Influences of aluminum ions on the determination of zpc (zero point of charge) of variable charge soils

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1 Soil Science and Plant Nutrition ISSN: (Print) (Online) Journal homepage: Influences of aluminum ions on the determination of zpc (zero point of charge) of variable charge soils Katsutoshi Sakurai, Akinori Nakayama, Tsutomu Watanabe & Kazutake Kyuma To cite this article: Katsutoshi Sakurai, Akinori Nakayama, Tsutomu Watanabe & Kazutake Kyuma (1989) Influences of aluminum ions on the determination of zpc (zero point of charge) of variable charge soils, Soil Science and Plant Nutrition, 35:4, , DOI: / To link to this article: Published online: 14 May Submit your article to this journal Article views: 75 View related articles Citing articles: 7 View citing articles Full Terms & Conditions of access and use can be found at Download by: [Kochi University] Date: 10 May 2016, At: 21:01

2 Soil Sci. Plant Nutr.. 35 (4) INFLUENCES OF ALUMINUM IONS ON THE DETERMINATION OF ZPC (ZERO POINT OF CHARGE) OF VARIABLE CHARGE SOILS Katsutoshi SAKURAI 1, Akinori NAKAYAMA 2, Tsutomu WATANABE', and Kazutake K YUMA Soil Science Laboratory. Faculty of Agriculture. Kyoto University. Sakyo-ku. Kyoto. 606 Japan Received December Influence of AI dissolution on soil ZPC (zero point of charge) measured by II potentiometric titration (PT) method and a modified salt titration (STPT) method was ex.amined using two strongly weathered soils from Thailand and two volcanic a~h soils from Japan. The amount of dissolved AI ions increased with the increase in the concentral1on of a supporting electrolyte for the strongly weathered soils. while the increase wa~ neal1g1ble for the volcanic ash soils. ZPC value orthe strongly weathered soils determined by the PT method WaS lower than that by the STPT method. due to the greater AI dissolution associated with the higher electrolyte concentration used in the PT method. AI ions adsorbed onto the soil surface would shift the ZPC to a higher ph value not as II result of the formation of hydro.. y AI polymers. but due to the blocking of permanent negative charge sites, '" hlch could otherwise lower the zpc. The CT p value. as a measure of permanent charge or the amount of II' or 011 adsorbed by a soil required to attain the ZPC. could be used to describe thi~ phenumenon. In the STPT method. the salt concentration was not high enough to cause a,ignificllnt AI dissolution at the ZPC. which is considered to be II more suitable condition than in the PT method because the ZPC value can be evaluated at a low salt concentration as In the cat.e of field conditions for crop production. Thus. the STPT method is recommended for the determination of the ZPC. Key Words: aluminum ions. STPT method, variable charge soil. Zrc. ZPC (zero point of charge) is the point, at which the net charge of variable <.:harge components becomes O. The amount of potential determining ions (PDI) adsorbed b)' a soil at the ZPC is designated as ct p ' which may be considered as an index. of the permanent charge that develops at the ZPC (SAKURAI el al 1988). ZPC and ct p <.:an be determined as definite values of a soil under a given condition. and the)' are u~eful in predicting the behavior of mineral ions in relation to the ph and sail concentration of the soil medium. Some investigators have pointed out that the enhanced dissociation of c~change. Present addresses: 1 Faculty of Agriculture. Kochi University. Nankoku jip'.. n. -~, iturata Manufacturing Co. Ltd. Nagaokakyo. 617 Japan. I cel Corporation. Scld Japan. 623

3 624 K. SAKURAI, A. NAKAYAMA, T. WATANABE, and K. KYUMA able AI when a concentrated salt solution such as I N was used as a supporting electrolyte would cause a decrease of the ZPC value of soils (V AN RAIJ and PEECH 1972; GILLMAN and UEHARA 1980). In order to remove exchangeable AI beforehand, a sample soil is often saturated with a non-specifically adsorbed index cation (HENDERSHOT and LAVKULICH 1978; W ADA and OKAMURA 1980). ZPC and O'p, however, could be of more practical significance, if they were measured without such a preliminary treatment. In a preceding paper (SAKURAI et al 1988), we also postulated the effect of aluminum dissolution on the ZPC and O'p determined by various titration techniques for Thai Oxisols and Ultisols, which exhibit a low ZPC value around 4.0, and contain a considerable amount of exchangeable AI. It was, however, not possible to determine to what extent AI ions could have affected the titration curves and ZPC. Thus, AI dissolution during the application of a few titration methods was examined in reference to the ZPC and O'p values.. SAMPLES AND METHODS Two volcanic ash soils from Japan and two strongly weathered soils from Thailand were used as representatives of variable charge soils. The volcanic ash soils (K42 and AOJ) are mainly composed of amorphous materials like allophane, allophane-like material, and/or imogolite and trace amounts of crystalline aluminosilicates. The strongly weathered soils (T6B, Oxisols; T7B, Ultisols) are mainly characterized by the presence of hematite as surface coatings of kaolinite and as aggregates. Some of their physico-chemical characteristics are listed in Table I. The amount of extractable AI with I N NaCl was 0 for the volcanic ash soils and 2.26 and 4.34 meq/ 100 g for the T6B and the T7B soils, respectively. Organic matter content was low to very low for all the sample soils (0.2 to 2.0%). Table I. Selected properties of sample soils. ex.ai ecec CEC Sample phh.o ph SSA T-C Clay N8C1 (meq/ (meq/ (meq/ 100 g) 100 g) 100 g) (m 2 /g) (%) minerals Strongly weathered soils T6B Kt T7B Kt Volcanic ash soils K A,A',Im AOJ A,A',Im ex.ai. I N NaCI extractable AI (exchangeable AI); ecec. effective CEC (exchangeable Ca+Mg+ K+ Na+ AI); CEC, I N ammonium acetate method at ph 7.0; SSA, specific surface area measured by the ethylene glycol monoethyl ether retention; T-C, total carbon content. Kt, kaolin; A, allophane; N, allophane-like materials; 1m, imogolite.

4 Influences of Aluminum Ions on the Determination of ZPC 625 Determination of ZPC and O'p. ZPC was determined as the ph value at the cross section of titration curves obtained with the use of two concentrations of supporting electrolytes, and O'p was measured as the net adsorption of H+ or 011- at the ZPC. Two titration techniques were used; a potentiometric titration method (PT) and a salt titration method including the calculation step used in the potentiometric titration (STPT) method (SAKURAI et al 1988). Two sets of soil suspensions were prepared with or without ph adjustment using 0.1 N HCI or NaOH, and allowed to stand for 4 days at room temperature with occasional hand shakings to reach an equilibrium. Then the ph value was recorded (ph I). One set of the soil suspension was centrifuged at 2.280xg for 10min. and the supernatant solution was used for the determination of Al ions. For the other set. to reach a concentration of N for the electrolyte. 0.5 ml of a 2.00 N NaCI solution was added and shaken reciprocally for 3 h. After ph reading (ph 2). the solution was centrifuged at 2,280x g for 10 min, and the supernatant was also kept for the analysis of Al ions. The initial salt concentrations of the supporting electrolyte solution in the PT method were 0.1 and 0.01 N, and that in the STPT method was very low (less than N) because water instead of a salt solution was used. These salt concentration states were expressed as PT(O.Ot), PT(O.t). and ST. In the STPT method. the final sail concentration after the addition of a salt solution was N. which was expressed as ST +s. The measured ZPC was expressed as PT-ZPC or STPT-ZPC specifying the method used. The term ZPC without any specification was used as the general term. including both PT-ZPC and STPT-ZPC. Preparation of Na-saturated (Na-sat) sample. Al ions of the T611 and T7U samples were removed with 1 N NaCI solution by reciprocal shaking for I h. and centrifugation for 10 min at 250x g. After the samples were dialyzed to deionized water until the electrical conductivity of the outer solution became less than 5 its/em. they were air-dried and ground to pass a 0.5 mm mesh sieve. Preparation of AI solution. A predetermined amount of 0.1 N NaOH solution was added drop by drop to 50 ml of a 0.2 N (6.6 X 10-2 M) AICI, solution. under constant stirring. and then the mixture was diluted to 100 ml to reach a molar ratio of NaOHI Al (hereafter, referred to as the OHI Al ratio) of O or 2.7. The ph values of these solutions were , and respectively. The Al concentration of all the resultant solutions was 0.1 N (3.3 x 10-2 M). The solutions were aged for 5 days to attain an equilibrium. Although some precipitation of At(OH), occurred in the Al solution at ph 5.64 while aging, the solution was stirred vigorously before use. Determination of AI ions. The total amount of aluminum in the supernatant solutions was determined by the aluminon method (llsu 1963). After using the method described by BERSILLON et al (1980). Al ions were grouped into four categories: monomeric ions, and low, medium. and high Oll-AI polymers. The monomeric Al ions were determined with ferron, and the tow 0111 At polymers were determined us

5 626 K. SAKURAI, A. NAKAYAMA, T. WATANABE, and K. KYUMA the difference between the amount of Al retained by the Na-resin and that of the monomeric Al ions. The high OH/ Al polymers precipitated with Na 2 S0 4, and the medium OH/ Al polymers were determined as the remainder. The OH content in the Al polymers was determined by titration with NaOH. Timing of AI addition. A \0 ml portion of water (in the STPT method) or NaCI solution (in the PT method) was added first to a soil sample. Then an aliquot of an Al solution was added to the soil suspension, which was kept for I h with occasional hand shakings. Finally, HCI or NaOH was added to adjust the suspension ph and the total volume was made to 20 ml with water. RESULTS AND DISCUSSION 1. Amount and composition of AI ions dissolved during the titration The composition of the Al ions was examined first, using the standard Al solution. When the OH/ Al ratio was 0 and the ph was 3.8, monomeric Al ions were the only species. As the OH/ Al ratio increased, the amount of monomeric Al ions decreased and that of the low OH/ Al polymers increased. When the OH/ Al ratio was 1.8 and the ph was 4.5, 30% of Al still remained as monomeric Al ions and 70% changed into low OH/ Al polymers, and when the OH/ Al ratio was 2.7 and the ph was 5.6, the monomeric Al ions disappeared and the proportion of low and high OH/ Al polymers was identical. The amount of medium OH/ Al polymers was negligible throughout the ph range examined. A linear relationship between the OH/ Al ratio and the proportion of the monomer Al ions in solution was observed as shown by WADA and WADA (1980), in which the OH/ Al ratio of the polymeric Al species was constant throughout the hydrolysis stage, and the change in the OH/ Al ratio of the polymer resulted from the change in the relative proportion of AP+, AI 2 (OHMH20)sH, and AI04AI 12 (OH)24 (H20)127+. Similar species may have been formed in the solution studied here. 5 --Total AI Monomer AI Salt 8 4 concentration cst < (N) -g,. est ,~ 1 3 "PT (0.01) c 2 ~ PT (0.1) i3 ph ~~ 0 3 Fig. I. AI dissolution from T6B during the application of the STPT and PT methods.

6 Influences of Aluminum Ions on the Determination of ZPC 627 Figure I shows the amount of the total and the monomeric Al dissolved during the titration of a strongly weathered soil (T6B). It is evident that the AI dissolution increased with the increase in the concentration of NaCl. When the soil sample was equilibrated with water (designated as ST in Fig. I), Al started to dissolve at a pjl below 4.3, and the amount increased with the decrease in ph, indicating an exchange of H for AI. As is shown in Fig. 1, the value of PT-ZPC was lower than that of STPT-ZPC. AI dissolution from T6B was less significant at ph close to STPT-ZPC (pb 4.3). but the amount increased gradually as the ph value decreased. Thus AI dissolution was larger at PT-ZPC, or under higher ionic strengths of both Na+ and 11+. than at STPT-ZPC. Although the results obtained for the volcanic ash soils are not shown in Fig. I, there was little AI dissolution throughout the ph range from 6.5 to 4.0. As for the composition of the Al ions dissolved. monomer AI ions were dominant throughout the ph range used to determine the ZPC. This applied to every soil regardless of the amount of Al dissolved. 2. AI addition to untreated and Na-saturated soil samples Effects of the composition and the amount of AI ions were examined in relation to the determination of the ZPC by the STPT method. When an Al solution with an OB/ AI molar ratio of O or 2.7 was added. the STPT-ZPC value was the same as that without Al addition. This applied to every soil examined (Table 2). In contrast, the magnitude of the value of O'p changed with the OH/ Al ratio of the AI solution. A positive sign of the O'p value implied that there was Table 2. Changes in the STPT-ZPC and -O'p values of the soil samples by the Itddllion of the AI solution with different Oil/AI ratios. +AI OH/AI (meq/ 1 ()() g) ratio ZPC T68.-~.-. -~ -.- O'p O'p (meq/ pllw b ZPC (mc:q/ pll.. b I ()() g) T71l 100,) ---_._" (ph 5.6) I.IS (ph 4.5) (ph 3.8) K (ph 5.64) S l 5.0 (ph 4.45) (ph 3.78) S AOJ ph of the AI solution added. b ph value in water measured after 4 days 5tanding. usin, I : 10 of soil: water ratio.

7 628 K. SAKURAI, A. NAKAYAMA, T. WATANABE, and K. KYUMA Fig. 2. OH- 1O.- +AI10.0 Sample meql100g 88 T6B AI 7.5 Z'.5.& c.2 +AI e-4 S +AI 2.5 '0 c 2 Do 0 H+ 2 Untreated 5 ph STPT curves of T6B with or without Al addition at various rates. 6 ~ Sample T7B 5 - CI ~.~ Untreated ~. ost ~3 \\.ST+. GI ST +AI ST+8 +AI c( 1 -. ~~ 'ti ~... ~ 0 ' is Na-8aturated 4 '" 3 2 "-6 '. '--- 1 ~ e... 4a., ph Fig. 3. Al dissolution from T7B during the application of the STPT method with or without addition of 5.00 meq/ioo g AI. +s, after salt addition. an excess of adsorbed H+ over OH- at the ZPC and a negative sign the opposite. Thus, the increase in the negative O'p value implied the increase of the positive charge. The variation in the O'p values will be discussed later. Regardless of the value of the OHj

8 Influences of Aluminum Ions on the Determination of ZPC 629 Al ratio, since the addition of Al solutions did not change the ZPC of the soils (Table 2), an Al solution with an OH/ Al ratio of 0 was used. Figure 2 shows the STPT curves oft6b when an Al solution with an OB/ Al ratio of 0 (ph = 3.8) was added at the rates of 2.50, 5.00, 7.50, 10.0 meq (0.833, , 3.33 mmol, respectively)/loo g soil. It was evident that the addition of varying amounts of Al did not change the ZPC value, but increased the CT p value as OU - adsorbed. On the other hand, the ZPC values of the Na-sat soil (T7B) which were determined at 2.78 by the STPT method and 2.54 by the PT method were much lower than those of the untreated soil which retained an amount of exchangeable Al of 4.34 mcq/ 100 g (Fig. 3 and Table 3). When Al ions were added at a rate of 5.00 meq/ 100 g of the Na-sat soil, the ZPC values which increased to 3.85 when determined by the STPT method and to 3.69 by the PT method were close to those of the untreated soils, Le and The amount of Al added to 100 g soil was only slightly larger than that of the exchangeable Al originally retained by T7B. Dissolution of Al from the Na-sat soil was negligible in the ph range of 3.0 to 4.0 (Fig. 3),. and even after Al addition, a smaller amount of Al was dissolved than that in the untreated soil. Furthermore, when Al ions were added to the Na-sat soil, they were dissolved much less in water than in the NaCI solution. while those added to the untreated soil were more dissolved in water, Le., in a similar amount to that in the NaCl solution. ZPC shifts and the dissolution pattern of Al ions may be related to each other. When exchangeable Al ions are substituted for Na ions, some sites may become a source of negative charge. The increase in the CT p value and downward shift of zrc caused by the Na-sat treatment (Table 3) would indicate that these changes are related to permanent negative charge sites as suggested by SAKURAI el al (1989a). The amount of exchangeable Al ions substituted for Na of T6B and T7B (Table I) was very clo!ltc Soil +AI (meq/ioo g) Table 3. ZPC Values of ZPC, C1 p ' and ph,... PT C1p (meq/ioog) ZPC STPT C1p (meq/ioo &) --~-~-~ I'll",' T6B T6B \ T6B (Na) \ T6B (Na) T7B \ \ T7B T7B (Na) S.68 T7B (Na) ph value in water measured after allowing the solution to stand for 4 days, u.in&, : 10 of soil: water ratio.

9 630 K. SAKURAI, A. NAKAYAMA, T. WATANABE, and K. KYUMA to the magnitude of the O'p value (Table 3), and therefore, it could be taken as a measure of an increase in availability for the reaction with POI in the permanent charge sites. Considering that AI ions added to the untreated soil could not cause an upward shift of ZPC, Al ions added to the Na-treated soils should have blocked preferentially the permanent negative charge sites as exchangeable ions. Only after occupying the permanent negative charge sites, would additional Al have been adsorbed by variable charge sites. But the latter would be lost relatively easily in response to the change in ph and salt concentration. 3. O'p and OH / Al molar ratio Although the ZPC values remained almost the same regardless of the composition (Table 2) and amount of AI added (Fig. 2), the magnitude of the O'p value changed in response to the amount of AI added. The phw in Table 2 corresponds to the ph value of the soil suspension measured 4 days after standing for equilibration with water and/ or an AI solution. When the phw value was higher than the ZPC value, H+ was required to decrease the suspension ph to reach the value of ZPC. On the contrary, when the phw value was lower than that of ZPC, OH- was required to increase the suspension ph to reach the ZPC value. GILLMAN and UEHARA (1980) stated that by the use of O'p (referred to as zero point of titration, ZPT in their paper) it was difficult to represent the permanent charge in that the ZPT depends on the soil ph and that specifically adsorbed ions influence the ZPC value (pho in their paper), and at the same time, the ZPT. However, the O'p value can be used in the following cases, (I) characterization of untreated soils and (2) comparative examination of the effects of some chemical treatments on a given soil. In the former case (I), the O'p value of soil samples makes it possible to compare the permanent charge revealed under a given condition, while in the latter case (2), the increase and decrease of the O'p value indicate the changes in the required amount of H+ or OH- adsorbed by the amended, removed, or recovered charge components at the zpc. The O'p value of the K42 sample was originally 0.10 meq/ioo g, indicating the occurrence of a net H+ adsorption at the zpc. When 5.00 meq (1.67 mmol) of AI/ I 00 g soil with an OH/ Al ratio of 0 was added, the O'p value became meq/ioo g, indicating the occurrence of a net OH- adsorption (Table 2). Since the ZPC value of K42 after the addition of Al was approximately 5.7 and was not different from that of the soil without Al addition, it is considered that OH- may have been consumed during the hydrolysis of Al ions. In addition, AI ions were not dissolved from K42 at the STPT -ZPC, suggesting their adsorption onto the soil as polymer ions. Thus, the OH/ Al ratio of the added Al ions at the ZPC can be calculated by the difference of O'p values before and after Al addition, that is, 0.10 and meq/ioo g, respectively. The net increase ofoh- adsorption by the Al addition amounted to 0.10-( -4.58)= 4.68 (meq/ioo g). Then, the resultant OH/ Al ratio of Al added at ZPC became 4.68/ 1.67 = 2.80, and almost all the Al ions should have been polymerized. This OH/ Al

10 Influences of Aluminum Ions on the Determination of ZPC 631 ratio was very close to that for the AI solution at ph 5.6. The difference between the phw and ZPC induced by the addition of AI solutions with OB/ AI ratios of 1.8 and 2.7 may control the O'p value, i.e., the higher the OH/ AI ratio of the AI solution or the more AI is polymerized, the less OH- is required by AI ions. For another volcanic ash soil, AOJ, similar results were obtained. On the contrary, the strongly weathered soil samples, T6B and T7B, had low ZPC values around 4.2. When the Al solution with a OH/ Al ratio of 0 was added. the net consumption of OH- at the ZPC was smaller than that of the volcanic ash soils. resulting in smaller negative O'p values. This phenomenon was attributed to the fact that the monomeric Al ions were present at a low ph around 4.2 and there was a small difference between the phw and ZPC values. When an Al solution with a pll of 5.6 and OH/ Al ratio of 2.7 was added. H+ was consumed by the AI polymers in solution to reach the ZPC. and therefore. the resultant O'p value was higher than that of the untreated soil. Thus, it can be concluded that the Al ions with various Oil/AI ratios added would be retained by an untreated soil in more or less identical forms at the ZPC. resulting in changes in the O'p values corresponding to the difference between the pllw and ZPC values. As stated before, it should be noted that. after all the permanent negative charge sites were blocked with artificially added AI. the CT p value indicated the amount of H+ or OH- required to bring the added Al ions to the form corresponding to the soil ZPC. The consumption ofoh- during the polymerization of added Al can be quantitatively represented in Fig. 2. where titration curves of the T68 with various amounts of Al added are shown. Since the O'p value increased by 2.00 meq for every 2.50 meq (0.833 mmol) of Al added to 100 g of soil. the OB/ Al molar ratio at the STPT-ZPC would be as high as 2.00/0.833=2.40. From T6B, which retained 2.26 meq/loo g of exchangeable AI ions. and to which 5.00 meq (1.67 mmol) of Al ions per 100 g soil were added. only I meq of monomeric Al ions per 100 g soil was dissolved at the STPT-ZPC. that is, 86% of the Al ions added and originally retained were adsorbed onto the soil. In contrast, since there was no Al dissolution from volcanic ash soils even after the addition of AI, it was assumed that the added Al was all adsorbed. HODGES and ZELAZNY (1983) and W ADA (1987a) noted the high affinity of Al polymer ions to variable charge clays or soils. It is obvious also from the results of the Al dissolution experiments that the Al ions added up to a rate of 10.0 meq/ioo g soil could be adsorbed on the soil at the STPT-ZPC. without causing any change in the position of the STPT-ZPC of the soils. 4. AI polymerization on the soil surface Figure 4 gives the titration curves of T7B with four treatments (Na-sat. Na-sat + AI. untreated, and untreated + AI) that were obtained by the f> A T method using II 0.1 N salt solution as the supporting electrolyte (SAKURAI el al 1989b). For the Nll-sat sample, there was no prominent increase in the adsorption of 011- for a ph range from

11 632 K. SAKURAI. A. NAKAYAMA. T. WATANABE. and K. KYUMA 12 OH ; ~6 II E. 4 -Q, :2 "C «I Q 0. 0 H+ / Fig. 4. o Untreated euntreated+ai DNa-sat Na-sat. +AI 6 ph PAT curves of T1B in N NaCI solution. 4.0 to 5.5, as observed for the other samples. This finding indicated that the Al ions were removed effectively by the Na saturation treatment. On the contrary, Al ions, if present, showed a strong buffering capacity to the addition of OH-. Besides, the addition of Al ions at a rate of 5.0 meq/ 100 g soil correspondingly increased the OHadsorption of the Na-sat and the untreated soil samples. Based on the titration curves, it appears that regardless of the nature of the charge of the sites, permanent or variable, the Al ions originally retained by the soils were hydrolyzed in the same way as those added (Fig. 4). Since Al dissolution did not occur at ph values above 4.5, it was inferred that the Al ions, either originally retained or added, would polymerize rather easily even when they were adsorbed onto the soil surface. Recently, WADA (I987a) pointed out that Al ions adsorbed on layer silicates could not be extracted quantitatively with a concentrated solution of a neutral salt (l M KCI) if materials exhibiting variable positive charges coexisted with the layer silicates. Furthermore, he noted that Al released from layer silicates was retained and polymerized on the variable charge materials, and variable charge clays (W ADA 1987b). These observations may be applicable to the strongly weathered soils used here, since the charges originating from Fe or Al oxides and hydroxides are the dominant source of the variable charge. If the Al ions were displaced from permanent charge sites to variable charge sites, a downward shift of the ZPC value should have occurred. Although a downward shift of the ZPC value was observed when exchangeable Al ions were removed from the permanent negative charge sites, Al addition in excess of the permanent charge did not affect the ZPC value. Final salt concentration of NaCI was N in the STPT 7

12 Influences of Aluminum Ions on the Determination of ZPC 633 method and 0.1 N in the PT and PAT methods. These concentrations would be too low to cause Al dissolution at the ZPC of the strongly weathered soils (T6B and T7B). Thus, Al 'ions remained on the permanent charge sites, and therefore. no funher reactions, as described by W ADA (1987a), occurred during the ~pplication of these titration methods. CONCLUSIONS Under the natural vegetation or actual field conditions for crop production. the concentration of the soil solution would not exceed that used in the PT method (0.1 N), whereby Al dissolution lowered the ZPC value. Thus, when the ZPC is evaluated as a practical index of the soil chemical fertility, the STPT method is recommended due to the relatively low salt concentration. Acknowledgments. This study was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (No ). REFERENCES BERSILLON, J.L., Hsu, P.H., and FIESSINGER, F. 1980: Characterization of hydroxy-aluminum.olutions. Soil Sci. Soc. Am. J., GILLMAN, G.P. and UEHARA. G. 1980: Charge characteristics of soils with variable and permanent charge minerals. II. Experimental. Soil Sci. Soc. Am. J., 44, HENDERSHOT, W.H. and LAVKULICH, L.M. 1978: The use of zero point of charse (Z.P.C.) to asi>esi pedogenic development. Soil Sci. Soc. Am. J., HODGES, S.c. and ZELAZNV, L.W. 1983: Influences of OB/ AI ratios and loading rate, on aluminumkaolinite interactions. Soil Sci. Soc. Am. J. 47, Hsu, P.H. 1963: Effect of initial ph, phosphate, and silicate on the determination of aluminum with aluminon. Soil Sci., 96, SAKURAI, K., OHDATE, Y. and KVUMA, K. 1988: Comparison of salt titration and potentiometric titration methods for the determination of zero point of charge (ZPC). Soil Sci. Plant Nutr., 3-1, SAKURAI, K., OHDATE, Y., and KVUMA, K. 1989a: Factors affecting zero point of charge (ZPC) of variable charge soils. Soil ScL Plant Nutr. 35, SAKURAI, K., OHDATE, Y., and KVUMA, K. 1989b: Potentiometric Auto-malic Titration (PAT) method to evaluate zero point of charge (ZPC) of variable charge soils. Soil ScL Plant Nutr VAN RAIJ, B. and PEECH, M. 1972: Electrochemical properties of some oxisols and alfisol of the tropics. Soil Sci. Soc. Am. Proc., 36, WADA, K. and OKAMURA, Y. 1980: Electric charge characteristics of Ando AI and buried AI horilon. soils. J. Soil Sci, WADA, SA. 1987a: Critical evaluation of I M KCI-c:xtraction method for determinina ClI.changeable AI ions in variable charge soils. Soil ScL Plant Nut'., 33, W ADA, S.-I. 1987b: Adsorption of AI(lII) on allophane, imogolite. goethite. and noncry~talline,iliel and the extractability of the adsorbed AI(III) in I M KCI solution. Soil SeL Plant Nutr W ADA, S.-1. and W ADA, K. 1980: Formation, composition and structure of hydro~y-lllumino'i1ic:ue ions. J. Soil Sci, 31,