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1.
Different particle-size fractions of soil clays from the semi arid north and the humid tropical south of Cameroon have been characterized with reference to their chemical composition, clay mineralogy and kaolinite crystallinity (Hinckley indices). Selected samples were also examined by Mössbauer spectroscopy. Hinckley indices of kaolinites, which were a major component of the coarse clays, varied considerably (< 0.1 - 0.69) and differed significantly as a function of the geographic and landscape positions of the soil profiles. The Hinckley indices averaged 0.31 in soils from southern Cameroon and 0.30 and 0.07 in soils from high and low landscape positions in northern Cameroon, respectively. Kaolinite crystallinity is therefore considered to vary as a function of transport and/or depositional environment of the kaolinite-containing material. Mössbauer spectra showed that kaolinite-dominated Vertisol coarse clays contained higher relative amounts of Fe2+ than the corresponding fine clay, which is dominated by smectite. It is conceivable that the Fe2+ content of the kaolinite reflects the redox environment of the samples.  相似文献   

2.
Soil degradation affects soil properties such as structure, water retention, porosity, electrical conductivity (EC), sodium adsorption ratio (SAR), and soil flora and fauna. This study was conducted to evaluate the response of contrasting textured soils irrigated with water having different EC:SAR ratios along with amendments: gypsum (G), farm manure (FM), and mulch (M). Water of different qualities viz. EC 0.6 + SAR 6, EC 1.0 + SAR 12, EC 2.0 + SAR 18, and EC 4.0 + SAR 30 was used in different textured soils with G at 100% soil gypsum requirement, FM at 10 Mg ha?1, and M as wheat straw was added on surface soil at 10 Mg ha?1. Results revealed that the applied amendments in soils significantly decreased pHs and electrical conductivity (ECe) of saturated paste and SAR. Four pore volumes of applied water with leaching fraction 0.75, 0.77, and 0.78 removed salts 3008, 4965, and 5048 kg ha?1 in loamy sand, silty clay loam, and sandy clay loam soils, respectively. First four irrigations with LF of 0.82, 0.79, 0.75, and 0.71, removed 5682, 5000, 3967, and 2941 kg ha?1 salts, respectively. The decreasing order for salt removal with amendments was FM > G > M > C with LF = 0.85, 0.84, 0.71, and 0.68, respectively. This study highlights a potential role of soil textures to initiate any mega program for reclamation of saline-sodic soils in the perspective of national development strategies.  相似文献   

3.
A low-swelling smectite exhibiting little intra-crystalline swelling even after saturation with Na was found in paddy soils derived from marine alluvium of Ariake Bay in Kyushu, Japan. The low-swelling smectite was considered to be beidellite-nontronite mineral containing as much as 10% Fe2O3. The low-swelling characteristics of this smectite are ascribed to the considerable substitution of Fe2+ for Al3+ in the ochtahedral layer which depresses the dissociation of unit layers of smectite. Transformation of low-swelling to high-swelling smectite is believed to be due to the oxidation of Fe2+ to Fe3+ in the octahedral layer.  相似文献   

4.
The electrokinetic behavior of colloidal particles in three waterlogged soils at 38°C was investigated with reference to the stability changes of soil colloidal suspensions under reductive conditions. The dispersed clay particles of the three soils exhibited a negative zeta (ζ) potential. The absolute value of the ζ-potential, |-ζ|, of these soils in the earlier period of waterlogging decreased, which caused the flocculation of clay particles. The concentrations of divalent cations, i.e., Fe2+ and Ca2+ in the soil solutions were estimated to be higher than their critical flocculation concentrations (CFCs) on the basis of the observed CFCs of Fe2+ and Ca2+ for the clay suspension of halloysite as a reference. With the progression of the reduction process, clay particles of one soil still exhibited a low |- ζ| and flocculated. The concentrations of Fe2+ and Ca2+ in the soil solutions were estimated to be higher than their CFCs, respectively. The clay particles of two sandy soils, however, showed an increase in |- ζ| due to the increase in pH and dispersed. The concentrations of Fe2+ and Ca2+ in the soil solutions were estimated to be lower than their CFCs, respectively. The stability changes of the soil colloidal suspensions by these divalent cations under sequential soil-reduction can be explained by the alteration of the Stern potential (- ψ s ), which determines the repulsion energy related to the potential energy of interaction between two particles. The apparent decrease in the Ca2+ concentration of the soil solutions in the later period of waterlogging was explained largely by the re-adsorption of water-soluble Ca2+ on the exchange sites of soil clays with the decrease in the Fe2+ concentration in the soil solution.  相似文献   

5.
Soil-shrinkage characteristics affect fluid transport and soil mechanical properties, with broad implications for environmental flows, crop production, and civil engineering designs. We quantified mild-saline-solutions effects on soil shrinkage curves and developed pedotransfer functions to predict curve parameters. Seven soil and soil mixes were equilibrated with solutions of 0.5-to-8 dS m?1 and 0-to-20 sodium adsorption ratios (SAR). Saturated paste rods were dried; water contents and isotropic shrinkage measured. Texture affected shape-forming factors when clay and smectite contents were >260 and 140 g kg?1, respectively. Solutions ≥2 dS m?1 affected the coefficient of linear extensibility for smectitic soils containing clay ≥300 g kg?1. Solution SAR affected only the highest clay content (530 g kg?1) and mixed mineralogy soils. However, the solution salinity levels were not high enough to affect shape factors of the shrinkage curves. Pedotransfer functions successfully described soil shrinkage with root-mean-squared-errors 1 to 4 magnitudes lower than the highest measured values.  相似文献   

6.
Lowland rice is a staple food for more than 50% world population. Iron toxicity is one of the main nutritional disorders, which limits yield of lowland rice in various parts of the world. The toxicity of iron is associated with reduced soil condition of submerged or flooded soils, which increases concentration and uptake of iron (Fe2 +). Higher concentration of Fe2 + in the rhizosphere also has antagonistic effects on the uptake of many essential nutrients and consequently yields reduction. In addition to reduced condition, increase in concentration of Fe2 + in submerged soils of lowland rice is associated with iron content of parent material, oxidation-reduction potential, soil pH, ionic concentration, fertility level, and lowland rice genotypes. Oxidation-reduction potential of highly reduced soil is in the range of –100 to –300 mV. Iron toxicity has been observed in flooded soils with a pH below 5.8 when aerobic and pH below 6.5 when anaerobic. Visual toxicity symptoms on plants, soil and plant tissue test are major diagnostic techniques for identifying iron toxicity. Appropriate management practices like liming acid soils, improving soil fertility, soil drainage at certain growth stage of crop, use of manganese as antagonistic element in the uptake of Fe2 + and planting Fe2 + resistant rice cultivars can reduce problem of iron toxicity.  相似文献   

7.
At present precise information on the b-fabric of Vertisols is inadequate for gaining a comprehensive knowledge about such soils formed in alluvium of basic igneous and metamorphic rocks and spread in varying agro-climatic zones of peninsular India. The aim of the present study, on five benchmark Vertisol series (Sarol, Aroli, Nimone, Bellary and Kovilpatti), was to assess the possible reasons for differences in the b-fabric of the subsoils despite their common shrink-swell properties, as evidenced by the presence of sphenoids and/or slickensides, and similar clay contents and amounts of fine clay smectite. In Sarol and Aroli soils from the sub-humid region, the b-fabric is porostriated, whereas in Nimone and Kovilpatti soils of the semi-arid and Bellary soils of the arid regions the fabric is either mosaic-speckled or granostriated, indicating weak plasma separation, a result of restricted swelling of clays. The related distribution pattern of the s-matrix in all these soils was open porphyric. Recent literature has suggested that weak plasma separation is an effect of dissolution and crystallization of calcite. However, generally low concentrations of soluble calcium (c. 1 mmol dm?3) in soil solutions of Vertisols in arid and semi-arid agroclimates suggest that the presence of calcite does not guarantee the presence of sufficient soluble calcium to impair swelling of clays by contracting the diffuse double layer. Weak plasma separation in Bellary and Kovilpatti soils compared with Sarol and Aroli soils is explained in terms of a decrease in the internal surface area of fine smectite, as evident from a very high degree of chloritization in smectite interlayers, and smaller proportions of coarse smectite. In Nimone soils the weak plasma separation is primarily due only to a decreased internal surface area of fine smectite.  相似文献   

8.
Laboratory experiments were conducted with sodic soils of varying exchangeable sodium percentage (ESP) (82, 65, 40, and 22) and a normal soil (ESP 4) to study the changes with time in soil pH, pCO2, Fe2+ and Mn2+ under submerged conditions with and without 1.0 per cent rice husk. In all the soils pCO2, Fe2+ and Mn2+ increased after flooding, reached the maximum value and then either maintained or declined slightly. The release of Fe2+ and Mn2+ was maximum in normal soil and decreased with increase of ESP in sodic soils. Addition of rice husk brought about a conspicuous increase in Fe2+ and Mn2+, the maximum increase being in lowest ESP soil. Flooding reduced the pH of all soils. The effect was more pronounced in the presence of rice husk. The kinetics of pCO2 indicated that accumulation of CO2 was higher in normal soil and least in highest ESP soil. The addition of rice husk showed an average increase of 0.0074 atm pCO2 in comparison to rice husk untreated soils.  相似文献   

9.
The long-term effects of intermittent flooding on soil properties were studied in field experiments on a Vertisol cropped with rice in Senegal. The dominant clay minerals were smectite and kaolinite. When the soil was reduced after flooding, its cation exchange capacity (CEC) increased to twice that of its oxidized, unflooded state. Mössbauer spectroscopy showed an increase in smectite structural FeII upon reduction, which explained a part of the increase in CEC. The rest of the increase was attributed to the removal of iron oxyhydroxide coatings by reductive dissolution. The reduction and dissolution of oxides under the field conditions were substantiated by analysis of the surfaces of vermiculites buried in the Ap horizons of the cropped and the non-cropped soils. The redox-induced CEC changes were found to be reversible after 22 cycles of rice cropping. Nevertheless, the structural Fe and free Fe contents of the rice field Ap horizon were less than those of soil in uncropped neighbouring land, suggesting that inundation induced weathering and eluviation of the minerals. The observed changes in CEC and related redox reactions may substantially modify proton, anion and cation balances in intermittently flooded soils.  相似文献   

10.
水溶性有机碳在各种粘土底土中的吸附：土壤性质的影响   总被引：3，自引：0，他引：3  
Clay-rich subsoils are added to sandy soils to improve crop yield and increase organic carbon (C) sequestration; however, little is known about the influence of clay subsoil properties on organic C sorption and desorption. Batch sorption experiments were conducted with nine clay subsoils with a range of properties. The clay subsoils were shaken for 16 h at 4 oC with water-extractable organic C (WEOC, 1 224 g C L-1) from mature wheat residue at a soil to extract ratio of 1:10. After removal of the supernatant, the residual pellet was shaken with deionised water to determine organic C desorption. The WEOC sorption was positively correlated with smectite and illite contents, cation exchange capacity (CEC) and total organic C, but negatively correlated with kaolinite content. Desorption of WEOC expressed as a percentage of WEOC sorbed was negatively correlated with smectite and illite contents, CEC, total and exchangeable calcium (Ca) concentrations and clay content, but positively correlated with kaolinite content. The relative importance of these properties varied among soil types. The soils with a high WEOC sorption capacity had medium CEC and their dominant clay minerals were smectite and illite. In contrast, kaolinite was the dominant clay mineral in the soils with a low WEOC sorption capacity and low-to-medium CEC. However, most soils had properties which could increase WEOC sorption as well as those that could decrease WEOC sorption. The relative importance of properties increasing or decreasing WEOC sorption varied with soils. The soils with high desorption had a low total Ca concentration, low-to-medium CEC and low clay content, whereas the soils with low desorption were characterised by medium-to-high CEC and smectite and illite were the dominant clay minerals. We conclude that WEOC sorption and desorption depend not on a single property but rather a combination of several properties of the subsoils in this study.  相似文献   

11.
Employing four mathematical models (first-order, parabolic-diffusion, Elovich and zero-order), kinetics of potassium desorption from eight soils with and without cropping were studied to evaluate their ability in explaining K release from soils. The decline in the soil test K in cropped soils over original soils was drastic in easily desorbable forms compared to that of strongly held forms like 3 M H2SO4 K. Results showed that parabolic diffusion as well as first-order kinetic equation explained the K release data well for both original and K depleted (cropped) soils. Elovich and zero-order equations were not suitable to describe the kinetic data. However, zero-order equation explained K release data better in case of K-depleted soils as compared to original soils. Soils with higher initial K contents registered higher release rate constants. Over the entire period of cropping the range of release rate (b) decreased from 1.26 to 1.53 × 10?2 to values ranging from 1.12 to 1.30 × 10?2 h?1. In contrast, the first-order equation, parabolic diffusion showed higher b values for cropped soils as they represent the diffusion gradient. Mica and its biotite content in both silt and clay fractions showed significant correlation (r) with b values. Similarly with the rate of K release, clay content of soils maintained significant r whereas the silt content did not.  相似文献   

12.
An experiment with two typical paddy soils from China and two clay minerals was conducted to study the effect of reduction of octahedral FeIII on fixation of NH4+ ions. Reduction of octahedral FeIII was achieved by treating soils and clay minerals with dithionite‐citrate‐bicarbonate (DCB) followed by dialyzing the samples under oxygen free conditions. Reduction of FeIII increased the negative charge of interlayers and resulted in a significantly higher ammonium fixation. Close positive correlations were found between the Fe2+ concentration or the ratio of Fe2+/Fe3+ and non‐exchangeable NH4+‐N. Therefore, it is concluded that the reduction of octahedral Fe induced by flooding is one of the important prerequisites for the pronounced ammonium fixation in flooded soils. However, the relation between ΔFe2+ and Δfix‐N was not stoichiometric.  相似文献   

13.
The effect of total electrolyte concentration (TEC) and sodium adsorption ratio (SAR) of water on ESR‐SAR relationships of clay (Typic Haplustert), clay loam (Vertic Haplustept) and silt loam (Lithic Haplorthent) soils was studied in a laboratory experiment. Twenty four solutions, encompassing four TEC levels viz., 5, 10, 20, and 50 mmolc l—1 and six SAR levels viz., 2.5, 5, 10, 15, 20, and 30 mmol1/2l—1/2 were synthesized to equilibrate the soil samples using pure chloride salts of calcium, magnesium, and sodium at Mg:Ca = 1:2. SAR of equilibrium solution decreased as compared to the equilibrating solution and more so in waters of low salt concentration and high SAR. At low electrolyte concentration, high SAR values were not attained in the equilibrium solution because of addition of calcium and magnesium from the mineral dissolution and from the exchange phase. Irrespective of TEC, exchangeable sodium in all the soils increased by about 4.5 to 5‐fold and irrespective of SAR, it increased by about 1.4‐ to 1.8‐fold. A positive interaction of TEC and SAR influenced the ESP build‐up and CEC played a major role in the visual disparity in sodication of these soils. At higher TEC levels, considerable increase in ESP was observed when it was corrected for anion exclusion and more so in silt loam followed by clay loam and clay soils. The values for Gapons' constant were in the range 0.0110—0.0176, 0.0142—0.0246, and 0.0189—0.0344 mmol—1/2l1/2 in clay, clay loam, and silt loam soils, respectively. Increase in TEC from 5 to 50 mmolc l—1 resulted in 5.84, 8.33, and 9.77 % decrease in Gapons' constant of clay, clay loam, and silt loam soils, respectively. The soils exhibited differential affinity for Ca2+, Mg2+ or Na+ under different quality waters. Regression coefficients of ESR‐SAR relationship were lower for low TEC as compared with high TEC waters. The exchange equilibrium was strongly affected by TEC of the solution phase. Variation in soil pH was gradual with respect to TEC and SAR of equilibrating solution and no sharp change was observed. Soluble salt concentration was doubled upon equilibration with low salt waters at all SAR levels in all the soils. However, the salt concentration remained unchanged upon equilibration with high salt waters. Considering pH 8.5 a boundary between soil salinity and sodicity, ESP values attained at TEC 5 mmolc l—1 were 7.34, 8.02, and 14.32 for clay, clay loam, and silt loam soils, respectively.  相似文献   

14.
The adsorption at pH's 4, 6 and 8 of adenine, guanine, cytosine, thymine and uracil on clays (montmorillonite, illite and kaolinite), Fe- and Al-oxides (goethite, hematite and gibbsite), a soil, and on a laboratory-prepared fulvic acid-montmorillonite complex was investigated. Portions of the clays and soil were saturated with H+, Fe3+ and Ca2+.Quantitatively, the extent of adsorption of nucleic acid bases by the clays was proportional to their exchange capacities, but the nature of the dominant cation had only minor effects. By contrast, the adsorption was strongly affected by pH, tending to decrease with increase in pH. Adsorption on goethite and gibbsite was lower than that on clays, while adsorption of nucleic acid bases on soils was slightly lower than that on oxides. The fulvic acid-montmorillonite complex adsorbed substantial, although smaller amounts of purines and pyrimidines, than did montmorillonite alone. The main adsorption mechanism at pH 4 appeared to be cation exchange whereas at pH 8 complex formation between the nucleic acid bases and cations on inorganic surfaces seemed to occur.The results of this and earlier work show that both inorganic and organic soil constituents adsorb nucleic acid bases. Which adsorption reaction predominates will depend on the clay and organic matter content and on the pH.  相似文献   

15.
Cesium-137 (137Cs) is strongly adsorbed on clay minerals, especially on illite. The adsorption of Cs+ on reference clay minerals, however, has not been fully investigated in relation to the presence of illite. The objective of this study was to clarify the effect of impurities (i.e., illite and vermiculite), present in reference smectite group minerals and kaolin minerals, on the retention of Cs+. The clay mineralogy of the reference minerals was characterized by X-ray diffraction (XRD). The radiocesium interception potential (RIP) was measured as an index of the Cs+ retention ability of clays. The content of illite in clay was represented by the total potassium (K) content given that illite is a major source of K in the clay fraction. The content of vermiculite in clay was represented by the Cs fixation capacity induced by Cs saturation followed by heating of samples at 110°C. Metabentonite and beidellite gave extremely high RIP values compared with other smectite group minerals, although a peak for illite (at 1.0 nm) was not observed in XRD analysis. The reference smectite and kaolin minerals showed a range of RIP values, even though their RIP values are theoretically zero. The RIP values had a significant positive correlation with the total K content of all the reference clay minerals (rs = 0.621*). This indicated that the retention ability for 137Cs depended more on the content of illite, as impurity, rather than the type of bulk mineral. Hence, the contribution of illite to the magnitude of the RIP was elucidated by the combination of measurement of total K content and XRD analysis.  相似文献   

16.
The microstructural stability of soils of different geneses (steppe soils, tropical soils, and subtropical soils) developed from marine clay, loess, and weathering crusts was studied by the method of successive treatments with chemical reagents destroying the particular clay-aggregating components. The following dispersing agents were used: (1) H2O (pH 5.5), (2) 0.1 N NaCl (pH 6), (3) 0.002% Na2CO3 (pH 8.7), (4) 0.1 N NaOH (pH 11.5), (5) the Tamm reagent (pH 3.2), and (6) 0.1 N NaOH (pH 11.5). The properties of the clay subfractions obtained in the course of these treatments were studied by a set of analytical methods, including X-ray diffractometry, Mössbauer spectroscopy, and magnetic measurements. It was shown that soil microaggregates are formed under the impact of a number of physicochemical processes; the content and properties of inorganic components (clay minerals in soils with a high CEC and iron oxides in soils with a low CEC) are the controlling factors. The structure of the parent materials is transformed to different degrees to form the soil structure. For example, autonomous nondifferentiated soils inherit, to some extent, the specific microorganization of the parent material. At the same time, the redistribution of substances in the soil profile and in the landscape may exert a substantial influence on the soil structure and microstructure. This is particularly true for autonomous differentiated soils, turbated soils, accumulative soils, polylithogenic soils, and polygenetic soils. The properties of the obtained subfractions of the clay (the mineralogical composition, the Fe2+/(Fe2+ + Fe3+) ratio, the magnetic susceptibility, and the Cha/Cfa ratio) attest to the spatial heterogeneity of the composition and properties of the mineral and organic aggregated compounds in soils.  相似文献   

17.
The pedological characteristics, and physical, chemical and mineralogical properties are investigated for soils developed in a 1 m to 1.5 m layer of lacustrine clay material deposited on a former sand plain in the Lake Chad basin in north east Nigeria. (These soils are being developed for intensive production of rice, wheat and other crops under irrigation.) The surface layer of these soils has about 60 per cent clay, a pH of about 7.9, and a coarse, strong prismatic structure. The sub-soil is often notably higher in clay content, pH and exch. Na. The smectite content of the clay fraction (surface soil) is 35 to 45 per cent and the CEC 350 meq kg-1. The levels of montmorillonite and CEC, and the size of the cracks in dry soil, although the latter are appreciable, are all notably less than the values reported for the clay soils of the Sudan Gezira. The Lake Chad basin clay soils are not considered to be vertisols according to the FAO Soil Map of the World, on grounds of profile morphology, though it seems the French pedologists would regard them as paravertisols.  相似文献   

18.
Soil organic carbon (SOC) storage and turnover is influenced by interactions between organic matter and the mineral soil fraction. However, the influence of clay content and type on SOC turnover rates remains unclear, particularly in tropical soils under natural vegetation. We examined the lability of SOC in tropical soils with contrasting clay mineralogy (kaolinite, smectite, allophane and Al-rich chlorite). Soil was sampled from A horizons at six sites in humid tropical areas of Ghana, Malaysian Borneo and the Solomon Islands and separated into fractions above and below 250 μm by wet sieving. Basal soil respiration rates were determined from bulk soils and soil fractions. Substrate induced respiration rates were determined from soil fractions. SOC lability was significantly influenced by clay mineralogy, but not by clay content when compared across contrasting clay minerals. The lability of SOC was lowest in the allophanic and chloritic soil, higher in the kaolinitic soils and highest in the smectitic soil. Our results contrast with conventional concepts of the greater capacity of smectite than of kaolinite to stabilize SOC. Contents of dithionite-citrate-bicarbonate extractable Fe and Al were inversely related to SOC lability when compared across soil types. A stronger inverse correlation between content of ammonium-oxalate extractable Fe and SOC lability was found when considering the kaolinitic soils only and we conclude that the content of active Fe (hydr-) oxides controls SOC stabilization in the kaolinitic soils. Our results suggest that the validity of predictive models of SOC turnover in tropical soils would be improved by the inclusion of soil types and contents of Fe and Al (hydr-) oxides.  相似文献   

19.
Abstract

Problems are invariably encountered when attempts are made to explain the variability in Bray percent yields or plant response in terms of soil or plant iron (Fe). To resolve this inconsistency, the present investigation was initiated to identify a combination of soil extractable Fe, soil properties and form of plant Fe that may be used as a measure of Fe deficiency. The study involved 16 diverse soils, using upland rice (Oryza sativa L.) as the test crop and Fe‐EDDHA [ferric ethylenediamine di (o‐hydroxyl‐phenyl acetic acid)] as source of Fe. The results showed that Bray percent yields were neither related to DTPA (diethylenetriamine pentaacetic acid) or EDTA (ethylenediamine tetraacetic acid) extractable Fe nor with total plant Fe. Even the inclusion of pH, lime, organic carbon and clay data in the regression equations was of no value. However, Bray percent yields were significantly and positively (r = 0.57* ) associated with ferrous Fe (Fe2+) in 40‐day‐old rice plants. The explanation concerning variability in Bray percent yields obtained on diverse soils could be increased about one and half 2 times (R2= 0.59*) if the contribution of lime and soil pH was also incorporated in the stepwise regression analysis. The individual contribution to R of lime, pi respectively. Thus, it appears that Fe2+ concentration in plants (along with soil pH) may identify Fe deficiency. The critical limit to separate Fe deficient from green rice plants was set at 45 ug Fe2+/g in the leaves.  相似文献   

20.
Ammonium chemistry in surface soils collected from major soil series of Bangladesh was studied in terms of quantity-intensity (Q/I) relationship. Soils were classified according to physiography and parent materials; calcareous floodplain soils were dominated by mica and smectite, while non calcareous floodplain soils, terrace soils, and hill soils were dominated by mica or kaolinite and lacked smectite as a major clay mineral. Different Q/I parameters were obtained between soils of smectitic and non-smectitic types. Soils of smectitic type had a high potential buffering capacity which kept the equilibrium activity ratio at a low and fixed level.. The content of labile ammonium was high and estimated to increase with increasing addition of NH4 +. In contrast, soils of non-smectitic type with a low potential buffering capacity showed a considerable variation of both the equilibrium activity ratio and labile ammonium content, as affected by the application of NH4 +. Based on the ammonium chemistry, expected N-fertilizer application was compared with the prevailing application method.  相似文献   

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