COMPARISON OF H/AL STOICHIOMETRY OF MINERAL AND ORGANIC SOILS IN BRAZIL 1071

R. Bras. Ci. Solo, 33:1071-1076, 2009

COMPARISON OF H/AL STOICHIOMETRY OF MINERAL ANDORGANIC SOILS IN BRAZIL(1)

Daniel Vidal Perez(2), Lúcia Helena Cunha dos Anjos(3), Adierson GilvaniEbeling(4) & Marcos Gervasio Pereira(3)

SUMMARY

Exchangeable Al has been used as a criterion for the calculation of limerequirement in several Brazilian States. However, the laboratory method withextraction by a 1 mol L-1 KCl solution followed by indirect alkaline titration is notaccurate for some Brazilian soils, mainly in the case of soils with high organicmatter content. The objective of this study was therefore to evaluate thestoichiometry of H+/Al3+ in KCl soil extracts. The results suggested that organicallycomplexed Al is the main contributor to exchangeable acidity in soils enrichedwith organic matter. Liming recommendations for organic soils based exclusivelyon exchangeable Al determined by the NaOH titration method should therefore berevised.

Index terms: acid-base titration; ICP-OES; KCl; Histosols.

RESUMO: COMPARAÇÃO DA ESTEQUIOMETRIA H/Al EM SOLOSMINERAIS E ORGÂNICOS BRASILEIROS

A determinação de alumínio trocável é utilizada como critério para cálculo de calagemem vários estados brasileiros. Contudo, a determinação indireta pela titulação com NaOH,após extração com solução de KCl 1 mol L-1, pode não ser adequada para certos tipos de solosbrasileiros, notadamente aqueles que apresentem altos teores de carbono orgânico. Dessaforma, o principal objetivo deste trabalho foi avaliar a estequiometria da relação H+/Al3+ emextratos de KCl. Os resultados obtidos sugerem que o Al complexado pela matéria orgânica,

(1) Work presented at the 18th World Congress of Soil Science hold at Philadelphia, Pennsylvania, USA. Recebido para publicaçãoem novembro de 2008 e aprovado em maio de 2009.

(2) Researcher, Embrapa Solos, Jardim Botânico 1024, CEP 22460-000 Rio de Janeiro (RJ). E-mail: daniel@cnps.embrapa.br(3) Professor, Soils Dept., Universidade Federal Rural do Rio de Janeiro – UFRRJ. Km 47 CEP 23890-000 Seropédica (RJ).

E-mails: lanjos@ufrrj.br; gervasio@ufrrj.br(4) Ph.D. Student, Soils Dept., UFRRJ. E-mail: adiersonge@gmail.com

NOTA

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em solos orgânicos, é o principal contribuinte para a acidez trocável obtida por titulação.Dessa forma, a recomendação de calagem em solos orgânicos baseado somente na determinaçãode alumínio trocável por titulação com NaOH deve ser revista.

Termos de indexação: titulação ácido-base; ICP-OES; KCl; Organossolos.

INTRODUCTION

The determination of Al is a permanent concern ofsoil fertility management. Soil chemists areconstantly testing methods that take into account theeffective soil exchange capacity, organic mattercontent, texture, and the pH-dependent charges ofvarious clays minerals (Menzies, 2003).

The titrimetric determination of Al3+ in 1 mol L-1

KCl soil extract using bromothymol blue as indicatorhas been widely used in Brazil (Raij et al., 1994;Bernardi et al., 2002). However, in fact, this proceduredetermines H+ in the KCl extract. This is recognizedas the exchangeable acidity (Sposito, 1989). In thiscontext, there is therefore no differentiation betweenthe “exchangeable H+” and H+ developed by Al3+ hydrolysis.

From very acid mineral soils with neutral saltsolutions, such as KCl, more Al3+ than H+ is extracted(Veith, 1977; Bloom et al., 2005), while in the organicsoil horizons, the quantity of extractable H+ typicallyexceeds Al3+ (Bloom et al., 2005). But Sparks (2003)observed that much of the H+ that appears to beexchangeable in organic soils may be due to Al3+

hydrolysis in the organic matter resulting in H+.An important tool for the differentiation between

H+ and Al3+ is based on the use of the titration/back-titration procedure (Bertsch & Bloom, 1996). Thismethod was first described by Yuan (1959). However,Coscione et al. (1998) showed the lower sensitivity ofexchangeable Al3+ determined by back-titration. Aldetermination by spectrophotometric methods, suchas ICP-OES, is doubtlessly more sensitive and reliable(Bertsch & Bloom, 1996). The purpose of this studywas therefore the evaluation of H and Al in 1 mol L-1

KCl extracts to determine whether the theoreticalvalue of 3 of Al hydrolysis predominates in theexchangeable acidity.

MATERIALS AND METHODS

The method for exchangeable Al extraction of mostBrazilian soil laboratories is based on the Al exchangefrom the surface of soil colloids using a non-buffered1 mol L-1 KCl solution (Embrapa, 1997; Bernardi etal., 2002). A soil:solution mixture, ratio 1:10, wasvigorously shaken for 5 min and left to standovernight. Then a 25 mL aliquot of the supernatantwas sampled and Al was indirectly quantified bytitration with 0.025 mol L-1 NaOH using

Bromothymol Blue as indicator. In this case, eachmol of NaOH neutralizes 1 mol H+. Thus, the numberof H+ mmoles in the 25 mL aliquot is (A x 0.025),where A is the volume (mL) of NaOH consumed intitration. Considering that the initial volume of KClwas 100 mL and the soil mass was 10 g, the resultcould be expressed as (A x 10) mmol kg-1 or A cmol kg-1

of H+. For further discussions in the text, this type ofmass concentration unit will be used, since it expressesthe real titration, i.e., H+ rather than Al3+. However,to clarify the use of titration in Al3+ routine analysis,the full calculation will be shown here. To expressthe result in terms of Al3+, it must be assumed thateach Al3+ produces 3 H+ after complete hydrolysis.Thus, the result would be expressed as (A x 10)/3 mmol kg-1 or (A/3) cmol kg-1 of Al3+. Since themmol Al3+ is equivalent to 3 mmolc Al3+, the finalresult will be expressed as (A x 10) mmolc kg-1 or Acmolc kg-1 of Al3+.

For a direct quantification of Al in the KCl extract,another aliquot of the supernatant was analyzed usingan inductively coupled plasma atomic emissionspectrometer (Perkin Elmer Optima 3000). Theoperating frequency was 50–60 MHz at a power of1,500 W with a torch argon flow rate of 15 L min-1.The selected Al atomic line was at 308.211 nm. Anitrogen gas purge flow rate of 5.0 L min-1 was usedthroughout the analysis. The sample flow rate was2.0 L min-1. In this case, the result was expressed asB mg kg-1 of Al3+, considering that the initial volumeof KCl was 100 mL and the soil mass was 10 g. Sincethe mmol of Al is, approximately, 27 mg, the resultcould be expressed as (B/27) mmol kg-1 or (B/270)cmol kg-1 of Al3+. For further discussions in the text,this kind of mass concentration unit will be used, sinceit expresses the real mass concentration of the element.However, to clarify the use of spectrophotometricdeterminations in Al3+ routine analysis, we will showthe full calculation. Since the mmol Al3+ is equivalentto 3 mmolc Al3+, the final result will be expressed as(B/9) mmolc kg-1 or (B/90) cmolc kg-1 of Al3+.

In the case of soils with high organic mattercontent, Al extraction by CuCl2 0.2 mol L-1 wasperformed according to Bertsch & Bloom (1996). TheAl determination followed the ICP-OES proceduredescribed above.

Thirty seven acid soil samples were selectedrepresenting different soil classes (Oxisol, Ultisol,Inceptisol, Entisol, Histosol) from different BrazilianStates (RJ, AM, MS, SP, AL, PB, PE, PR, MG, andDF). Their main properties are shown in table 1.

COMPARISON OF H/AL STOICHIOMETRY OF MINERAL AND ORGANIC SOILS IN BRAZIL 1073

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RESULTS AND DISCUSSION

In a first approach, it is possible to arrange thesoil samples in two groups (Tables 2 and 3). The first(Table 2) consisted of 18 horizons of mineral soilswhere the main dominant clay mineralogy is kaoliniteand/or iron/aluminum oxy-hydroxides. In this group,the mean H/Al ratio was around 3.4. In addition tothe potential titration errors in a routine aluminumanalyses, the presence of soluble Mn2+ and Fe3+ inKCl soil extract (Table 2) could represent a smallcontribution of H+ through the hydrolysis of thesemetals, which would explain a positive variation fromthe expected 3.0 ratio. Our results corroborate thefindings of McLean (1976), Veith (1977) and Bloom etal. (2005) that most of the protons measured in the

salt extract of mineral soils, i.e., the exchangeableacidity, may come from the hydrolysis of exchangeableAl3+.

The second group (Table 3) was composed of organichorizons with a H/Al ratio of around 0.4. In this case,the lower H+ concentration suggested that the“exchangeable H+” pool was not the major part ofexchangeable acidity in the studied organic soils. Thismay be explained by the findings of Ross et al. (2008).They showed that the quantity of exchangeable H+

may be insignificant at a pH above 4.5. The mean pHof the studied organic soils was around 4.9 (Table 3).

Another fact that corroborates the insignificantcontribution of exchangeable H+ to exchangeableacidity is related to the stoichiometry of the organic-

Table 1. Soil classification and localization of 37 selected soil samples

(1) SiBCS: Brazilian Soil Classification System (Embrapa, 2006); Soil Taxonomy (Soil Survey Staff, 1999).

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Table 2. Some chemical analyses of 18 mineral soil samples

(1) Extracted by 1 mol L-1 KCL and determined by ICP-OES. (2) Extracted by 1 mol L-1 KCL and determined by acid-based titration.(3) Determined by dichromate wet oxidation (Embrapa, 1997). (4) SD mean standard deviation. (5) CV mean coefficient of variation.

Table 3. Some chemical analyses of 19 organic soil samples studied

(1) Extracted by 1 mol L-1 KCL and determined by ICP-OES. (2) Extracted by 1 mol L-1 KCL and determined by acid-based titration.(3) Determined by dichromate wet oxidation (Embrapa, 1997). (4) SD mean standard deviation. (5) CV mean coefficient of variation.

COMPARISON OF H/AL STOICHIOMETRY OF MINERAL AND ORGANIC SOILS IN BRAZIL 1075

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bound Al. The high values of CuCl2-extracted Alcompared with those obtained by KCl (Table 3) aregenerally related to organic-bound Al (Shuman, 1990;Soon, 1993; Barra et al., 2001). In this case, there isa relative preference for hydroxyl-Al ions over Al3+

(Schnitzer & Skinner, 1963; Bloom et al., 1979).Hargrove & Thomas (1982) showed that as the Alcontent of the organic matter increased, the effectivevalence of the adsorbed Al varied from 2.12 to 0.93.Nissinen et al. (1999) and Simonsson (2000) reportedbinding stoichiometry values ranging from 1.1 and2.4 in literature. Therefore, the forced hydrolysis oforganic-bound Al by alkaline titration should generatelower H/Al ratio values.

The results obtained in the mineral soils showedthat most of the soil exchangeable acidity is primarilydue to Al3+. However, in the case of the studied organicsoils, the contribution of hydroxyl Al bound to theorganic matter is probably the main pool ofexchangeable acidity. The contribution ofexchangeable H+ does not seem to be relevant for thestudied soils. Considering that most authors indicatethat the toxic aluminum is Al3+ and the monomericAl-hydroxy species (Bloom et al., 2005), all limingrecommendations for organic soils based exclusivelyon exchangeable Al determined by NaOH-titrationshould therefore be revised. These findings corroboratethe studies of Pereira et al. (2005) and Ebeling et al.(2008). They pointed out the need of new analyticalroutine methods to properly study and monitor themanagement of Histosols in Brazil. Besides, therelationship between exchangeable acidity determinedby acid-based titration and Al determined directly byspectrophotometry (AAS or ICP) could be a usefulindex to characterize organic horizons.

ACKNOWLEDGEMENTS

The authors are grateful to CNPq and FAPERJfor the grants and funding support.

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