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Volume 40, 2002© CSIRO 2002

Australian Journalof Soil Research

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Aust. J. Soil Res., 2002, 40, 115–126

© CSIRO 2002 0004-9573/02/01011510.1071/SR00097

Clay mineral associations in melanic soils of South Africa

Gertruida M. E. van der MerweAC, Michiel C. LakerB, and Christl BühmannA

AARC-Institute for Soil, Climate and Water, Private Bag X79, Pretoria 0001, South Africa; e-mail: [email protected]; [email protected]

BDepartment of Plant Production and Soil Science, University of Pretoria, Pretoria 0002, South Africa; e-mail: [email protected]

CPreferred contact for inquiries

Abstract

The melanic horizon is 1 of 5 diagnostic topsoil horizons distinguished in the South African soilclassification system. Melanic soils span a wide spectrum, ranging from those that intergrade with a verticto those that intergrade with a humic horizon. Melanic soils are therefore expected to vary considerably withrespect to a variety of physical, chemical, and clay mineralogical properties. The objective of the presentstudy was to determine the clay mineral compositions of melanic horizons from 58 modal profiles and toestablish to what extent melanic soil properties are related to clay mineralogy. Special emphasis was placedon the characterisation of the clay fraction in terms of group and species identification. X-raydiffractometry was employed almost exclusively as the investigative technique in mineral identification andquantification. Melanic A horizons showed a large degree of variation with regard to their clay mineralassociations. More than half of the soils were dominated by smectite, 30% by kaolinite, and the rest by anassociation of about equal proportions of mica, kaolinite, and smectite. Talc and hydroxy-interlayeredvermiculite occurred in a number of soils while one horizon was dominated by an illite/smectiteinterstratification. The smectite component was identified as belonging to either beidellite or vermiculitespecies, depending on the method employed. About a quarter of the smectitic soils containedmontmorillonite as well but not as the dominant swelling phase.

Additional keywords: mollisols, clay mineralogy, layer charge.

SR00097G. M. E. van der M er weet al .Clay m ineral associ ations i n melanic soil s

Introduction

Well-defined diagnostic horizons form the basis for present-day taxonomic soilclassification systems (Soil Classification Working Group 1991; Soil Survey Staff 1998;WRB Working Group 1998). For the sake of practical feasibility, diagnostic horizons aredefined in terms of morphological features and easy-to-conduct physical and chemicallaboratory analyses.

Clay mineralogical criteria are generally not included in the definitions of diagnostichorizons. This may be due to cost and time factors; however, the influence of clay mineralson soil characteristics is not always straightforward. The mineralogical composition of theclay fraction of a soil is one of the critical factors determining many chemical and physicalproperties. Not only is there a close interrelationship between clay mineralogy and thecriteria used for differentiating and classifying soils such as base status, colour, or structure,but other soil characteristics such as erodibility, water infiltration capacity, sorptionpotential for heavy metals and/or pesticides and herbicides, or K-fixation are closely linkedto the clay minerals. Soils which are morphologically and chemically similar may displaydrastically different physical properties due to differences in their clay mineralogical suites.This is not only related to the dominant clay mineralogy, but often to effects of the presenceor absence of small amounts of other clay minerals (Stern 1990; Bloem 1992).

The melanic horizon is 1 of the 5 diagnostic topsoil horizons distinguished in the SouthAfrican soil classification system (Soil Classification Working Group 1991). The melanic

116 G. M. E. van der Merwe et al.

horizon of this system is similar to the mollic horizon of the international classificationsystems (WRB Working Group 1998). It is by definition a well-structured, dark-colouredhorizon with a high base saturation and a moderate to high organic matter content that lacksthe swell–shrink properties of vertisols soils (Soil Classification Working Group 1991).

Few (Fitzpatrick and Le Roux 1977) basic studies have been conducted on the claymineral association in melanic horizons in South Africa. The South African soilclassification system (Soil Classification Working Group 1991) states that ‘the absence ofvertic properties in melanic horizons is usually attributable to either a lower clay content(than in vertic horizons) or, if the clay content is high, a predominance of micaceous,vermiculitic or even kaolinitic rather than highly expansive clay minerals’. Preliminaryfindings by Bühmann (1987) indicate, however, that some of the melanic soils had smectiteproportions (in combination with a high clay content) identical to those of Vertisols. Asmelanic soils range in compositional characteristics between the highly swelling vertic soilsat one end and orthic, humic, or organic soils at the other, it can be assumed that the mineralcomposition of melanic horizons may display a considerable degree of variation.

Swelling is synonymous with smectite. All smectites swell, i.e. their ‘c’ spacing changeswith treatment. The extent of swelling, however, may vary dramatically and depends,among other parameters, on the nature of the interlayer cation (MacEwan and Wilson 1980)and on layer charge characteristics (Lagaly et al. 1976; Wilding and Tessier 1988). Theseinclude aspects such as the magnitude of the negative charge, its location, i.e. whether it ispredominantly tetrahedral or octahedral (Harward and Brindley 1965), and chargeheterogeneity (Lagaly et al. 1972), i.e. whether there are differences in layer chargecharacteristics between the 2 layers that sandwich the interlayer. In smectites with a highlayer charge and divalent cations in interlayer positions the unit cell distance changes from1.5 nm in the air-dry state to about 1.8–2.0 nm when fully dispersed in water. In smectiteswith a low layer charge and monovalent counterions, however, dispersion may result in anincrease in the ‘c’ spacing from 1.24 nm to >100 nm (Norrish 1954). Only the second typeof smectite will display a high degree of physical swelling. In the first type, little expansionmay be noticed as smectite in a soil generally does not reach the fully dispersed andsometimes also not the air-dry state. Some smectites consequently may show little changein their interlayer distance and thus in their swelling capacity, while others may beextremely expansive.

An impressive amount of information concerning the soil-forming factors, viz. climate,parent material, and relief, are contained in the memoirs accompanying land type maps inSouth Africa (Land Type Survey Staff 1984–1998). As far as mineralogy is concerned,however, virtually no basic studies have been conducted on the phyllosilicate associationsof melanic horizons in South Africa. The objectives of the present study, therefore, were todetermine the clay mineral compositions of melanic horizons from a large number of modalprofiles and to establish to what extent melanic soil properties are related to claymineralogy.

Materials and methods

A systematic land type survey initiated in 1971 delineates the areas into climate, terrain, and soil classes.The results are now available in the form of maps and accompanying memoirs for the whole of South Africa(Land Type Survey Staff 1984–1998). Evaluation of the above data (Van der Merwe 2000) showed thatmelanic soils in South Africa cover an area of about 2.34 million ha or 2% of the land surface. They occurthroughout the semi-arid to subhumid parts and correlate particularly with an average annual precipitationof 550–800 mm. Melanic soils formed from a variety of parent materials and are not restricted to a specifictopographic position. Melanic soils are essentially absent from the western half of South Africa, which

Clay mineral associations in melanic soils 117

receives <500 mm rain (Van der Merwe 2000). During the above land type survey, 89 modal profiles withmelanic A horizons were sampled of which 58 profiles were investigated in the present study. Samplelocations are presented in Fig. 1. The soils were disaggregated and individual clay (<2 µm) fractions separated by centrifugation. The clayswere saturated with Mg, Li, and K by shaking in a 1 M (3 M for Li) chloride solution for 1 h, followed byovernight equilibration. The suction-through method was used for orientation. Expansion tests wereperformed by solvation with ethylene glycol and glycerol (vapour at 60°C and 90°C, respectively, for 16 h).K-saturated samples were heated to 550°C and Li-saturated clays to 280°C for at least 4 h.

For alcylammonium intercalation, dodecylammoniumchloride (C12) was dissolved in a small amount ofethanol (to give a final concentration of a 0.1 M solution). A water:ethanol mixture (1:1) was slowly added,avoiding intense clouding, and the pH adjusted to 6–7 with HCl (Lagaly 1979). A 7.5 cm3 aliquot of thisdodecylammoniumchloride solution was added to about 30 mg of clay. The suspension was heated at 65°Cfor 2 days, the solution being replaced after 1 day. Excess dodecylammoniumchloride was removed by 10washings with a water-ethanol (1:1) mixture and one final washing with pure ethanol. The paste was thensucked through a ceramic tile for orientation, dried at 65°C and stored in a desiccator.

Clay minerals were identified and quantified via X-ray diffractometry, using a Phillips diffractometerand graphite monochromated CoKα radiation, generated at 45 kV and 40 mA. Specimens were scannedfrom 2° to 35° 2θ. Mineral percentages were determined from the intensity of their 001 reflections.Nomenclature follows AIPEA recommendations (Bailey 1980, 1982).

Mineral identification of various clay minerals is based on the position and possible shift of a series ofbasal reflections, applying auxiliary tests (Bailey 1980). Discrete minerals had to give a rational series ofbasal reflection with d005 × 5 = d004 × 4 = d003 × 3 = d002 × 2 = d001 (A) with a low background to both sidesof the peak maximum.

Layer charge characteristics of the swelling clay minerals were determined from peak positions afterapplication of the treatments, listed below.

Glycerol and ethylene glycol solvation of the Mg-saturated clay

Glycerol and ethylene glycol solvation permits differentiation of swelling clays on the basis of the layercharge (Harward and Brindley 1965); in discrete smectites a high, vermiculite-type layer charge leads tomonolayer formation (1.42 nm) with glycerol and generally also with ethylene glycol, while a low, smectite-

Fig. 1. Locality map of the South African melanic A horizons included in the study.


type layer charge results in bi-layer formation with both solvating agents with resultant spacings of 1.69 nmfor ethylene glycol and 1.8 nm for glycerol.

Greene–Kelly test

The Greene–Kelly (1953) test differentiates dioctahedral smectites on the basis of the seat of layer charge.After Li-saturation and heating to 280°C, irreversibly collapsed interlayers (1.0 nm) denotemontmorillonite, while re-expanding interlayers (1.69 nm) are ascribed to beidellite. The proportion ofmontmorillonite interlayers in a smectite crystal was estimated by comparing the position of the reflectionbetween 0.8 and 1.0 nm from the Mg-saturated, ethylene glycol solvated clay with that of the correspondingpeak produced by the Li-saturated, heated, ethylene glycol solvated material (Reynolds 1980). Any increasein the value of this peak is attributed to the presence of irreversibly collapsed interlayers (montmorillonite)in a smectite crystallite. Mixtures of montmorillonite and beidellite would be characterised by 2 distinctpeaks at 1.0 nm and 1.69 nm, respectively.

Intercalation with dodecylammoniumchloride

The C12 method permits identification of interlayer charge density (Lagaly et al. 1976; Lagaly 1982). Low-charge montmorillonites are characterised by basal spacings of 1.36 nm (monolayer), beidellites by 1.75 nm(bilayer), and vermiculites by >2.0 nm (paraffine-type structures).

K-saturation

Measurement of the spacing of the K-saturated and air-dried samples indicated a 1.24 nm line as acharacteristic of beidelite or low-charge vermiculite, whereas a spacing of 1.52 nm was taken to indicatemontmorillonite (Machajdik and Cicel 1981) and a 1.0 nm peak was ascribed to vermiculite. A peakposition between the two values was regarded as indicative of charge heterogeneity within the smectitecrystallite, i.e. water monolayer/water bilayer interstratification.

Results and discussion

Clay mineral associations

The melanic A horizons investigated in the present study contained one or more of the clayminerals, viz. kaolinite, mica, talc, smectite, illite/smectite interstratifications, vermiculite,and hydroxy-interlayered vermiculite (HIV), also often referred to as pedogenic chlorite(Fig. 2).

Melanic soils, per definition, must exhibit a dark colour, a relatively high base status anda blocky structure (MacVicar et al. 1977). A dark colour is closely related to the organicmatter (OM) content, while the base status reflects the presence of charged sites (CEC).Structure refers to the natural aggregation of primary soil particles into compound units.

Aggregates are composed of minerals, organic matter, water and air. Some of theseconstituents like quartz, lime, feldspar, and air are chemically inert. Others possess a strongdipole character but are otherwise uncharged, like water, while still others carry a layercharge which may be permanent (mica, HIV, smectite, vermiculite) or variable (secondaryFe and Al phases, edge sites in phyllosilicates). Organic matter may positively or negativelycharged or be inert, depending on its nature. In order to form aggregates, the primary soilconstituents have to be bound together, which requires the existence of bonding sites. Asinert phases do not possess bonding sites, they cannot be involved in aggregate formation.The interaction of charged and/or polar sites, on the other hand, may lead to cohesivebonding and result in the formation of heterogeneous aggregates (Bühmann et al. 1998).Charged sites are restricted almost exclusively to the soil organic matter and phyllosilicatefractions which are intimately associated. The presence of minerals with charged sites,particularly permanently net negative sites, is therefore an important factor in soil structureformation as well as in CEC.


For this reason the clay mineral associations in the melanic horizons have been groupedon the basis of their layer charge characteristics (Fig. 2) into:

(a) essentially low layer charge minerals representing non-swelling type, viz. kaoliniteand talc with a net negative charge of <0.2 per O10 unit cell;

(b) clays representing non-swelling minerals, viz. mica and HIV with a high layer chargeof >0.8 per O10 unit cell; and

(c) clays representing swelling minerals, viz. smectite, vermiculite, and/or illite/smectiteinterstratifications with a medium to high layer charge with a net negative charge of0.2–0.8 per O10 unit cell.

The soils showed considerable variations with regards to the dominant clay mineralassociations (Fig. 2). Some striking patterns were evident, however. The first is theextremely small number of cases (5) in which mica + HIV constitute the dominant claymineral association. In four of these HIV makes the sole contribution, with no mica present.In all four cases the soils also contained no swelling clay minerals. No less than 24 (>40%)of the melanic horizons were devoid of mica and HIV, the vast majority of them beingdominated by smectite and vermiculites.

Soils dominated by clay minerals with a low layer charge

Nearly 30% of the melanic horizons studied had kaolinite as the dominant clay mineral.Some of these were devoid of swelling clay minerals, others of mica and HIV (Figs 2, 3a).Few kaolinite-dominated horizons contained both smectite + vermiculite, and mica + HIV.

� Smectite + Vermiculite(medium to high layer charge; swelling)

100 90 80 70 60 50 40 30 20 10

10

20

30

40

50

60

70

80

90

100

Mic

a +

HIV

�

(hig

h la

yer c

harg

e; n

on-s

wel

ling)

10

20

30

40

50

60

70

80

90

100

Kaolinite + Talc �

(low layer charge; non-sw

elling)

Fig. 2. Clay mineral associations in melanic soils from South Africa.


All melanic soils, which were dominated by kaolinite and talc, also contained at leastsome charged phases, either associated or interstratified. Even the soil with 100% kaolinitehad a high background at the low angle side of the kaolinite diffraction line, a clearindication of the presence of 2:1 layer interstratification components. These components,unfortunately, could not be quantified and were very difficult to qualify. The CEC of thesoil clay of 40 cmol(+)/kg clay also indicates the presence of charged material, as accordingto Delvaux et al. (1990) a CEC >10 cmol(+)/kg clay points to contamination by charged2:1 clays. A small amount of charged minerals, in combination with an average (1.9%) OMcontent and major proportions of sesquioxides (Fe: 6.8%), may induce aggregation.

The kaolinite-dominated melanic A horizons could logically be expected to be the onesthat grade towards humic A horizons. Some melanic and humic horizons morphologicallyresemble each other closely. They are purely distinguished from each other according to the

15 10 5 2

2θ degrees

(a)

(b)

(c)

(d)

Fig. 3. X-ray traces of the clay fractions of selected melanic soil horizons (<2 µm; oriented specimen, Mg saturated, ethylene glycol solvated), which are dominated by (a) kaolinite, (b) an association of smectite and vermiculite, (c) an association of mica and HIV, and (d) an illite/smectite interstratification.


base status limits in the definitions (Soil Classification Working Group 1991). Thegradation of melanic to humic horizons on the one (more highly weathered) side to vertichorizons on the other side is illustrated by the fact that one of the horizons, classified as‘melanic’ according to the original criteria (MacVicar et al. 1977) had to be reclassified ashumic according to the slightly revised criteria (Soil Classification Working Group 1991),while 11 others had to be reclassified as vertic (these reclassified soils were not includedin the present study).

Soils dominated by non-swelling clay minerals with a high layer charge

Mica and chlorite are generally assumed to be associated with the formation of melaniccharacteristics, being non-swelling and therefore not conducive to the development ofvertic properties, but having a sufficient number of charged sites available for binding soilconstituents into aggregates. The present study, however, contradicts the above hypothesis.Only very few of the soils studied were dominated by mica and/or HIV, while chlorite wasabsent from all soils (Figs 2, 3b).

Viewing micaceous clay mineralogy as a predominant factor in creating melanic featuresis in all probability a misconception. In fact, in the Eastern Cape (a Province of SouthAfrica) the clay fractions of the dense, structureless, bleached, orthic A horizons developedfrom mudstones and shales, are dominated by mica. In this area the melanic soils found inbetween the above are associated with dolerite as the parent material.

Soils dominated by swelling clay minerals

Soils dominated by smectite in combination with a high clay content are generally assumedto develop shrink–swell characteristics often referred to as vertic properties. In the presentstudy, however, the clay fractions of more than half of the melanic horizons are dominatedby swelling clay minerals (smectite, vermiculite, illite/smectite interstratifications), whichcomprise more than 75% of the clay fraction in about half of these soils (Figs 2, 3c, 3d).Since many of these soils also have high clay contents they could be expected to showstrong swell–shrink characteristics. The clay mineral association of the melanic soils in thepresent study in fact is almost identical to those found by Bühmann and Schoeman (1995)for vertic soils in the northern regions of South Africa; most of the soils showing verticproperties are dominated by smectite, accompanied by some proportions of kaolinite (up to65%) and little amount of mica. Since melanic horizons cover the range between humichorizons on the one hand and vertic on the other, it is logical to expect a clay mineralogyranging from predominantly kaolinitic to smectitic. The fact that some melanic horizonshave a clay mineralogical composition identical to that of vertic horizons was unexpectedand difficult to explain since vertic horizons by definition have strong swell–shrinkproperties.

It should be borne in mind, however, that swell–shrink phenomena are influenced by avariety of interrelated chemical and physical parameters like organic matter, carbonates,sesquioxides (Fe and Al) and silica, which bind the soil fabric, apart from the content ofexpansible clay minerals, particularly that of smectite. Wilding and Tessier (1988) indicatedthat high-charge smectites have a lower swell–shrink potential than low-charge smectites.The extent of swelling in smectites can vary greatly and natural smectites range fromstrongly swelling to non-swelling, depending on the magnitude and location of the layercharge (Wilding and Tessier 1988) and on charge heterogeneity (Lagaly et al. 1972). Thetendency of clay interlayers to take up water decreases with increasing interlayer chargedensity and charge homogeneity.


Layer charge characteristics

Layer charge characteristics of smectites are commonly determined by solvation withethylene glycol and glycerol, saturation with K, intercalation with C12 and the Greene–Kelly test. These tests were therefore performed on those melanic soils that were dominatedby smectite. Different treatments yielded slightly different results (Table 1).

According to the ethylene glycol treatment, 60% of the swelling clays were essentiallysmectitic, i.e. they formed a double layer with a regular series of subsequent orderreflections. In 22% of the samples, a peak position between 1.5 and 1.7 nm pointed to aninterstratification of smectite and vermiculite, while 18% of the soils contained anassociation of vermiculite and smectite.

Glycerol solvation provided a slightly different picture. An association of smectite andvermiculite was identified in 48% of the samples, discrete smectite in 41% and discretevermiculite in 11% of the soils.

Intercalation with dodecylamine also points to a high interlayer charge density andrelated low swelling capacity. Only one of the soils investigated contained somemontmorillonite, while 44% were composed of vermiculite and 26% of an association ofbeidellite and vermiculite.

According to the Greene–Kelly test most soil smectites were characterised as beidelliteand only 23% of the melanic soils contained montmorillonite, the highest amount being24%. Montmorillonite occurred in association with beidellite. This composition differsmarkedly from that reported for South African vertisols, which all containedmontmorillonite, average proportions ranging from 20% to 30%.

K-saturation points to beidellite as the dominant, in most cases the sole, swellingcomponent. Discrete montmorillonite was absent as was discrete vermiculite.

From the above treatments it must be concluded that a significant percentage of thesmectite species, which dominate South African melanic soils, is of the high-charge variety(high-charge smectite and/or low-charge vermiculite) and should therefore not be prone toosmotic swelling.

As a high degree of swelling is generally associated with a low layer charge, arisingpredominantly from octahedral substitutions, i.e. montmorillonite sensu stricto and notwith beidellite or vermiculite, Vertisols in South Africa must have a considerably higherswelling capacity compared with melanic soils. This finding may explain the presence of ahigh proportion of swelling clay despite an absence of a high degree of expansiveness.

Melanic soils develop from a variety of parent materials (Van der Merwe 2000),including arkosic sandstone, siltstone, mudstone, shale, diamictite, dolerite, basalt, notite,amphibolite, granite, gneiss, rhyodacite, and carbonates. There were distinctive differencesin clay mineral associations between melanic soils formed from different precursors(Table 1). The mica content was on average much higher in sediment-derived pedons, whilegranitic substrates resulted in high kaolinite contents. Talc and HIV were restricted to soilsformed from mafic rocks.

Table 1. Average clay mineral contents (%) in relation to parent material

Swelling clays Kaolinite Mica Talc HIV

Average 46 36 11 2 5Sediment 49 25 26 0 0Mafic rocks 43 38 8 2 9Granite/gneiss 29 61 10 0 0


Clay mineral associations in soil taxonomy

Taxonomic soil classification is essential for proper communication about soils. As intaxonomic classification in other fields, like plants and animals, taxonomic classificationsof soils are not practical classifications upon which detailed decision making or planningcan be based. Especially in higher categories each taxon usually includes a wide range ofsoils with different properties around limited commonalities in their features. In the SouthAfrican taxonomic system the soil form is characterised by a specific assemblage ofdiagnostic horizons.

It has already been pointed out that melanic A horizons span the spectrum from thosethat integrate with humic A horizons to those that intergrade with vertic A horizons. Thechallenge is to make meaningful subdivisions of these for classification in lower categories.Thus far, only the absence or presence of CaCO3, clay content, and colour have been usedfor series or family classification in soils with melanic horizons (MacVicar et al.1977; SoilClassification Working Group 1991). When looking at international trends, it would seemthat it will be important in South Africa to also distinguish between the ‘soft’ granularstructured melanic soils and the ‘hard’ ones which show vertic properties, but not stronglyenough to qualify as vertic horizons. N Khitrov (pers. comm., 1996) has attempted to derivecriteria that distinguish between ‘hard’ Chernozems (those that tend towards Vertisols) and‘normal’ Chernozems. The mollic (= melanic) horizon is the key horizon in Chernozems.In the WRB (1998) system ‘vertic’ is at the top of the priority listing of lower level units inthe Chernozem, Kastanozem, and Phaeozem reference units. These are the reference unitswith mollic horizons as key horizons.

Clay mineralogical studies may be important in this regard. Very detailed studies of theclay mineralogy will be required, however, which go beyond mineral group identificationsand focus on different species and the specific nature of interstratifications.

Selected soil textural and chemical properties

Though the study focused on clay mineral associations, some selected textural andchemical soil properties are presented.

Particle size distribution

A minimum amount of clay (15%) is required to impart melanic characteristics(MacVicar et al. 1977). Most of the melanic soils studied contained a considerably higherclay percentage and about half of them fell into the clay textural class (Fig. 4). Averagevalues for silt were low (19%), while those of clay (39%) and sand (40%) were almostequal. The degree of variability within the various particle size fractions was significant,however. Sand, silt, and clay contents ranged from 8 to 78%, 3 to 49%, and 15 to 65%,respectively. Clay mineral associations were not related to texture and soils with a low claycontent did not contain higher smectite proportions (Fig. 4). Melanic soils are associatedwith widely contrasting clay properties. These may vary from kaolinite-dominated horizonswith clay contents as low as 18% to essentially smectitic horizons with a clay contentexceeding 60%.

Exchangeable cation population

Smectites display a particularly high swelling and dispersion capacity when Na is presentin interlayer positions, while divalent cations, particularly Ca, ‘stabilise’ a soil. Of themelanic soils of South Africa Ca was the dominant interlayer cation, followed by Mg, while K


and Na contents were low. As much as 78.5% of the soils had an exchangeable Na percentageof <5, a value regarded as being below the ‘dispersion level’ for Australian (Emerson 1983)and South African (Gerber and Harmse 1987) soils. Most of the melanic horizons shouldtherefore be stable structurally and of low swelling potential. The cation exchange capacity(CEC) (cmol(+)/kg clay) ranged from 18 to 165, averaging 62. The extreme values arecharacteristic of kaolinite, which has a low layer charge and accordingly CEC, andvermiculite, which has a high proportion of exchangeable cations in interlayer positions. Anaverage value of 62 reflects an association of swelling clays with non-swelling ones.

pH

The soil pH is an indicator of the degree of weathering, being low in highly leached soilsand neutral to alkaline in soils with a low degree of weathering. The pH (H2O) of SouthAfrica’s melanic soils ranged from 9.3 to 5.4, averaging 6.9 (Fig. 5). These values reflect adegree of weathering which is low to moderate, a finding in line with the hypothesis ofmelanic soil formation (MacVicar et al. 1977). Kaolinite proportions increased with

0

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100

0 20 40 60 80

Clay (%)

Sm

ectit

e (%

)

Fig. 4. Relationship between clay and smectite contents.

R2 = 0.3086

4.5

5.5

6.5

7.5

8.5

9.5

10.5

0 50 100

Kaolinite (%)

pH

Fig. 5. Relationship between the kaolinite content in the clay fraction andsoil pH (H2O).


decreasing pH, particularly at the expense of smectite, a finding expected from an increaseddegree of weathering (Fig. 5).

Organic matter

The OM content varied between 0.5% and 4.3% with an average of 1.8%. Half of thesoils had an OM content between 1% and 2%. In 15% of the soils, OM percentages were<1, in 21% they ranged between 2 and 3, and in 10% they exceeded 3. The OM contentincreased with increasing kaolinite percentage and decreasing pH, but there was a highlysignificant scatter.

Conclusions

While smectite was established as the dominant clay mineral group in about half of themelanic horizons, the dominant smectite species was classified as beidellite or low-chargevermiculite, depending on the method employed. Few soils contained montmorillonite. Allmelanic horizons contained kaolinite, a significant number of them as the dominant claycomponent. The central position of melanic horizons between vertic, humic and orthic Ahorizons was confirmed by the results of the clay mineralogical data. The results of thisstudy may help to derive basic criteria for distinguishing between normal ‘soft’, granularstructured melanic A horizons and ‘hard’ or ‘vertic’ melanic horizons, for classification atlower categories (family or series level).

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Manuscript received 9 November 2000, accepted 15 May 2001

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