Smectitic Soils Research Articles

Tensile strength of dry, remoulded soils as affected by properties of the clay fraction

Changes in tensile strength with clay type, clay particle size, and amounts of spontaneously and mechanically dispersible clays were assessed for dry, remoulded samples of eight Australian Vertisols and Alfisols varying in clay mineralogy. The effects of different clay content and particle size on tensile strength were also measured. The average particle sizes of whole clay, and of spontaneously and mechanically dispersible clays were also determined. Tensile strength of soils was influenced by the type and amount of clay present, clay particle size, and amount of dispersible clay. Soil strength increased with increasing clay content. When the clay content increased beyond 20%, there was a dramatic increase in soil strength indicating the possible role of particle arrangement. Correlations between different clay types, clay particle sizes, cation exchange capacity and tensile strength clearly illustrated that the sensitivity of tensile strength to these factors was greatest in soils dominated by smectite, followed by illitic and then kaolinitic soils. Tensile strength of soils was positively and significantly correlated with both spontaneously and mechanically dispersible clays. A significant negative correlation was obtained between clay particle size and tensile strength for illitic soils, whereas the same linear relationship was not significant for smectitic soils. However, a significant negative exponential regression was obtained between the cube of clay particle size and tensile strength when the results of all soil samples were pooled together. Transmission electron micrographs (TEM) showed a wider clay particle size distribution in smectites than in illites. Soils with the highest amounts of fine clay and the widest clay particle size distribution had higher tensile strengths. The size of spontaneously dispersed clay particles was smaller than that of mechanically dispersed clay. No difference in clay particle size was found between spontaneously and mechanically dispersible clays from mixtures of a fine clay with a basic sand and silt matrix. Regression, collinearity diagnostics and principal component analyses were used to analyse the data. A high correlation was found between the cation exchange capacity (CEC) of clays and soil strength. The principal component analysis indicated that spontaneously dispersible clay, percentage of randomly interstratified minerals (RIM) and CEC were important factors in predicting the strength of remoulded soils.

Ca-K-Na exchange equilibria on a smectitic soil: Modeling the variation of selectivity coefficient

Abstract We conducted binary and ternary cation exchange experiments on a smectitic soil and calculated the Vanselow selectivity coefficients. The values of the natural logarithm of the Vanselow selectivity coefficient for Ca→Mg exchange, In cKMg/Ca, slightly scattered around an average of −0.553. The values of In cKK/Ca and In cKNa/Ca in respective binary systems, on the other hand, significantly varied as the mole fraction of K in the exchanger phase increased. In the Ca-K-Na ternary system, cKK/Ca varied in quite a similar manner as in the binary system whereas In cKK/Ca behaved differently. Based on these observations, we modeled the variation of the selectivity coefficients using the data from ternary exchange experiments. The obtained equations are: In cKK/Ca =−4.271 .NK,+4.220, In cKNa/K =2.417 .NK,−2.742, and In cKK/Ca −In cKNa/Ca + 2ln cKNa/K = 0, where NK is the mole fraction of K in the exchanger phase.

Atrazine Desorption from Smectites

AbstractIn soils low in organic matter, pesticide adsorption and desorption by clay minerals may strongly influence the fate of pesticides in soil environments. Atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1,3,5‐triazine) adsorption‐desorption was determined on 11 reference smectites (<2‐µm size fraction), and a soil smectite (<0.02‐µm size fraction from the Ap horizon of a fine‐loamy, mixed, mesic Typic Haplaquoll). For each clay sample, adsorption and desorption isotherms were determined using batch equilibration. Atrazine adsorption on the clays decreased with increasing surface change density (SCD) of the smectites. The desorption isotherms indicated that adsorption was generally reversible. This suggests that atrazine is primarily adsorbed on smectite surfaces through relatively weak van der Waals or H bonds. However, a small positive hysteresis was observed with some clays, and the magnitude of the hysteresis, evaluated using the ratio of the Freundlich isotherm coefficients for adsorption and desorption, increased with SCD. This suggests that atrazine is retained by stronger binding mechanisms on smectites with high SCD, in spite of its lower adsorption capacity. On clays with high atrazine adsorption coefficients, the amount of atrazine desorbed was larger than would be predicted from the adsorption isotherms, resulting in a “negative hysteresis” whereby the desorption isotherm slope was greater than the adsorption isotherm slope. “Negative hysteresis” can be explained if atrazine is assumed to be excluded from interlayer or intraquasi‐crystal water, where only external or interquasi‐crystal water would be available during the desorption process.

Cadmium Sorption on Specimen and Soil Smectites in Sodium and Calcium Electrolytes

AbstractAlthough specimen smectities (e.g., SWy‐1) are often used as analogues of the exchanger phase in smectitic soils, few comparisons of metal ion sorption on specimen and soil smectites have been made. In this study, the sorption of Cd was measured on SWy‐1 and on clay‐sized separates from two smectitic subsoils to evaluate the selectivity of specimen and soil‐derived smectites for Cd. Sorption was measured in clay suspensions (≈1 mmolc L−1 equivalent charge concentration at pH 6.0) in Na+, Ca2+, and Na+‐Ca2+ perchlorate solutions across pH 4.5 to 8.5 and at ionic strengths (I) ranging from 0.005 to 0.1. Ionic strength and electrolyte cation valence strongly influenced Cd sorption by SWy‐1 and the soil smectites. Ion exchange dominated Cd sorption at low ionic strength in Na+ electrolyte (I = 0.005–0.014). Increasing Na+ concentrations to I = 0.1 or changing the electrolyte cation to Ca2+ at I = 0.003 to 0.006 suppressed ion exchange. When ion exchange was suppressed, Cd sorption to both specimen and soil smectites showed little dependence on ionic strength and increased with pH. Except at the lowest Na+ concentration (I = 0.005), conditional equilibrium constants (Kv) for Cd2+ exchange increased with increases in both ionic strength and pH. These increases were ascribed to Cd complexation reactions to edge sites on the layer silicates whose effects became evident only under conditions that suppressed ion exchange. At pH 6 and I = 0.05–0.01, SWy‐1 did not exhibit any preference for Na+, Ca2+, or Cd2+. The smectitic soil separates, in contrast, showed (i) sorption behavior that increased sharply with pH, (ii) preference for Cd in Na+ and Ca2+ electrolytes, and (iii) variation in Kv with ionic strength, pH, and surface coverage. The contrasting sorption behavior of the soil smectites was hypothesized to result from (i) a greater edge surface area, which increased the contribution of oxidelike complexation reactions to Cd sorption, and (ii) the presence of minor associated organic material and Fe oxides that functioned as co‐complexants for Cd.

TEMPERATURE EFFECTS ON BORON ADSORPTION BY REFERENCE MINERALS AND SOILS

Information on the effect of temperature on B adsorption by soils and soil minerals is scant. These data are needed to understand B availability. Boron adsorption on goethite, gibbsite, kaolinite, montmorillonite, calcite, and two arid zone soils was investigated as a function of solution pH (3–12) and reaction temperature (10, 25, and 40|MoC) after 2 h of reaction time. Boron adsorption on all materials increased from pH 3 to 7, exhibited a peak at pH 7.5 to 10, and decreased from pH 10.5 to 12. Temperature dependence measured as the increase of the B adsorption maximum at 10|MoC compared with 40|MoC on reference minerals increased in the order: calcite (3|X%) < goethite (7|X%) < gibbsite (18|X%) < montmorillonite (-20|X%) < kaolinite (26|X%). The kaolinitic soil exhibited greater temperature dependence than the smectitic soil. The B adsorption reaction was exothermic since B adsorption decreased with increasing temperature for all materials, except for montmorillonite at high pH. Highly specific ion adsorption is expected to be exothermic, suggesting an inner-sphere adsorption mechanism for B on all reference minerals except montmorillonite.

Improved Calibration of Time Domain Reflectometry Soil Water Content Measurements

Time domain reflectometry (TDR) is becoming a widely used method to determine volumetric soil water content, θ, from measured effective relative dielectric constant (permittivity), ε, using the empirical θ(ε) Topp-Davis-Annan calibration equation. This equation is not adequate for all soils. The purpose of this study was to compare the Topp calibration equation with a theoretical (Maxwell-De Loor) and an empiricial (fitting exponent α) mixing model for the four components: solid phase (s), tightly bound water (bw), free water, and air. Water content permittivity were measured, gravimetrically and by TDR, on packed columns of 11 soils ranging from loess to pure bentonite. Measured specific surfaces were S = 25 to 665 m2 g−1 and bulk densities ρb = 0.55 to 1.65 g cm−3. Topp yielded accurate ε(θ) values only for the four soils with ρb > 1.30 g cm−3, including illite (S = 147 m2 g−1). Maxwell-De Loor gave similar accuracy for seven soils, including attapulgite (S = 270 m2 g−1, ρb = 0.55 g cm−3), assuming a monomolecular tightly bound water layer (thickness δ = 3 × 10−10 m; θbw = δ ρbS), εbw = 3.2, and εs = 5.0. The ε(θ) curve of these soils had the same shape as Topp. Two gibbsite soils with dissimilar curves required εbw = 3.2 and εs = 16 to 18, and two smectite soil materials required εbw = 30 to 50 and εs = 5.0, to obtain good fits. Deviations from Topp appear generally due more to the lower ρb and thus higher air volume fraction at the same θ associated with fine-textured soils than to tightly bound water with low ε. Both effects, as well as apparent anomalous behavior such as decreasing effective ε with increasing εs, can be accomodated by the Maxwell-De Loor equation. This makes it a better calibration equation than Topp. The empirical α model is sensitive to the unpredictable value of α and cannot accomodate anomalous behavior.

Potassium-calcium exchange in five Ap soils from paddy fields and its effect on potassium concentration in soil solution

Abstract We measured the selectivity coefficient (K V) for K-Ca exchange equilibrium as a function of K-saturation on two smectitic soils, two halloysitic soils and a soil dominated by noncrystalline aluminum silicates. The selectivity coefficients for all the soils decreased as K-saturation increased. Among the five soils, the halloysitic soils exhibited higher K V values than the smectitic soils, while the K V values for the soils dominated by noncrystalline aluminum silicates were comparable with those of the halloysitic soils at low K-saturation but the K V values were even lower than those for the smectitic soils at higher K-saturation. Numerical simulations showed that the K concentration in the soil solution after addition of a K-salt differed significantly among the soils, depending on the exchangeable K status and K-selectivity of the soils.

Predicting Cation Transport in Smectitic Soils

Predicting Cation Transport in Smectitic Soils

Genesis and transformation of minerals in the formation of red (Alfisols) and black (Inceptisols and Vertisols) soils on Deccan basalt in the Western Ghats, India

SUMMARYSix noncalcareous pedons from the basaltic terrain of the Western Ghats in Maharashtra, India, were identified for the present study. Of these, two red‐soil pedons (Typic Haplustalfs) and one black‐soil pedon (Vertic Ustropept) are from the Bhimashankar plateau at an elevation of 1000 m above mean sea level, experiencing a humid (&gt;5000 mm rainfall) tropical climate. The other three pedons of black soils (Typic Chromusterts and Typic Ustropept) are from the semi‐arid zone (500–1000 mm rainfall), at an elevation of 825–893 m.Clay mineralogical investigation indicated that interstratified smectite‐kaolin (Sm/K) is dominant in red soils whereas smectite is dominant in black soils. The Sm/K is formed by the transformation of montmorillonite, the first weathering product of Deccan basalts in a humid tropical climate. We suggest that the interstratification of kaolin with chloritized smectite may also be an important ephemeral stage during the transformation of smectite to kaolinite. The presence of zeolites provided sufficient bases to prevent the complete transformation of Sm/K to kaolinite. The presence of smectites and zeolites made the formation of black soils possible in microdepressions even in a tropical humid climate. The genesis of Sm/K and smectite in red and black soils, respectively, suggests that these soils formed through a progressive landscape reduction process. The presence of both Sm/K and smectite in black soil clays of semi‐arid climate suggests that the smectite of these soils was formed in an earlier humid climate.

X-ray diffraction characteristics and related properties of Smectites in some Canadian soils

The X-ray diffraction (XRD) peak intensities of smectites in Chernozemic and related soils of Western Canada are generally low, in contrast to the high peak intensities of smectites in Podzolic soils of Eastern Canada and those of standard smectite samples. Consequently, X-ray quantitative analysis based on standard smectite samples may underestimate the amount of smectite and overestimate the amount of noncrystalline material in western Canadian soils. This study was undertaken to find the reasons for the weak XRD peak intensities of western soil smectites in terms of their purity and crystallinity. The Tiron dissolution method extracted only small amounts of noncrystalline material from the western soil clays and had little effect on XRD characteristics. The cation exchange capacity (CEC) hysteresis (or pH dependent CEC) between pH 3.5 and 11.0 of the western soil clays was also relatively small which confirmed the absence of significant amounts of noncrystalline material in these soil clays. Observed deviations of XRD positions from true basal spacings indicated that the western soil smectite particles were consistently thinner than the eastern soil smectite and Wyoming montmorillonite particles. Electron microscope observations supported these results. Because the diffraction intensity is proportional to the square of the particle thickness, the thinness of the western soil smectite particles appears to be a major factor in reducing their peak intensities. Thus, to obtain comparable peak intensity data from different soil smectites, particle thickness should be taken into account. Since it is often not practical to measure particle thickness, a procedure for quantifying smectite in soils that avoids this measurement is proposed. Key words: Peak intensity, crystallinity, particle size, particle thickness, Chernozemic smectite, Podzolic smectite

Influence of clay minerals and organic matter on effects of sodicity on soils

This review shows that the permeability of soils decrease because exchangeable sodium causes aluminosilicate clay minerals to swell and disperse in water. Among common soil minerals, only smectites with a high percentage of exchangeable Na show extensive swelling. Illites often remain dispersed in solutions of high electrical conductivity, partly because the shapes of their particles prevent strong cohesion. Smectites are often highly mobile, consistent with their small particle sizes. The dispersion of kaolinites varies with solution pH since a significant proportion of their charge can be variable. The sensitivity of soils to pH may be more strongly influenced by other, variably charged components than by the dominant aluminosilicates. Of these, metal oxides, hydroxides and oxyhydroxides can restrain the dispersion of aluminosilicate clay minerals. Organic matter can aid the dispersion of sodic soils while suppressing swelling. It can also be mobilized in a soluble form and probably also in a colloidal form under sodic conditions. Exchangeable Mg enhances dispersion from sodicity, but apparently to a greater extent in illitic soils than in smectitic soils. Exchangeable Al tends to prevent dispersion but the influence of clay mineralogy on its effect is apparently untested. The effect of sodicity upon soils is amplified by fineness of texture and by mechanical disturbance. Studies of soil fabric should increase the current poor understanding of the macroscopic effects of sodicity in terms of interparticle interactions of the pure components.

CLAY DISPERSION AS RELATED TO SOIL PROPERTIES AND WATER PERMEABILITY

Seal formation is affected by clay dispersion, and both parameters depend on the same soil and soil solution properties. Since clay dispersion can be determined easily in the laboratory, its use for predicting soil susceptibility to seal formation was evaluated. Laboratory clay dispersion tests were conducted on soil samples from semi-arid (northern Cameroon and Israel) and humid (Georgia, USA) regions. Clay dispersion (presented as percent of total clay) in distilled water (to simulate rain-water) was significantly correlated with silt and clay content. Clay dispersion was severe (>40%) in soils with silt and clay content >15%, which were the kaolinitic soils from the humid region and the predominantly smectitic soils from semi-arid regions. Kaolinitic and illitic soils from the semi-arid regions, whose silt and clay content was <15%, were less dispersive. Deflocculation of kaolinitic clay by decomposed organic matter present in humid conditions was suggested as an additional cause for the dispersivity of the kaolinitic soils from Georgia. In stable soils, clay dispersion was affected by exchangeable sodium percentage (ESP) at all levels. In dispersive soils, only high ESP levels increased clay dispersion. Clay dispersion decreased in electrolyte solution (EC = 0.8 dS m−1) and was significant mainly in the kaolinitic soils from both northern Cameroon and Georgia. The degree of clay dispersion depended also on the energy applied; different soils needed different amounts of energy for reaching their utmost dispersion. The correlation obtained between clay dispersion and the final infiltration rate or percent runoff explained only a relatively small proportion of the variance measured and, hence, prevented an accurate prediction of these two variables from percent dispersed clay.

Relationship of K forms and release characteristics with clay mineralogy

The relationship between K forms (water soluble K, NH 40Ac- and HN0 3-extractable K) and characteristics of laboratory release of non-exchangeable K were examined as a function of clay mineralogy and taxonomy, using samples from some agriculturally important bench mark soil series of India. Potassium release characteristics were studied in samples of soils having NH 40Ac- and HN0 3-extractable K in the vicinity of the modal values in the various series. The release of “external” and “lattice” K was was computed from the “K release curve” and computed estimates were compared within and across the soil groups. Generally, kaolinitic soils were low, mixed and illitic soils WERE medium, and smectitic soils were high in NH 40Ac-K. In the estimates of HN0 3-K, kaolinitic, mixed and smectitic soils were low, and illitic soils were high. The amounts and rates of K release were positively and significantly correlated with the measured K reserves. The K release was greatest in illitic and least in kaolinitic soils. The results show that while kaolinitic soils are low in both NH 40Ac-K and HN0 3-K and also in the release of their non-exchangeable K, the situation is different in smectitic and illitic soils. Although smectitic soils are high in NH 40Ac-K, their HN0 3-K is modest and the rate of K release is moderate. On the other hand, illitic soils have modest amounts of NH 40Ac-K but high amounts of HN0 3-K, and are associated with high K release rates. Hence, it is advisable to consider the amount of both the exchangeable and non-exchangeable K and clay mineralogy of soils in arriving at a judgement of K release and availability in soils.

Kinetics of release of potassium by sodium tetraphenyl boron from some top soil samples of Red (Alfisols), Black (Vertisols) and Alluvial (Inceptisols and Entisols) soils of India

Alluvial (Inceptisols and Entisols) soils containing illite as dominant clay mineral released significantly higher amounts of K to 0.3 M NaTPB than smectitic Black (Vertisols) and kaolinitic Red (Alfisols) soils. Exchange rate indices derived from the plots of K released to 0.3 M NaTPB versus log t were also higher for illitic soils than for kaolinitic and smectitic soils. Magnitudes of both surface and internal K released from different soils were in the sequence: illitic > smectitic > kaolinitic. Data of K release to 0.3 M NaTPB could be described by first order rate equation. In parabolic plots, existence of a linear relationship throughout the period of exchange with NaTPB indicated the absence of film diffusion controlled processes. Diffusion coefficients calculated from radial diffusion equation were dependent upon texture and clay mineralogy of soils and decreased sharply with time for kaolinitic and smectitic soils. Illitic soils, however, maintained higher values of diffusion coefficients over extended periods of time.

Effect of aluminum treatments on permeability and exchangeable cation status of a Sodic Smectitic Soil

Abstract A smectitic soil adjusted to different levels of exchangeable sodium percentage (ESP≤40) was treated with 5 and 15 mmol AI/100 g added as Al2(SO4)3 or Al2(SO4)3 and CaCO3 mixture (1:1.5 molar ratio). The treated and untreated soils were leached with a 0.01, 0.059 or 0.1 M Cl¯ NaCl-CaCl2 solution (sodium adsorption ratio (SAR) = 20) and subsequently with water. The Al treatment prevented the increase of ESP upon the leaching with the SAR 20 solution, increased the soil permeability, and prevented clay dispersion when the soil was leached with water. The 5 mmol Al/100 g treatment with the Al2(SO4)3 and CaCO3 mixture was recommended for this soil with a cation exchange capacity of 28 meq/100 g, because the added Al was effective in increasing and maintaining the soil permeability and preventing the increase of ESP by leaching with Na+-containing water. The increase in the amount of exchangeable Al and the decrease in the amount of exchangeable Ca, Mg, and K caused by the added Al were small.

Mobilization of non-exchangeable K by ryegrass in five Moroccan soils with and without mica

Intensive cropping of Italian ryegrass (Lolium multiforum L.) in pots was used to assess the contribution of non-exchangeable K to plant uptake. The soils used were: two soils high in mica (illite) developed on recent alluvium plus two smectitic (beidellitic) soils and a soil of mixed mineralogy rich in mica. Four K treatments were used (0, 28.6, 143, and 286 mg kg-1 soil) with 8 successive monthly cuttings. A response of plant K uptake to added K was observed in all soils. Both 1.0 M NH40Ac and 0.2 M CaCl2 extractable K were depleted to a minimum level specific for each soil. The minima were lower in the old upland soils compared to the young alluvial soils. Uptake of K by Italian ryegrass induced K release from the non-exchangeable K to replenish the plant available pool of K ions. The release of mica interlayer K in the alluvial and in the high K smectitic soil supplied sufficient K to plants even under intensive cropping. The rate of mobilization of interlayer K was low in the smectitic soil with lower K. The lowest release rate was in the old high mica soil. Iron coatings may have inhibited mobilization of interlayer K. The rates of mobilization cannot be predicted from mineralogical and K-extraction data only. The rates of K uptake and the rates of K release by ryegrass under intensive cropping are potential values which can be used for modelling K availability to plants in the soils studied.

Paleopedology and Stable Isotope Chemistry of Late Silurian Vertic Paleosols, Bloomsburg Formation, Central Pennsylvania

ABSTRACT Paleosols with vertic (Vertisol-like) features occur in repetitively stacked fining-upward sequences bounded by marine erosional surfaces in the Bloomsburg Formation (Ludlovian to Pridolian) of central Pennsylvania. Paleosol development was strongly influenced by repeated changes in relative sea level, which resulted in subaerial exposure and pedogenesis of marginal-marine sediment accumulating along a low-energy, mud-dominated coastline in a delta-plain environment. Bloomsburg paleosols are characterized by very weak horizonation, abundant pedogenic slickensides, and a micromorphology dominated by sepic fabrics and peds bounded by stress cutans. Pedogenic carbonate is sparse, occurring principally as micrite nodules. Stable carbon isotope compositions of stratigraphically-low pedogen c carbonate (mean 13C = -1.92 PDB) are attributed to pedogenesis proximal to a marine coastline. In stratigraphically-high paleosols showing evidence of rhizolith development, isotopic compositions of pedogenic carbonate suggest a significant contribution of carbon from C3 flora. Atmospheric P(CO2) levels estimated for these calcretes (4500-5500 ppmV) are significantly greater than present-day values. Low 18O values for pedogenic carbonate (mean 18O = -12.12 PDB) record isotopic exchange between paleosol carbonate and 18O-enriched fluids associated with the conversion of original soil smectites to illite and chlorite during burial diagenesis. Paleosols bounded by marine erosional surfaces show variable degrees of marine hydromorphism attributable to rising water table during transgression', evidence for hydromorphism includes extensive reduction of iron as well as relative increases in CaO, MnO, MgO, P2O5, Na2O, and Sr at the tops of paleosols, due to introduction of marine fluids and sediment into open soil fractures and burrows. The abundance of vertic features in all Bloomsburg paleosols, by analogy with Holocene Vertisols, indicates the recurrence of a seasonally wet-dry (dry 4-8 months/year) paleoclimate n the central Appalachian region at a paleolatitude of about 20° south.

Adsorption of Atrazine on Smectites

AbstractSmectites may strongly influence the fate of pesticides in soils due to their large surface area and abundance in agricultural soils. This research was undertaken to determine the effects of smectite properties on the affinity of smectitic clays for atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1, 3, 5 triazine). Samples of the &lt;2‐µm fraction were separated by sedimentation from 13 reference smectites and treated with H2O2 for removal of organic matter. A soil smectite sample was separated (&lt;0.02‐µm fraction) from a H2O2 and dithionite‐citrate‐bicarbonate‐treated Webster (fine‐loamy, mixed, mesic Typic Haplaquoll) soil by a dispersion‐centrifugation‐decantation technique. All 14 samples were Ca saturated, dialyzed free of excess electrolyte, and freeze dried. The mineralogy of each sample was evaluated by a combination of x‐ray diffraction and chemical analysis. Values for cation‐exchange capacity (CEC), surface charge density (SCD), surface area (SA), and percent tetrahedral charge (TC) were based on structural interpretations of the chemical analyses. Freundlich adsorption isotherms for atrazine adsorption on each sample were determined using the batch‐equilibration method. Smectite was the dominant mineral in 13 of the clay samples; the 14th sample was a mixture of interstratified smectite‐illite, quartz, and kaolinite. For the smectitic clays, atrazine adsorption ranged from 0 to 100%, and the logarithm of the Freundlich adsorption constant [log(Kf)] decreased linearly with the CEC (r2 = 0.82) of the clays. Stepwise multiple‐regression analysis indicated that log (Kf) values were correlated with SCD and SA values (r2 = 0.83). Inclusion of TC and suspension pH as independent variables in the regression analysis did not significantly improve the correlations. The results indicate that the affinity of smectites for atrazine decreases with increasing SCD, which suggests that atrazine is primarily adsorbed by smectites as a neutral species.

Chemistry of Smectitic and Illitic Phases in Interstratified Soil Smectite

AbstractThe objective of this investigation was to determine the elemental composition and mineralogy of the smectitic and illitic phases in interstratified smectite‐illite from a typical upper Midwestern agricultural soil. A sample of soil clay (&lt;2‐µm fraction) from the Ap horizon of a Webster (fine‐loamy, mixed, mesic Typic Haplaquoll) pedon was fractionated by centrifugation to prepare various fine‐clay (&lt;0.020‐, &lt;0.026‐, &lt;0.036‐, &lt;0.045‐, &lt;0.060‐, and &lt;0.090‐µm size fractions) samples. X‐ray diffraction analyses indicated that interstratified smectite‐illite was the only mineral present in significant quantities in the fine‐clay samples. Elemental analyses of the fine‐clay samples were performed by inductively coupled plasma‐atomic emission spectroscopy using suspension nebulization. The elemental analyses revealed significant linear correlations for the K vs. other element relationships (P &lt; 0.001 for K vs. Si, Al, Fe, Ca, Ti, Mn, and Zn, and P &lt; 0.01 for K vs. Mg). A nonlinear parameter optimization technique, based on elemental mass balance, was used to estimate the elemental compositions and proportions of the smectitic (K‐free) and illitic (K‐bearing) phases in the fine‐clay samples. The results for the elemental mass balance optimization were independently verified by water mass balance. The smectitic phase was a high‐charge (0.482 per formula unit), Fe‐rich montmorillonite (46.6% tetrahedral charge). The illitic phase consisted of dioctahedral, tetrahedrally charged (86.9% tetrahedral charge) elementary illite particles. Layer charge for the illitic phase was 0.473 per formula unit, which is substantially below the lower limit for classification of illite. The interstratified smectite‐illite comprised at least 60% of the whole (&lt;2‐µm fraction) soil clay and the illitic phase comprised at least 36% of the interstratified smectite‐illite.

Effect of water quality and amendments on the hydraulic properties and erosion from several mediterranean soils

The response of six sandy-loam soils from Portugal and Israel to leaching with sodic and saline water and to simulated rain was studied. The dominant clay mineral in the soils from Portugal was kaolinite, whereas smectite predominates in the soils from Israel. The permeability of the soils depended on the soil texture: it decreased with an increase in the silt and clay content. The response of the soils to sodicity depended on the electrolyte concentration; salt concentrations exceeding 10 mmol c ·L −1 was enough to prevent the deleterious effect of exchangeable sodium (≤20%). When leaching with distilled water (stimulating rain water), the presence of primary minerals and lime determine the susceptibility of the soils to sodicity. The calcareous loess from Israel was the least susceptibility to sodicity. The six soils were susceptible to sealing, high runoff and erosion when exposed to rain. The soil surface was particularly vulnerable to sealing due to both the mechanical impact of raindrops and the low concentration of electrolytes in the rainwater. Seal formation was due to two mechanisms: 1. (i) physical disruption of aggregates at the soil surface which depended on the impact energy of raindrops and the inherent aggregate stability; and 2. (ii) chemical dispersion which depended on the mineralogy of the clay, the ESP, and the electrolyte concentration. When the impact of the drops was prevented, or when the anionic polymer was sprayed at the soil surface, physical breakdown of the aggregates was reduced and runoff and erosion were slight. When the electrolyte concentration was high, the chemical dispersion was small and runoff and erosion decreased, compard with the control. The smectite soils from Israel were more susceptible to sealing than the kaolinitic soils from Portugal.