Soil Chemistry and Mineralogy A&. J. Soil Res., 1992, 30, 119-30

Relationships between Extractable Al, Selected Soil Properties, pH Buffer Capacity and Lime Requirement in some Acidic Queensland Soils

R. L. Aitken

Agricultural Chemistry Branch, Queensland Department of Primary Industries, Indooroopilly, Qld 4068.

Abstract

The objectives of this study were to examine (1) interrelationships between various forms of extractable A1 and selected soil properties, (2) the contribution of extractable A1 to pH buffer capacity, and (3) investigate the use of extractable A1 to predict lime requirement.

Aluminium was extracted from each of 60 Queensland soils with a range of chloride salts: 1 M KC1 (A~K), 0.5 M CuClz (AlCU), 0.33 M Lac13 (AIL,) and 0.01 M CaC12 (Alc,). The amounts of A1 extracted were in the order Alc, > AIL, > AIK > Alca. Little or no A1 was extracted by KC1 or Lac13 in soils with pHw values greater than 5 ~ 5 , whereas CuClz extracted some A1 irrespective of soil pH. The greater amounts of A1 extracted by CuClz were attributed mainly to A1 from organic matter, even though all of the soils were mineral soils (organic carbon 54.7%). Both Alcu and AIL, were significantly (P < 0.001) correlated with organic carbon, whereas none of the extractable A1 measures was correlated with clay content.

A ~ K and A~L, were poorly correlated to pH buffer capacity. The linear relationship between Alcu and pH buffer capacity ( r2 = 0.49) obtained in this study supports the view of previous researchers that the hydrolysis of A1 adsorbed by organic matter is a source of pH buffering in soils. However, the change in CEC with pH accounted for 76% of the variation in pH buffer capacity, indicating that other mechanisms such as deprotonation of organic groups and variable charge minerals are also involved in pH buffering.

The ability of CuCl2 and LaC13 extractable A1 to estimate lime requirement depended on the target pH. The results suggest that lime requirements based on neutralization of AIL, would be sufficient to raise pHw to around 5.5, whereas requirements based on neutralization of AlcU substantially overestimated the actual lime requirement to pHw 5.5, but gave a reasonable estimation of the lime requirement to pHw 6 5.

Keywords: extractable aluminium, pH buffer capacity, lime requirement.

Introduction

Previous studies on acidic Queensland soils (Aitken et al. 1990a, 1990b) have shown that neither the lime requirement nor pH buffer capacity was well correlated with exchangeable (1 M KC1 extractable) Al. For most of the soils studied, the lime required to reduce exchangeable A1 to a predetermined level far exceeded the lime requirement calculated from the exchangeable Al value, even when the latter was multiplied by factors of 1.5 or 2. The development of negative charge with increasing pH on liming was considered to be the main mechanism for pH buffer capacity in the soils studied with exchangeable A1 contributing little

R. L. Aitken

to pH buffer capacity (Aitken et al. 1990~) . Although the soil properties of organic carbon, clay content and effective cation exchange capacity were all highly correlated with the change in CEC with pH, organic carbon made the greatest contribution to the variation in charge with pH. Aluminium forms complexes with organic matter, by reaction mainly with carboxyl and phenolic hydroxyl groups (Thomas and Hargrove 1984; Stevenson and Vance 1989), and is not readily displaced by KC1 (Bloom et al. 1979a; Hargrove and Thomas 1981). Hydrolysis of A1 in soil organic matter as the pH is increased would result in an increase in CEC and A1 complexed with organic matter may be an important mechanism of pH buffering in soils. The possibility that forms of less readily extractable A1 (for example, organically bound, hydroxy-Al) may contribute to pH buffer capacity in Australian soils and provide a means of predicting lime requirement warrants investigation.

Aluminium extracted with 0.5 M CuClz has been used to estimate both organically bound A1 and a portion of the A1 in hydroxy-A1 polymers and clay minerals (Juo and Kamprath 1979). Lanthanum chloride has also been suggested as being more effective than KC1 in extracting A1 from organic matter (Bloom et al. 1979~) . On the other hand, dilute (e.g. 0.01 M) CaC12, proposed as a means of diagnosing phytotoxic A1 in soils (Hoyt and Nyborg 1971; Bromfield et al. 1983), would be expected to extract less A1 than 1 M KC1 because of its lower ionic strength and reduced capability in displacing exchangeable A1 from strongly acidic soils (Gillman and Sumpter 1986). The objectives of this study were to examine (1) the interrelationships between various forms of extractable A1 and selected soil properties, (2) the contribution of less readily extractable A1 to pH buffer capacity, and (3) the utility of extractable A1 for predicting lime requirement in a range of acidic soils.

Materials and Methods The data used in this study were obtained from 60 surface (0-15 cm) soils collected from

eastern Queensland. The soils collected included the following Great Soil Groups: podzols (3), podzolics (15), red earths (6), krasnozems (18), euchrozems (2), red-brown earths (3), black earths (3), humic gleys (I), solodics (I), alluvials (1) and 7 soils that were either intergrades or for which there was no suitable group. Each soil was air dried and sieved <2 mm prior to analysis. In addition to measurements of pH, organic carbon, particle size analysis, and effective cation exchange capacity (ECEC) using methods previously described (Aitken et al. 1990a), the variation in ECEC per unit change in soil pH (ACEC) was determined using the method of Gillman and Sumpter (1986). For this suite of soils pHw (1 : 5 soil :water) ranged from 3.7 to 7.6, organic carbon ranged from 0.3% to 4.7%, and clay content ranged from 1% to 77%.

Aluminium extracted (1 : 10 soil : solution, 1 h equilibration) with 1 M KC1 (AIK) was determined by autc-titration using a modification of the Yuan (1959) method. Each soil was also extracted with 0.5 M CuClz (Alc,), 0.33 M Lac13 (A~L,) and 0.01 M CaClz (Alc,), the suspension centrifuged and the A1 concentration in the filtered (Whatman No. 42) supernatant determined by inductively coupled plasma atomic emission spectrometry. A 1 : 10 soil : solution ratio was used for extractions with CuC12 and Lac13 (Juo and Kamprath 1979; Oates and Kamprath 1983a), whereas extraction with 0.01 M CaClz was carried out at a 1 : 5 soil : solution ratio (1 h equilibration). For comparative purposes all extractable A1 values were expressed as mg A1 kg-' soil.

To determine the equilibration time for extraction with CuC12 and Lac13 the effect of various shaking times (0.5, 1, 2, 4, 8, 16 and 24 h) on extractable A1 was studied on each of three soils. Extraction of each of the 60 soils with CuClz and Lac13 was conducted at each of two equilibration times, I h (for comparison with 1 M KC1 extractable Al) and at the

Aluminium in Queensland Soils

optimum time from the time trial. No time trial was conducted for 1 M KC1, as Bruce (1986) had concluded that a I h shaking time for 1 M KC1 gave satisfactory results for exchangeable A1 in a range of acidic soils. Ten representative soils of varying organic matter and extractable A1 status were used to evaluate the relative effects of extracting solution cation and pH on extractable Al. Each soil was equilibrated (16 h) with 0.5 M CuCl2 (1: 10 soi1:solution) and the pH (pHc,) of the suspension recorded. Separate samples of these soils were then equilibrated with 1 M KC1 and 0.33 M LaC13 (1 : 10 soil: solution) and the pH adjusted with dilute HCl to the respective pHcu values for each soil. After the pH values had stabilized the suspensions were centrifuged, supernatants filtered and the A1 concentration determined as previously described.

The pH buffer capacity of each soil was determined from the relationship between pH and added lime (obtained from batch equilibrations of soil-water-Ca(0H)z suspensions; Dunn 1943). The relationships between pH ( Y axis) and added Ca(OH)2 (X axis) were linear for pH values <6.5 (r2 2 0.91). Because buffer capacity is defined as the change in quantity with intensity, the reciprocal of the slope of the regression line was taken as a measure of the buffer capacity and expressed as g CaC03 (kg soil)-' (unit p ~ ) - ' . The lime requirements to pH 5.5 (for soils with initial pHw <5.5) and pH 6.5 were also determined from these relationships.

Results and Discussion Effect of Equilibration Time, Extracting Cation and Suspension pH on Extractable A1

The effect of equilibration time on the amount of A1 extracted with 0.5 M

CuC12 and 0.33 M LaC13 is shown in Fig. 1. Although the soils contained different levels of extractable All the amount of A1 extracted tended to reach a plateau value around 16 h equilibration for each soil and extracting solution. These equilibration times are in contrast to the result of Juo and Kamprath (1979), who observed that an equilibration period of 2 h was sufficient for maximum extraction of A1 using 0 - 5 M CuC12. The higher clay content of the soils used in the present study (Fig. 1 ) compared with that of the soil used by Juo and Kamprath (1979) may possibly explain the longer equilibration time. A longer equilibration time for CuC12 and LaC13 extractants compared with KC1 would be compatible with the view that CuC12 and LaC13 extract both readily and nonreadily exchangeable A1 (Bloom et al. 1979a; Oates and Kamprath 1983a; Hargrove and Thmas 1984), whereas KC1 extracts only readily exchangeable Al.

Table 1. The range and mean levels of aluminium extracted from 60 soils with various extractants and extraction times

Equilibration Extractable A1 (mg kg-') Extractant time (h) Range Mean

1 M KC1 1 <O. 1-526 109 0.5 M CuCl2 1 17.4-1143 305

16 17-5-1520 503 0-33 M Lac13 1 1-761 148

16 1-855 200 0-01 M CaCl2 1 <O el-170 16

Table 1 shows the range and mean extractable A1 levels for the soils for each extractant and extraction time. The amounts of A1 extracted were in the order Alc, > AIL, > AIK > Alca, irrespective of whether the extractants were compared at equivalant extraction times (I h) or at optimum extraction times.

R. L. Aitken

Although 16 h equilibration was required for maximum extraction (Fig. I), the amount extracted after I h was highly linearly correlated, for a given extractant, with that extracted after 16 h with coefficients of determination for Alc, and AILa of 0.95 and 0- 98, respectively.

I O 0

Soil 36.432 C, 72% clay I A A

Soil 57, 3% C, 41% clay

0 0 Soil 18,122 C, 15% cloy

Fig. 1. The effect of equilibration time on aluminium extracted from each of three soils with (a) 0 5 M CuClz and (b) 0.33 M LaCl3.

Equilibration time (hours)

Factors such as suspension pH, nature of cation and ionic strength are responsible for the differences in extractable A1 shown in Table 1. The effect of suspension pH and extracting cation on extractable A1 is shown in Table 2. Suspension pH values in 0 - 5 M CuC12 ranged from 2 -90 to 3.37, whereas in 1 M KC1 and 0 33 M Lac13 suspensions pH values ranged from 3 71 to 4.62 and 3.55 to 4.28, respectively. The 0.5 M CuClz extracting solution (pH 2-9) is buffered, whereas the KC1 and LaC13 solutions are unbuffered. Although there was an increase in extractable A1 when the pH of the KC1 suspensions was adjusted to that of the corresponding CuC12 suspension, the amounts extracted were substantially lower than those extracted by CuClz (Table 2). The greater amounts of A1 extracted (at equivalent pH values) by CuClz are a result of the effects of extracting cation and ionic strength. Ionic strengths of the extracting solutions were in the order Lac13 (1.98 M) > CuC12 (1.5 M) > KC1 (1 M). The ability of Cu to form strong coordination complexes with organic matter has been well documented (Thomas 1981; Stevenson and Ardakani 1972; Bloom 1981), and the higher ionic strength of CuClz compared with KC1 would also enhance displacement of A1 from organic matter (Stevenson and Vance 1989). Aluminium extracted by Lac13 at the same pH as CuClz (Table 2) was either less than or approximately equal to the A1

Aluminium in Queensland Soils

extracted with CuC12 despite the higher ionic strength of LaC13. This highlights the ability of Cu to form complexes with organic matter since, for any given pH and ionic strength, trivalent cations are usually bound to organic matter and clay minerals to a greater extent than divalent ions (Chen and Stevenson 1986). Soil: Lac13 suspension pH values were generally 0.1 to 0 . 3 pH units lower than those of soil: KC1 suspensions (Table 2) explaining, in part, the greater amounts of AIL, compared with AIK (Tables 1 and 2). However, a combination of higher ionic strength and cation valence in the case of Lac13 would be the main reason for the greater amounts of AILa compared with AIK (Tables 1 and 2) since, when both KC1 and Lac13 suspensions were adjusted to the same pH value (pHcu), Lac13 extracted more A1 than KC1 in all soils (Table 2).

Table 2. Effect of extracting cation and suspension p ~ A on extractable A1 in selected soils

Soil Organic Extractable A1 (mg kg-') carbon (%) CuC12 KC1 Lac13

At pH KC1 At pH CuClz At pH Lacla At pH CuClz

A pH values shown in parenthesis.

Interrelationships between Extractable A1 and Selected Soil Properties

The relationships between soil pH and each of AIK, AIL,, and Alc, are shown in Fig. 2. As previously shown in many studies of acid soils (Coleman and Thomas 1967; Sanchez 1976; Juo 1977), there is little or no readily exchangeable (KC1 extractable) A1 above pHw 5.5. Similarly the amounts of A1 extracted by Lac13 were very low at pHw values >5.5 (Fig. 2b), despite the much lower pH of soil: LaC13 suspensions compared with soil: water pH values (Table 2). On the other hand, CuC12 extracted significant amounts of A1 at pH values >5.5 presumably for the reasons discussed above. It is apparent that, despite the absence of significant amounts of KC1 extractable A1 at pHw values >5 5, there is a considerable amount of A1 bound to organic matter at these pH values in many soils. As previously discussed, the ability of CuC12 to extract A1 from soils with pHw >5.5 may also be partially attributable to the dissolution of hydroxy-A1 polymers and clay minerals by the acidic CuC12.

Interrelationships between extractable A1 and selected soil properties are shown in Table 3. Although Lac13 extracted more A1 than KC1 (Table I), AILa and A ~ K were highly correlated (r2 = 0-81), whereas Alcu was less well correlated with A ~ K (r2 = 0.43). Aluminium extracted with 0.01 M CaC12 was also linearly

R. L. Aitken

correlated with A ~ K ( r2 = 0.58). When data for two soils were ignored, A ~ K was highly linearly related to Alc, (r2 = 0.85, n = 58), suggesting that if 1 M KC1 extractable A1 is an unsatisfactory indicator of phytotoxic A1 (Adams 1984; Wright 1989), then Alc, is unlikely to prove a substantially better indicator of the levels of phytotoxic Al.

3.6 4.6 6.6 6.6 7.6

Soil pH (1:5 water)

900

600

300

0

Table 3. Correlation coefficients (7) for the relationships between measures of extractable A1 and selected soil properties for 60 soils

ACEC A l c u - ~ AILa Alcu Alca A ~ K Clay

Organic C 0-540* 0.755* 0.500* 0.694* 0.075 0.223 0-349 Clay 0.588" 0.422* 0.100 0.347 -0.125 0.011 A ~ K 0.312 0.364 0.898* 0.652* 0.759* Aka 0.087 0.167 0.674* 0.407 Alcu 0.628" 0.943* 0.862* AIL, 0.453* 0.667* A h - K 0.635*

3.6 4.6 6.6 6.6 7.6

- o

fb) 0.33M -

0 a - " "oO;Oo - 0

O O * 0 .

moo 0983 080

- 0 o0 04 000 p4 * 0.

Fig. 2. Relationships between soil pH (1 : 5 soil: water) and aluminium extacted with (a) 1 M KCl, (b) 0.33 M

Lac13 and (c) 0.5 M CuClz for 60 soils.

Aluminium in Queensland Soils

Alcu and AIL, were significantly ( P < 0.001) correlated with organic carbon, whereas none of the extractable A1 measures was correlated with clay content. In addition to exchangeable Al, A1 associated with organic matter and soil minerals would act as reservoirs for Al. The difference Alcu-AIK (AlcU-~) might therefore be expected to be related to either, or both, of organic carbon and clay content. Both organic carbon and clay content were correlated with Alc,-~ (Table 3). However, step-up multiple linear regression indicated that the contribution of organic carbon to AlcUFK was highly significant ( P < 0.001), whereas that of clay was less important ( P < 0.05) and the relationship was given by:

A l c u - ~ = 203(organic carbon) + 2.2(clay) - 71 ( R ~ = 0.60, n = 60),

in which A l c u - ~ is expressed as mg A1 kg-I soil and both carbon and clay are as % by weight.

All of the soils used in this study were mineral soils (organic carbon I 4.7%). Therefore, despite the likelihood of increased dissolution of clay minerals at the lower pH of CuC12 suspensions, the greater amounts of A1 extracted with CuC12 (Table 1) are attributable mainly to the displacement of A1 associated with organic matter. Conyers (1990) also reported a good relationship between Alc , -~ and organic carbon for some southern New South Wales soils and concluded that A l c u - ~ mainly represents organically bound Al. Previous studies, using soils with considerably higher organic matter contents than those used in this study, have also demonstrated the ability of CuClz to displace A1 from organic matter (Hargrove and Thomas 1981; Oates and Kamprath 1983b).

The A1 bound to organic matter might be expected to have implications with respect to the effective cation exchange capacity (ECEC) of the soil. The ECEC would be affected by clay content, organic carbon content and soil pH, with ECEC increasing as one or more of clay, organic carbon and pH increased. However, since A1 bound to organic matter is not readily exchangeable, it would be expected to block cation exchange sites and have a negative effect on ECEC (Thomas and Hargrove 1984). This possibility was evaluated for this suite of soils by step-up multiple linear regression of clay, organic carbon, pH and Alc , -~ against ECEC on the assumption that all of the A l c U - ~ was associated with organic matter. Each of clay, organic carbon (OC) and pH were significantly (P < 0.05) positively correlated with ECEC, whereas Alcu-K was significantly ( P < 0.05) negatively correlated with ECEC. The higher the Alcu-~, the lower the ECEC for a given level of clay, organic carbon and pH. The relationship was:

ECEC = 0.0779(clay) + 5.82(OC) + 4-95(pH) - 0-02(Alcu-K) - 24.5

This result is in agreement with that of Martin and Reeve (1958), who showed that the apparent exchange capacity of an organic-matter suspension decreased with increasing A1 content. The beneficial role of organic matter in mitigating A1 toxicity in soil solution of acid soils is countered, to a small extent, by the bound A1 reducing the ECEC.

R. L. Aitken

Both Alcu and Alcu-K were correlated with ACEC (Table 3). This may result from the development of charged sites on organic matter as organically bound A1 is hydrolysed and/or the fact that organic carbon, ACEC, Alcu and Alc , -~ were intercorrelated. Step-up multiple linear regression of the measures of extractable Al, AlcU-~, organic carbon and clay content against ACEC indicated that only Alcu-~ , organic carbon and clay content significantly (P < 0.001) contributed to ACEC.

Relationships between pH Buffer Capacity, Lime Requirement and Extractable A1

Although AIK is reduced to negligible levels at pHw values >5-5 (Fig. 2a), pH buffering in soils does not cease as the pH increases above pH 5.5 and mechanisms other than AIK must contribute to pH buffering. Scatterplots of A ~ K , AILa, Alcu, A l c u - ~ and ACEC against pH buffer capacity are presented in Fig. 3. Although Alcu, Alc,-K and ACEC were all linearly correlated with pH buffer capacity with coefficients of determination of 0.49, 0.52 and 0.76, respectively (Figs 3c, d and e), the relationship between ACEC and pH buffer capacity was superior (Fig. 3e). Thomas and Hargrove (1984) observed that the degree of hydrolysis of A1 bound to organic matter increased with increasing pH, effectively increasing the CEC as the hydrolysed A1 counters less negative charge on the functional groups. The results obtained in this study suggest that the hydrolysis of A1 associated with organic matter makes a contribution to pH buffering in these soils. In a study of three soils and one Al-substituted peat, Bloom et al. (19793) concluded that the hydroloysis of A1 adsorbed by organic matter was a major source of pH buffering. The better relationship between ACEC and pH buffer capacity compared with that between Alcu and pH buffer capacity (Fig. 3) indicates that other mechanisms such as deprotonation of organic groups and variable charge minerals are also involved in pH buffering. This view is supported by the results of Hargrove and Thomas (1984), who observed that, while CuC12 extracted all of the A1 from organic matter, Alcu accounted for only a portion of the titratable acidity when the organic matter contained low to moderate A1 contents.

Studies by Oates and Kamprath (19833) and Hargrove and Thomas (1984) have proposed that A1 extracted with LaC13 and CuC12 be evaluated as a basis for calculating lime requirements. Oates and Kamprath (19833) concluded that lime requirement calculated from AIL, was a good indicator of the lime required to neutralize soil Al, whereas if a target soil pH of around 6 was used as the criterion for lime requirement, then Alcu should be used to calculate lime requirement. Fig. 4 shows scatter plots of lime requirements calculated from Alcu and AILa against lime requirements to pH values of 5 - 5 and 6.5. Theoretical 1 : 1 lines and linear regression equations for each relationship are also shown. The ability of CuC12 and LaC13 extractable A1 to estimate lime requirement (LR) depended on the target pH (Fig. 4). Liming to pHw 5-5 would be sufficient to reduce KC1 extractable A1 to negligible levels (Fig. 2), and the relationship shown in Fig. 4a provides some support for the finding of Oates and Kamprath (1983b) that LR based on AILa was sufficient to neutralize KC1 extractable Al. Most points tended to be close to the 1: 1 line. The underestimation of actual lime requirement to pH 6.5 by LR calculated from AILa (Fig. 4b) would be expected, since there was little or no AIL, extracted from soils with pHw values >5-5

Aluminium in Queensland Soils

(Fig. 2). Lime requirement calculated from Alc, was poorly correlated with actual lime requirement to pH 5 . 5 and, in most soils, substantially overestimated actual lime requirement (Fig. 4c). In contrast, Fig. 4d shows that in most soils lime requirement calculated from Ale, was a reasonable approximation of acutal lime requirement to pH 6 .5 . The amount of lime required to raise soil pH to some predetermined value depends on both the pH buffer capacity and initial soil pH. Because of this and the likelihood that buffering in soils results from a combination of chemical reactions, it is unlikely that extraction of a

pH buffer capacity (g CaC03 kg" unit-' pH)

Fig. 3. Relationships between soil pH buffer capacity and (a) 1 M KC1 extractable Al, (b) 0-33 M Lac13 extractable Al, (c) 0.5 M CuClz extractable Al, (d) Alcu-~ and (e) the change in CEC per unit pH (ACEC).

R. L. Aitken

single component (for example, Al) will provide a satisfactory index of lime requirement. Moreover, in a number of acidic Queensland soils, Mn toxicity can be the limitation to plant growth (Fergus 1954; Elpinstone and Hall 1986; Aitken et al. 1989) and extractable A1 can be quite low. In cases such as these, lime would be applied to achieve some target pH and lime rates calculated from extractable A1 would not prove satisfactory. Lime requirements obtained from buffer methods (which integrate both the initial soil pH and the soil pH buffer capacity) may prove more reliable.

Fig. 4. Relationships between lime requirements calculated to neutralize 0.33 M LaC13- extractable A1 and actual lime requirements to (a) pH 5.5 and (b) pH 6.5, and between lime requirements calculated to neutralize 0.5 M CuC12-extractable A1 and actual lime requirements to (c) pH 5.5 and (d) pH 6.5. Correlation coefficients for linear functions (solid lines) fitted to the data are shown. Broken lines show theoretical 1 : 1 relationships.

Acknowledgments

The technical assistance of Peter McKinley and Cornelia Rahardjo is gratefully acknowledged.

Aluminium in Queensland Soils

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Manuscript received 21 June 1991, accepted 24 September 1991