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The use of Ca-modified, brown-coal-derived humates and fulvates for treatment of soil acidity.

Introduction

Soil acidity, particularly in the subsoil of the root-zone, has been identified as a major limiting factor in crop productivity in Australia. Acidic soils are deficient in Ca and Mg (Leeper and Uren 1993). Traditional treatment with lime and/or dolomitic lime is effective in increasing the pH of the root-zone c. 10 years after surface application (Scott et al. 1999; Whitten et al. 2000) or when applied over many years (Noble and Hurney 2000) in some situations, but has little short-term effect on root-zone pH unless deep incorporation using specialised equipment is used. An extensive list of references to these problems is available in recent publications (Noble et al. 1995; Smith et al. 1995). The use of Ca-saturated organic materials has shown considerable promise in the treatment of subsoil acidity, especially Ca-treated humic and fulvic acids (Van der Watt et al. 1991; Noble et al. 1995) prepared by wet oxidation of a South African coal at elevated pressure and temperature (Cronje 1989).

Australia is rich in coal deposits and the very large reserves of easily winnable brown coals in the La Trobe Valley of Victoria represented an opportunity to prepare other products rich in fulvic and humic acids. Brown coals have a high intrinsic oxygen content and considerable quantities of humates and fulvates can be obtained by simple extraction with alkali (Woskoboenko et al. 1991). In addition, brown coals are more susceptible to oxidation than the higher rank coals described in the South African patent (Hayatsu et al. 1982; Verheyen and Johns 1983) so that it is easy to prepare samples of various humate and fulvate contents by varying the strength of the oxidant. Thus, samples with different proportions of humates and fulvates were readily prepared by oxidation of brown coals and their conversion to Ca salts provides the opportunity to evaluate these materials as reagents for the treatment of acidity in Australian soils.

Materials and methods

Soil

The soil used in these experiments was an acid red podzol collected near Book Book, New South Wales, Australia (35[degrees] 22'S, 147[degrees] 30'E). Noble et al. (1995) gave a full description of the soil, including the following analytical data for the [A.sub.1] horizon. The exchangeable cation contents (in [mmol.sub.c]/kg) were: Ca 8.9, Mg 1.4, K 0.6, Na 1.1, Al 7.7; and the pH, measured as described below, was 3.95. The organic carbon and mean clay content (in g/kg) were 15.2 and 100, respectively. In this study, an attempt was made to measure the exchangeable Fe and Mn, but the values obtained were of the order of the limits of detection (3 [mmol.sub.c]/kg for Fe, 0.4 [mmol.sub.c]/kg for Mn) and so are not reliable. Samples from the [A.sub.1] horizon (0-10 cm depth) were used after being air-dried and ground to pass a 2-mm mesh sieve.

Coal and coal products

Coal

A Loy Yang run of mine coal was obtained pre-ground to -10 mm from HRL Pty Ltd. It had a moisture content of 63 wt% and an ash content of 1.2wt% (dry basis, db). The coal was dried at 25[degrees]C, 0.1 kPa to c.20 wt% moisture and then ground to pass a 250-[micro]m sieve. It was then coned and quartered to divide off a representative sample. Any excess not required was stored under water to minimise oxidation.

Coal oxidation and analysis of the oxidation product

The wet coal (c. 160 g, 60 g db) was slurried with hydrogen peroxide (100 vol) in ratios varying from 1:1 to 1:5 (dry coal weight:[H.sub.2][O.sub.2] volume, w/v) in a 3-L three-necked flask and stirred at 80[degrees]C with an overhead stirrer until the hydrogen peroxide had all reacted (potassium iodide was used as an indicator). The slurry was cooled and transferred to a beaker.

The proportions of humin (defined as the alkali-insoluble fraction), humic acid (the alkali-soluble, acid-insoluble fraction), and fulvic acid (the alkali- and acid-soluble fraction) were then determined on a representative subsample (c. 50 g) of the slurry, by the following procedure. The pH of the subsample (c. 1.4) was adjusted to 2.0 with 1 M NaOH. The slurry was then filtered through a glass microfibre filter paper (Whatman type C, 9 cm dia.) and the filtrate (fulvic acid) freeze-dried. The residue was slurried with sufficient 1 M NaOH to increase the pH to 14. The suspension was centrifuged at 500g and the solubles decanted. The insoluble material (humin) and solubles (humic acid) were then separately freeze-dried. The masses of humin and humic and fulvic acids were recorded separately for each of the oxidising reactions.

Selected fractions were analysed for C, H, and N with a LECO CHN Analyser (HRL Pty Ltd). The dried fractions were characterised by FTIR spectroscopy (as KBr discs) using a Perkin-Elmer 1600 series spectrometer. Solid-state [sup.13]C CP/MAS NMR spectra were recorded for freeze-dried samples, using a Varian Unity Plus 300 NMR spectrometer fitted with a Doty supersonic solid-state probe. [sup.1]H NMR spectra were recorded for water-soluble materials as solutions in [D.sub.2]O with sodium 2,2,3, 3-[d.sub.4]-3-trimethylsilylpropionate as internal reference at 400 MHz using a Bruker DRX 400 spectrometer.

The total concentration of acid groups was determined for the fulvic acid fraction of the 1:5 coal: [H.sub.2][O.sub.2] (w/v) product and for the acid-washed (2 M HCl, then 0.1 M HCl) humic acid fractions of the 1:2 and 1:5 coal:[H.sub.2][O.sub.2] (w/v) products. The method was similar to that of Bailey and Lawson (1965). A sample containing ~60 mg of humic or fulvic acids was weighed into a beaker and an excess of 0.1 M NaOH (10 mL for humic acid, 20 mL for fulvic acid) was added. The solution was made up to c. 80 mL with distilled water, stirred for 1 h under nitrogen, and back-titrated with 0.1 M HCl. In all cases one inflection point was observed, and the concentration of acid groups was calculated from the original amount of NaOH added and the amount of HCl added to the inflection point.

Ca-containing products

A saturated solution containing 27 mmol Ca[Cl.sub.2] (laboratory grade) was added to a slurry of the oxidised coal and water (5.5 g db) and the pH adjusted to 5 by addition of 1 M NaOH. The slurry was allowed to equilibrate overnight and then freeze-dried.

Soil leaching experiments

The leaching experiments were based on the method used by Smith et al. (1995). Perspex columns (7 cm internal diameter, 20 cm height), with 2-cm-thick adjustable bases containing an 8-mm-diameter hole covered with a 0.45-[micro]m Millipore filter, were packed to uniform bulk density with c. 860 g of soil to a depth of 15 cm. The total pore volume was c. 240 mL. A sample of each Ca-incorporated oxidised product containing 3.8 g of Ca-organic mixture (15.5 mmol Ca or 160 g Ca/[m.sup.2] of column surface) was evenly spread on the top of each column to produce a ~4-mm-thick layer. Lime, AR grade CaC[O.sub.3], was spread on top of another column (1.55 g to give 15.5 mmol Ca). One column was left as a control. The soil, lime, and coal-derived products were covered with an inert sponge and 0.01 M NaCl solution was applied at a rate of 5 mL/h using a peristaltic pump until approximately 3 pore volumes had passed through each column (10 days). The effluent was collected every 6 h during the day and every 12 h overnight. The collected effluent was split into 2 samples. Electrical conductivity and pH values were measured immediately after collection on one of the samples and the other sample was used for inductively coupled plasma-atomic emission spectrophotometry (ICP-AES) determination of K, Mg, Ca, Al, Fe, and Mn. Once leaching was complete the columns were allowed to dry. Eight 1-cm slices were taken from depths of 0-1, 1-2, 3-4, 5-6, 7-8, 9-10, 11-12, and 14-15 cm.

pH and conductivity measurements

The pH of the leachates was measured directly using an Activon digital pH/mV meter with a calomel electrode which had been calibrated using pH 7.4 and 4.0 buffers. Representative soil samples (4 g) were stirred with 0.01 M Ca[Cl.sub.2] solution (20 mL) for 15 min, the soil was allowed to settle, and the pH of the supernatant was determined (Rayment and Higginson 1992 method 4B1).

The conductivity of the leachates was measured using a Hanna Instruments H1 8820 N conductivity meter calibrated against a standard KCl solution with conductivity 12.86 mS/cm and the results used to calculate ionic strengths using I = 0.013EC, where I is ionic strength ([mol.sub.c]/L) and EC is electrical conductivity (mS/cm at 25[degrees]C) (Griffin and Jurinak 1973).

Metal concentrations

The metal concentrations in the coal (K, Mg, Ca, Al, Fe, Mn), in the soil column leachates (K, Mg,Ca, Al, Fe, Mn), and in the soil extracts (Ca, Al) were determined on solutions using a Varian Liberty 200 ICP-atomic emission spectrophotometer.

Samples of the coal ash were prepared for ICP-AES analysis by heating 4.4 mg of ash with 5 M HCl (20 mL) until no further dissolution occurred. The mixture was filtered and the residue washed with 5 M HCl (10 mL) and then with distilled water. The filtrate and washings were made up to a known weight (c. 60.0 g) to give a constant HCl concentration, which is required for valid analyses.

A known volume of each leachate fraction was analysed. The exchangeable cations in the soil samples were extracted into aqueous solution following the first part of method 15E2 of Rayment and Higginson (1992). Soil samples (2 g) were stirred with 60% (w/w) aqueous ethanol (2 x 25 mL) for 30 min to remove water-soluble ions followed by centrifugation (500g) with the supernatant decanted. The washing, centrifugation, and decanting were repeated with 20% (w/w) aqueous glycerol (2 x 25 mL) to ensure more complete removal of soluble salts. Exchangeable cations (including [Ca.sup.2+] and [Al.sup.3+]) were then extracted twice using 0.1 M Ba[Cl.sub.2]/0.1 M N[H.sub.4]Cl solution (20 mL) by mechanical shaking of the samples for 2 h followed by centrifugation and the supernatants analysed by ICP-AES as above.

Results

Characterisation of the oxidation products

The proportions of the solubility fractions in the products from oxidations of coal using varying amounts of hydrogen peroxide are summarised in Table 1. As expected the proportion of fulvic acids in the product increased with increasing proportion of hydrogen peroxide used. Typical elemental analyses of the fractions (wt% db) were: for fulvic acid from the 1:5 coal:[H.sub.2][O.sub.2] (w/v) oxidation, C 44.3, H 3.5, N 1.0, and for humic acid from the 1:2 coal:[H.sub.2][O.sub.2] (w/v) oxidation, C 50.6, H 3.6, N 1.0.

The solid state [sup.13]C NMR spectra showed that the humic acids had a higher fraction of aromatic carbon (0.24, 0.23, 0.33 [+ or -] 0.02 for products of the 1:5, 1:2, and 1:1 coal: [H.sub.2][O.sub.2] (w/v) oxidations, respectively) than the humins (0.16, 0.17, 0.25 [+ or -] 0.02 for products of the 1:5, 1:2, 1:1 coal [H.sub.2][O.sub.2] (w/v) oxidation, respectively) or, in the one case where sufficient sample was available for measurements, fulvic acid (0.09 [+ or -] 0.02 for the product of the 1:5 coal:[H.sub.2][O.sub.2] (w/v) oxidation). The low aromatic content of the humin may be attributed to the concentration in this fraction of the hydrophobic, not readily oxidised non-aromatic resin acids and long-chain waxes. Comparison of the aromatic carbon concentrations of the fractions and their proportions given in Table 1 shows that the overall aromaticity of the total products decreased with increasing oxidation severity, in agreement with the proposal that hydrogen peroxide oxidation not only leads to the formation of acid groups by cleavage of inter-ring bonds but also to the destruction of aromatic rings (Miura et al. 1996). The concentration of carbonyl carbon was higher for the fulvic acid from the 1:5 coal: [H.sub.2][O.sub.2] (w/v) oxidation than for the corresponding humic acid, and for the humic acids increased with oxidation severity.

The [sup.1]H NMR spectrum in aqueous solution of the fulvic acid from the 1:5 coal:[H.sub.2][O.sub.2] (w/v) oxidation indicated that major individual components were malonic, glycolic, succinic, and acetic acids in the approximate molar ratio 1:0.3:0.25:0.2. However, most of the mass was in the form of a wide range of other components. No formic or glyoxylic acid was detected and the pyruvic:malonic acid molar ratio was <0.05. The [sup.1]H NMR spectra of the other fulvic acids were similar, with broader peaks indicating an even wider range of compounds was present.

FTIR spectra of the products were in agreement with the NMR data. In particular the carbonyl bond in the IR spectrum of the humic acid separated from the 1:5 coal :[H.sub.2][O.sub.2] (w/v) product was stronger than that in the spectrum of the humic acid from the 1: 2 coal :[H.sub.2][O.sub.2] (w/v) product. The titration results indicated a higher acid group content (18 mol/kg db) for the fulvic acid from the 1:5 coal:[H.sub.2][O.sub.2] (w/v) product, and lower but still appreciable concentrations in the humic acid fractions, 3.3 mol/kg db (1:2 coal:[H.sub.2][O.sub.2] (w/v)) and 4.1 mol/kg db (1:5 coal:[H.sub.2][O.sub.2] (w/v)). The trends for acid group content paralleled those for carbonyl carbon and were consistent with FTIR. Comparison with the elemental analyses (finding oxygen by difference) suggests humic acid fractions contained more non-acidic oxygen than fulvic acids. For the fulvic acid sample, the distribution of [pK.sub.a] values was estimated from the titration curve. The distribution can be defined by associating with every [pK.sub.a] value the concentration C of acid groups with [pK.sub.a] less than or equal to this value, so that [pK.sub.a] is a function of C. The lowest [pK.sub.a] value will be [pK.sub.a] (0), and the highest will be [pK.sub.a] ([C.sub.o]), where [C.sub.o] is the total acid group concentration. It was assumed that [dpK.sub.a]/dC was constant in each of the regions 0 < C < 0.5[C.sub.o] and 0.[C.sub.o] < C <.[C.sub.o], with a suitable continuity condition at C = 0.5[C.sub.o]. There are then three unknowns, the [pK.sub.a] values at C = 0, 0.[C.sub.o], and [C.sub.o], which were determined by least squares fitting of the experimental titration curve to the theoretical form calculated from the assumed distribution. It was found that about half of the acidic groups had a [pK.sub.a] value of 3-4, with the [pK.sub.a] value of the remainder ranging from 4 to above 7. Such a distribution excludes oxalic acid as a major component [first [pK.sub.a] about 1.25 at the low ionic strength of the titrations, according to Martell and Smith (1977)].

In view of the high humin content from the 1:1 coal:[H.sub.2][O.sub.2] (w/v) preparation, only the 1:2 and 1:5 coal:[H.sub.2][O.sub.2] (w/v) products were converted into Ca salts for evaluation. It may be shown by the arguments that follow that fulvic and humic acid fractions and hence these 2 products differ markedly in the average distance between nearest neighbour acidic groups in a molecule. Taking fulvic acid first, for the fulvic acid from the 1:5 coal:[H.sub.2][O.sub.2] (w/v) oxidation, the elemental analysis and titration results imply that half of the carbon atoms were in acidic groups. The observed [pK.sub.a] distribution and the composition of this fraction implied by NMR then indicated that molecules containing more than one acidic group must have predominated in this fraction. The high proportion of carbon atoms in acidic groups then further implies that nearest neighbour acid groups in a molecule must have been separated on the average by only a small number of carbon atoms. Fulvic acid fractions of the other oxidation products will be similar. For the humic acid fractions, the proportion of carbon atoms attached to acidic groups (e.g. 0.08 for the humic acid from the 1:2 coal: [H.sub.2][O.sub.2] (w/v) oxidation) was much lower than for fulvic acids. This smaller proportion of acidic carbon atoms implies that the average distance between nearest-neighbour acidic groups in the humic acid molecules will be greater than in the fulvic acid molecules if 3 conditions are satisfied:

(1) The distribution of acidic group concentrations in the humic acid molecules is smooth. Solubility constraints require that this distribution be broadly similar to that for molecular weights, which for brown-coal-derived humic acids is roughly bell-shaped (Woskoboenko et al. 1991, p. 159).

(2) The range of acidic group concentrations in humic acid molecules is sufficiently small. Again, solubility constraints require that the range be similar in order of magnitude to that of molecular weights, which for brown-coal-derived humic acids is about a factor of 10 (Woskoboenko et al. 1991, p. 159).

These conditions and the lower limit to the coal humic acid molecular weight of about 1 kDa (Woskoboenko et al. 1991, p. 160) imply that most humic acid molecules contained more than one acidic group.

(3) The spatial distribution of the acidic groups in the humic acid molecules is random. The structure of brown coal (Verheyen and Perry 1991) and changes during oxidation (Miura et al. 1996) would be expected to ensure this.

Thus, nearest-neighbour acidic groups in the product of the 1:5 coal: [H.sub.2][O.sub.2] (w/v) oxidation will on the average be closer to one another than in the product of the 1:2 coal:[H.sub.2][O.sub.2] (w/v) oxidation, mainly because of the higher proportion of fulvic acid in the former (Table 1). The average molecular weight and aromaticity of the product of the 1:5 coal:[H.sub.2][O.sub.2] (w/v) oxidation will also be lower.

Untreated soil

Control experiments were carried out using columns of untreated soil. The range of pH values gave an average of 3.8, very similar to those quoted previously by Noble et al. (1995). The average exchangeable Ca content of 8 [mmol.sub.c]/kg after the experiment agreed with the value of 9 [mmol.sub.c]/kg found by Noble et al. (1995), but the exchangeable Al content (10 [+ or -] 1 [mmol.sub.c]/kg) was higher than the 6 [+ or -] 1 [mmol.sub.c]/kg measured by Noble et al. (1995). The discrepancy could be due to the difference in Al analysis methods, ICP-AES in this work, pyrocatechol violet in that of Noble et al. Even for long contact times, the latter method can give low results, depending on the anion present (Bartlett et al. 1987), whereas ICP-AES is generally reliable. This result suggests that analyses of soil exchangeable Al in the 2 studies should differ by approximately a factor of 2.

Treated soils after leaching

Leaching of the control column and soil columns with Ca incorporated materials applied at pH 5 resulted in variations in pH, Ca, and Al (Figs 1, 2 and 3). The pH of the control remained virtually constant (3.7-3.9) along the length of the column, whereas the pH of the column treated with humate-rich material from a 1:2 coal:[H.sub.2][O.sub.2] (w/v) oxidation (humate-treated column) decreased from 4.7 near the surface to 4.0 at a depth of 15 cm and that of the column treated with fulvate-rich material from a 1:5 coal:[H.sub.2][O.sub.2] (w/v) oxidation (fulvate-treated column) from 5.4 near the surface to 4.5 at a depth of 15 cm. The fulvate-treated column showed increases in pH ranging from 1.6 near the surface to 0.4 at a depth of 15 cm, which were all significant (P < 0.05) relative to the control, using the statistical test suggested by Dunnett (1955). In contrast, the humate-treated column showed significant increases relative to the control (P < 0.05, Dunnett's test) only to a depth of 6 cm. The lime-treated column only showed a significant pH increase over the control (P < 0.05, Dunnett's test) in the top 1 cm of the column. The distribution of exchangeable Al with depth (Fig. 2) was that expected from the usual model of soil acidification, in which the soluble Al increased as the pH decreased. The fulvate-treated column showed significant (P < 0.05, Dunnett's test) decreases in Al and increases in Ca relative to the control over the whole column, whereas the humate-treated column showed significant (P < 0.05, Dunnett's test) changes to a depth of 6 cm (Ca) and 12 cm (Al) and the limed column, only in the top 1 cm (Figs 2 and 3).

[FIGURES 1-3 OMITTED]

The leachates

The variations in pH of leachate samples with cumulative volumes of leachate are shown in Fig. 4. The initial pH values of the leachates of untreated and limed columns were about 3.5, and the values increased to 4.7-5.0 as leaching progressed. For columns treated with Ca-containing oxidised products, the initial pH of the leachates was slightly lower (3.2) and remained below that of the leachates from the untreated and limed columns over the first ~0.5 pore volumes, but subsequently increased to 4.7-5.5.

[FIGURE 4 OMITTED]

The lower pH of the early leachates from columns treated with soluble Ca preparations reflected differences in the flux of ions out of the column. The ionic strengths of the leachates from columns treated with soluble Ca preparations were greater than those of the leachates from the untreated or limed columns (Fig. 5) over the first ~0.5 pore volume, although the results for the 2 treatments were not distinguishable.

[FIGURE 5 OMITTED]

A clear difference between the fulvate and humate-treated columns was, however, observed for the total amount of Al in the leachate (Fig. 6). The fulvate-treated column lost significantly more Al in the leachate than the humate-treated column (P < 0.05, Scheffe's test). Both the fulvate-rich and the humate-rich preparations led to major, statistically significant (P < 0.05, Dunnett's test), increases in the elution of Al in the first 0.5 pore volumes over that from the untreated columns, whereas adding lime had no significant effect.

[FIGURE 6 OMITTED]

Addition of lime or fulvate to the column had no statistically significant effect on Ca leaching but a significant increase (P < 0.05, Dunnett's test) was observed for the column treated with humate in the total amount of Ca in the leachate, even when <0.5 pore volumes had been collected (Fig. 7). Thus, in contrast to what was observed for Al, more Ca was lost from the humate-treated than from the fulvate-treated column.

[FIGURE 7 OMITTED]

The leaching of Fe from the column also varied with the treatment, in that the fulvate-rich material led to a much greater elution of Fe than any of the other treatments, all of which gave slightly higher results than the control (Fig. 8). Leaching of Mn, K, and Mg (not shown) did not change with treatment.

[FIGURE 8 OMITTED]

Discussion

Previous studies of organic Ca salts as ameliorants of soil acidity have concentrated on salts of single organic acids (e.g. Hue et al. 1986; Hern et al. 1988) or mixtures of low molecular weight acids (Van der Watt et al. 1991; Noble et al. 1995; Smith et al. 1995) because these are water-soluble and thus are easily carried down the soil column. Hue et al. (1986), Hern et al. (1988), and Smith et al. (1995) noted that water-solubility is not sufficient; the organic functional group must interact strongly with the exchangeable Al species in the soil so that the Ca in the organic salt exchanges with the Al. The Al exchanged with the Ca may be carried down and out of the root-zone as a soluble complex or immobilised as an insoluble organic or inorganic complex; even an immobilised Al complex will generally be less phytotoxic than monomeric Al (see Hue et al. 1986). In general, the stability constants of trivalent ions such as Al are higher than those of divalent ions such as Ca for complexes with carboxylate ligands, but their relative magnitude varies widely (Martell and Smith 1977; Tam and Williams 1984), so that in considering the effect of organic structure on the effectiveness of the organic additive (see Hue et al. 1986; Tam and McColl 1990), it is preferable to take the binding of both Ca and Al into account.

The 2 organic additives tested in this study differed in average distance between nearest-neighbour acidic groups, aromaticity, and the proportion of non-acidic oxygen. An indication of the effect of these differences on Al binding by the organic additives can be obtained by calculating the proportion of free and hence phytotoxic Al in aluminium-containing solutions of calcium salts of simple organic acids differing in structure and composition under conditions (pH, Al: Ca ratio) similar to those of this study. The Ca and Al binding to soil species and the precipitation of Al compounds have not been considered because it is difficult to estimate the relevant parameters and the neglect of these factors should not alter the general trends. Table 2 gives the proportion of free [Al.sup.3+] at pH 3.5 using stability constants from the literature and an Al:Ca ratio of 1:8, the ratio of exchangeable Al to Ca in the columns before leaching began. Under these conditions the neglect of Al-hydroxide species and 1:2 Al:ligand complexes and differences between the actual Al and Ca concentrations and those taken for Table 2 should not greatly affect the relative binding effectiveness calculated for different organic acids. The stability constants used should be for the ionic strength of the mixture, but since interpolation from the measured values of stability constants at different ionic strengths is uncertain and the stability constants vary little above an ionic strength of 0.1 M, well-supported literature values at various ionic strengths were used.

The results suggest that the proportion of complexed Al decreases in the order oxalate > malonate > succinate, that is, as the distance between adjacent acidic groups increases, consistent with the order of Al detoxification found by e.g. Hue et al. (1986), and agreeing with their suggestion that the effectiveness of an ameliorant will increase with decreasing average distance between nearest-neighbour acidic groups. The effect of aromaticity can be deduced from a comparison of the proportion of complexed Al in the pairs malonate/salicylate and succinate/o-phthalate. In each pair, the number of carbon atoms between the acidic groups of both members is the same, but the first member of each pair is aromatic and the second is not. The results suggest that the effect of attachment of acidic groups to aromatic carbons is small. Comparison of the results for glycolic and pyruvic/2-oxobutanoic acid with that for oxalic acid shows that the alcoholic and ketonic groups are relatively ineffective in promoting Al binding so that the large non-acidic oxygen content of the humic acid component of the additives will not greatly influence detoxification by the Al-binding mechanism.

The changes in soil exchangeable Al in the experimental system of this study will roughly parallel the changes in free [Al.sup.3+] and phytotoxic Al in general. Phytotoxic Al will be exchangeable; non-phytotoxic soluble Al complexes will be washed out with the leachate and strongly bound Al in immobile complexes will not be displaced by the [Ba.sup.2+] and N[H.sup.4+] used in the determination of exchangeable Al.

The above discussion suggests that the Ca salts of higher molecular weight humic materials with a lower concentration of acid groups are relatively ineffective in detoxifying Al and ameliorating soil acidity, and such considerations have probably guided the choice of trial materials in the past. However, these generalisations must be treated with caution, because the measured stability constants can vary significantly with the method of determination (Kerven et al. 1995) and the ratio of mono- to bi-dentate complexes can vary with chemical characteristics of the soil, such as pH and ionic strength. Furthermore, only equilibrium conditions have been considered, not kinetics, which may be as important. In any case, the considerations above only determine an order of merit; organic humates of higher molecular weight can still exhibit some water solubility and Al complexing capability and have been shown to have beneficial effects (e.g. Noble et al. 1995). Thus it is worthwhile to test organic ameliorants with other advantages (e.g. cost) even if their predicted solubility and Al complexing ability are not ideal.

This study compared the effects of 2 materials of different humate/fulvate concentrations of different solubility and average distance between nearest-neighbour acidic groups. Both materials contained at least some material (humate) which is generally considered to be of high molecular weight (see Piccolo and Conte 2000). The fulvate-rich preparation, the product of a 1:5 coal:[H.sub.2][O.sub.2] (w/v) oxidation, would have been expected to be more effective in reducing phytotoxic Al in the soil solution in the upper layers, and hence ameliorating acidity, than the humate-rich preparation, the product of a 1:2 coal:[H.sub.2][O.sub.2] (w/v) oxidation, because it was more soluble and nearest-neighbour acidic groups were on the average closer to each other.

In many respects the fulvate-rich preparation was indeed the better ameliorant. For the fulvate-rich preparation, the concentration of Ca in the leachate fell to low levels after only a small amount of leachate had passed through the column, whereas removal of Al in the leachate continued for a longer time. For the humate-rich preparation, more of the added Ca was leached out of the column and the leaching of Al practically ceased at the same time as that of Ca. The pH of the column treated with the fulvate-rich product was much higher than that of the control over the whole 15 cm of the column and exchangeable Al was <2 [mmol.sub.c]/kg to a depth of 8 cm, whereas the pH of the column treated with the humate-rich product was only higher than that of the control to a depth of 6 cm and the exchangeable Al level was >4 [mmol.sub.c]/kg below 2 cm. These results indicate much stronger complexing of Al and a larger rise in pH for the fulvate-rich product. The high affinity of this product for multivalent ions is further evidenced by its ability to remove Fe from the soil.

However, the humate-rich product was still markedly superior to lime with respect to pH and exchangeable Al and Ca concentration over the top 6-12 cm of the column after the experiment. Furthermore, the effect on the general long-term condition of the soil (as distinct from the specific effect on acidity) of the high solubility and complexing ability of the fulvate-rich additive may be harmful. This is because transport of soluble or pseudo-soluble organic matter is the driving force of a proposed mechanism of podzolisation (Duchaufour 1998, p. 137) so that application of the coal-derived additives could have the same undesirable consequences. These include transport of plant nutrients out of the range of the roots, formation of a barren, silica-rich [A.sub.2] horizon, and deposit of organics, Al, and Fe to form B-horizons of limited permeability (Duchaufour 1998, pp. 176-179). These effects would be expected to be more important, the higher the solubility and complexing ability of the additive, but in fact the fulvate-rich additive did not enhance leaching of Ca or Mg more than the humate-rich additive. However, it did leach more Fe than the humate-rich additive.

Furthermore, the material balance for the exchangeable Al implied that the fulvate-rich product reduced the level of exchangeable Al by leaching it down, whereas application of the humate-rich mixture tended rather to immobilise it, which might be preferable in this connection. For the fulvate-rich material, over 90% of the decrease in exchangeable Al was accounted for by the amount in the leachate (7.0 [+ or -] 0.3 [mmol.sub.c]/kg lost, 6.4 [+ or -] 0.3 [mmol.sub.c]/kg soil eluted), but for the humate-rich material only 65% of the decrease in exchangeable Al appeared in the leachate (4.0 [+ or -] 0.3 [mmol.sub.c]/kg lost, 2.6 [+ or -] 0.3 [mmol.sub.c]/kg soil eluted). The chemical structure of the insoluble Al compounds is unknown; apart from complexes with organic ligands, the higher pH could promote hydrolysis reactions to give ultimately e.g. gibbsite, alunite (Noble et al. 1995), or amorphous Al[(OH).sub.3].

The fulvate-rich preparation would be expected to be less effective than additives of low average molecular weight tested in the past because of its high humate content. Direct comparisons can be made with results of Noble et al. (1995) for Ca oxyfulvate; the same soil and Ca concentration were used as in this work, and the oxyfulvate would be expected to be chemically similar to the fulvate fractions of the products tested in this study, because all were obtained by coal oxidation. Though the fulvate-rich preparation had a lower proportion of fulvate than the Ca oxyfulvate used by Noble et al. (1995), it was as effective as the Ca oxyfulvate in many respects. The rise in pH in the top 10 cm of soil was the same as that noted by Noble et al. (1995). The exchangeable Al content of the column treated with the fulvate-rich preparation (<2 [mmol.sub.c]/kg to a depth of 8 cm) was comparable to that of the Ca oxyfulvate-treated column of Noble et al. (1995) (1 [mmol.sub.c]/kg at this depth) when the difference in Al analysis methods is taken into account (see above). In one respect the product of Noble et al. (1995) differed from the fulvate-rich preparation, in that for Ca oxyfulvate, substantial leaching of Ca and Al occurred over the same time interval, whereas leaching of Al from the column treated with the fulvate-rich additive continued after leaching of Ca had stopped. The same prolonged leaching of Al was observed for the superior Ca citrate additive studied by Smith et al. (1995). Thus, these comparisons again suggest that these additives are more effective than considerations based on the properties of simple inorganic acids would predict.

In conclusion, these results suggest that organic ameliorants of acid soils need not be of low molecular weight or very high acid group concentration. Despite their theoretical disadvantages, mixtures of quite high apparent average molecular weight can be used provided they have sufficient functionality to remove and complex the undesirable exchangeable ions in the soil and show some mobility in soil-water systems. Additives superior to lime can easily be obtained. The high-molecular weight organic additives have the practical advantage over low molecular-weight mixtures of being less liable to bacterial attack (Kerven et al. 1991).
Table 1. Solubility distribution of products (proportions, wt%
dry basis) from hydrogen peroxide oxidation of Loy Yang brown coal

Coal/[H.sub.2][O.sub.2] Humin Humic acid Fulvic acid
(dry coal wt/vol.)

1:1 46 52 2
1:2 11 84 5
1:3 13 72 15
1:5 14 41 45

Table 2. The stability constants and proportion of free [Al.sup.3+] for
solutions containing the anions of simple organic acids, Ca, and Al at
a pH of 3.5

The calculations were made for [total Ca] = 0.08 M,
[total Al] = 0.01 M, [total anion] = [total Ca] (oxalate, malonate,
succinate, o-phthalate) or [total anion] = 2[total Ca] (salicylate,
glycolate, pyruvate/2-oxobutanoate), corresponding to the [total Ca]:
[total anion] ratio for the treatments

Anion
 [H.sub.2]L
 HL.H

Oxalate 0.97(1.0) (B)

Malonate 2.70(0.25) (C)

Succinate 4.05(0.25) (C)

o-phthalate 2.82(0.25) (C)

Salicylate 2.69(0.1) (G)

Glycolate

Pyruvate/2-oxobutanoate

Anion [Log.sub.10] stability constant
 (ionic strength in M) (A)

 HL Ca HL
 H.L Ca.HL

Oxalate 3.57(1.0) (B) 1.38(0.1)

Malonate 5.36(0.25) (C) 3.20(0.25) (C)

Succinate 5.26(0.25) (C) 1.91(0.25) (C)

o-phthalate 5.09(0.25) (C) 2.89(0.25) (C)

Salicylate 0.15(0.16)

Glycolate 3.586(0.5) (H)

Pyruvate/2-oxobutanoate 2.37(0.1,av)

Anion [Log.sub.10] stability constant
 (ionic strength in M) (A)

 CaL AlL
 Ca.L Al.L

Oxalate 1.66(1.0) 6.03(0.6) (B)

Malonate 1.64(0.25) (C) 6.71(0.1) (D)

Succinate 1.45(0.25) (C) 3.2(0.5) (E)

o-phthalate 1.72(0.25) (C) 2.94(0.6) (F)

Salicylate

Glycolate 0.92(0.5) (H) 1.544(0.1) (I)

Pyruvate/2-oxobutanoate 0.8(0.16) (J) 2.09(0.1) (K)

 All other [[Al.sup.3+]]/
Anion equilibria [Total Al]

 Other Al
 equilibria

Oxalate 1.40(0.6) AlHL.H (B)/ 7 x [10.sup.-5]
 Al.[H.sub.2]L

Malonate 4.5 x [10.sup.-3]

Succinate 1.42(0.5) AlHL (E)/ 0.61
 Al.HL

 3.75(0.5) AlL (E)/
 AlOHL.H

o-phthalate 0.85

Salicylate 0.124(0.1) AlL.H (G)/ 2.0 x [10.sup.-3]
 Al.HL

Glycolate 0.33

Pyruvate/2-oxobutanoate 0.07

(A) The notation for the equilibria follows that of Martell and Smith
(1977), with L = ligand without acidic hydrogens. The constants are
from Martell and Smith (1977) unless otherwise stated. All
concentrations are in M. All stability constants were measured at
25[degrees]C.

(B) Sjoberg and Ohman (1985).

(C) Daniele et al. (1985).

(D) Powell and Town (1993).

(E) Charlet et al. (1984).

(F) Hedlund et al. (1988).

(G) Secco and Venturini (1975).

(H) Piispanen and Lajunen (1995).

(I) Kereichuk et al. (1980).

(J) Pyruvate.

(K) 2-oxobutanoate; Smith and Martell (1989).


Acknowledgments

We thank HRL for funding, including provision of a postgraduate research award to D.P., and the referees for their comments.

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Manuscript received 9 April 2001, accepted 16 May 2002

Damayanthi Peiris (A), Antonio F Patti (AB), W. Roy Jackson (A), Marc Marshall (A), and Christopher J Smith (C)

(A) Centre for Green Chemistry and School of Chemistry, PO Box 23, Monash University, Vic. 3800, Australia.

(B) School of Applied Sciences, Monash University, Vic 3800, Australia.

(C) CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia.
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Author:Peiris, Damayanthi; Patti, Antonio F; Jackson, W. Roy; Marshall, Marc; Smith, Christopher J.
Publication:Australian Journal of Soil Research
Geographic Code:8AUST
Date:Dec 1, 2002
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