TECHNICAL ARTICLE

Components of Surface Charge in Tropical Soils With Contrasting Mineralogies

Luís Reynaldo Ferracciú Alleoni,1 Ricardo Trippia dos Guimarães Peixoto,2 Antônio Carlos de Azevedo, and Leônidas Carrijo Azevedo Melo1

Abstract: Study o f surface electric charges o f soi l col lo ids helps in understanding the phys icochemica l phenomena that influence the capacity o f retaining nutrients and their availabil i ty to plants. The structural charge ( σ 0 . m i n ) , the variable charge ( σ q . m i n ) , and the organic matter ( O M ) charge ( σ t o t a l OM) o f 12 tropical soils with contrasting mineralogies were evaluated based on the difference o f selectivity for C s + and Li- between the functional ionizable surfaces groups and the mineral 2:1 siloxane surface. Soi ls were d iv ided into three groups: G l , soils wi th a predominance o f kaolinite and gibbsite in the clay fraction; G 2 , soils with hydroxy-interlayered 2:1 minerals; and G 3 , soils wi th smectite but without hydroxy-interlayered 2:1 minerals. The C s ab­sorption method was efficient for detecting the charge components in tropical soils. The mineral structural charge contributed 11% and 16%, 2 8 % and 3 1 % , and 5 2 % and 5 9 % o f total soi l charge o f A and B horizons o f soils from groups G 1 , G 2 , and G 3 , respectively. O n the other hand, O M contributed 5 3 % and 37%, 4 8 % and 4 1 % , and 2 1 % and 2 0 % o f total charge for the same samples, respectively. In highly weathered soils o f group G1 and, to a lesser extent, in soils in group G 2 , surface charges depended mainly on their variable components, resulting from the O M ( σ t o t a l OM). as well as from imperfections found in 1:1 minerals ( σ q . m i n ) . The importance o f O M in determining the magnitude o f electric charges in humid tropical soils is highlighted.

Key word s : C s complexat ion method, cation exchange capacity,

smectite, HIV, soi l organic matter.

(Soil Sci 2009; 174: 629-638)

Many physicochemical soil attributes are influenced by the nature and the amount of charge on their colloid surfaces.

A better understanding of the amount and distribution of charge components on the colloid's surface may be useful in evaluating the cationic and anionic exchange capacity (Gillman, 1984; Fontes and Alleoni, 2006), as well as the pedogenetic develop­ment of soils (Charlet and Sposito, 1987; Chorover et al., 2004).

The net structural surface charge density (σ 0 ) , derived from isomorphic replacement, is frequently underestimated in studies of soils from tropical and humid subtropical regions, where 1:1 layer-type clay minerals and Fe and A1 oxides are predominant. In such colloids, σ 0 is small, generally less than 20 mmol kg-1

(Sposito, 1989). However, some Brazilian soils exhibit variable amounts of hydroxy-interlayered 2:1 minerals (2:1 HI) such as vermiculites (HIV) and smectites (Potter and Kämpf, 1981), in which σ0 has a determining relevance in the total surface charge density of particle (σ p ) .

Based on the development of the surface complexation theory (Sposito, 2004) and on the equation for charge balance. Sposito and contributors developed two methods that allow the measurement of net surface structural-charge density ([σ 0] per­manent charge, derived from isomorphic replacement) and the net ionic-surface charge density ([σ q] variable charge, dependant on conditions of the medium, mainly the pH and ionic strength of the soil solution) (Charlet and Sposito, 1987; Anderson and Sposito, 1991; Chorover and Sposito, 1993). The measurement of structural charge σ 0 in 2:1 phyllosilicates can be based on the difference of selectivity for Cs + and L i + between the siloxane surface and the ionizable-surface groups because of formation of inner-sphere complexes on the ditrigonal pores of the siloxane surfaces of these minerals. This method was successfully used by Anderson and Sposito (1991), Chorover and Sposito (1993), Peixoto (1995), and Weber et al. (2005) in Brazilian soils. Fontes and Sposito (1995) studied samples from B horizons of Oxisols with low organic carbon contents and observed that the method allowed the detection of small amounts of permanent-charge minerals, many times undetectable by X-ray diffraction.

Although soils from tropical and subtropical regions have undergone an intense weathering process leading to a predomi­nance of kaolinite and Fe and A1 oxides in the clay fraction, significant amounts of 2:1 layer-type minerals are found in some situations. Among these minerals, the most commonly found are those with A1 and Fe interlayer hydroxy polymers (Moller and Klamt, 1982; Curi et al., 1984). The most probable formation of these minerals is caused by A1 polymerization (or Fe, more rarely) in an acidic medium in the presence of clayey-silicate minerals of 2:1 layer type, mainly vermiculite and smectite. Muscovite weathering in acidic medium can, for example, produce HIV These clayey-silicate minerals are intermediate between vermi­culites and smectite on one side and chlorites on the other. Therefore, their chemical, physical, and mineralogical properties differ from these minerals; as a result, soils that contain them should be studied, described, and classified in a specific way (Fontes, 1990).

The 2:1 HIV minerals can play an important role in soil fertility (Peixoto et al., 1995). The selectivity of these minerals for K (but not the fixation) may depend on the intensity of having the interlayers filled with A1 (or Fe) hydroxy polymers. Peixoto et al. (1995) verified in Brazilian Hapudox soils that the σ0 decreased proportionally with the increase in blockage of permanent-charge sites by hydroxy polymers, which could be related to the A1 saturation values of the soils. Barré et al. (2008) supported the view of 2:1 clay minerals as a K reservoir and specified the role of each of the 2:1 clay mineral populations in K absorption and release. A l l of the 2:1 clay minerals commonly found in temperate grassland soil clay assemblages containing illite, interstratified illite/smectite, and soil vermiculite take part in the soil K reservoir. The five clay mineral populations com­monly found in temperate grassland soil clay assemblages are modified through K absorption or removal, and these modifica­tions seemed to be reversible.

1Department o f Soil Science, University o f São Paulo-CP. 09-13418-900-Piracicaba, São Paulo, Brazi l . Dr. Luis Reynaldo Ferracciú Al leoni is corre­sponding author. E-mail: alleoni@esalq.usp.br 2 E M B R A P A - N a t i o n a l Center o f Soil Research, Rua Jardim Botânico, Rio de Janeiro, Brazi l . Received March 20, 2009, and in revised form September 14, 2009. Accepted for publication September 16. 2009. Copyright © 2009 by Lippincott Will iams & Wilkins , Inc. ISSN: 0038-075X DOI : 10.1097/SS.0b013e3181c17a93

Soil Science • Volume 174, Number 11, November 2009 www.soilsci.com | 629

Alleoni et al. Soil Science • Volume 174, Number 11, November 2009

An evaluation of the contribution of permanent charges to Brazilian soils with different mineralogical compositions was carried out through the use of the Cs absorption method. The effects of organic matter (OM) and the presence of 2:1 HI minerals on the composition of charge components in these soils were also evaluated.

MATERIALS AND METHODS Twelve soil samples were collected in areas traditionally

cropped in Brazil (Fig. 1): (i) Acrudox ([Acr 1] Guaira, state of São Paulo, 20°19'06"S, 48°18'38"W), (ii) Acrudox ([Acr 2]

Uberlândia, state of Minas Gerais, 18°55'07"S, 48°16'38"W), (iii) Acrudox ([Acr 3] Uberlândia, Minas Gerais, 18°54'03"S, 48°18'33"W), (iv) Hapludox ([Hap 1] Rondonópolis, state of Mato Grosso, 16°28'15"S, 54°38'08"W), (v) Eutrudox ([Eut] Ribeirão Preto, São Paulo, 21°10'39"S, 47°48'37"W), (vi) Kandiudult ([Kand] Otacílio Costa, state of Santa Catarina, 27°28'59"S, 50°07'19"W), (vii) Hapludox ([Hap 2] Ibirubá, state of Rio Grande do Sul, -28°37'39"S, 53°05'23"W), (viii) Haploperox ([Haplo] Santa Cecilia, 26°57'39"S, 50°25'37"W), (ix) Argiudoll ([Arg 1] Bagé, Rio Grande do Sul, 31°19'53"S, 54°06'25"W), (x) Argiudoll ([Arg 2] São Sepé, Rio Grande do

FIG. 1. Distribution of the soils inside the Brazilian territory.

630 | www.soilsci.com © 2009 Lippincott Williams & Wilkins

Soil Science • Volume 174, Number 11, November 2009 Surface Charge of Tropical Soils

TABLE 1. Physicochemical Attributes and Mineralogy of Surface (A) and Subsurface (B) Horizons of the Soils

Total C R E M C § C l a y V m C E C

G r o u p Soil † p H H2O Δ p H ‡ g kg-1 % - - - mmolc kg-1 Ki Mineralogy

A Horizon 1 Acr 1 4.6 -0.5 18.6 2.7 350 17 45 31 0.8 Kt > Gb > A(Qz)

Acr 2 4.6 -0.5 17.3 1.6 786 9 50 24 0.9 Kt > Gb > A(Qz) Acr 3 4.5 -0.3 22.0 3.0 774 8 58 19 0.8 Kt > Gb > Qz Hap 1 4.7 -0.9 27.2 2.9 650 5 75 24 1.5 Kt >>> Gb > Qz

2 Eut 5.7 -0.9 30.0 3.2 820 (.4 2 171 1.4 Kt > Gb >> HIV > A Kand 3.9 -0.3 39.3 4.6 606 89 2 75 1.4 Kt > HIV >> Gb > Qz Hap 2 4.9 -0.6 29.2 1.5 573 30 5 69 1.7 Kt > HIV > Qz Haplo 4.9 -0.9 23.1 1.4 659 16 46 47 1.6 Kt > HIV

3 Arg 1 4.6 -0.6 14.0 1.2 101 54 8 37 4.3 Kt-S > Kt Arg 2 4.5 -0.7 15.7 1.7 163 58 12 70 3.7 S > Il > Kt-S Haplus 8.1 -1.1 19.8 15.2 510 100 0 301 3.0 S >> Kt Haplud 6.2 -1.0 19.6 2.4 460 84 0 281 3.2 Kt > S Group 1|| 4.6 -0.6 21.3 2.6 640 10 57 25 1.0 Group 2 4.9 -0.7 30.4 2.7 665 55 14 91 1.5 Group 3 5.9 -0.9 17.3 5.1 309 74 5 172 3.6

B Horizon 1 Acr 1 5.1 +0.6 6.1 2.1 450 21 0 17 0.8 Kt > Gb > A(Qz)

Acr 2 5.1 +0.6 10.1 4.5 810 15 19 19 0.8 Kt > Gb > A(Qz) Acr 3 4.8 +0.3 11.8 2.5 801 7 26 19 0.8 Kt > Gb > Qz Hap 1 5.1 -0.9 6.4 2.9 800 5 68 25 1.5 Kt >>> Gb > Qz

2 Eut 6.0 -0.7 11.4 2.6 700 67 0 88 1.6 Kt > Gb >> HIV > A Kand 4.8 -0.9 9.0 2.6 687 53 3 34 1.6 Kt > HIV > Gb (Gt.Il) Hap 2 5.1 -0.9 8.3 0.7 783 43 13 39 1.6 Kt > HIV > Qz Haplo 4.8 -0.7 13.2 0.9 640 26 4 40 1.5 Kt > HIV

3 Arg 1 7.0 -1.7 8.1 1.1 270 94 0 144 3.2 Kt-S > S > Kt Arg 2 5.3 -1.6 9.9 2.4 460 53 34 153 2.5 S > Kt-S > 11 Haplus 8.1 -1.1 15.4 11.4 530 100 0 312 3.0 S >> Kt Haplud 6.7 -0.6 4.2 1.2 210 91 0 258 4.2 Kt > S Group 1 5.0 0.2 8.6 3.0 715 12 28 20 1.0 Group 2 5.2 -0.8 10.5 1.7 703 47 5 55 1.6 Group 3 6.8 -1.3 9.4 4.0 368 84 9 217 3.2

†Soil, see Fig. 1. ‡ΔpH, pH KCl 1 mol/L - pH H 2O.

§Remaining C after ashing at 400 °C. "Average of each group of soils. V: basis saturation (Ca + Mg + K + Na) x 100/CEC; m: aluminum saturation (Al x 100/effective CEC); CEC, compulsive exchange method using

BaCl 2; Ki: (%SiO2/%AI2O3) x 1.7. Relative abundance of identified minerals: Kt, kaolinite; Gb, gibbsite; A, anatase; Qz, quartz; HIV, hydroxy-interlayer vermiculite; Gt, goethite;

Il, illite; Kt-S, interstratified kaolinite-smectite; S, smectite.

Sul, 30°09'38"S, 53°33'55"W), (xi) Haplusterts ([Haplus] Juazeiro, state of Bahia, 09°24'42"S, 40°29'55"W), (xii) Hapludoll ([Haplud], Piracicaba, São Paulo, 22°43'31"S, 47°38'57"W).

Soils were divided into three groups with four soils each, and each group had soils with similar mineralogy and texture. Group 1 was composed of highly weathered soils, with predominance of kaolinite (Kt) and gibbsite (Gb) and absence of 2:1 layer-type clay minerals. Group 2 was composed by 2:1 layer-type clay minerals with hydroxy interlayers (2:1 HI), whereas Group 3 contained soils with smectite, without hydroxy interlayers (Table 1).

Samples were collected from the surface layer (0-0.2 m) and from the highest expression of the B horizon. Soils were air

© 2009 Lippincott Williams & Wilkins

dried, sieved through a 2-mm sieve, and characterized by chemical, physical, and mineralogical analysis (Camargo et al., 1986). Total carbon was determined by dry combustion using a C N H LECO equipment (Nelson and Sommers, 1982). Soil pH was determined potentiometrically in both H2O and 1.0 mol/L K C l solutions (1:2.5 solid-liquid ratio) using a combined glass-calomel electrode; the equation ΔpH = p H K C l - p H H 2 O was used to verify the sign of the net charge (Mekaru and Uehara, 1972). Iron, aluminum, and silica were extracted using 9.0 mol/L H 2 S O 4 , and the contents of Fe and A1 were determined using atomic absorption spectrophotometry (AAS), whereas Si was determined by gravimetry (Camargo et al., 1986). The Ki weathering index was calculated with the molar relation method where Ki = (%SiO2/60)/(%A12O3/102).

www.soilsci.com | 631

Alleoni et al. Soil Science • Volume 174, Number 11, November 2009

FIG. 2. Typical diffratograms of soils from Group 1 (Acrudox 1) (A), Group 2 (Haploperox) (B), and Group 3 (Haplusterts) (C). Kt, kaolinite; Gb, gibbsite; Qt, quartz; 2:1 HI, hydroxy-interlayered 2:1.

632 I www.soilsci.com © 2009 Lippincott Williams & Wilkins

Soil Science • Volume 174, Number 11 , November 2009 Surface Charge of Tropical Soils

Exchangeable Ca, Mg, K, and Na were extracted with an anionic exchange resin. The contents of Ca and Mg were quan­tified using an A A S , whereas K and Na were determined using a flame photometer. Granulometric analysis was performed by the densimeter method (Camargo et al., 1986). The cation exchange capacity (CEC) was determined using the compulsive exchange method (Gillman, 1979). Two grams of each soil sample were saturated with 20 g of an unbuffered 0.05-mol/L BaC12 solution, after which 20 g of 0.0025-mol/L M g S O 4 solution was added. The mixture was then gently shaken overnight to replace any Ba present by Mg. The supernatant was decanted, and Mg was measured by A A S .

Mineralogical analysis of the clay fraction after treatment with Na-CDB (dithionite-citrate-bicarbonate) was conducted using X-ray analysis of oriented mounts on glass slides. Samples were saturated with either Mg or K, solvated with glycerol, heated to 350 °C and 550 °C, and then X-rayed. X-ray dif-fractograms were obtained with a diffractometer (Philips X'pert System) operating at 40 kV and 40 mA using Ni-filtered Cu-Kα radiation, at 1.2° 2 θ min-1 in the 3 to 35° 2 θ range. Figure 2 shows a typical diffractogram of each soil group.

The permanent (σ 0 ) and variable (σ q ) charges were quantified by the Cs+ complexation method based on the strong preference of permanent-charge sites for C s + over L i + (Peixoto, 1995; after Anderson and Sposito, 1991; Chorover and Sposito, 1993). The soil mass and solutions were reduced 10-fold in relationship to that used in the original method (Weber et al., 2005). To determine the variable charge of the O M ( σ t o t a l OM) and the mineral variable charge (σ q . m i n ) , electric charges of natural soil samples were measured before and after removing O M , using ashing at 400 °C for 2 h (Peixoto et al., 1995). The Cs method measures the number of permanent-charge sites accessible to C s + in 2:1 phyllosilicates (operationally defined as the structural charge σ0) and it is based on the preference for C s + over L i + (difference of selectivity for Cs + and L i + ) between the functional groups of ionizable surface and siloxane surface caused by formation of inner-sphere complexes on the ditrigonal pores of the siloxane surfaces of these minerals. The Cs method was composed of the following four stages:

1. Preparation of CsCI-Saturated Adsorbents A 3.5-g portion of soil was placed into a 50-mL centrifuge

tube containing 24 g 0.5 mol k g - 1 CsCl. This suspension was equilibrated for 30 min. The suspension was then centrifuged at 9000 r.p.m. for 15 min, and the supernatant solutions were dis­carded. The remaining soil was added with 20 g of 0.1 mol kg-1

CsCl. The procedure was repeated once. In the last wash, a 0.05-mol kg-1 CsCl solution was used. The saturated paste was refrigerated at 4 °C for 48 h. After soil saturation, 0.35 g of paste (dry basis) was placed into 50-mL centrifuge tubes (in triplicate) and equilibrated with 35 g of CsCl 0.05 mol k g - 1 . The pH of the supernatant solution was adjusted to 6.0. Subsequently, the suspension was equilibrated for 1 h in a horizontal shaker, centrifuged at 8600 r.p.m. for 30 min, and the supernatant solution was discarded. To determine the water content in the soil paste, duplicates of 0.5 g were weighed and oven dried at 110 °C for 48 h.

2. Formation of Cs + Inner-Sphere Complexes Thirty grams of 95% ethanol was added to the suspension,

which was shaken three times for 10 sec in a shaker and cen­trifuged at 8600 r.p.m. for 30 min. This procedure was repeated until C1- was no longer detected in the supernatant solution by adding 1 mol kg-1 A g N O 3 solution. Washing soil with ethanol was necessary to reduce the concentration of entrained C s + (not adsorbed). This procedure facilitated the subsequent C s + ex­

change for Li+ in the variable-charge sites; however, it does not change the siloxane surfaces (Anderson, 1989). Tubes were transferred to an oven for 48 h at 65 °C to allow the formation of C s + inner-sphere complexes with the siloxane surfaces.

3. Determination of Variable-Charge Surface Density (Net Ionic) ( σ q )

The extraction of Cs+ from sites with lower selectivity was carried out by adding 30 g of a 0.01-mol k g - 1 L i C l solution (pH 6.0) to dry soil pastes. The suspension was then equilibrated for 30 min and centrifuged at 8800 r.p.m. for 1 h. The super­natant was transferred to glass flasks and kept under refriger­ation for later σ q measurement.

4. Determining Effective Permanent-Charge Surface Density ( σ 0 )

Cs + from sites of high selectivity was extracted by adding 30 g of 1.0 mol k g - 1 N H 4 A c to the remaining pastes. The suspension was equilibrated for 30 min in a horizontal shaker and centrifuged at 5400 r.p.m. for 15 min. The supernatant solution was then transferred to 250-mL flasks. The wash with N H 4 A c 1.0 mol kg-1 was repeated more than twice. Cs + concentration in the L iC l and N H 4 A c extracts were measured by A A S .

The methodological process followed the approach pro­posed by Peixoto (1995) based on concepts of the theory of surface complexes (Sposito, 1989). It was assumed that besides the formation of inner-sphere complexes of Cs + with the siloxane ditrigonal cavities of 2:1 phyllosilicates (sites of high selectivity), there is complexation of Cs + by other reactive sites on the surface of soil particles, mainly the soil O M . It is con­sidered that Cs + is bound to soil clays in sites of differing selectivity, and its removal from homoionic saturated soil par­ticles should become progressively difficult as Cs + is displaced from electrostatic interaction and retained, in increasing relative proportions, at sites of higher binding energy. In the soil HI clays, it was expected that the hydroxy-A1 interlayer groups act as "props" inhibiting layer collapse about ions such as K + , N H 4

+ , Rb + , and C s + in the natural Brazilian soil conditions. Therefore, only a part of the surface structural-charge density ([σ0] permanent charge) of these mineral wedge zones may be available for adsorption reactions.

It was assumed that the set of experimental conditions allowed the exchange all Cs complexed on the most accessible interlayered sites (e.g., wedge and "frayed edge "-like) in soil 2:1 clay minerals, considering that the determination procedure is an operational method for the characterization of Cs exchange complexes. For example, the number of N H 4 O A c extraction-washing steps applied in our study created a situation analogous to the finding of de Koning and Comans (2004), when they extracted 100% of the bound radiocaesium from illite by 1 mol/L N H 4 after 5 min of contact time with a Cs-selective adsorbent despite the Cs fixation caused by the collapse of the frayed edges of the clay mineral. Based on previous studies and on procedures that were used to diminish the hysteresis phenomenon of Cs +

sorption on 2:1 clay minerals (e.g., maintain at 4 °C the CsC1-saturated soil samples pastes for up to 2 weeks before initiation of limited-duration [0.5 h] desorption reactions), we assumed that ion exchange reactions had achieved equilibrium following the LiC1 and the N H 4 O A c steps for the batch experimental con­ditions used (e.g., Cs saturation step, soil-solution ration, pH 6, ionic strength, ethanol washing and 110 °C sample treatment, shaking time, number of washings-extractions in each step etc). The ammonium acetate has been considered to be a good extractor of C s + because it is a soft Lewis acid with low hydra­tion energy and because it allows a better atomic emission analysis of Cs + (Anderson and Sposito, 1991).

© 2009 Lippincott Williams & Wilkins www.soilsci.com | 633

Alleoni et al. Soil Science • Volume 174, Number 11, November 2009

In the soil environment, we have a complex and dynamic system, whose mineralogy structural composition depends on the environmental conditions. Therefore, it was considered that the Cs adsorption method is a strictly macroscopic concept that is consistent with several different kinds of exchange mechanism at the molecular level. The mechanism of surface complexation is used to interpret the Cs exchange reactions, but the mech­anism itself must be established by separate experiments. The chosen method should balance easy-to-use features and be representative of the studied natural conditions.

The following surface charge components were then operationally defined: σ q , natural soil = variable surface charge density of natural soil

(Cs + extracted from sites of low selectivity). σ q , m i n

= mineral variable surface charge density (Cs + extracted from sites of low selectivity = C s + determined after the ashing of OM). Reactive sites of low selectivity hold inner-sphere complexes not strong enough to avoid the C s + exchange for the hydrated L i + . These functional groups may be related to imperfections in the lattice, including isomorphic substitution) and/or surface fea­tures at the micrometer to nanometer scale (kinks, vacancies, steps, adatoms; Hochella Jr, 1990), both in silicates and oxides in the clay fraction (Parfitt, 1980; Sposito, 1989).

σ 0 , n a t u r a l soil

= effective permanent surface charge density in natural soil (Cs* extracted from sites of high selectivity). In these reactive sites, inner-sphere complexes are formed from Cs+ with siloxane ditrigonal cavities of 2:1 phyllosi­licates and also are derived from complexes formed by the reactive functional groups to soil O M .

σ 0 , m i n

= effective mineral permanent surface charge den­sity (Cs + extracted from sites of high selectivity deter­mined in the soil after the ashing of OM). This charge is derived from the formation of inner-sphere complexes from Cs+ only with siloxane ditrigonal cavities of 2:1 phyllosilicates.

σ M O , n a t u r a l s o i l = O M charge density in natural soil ( σ N A T S O I L OM

= σq, n a t soil - σ q . m i n ) derived from organic functional groups with low affinity for Cs + .

σ m i n O M = O M charge density in the mineral (σr m i n O M = σ0, nat

s o i l - σ 0 . m i n ) derived from organic functional groups with high affinity for Cs + .

σ t o t a l O M = total O M charge density in the soil (σ T O T A L OM = σ n a t

s o i l OM + σmin OM) derived from organic functional groups, σtotal SOIL

= total surface charge density of the soil (σ t o t a l s o i l = σ0, m i n + ΣQ. M I N + σ t o t a l OM).

The electric charge of O M per kilogram of total carbon in the soil (TC = C t o t a l , in g kg-1 of soil) was calculated based on the following equation: Σ S O M . C t o t a l

= ( σ t o t a l OM / C t o t a l )

* 1000. The electric charge derived from O M of soil per kilogram of soil TC (σSOM. total C) represents the potential reactivity of func­tional groups existing in the soil O M normally related to its degree of humification. The total acidity of humic substances in soils of several climatic regions ranges from 3,000 to 17,000 mmol c kg-1 (Stevenson, 1994, chapters 1 and 9; Sposito, 1989, chapter 3). In general, CEC determined by Ba(OH) 2 of colloidal humic acids typically range from 4,000 and 9,000 mmol c k g - 1 . Conversely, it ranges from 1,000 to 4,000 mmol c kg-1 for peat materials (Sposito, 1989). Dobbss et al. (2008) evaluated elec­trochemical properties of the surface horizon of seven Brazilian Oxisols and observed that the removal of the humic and fulvic acids reversed the soil surface charge and also led to a marked loss in the buffering capacity of the soil charge.

RESULTS AND DISCUSSION Values of pH (measured in water) from surface horizons

ranged from 4.5 to 4.9 in nearly 70% of the soils. However, there were extreme situations of pH 3.9 in the Kandiudult (Kand) and of pH 8.1 in the Haplusterts (Haplus) (Table 1). A l l samples from the surface layer exhibited negative ΔpH, indicating a negative net charge in the soil. The same relationship was observed in the subsurface layers, with exception to acric Oxisols of Group 1 (Acrudox), which presented a positive charge balance. These soils represent the extremes of the weathering scale (Alleoni and Camargo, 1994), with the predominance of kaolinite and gibbsite (Table 1) in the clay fraction. As a consequence, the levels of CTC were commonly very low (Table 1). In the B horizon of these soils, where the O M content was negligible and the contents of Fe and A1 oxides were high, the zero point of charge in many cases exceeded the original pH, providing a net positive balance of charge in the soil, which acts predominantly as an anion ex­changer (Fontes and Alleoni, 2006). The presence of gibbsite in highly weathered soils can explain the occurrence of positive sur­face charge caused by the high affinity for H+ of their surface hydroxyls groups (aluminol), whose linking sites act as Lewis acid at acidic conditions (low pH) (Peixoto, 1995).

The CEC of soils from Groups 2 and 3 were higher than those of soils from Group 1, probably because of the presence of 2:1 layer-type clay minerals (Table 1). Minerals 1:1-type such as kaolinite and the gibbsite were also present in soils from Group 2, showing, however, predominance of 2:1 HI minerals. In Group 3, there was the 2:1 layer-type clay minerals without hydroxyls in the interlayers, being the smectite, illite, and interstratified kaolinite-smectite (Kt-E) the principal ones (Table 1). There was a high positive correlation between the CEC and the total surface charge density of soil, σ t o t a l soil (CTC = 0.95 σ σ t o t a l soil + 26; R2 = 0.95), apparently because of the contribution of permanent charge and consequently being used as a consistency test.

The total C average value was higher in the A horizon of Group 2 soils, with 30.4 g k g - 1 , compared with Group 1 and Group 3, respectively, with 21.3 and 17.3 g kg-1 . The total C values in the B horizon were 8.6, 10.5, and 9.4 g k g - 1 , re­spectively for Groups 1, 2, and 3 soils. The soil thermal treat­ment with a muffle furnace was not sufficient to totally eliminate the O M ; their levels of remaining total C, however, were dras­tically reduced to 2.8, 2.2, and 1.7 g kg-1 of general average values for Groups 1, 2, and 3 (without the Haplusterts) soils, respectively (Table 1). The Haplusterts (Group 3) had the highest total remaining C contents of 15.2 and 11.4 g k g - 1 , for A and B horizons, respectively, probably because of greater amounts of Ca carbonate as indicated by the level of Ca (1,350 mmol c k g - 1 ) and pH (8.1) found (Table 1), as well as their relatively lower Σ M O S . T C of 387 and 1,285 mmol c kg-1 TC for A and B horizons, respectively (Table 2).

As expected, the highest levels of base saturation (V%) were found in soils from Group 3, which were less weathered and retained larger amounts of basic cations. The weathering level of soils from Group 3 was lower than that from soils in Group 2, with K i values greater than those from more weathered soils of Group 1 (Table 1).

The variable-charge density (σ q . m i n ) and the effective mineral permanent-charge density (σ 0 , m i n ) were similar between A and B horizons for all soils (Table 2). Thus, it may be attributed to the similarity of the mineralogy in both horizons (Table 1). The only exception was the Eutrudox (Eut) in Group 2, in which higher levels of σq. min and σ0. min were observed in A horizon. On the other hand, higher levels of σq, min and σ0. min were found in the B horizon of Argiudolls (Arg1 and Arg2) in Group 3. These results may be related to higher levels of clay (Table 1) associated with the

634 | www.soilsci.com © 2009 Lippincott Williams & Wilkins

Soil Science • Volume 174, Number 11, November 2009 Surface Charge of Tropical Soils

TABLE 2. Effective Surface Charges in Surface and Subsurface Samples of the Soils

Soil W i t h OM‡ Soil W i t h o u t O M §

σ q . n a t u r a l s o i l natural soil σ q . m i n Σ 0 . m i n Σ O M . t o t a l σ s o l o . t o t a l

Σ t o t a l O M

m m o l c k g - 1 CT G r o u p S o i l † m m o l c kg-1 solo

Σ t o t a l O M

m m o l c k g - 1 CT

A Horizon

1 A c r 1 28 12 11 4 24 40 1,317 A c r 2 22 12 15 4 14 33 824 A c r 3 25 9 14 3 16 33 733 Hap 1 30 16 14 5 26 46 974

2 Eut 97 80 50 54 73 177 2,436 Kand 54 70 25 31 67 124 1,712 Hap 2 55 53 25 30 54 108 1,855 Haplo 45 43 24 24 40 88 1,723

3 A r g 1 23 18 12 1 1 17 40 1,225 A r g 2 31 45 21 40 15 76 961 Haplus 104 202 99 204 8 311 387 Haplud 83 170 41 163 49 253 2,494 Group 1|| 26 12 14 4 20 38 962 Group 2 62 62 31 35 59 124 1,932 Group 3 60 109 43 105 22 170 1,267

B Horizon

1 A c r 1 16 11 10 4 13 27 2,082 A c r 2 18 10 15 4 8 28 809 A c r 3 19 7 14 4 8 26 693 Hap 1 21 14 15 6 14 35 2,215

2 Eut 61 67 36 46 46 127 4,066 Kand 52 61 27 35 51 112 5,648 Hap 2 45 49 28 30 36 94 4,359 Haplo 41 42 25 23 35 82 2,637

3 A r g 1 45 105 34 83 34 151 4,185 A r g 2 54 146 36 122 42 200 4,205 Haplus 103 218 97 203 20 320 1,285 Haplud 51 241 34 164 95 292 22,285 Group 1 19 10 14 5 11 29 1,450 Group 2 49 55 29 33 42 104 4,178 Group 3 63 178 50 143 47 241 7,990

Electric surface charges: σq. m i n : mineral variable-charge surface density; σ0. m i n : mineral permanent-charge surface density; σMOS. total: total organic matter charge density in the soil; σ s o l o . t o t a l : total charge density in the soil = σ0. min

+ σq. min + σMO. total. †Soil: see Fig. 1. ‡Natural soil with O M . §Soil after ashing at 400 °C.

||Average of each group of soils.

σ q . n a t u r a l s o i l , variable surface charges density of natural soil; σ0. n a t u r a l soil, permanent surface charges density in natural soil; σ total OM, total O M charge density in the soil.

minerals found in these horizons: in Eut, the A horizon had Kt > Gb >> 2:1 HI > A; in Arg1, the Btgx2 horizon had Kt-S > S > Kt; and in Arg2, the Btgx 1 horizon has S > Kt-S > II (Table 1). Alleoni et al. (2005) verified higher levels of CEC in samples from B horizon than in samples from A horizon of an Alfisol and an Ultisol in Brazil. The authors also attributed this fact to the increase of clay level in the subsurface layer, which may have masked the reduction effect of lesser O M content.

The operational procedure adopted to the Cs + complexation allowed the detection of differences in the permanent-charge densities ( σ 0 , min) among the groups of soils, being homogenous among soils within each group caused by the predominant min-

erals present in the soils (Table 2). In Group 3, particularly, the effect of the mineralogy and of the clay content on the value of σ0, m i n was meaningful (Tables 1 and 2); it was higher in soils with high amounts of smectite (204 mmol c k g - 1 in Haplusterts and 163 mmol c k g - 1 in Hapludoll) regardless of the amount of clay. The Argiudoll (Arg 2) was an exception because the mag­nitude of σ0, min was related to the higher clay contents in B horizon (σ 0 , min of 40 mmol c k g - 1 , 163 g kg-1 of clay). The Argiudoll (Arg 1) presented lower levels of σ0, m i n compared with the other soils in Group 3.

Soils from Group 2, which have 2:1 HIV, contained σ0, min

higher than those from highly weathered soils in Group 1, where

© 2009 Lippincott Williams & Wilkins www.soilsci.com | 635

Alleoni et al. Soil Science • Volume 174, Number 11, November 2009

the clay fraction was predominantly composed of 1:1 layer-type clay minerals (kaolinite) and Fe and A1 oxides (hematite, goethite, and gibbsite) (Table 2). This shows the importance of the presence of 2:1 HIV clay minerals in the composition of permanent charges in highly weathered soils from humid tropi­cal regions. The average Σ0. MIN in B horizon were 5, 33, and 143 mmol c kg-1 of Groups 1, 2, and 3 soils, respectively. Soils from Groups 2 and 3, which had 2:1 layer-type clay minerals with or without hydroxy interlayers, had higher charge densities than the soils from Group 1. Therefore, the total charge density of

these groups was more dependent on the activity of their clay than on the content of O M in the soil.

The Cs + complexation method detected permanent charges in the highly weathered soils from Group 1 (Table 2), whose values are in agreement with Peixoto (1995) who discounted the O M contribution and obtained σ 0 levels ranging from 6 to 20 mmol c kg-1 in three Hapludox soils derived from basalt with diverse mineralogies (ranging from soils rich in Gb and Kt to soils with HIV). Weber et al. (2005) obtained values ranging from 4 to 12 mmol c kg-1 in an Oxisol with acric properties.

FIG. 3. Percentage of surface charge components (Σ T O T A L OM/ total O M charge density in the soil; σq, m i n , effective mineral variable-charge density; σ0, MIN, mineral permanent-charge density) of the soils measured by C s + adsorption method in surface (A horizon) and subsurface (B horizon) samples. For soils' identification, see Materials and Methods section.

636 | www.soilsci.com 2009 Lippincott Williams & Wilkins

Soil Science • Volume 174, Number 11, November 2009 Surface Charge of Tropical Soils

These σ 0 levels have been attributed to the presence of HIV and impurities in mica shape, undetectable by the X-ray analysis. Moore and Reynolds ( 1997) highlight the fact that mineralogical phases present at amounts lower than 5% in the mass basis may not be detectable in routine analyses.

The presence of hydroxy-A1 islands tends to prop open the silicate layers in those soil clay minerals (Rich, 1968; Barnhisel and Bertsch, 1989), and consequently, it is expected to affect the fixation and selectivity of ions by permitting entry of K-size ions into partially opened HIV layers, without hindrance by collapse at the edges of the mineral. The selectivity of these minerals for Cs+ should depend on the intensity of the interlayers filled with A1 hydroxy, so that the σ0 could decrease propor­tionally to the increase of blockage of permanent-charge sites by A1 hydroxy, whose stability should be related to the envi­ronmental conditions for the HI clay natural existence in soils (e.g., pH, A1 saturation values, soil solution composition, etc). Although there may be an underestimation of the true amount of isomorphic substitution and perhaps some of the accessible permanent charge, the proposed operational determination pro­cedure was designed to quantify the net surface charge density components normally related to minerals present in the soils in a way that this understanding could be used for a better agri­cultural soil fertility management (e.g., at a pH of about 6.0), taking into account the relationship between the soil mineralogy and the soil CEC, and not actually to characterize the more specific clay mineral structure, which could be done with complementary mineralogical studies.

Another factor that may have influenced the σ0 of the studied soils was the C s + complexation by O M covering the sur­face of the mineral particles, which may not have been com­pletely eliminated in the ashing (Table 1). Therefore, although the O M level was low, σ 0 may have been overestimated. The decrease of σq. min after O M removal was more pronounced in A than in B horizon (Table 2) because of the higher amounts of negative charge sites on the surface layer of the soils. In tum, the exposition of variable-charge sites of the inorganic fraction prevailed in the subsurface horizon.

The effective mineral permanent-charge density ( Σ 0 . m i n ) contributed to A and B horizons, on average, with 11% and 16%, 28% and 31%, and 52% and 59% of total soil charge ( σ t o t a l S O I L) of Groups 1, 2, and 3, respectively. The variable-charge density (σ q . m i n ) contributed 36% and 47%, 25% and 28%, and 27% and 21%; whereas the total O M charge of soil ( Σ O M , t o t a l ) contributed 53% and 37%, 48% and 41%, and 21% and 20% of total soil charge (Fig. 3). These differences in the contribution among the charge components of soil occurred because of the differences in both mineralogy and O M contents among the soil groups (Tables 1 and 2).

The mean contribution of variable charges of soils, which cor­responds to the sum of O M to mineral components (σrOM, total +

σq. m i n ) , accounted for 89% of the total electric charges in the samples from the surface layers of soils in Group 1. In Group 2, they accounted for 72%, whereas in Group 3. the variable charges accounted for 48% of the total electric charges. In the subsurface horizons, the participation of total variable charges in relation to the total electric soil charge, on the average, was similar to that in the surface horizon, that is, 84%, 69%, and 41% for Groups 1, 2, and 3, respectively (Fig. 3).

The average levels of σ t o t a l soil in A and B horizons were, respectively, 38 and 29 mmol c kg-1 for G1, 124 and 104 mmol c kg-1 for G2, and 170 and 241 mmol c kg-1 for G3. Peixoto (1995) found σ t o a l soil ranging from 26 to 126 mmol c k g - 1 in profiles of three Brazilian Hapludoxes derived from basalt. Despite the preponderant role of the soil mineralogical con­

stitution in the expression of total soil charges, especially related to the presence of 2:1 minerals, it is noted that O M has an expressive contribution in the magnitude of variable charges in all soil groups. However, the importance increases considerably with the increase of the degree of weathering in tropical and subtropical soils (Fig. 3).

Levels of σMOS, C total ranged from 387 to 22,285 mmol c kg-1 TC. The mean levels in A and B horizons were 962 and 1,450 mmol c kg-1 for G1, 1,932 and 4,178 mmol c kg-1

for G2, and 1,267 and 7,990 mmol c kg-1 for G3 (Table 2). Extreme levels were found in Group 3, with lower levels in both horizons of the Hapludox (A, 387 mmol c kg-1 ; B , 1,285 mmol c k g - 1 ) and a very high level in B horizon of Hapludoll (22,285 mmol c k g - 1 ) . In Brazilian soils derived from basalt, Peixoto (1995) found levels of σ M O S . C t o t a l rang­ing from 308 to 2,068 mmol c kg-1 TC, with an average of 1,655 mmol c kg-1 in a kaolinitic Rhodic Hapludox; of 479 mmol c kg-1 in a ferruginous/sesquic Typic Hapludox, and of 543 mmol c kg-1 in a sesquic/gibbsitic Humic Rhodic Hapludox. The σ M O S , C total may represent the reactivity po­tential of the functional groups on SOM normally related to its humification degree. The total acidity of soil humic sub­stances of various climatic regions ranged from 3,000 to 17,000 mmol c kg-1 (Stevenson, 1994, Chapters 1 and 9; Sposito, 1989, Chapter 3). Generally, the C E C of colloidal humic acids determined by Ba(OH) 2 ranged from 4,000 to 9,000 mmol c kg-1 and from 1,000 to 4,000 mmol c kg-1 for turf materials (Sposito, 1989).

Oorts et al. (2000) verified a linear increase in CEC with the rise of C content in soil and in loam and clay amounts in a sample from the surface horizon of a Ferric Lixisol. According to Oorts et al. (2000), the variations among six arboreal species were correlated with differences in the biochemical composition of the litter. This highlights the importance of organic residues with slow decomposition rates to the CEC of highly weathered soils, reinforcing the important effect of O M on the generation of variable charge in these soils. This fact occurs in the no-till system (NTS) (Caires et al., 2006), in which the addition of crop residues on soil surface contributes to the increase of variable charge, mainly in well-drained Oxisols cultivated for several years under this system, where the amount of total charges has increased with time. Ciotta et al. (2003) compared the NTS with the conventional one and obtained increases of 15.2 mmol c kg-1

in effective CEC and of 20.7 mmol c k g - 1 in total C E C (at pH 7) on the layer of 0 to 8 cm, which represented 44% and 21% relative increases, respectively, after 21 years of adopting the NTS in an Hapludox south of Brazil.

CONCLUSIONS The C s + complexation method operational adaptation

characterized the soil surface charge components ( σ s o i l , t o t a l

[mmol c kg-1 solo] = σ 0 . m i n + σ q . m i n + σ O M . t o t a l ) and was efficient in detecting structural charges in tropical soils, including highly weathered ones, noting the contribution of 2:1 layer-type clay minerals existing in those soils and generating an expressive increase in the σ0. m i n levels (Group 1 < Group 2 < Group 3 ).

In the Eutrudox (Eut) of Group 2, as well as in the Argiudoll (Arg1 and Arg2) of Group 3, an effect of clay levels and the presence of 2:1 layer-type clay minerals (2:1 HI and Kt-S) on the increase of σq, min and of σ0, m i n in these soils was observed.

In less-weathered soils of Group 3, the presence of 2:1 layer-type clay minerals (smectite) contributed nearly 60% of the total soil charges ( σ s o l o , t o t a l ) , whereas the O M contributed to 20%.

© 2009 Lippincott Williams & Wilkins www.soilsci.com | 637

Alleoni et al. Soil Science • Volume 174, Number 11, November 2009

The soil O M had an expressive contribution in the mag­nitude of variable charges in all soil groups. However, the importance increased considerably with the increase of the degree of weathering in tropical and subtropical soils (Group 3 < Group 2 ≤ Group 1).

The soil O M charge determined in A and B horizons ranged from 387 to 22,285 mmol c kg-1 TC, what might be related to its reactivity and degree of humification.

ACKNOWLEDGMENT The authors thank Dr Samuel J. Traina from the University

of California, Merced, CA, for his close reading of the manu­script and valuable suggestions.

REFERENCES

Alleon i , L . R. F., C . S. M . Iglesias, S. C . Me l lo , O. A . Camargo, J. C . Casagrande, and N . A . Lavorenti. 2005. Atributos do solo relacionados a adsorção de cádmio e de cobre em solos tropicais. Acta Scientiarum 27:729-737.

Alleoni , L . R. F., and O. A . Camargo. 1994. Potencial elétrico superficial e carga elétrica líquida de Latossolos ácricos. R. Bras. C i . Solo 18:181-185.

Anderson, S. J. 1989. Variable Charges Properties o f Soil Containing 2:1 Phylosilicates. Dissertation, University o f California, Riverside, C A , p. 100.

Anderson, S. J., and G . Sposito. 1991. Cesium-adsorption method for measuring accessible structural surface. Soil Sci . Soc. A m . J 55: 1569-1576.

Bamhisel , R. I., and P. M . Bertsch. 1989. Chlorites and hydroxy-interlayered vermiculite and smectite. In Minerals in Soil Environment. J. B . Dixon . S. B. Weed, and R. C . Dinauer (eds.). Soil Science Society o f America, Madison, WI , pp. 729-788.

Barré, P., B . Velde, C . Fontaine, N . Catel, and L . Abbadie. 2008. Which 2:1 clay minerals are involved in the soil potassium reservoir? Insights from potassium addition or removal experiments on three temperate grassland soil clay assemblages. Geoderma 146:216-223.

Caires, E. F., F. J. Garbuio, L . R. F. Al leoni , and M . A . Cambrì . 2006. Surface lime application and black oat cover preceding corn and soybean crops under a no-till system. R. Bras. C i . Solo 30:87-98.

Camargo, O. A . , A . C . Moniz , J. A . Jorge, and J. M . A . S. Valadares. 1986. Métodos de análise química, mineralógica e fisica de solos do Instituto Agronômico de Campinas. Instituto Agronômico . Campinas, Brazi l , p. 94 [Boletim Técnico, 106].

Charlet, L . , and G . Sposito. 1987. Monovalent ion adsorption by an Oxisol . Soil Sci . Soc. A m . J 51:1155-1160.

Chorover. J., M . K . Amistadi, and O. A . Chadwick. 2004. Surface charge evolution of mineral-organic complexes during pedogenesis in Hawaiian basalt. Geochim. Cosmochim. Acta 68:4859-4876.

Chorover, J., and G . Sposito. 1993. Measurement o f Surface Charge Components. Technical Reports, N S F grant E A R 9221258. University of California at Berkeley, Berkeley, C A , p. 48.

Ciotta, M . N . , C . Bayer. S. M . V. Fontoura. P. R. Ernani, and J. A . Albuquerque. 2003. Matéria orgânica e aumento da capacidade de troca de cátions em solo com argila de atividade baixa sob plantio direto. Ciência Rural 33:1161-1164.

Cur i , N. N . Kämpf, and M . Resende. 1984. Mineralogia, química, morfologia e geomorfologia de solos originados de rochas efusivas das encostas superior e inferior do Nordeste do Rio Grande do Sul. R. Bras. C i . Solo 8:269-276.

de Koning, A . , and R. N . J. Comans. 2004. Reversibility of radiocaesium sorption on illite. Geochim. Cosmochim. Acta 68:2815-2823.

Dobbss, L . B . , L . P. Canellas, L . R. F. Al leoni , C . E . Rezende, M . P. F.

Fontes, and A . C . X . Velloso. 2008. Electrochemistry of Brazilian Oxisols after removal o f soluble organic matter. R. Bras. C i . Solo 32: 985-996.

Fontes, M . P. F., and G . Sposito. 1995. Medição da densidade de carga estrutural acessível em Latossolos brasileiros com diferentes miner­alogias. In Congresso Brasileiro De Ciência Do Solo, 25. Anais, Viçosa, Minas Gerais 1:292-294.

Fontes, M . P. F. 1990. Vermiculi ta ou esmectita com hidróxi nas entrecamadas: Proposição de nomenclatura. Boletim Informativo da Sociedade Brasileira de Ciência do Solo 15:24-28.

Fontes, M . P. F., and L . R. F. Al leon i . 2006. Electrochemical attributes and availability of nutrients, toxic elements, and heavy metals in tropical soils. Sci . Agr ic . 63:589-608.

Gi l lman, G . P. 1979. A proposed method for the measurement o f exchange properties of highly weathered soils. Aust. J. Soil Res 17:129-139.

Gi l lman, G . P. 1984. Using variable charge characteristics to understand the exchangeable cation status o f Oxisols. Aust. J. Soil Res 22:71-80.

Hochella, M . F. Jr. 1990. Atomic structure, microtopograph, composition and reactivity o f mineral surfaces. Reviews in Mineralogy and Geo­chemistry 23:87-132. In Mineral-Water Interface Geochemistry. M . F. Hochella Jr, and A . F. White (eds.). Washington, D C , p. 603.

Mekaru, T , and G . Uehara. 1972. Anion adsorption in ferruginous tropical soils. Soil Sci. Soc. A m . J. Proc 36:296-300.

Mol le r , M . R. F., and E . Klamt . 1982. Identificação e gênese de argilominerais em Latossolo Roxo de Santo Ângelo (RS). R. Bras. C i . Solo 6:161-166.

Moore, D. M . , and R. C . Reynolds. 1997. X-ray Diffraction and the Identification and Analysis o f Clay Minerals, 2nd Ed. Oxford University Press, New York, N Y , p. 378.

Nelson, D. W , and L . E. Sommers. 1982. Total carbon, organic carbon, and organic matter. In Methods o f Soil Analysis. Part 2 Agron. Monogr. 9, 2nd Ed. A . L . Page, et al (ed.). A S A and S S S A , Madison, WI , pp. 539-580.

Oorts, K . , B. Vanlauwe, O. O. Cofie, N . Sandinga, and R. Merckx. 2000. Charge characteristics o f soil organic matter fractions in a Ferric Lix iso l under some multipurpose trees. Agrofor. Syst. 48:169-188.

Parfitt, R. L . 1980. Chemical properties o f variable charge soils. In Soils With Variable Charge. B . K . G . Theng (ed.). Palmerston North, New Zealand, pp. 167-194.

Peixoto, R. T. G . 1995. Surface Chemical Properties o f Oxisols Derived From Paraná Basalt (Brazil). Dissertation, Ohio State University, Columbus, O H , p. 188.

Peixoto, R. T. G . , S. J. Traina, and J. M . Bigham. 1995. Aplicabilidade do método de adsorção do Cs na determinação da carga elétrica permanente em Latossolos derivados do basalto no Paraná. In X X V Congresso Brasileiro de Ciência do Solo. Viçosa, Minas Gerais 1:295-297.

Potter, R. O., and E. Kämpf. 1981. Argilo-minerais e óxidos de Fe em Cambissolos e Latossolos sob regime climático similar na encosta inferior do Nordeste do Rio Grande do Sul. R. Bras. C i . Solo 5:153-159.

Rich , C . I. 1968. Hydroxy interlayers in expansible layer silicates. Clays Clay M i n . 16:15-30.

Sposito, G . 2004. The Surface Chemistry o f Natural Particles. Oxford University Press, New York, N Y , p. 242.

Sposito, G . 1989. The Chemistry o f Soils. Oxford University Press, New York, N Y , p. 277.

Stevenson, F. J. 1994. Humus Chemistry: Genesis, Composition and Reactions. John Wiley, New York, N Y , p. 496.

Weber, O. L . S., J. C . Chitolina, O. A . Camargo, and L . R. F. Al l eon i . 2005. Cargas elétricas estruturais e variáveis de solos tropicais altamente intemperizados. R. Bras. C i . Solo 29:867-873.

638 I www.soilsci.com © 2009 Lippincott Williams & Wilkins

COPYRIGHT INFORMATION

TITLE: Components of Surface Charge in Tropical Soils WithContrasting Mineralogies

SOURCE: Soil Sci 174 no11 N 2009

The magazine publisher is the copyright holder of this article and itis reproduced with permission. Further reproduction of this article inviolation of the copyright is prohibited. To contact the publisher:http://www.buymicro.com/rf/dih/williams_and_wilkins.htm