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Soil Science Society of America Journal 63:1116-1123 (1999)
© 1999 Soil Science Society of America

DIVISION S-2-SOIL CHEMISTRY

Solubility of Aluminum and Silica in Spodic Horizons as Affected by Drying and Freezing

M. Simonssona, D. Berggrena and J.P. Gustafssonb

a Dep. of Soil Sciences, Swedish Univ. of Agricultural Sciences (SLU), Box 7014, SE-750 07, Uppsala, Sweden
b Division of Land and Water Resources, Royal Inst. of Technology (KTH), SE-100 44, Stockholm, Sweden

magnus.simonsson{at}mv.slu.se


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
For convenience, soil samples are often dried before storage and experimental use. However, the literature offers examples of drying that results in changes in pH, solubility of organic matter, and dissolution rates of Al. In this study we examined the solubility of Al and Si in fresh soil and in soil that had been dried or deep-frozen. Five Spodosol B horizon soils were subjected to batch titrations, where portions of each soil were equilibrated with solutions with varying concentrations of acid or base added. Extractions with acid oxalate and Na pyrophosphate indicated the presence of imogolite-type materials (ITM) in three of the soils. In the other two soils most secondary solid-phase Al was associated with humic substances. Deep-freezing did not significantly change pH nor the concentration of Al or Si as compared with fresh soil. Drying, on the other hand, yielded pH increases of up to 0.3 units at a given addition of acid or base, whereas Al3+ changed only slightly, implying a higher Al solubility in all of the soils. Furthermore, dissolved silica increased by up to 200% after drying, except in a soil that almost completely lacked oxalate-extractable Si. We suggest that drying enhanced the dissolution of ITM by disrupting soil organic matter, thus exposing formerly coated mineral surfaces. In the soil where dissolved Si did not change with drying, it had been demonstrated that Al–humus complexes controlled Al solubility. We suggest that fissures in the organic material caused by drying may have exposed formerly occluded binding sites that had a higher Al saturation than had sites at the surface of humus particles.

Abbreviations: AAS, atomic absorption spectometry • ANOVA, analysis of variance • FIA, flow injection analysis • IAP, ion-activity products • ITM, imogolite-type materials


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
THE RELEASE OF TOXIC AL3+ is one of the most serious consequences of anthropogenic soil acidification (Cronan and Grigal, 1995; Havas and Rosseland, 1995). Therefore, the mechanisms controlling Al solubility have been a topic of intense research for more than a decade. The Spodosol B horizon offers a special challenge to soil chemists dedicated to Al since it contains inorganic Al precipitates, organic matter, and Al associated with humic substances in the solid phase. All of these are secondary materials that may control the solubility of Al. As a result, much effort has focused on this particular horizon. Many of these studies have been performed using soil materials brought to the laboratory. Although some researchers have performed their experiments on field-moist samples, a majority dried their samples before the experiments (Mulder et al., 1989; Reuss et al., 1990; Dahlgren and Walker, 1993; LaZerte and Findeis, 1994; Simonsson and Berggren, 1998). Freezing was less frequently used as a sample storage method, but it has been used; e.g., Berggren and Mulder (1995). Although some reports indicate altered surface reactivity, particularly as a consequence of drying, little attention has been paid to any possible effects on Al solubility that are due to sample pretreatment.

Drying has been reported to depress soil pH (Courchesne et al., 1995), although the opposite effect has sometimes been found (Bartlett and James, 1980; Payne and Rechcigl, 1989). Drying may also alter the properties of the organic matter. For instance, enhanced solubility of organic matter, especially in the fraction with a relatively low molecular weight (<3500 g mol-1), has been reported (Homann and Grigal, 1992). These changes may be due to organic structures being torn apart by the physical stress induced by the desiccation (Bartlett and James, 1980). It has been suggested that a decreased pH may be related to the exposure of new acidic groups that is owing to ruptures in the organic matrix, or it may be related to the partial oxidation of the organic matter that results in the formation of new acidic groups (Raveh and Avnimelech, 1978).

The sorption capacity for phosphate increased after drying in a study on Fluvisols, Cambisols, and Spodosols (Haynes and Swift, 1985). Sulfate adsorption may also increase after drying (Comfort et al., 1991). Since sulfate adsorption takes place on oxide surfaces rather than in soil organic matter (Karltun and Gustafsson, 1993), this indicates an increased reactivity of mineral surfaces in the soil after drying. Freezing, on the other hand, caused no significant change in sulfate adsorption in the study of Comfort et al. (1991).

Because drying may alter the reactivity of both soil organic matter and mineral phases, an effect on Al solubility can be expected. In an experiment where a Bs horizon soil was subjected to acid leaching (Dahlgren and Walker, 1993), drying increased the initial dissolution rate of monomeric (mainly inorganic) Al by 9%, although after prolonged leaching the dissolution rate approached that of undried soil. A similar trend was observed for dissolved organic C by Berggren et al. (1998). The concentration of dissolved organic C was increased by freeze-drying, but it approached the concentrations of undried soil after repeated acid leachings. Freezing also may affect the solubility of soil organic matter. Ross and Bartlett (1990) obtained enhanced concentrations of dissolved organic C and total dissolved Al after freezing and thawing Spodosol Oa and Bhs soils, although the authors did not report any estimates of inorganic monomeric Al.

To our knowledge, the literature offers no systematic study on what effects sample pretreatment gives regarding the solubility of Al and Si under conditions reasonably similar to those of the soil solution. Based on the literature reviewed, it seems reasonable to hypothesize that drying and/or freezing will affect Al solubility. The objective of the present paper was to investigate the effect of drying and freezing (-20°C) on the solubility of Al and Si in spodic B horizons. We used five samples, of which three had been used in previous research on Al solubility. The solid phases controlling the solubility of Al included (i) Al complexes with soil organic matter, and (ii) inorganic phases like Al(OH)3 or imogolite-type materials (Berggren and Mulder, 1995; Berggren et al., 1998; Gustafsson et al., 1998). The solubilities of Al and Si, as a result of sample pretreatment, were investigated in batch-wise acid–base titrations. Secondary solid phases were characterized by extractions with Na pyrophosphate and acid oxalate.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Soils
Soil samples were taken in spodic B horizons of Spodosols at three sites in southern and central Sweden. All soils were developed in quaternary glacial till. Granitic minerals dominate the parent materials of Sknes Värsjö (Olsson and Melkerud, 1989) and Strsan (Johnsson, unpublished data, 1989). Lövkullen Bs2 contained imogolite-type materials (ITM) as shown by infrared spectroscopy and transmission electron microscopy (Gustafsson et al., 1999). The Sknes Värsjö profile (56°40'N, 13°30'E) is in a region of considerable industrial acidification and intense acid leaching (total deposition of N and S is 25 kg ha-1 yr-1 each), whereas the soils of Strsan (60°55'N, 16°01'E) and Lövkullen (60°13'N, 14°35'E) are less affected (total deposition of N is 7 and of S, 5 kg ha-1 yr-1). Deposition data are those of Lövblad et al. (1995).

Sample Pretreatment
Soils referred to as fresh were sieved moist (<2 mm), put in polyethylene bags, and kept at 2°C in a cold-storage room until the time of the experiment (batch titrations discussed below), which were started within 9 to 18 d after the sampling. Dried soils were obtained by leaving the samples in a vigorously ventilated drying-room for 1 to 4 d. Within that time the samples attained dry-matter percentages >95% on an oven-dry (105°C) basis. The temperature of the drying-room was 30 to 35°C; i.e., <40°C, which is in accordance with the recommendations of the International Organization for Standardization (ISO, 1994). Dried samples were sieved (<2 mm) and stored in plastic containers at room temperature (normally 22°C) until batch titration, four months later. For Lövkullen Bs2 the fresh sample was stored at 2°C for 5 mo before use, and the dried sample was used on the same day that drying was finished. In all soils but the Lövkullen Bs2, we also made titrations on samples that had been frozen. They were prepared by leaving portions of the sieved field-moist soils (100–200 g fresh wt) in a freezing-room (-18 to -20°C), where they were stored for 6 mo. They were then thawed during two days at 2°C and immediately used for batch titrations. We observed no consistent change in water content that was due to storage in the freezing-room.

Batch Titrations
For the batch titrations, we put moist or dried soil corresponding to 2.0 g dry wt (<2 mm) into 35-mL polypropylene centrifuge tubes with screw cappings and added 20 mL of solution containing HCl, NaOH, or only the background solute, NaCl, which was added so that a final ionic strength of 20 ± 3.4 mM was obtained. We used the titrations to see how drying or freezing affected the reactivity of the solids, rather than to simulate any liming or any acidifying deposition of HNO3 or H2SO4. We therefore chose HCl and NaOH as titrants and used NaCl as a background solute since Na+ and Cl- cause relatively small side effects related to soil–microbial activity or sorption processes. A scheme of the solutions added is shown in Table 1 . Fresh Lövkullen Bs2 samples were run in duplicate; dry Lövkullen Bs2 and all of the other soils were not replicated. The suspensions were placed horizontally in a reciprocating shaker equipped with a water bath thermostatted at 8.0°C and shaken at a rate of 60 turns per minute (amplitude 2.5 cm). We chose a temperature of 8.0°C, rather than room temperature, to avoid triggering any excessively high microbial activity during the equilibration. This is about 2 to 5° warmer than the annual mean air temperature at the sites. The Strsan and Sknes Värsjö samples were shaken for 5 d, which we had shown to be sufficient for the most reactive Al phases to equilibrate; i.e., maximum dissolved inorganic-Al concentrations were attained within that time (Berggren and Mulder, 1995; Simonsson and Berggren, 1998). Lövkullen Bs2 was shaken for 13 d since ITM, which was present in this soil, reacts more slowly than does, for instance, Al(OH)3 (Dahlgren et al., 1989; Gustafsson et al., 1998).


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  		Table 1 Solutions added to the soils in the batch titrations. In all cases, soil corresponding to 2.0 g of dry matter (<2 mm) was used

 



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  		Fig. 3 Titration curves with fresh, frozen, and dried soils. Plots of added base - acid (OH- - H+) vs. free H+, dissolved Alqr, base cations other than Na+, and monomeric silica; 1 mM of acid or base corresponded to {approx}1 cmolc kg-1. Drying caused substantial changes in dissolved H+ and Si

 
After equilibration, we centrifuged the samples for 30 min at 16000 g. The pH was measured at 8°C with a combined glass electrode (GK2401C, Radiometer, Copenhagen). The remainder of each solution was membrane-filtered [0.2-µm Gelman Sciences (Ann Arbor, MI) Acrodisc PF polyethersulfone- or Sartorius (Göttingen, Germany) Minisart N cellulose acetate filters]. Aluminum was measured using flow injection analysis (FIA) according to Clarke et al. (1992). In this method the sample is allowed to react with oxine (8-hydroxyquinoline) for 2.3 s. The Al trioxinate formed is then extracted into chloroform and determined spectrophotometrically. Before analysis, all solutions were kept at 8°C. The Al fraction measured is referred to as quickly reacting Al, Alqr, and includes inorganic monomeric Al complexes with the exception of those with fluoride. Clarke et al. (1995) showed that virtually all Al complexes with dissolved humic substances are excluded from Alqr. Where Alqr exceeded 2 mg L-1, we modified the analysis as described by Berggren and Sparén (1996). Molybdate-reactive silica was measured at 22°C with a FIA method modified from Thomsen et al. (1983), using stannous chloride as the reducing agent (Tecator, 1992). We assumed that the fraction measured consisted of orthosilicic acid and its monomeric complex with Al. Concentrations of Ca, Mg, K, and Na were measured with flame atomic absorption spectrometry (AAS) (IL 551, Instrumentation Laboratory, Wilmington, MA).

Activities of Al3+ were calculated with our computer program, SpeciAl. Input variables to SpeciAl were temperature, pH, and concentrations of Alqr, Si, Ca, Mg, K, Na, and Cl. The program adjusted the constants for temperature using van't Hoff's equation, and it calculated activity coefficients of charged species according to the Davies approximation. The uncharged species H4SiO4 was assigned an activity coefficient of unity. The solubility of Al and Si was evaluated in terms of logarithmic (base 10) ion-activity products (IAP) with respect to (i) gibbsite, ; and (ii) imogolite, . Table 2 lists chemical reactions and thermodynamic data used for speciation.


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  		Table 2 Equilibrium constants (log10 values) and standard enthalpies of reactions used in computations{dagger}{dagger}{ddagger}{ddagger}§§

 
We did not measure anions in the present experiment, so we used total added Cl as the only anion concentration and assumed that it did not react with the soil. However, in supplementary equilibrations (Titration steps no. 5 and 9 of all dried soils, and of fresh Sknes Värsjö Bh and Bs1) sulfate was measured (2000i/SP ion chromatographer, Dionex, Sunnyvale, CA). Dissolved SO2-4 never exceeded 0.15 mM, leaving at least 97% of Alqr uncomplexed with SO2-4.

Solid-Phase Characterization
Secondary solid-phase constituents were determined by separate extractions using (i) 0.2 M ammonium oxalate–oxalic acid, pH 3.0 (Alo, Feo, Sio), and (ii) 0.1 M sodium pyrophosphate, pH 10 (Alp, Cp, Fep) (Reeuwijk, 1993). All extractions were performed on dried (<2 mm) soils. Pyrophosphate efficiently extracts those Al forms that are associated with organic matter. Oxalate dissolves noncrystalline minerals of Al, Fe, and Si, and in addition, Al complex-bound by organic matter. The difference between Al extractable with oxalate and pyrophosphate (Alo - Alp) may be used to determine Al of inorganic secondary phases (Parfitt and Childs, 1988). Silicon in oxalate-extracts, Sio, generally originates mainly from imogolite-type materials (ITM), and the ratio of (Alo - Alp)/Sio obtained in soils with secondary Al dominated by ITM ideally is close to 2.0 (Dahlgren, 1994). We used plastic centrifuge bottles for the oxalate extractions and glass bottles with a silicone stopper for the pyrophosphate extractions. After extraction, the samples were centrifuged at 1100 g for 30 min and filtered through 0.45-µm membrane filters (ME 25, Schleicher & Schuell, Dassel, Germany). Aluminum, iron, and silicon in oxalate extracts (Alo, Feo, and Sio) were measured by inductively coupled plasma atomic emission spectrometry, (JY 24, Jobin Yvon Instruments, Longjumeau, France). In the pyrophosphate extracts, Al and iron (Alp and Fep) were measured by flame AAS (IL 551); organic carbon (Cp) was determined using a total organic carbon analyzer (TOC-5000A, Shimadzu, Duisburg, Germany). In most soils we also measured the loss on ignition by heating to 600°C.

Statistical Analysis
An analysis of variance (ANOVA) was made with the results of the batch titrations to test differences obtained between (i) frozen and fresh, and (ii) dried and fresh soils. All soils except Lövkullen Bs2 were subjected to a three-way ANOVA with the factors soil (four individual horizons), pretreatment (fresh, frozen, dry), and acid–base addition (Titration steps no. 1–4 were excluded to obtain balance because they were not applied to all soils). Since the titration points were not replicated, the three-factor interaction (soil x pretreatment x acid–base addition) was used as error term. Lövkullen Bs2, in which fresh samples were duplicated, was subjected to a two-way ANOVA with the factors pretreatment (fresh and dry) and acid–base addition (only no. 1–8, which were duplicated in the fresh soil). Based on the assumption that errors in the concentrations were proportional rather than absolute, the ANOVA was performed with logarithmic variables.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Effects of Soil Pretreatments
The solubility of Al with respect to solid-phase Al(OH)3 (in terms of IAPgibbsite) increased significantly (P < 0.001) in response to drying. This was shown both by (i) the overall ANOVA performed with the soils of Sknes Värsjö and Strsan, and (ii) the separate ANOVA made with Lövkullen Bs2 only. As can be seen in Fig. 1 , drying resulted in an increased Al solubility (greater log{Al3+} at any given pH) in all soils. In the Bs soils, this was mainly an effect of the pH, which for a particular acid–base addition increased on drying by up to 0.31 units, whereas log{Al3+} was only slightly affected (except when base was added). By contrast, freezing did not significantly change Al solubility. The effects of soil pretreatment on the solubility of Al relative to Al(OH)3 was paralleled by the effects on the solubility of Al and Si relative to imogolite. Drying yielded a significant (P < 0.001) increase in IAPimogolite, whereas freezing resulted in no significant change. Lövkullen Bs2, which has a documented occurrence of ITM, and Strsan Bs1 appeared to be undersaturated in their fresh states and close to equilibrium with imogolite after drying (Fig. 2) . In all the soils except Sknes Värsjö Bh, dissolved Si increased markedly as a result of drying (Fig. 3) . However, the increase in pH accounted for up to 90% of the increase in IAPimogolite, whereas log{H4SiO4} accounted for between 15 and 50%. Although significant changes in dissolved base cations did occur, they were small; generally, the effect of drying was not greater than that of freezing (Fig. 3).



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  		Fig. 1 Solubility of Al relative to gibbsite (Al(OH)3) in fresh, frozen, and dried soils; logAl is log10 activity of dissolved Al3+. Broken lines indicate theoretical equilibrium with respect to crystalline gibbsite and microcrystalline gibbsite . Arrows indicate no addition of acid or base. Al solubility increased as a result of drying the soils

 


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  		Fig. 2 Solubility of Al and Si relative to imogolite (Al2SiO3(OH)4) in the Bs horizons investigated; logSi is log10 activity of dissolved H4SiO4. The broken lines indicate equilibrium with respect to imogolite . Arrows indicate no addition of acid or base

 
Acid Neutralization and Aluminum Solubility Control
In the lower (Bs) horizons, inorganic solid-phase Al was relatively abundant, according to (Alo - Alp) (Table 3) , as was oxalate-extractable silica Si (Sio). Furthermore, the Bs horizons were in the range of equilibrium with gibbsite (Al(OH)3) (Fig. 1); that is, the control of Al solubility by equilibrium with secondary mineral phases was possible. A ratio of (Alo - Alp)/Sio close to 2.0 indicated the presence of ITM in all Bs horizons of this study (Table 3). However, Gustafsson et al. (1998) demonstrated that the short-term solubility control of Al may be mediated by the precipitation of Al(OH)3, rather than by the dissolution of imogolite only. The buffering of the Bs soils was dominated by the release of Alqr, which (summed up as 3 + 2 + + 2) balanced between 66 and 100% of the H+ added in the most acid batch of each soil (Fig. 3).


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  		Table 3 Classification, water content (fresh), loss on ignition, pH, and contents of oxalate- and pyrophosphate-extractable (index o and p) Al, Fe, C, and Si

 
By contrast, Sknes Värsjö Bh and most batches of Strsan Bhs (all batches except fresh soil with no acid added) were undersaturated with respect to gibbsite. These horizons (Bh, Bhs) also had much less inorganic Al than the underlying Bs. Furthermore, Alp was much greater than (Alo - Alp) (Table 3), indicating that most Al was associated with organic matter. The lower gibbsite line of Fig. 1 refers to a macrocrystalline phase with low solubility (Palmer and Wesolowski, 1992), so points lying below the line clearly indicate undersaturation with respect to any solid Al(OH)3 phase, making Al solubility control by complexation with solid-phase organic matter probable. The best example of this was Sknes Värsjö Bh, with a low pH (3.76 in 0.02 M NaCl) and a strong undersaturation with respect to gibbsite throughout the titration curve. In this soil, the likeliness of Al solubility control by complexation to soil organic matter was demonstrated by Tipping et al. (1995) and Berggren and Mulder (1995), who obtained satisfactory fits with solubility data using the mechanistic Windermere humic aqueous complexation model (WHAM, Tipping, 1994). Although Al buffering also dominated in Sknes Värsjö Bh, its low Al solubility resulted in up to 39% of the added acid remaining as free H+ in the solution of the most acid batch.

Enhanced Solid-Phase Reactivity
The increased pH observed after drying was the result of an enhanced ability of the soils to neutralize added or inherent acidity. This can be explained by the increased solubility of Al, which we observed in all soils after drying. Also, most dried soils had much more dissolved Si (up to 200% increase) compared to the fresh soils (Fig. 3). This indicates an increased mineral dissolution due to drying. Most probably, ITM was the source of dissolved Si in the Bs soils since the (Alo - Alp)/Sio ratio, which was {approx}2.0, indicated that oxalate extractable Si derived from ITM in these soils (Table 3). Furthermore, all solutions were undersaturated with respect to amorphous SiO2; the concentration of dissolved H4SiO4 was between 0.5 and 1.5 orders of magnitude less than 1.3 mM, which is expected in the case of equilibrium with that solid phase at 8°C (Table 2). Concentrations of dissolved H4SiO4 are commonly too low in Spodosols to permit the formation of amorphous SiO2 (e.g., Manley et al., 1987). Thus, we do not expect amorphous silica to be a source of dissolved Si in our soils. However, given the expected ratio of Al to Si in ITM ({approx}2.0), Alqr did not increase to a level corresponding to the increase in dissolved Si. If Si and Al were released by congruent dissolution of ITM, then the extra 0.1 mmol L-1 of Si mobilized in, for instance, Lövkullen Bs2 as a result of drying (Fig. 3) should be accompanied by {approx}0.6 mmolc L-1 of Alqr. Instead, Alqr decreased slightly due to drying (Fig. 3): therefore, it appears that most of the Al released as a result of drying was reprecipitated in the soils as phases without silica, such as Al(OH)3, or was retained in complexes with soil organic matter, in both cases leaving a surplus of H4SiO4 behind in the solution.

We suggest that the dried Bs soils, when rewetted, had an enhanced dissolution rate of ITM compared with soils that had been kept moist or frozen since the sampling. Consequently, the solutions of dried Strsan Bs1 and Lövkullen Bs2 ended close to equilibrium with imogolite (Fig. 2), whereas the fresh soils probably did not have sufficient time for this. Based on the literature reviewed (Raveh and Avnimelech, 1978; Bartlett and James, 1980; Homann and Grigal, 1992), we think that disruption of organic matter was responsible for the enhanced mobilization of Si, probably by exposing surfaces of ITM that were originally coated with soil organic matter. This would have given way to an acid attack on those minerals when the soils were rewetted. In this context, it is interesting to recall that the sorption capacity for phosphate (Haynes and Swift, 1985) and sulfate (Comfort et al., 1991) may increase when a soil is dried. For pure Fe and Al oxides, on the other hand, the sorption capacity decreases upon drying, owing to an increased crystallinity and a decreased specific surface area (McLaughlin et al., 1981). Thus, the increased anion adsorption in soils due to drying supports the idea of an increased exposure of mineral surfaces. Regarding the interplay between ITM and Al(OH)3 controlling dissolved Al, we hypothesize that the enhanced dissolution of ITM caused by drying results in the precipitation of Al(OH)3 with a greater solubility than any phases in equilibrium with the solutions of the fresh and frozen soils.

Notably, however, the solubility of Al was higher after drying also in Sknes Värsjö Bh, although the concentration of dissolved Si did not increase. Here, we hypothesize that organic matter in a soil may be a three-dimensional matrix that contains complex-bound metal ions at sites exposed at the surface, as well as at sites occluded in the matrix. In a soil that has been subject to prolonged acid leaching in the field, surface sites might have a lesser average Al-saturation than interior sites. Thus, ruptures in the organic structure produced by desiccation in the laboratory may facilitate the access to sites with a higher Al saturation. Since Al solubility, as controlled by humic complexation, is a function of the relative abundance of Al at the binding sites (Cronan et al., 1986; Wesselink et al., 1996; Simonsson and Berggren, 1998), this would increase the apparent Al solubility. This would also explain the increased Al solubility in Strsan Bhs, although in this case additional Al was probably released from ITM, as indicated by the increased dissolved Si after drying.

Given that freezing causes severe drought between the forming ice crystals, we were surprised to notice the small effects caused by this pretreatment. However, Comfort et al. (1991) observed no significant change in sulfate adsorption as a result of freezing, indicating unchanged reactivity of secondary oxide surfaces in their soils.


   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Drying led to increased solubility of Al and enhanced neutralization of added H+ in all the Spodosol B horizon soils tested. Freezing, on the other hand, did not affect pH nor Al solubility. In most soils, drying increased the release of Si, indicating that the reactivity of the mineral solid phases, probably ITM, was enhanced. Since Spodosol B horizons often contain both Al–humus complexes and Al in secondary minerals, experiments carried out with dried soils will tend to overemphasize the role of mineral dissolution in Al solubility control; however, drying may also slightly increase the apparent solubility of Al in Bh horizons that virtually lack reactive secondary minerals.

Received for publication May 4, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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