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J Plant Nutr Soil Sci 2009, 172, 477–486 DOI: 10.1002/jpln.200700217 477 Clay dispersion and its relation to surface charge in a paddy soil of the Red River Delta, Vietnam Minh N Nguyen1*, Stefan Dultz1, Jörn Kasbohm2, and Duc Le3 Institute of Soil Science, Leibniz University of Hannover, Herrenhäuser Str 2, 30419 Hannover, Germany Institute of Geography and Geology, Greifswald University, Friedrich-Ludwig-Jahn Str 17a, 17487 Greifswald, Germany Department of Pedology and Soil Environment, Faculty of Environmental Sciences, University of Science, 334-Nguyen Trai, Hanoi, Vietnam Abstract Dispersion is an important issue for clay leaching in soils In paddy soils of the Red River Delta (RRD), flooding with fresh water and relatively high leaching rates can accelerate dispersion and the translocation of clay For the clay fraction of the puddled horizon of a typical paddy soil of the RRD, the effect of various cations and anions as well as humic acid (HA) at different pH values on the surface charge (SC) were quantified and the dispersion properties were determined in test tubes and described by the C50 value In the 4, where SC is > –18 mmolc kg–1 The flocculation efficiency of Ca strongly depends on the pH At pH 4, the C50 value is 0.33, 0.66 at pH 5, and 0.90 mmol L–1 at pH At pH 6, close to realistic conditions of paddy soils, the effect of divalent cations on the SC and flocculation decreases in the order: Pb > Cu > Cd > FeII > Zn > Ca > MnII > Mg; FeII was found to have a slightly stronger effect on flocculation than Ca An increase in concentrations of Ca, MnII, and Mg from to mmol L–1 resulted in a change in SC from –25 to approx –15 mmolc kg–1 In comparison, the divalent heavy-metal cations Pb, Cu, Cd, and Zn were found to neutralize the SC more effectively At a Pb concentration of mmol L–1, the SC is –2 mmolc kg–1 From pH to – 5, the dispersion of the clay fraction is facilitated rather by SO2À than by Cl , which can be explained by the higher affinity of SO2À to the positively charged sites With an increase of the amount of HA added, the SC continuously shifts to more negative values, and higher concentrations of cations are needed for flocculation At pH 3, where flocculation is usually observed, the presence of HA at a concentration of 40 mg L–1 resulted in a dispersion of the clay fraction While high anion concentrations and the presence of HA counteract flocculation by making the SC more negative, FeII and Ca (C50 at pH = 0.8 and 0.9 mmol L–1, respectively) are the main factors for the flocculation of the clay fraction For FeII and Ca, the most common cations in soil solution, the C50 values were found to be relatively close together at pH 4, 5, and 6, respectively Depending on the specific mineralogical composition of the clay fraction, SC is a suitable measure for the determination of dispersion properties and for the development of methods to keep clay particles in the soil in the flocculated state Key words: paddy soil / surface charge / clay dispersion / heavy metals / anion effects / humic acid Accepted February 18, 2008 Introduction The Red River Delta (RRD) is typically used for the cultivation of rice In this area, high rainfall (1600–1900 mm y–1) and human activities like tillage and irrigation can induce a dispersion of clay Due to harvesting 2–3 times per year, there are seasonal changes of the water regime causing a change of oxidizing and reducing conditions In the RRD, there are also many so-called handicraft villages with a large number of small metalworking factories, which have impacts on the environment The soil under investigation is in the vicinity of such a handicraft village where, in addition, nonferrous– heavy metal (HM) recycling has been carried out for many decades As a consequence, increased HM concentrations are found in the soils and especially in the channels around the village (Le and Nguyen, 2004) Increases of HM contents below the puddled horizon were also reported by Nguyen et al (2006) Besides solute transport, leaching of HMs may also be facilitated if they are adsorbed on clay minerals and dissolved organic C (DOC) (Karathanasis, 2003; de Jonge et al., 2004) The leaching of clay is a common observation in paddy soils (Boivin et al., 2004; Nguyen et al., 2006) * Correspondence: M N Nguyen; e-mail: minh@ifbk.uni-hannover.de 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.plant-soil.com 478 Nguyen, Dultz, Kasbohm, Le J Plant Nutr Soil Sci 2009, 172, 477–486 For clay flocculation and the formation of stable aggregation, type and concentration of cations in the soil solution are wellknown as decisive factors The presence of di- and trivalent cations in the electric double layer can decrease zeta potential by reducing net negative charge of the clay-minerals surfaces and thus accelerate flocculation more effectively than monovalent cations At a given pH, increased cation concentrations can make the surface charge (SC) of clay fraction less negative and accelerate the flocculation The cation concentration affects the thickness of the electric double layer, which plays a decisive part, because the overlap of this layer from two particles is a prerequisite for flocculation (Goldberg and Forster, 1990; Sumner, 1993) A decrease in SC to more negative values is known as the primary factor for clay dispersion The pH affects dispersion by changing the SC of the clay particles through protonation and deprotonation reactions of variable charged sites (Thellier et al., 1992; Kaya, 2006) Chorom and Rengasamy (1995) reported a positive relationship between pH and the percentage of dispersible clay The increase in SC is also positively correlated with pH persion of clay Additions of lime and soluble salts from different sources such as fertilizers (Haynes and Naidu, 1998) and the release of FeII and MnII under reducing conditions (Wada et al., 1983; Saejiew et al., 2004) are expected to be factors for clay flocculation In this study, the SC and flocculation properties of the clay fraction of the puddled horizon from a typical paddy soil of the RRD were determined at different pH and concentrations of cations and anions, which are found in the soil solution Soaking of a soil sample in water under anaerobic conditions, representing reducing conditions in paddy soils were conducted in order to determine decisive factors for clay dispersion in the flooded period As the streaming potential, which is utilized to evaluate the zeta potential, may show a weak reproducibility (Böckenhoff and Fischer, 2001), the use of the SC, quantified by polyelectrolyte titration, as a parameter for clay dispersion was determined Materials and methods 2.1 Materials In other studies on clay flocculation, it is tried to bracket the equilibrium state between solution and the exchange complex either by presaturating the clay with the targeted SAR (Goldberg and Forster, 1990) or by using both Na- and Casaturated clays (Saejiew et al., 2004) considering various SAR and ionic strength As the focus of this study was the quantification of the effect of many different cations on SC and flocculation, only the Na-saturated clay fraction was considered The soil used in this study was selected from a soil series taken at the end of the dry season in March 2005 in flooded rice fields, in direct vicinity of the handicraft village Tong Xa with nonferrous-HM recycling activities, Red River Delta, Vietnam (106°1′12″ E and 20°19′48″ N) The soil sample was taken from the puddled horizon (0–25 cm depth) The sample was air-dried and passed through a mm sieve The clayloam soil (sand: 22%, silt: 45%, clay: 33%) has a slight acidic reaction (pH 5.6) The cation-exchange capacity (CEC) is 125 mmolc kg–1 Dithionite-soluble Fe is 1.7% The organic-C content is 2.2%, which is typically high for paddy soils The C : N ratio is 9.7 The DOC concentration, quantified with a TOC-analyzer (elementar, liqui TOC trace) in a 1:10 aqueous extract, is 186 mg kg–1 Soluble cations and anions (Tab 1) were extracted with de-ionized water (1:10) and detected by inductively coupled plasma (ICP-OES) and anion chromatography (DIONEX ICS-90) In the solution, an abundance of soluble Ca and Mg was observed while the concentration of – SO2À was found to be much higher than that of Cl (Tab 1) In the charge balance, where the charge of dissolved organic matter (DOM) is not considered, the sum of cations corresponds satisfactorily with the sum of anions (Tab 1) Also at the exchange sites (Ag-thiourea method performed at soil pH according to Pleysier and Juo, 1980), Ca (62%) and Mg (30%) are the most common cations In paddy soils, flooding and tilling for a new crop can facilitate the dispersion of clay In several periods of a year, a change of pH due to the alternation of soil condition from dry to flooded or in the reverse direction can also influence the dis- For the preparation of soil solution, an experiment of soaking the sample in water under anaerobic conditions was performed After a period of 15 d, the FeII content released from the sample to the solution is 3.6 mmol kg–1, which is approx The effect of inorganic and organic anions on clay dispersion is also an important issue Anions can be adsorbed on positively charged edge sites of clay minerals and counteract flocculation (Gu and Doner, 1993) Salts with multivalent inor3À ganic anions such as SO2À and PO4 were found to increase the critical coagulation concentration of clay-mineral suspensions (Penner and Lagaly, 2001) Also the addition of small amounts of humic acid (HA), citrate, formate, carbonate, and silicate can increase the critical flocculation concentration of kaolinitic soil clays (Frenkel et al., 1992) At pH 4, small additions of HA to kaolinite resulted in edge charge reversal from positive to negative and substantially reduced coagulation rates (Kretzschmar et al., 1998) As an effect of organic anions, soil-structure stability was found to decrease (Goldberg et al., 1990; Tejada and Gonzalez, 2007) Table 1: Soluble cations and anions in the 1:10 aqueous extract (pH 5.6) of the puddled horizon of a paddy soil from the Red River Delta, Vietnam Na+ K+ NH4+ Ca2+ Mg2+ Mn2+ Fe2+ Cu2+ Zn2+ Cl– SO42– R cations R anions 0.02 4.7 46.2 58.18 50.9 (mmolc kg–1) 8.0 0.66 2.9 31.4 15.0 0.14 0.05 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim 0.01 www.plant-soil.com J Plant Nutr Soil Sci 2009, 172, 477–486 Clay dispersion and its relation to surface charge 479 tion and decantation The suspension was flocculated with NaCl, centrifuged, washed until salt-free, and freeze-dried In order to determine the effect of HA on dispersion, the clay fraction was pretreated with H2O2 in order to remove organic matter (OM) 0.334 | Intensity (rel.) d-values (nm) 0.717 | 1.00 | 1.43 | 0.358 | 0.499 | 0.473 | 0.426 | 2.2.2 Preparation of clay suspensions a 0.485 | b c |0.325 d 10 15 20 °2 Theta 25 a) Mg saturation c) Mg saturation, ethylene glycol treatment b) K saturation d) K saturation, 550°C treatment 30 Figure 1: X-ray diffraction patterns of the clay fraction ( Cu (0.45) > Cd (0.7) > Fe (0.8) > Zn (0.85) > Ca (0.9) > Mn (1.1) > Mg (1.5) > K (27) > Na (40) This order is well-fitted to the results of other studies (Arora and Coleman, 1978; Heil and Sposito, 1993; Sumner, 1993) in which the strength of cation was reported to be controlled by the valency, ionic 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Surface charge (mmolc kg-1) -25 b -20 -15 -10 -5 0 0.2 0.4 0.6 0.8 1.0 Cation concentration (mmol L-1) Al Pb Cu Cd FeII Zn Ca Mn Mg Figure 4: Flocculation (a) and corresponding surface charge (b) of the clay fraction in dependence on the kind of cation at pH radius, and hydrated-ion size In comparison with the findings of Saejiew et al (2004), slight differences in the effectiveness of Ca and Fe were observed However, there is a general difficulty when comparing the results, as Saejiew et al (2004) applied other methods (settling time of 24 h, different SAR values, results are presented in grams of suspended clay) An increase of the concentration of Ca, MnII, and Mg from to mmol L–1 results in a change in SC from –25 to approx –15 mmolc kg–1 (Fig 4b) In comparison with these divalent cations, the divalent heavy-metal cations Cu, Pb, Zn, and Cd were found to neutralize the SC more effectively Especially, the presence of Pb can increase the SC, which is at a concentration of mmol L–1 at –2 mmolc kg–1 The addition of Al can completely neutralize the negative SC The point of zero charge is reached, when the Al concentration is 0.19 mmol L–1 In comparison of the most common cations of the investigated soil, FeII has a slightly stronger effect on flocculation than Ca This is especially true at lower concentrations, where the transmission of the suspension is increasing quite earlier in presence of FeII than of Ca From SC determination, also the trend for a stronger increase of the value at the same concentrations is observed Despite the fact that Ca and FeII (Tab 1, section 3.5) are the most common cations in soil solution, the pronounced effect of some heavy-metal cations like Pb on flocculation and SC is an interesting issue This www.plant-soil.com Nguyen, Dultz, Kasbohm, Le J Plant Nutr Soil Sci 2009, 172, 477–486 might be of special importance for the channels around Tong Xa village where in comparison with the adjacent soils highest concentrations of HMs are found (Le and Nguyen, 2004) 3.3 Anion effects Whereas at pH and 3.5, clay flocculation was observed in the NaCl as well as in the Na2SO4 solution in the concentration range up to 10 mmol L–1, at pH 4, 4.5, and 5, flocculation is observed only in case of addition of the NaCl solution (Fig 5a, b) From pH to 5, the dispersion of the clay fraction – is facilitated by SO2À rather than by Cl Whereas at pH the change of transmission is the same for both anions, at higher pH (3.5–5) the transmission of the suspension increases quite earlier for Cl– than for SO2À This might be due to the higher affinity of SO2À to the positively charged sites Obviously, SO2À can neutralize positive charges more effectively, which consequently counteracts flocculation Finegrained permanent negatively charged muscovite that was used to affirm this effect showed a similar trend Furthermore, – in this experiment SO2À much more than Cl facilitated the dispersion at low pH The stronger effect of multivalent anions on dispersion was also confirmed by Penner and Lagaly (2001), where the addition of SO2À and PO3À 4 severely increased the critical flocculation concentration of clay suspensions Phosphate is known to form innersphere complexes on surfaces, which decreases SC of the clay fraction and, as a consequence, facilitates dispersion (Jose et al., 2000) 100 Transmission (%) 80 Cl SO4 30 20 10 3.0 3.5 4.0 4.5 5.0 5.5 6.0 pH Figure 6: pH-dependent C50 values of the suspensions of the clay fraction for Cl– and SO2À (Na salts added) At pH 5, where the SC of the clay fraction decreased to –17.5 mmolc kg–1 (Fig 2), a change in the effect of Cl– and SO2À on dispersion is observed From the C50 values at pH 5.5 and it can be deduced that a higher amount of NaCl in comparison with Na2SO4 is needed for flocculation (Fig 6) This means that the divalent SO2À is more strongly attracted at pH < 5, where increased protonation of functional groups induces a higher density of positively charged sites On the other hand, the stronger attractive force for SO2À can also depend on the kind of functional groups protonated in this pH region At pH < 5, there is indication that the repulsion between the particles is accelerated if SO2À is adsorbed on the surfaces, especially at the edge sites At pH 6, close to realistic conditions in paddy soils, SO2À , the most common anion in the soil solution (Tab 1), accelerates flocculation more strongly than Cl– 60 a) Na2SO4 3.4 Effects of humic acid b) NaCl For both, purified HA as well as the clay fraction, where OM was previously removed, an adjustment to different pH resulted in a severe increase in SC at low pH (Fig 7) In comparison with the SC of the original clay fraction (Fig 2), at pH < 4, a slightly higher SC was observed after the removal of OM, and at pH > 5, the values were quite similar For HA, the point of zero charge was observed at pH 1.5 When the pH was increased to 10, the SC of HA decreased to –2720 mmolc kg–1 In comparison with the clay fraction, at pH 10 the SC of HA is two orders of magnitude higher 40 20 100 80 Transmission (%) 40 C50 value (mmol L-1) 482 60 40 20 10 Na concentration (mmol L-1) pH: 3.5 4.5 Figure 5: Effects of Na2SO4 and NaCl on the flocculation of the clay fraction 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim In presence of HA, the SC of the suspension was observed to be more negative At pH 4, 5, and 6, an addition of HA in amounts from to 30 mg L–1 resulted in a decrease of the SC of the clay suspensions from –17.6 to –29.8, from –19.2 to –46.6, and from –31.3 to –52.1 mmolc kg–1, respectively With an increase of the amount of HA added, the SC continuously shifted to more negative values At pH 3, where flocculation was usually observed (Fig and 5a, b), the presence of HA at a concentration of 40 mg L–1 resulted in a dispersion of the clay fraction (Fig 8) Most probably, this is due to a reversal of edge charge of the clay fraction from positive to negative in the presence of HA (Kretzschmar et al., 1997), so that the forwww.plant-soil.com Clay dispersion and its relation to surface charge 483 -40 100 Humic acid Clay fraction, organic matter removed Transmission (%) Surface charge: humic acid (dmolc kg-1) -1 clay fraction (mmolc kg ) J Plant Nutr Soil Sci 2009, 172, 477–486 -30 -20 -10 80 60 40 20 0 0.2 Figure 7: pH-dependent surface charge of humic acid and the clay fraction, where organic matter was removed mation of edge-to-face structures (card houses) and the resulting flocculation is prohibited By adding HA to the suspension, the amount of FeII needed for flocculation is increased severely (Fig 9) At a HA concentration of 20 mg L–1, the C50 values for FeII were found to increase from 0.23 to 0.61 at pH and from 0.49 to 0.82 at pH With increasing pH values, the SC of HA and clay minerals becomes more negative As a result, higher FeII concentrations are required for the flocculation of the clay fraction Kretzschmar et al (1998) reported that already small additions of HA caused pronounced increases in colloidal stability and the coagulation rates became strongly dependent on ionic strength The increase in ionic strength generally resulted in decreased colloidal stability indicating that the suspensions were stabilized by electrostatic repulsive forces With an increase in pH from to and HA concentration from to mg L–1 (TOC), colloidal stability of clay suspensions increased Transmission (%) 100 80 60 40 20 20 40 0.6 0.8 1.0 -1 Fe concentration (mmol L ) pH 0.4 II 60 80 -1 Humic acid concentration (mg L ) Figure 8: Effect of humic acid on the dispersion of the clay fraction at pH 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Clay fraction: pH: Original 10 mg L-1 HA added 20 mg L-1 HA added Figure 9: pH-dependent flocculation of the clay fraction in presence of 10 and 20 mg L–1 HA and increasing FeII concentrations 3.5 Flocculation by soil solutions After 24 h soaking time of the soil in water under anaerobic conditions (N2 atmosphere), a remarkable increase of the Ca and Mg concentration was observed (1.7 and 0.3 mmol L–1), which on the one hand might be due to soluble salts (Tab 1) and on the other hand to a desorption from cation-exchange sites of clay minerals and OM Up to day 10, increasing amounts of Fe and Mn were determined in the supernatant Afterwards, only slight changes in the chemistry of the solution occurred On day 15, the final concentration for Fe, Ca, Mg, and Mn were 1.84, 2.01, 0.29, and 0.17 mmol L–1, respectively During the experiment, an increase of the pH of the solution from to was observed, which is most probably due to the dissolution of Mn and Fe oxides under reducing condition The shift of pH can facilitate the dispersion of clay and increase the critical flocculation concentration of cations From the transmission of the suspensions determined at different dilution ratios, it can be concluded that the amount of cations corresponding to a dilution ratio 3:10 is sufficient to induce flocculation (Fig 10) The C50 value is reached at a dilution ratio of 2:10 where the concentrations of FeII, Ca, Mg, and MnII are 0.37, 0.40, 0.06, 0.04 mmol L–1, respectively In comparison with the C50 values of these cations at pH (cf., section 3.2), it can be concluded that FeII (C50 value at 0.8 mmol L–1) and Ca (C50 value at 0.9 mmol L–1) are the main factors for the flocculation of the clay fraction in the selected paddy soil The sum of the FeII and Ca concentration at a dilution ratio of 2:10 is 0.77 mmol L–1 This critical concentration is smaller than that of Ca or FeII alone and is most probably due to other cations like MnII and Mg in the soil solution, even if they have low concentrations and relatively high C50 values (1.1 and 1.5 mmol L–1, respectively; cf., section 3.2) and only slight effects on flocculation can be assumed Dissolved organic matter, which is 186 mg kg–1 in a www.plant-soil.com 484 Nguyen, Dultz, Kasbohm, Le J Plant Nutr Soil Sci 2009, 172, 477–486 Surface charge (mmolc kg-1) Transmisson (%) 100 80 60 40 20 -15 -14 II -13 s fo alue 0v r Fe C5 -12 -11 C 50 -10 a rC s fo e u val pH pH pH -9 -8 2/10 6/10 4/10 10/10 8/10 Dilution ratio (soil solution / deionized water) 0.8 Cation concentration (mmol L-1) 0.2 0.4 0.6 1.0 Figure 10: Flocculation of the clay fraction by a soil solution representing reducing conditions Figure 12: pH-dependent C50 values and corresponding SC of the clay fraction for FeII and Ca 1:10 aqueous extract of the soil sample under investigation (cf., section 2) and a common feature in paddy soils (Maie et al., 2004; Chien et al., 2006), can facilitate clay dispersion and increase the amount of cations needed for flocculation The saturation of the exchange sites by different cations resulted in different SC at the C50 value Therefore, the use of SC to predict flocculation might be limited, if different cations are present in the soil solution For divalent cations, also the formation of monovalent metal chloride ion pairs on the external surface of montmorillonite was reported (Sposito et al., 1983) The possible sorption of FeIICl+ on clay-crystal edges was confirmed with 57Fe Mưßbauer experiments (Charlet and Tournassat, 2005), which might affect SC and flocculation properties However, for FeII and Ca, the most common cations in the soil solution of paddy soils, the SC at the C50 is relatively close together (Fig 11) The quantification of the SC at the C50 value for FeII and Ca at pH 4, 5, and shows that at pH 4, the SC is highest (Fig 12) At pH 4, SC becomes less negative, which allows flocculation already at lower concentrations of FeII and Ca An increase of the pH from to resulted in a change in SC from –10.3 to –13.1 mmolc kg–1 and from –10.4 to –14.6 mmolc kg–1 for FeII and Ca, respectively At pH 4, 5, and 6, all the values for SC are relatively close together in the FeII, Ca soil clay system and SC is— under consideration of the pH—a valuable parameter for prediction of dispersion properties 3.6 Relationship between surface charge and clay dispersion Surface charge (mmolc kg-1) The effect of cations on SC and flocculation of the clay fraction is given by the relation between SC and C50 value (Fig 11) From the cations under investigation, highest SC of the clay fraction at the C50 value is found for Al and Pb The SC has similar values (–5.0 and –5.9 mmolc kg–1, respectively) For these cations already at relatively low concentrations the C50 value is reached (0.11 and 0.30 mmol L–1, respectively) The SC of Cu is –11 mmolc kg–1 From the other divalent cations under investigation, SC at the C50 value is relatively close together (–13.0 to –16.5 mmolc kg–1), but distinct differences in the cation concentration at C50 value are observed The highest value is obtained for Mg with 1.5 mmol L–1 The strength of cations on flocculation (measure: C50 value) is related with SC and has the order: Al > Pb > Cu > Cd > FeII > Zn > Ca > MnII > Mg -16 C 50 s lue va Al Pb Cu Cd -12 II -8 Fe Zn Ca Mn Mg -4 Conclusions 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Surface charge is highly dependent on the content of 2:1 and 1:1 clay minerals in the sample, which affects the proportion of variable to permanent charge Hence, results on SC determinations depend strongly on the kind of clay minerals present in a sample The results reported in this study are valid for the clay fraction under investigation, where the proportion of 2:1 vs 1:1 clay minerals is 5:1 With an increasing number of variably charged sites due to higher amounts of 1:1 clay minerals and/or secondary oxides, the effect of slight changes in pH and ion concentrations on SC is increasing 1.6 Cation concentration (mmol L-1) Figure 11: C50 values and corresponding SC for Al and different divalent cations 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Leaching processes causing downward movement of clay can be an important factor in rice fields with flooding irrigation water and may result in environmental risks due to losses of contaminants to the groundwater It is well-known, that the zeta potential is an important parameter for characterizing clay dispersion The physicochemical mechanisms of clay www.plant-soil.com J Plant Nutr Soil Sci 2009, 172, 477–486 dispersion which is the major prerequisite for clay leaching were reevaluated in this study for a paddy soil of the RRD The kind and concentration of cations, the pH, and, to a lower extent also the presence of HA and certain anions, affect clay dispersion in soils primarily by changing the negative SC of the clay fraction Inorganic ions and HA were found to have strongly distinguishable effects on the SC of the clay fraction This demonstrates the high accuracy of the applied PCD technique with polyelectrolyte titration for quantifying dissociated charge For FeII and Ca, the most common cations in paddy soils, the strength on flocculation and the related SC were found for both cations from almost the same value at certain pH For such defined soil systems, SC quantified in knowledge of the pH is another suitable measure for the determination of dispersion properties and helpful for the management of dispersive soils A careful management such as split dressing of chemical fertilizers can be a helpful tool in addition to other agricultural practice like regular addition of organic and Fe-supplying amendments, to decrease clay leaching Also predictions of particulate mediated transport of contaminants can be ameliorated by considering decisive soil properties for dispersion in modeling Acknowledgment This work was granted by the 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www.plant-soil.com ... concentration of cations, the pH, and, to a lower extent also the presence of HA and certain anions, affect clay dispersion in soils primarily by changing the negative SC of the clay fraction Inorganic... humic acid (HA), citrate, formate, carbonate, and silicate can increase the critical flocculation concentration of kaolinitic soil clays (Frenkel et al., 1992) At pH 4, small additions of HA to kaolinite... flocculation more effectively than monovalent cations At a given pH, increased cation concentrations can make the surface charge (SC) of clay fraction less negative and accelerate the flocculation The