Clay transformations following a leaching experiment on an acid brown soil

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1 Clay Minerals (1997) 32, Clay transformations following a leaching experiment on an acid brown soil M.-P. TURPAULT, Q. PONETTE, S. BELKACEM AND C. NYS INRA, Cycles Biog~ochimiques, Champenoux, France (Received 20 December 1995; revised 24 July 1996) ABSTRACT: The chemical and mineralogical properties of the A1 and B horizons of an acid brown soil (typic Dystrochrept) were compared before and after a 20 month column leaching experiment. At the end of the leaching period, the organic carbon content of the A1 horizon had decreased by ~20% compared with the original horizon. In the leached A1 horizon, the vermiculite layers of both the vermiculite and interstratified minerals were transformed into high-charge expanding layers, with the charge mainly located in the octahedral sheet. The inhibition of expansion properties by pretreatment with dithionite-citrate-bicarbonate (DCB) was explained by an increase in total layer charge induced by Fe reduction. In the B horizon, leaching resulted in a loss of sesquioxide materials, especially in the interlayers of vermiculites. In both horizons, the amounts of A1 extracted by Na citrate and the amounts of Fe extracted by DCB decreased. Analysis of solution composition showed that A1 mobilization was closely associated with NO3 leaching. Detailed studies on soil processes commonly involve the use of reconstructed soil columns (e.g. James & Riha, 1989; Liu et al., 1990; Smith et al., 1995). This approach reduces the inherent variability associated with field experiments, allows the simultaneous monitoring of solution and solid phases, and limits the interactive effects from concomitant processes. Sample pretreatment (airdrying, sieving, mixing, aeration) (Runge, 1974) together with leaching conditions and laboratory procedures may, however, result in a varying degree of disturbance, which in turn could cast doubt on the extrapolation of results to field conditions. Until now, most reports of disturbance associated with leaching column experiments have focused on soil solution composition, such as increased nitrate leaching (Boutin & Robitaille, 1989; David et al., 1991). Little attention has been given to the chemical evolution of the solid soil phase, particularly the <2 lam fraction. In this paper, the general soil properties (<2 mm fraction) as well as the mineralogy of the clay fraction (<2 I.tm) were determined for the A1 and B horizons of an acid brown forest soil, before and after a leaching experiment with reconstructed soil profiles. An attempt was made to relate the modifications of the soil solid phase to the chemical evolution of the leachates. MATERIALS AND METHODS Soils and sampling site The Oi, Oe, A1 and B horizons were collected from an acid brown soil with a mull humus (typic Dystrochrept; USDA, 1992), located in the French Ardennes (Croix Scaille massif). The parent material of the soil consisted of weathered Revinien shales, covered by a silty loam layer. The vegetation was a coppice with Oak (Quercus petraea Liebl.) standard. Leaching experiment The soils were used in a column experiment in order to study the downward movement of Ca and Mg fertilizers through acid forest soils (Belkacem, 1993; Belkacem & Nys, 1995). The columns were made by packing successively 5600 g of B (4810 g of the <2 mm fraction and 1191 g of the <2 ~tm The Mineralogical Society

2 290 M.-P. Turpault et al. fraction), 1900 g of A1 (1725 g of the <2 mm fraction and 495 g of the <2 gm fraction), 55 g of Oe and 17 g of Oi (105~ oven-dry weights) in polyethylene containers (23.5 cm i.d.), so as to recreate the natural stratification in the field. A quantity of CaCO3 + MgO (46.7% CaO, 11.8% MgO) was applied uniformly to the surface of the columns, without mixing, at rates of 0 (control treatment) and 5.60 t ha -1 equivalent CaO (liming treatment), with four replicates. The columns were installed in an open-air nursery at Nancy, in the northeast of France for a 20 month leaching period. The 800 mm mean annual precipitation at the experimental site was completed by additional local rainfall to give a total application equivalent to 1126 mm year -1 rainfall, close to that of the collecting site in the Ardennes, After eight months leaching, 17 g of Oi layer were added to the top of each column to simulate mean annual litterfall. The original soil horizons which were air-dried and analysed immediately are referred to as materials I. In the following text the samples from the control and limed columns were designated as materials II and III, respectively. Solution analyses The leachates were collected at monthly intervals. After determination of volume, double filtration (Whatman no. 42 filterpaper and 0.45 gm Gelman metricel filter successively) and individual ph measurement with a glass electrode, the four replicates were combined in equal volumes. All solutions were stored at I~ prior to chemical analyses. Total Ca, Mg and AI were determined by inductively coupled plasma (ICP) spectrometry and NO3 was measured colorimetrically on a Technicon II Auto-Analyser. The concentrations of mineral cations and anions were expressed in gmolc 1-1, assuming the following valences: Ca 2+, Mg 2+, AI 3+ and NO3. Soil analyses The soil solid phase from the A1 and B horizons was characterized at the beginning of the experiment and at the end of the leaching period. After air-drying, subsamples from the <2 mm fraction (fine earth) were extracted with Na pyrophosphate (py; McKeague et al., 1971), NH4 oxalate (ox; Tamm, 1922), Na citrate (ci; Tamura, 1958) and dithionite-citrate-bicarbonate (DCB or d; Mehra & Jackson, 1960). The Si, A1 and Fe in the extracts were analysed by ICP-AES. The total iron (Fet) and total aluminium (Alt) contents in the <2 mm fraction were analysed. The exchangeable cations were extracted with 0.5 M NH4C1. The Na, K, Ca, Mg and A1 were determined by ICP-AES. Extractable H and A1 (Hex and Alex) were quantified by titrimetry (Tmby, 1989). Effective cation exchange capacity (ECEC) was calculated as the sum of Na + + K + + Ca z + Mg 2+ + Alex + Hex. The KC1 ph was measured in a 1/2.5 mass/volume ratio. The organic carbon content was estimated from loss on ignition, using the regression equation: (organic carbon content) = (loss on ignition) ; n = 66 and R 2 = (Belkacem & Nys, 1995). The clay fractions (<2 I.tm) were obtained by sedimentation after destruction of organic matter with H202 and dispersion with NaOH. After saturation with Mg, subsamples were treated with DCB or with ci in order to remove short-range ordered minerals, hydroxy-interlayers in 2:1 phyllosilicates and iron oxides. Both extracts (DCB and ci) were analysed for Si, A1 and Fe by ICP-AES. Mineralogical analyses were performed using a Siemens D5000 diffractometer fitted with a graphite monochromator which selected Cu-Ka radiation. Oriented samples for X-ray diffraction (XRD) were prepared on glass slides by sedimentation from untreated, DCB extracted and Na citrate extracted specimens. Further treatments on these samples included Mg-saturation, solvation with ethylene glycol (EG), and K-saturation followed by heating to 110, 220, 330, 440 and 550~ High-charge and low-charge smectite layers were identified from the re-expanding properties of the K-saturated samples after EG solvation. The octahedral vs. tetrahedral charge distribution in the smectite layers was determined from Li-saturated samples which were heated at 300~ for 24 h (Greene-Kelly, 1953) on oriented quartz slides. Effective cation exchange capacity (ECEC) of the clay fraction was obtained from 1 M KCI extraction of the Mg-saturated samples. RESULTS Original soil (materials I) Original A1 and B horizons were strongly acidic, base-poor materials (Table 1). In the <2 mm fraction, exchangeable acidity (Alex + H~x) dominated the exchange complex of each horizon,

3 Clay transformations during leaching 291 TABLE 1. Selected properties of the <2 mm and <2 Ixm fractions. Materials I II IIl Measurements Horizons A 1 B A 1 B A 1 B In KC1 ph Alex (cmolckg-1) the Hex (cmolckg-1) 3.92 tr 1.83 tr 0.19 tr ECEC (cmolckg-1) <2 mm C (g kg -~) Alox(g kg -1) fraction Feox(g kg -1) In ECEC (cmolckg -l) Citrate- Si the bicarbonate- Fe dithionite AI extractable Ti (g kg -1) P <2 I.tm Na citrate Si extractable Fe (g kg -1) A fraction ECEC (cmol~kg -1.) Alci(g kg -1.) Fecb(g kg -1.) I: original materials. II: control columns. III: limed columns. C: organic carbon. Alex, Hex: exchangeable Al, H. ECEC: effective cation exchange capacity. Alo Feox: oxalate extractable AI, Fe. tr: traces. *: kg -1 of the <2 mm fraction, extrapolated from measurement on the clay fraction (ECECclay = ECEC of <2 I.tm fraction * proportion of clay fraction in the <2 mm fraction). representing up to 87% of the ECEC in the B. The KC1 ph ranged from 2.9 in the A1 to 3.9 in the B. For all the extractants, the mobilization of AI and Fe was higher in the B horizon. In both horizons, the total Fe pool was largely dominated by DCBextractable forms, which indicated a very low fraction of silicate-bound Fe (Jeanroy, 1983). In this soil, the distribution of Fe forms was approximately 15% in silicates, 55% in crystalline oxides and 30% in short-range ordered inorganic or organo-metallic phases. In these short-range ordered phases, Fe may be associated with Si and A1 because large amounts of these elements were generally co-extracted. The ECEC of the clay fraction (<2 ~m) was 38.4 and 19.4 cmolc kg -1 in the AI and B horizons, respectively (Table 1). The amounts of A1 and Fe extracted by DCB and Na citrate were higher in the B horizon, indicating large amounts of short-range ordered minerals and Fe oxides in this horizon. The XRD diagrams exhibited mica (1.00; 0.500; nm), vermiculite (1.42 nm), interstratified mica-vermiculite (2.40; 1.20 nm) and kaolinite (0.717; nm) reflections. Only traces of quartz (0.424; nm) and feldspars (0.324 and nm) were detected (Fig. la-d). The XRD diagram of the clay fraction random powder (not shown) exhibited intense reflections at and nm associated with dioctahedral minerals; the small peak at nm may be due to the 211 quartz reflection but also to trioctahedral phyllosilicates. The lack of any swelling after EG solvation indicated that there were no smectite layers. As shown by heating of the K-saturated clays (Fig. le), the collapse of interlayers was progressive and incomplete for all the samples even at ~ The stronger asymmetry of the peak at 1.00 nm towards the low 20 angles in the B horizon compared with the A1, indicated larger amounts of hydroxyinterlayers in vermiculite and interstratified mica-

4 i 292 M.-P. Turpault et al. I II III ~ K3 K4 K5 K K1 K2 K3 K4 K5 em oq ee~ r t-- / tt3 B B e I r r f d C b a d B B ~-C ~ o20 ~ FIG. 1. X-ray diffractograms of the A1 and B horizons (<2 Ixm fraction). (I) original material; (II) control columns; 01D limed columns. (a-d) Mg-saturation (a,b) without pretreatment; (c,d) after Na citrate extraction; (b,d) after ethylene glycol solvation, (e) K-saturation (K) at room temperature; (Kl) at ll0~ (K2) at 220~ (K3) at 330~ (K4) at 440~ (K5) at 550~ spacings in rim; Cu-Kct radiation,

5 Clay transformations during leaching 293 vermiculite minerals, in agreement with DCB and Na citrate extract data. After DCB (not shown) or Na citrate treatments, the vermiculite reflection (1.42 nm) was clearer and the interstratified micavermiculite (2.40 and 1.20 nm) reflections were more clearly defined (Fig. lc-d). According to the peak positions (2.40 and 1.20 nm), the interstratified micavermiculite contained 50% mica layers (Reynolds, 1980). Compared with those of the B horizon, the XRD diagrams of the A1 horizon displayed more intense peaks at 1.20 and 1.40 nm. Soil evolution following leaching (materials H and III) Leaching induced various modifications in the chemical properties of the <2 mm fraction, The organic carbon content of the A1 horizon decreased for both treatments (Table 1). The rise in KC1 ph in the A1 horizon of the limed columns was associated with a strong decrease in extractable acidity, while the ECEC more than doubled. Both exchangeable AI and the ECEC of the B horizon increased. The amounts of Fe extracted by DCB were slightly reduced in both horizons. The decrease in mobilization of A1 following leaching, if any, was greater in the limed columns. After 20 months of leaching, the mineralogy of the clay fraction (<2 lam) was modified. As for the <2 mm fraction, the ECEC of the <2 p.m fraction increased in the B horizon. In the A1 horizon, the ECEC remained nearly constant (Table 1). The amounts of Fe and A1 extracted by DCB decreased strongly. The amounts of AI extracted by Na citrate decreased in both horizons while Fe amounts only decreased in the A1 horizon. Leaching had little influence on the amounts of Si extracted by Na citrate. Leaching mostly affected the clay mineralogy of the A1 horizons, with little difference between materials II and III. After EG solvation of the Mg saturated samples, the peak at 1.40 nm partly shifted to 1.70 nm, and the peaks at 2.40 and 1.20 nm moved to 2.70 and 1.35, respectively (Fig. lb). Similar XRD diagrams were obtained for Na citrate specimens; in this case, however, the peak at 1.40 nm moved completely to 1.70 nm (Fig. ld; Fig. 2c,f). These observations indicate that all the vermiculite layers in both the vermiculite and interstratified minerals were transformed into expanding layers. After K-saturation, heating to ll0~ and EG solvation, the smectite-like layer did not re-expand which is the characteristic pattern for high-charge layers (not shown). The Greene-Kelly (1953) test performed on the A1 horizon from material III clearly indicated that the expanding layers were essentially montmorillonitic (Fig. 2g). Contrary to Na citrate treatment, DCB-extracted specimens did not swell after EG solvation (Fig. 2d). Heating the K- saturated clays from the A1 horizon revealed the complete disappearance of the asymmetry of the 1.00 nm peak towards the low 20 angles (Fig. le). Despite the decrease in the amounts of A1 and Fe extracted by DCB and Na citrate treatments, the ECEC values remained nearly constant (Table 1). For the B horizon, the asymmetry of the 1.00 nm peak towards the low 20 angles either decreased (simple K-saturation) or disappeared completely (K-saturation following Na citrate extraction -- not shown). These observations together with the decrease in the amounts of A1 extracted by Na citrate (Table 1) indicated losses of amorphous materials, especially hydroxy-a1 interlayers. This conclusion was further supported by the XRD patterns of materials II (Fig. ld): after EG solvation of Na citrate treated specimens, the 1.40 nm peak moved partially to 1.70 nm. These mineralogical transformations were associated with a large increase of ECEC, from 19.4 cmolc kg -1 in the original B horizon, to 28.6 and 34.8 in materials II and III, respectively. Comparison of the <2 I, tm and the <2 mm fractions There was generally excellent agreement between ECEC extrapolated from measurement of the clay fraction (<2 ~tm) and that derived from direct measurement of the fine earth fraction (<2 mm) as shown in Table 1. This suggests that the contribution of organic matter to ECEC was minor in most cases. The only exception was the A1 horizon from the limed columns, in which measured ECEC of the fine earth was about three times higher. This can be attributed to the enhanced dissociation of organic matter functional groups on rising ph, as well as to a possible dissolution of residual lime particles during the extraction. Soil solution chemistry during experiment As shown in Fig. 3a, leaching of AI from the control columns was concomitant with that of NO3 anions. This suggests that the mobilization of A1

6 294 M.-P. Turpault et al. A1 I o 7 A1 II w. A1 III 1 o? e,r,~" ~.rq c5 [ t ;02, g a w ~ cimg ]l ~ li? 2 ~ 1'0 2 1'0 ~ FIG. 2. X-ray diffractograms of the <2 gm fraction from A1 horizons after ethylene glycol solvation. (I) Mgsaturated specimens from original material (a) without or (b) with oxidation by H202 ; (II) Mg-saturated specimens from control column, (c) after Na citrate treatment, (d) after dithionite-citrate-bicarbonate treatment, (e) after Na citrate treatment and oxidation by H202; (III) Na citrate treated specimens from limed columns (f) after Mg-saturation or (g) after Li-saturation and heating at 300~ spacings in nm; Cu-Kc~ radiation. from these columns was, at least partly, affected by nitrification. Indeed, the protons arising from the oxidation of NH~ ions to NO;- may be partly consumed in dissolution or exchange reactions. The released ions may then be co-eluted with anions, when not immobilized. The same process was thought to be active in the limed columns (Fig. 3b). In this case, however, the decreased mobilization of A1 from about the seventh month of leaching relative to the control was accompanied by increased leaching of Mg. The Ca concentrations (Fig. 3) in the leachates remained comparable in both treatments, while ph values tended to be slightly higher after liming (Fig. 4). The limited mobility of Ca compared with Mg ions after liming can be attributed primarily to the greater preference of organic exchanges for Ca, relative to Mg (Salmon, 1964). For the two treatments, a second transport process for A1 could be chelation with organic molecules. Without any collection of solutions below the A1 horizon, it cannot be established whether the decrease in the organic carbon content in this horizon (Table 1) originated from organic leaching in solution, from CO2 release, or from both processes. DISCUSSION AND CONCLUSIONS 2:1 layer silicate evolution in the A1 horizon As indicated by XRD diagrams, vermiculite minerals and interstratified mica-vermiculite minerals were found in the original AI horizon. Following leaching, the vermiculite layers were partially transformed into expanding layers. The

7 (a) CONTROL NO 3 Clay transformations during leaching 4.50 [ 295 +ooo ;\...: :; +,ooo. ~.2ooo- / / \\ ~-o ' ~ 3.75 ' r Control o ~.so. ~ Ume i 5000 " (b) LIME MA M J J A S O N D J F M A M J J A S Time (months) ? J FIG. 4. Temporal variation of ph in the leachates from the control (O) and limed (O) columns. o E 2000" \ j, /,,% ' )i" ' o "'n, " " - " 1 0,,,,,,,,,,,,,,,,,, 150t(c) INFILTRATION /~I -+ I / +,+, F M A M J J A S O N D J F M A M J J AS Time (months) FIG. 3. Temporal variation of NO3 (O), A1 (O), Mg (A) and Ca (I) concentrations in the leachates from the control (a) or limed (b) columns. lack of re-expansion after K-saturation and EG solvation (not shown) indicated a layer charge greater than 0.43 per formula unit (Schultz, 1969). Finally, the Greene-Kelly test showed that the charge was mainly localized in the octahedral sheet. It remains to be determined whether the difference between the expansion properties of the original and leached A1 materials is connected with a major mineralogical change. Because we did not use the alkylammonium ion exchange technique (Lagaly & Weiss, 1969), the total layer charge of 2:1 phyllosilicates was not determined. Malla & Douglas (1987a) demonstrated that the boundary of 0.6 for total layer charge cannot be inferred from the expansion properties following EG or glycerol solvation. According to these researchers, the reliable test to differentiate smectite from vermiculite is the re-expansion of K-saturated clays following heating to 300~ and glycerol solvation. After performing this test on the leached samples, the peak at 1.00 nm did not move, indicating a total layer charge >0.6 (Malla & Douglas, 1987a). Malla & Douglas (1987b) also indicated that the expansion properties depended on the distribution of charge between octahedral and tetrahedral sheets; for a similar total layer charge, a vermiculite with predominantly octahedral charge forms two-layer glycerol complexes more easily. From the above discussion, it follows that two processes could be associated with the vermiculite transformation during this experiment: (i) a total layer charge decrease and/or (ii) a relative octahedral charge increase. Contrary to Na citrate extraction, DCB treatment prevented the phyllosilicate swelling for the leached AI horizons. A possible explanation would be the reduction of Fe 3+ to Fe 2+ following DCB, resulting in an increased total layer charge (Stucki et al., 1984). This hypothesis was confirmed by the partial recovery of the re-expansion properties after the oxidation (H202, for two days) of DCB-treated clays (Fig. 2e). Conversely, Fe oxidation could explain a total charge decrease from the leached A1 horizons compared with the original material. This process alone was, however, apparently not sufficient;

8 296 M.-P. Turpault et al. oxidation of the original A1 horizon with H202, whether Mg-saturated or extracted by Na citrate did not result in any phyllosilicate swelling (Fig. 2b). With respect to a possible charge redistribution between tetrahedral and octahedral sheets, only the Greene-Kelly test followed by n-alkylammonium ion exchange (Malla & Douglas, 1987b) could verify the hypothesis. Soil processes Chemical and mineralogical transformations in the A1 layer of the leached materials were close to those found in surface horizons of podzols (Righi et al., 1988). In both cases, considerable amounts of A1 and Fe are removed from amorphous phases and interlayered polycations, expandable high-charge clay minerals are found (Gjems, 1967). In podzols, the expandable minerals called 'transformation smectites' (Wilson, 1987) are identified as beidellites (Douglas, 1982) or montrnorillonite (Righi & Meunier, 1991). In the present case, however, the total layer charge of the expandable minerals was found mainly in the octahedral sheets. The lack of any major difference between control and limed columns suggests that mineralogical changes were primarily induced by soil pretreatment (drying, sieving, aeration, mixing) and leaching procedures (temperature, light, humidity etc. Biological activity was greatly stimulated, which resulted in strong C and N mineralization. The extent of the transformations, as well as their rapid kinetics (20 months), implies that special attention must be given to the design of laboratory studies, particularly if the results are to be extrapolated to the field. ACKNOWLEDGMENTS The authors are grateful to P. Bonnaud for technical assistance, D. Righi and three reviewers for useful comments on the manuscript and an English colleague for reviewing the text. REFERENCES Belkacem S. (1993) Etude de la resaturation des sols acides soumis it de forts apports acides: effet des formes et doses d'amendements sur le fonctionnement d'un sol acide forestier. Thbse, Univ. Nancy I, France. Belkacem S. & Nys C. (1995) Consequences of liming and gypsum top-dressing on nitrogen and carbon dynamics in acid forest soils with different humus forms. Plant Soil 173, Boutin R. & Robitaille G. (1989) Effets de l'acidification in vitro d'un podzol sur la chimie des percolats et des horizons. Information report LAU-X-91, Forestry Canada, Quebec region, Laurentian Forestry Centre. David M.B., Vance G.F. & Fasth W.J. (1991) Forest soil response to acid and gait additions of sulfate: II aluminum and base cations. Soil Sci. 151, Douglas M.L. (1982) Smectites in acidic soils. Proc. Int. Clay Conf, Bologna & Pavia, Gjems O. (1967) Studies on clay minerals and clay minerals formation in soil profiles in Scandinavia. Med. Nor. Skogsforsoksvesen, 21, Greene-Kelly R. (1953) Irreversible dehydratation in montmorillonite. Clay Miner. Bull. 2, James B.R. & Riha S.J. (1989) Aluminum leaching by mineral acids in forest soils: I. Nitric-sulfuric acid differences. Soil Sci. Soc. Am. J. 53, Jeanroy E. (1983) Diagnostic des formes du fer dans les pddogenkses tempdr~es. Evaluation par les rgact~fs chimiques d'extraction et apports de la spectromdtrie Mossbauer. Th6se, Univ. Nancy I, France. Lagaly G. & Weiss A. (1969) Determination of layer charge in mica-type layer silicates. Proc. Int. Clay Conf Tokyo, Liu K.H., Mansell R.S. & Rhue R.D. (1990) Cation removal during application of acid solutions into airdry soil columns. Soil Sci. Soc. Am. J. 54, Malla P.B. & Douglas L.A. (1987a) Identification of expanding layer silicates: layer charge vs. expansion properties. Proc. Int. Clay Conf, Denver, Malla P.B. & Douglas L.A. (1987b) Layer charge properties of smectites and vermiculites: tetrahedral vs. octahedral. Soil Sci. Soc. Am. J. 51, McKeague J.C., Brydon J.E. & Miles N.M. (1971) Differentiation of extractable iron and aluminum in soils. Soil Sci. Soc. Amer. Proc. 35, Mehra O.P. & Jackson M.L. (1960) Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Clays Clay Miner. 7, Reynolds R.C. (1980) Interstratified clay minerals. Pp, in: Crystal Structures of Clay Minerals and their X-ray Identification (G.W. Brindley & G. Brown, editors) Miner. Soc., London. Righi D. & Meunier A. (1991) Characterization and genetic interpretation of clays in an acid brown soil (dystrochrept) developed in a granitic saprolite. Clays Clay Miner. 5, Righi D., Ranger J. & Robert M. (1988) Clay minerals as indicators of some soil forming processes in the temperate zone. Bull. Mineral. 111,

9 Clay transformations during leaching 297 Runge M. (1974) Die Stickstoff-Mineralisation im Boden eines Sauerhumus-Buchenwaldes: I. Mineralstickstoff-Gehalt und Netto-Mineralisation. Oecol. Plant. 9, Salmon R.C. (1964) Cation-activity ratios in equilibrium soil solutions and the availability of magnesium. Soil Sci. 98, Schultz L.G. (1969) Lithium and potassium absorption, dehydroxylation temperature, and structural water, content of aluminous smectites. Clays Clay Miner. 17, Smith C.J., Goh K.M., Bond W,J. & Freney J.R. (1995) Effects of organic and inorganic calcium compounds on soil-solution ph and aluminium concentration. E. J. Soil Sci. 46, Stucki J.W., Low P.F., Roth C.B. & Golden D.C. (1984) Effects of oxidation state of octahedral iron on clay swelling. Clays Clay Miner. 32, Tamm O. (1922) Um best amning ow de oorganiska kompenterna i markens gelcomplex. Meddn. Starens SkogsfOkAnst, 19, Tamura T. (1958) Identification of clays minerals from acid soils. J. Soil Sci. 9, Trtiby P. (1989) Eine Titrationsmethode zur simultanen Bestimmung von H und Aluminium in NH4C1 Bodenextrakten. Z. Pflanzenerngihr. Bodenk. 152, USDA (1992) Keys to Soil Taxonomy. 5th edn. Technical Monograph 19, Soil Management Support Service. Pocahontas Press, Blacksburg, VA. Wilson MJ. (1987) Soil smectites and related interstratified minerals: recent developments. Proc. Int. Clay Conf., Denver,