Published in Soil Sci. Soc. Am. J. 68:1578-1585 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
DIVISION S-2—SOIL CHEMISTRY
A New Method for the Simultaneous Measurement of pH-Dependent Cation Exchange Capacity and pH Buffering Capacity
K. Oortsa,*,
B. Vanlauweb,c,
J. Pleysierb and
R. Merckxa
a Lab. for Soil and Water Management, Faculty of Agricultural and Applied Biological Sciences, Kasteelpark Arenberg 20, 3001 Leuven/Heverlee, Belgium
b Soil Microbiology Unit, International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria, c/o Lambourn, Carolyn House, 26 Dingwall Road, Croydon CR9 3EE, UK
c Present address Tropical Soil Biology and Fertility Program, Unesco-Gigiri, PO Box 30597, Nairobi, Kenya
* Corresponding author (koen.oorts{at}agr.kuleuven.ac.be)
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ABSTRACT
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The pH dependent cation exchange capacity (CEC) and pH buffering capacity (pH BC) are two crucial properties in soil fertility management of variable charge soils. However, they are rarely measured, since most of the existing methods are cumbersome and time-consuming. We propose a new method, based on the silver-thiourea (AgTU) method, for the simultaneous measurement of pH dependent CEC and the pH BC. In general, this method consists of first increasing the pH of the soil to about pH 7 and then gradually acidifying the soil by titrating with HNO3, while measuring pH and CEC after each equilibration. This way, both CEC at different pH values and the pH BC are measured on the same sample, without the multiple washing steps needed in other procedures. Various aspects of the proposed method were tested and the results obtained were compared with the ion adsorption method. The modified AgTU method obtains field relevant results and is suitable for the routine analysis of large numbers of samples.
Abbreviations: AgTU, silver-thiourea • CEC, cation exchange capacity • pH BC, pH buffering capacity
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INTRODUCTION
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MANY SOILS of both temperate and tropical regions have variable or pH dependent charge properties (Theng, 1980). The charge characteristics of a soil directly control important agronomic aspects for example, nutrient retention and pH BC. A correct determination of the in situ CEC and a full understanding of the variation of charge with pH are therefore essential for the sustainable management of these soils.
Some specifics of variable charge soils have important implications for the choice of the methodology to measure CEC. The magnitude and sign of surface charges of these soils depend on pH, electrolyte concentration, and valency of the counter ion, hence CEC has to be measured at a pH and ionic strength similar to that of the soil solution to obtain field relevant results (Van Raij and Peech, 1972; Gallez et al., 1976; Morais et al., 1976; Gillman, 1981). Therefore, some widely used CEC methods (e.g., with 1 M CH3COONH4 at pH 7 or with 0.5 M BaCl2 at pH 8) do not yield field relevant CEC values for variable charge soils.
In contrast to the determination of the total CEC of the soil, the measurement of CEC changes with pH is not a routine analysis. One of the major reasons for this is that all existing methods are time-consuming. Most methods for the measurement of variable charge are modifications of the ion adsorption method of Schofield (1949) (Gillman and Sumpter, 1986; Itami et al., 1996; Van Ranst et al., 1998). All these methods start with the saturation of the exchange sites with an index ion by equilibrating the soil with a concentrated salt solution. Thereafter, the soil samples are equilibrated with dilute solutions of the same salt to obtain a field relevant ionic strength. During the final equilibrations, the pH is adjusted by adding small amounts of acid or base to the cover the desired pH range. Finally the adsorbed index ions are replaced by extracting the samples with a concentrated solution of another salt and the concentrations of the index ions are analyzed in this solution. The advantage of these methods is that positive and negative charge can be measured simultaneously. However, all these methods rely on multiple washing and centrifugation steps, and therefore they do not only involve much time and labor but also entail a risk for some charge bearing components being dissolved and lost during the procedure. Further, each CEC-pH combination requires a separate sample, which makes the method unsuitable if only limited amounts of material are available.
The AgTU method proved to be a valuable method for measuring the CEC of soils at low ionic strength (I = 0.01 M) and at any desired pH (Chhabra et al., 1975; Pleysier and Juo, 1980). The method relies on the very strong adsorption affinity of the AgTU complex for the cation exchange sites. Therefore, neither the saturation with a concentrated salt solution nor the subsequent equilibration at lower ionic strength is needed. Only some specific exchange sites (for K+, NH+4, and Cs+) on the interlayers of illite-type clay minerals are inaccessible for the bulky AgTU complex, either for steric or selectivity reasons (Cremers et al., 1988). For that reason, the AgTU complex also can be used to discriminate between the regular (planar) cation exchange sites and these highly specific interlayer sites on illite-type clay minerals (Cremers et al., 1988). This does not affect the overall result for the CEC of the soil as these specific sites only contribute about 1% to the total CEC of these clays. After a single extraction of the soil with AgTU, the CEC is determined by difference (Ag added – Ag in solution). It was our purpose to adapt this method so that CEC could be measured at different pH values on the same sample, avoiding all the equilibration steps needed in other methods.
The pH BC is another important physicochemical characteristic of highly weathered soils. The pH BC of a soil is defined as the amount of acid or base needed to change the pH of the soil with 1 unit. Many variable charge soils in the tropics have pH values lower than 5.5 and have relatively high concentrations of exchangeable Al (Fox, 1980), so liming is often required for good crop yields. Because of the variable charge, acidification often has severe consequences for the overall fertility of these soils (e.g., decrease in CEC, lower storage capacity for base cations, Al and Mn toxicity). Therefore it is essential to know the sensitivity of the soil for pH changes to design proper soil pH management schemes when these soils are cultivated.
The pH BC is generally determined with a titration procedure (Aitken and Moody, 1994; Curtin et al., 1996; Conyers et al., 2000). Such a titration technique is time-consuming and therefore, has largely been replaced by buffer solution methods for the measurement of lime requirement (Aitken and Moody, 1994). However, these buffer solution techniques do not allow the ready prediction of pH-changes due to various inputs (e.g., fertilizers, crop residues, acid deposition) or biological processes. Therefore, if changes in soil pH are a concern, a pH BC, achieved by a titration procedure, is still required. Nevertheless, there still appears to be no standard method for the determination of this pH BC.
The pH BC depends both on mineral dissolution and protonation–deprotonation reactions of organic matter and variable charge surface functional groups. These protonation and deprotonation reactions also determine the variable charge of the soil and some authors found correlations between pH BC and CEC across a range of Australian soils (Aitken et al., 1990, 1998; Moody et al., 1997; Conyers et al., 2000). Aitken and Moody (1994) also found that pH BC increased with increasing ionic strength, as did the variable charge. Since both CEC and pH BC are to a large extent controlled by the same reactions, it sounds logical to determine both characteristics in the same conditions. In the procedure proposed in this study, pH BC is determined concurrently with pH-CEC measurements without requiring additional steps.
The objectives of this study were (i) to adapt the AgTU method in a way that CEC could be measured as a function of pH on one sample and at low ionic strength; (ii) to evaluate the buffering capacity calculated from the pH changes induced for the measurement of the pH dependent CEC; and (iii) to verify this method against earlier proposed methods.
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MATERIALS AND METHODS
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Soils
Topsoils (0–10 cm) from three different soil types were selected (Table 1). These soils were representative for major soil types in Sub-Saharan West Africa. They differ in texture and soil organic matter content, while they all have low CEC values, typical for highly weathered soils. These soils are dominated by kaolinite and sesqui-oxides (Gallez et al., 1975). Therefore, they display low permanent charge and most CEC was due to the variable charge components. The soils were air dried and sieved through 4 mm before use to remove stones and large organic debris.
General Outline of the Modified AgTU Method
Soil pH was initially increased to obtain a first CEC value at about pH 7. Then adding small amounts of HNO3 gradually acidified the soil and the CEC was measured after each equilibration until the pH was decreased to about 3. The pH was increased before addition of the AgTU solution to the soil, as preliminary tests showed that precipitation of Ag2S occurred when adding small amounts of a strong base to a soil–AgTU mixture to try to increase the pH without changing the Ag concentration.
The amount of soil for the CEC analysis was chosen, in such a way that after equilibration, between about 50 and 90% of the AgTU was still in solution, assuring, on the one hand, an over saturation with AgTU and therefore complete occupation of the exchange sites and, on the other hand, resulting in a decrease in Ag concentration in solution that was large enough to detect AgTU adsorption. A rough estimate of the CEC can be made based on texture and organic matter content of the soils. For the soils used in this study, the optimal amount was 3.0 g for the Ibadan soil, 4.0 g for the Ferke soil, and 15.0 g for the Niaouli soil. Fifteen milliliters of 10–3 M NaOH was added to the soil samples in a centrifuge tube. The pH was measured after 2 h of shaking (end over end) and when the pH was still below 8, a few drops of 1 M NaOH were added and the tube was shaken again. A pH value between 8 and 9 was needed to obtain a first CEC reading at around pH 7, since the pH immediately dropped by one or two units after addition of 15 mL AgTU solution (pH ± 5.5). When the pH was between 8 and 9, 15 mL of an unbuffered AgTU-solution (0.2 M TU, 0.02 M Ag+ as AgNO3) was added to obtain a concentration of 0.01 M AgTU. One liter of the added AgTU mixture was prepared by first adding 0.25 L deionized water to 0.5 L of a 0.4 M TU solution and then adding slowly 0.25 L of a 0.08 M AgNO3 solution under vigorous stirring. After the AgTU was added to the soil samples, the tubes were shaken overnight (16 h). Subsequently, the pH was measured, the samples were centrifuged (15 min, 1000 x g), and a first 1-mL subsample was taken from the clear supernatant for Ag-analysis. Then, a small amount (0.01–0.1 mL) of 1 M HNO3 was added to the samples. After 4 h of shaking, pH was measured, samples were centrifuged, and a second subsample taken. This procedure was continued, shaking the samples alternately overnight or for 4 h after addition of HNO3, until pH 3 was reached. The amount of acid added was selected based on a rough estimate of the range in pH BC of the soil types studied and on the drop in pH caused by previous additions of acid. All volumes added to or removed from the supernatant were carefully recorded. Silver was analyzed by atomic absorption spectrometry. The CEC was calculated by difference, taking into account all changes in Ag concentration due to removal or addition of solution and due to removal of Ag for previous CEC measurements. The whole procedure resulted in six CEC measurements for each sample at low ionic strength (0.01 M) between pH 3 and 7.
Testing of the Method
Changes in pH and Cation Exchange Capacity with Time After Addition of an Amount of Acid
To test when equilibrium was reached after acidification of the suspensions, the changes in pH and CEC were monitored with time after addition of HNO3. Samples were weighed in a centrifuge tube (same weights as reported above) and 30 mL of an unbuffered AgTU-solution (0.01 M Ag+, 0.1 M TU) was added. Two different amounts of 1 M HNO3 (0.05 and 0.1 mL) were added to each soil in duplicate. The samples were shaken and after 2, 5, 8, 24, 28, 32, and 48 h, pH was measured, the samples were centrifuged (15 min, 1000 x g) and subsamples were taken from the supernatant solution. Silver was analyzed by atomic absorption spectrometry and CEC was calculated by difference.
Effect of Varying Shaking Times in the Proposed Protocol
It was tested if the proposed use of different equilibration times (overnight and 4 h) after addition of acid did not affect the results. Therefore the CEC was measured as a function of pH as described above, but with shaking overnight (16 h) after every addition of HNO3, thus determining 1 CEC-pH combination per day. The results were compared with CEC-pH relationships obtained by the proposed procedure. Four replicates were performed per treatment.
Effect of Intermediate Measurements on the Results for the Cation Exchange Capacity at Low pH
There was a potential risk that the multiple actions and measurements (pH and Ag) involved in obtaining the CEC values at the lowest pH, did affect the accuracy and precision for these CEC measurements. Therefore, the CEC was also determined at these low pH values without all the intermediate CEC measurements. After increasing the pH as described above, an amount of 1 M HNO3 corresponding to the cumulative amount of acid used to obtain the second, fourth, or sixth CEC measurement in the proposed procedure was added. The samples were shaken overnight, the pH was measured, samples were centrifuged and the clear supernatant was sampled for Ag analysis. Results were compared with the CEC-pH relationships obtained by the proposed modified AgTU method. Four replicates were performed per treatment.
Comparison with Other Methods
Buffered Silver-Thiourea Method
The CEC-pH relationships of the soils were also determined with a series of buffered AgTU solutions (Chhabra et al., 1975). These solutions consisted of 0.01 M Ag+ (as AgNO3), 0.1 M TU and 0.1 M CH3COONH4 at a specific pH (pH 3, 4, 5, 6. or 7) obtained by adding NH4OH or acetic acid. A separate soil sample was needed for each CEC-pH combination. Samples were weighed in centrifuge tubes, the AgTU-solutions were added and the tubes were shaken overnight. The pH was subsequently measured, the samples were centrifuged and the supernatant was sampled for Ag analysis. All measurements were done in triplicate.
Ion Adsorption Method
The CEC-pH relationships were also obtained by the ion adsorption method by Gillman (Gillman, 1984; Gillman and Sumpter, 1986). Samples were weighed in a centrifuge tube (2.0 g for the Ibadan and Ferke soils, 5.0 g for the Niaouli soil) and 20 mL 0.1 M CaCl2 was added to each tube. After 2 h of shaking, the samples were centrifuged and the supernatant discarded. The soils were washed twice with 0.002 M CaCl2. After a third addition of 20 mL 0.002 M CaCl2, the pH was adjusted to between 3 and 7 by adding 0.5 M HCl or 0.01 M Ca(OH)2. After 20 h shaking, pH was measured, the tubes were centrifuged and the supernatant solution was removed and retained for analysis of Ca and Al. The tubes were weighed to determine the amount of entrained solution. Next, the soils were extracted overnight with 20 mL of 1 M NH4NO3 and Ca and Al were measured in this extract. The amounts of cations adsorbed were calculated by subtracting the quantities of Ca and Al in the entrained solution. Both Ca and Al were determined by ICP-OES (PerkinElmer 3300, Dual View). The total CEC (CECT) was defined as the Ca2+ and Al3+ adsorbed and base CEC (CECB) as the Ca2+ adsorbed. For each soil, CEC was measured at five pH values between 3 and 7 and all measurements were made in triplicate.
pH Buffering Capacity
Taking into account the amount and concentration of NaOH and HNO3 added to the soil, and the measured pH values, it was possible to calculate the pH BC of the soils, without any additional measurement. The relationship between pH and H+ or OH– added was linear within at least 1 pH unit of the native pH of the soil. Linear regressions were fitted to this linear part of the buffer curve. The reciprocal of the slopes of the regression lines was a measure for the pH BC. Buffering capacity was expressed as the amount (cmol) of H+ or OH– required to change the pH of one kilogram of soil by one pH unit. Results from the AgTU method were compared with pH BC calculated similarly from the pH changes in the ion adsorption method.
Data Analysis
Cation exchange capacity-pH relationships and buffer curves were described by linear regression. Intercepts and slopes of the regressions were compared by the general linear hypothesis (Sen and Srivastava, 1990). Differences are considered significant at P 
0.05.
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RESULTS AND DISCUSSION
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The modified AgTU method resulted in linear CEC-pH relationships (Fig. 1)
. The three soils all had low CEC values, typical for highly weathered soils, dominated by kaolinite and oxides (Gallez et al., 1976; Duxbury et al., 1989). The linear regressions explained more than 94% of the variation and showed the significant (P 0.001) effect of pH on the charge characteristics of these soils.
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Fig. 1. Cation exchange capacity-pH relationships obtained by the modified silver-thiourea (AgTU) method. Regressions lines shown are fitted for the results from the proposed procedure.
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Tests of the Proposed Procedure
The pH increased significantly up to 24 h after the addition of HNO3 (Fig. 2)
. After 24 h, there was still a slight, but nonsignificant, increase. All three soils showed the same response to both treatments. These results clearly showed that for all soils and treatments, equilibrium was not reached and the pH still changed after the proposed 16- or 4-h equilibration time.
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Fig. 2. Effect of equilibration time after addition of 0.05 or 0.1 mL of HNO3 (1 M) on the pH and cation exchange capacity (CEC) of the soil. Error bars indicate the standard error of the difference (SED).
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Also CEC increased with time after the addition of an amount of 1 M HNO3 (Fig. 2). The CEC generally showed the same trends as pH: a significant increase up to 24 h after acidification, and thereafter values leveled off and differences were not significant anymore. It was assumed that these differences in CEC were caused by the differences in pH and that the CEC-pH combinations measured were points on the general CEC-pH relationships of these soils. To confirm this, these CEC-pH combinations were compared with the results of the CEC-pH relationships obtained with the proposed procedure (Fig. 1). The results showed that for all soils, these points agreed well with the general variation of CEC as a function of pH. It was therefore concluded that equilibration time after addition of acid had little effect on the overall CEC-pH relationships.
The results for the CEC-pH relationships determined while shaking overnight (16 h) between all measurements and those obtained after alternately overnight (16 h) and 4-h shaking (Fig. 3)
also confirmed that different equilibration times shifted the individual CEC-pH observations but had little effect on the obtained CEC-pH relationships. No significant differences were found for both intercepts and slopes from the two regression lines. Shaking for alternately 16 and 4 h between CEC measurements allowed determining two CEC-pH combinations per day and the complete measurement of the CEC-pH relationship for a sample within 1 wk. This made this method more convenient for routine analysis.
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Fig. 3. Cation exchange capacity (CEC)-pH relationships after shaking alternately for 16 or 4 h (solid lines) or after always shaking overnight (broken lines).
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The CEC-pH combinations obtained without intermediate CEC measurements agreed very well with the general CEC-pH relationships obtained by the proposed procedure (Fig. 1). Further, the variation in CEC and pH between replicates did not increase with increasing amount of prior measurements. Therefore, it was concluded that the accuracy and precision for the last CEC-pH combinations were not affected by previous measurements. This data also proved that the long equilibration time between the AgTU cations and the soil colloids before the last CEC measurements did not result in any significant additional, slow adsorption of the AgTU cation to the soil, apart from the exchange reactions.
Comparison with Other Methods
Figure 4
shows a very good agreement between the modified and the buffered AgTU methods. Except for the intercepts for the Ferke soils, both intercepts and slopes from the regression lines were not significantly different. The higher ionic strength of the buffer solutions did not seem to have an effect on the measured CEC values. However, the excess NH+4 in the buffered AgTU-method may have occupied a small part (up to 5%, Chhabra et al., 1975) of the exchange sites, and therefore mask the increase in CEC due to the higher ionic strength.
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Fig. 4. Comparison of cation exchange capacity (CEC)-pH relationships determined by the modified silver-thiourea (AgTU) method (solid lines), the buffered AgTU method (broken lines) and the ion adsorption method (dotted lines). Data points presented are averages of four (modified AgTU method) or three (buffered AgTU and ion adsorption method) replicates. All regressions are significant at P 0.0001.
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Above pH 5.5, CEC values obtained by the ion adsorption method agreed well with the results from the AgTU methods, but at lower pH values, the ion adsorption method resulted in significant lower CEC values for all three soils (Fig. 4). Therefore, the fitted regressions for the ion adsorption method had a steeper slope and smaller intercept and differences were highly significant (P 0.001). This discrepancy was explained by the higher selectivity of the AgTU cation for the exchange sites of the soil. Since the ion adsorption method only resulted in a lower CEC at pH values below 5.5, which is the pH range where exchangeable Al3+ and H+ occupy a substantial part of the exchange complex, it was assumed that NH+4 did not replace all exchangeable Al3+ and H+ from the soil complex during the last step in the ion adsorption procedure. Gillman (1984) also found that <70% of the KCl extractable Al3+ was exchangeable by this ion adsorption method. The maximal amount of exchangeable Al3+ extracted by 1 M NH4NO3 was 0.2 cmolc kg–1 for the Ibadan soil and 0.5 cmolc kg–1 for the Ferke and Niaouli soils. Only for the Ferke and Niaouli soils at pH 3.5, exchangeable Al3+ occupied more than 20% of the total CEC as measured by the ion adsorption method.
From all these observations, it was concluded that the modified AgTU method gave reliable and reproducible results for the CEC-pH relationships of highly weathered soils. The most important advantage of the method is the fact that it avoids all washing and equilibration steps involved in other methods required to saturate the exchange complex of the soil with an index cation at the desired pH and ionic strength. Therefore, the risk for dissolution and subsequent loss of charge bearing components is reduced during the determination of pH dependent charge with the modified AgTU method. Moreover, the time and labor involved were greatly reduced. For the determination of the CEC-pH relationship (six CEC measurements) for one soil, only one sample, six centrifugations, and six Ag analyses were needed with the modified AgTU method, while the ion adsorption method involved at least six samples, 30 centrifugations, and 12 Ca and Al analyses. The proposed procedure allowed determining the complete CEC-pH relationship for 50 different soil samples in 1 wk. With the ion adsorption method, only about 10 samples could be analyzed in the same time. A complete CEC-pH relationship can be determined on one small subsample, which makes the proposed method suitable when only limited amounts of a sample are available. It was concluded that the modified AgTU method is an attractive method for the measurement of CEC-pH relationships, suitable for analysis of large amounts of samples.
pH Buffering Capacities
A standard method for the measurement of pH BC does not exist, with little agreement on the equilibration time needed after the addition of acid or base. Equilibration periods ranging from 1 h to 30 d have been reported (Conyers et al., 1995), with a shaking time of 16 h most widely used (Curtin et al., 1996; Tang et al., 1999; Paul et al., 2001). Conyers et al. (2000) concluded that shaking for 16 h was a reasonable approximation to longer-term equilibria (5-wk incubation with limestone). Moreover, Conyers et al. (1995) reported that incubation for 30 d was not suitable for the determination of the pH BC of the soil, since the processes during that period had largely overridden the additions of mineral acid.
Figure 5
shows the linear part of the buffer curves obtained by the proposed modified AgTU method for the three soils used in this study. All three soils showed the same effect of shaking time. Differences in pH BC between soils were more pronounced than differences between treatments (Table 2). Buffer curves obtained by always shaking overnight had a slightly larger intercept and less negative slope than the buffer curves obtained by the proposed procedure for the modified AgTU method (Table 2). Consequently, pH BCs for the overnight shaking treatment were slightly larger than the pH BCs obtained by shaking alternately for 16 or 4 h. This agreed with the observation that after addition of acid and shaking for 4 h, the equilibrium pH was not yet reached and pH still increased significantly with time (Fig. 2). Therefore, pH was generally higher after always shaking overnight than after alternately shaking for 16 or 4 h, although the same amount of acid was added for both treatments. Despite these differences in individual pH values, the differences in slopes of the buffer curves between both treatments were small and not significant. Since the modified AgTU method resulted in pH BCs comparable with those obtained after shaking for 16 h, which seemed to be acceptable according to literature (Conyers et al., 2000), it was concluded that the method resulted in a good estimate of the pH BC of the soils.
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Fig. 5. Linear part of the buffer curves obtained by the proposed modified silver-thiourea (AgTU) method for the three soils selected. The pH buffer capacity (pH BC) was estimated as the reciprocal of the slope of the regression lines. Regression equations and pH BC are reported in Table 2.
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Table 2. Regression parameters for the pH buffer curves obtained by the different methods. All regressions are significant at P 0.0001.
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Theoretically, the intercept of the fitted regression lines of pH as a function of cmol H+ or OH– added kg–1 soil, that is, the pH of the soil without addition of acid or base, corresponds with the pH of the soils measured in a neutral salt solution of similar ionic strength. The measured native pH of the three soils was intermediate to the intercepts of both shaking treatments of the modified AgTU-method (except for the Ibadan soil), and the differences were generally small. We found a very good agreement between the intercept of the buffer curves and the pH measured in 0.01 M CaCl2 for 28 samples from the Ibadan soil varying in organic matter content:
This observation is a second indication of the suitability of the modified AgTU method for the determination of the pH BC of the soil.
Results for the pH BC obtained by the proposed modified AgTU method also agree well with pH BCs derived from the ion adsorption method (Table 2). Slopes from the buffer curves were not significantly different between both methods, except for the Niaouli soil, where a small significant (P 0.005) difference was observed. In contrast to the slopes, the intercepts from the buffer curves were highly significantly different (P 0.0001, except for Ibadan, where P 0.01) between both methods. The larger intercepts obtained by the ion adsorption method were probably caused by the removal of some exchangeable acidity during the saturation of the exchange complex with Ca2+. This agreed with the observation that differences in intercept between the two methods were largest for the Niaouli soil (with lowest native pH: 3.95) and smallest for the Ibadan soil (with highest native pH: 5.94).
The pH dependent charge of a soil acts as a sink for applied H+ or OH– and is therefore an important source of pH BC in the soil (Aitken et al., 1990; Aitken and Moody, 1994; Curtin and Rostad, 1997). For all soils studied, the pH BC was significantly larger than the slope of the CEC-pH regression lines, which is an estimate for the pH dependent CEC (Fig. 3 and Table 2). The contribution of the variable charge to the pH BC of these soils, determined by the proposed method, ranged from 25% for Niaouli to 41% for Ferke, demonstrating the importance of mineral dissolution reactions in buffering the pH of these soils (Aitken, 1992; Conyers et al., 1995, 2000).
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CONCLUSIONS
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The modified AgTU method offers an attractive alternative to the ion adsorption methods for the measurement of CEC-pH relationships and pH BCs. The method leads to CEC results that are consistent with the results obtained by buffered AgTU solutions. The proposed method yields larger CEC values than the ion adsorption method at pH values below 5.5, whereas above this pH value both methods yield similar CEC values. This is ascribed to the very high selectivity of the AgTU cation for the exchange sites in the soil. Further, the modified AgTU method avoids the multiple washing and equilibration steps involved in the classic ion adsorption methods, so eliminating the risk of dissolution and loss of charge bearing components during the procedure. Since both variable charge and pH buffering are partly controlled by the same exchange reactions in the soil, it is important to determine both properties simultaneously in similar experimental conditions. Results for the pH buffering capacities obtained by the modified AgTU method agree well to values obtained by the ion adsorption method. The proposed modified AgTU method requires only one small soil sample for the simultaneous determination of both a complete picture of CEC variation with pH and a pH buffer curve. Compared with other methods, the proposed modified AgTU method strongly reduced the time and labor involved in determining the CEC-pH relationship and pH BC of a soil.
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ACKNOWLEDGMENTS
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K. Oorts thanks the Fund for Scientific Research–Flanders (Belgium) for a grant as Research Assistant.
Received for publication March 20, 2003.
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