diesel and biophysicochemical alterations caused by diesel degradation was investigated during 12months. The research was performed on a slightly alkaline soil, with high content of organic matter (148g/kg) and silty clay texture. Soil clay mineralogy was mainly composed of plagioclase, amphibole, biotite, interstratified mica-smectite, goethite and some vestigial kaolinite. A decrease on resistivity and induced polarization (IP) in contaminated soil was observed during the 12months. This reduction on geoelectrical parameters was related to: 1) the increase on the electrolyte conductivity; 2) the increase on interfacial surface area, as result of biotite transformation and weathering of other ferromagnesian minerals and 3) a slight increase in soil aggregation, caused by diesel-degrading microorganisms (a significant increase of the numbers of specific carbon degraders was observed). After 4months, the IP response was lowest in the contaminated soil which was explained by inhibition of the cation exchange capacity due to two possible processes: 1) clay particles coating by organic molecules and 2) attachment of microbial cells (biofilms) to clay particles and/or soil aggregates. The results suggest that the content and mineralogy of the clay fraction as well as the aggregation state of the soils contaminated with LNAPL's affect the IP response. This response is a diagnostic of the biophysicochemical alterations occurring during diesel degradation as a result of biological activity.

Influence of soil texture and mineralogy on organic matter content and composition in physically separated fractions soils of Thailand

There is a limited knowledge on the role of soil texture and clay mineralogy in governing the content and composition of organic matter (OM) associated with physically-separated fractions in major Thai soils. To extend the understanding in this context, we sampled a range of Thai soils representing four major mineralogy groups, i.e. siliceous, mixed mineralogy, kaolinitc, and smectitic soils. The clay fraction and four density fractions [two particulate OM, free-light (fPOM) and occluded (oPOM), <1.85gcm−3; and two heavy fractions, 1.85–2.60gcm−3 (HFOM1.85–2.60) and >2.60gcm−3 (HFOM>2.60)] of soils from the top- and sub-soil horizons with varying clay content (58–967gkg−1) and mineralogy were analyzed for organic carbon (OC) and total nitrogen (N) contents. Additionally, chemical composition of the OM in the clay fraction of soils from 0–20cm, 20–40cm and 40–70cm and the four density fractions from soils at 20–40cm depth was characterized using diffuse reflectance infrared Fourier transformed (DRIFT) spectroscopy. The OM associated with oPOM (OC only) and HFOM1.85–2.60 (OC and total N) fractions was significantly increased with increasing clay content among soil groups and the degree of association depended on the dominating clay mineral (albeit significantly at P<0.05 for some soil groups only). The relative proportion (%rA) of aliphatic-C in the clay and HFOM1.85–2.60 fractions significantly increased with increasing sand content, and significantly decreased with increasing clay content. Whereas, opposite relationships were observed for the aromatic-C and amide N (amide-II) with sand and clay contents, respectively. The organic-mineral fractions (clay or HFOM1.85–2.60) of the coarse-textured siliceous soils were enriched with aliphatic-C and depleted in amide N, whereas the organo-mineral fractions of the fine-textured soils were enriched with aromatic-C and amide N. Aliphatic-C represented the dominating organic structure in the siliceous soils, whereas aromatic-C and amide N represented typical forms of OM in the fine-textured soil groups. The present study provided insights into the role of soil texture and clay mineralogy in governing the content and composition of OM pools in Thai soils with relevance for sustaining soil fertility and mitigating C emissions.

Discussion of “Mechanics of the Casagrande liquid limit test” 1 Appears in the Canadian Geotechnical Journal, 49(9): 1015–1023 [doi:10.1139/t2012-066

The discusser wishes to express his appreciation to the author for shining a more useful light on the theoretical issues related to the conventional Casagrande percussion method, which is used for determining the liquid limit of fine-grained soils worldwide with the exception of a few countries where the fall cone method is preferred. Particularly, the author deserves a compliment for demonstrating theoretically that the results obtained from the Casagrande percussion and fall cone methods represent two different mechanical properties of the soil. This finding is in concurrence with the outcome of research by the discusser and his colleague presented in many of their publications (e.g., Sridharan and Prakash 1998, 1999, 2000; Prakash and Sridharan 2006). However, the basis and approach adopted by the two groups (i.e., the author on one side and the discusser with his research colleague on the other) to achieve the same goal are different. The author has shown theoretically through the Newmarkian analysis of the Casagrande percussion test that the Casagrande percussion liquid limit is a measure of soil specific strength, while the fall cone liquid limit is a direct measure of the undrained strength of the soil. The clay minerals composing the fine-grained soils are no longer inert, but are physicochemically active by virtue of their surface charge deficiency. The extent to which the fine-grained soils are active depends upon their clay mineralogical composition, which controls their behavior. It is unfortunate that the clay mineralogy of soils is ignored in most of the present-day research dealing with fine-grained soils even though most of the field problems associated with these soils can be attributed to their clay mineralogical composition. The liquid limit behavior has been proven to be a dominant function of the soil clay mineralogy. While the liquid limit of montmorillonitic soils is due primarily to the diffuse doublelayer-held water (Sridharan et al. 1986), that of kaolinitic soils is mainly due to the soil fabric as governed by the interparticle attractive forces (Sridharan et al. 1988). The undrained shear strength of fine-grained soils has been shown to have two components: viscous shear resistance and frictional shear resistance (Sridharan and Prakash 1999, 2000). An exhaustive experimental study of the mechanisms that come into play during the Casagrande percussion and fall cone methods of determining the liquid limit of soils has shown that the value of the liquid limit obtained by the Casagrande percussion method is controlled primarily by the viscous shear resistance while that obtained by the fall cone method is controlled essentially by undrained frictional shear resistance (Sridharan and Prakash 2000; Prakash and Sridharan 2006). The above illustrations reveal very clearly that the liquid limit values obtained by the Casagrande percussion and fall cone methods cannot be the same and hence, the liquid limits obtained by the two methods represent two different soil properties controlled by two different mechanisms. This conclusion is very much in line with that of the author, proven by a different approach altogether. The discusser also agrees with the author in that there is a growing tendency towards using the fall cone method to determine the liquid limit without understanding its limitations. In that direction, the author’s effort in proposing a methodology using the fall cone method itself to arrive at the liquid limit equivalent to that by the Casagrande percussion method is welcome. However, while doing so, the author is trying to equate the undrained shear strengths of the soils at the Casagrande percussion and fall cone methods’ liquid limits, which is not fully justifiable. This is because the undrained shear strengths mobilized during testing by the two methods are not of the same nature, as indicated by the discusser in the previous paragraphs. The discusser is aware that the author’s approach is theoretical, which does not take into account the physicochemical nature of liquid limit of fine-grained soils and the controlling mechanisms that are operative during the two testing procedures. Bringing these complex parameters into the theoretical formulation is not that simple. Apart from this issue, the discusser wishes to bring the following major discrepancies in the paper under discussion to the attention of the author for his comments: • Equation [6] appears to be incorrect. For a saturated soil mass, the correct relation should have been the following equation (where w is the mass density of water):