Pertanika J. Trop. Agric. Sci. 19(1): 43-53 (1996)ISSN: 0126-6128

© Penerbit Universiti Pertanian Malaysia

Charge Characteristics in Relation to Mineralogy of Selected SoilsfroID South-east Asia

J. SHAMSHUDDIN, RUZIAH SALLEH, M.H.A. HUSNI and KAMIS AWANG 1

Department of Soil ScienceFaculty of Agriculture

Universiti Pertanian Malaysia43400 UPM Serdang, Selangor Darul Ehsan, Malaysia

JDepartment of Forest ProductionFaculty of Forestry

Universiti Pertanian Malaysia43400 UPM Serdang, Selangor Darul Ehsan, Malaysia

Keywords: charge characteristics, Inineralogy, weathering, X-ray diffraction

ABSTRAKSatu kajian mengenai mineralogi dan ciri-ciri cas telah dijalankan terhadap 7 jenis tanah dari Filipina, Indonesiadan Malaysia. Tanah-tanah tersebut termasuk 1 Entisol, 3 Alfisol, 1 Andisol dan 2 Oksisol. Smektit, mika dankuarza didapati wujud di bahagian lempung pada tanah Entisol. Smektit dan kaolinit ialah mineral-mineraldominan di tanah Alfisol. Kebanyakan mineral di tanah Andisol ialah haloisit, manakala kaolinit dan oksidabanyak didapati di tanah Oksisol. Kuantiti kaolinit meningkat dari bawah ke permukaan tanah, sedangkan kuantitihaloisit semakin menurun. Ini menunjukkan haloisit telah ditukarkan kepada kaolinit semasa proses luluhawa.Perbezaan mineralogi bagi tanah yang berlainan adalah jelas dipengaruhi oleh perbezaan ciri-ciri cas. Tanah yangmengandungi smektit (Entisol dan Alfisol) mempunyai cas negatifyang tinggi. Tanah Andisol mengandungihaloisit dan tanah Oksisol mengandungi kaolinit dan oksida, mempunyai jumlah cas positifyang tinggi tetapijumlah cas negatifyang sederhana. Ketersediaan Ca dalam tanah bergantung kepada Ca tukarganti dan keupayaanpertukaran kation berkesan.

ABSTRACT

The mineralogy and charge characteristics of 7 soils from the Philippines, Indonesia and Malaysia were studied.The soils consisted of an Entisol, 3 Alfisols, an Andisol and 2 Oxisols. Smectite, mica and quartz were present inthe clay fraction of the Entisol. In the Alfisols, smectite and kaolinite were the dominant minerals. The Andisol wasdominated by halloysite, whereas the Oxisols were dominated by kaolinite and oxides. The amount of kaoliniteincreased towards the surface, while halloysite decreased, indicating the transformation of halloysite to kaoliniteduring the course of weathering. Differences in mineralogy of the various soil types were reflected clearly in thedifferences in charge characteristics. Soils with smectite (Entisol and Alfisol) had high a negative charge. TheAndisol, which contained halloysite, and the Oxisol, with kaolinite and oxides, had high amounts ofpositive charge,but moderate amounts ofnegative charge. The availability ofCa in the soils depended upon exchangeable Ca and theefJective cation exchange capacity (ECEC).

INTRODUCTION

Many soils in the Philippines and Indonesiaand some soils in Malaysia are derived fromvolcanic rocks of recent to Pleistocene age.Depending on the age and composition ofthe parent rock, and the stage of weath-

ering, volcanic soils in the tropics containallophane, halloysite, smectite, kaolinite,goethite and gibbsite (Eswaran 1979;Delvaux et at. 1989). Volcanic soils classi­fied as Andisols are known to contain largeamounts of halloysite (Mohr et at. 1972;

J. SHAMSHUDDIN, RUZIAH SALLEH, M.H.A. HUSNI AND KAMIS AWANG

Allen and Hajek 1989). Imogolite and x­ray amorphous hydrous aluminosilicates(collectively known as allophane) are com­mon in these soils (Wada 1989). Geographicdistribution of allophane and imogolite hasbeen connected with areas of recent volcanicactivity throughout the Pacific ring.

The charge of Andisols containingallophane and imogolite is pH dependent(Okamura and Wada 1983). The cationexchange capacity (CEC) is known toincrease with increasing soil pH and/orionic strength (Gillman and Hallman1988). For instance, the CEC increasedwhen fertilizer such as sulphate of ammoniawas applied, via replacement of OH- bySO/- (Guadalix and Pardo 1991). TheCEC of these soils can be determinedaccurately by Ca or Ca plus Al adsorption(Gillman and Sumpter 1986) or by acompulsive exchange method (Gillmanand Hallman 1988). Such an estimategives a true CEC value under field

conditions; this information is considereduseful in soil management. The charge andion retention properties chiefly govern thesoil cation dynamics. In view of the lack ofbaseline data for these soil types, theobjectives of this research were to char­acterize the mineralogy of a range ofvolcanic soils, occurring under differentclimatic conditions in the Philippines,Indonesia and Malaysia, and to establishthe relationship between their mineralogyand charge characteristics.

MATERIALS AND METHODS

The Soils

An Entisol (Philippines), 3 Alfisols (Phi­lippines), an Andisol (Indonesia) and 2Oxisols (one each from the Philippines andMalaysia) were examined in the field,classified (Soil Survey Staff 1990) andsampled. Table 1 gives their location,annual rainfall, rock type and geologic

TABLE 1Location, annual rainfall, rock type, geologic age and classification of soils from Indonesia,

Malaysia and the Philippines

Country Location Annual Rock Type Geologic ClassificationRainfall Age(mm)

Philippines Setio Bueno, 1986 basalt Holocene LithicTarlac Ustorthents

UPLB, Laguna 2006 doleri te, tuff Pleistocene Typic Hapludalfs

Rosario, 2297 basalt Pleistocene Typic HapludalfsLa Union

VISCA, Leyte 2499 basalt Pleistocene Typic Palendalf

eMU, Bukidnon 1828 basalt Pleistocene KandiustalficEutrustox

Indonesia Carita 11, 2874 basalt Tertiary TypicJava Hapludands

Malaysia Kuantan, 2757 basalt Tertiary Typic AcrudoxPahang

44 PERTANIKA J. TRap. AGRIC. SCI. VOL. 19 NO.1, 1996

CHARGE CHARACTERISTICS IN RELATION TO MINERALOGY OF SELECTED SOILS FROM SOUTH-EAST ASIA

TABLE 2Chemical properties of soils from Indonesia, Malaysia and the Philippines

Soil Hor pH(l: 1) pHo Exch Ca ECEC B.S Fe20g Corg Clay(H2O) cmolc/kg (%)

Entisol A 5.2 2.7 16.3 23.5 97.2 2.4 1.8 28.6Alfisol Ap 6.2 3.7 25.1 34.4 99.9 2.1 1.1 37.7

Bt2 5.3 nd 29.7 42.7 99.3 1.6 1.3 53.2Andisol Al 4.3 3.5 1.4 2.3 95.6 7.3 1.4 76.4

BW2 4.7 nd 1.3 3.6 44.3 7.8 1.0 73.5Oxisol A 5.7 3.3 12.4 17.1 99.4 8.6 3.3 65.1(Eutrustox) Bt2 4.7 nd 6.7 8.1 91.4 10.6 1.3 74.3

nd = not determined

age. Four soils were selected for detailedinvestigation into their mineralogy andcharge properties. Chemical properties aregiven in Table 2. Note that for Table 2 boththe Alfisol (UPLB) and Oxisol (CMU)were from the Philippines, and the Entisolis a shallow soil without Band C horizons.In addition, the Oxisol is a special type,Eutrustox, which has a very high base value(91.4 - 99.40/0) compared with normalOxisols, which have a base value of lessthan 350/0.

Soil AnalYsisThe pH of a 1: 1 solution ofsoil in water wasdetermined after 1 h of intermittent shakingand standing overnight. Exchangeable Caand Mg were extracted by 1 M NH40Acand determined by atomic absorptionspectrophotometry, while exchangeable Kand a in the same extract were deter­mined by flame photometry. ExchangeableAl was extracted by 1 M KCl anddetermined colorimetrically (Barnhisel andBertsch 1982). Effective cation exchangecapacity (ECEC) was calculated as the sumof basic exchangeable cations and ex­changeable AI, while base saturation (BS)was calculated on the basis of ECEC. Freeiron oxide content was determined by themethod of Mehra and Jackson (1960). TheWalkley-Black method (Nelson and Som­mers 1982) was used to measure organic

carbon (Corg)'

Clay content of the soils was deter­mined by the pipette method of Day(1965). To obtain the clay, the soil wasfirst treated with H 20 2 to remove organicmatter. It was later dispersed with diluteNa2C03 solution. This clay « 2~m) waslater used to study the mineralogy of thesoils by X-ray diffraction (XRD) andtransmission electron microscopy (TEM).The XRD analysis was conducted by anautomated Phillips diffractometer equippedwith a graphite monochromator, operatedfrom 3 to 50 degrees 2-theta, using Cu Kcxradiation and scanning speed of half adegree per minute. The X-ray diffractionanalysis was carried out on Mg-saturatedand glycolated samples.

Charge characteristics of the soils weredetermined by the method of Gillman andSumpter (1986). In this method, negativecharge as measured by Ca adsorption wastermed CECB, while that measured by Caand Al adsorption was termed CECT . Thepositive charge as measured by Cl adsorp­tion was termed AEC. Soil weatheringindex (WI) was calculated (at soil pH) asfollows (Tessens and Shamshuddin 1983):

WI = negative charge - positive charge x 100negative charge

The negative charge (CECB ) and thepositive charge required for WI determina-

PERTANIKA J. TRap. AGRIC. SCI. VOL. 19 NO.1, 1996 45

J. SHAMSHUDDIN, RUZIAH SALLEH, M.B.A. HUSNI AND KAMIS AWANG

tion were estimated from the charge curves.PHo , defined as the pH of the variablecharge colloid at which the net charge iszero, was determined by the method ofGillman and Sumpter (1986).

Extraction of Soil Solution and AnalYsis

Distilled water was added to the air-driedsoils and the wetted soils were subsequentlyincubated for 1 day at a matric suction of10 kPa (Menzies and Bell 1988). This studyassumed that a state of equilibrium wasreached between the liquid and solid phaseof the soils during the incubation period.Soil solutions were extracted by centrifuga­tion at 2000 rpm for 1 h using speciallydesigned centrifuge tubes. The pH andelectrical conductivity (EC) of each soilextract were immediately determined from2-m1 subsamp1es. The remainder of theextract was stored at 5°C for later determi­nation of Ca, Mg, K, a, Fe, AI, Sand Pby inductively coupled plasma atomicemission spectroscopy (ICPAES) .

RESULTS AND DISCUSSION

Soil TypesDifferences in the lithology, age of parentmaterial, climate, drainage conditions andvegetation for the soils in the differentregions (Table 1) were reflected by thedifferences in their chemical properties(Table 2), as the soils had undergonedifferent rates of chemical weathering.Hence, several groups of soils ranging fromEntisol to Oxisol are found in the region.According to their chemical properties andprofile morphology, the least weathered ofall the study soils was sited at Setio Bueno,Tarlac, the Philippines (Entisol), while themost weathered soil was found at Bukid­non, the Philippines and Kuantan, Malay­sia (Oxisol). The other soils, which hadhigh basic exchangeable cations, wereclassified as Alfisols (Philippines). The soilwhich had low bulk density and a high

amount of amorphous materials was classi­fied as an Andisol (Indonesia). The Oxisoland Andisol also contain high amounts ofFe20g.

Mineralogy

Overall, the peaks on the X-ray diffracto­grams of the soils were clear although theclay samples were likely to be coated withthe amorphous materials (including allo­phane and/or sesquioxides). Furthermore,the low scanning speed was slow enough toallow differentiation of the individualminerals on the X-ray diffractograms.Although allophane is common in volcanicsoils (Mohr et al. 1972; Wada 1989) it wasnot confirmed by the XRD analysis orTEM observation.

The X-ray diffractogram of the Entisolhad reflections at 1.52, 1.0, 0.713, 0.5,0.445, 0.426, 0.418, 0.356, 0.334, 0.245 and0.212 nm (Fig. 1). The strong and clearreflections at 0.426 and 0.334 nm indicatedthe presence of quartz in the clay fraction.Mica (1.0, 0.5 nm), smectite (1.52 nm) andkaolinite (0.713, 0.356 nm) were also pre­sent, but in lesser amounts. A small amountof goethite (0.418, 0.245 nm) and hydratedhalloysite (A12Si20s(OH)4.2H20) werepresent. The presence of the hydratedhalloysite was shown by the 1.0, 0.445and 0.346 nm reflections (Dixon 1989). TheTEM micrograph (Plate 1A) gives a visualillustration of the presence of kaolinite,mica and/or smectite in the soil. Thesmectite in the soil was probably analteration product of mica weatheringunder impeded drainage condi tions(Velde 1992). This smectite can be inter­stratified with the hydrated halloysite asshown by Delvaux et al. (1989) in volcanicsoils of Cameroon.

Smecti te, halloysi te, kaolinite andgoethite were present in the topsoil of theAlfisol (Fig. 1). The 1.5 - 1.8 nm peak wasstrong and intense, suggesting that smectite

46 PERTANlKA J. TRap. AGRIC. SCI. VOL. 19 O. 1, 1996

CHARGE CHARACfERISTICS IN RELATION TO MINERALOGY OF SELECfED SOILS FROM SOUTH-EAST ASIA

24>i • I i i i I I • i

I"'- ~ T"" IX) 10 C\I a> <0 C') 0

--i ci ~ eO C"i eO C\i ,...: C\i ,...:T"" C\I C\I C') C') ~ ~

Fig. 1 X-ray diffraction patterns of Mg-saturated clayfraction from the A horizon of the Entisol and. Alfisol.

Plate lA. TEM micrograph of the clay fraction from theA horizon of the Entisol

was present in large amounts. This isconsistent with the high ECEC value(Table 2) and is a common feature of avolcanic soil with moderately well drainedconditions existing under a udic moistureregime. The presence of amorphous mate-

Plate lB. TEM micrograph of the clay fraction from theAp horizon of the Alfisol

rials in the clay sample tended to reduce thesharpness of the XRD peaks. Plate lB showsthe occurrence of halloysite, kaolinite and/or smectite being coated by the amorphousmaterials. The halloysite in the soil wassimilar in morphology to the ferruginoushalloysite from Hokkaido, Japan (Wadaand Mizota 1982).

The X-ray diffractogram of the Andisol(Fig. 2) showed the dominance ofhalloysite(0.73, 0.445 nm) with minor amounts ofkaolinite (0.358 nm), goethite (0.418,0.269, 0.245 nm) and hematite (0.270,0.252 nm). The sample from the C horizon(not shown) gave a similar XRD pattern,indicating a similar mineralogy in bothhorizons. The peaks 0.73 and 0.445 nmbelong to dehydrated halloysite, which hasa formula of A12Si20 s(OH)4 (Dixon 1989).This halloysite is common in youthful soildeveloped from volcanic rock (Dixon 1989;Allen and Hajek 1989). TEM observationfurther indicated that the clay fraction wasdominated by halloysite, without clearmanifestation of the presence of allophaneand/or imogolite (Plate lC). Further TEMobservations (not illustrated here) suggest

PERTANIKA J. TRap. AGRIC. SCI. VOL. 19 NO.1, 1996 47

J. SHAMSHUDDIN, RUZIAH SALLEH, M.H.A. HUSNI AND KAMIS AWANG

Fig. 2 X-ray diffraction patterns of Mg-saturated clayfraction from the A horizon of the Andisol and Oxisol

2<1>, i i i i i i , i"- ~ .... co It) C\l 0) <0 C') 0-.i m ~ eO M eO N ,..: N ,..:.... C\l C\l C') C') ~ ~

that halloysite was more dominant in thesubsoil than in the topsoil.

The XRD peaks on the diffractogramof the Gxisol were different from those ofthe Andisol in that clear peaks wereobserved at 0.72 and 0.358 nm, showingthat kaolinite was more dominant than

halloysite (Fig. 2). The amounts of goethite(0.416, 0.269, 0.245 nm) and hematite(0.269, 0.251 nm) in the'soil were higherthan in the Andisol, presumably as a resultof the more advanced stage of weathering.A small amount of gibbsite (0.486 nm) wasdetected in this soil. Chemical weatheringunder well-drained conditions has resultedin the formation of the sesquioxides. Similarminerals were found in the B04 horizon ofthe soil. TEM observation (Plate ID)further indicated the presence of a mixtureof kaolinite (hexagonal) and halloysite(tubular) in the Gxisol. It was noted thatthe amount of kaolinite increased towardsthe surface, while the halloysite decreased.This is consistent with the report ofDe1vaux et al. (1989) who found thathalloysite in volcanic soils in WesternCameroon was formed earlier than kaoli­nite in the weathering sequence.

Clear X-ray reflections at 0.404 and0.248 nm were observed in the diffracto­grams of the Andisol and Alfisol. The mine­ral which gives the reflections was identifiedas cristobalite, which is commonly asso­ciated with volcanic deposits' of Tertiary

ANDISOLcoIt)C')

c::i

Plate 1C. TEM micrograph of the clay fraction from theA horizon of the Andisol

Plate 1D. TEM micrograph of the clay fraction from theA horizon of the Oxizol.

48 PERTANlKA J. TROP. AGRIC. SCI. VOL. 19 NO. I, 1996

CHARGE CHARACTERISTICS IN RELATION TO MINERALOGY OF SELECTED SOILS FROM SOUTH-EAST ASIA

age having formed from dissolution ofvolcanic glass (Drees et at. 1989). Both soilsalso have low bulk density, an attributeusually associated with andic properties.

The XRD analyses showed the pre­sence of admixtures of halloysite, kaolinite,smectite, gibbsite and goethite in variousproportions in the clay fraction of the soilsunder investigation. The majority of thoseminerals are variable-charge minerals.Primary minerals such as mica and quartzwere present in small amounts in the clayfraction of the Entisol. The clay fraction ofthe Andisol was dominated by halloysite,while that of the Oxisol was dominated by amixture of kaolinite and halloysite. TheAlfisol contained large amounts of smectiteand kaolinite.

Fig. 3 shows the suite of clay minerals inthe different soil types arranged in order ofdecreasing abundance. Volcanic materialsfirst weather to form either Andisol orEntisol, depending on the mineralogicalcomposition and moisture regime. Accord­ing to Mohr et at. (1972), volcanic glassunder well-drained conditions first changesto allophane, which on further weathering

is transformed to halloysite, and subse­quently to kaolinite. This transformationcan be visualized in the development of anAndisol.

In the other pathway, mica in theEntisol first weathers to smectite. Furtherweathering results in a complete destruc­tion of mica and the formation of smectite,which in turn weathers to halloysite and/orkaolinite. Such a sequence typifies thedevelopment of an Alfisol. In the Oxisol,kaolinite, oxides and halloysite are present,but smectite is absent. We found that theamount of goethite and hematite increasedwith weathering. Additionally, kaolinite inthe Alfisol, Andisol and Oxisol increasedtowards the surface, while halloysite de­creased. These observations support thescheme of mineral weathering shown inFig. 3.

Charge Properties

The CECB of the Entisol and Alfisol washigh, with values comparable to thosereported by Gillman and Hallman (1988)for the volcanic soils of Papua New Guinea.The high negative charge in the Entisol and

Volcanic ... Entisol ... Alfisol ... OxisolMaterials

,

Smectite Smectite KaoliniteMica Kaolinite OxidesKaolinite Halloysite Halloysite

Quartz Oxides

Oxides Cristobalite

Andisol

HalloysiteOxidesKaolinite

Cristobalite

Fig. 3. Soil minerals arranged in order of decreasing abundance in the various soil types

PERTANIKA J. TROP. AGRIC. SCI. VOL. 19 NO.1, 1996 49

J. SHAMSHUDDIN, RUZIAH SALLEH, M.H.A. HUSNI AND KAMIS AWANG

Alfisol was attributed partly to the presenceof smectite in the soils (Fig. 4). The ECECof these two soils was high (Table 2). TheCECB value of both soils increased as thepH increased. CECT appeared to increasein the Entisol and Alfisol at pH below 3.5(Fig. 4). A portion of the Al measured in thesoil solution could have been dissolved fromthe inner part of the phyllosilicates(smectite or mica) rather than on theexchange sites. At very low pH some ofthe soil minerals are no longer stable, andAl in the minerals readily goes into the soilsolution.

The Andisol had a net positive chargeat the soil pH of 4 (Fig. 4). Halloysite wasthe dominant clay mineral in the soil(Fig. 2; Plate lC). The changes of CECB

and AEC with pH were similar to thosereported for the volcanic soil with some

allophane and/or imogolite (Okamura andWada 1983). Like the Entisol and Alfisol,the CECT of the Andisol increased belowpH 3.5. The high positive charge present inthe soil was partly caused by the Fe oxides(goethite and hematite), as indicated bytheir 7.30/0 content in the topsoil (Table 2).On the other hand, the Entisol and Alfisol,which had quartz, mica and smectite withminor amounts of kaolinite, exhibiteddifferent charge characteristics to those ofthe Andisol. The negative charge in theEntisol and Alfisol was, however, higherthan the positive charge at the soil pH.

The positive charge in the Oxisol wasslightly lower than in the Andisol. Addi­tionally, the negative charge in the soil waslow and is a ttribu ted partly to thedominance of low activity clays. XRDanalyses (Fig. 2) and TEM observations

Entisol Andisol15

15CECT

1010

CECrCECs 5

0'1 5~

"-6.5 0 5·5 6·5(.) 0 5·5 3·5 4·5

"0 3·5 pHE 5(.) 5

1010

AECAEC

1515

Alfisol Oxisol

20

15 15

~ 10 10

"-CECS

(.)

0 5 5

E CECs(.)

00 5·5 6·53·5 4·5 5·5 6·5 3·5 4·5

pH pH5 5

10 10

Fig. 4. Charge characteristics of the Entisol, Alfisol, Andisol and Oxisol

50 PERTANIKA J. TRap. AGRIC. SCI. VOL. 19 NO.1, 1996

CHARGE CHARACTERISTICS I RELATION TO MINERALOGY OF SELECTED SOILS FROM SOUTH-EAST ASIA

(Plate Ie and ID) showed that kaolinite,halloysite, hematite and goethite werepresent in large amounts. The high amountof Fe20g contributed a certain amount ofpositive charge to the soil surfaces. TheCECB and AEC of the topsoil are quitesimilar to those of the Andisol. However, theCECT did not increase below pH 3.5. Theposi tive charge in the B04 horizon of theOxisol (data not shown) was higher than thenegative charge at pH of 3.0 to 6.5. Thischarge pattern was similar to that of the soilat Kuantan, Malaysia. The Kuantan soil,classified as Typic Acrudox (Tessens andShamshuddin 1983), is one of the mosthighly weathered soils in Malaysia.

The weathering index (WI) values forthe Entisol and Alfisol were high, withvalues ranging from 100 to 50. The highvalue means that the soils are in the recentor intermediate stage of weathering (Tes­sens and Shamshuddin 1983). The low pHais directly related to the high WI and ischaracteristic of newly developed soils(Gallez et al. 1976; Uehara and Gillman1981) .

The WI value of the Oxisol was 50 - 0,which fits very well into the category of anadvanced stage of weathering, although thepHa of the soil was low. The low pHo waspresumably caused by the presence ofsignifican t amounts of aluminosilica tes(Uehara and Gillman 1981).

The WI of the Andisol was < 0, whichis typical of a soil dominated by halloysiteand oxides of Fe and AI. Accordingly, thesoil can be placed in the category of anadvanced stage of weathering. However, webelieve that the WI calculated from chargecharacteristics cannot be applied to thiskind of soil. The effect of amorphousmaterials (most likely allophane) was nottaken into consideration in the formulationof WI. Allophane is normally associatedwith relatively young volcanic soil (Mohr etal. 1972).

Soil Solution Attributes

The soil solution Ca concentrations in thesoils of the Philippines were high, but low insoils from Indonesia and Malaysia. Thehighest value of soil solution Ca concentra­tion was observed in the Bt2 of the Alfisol.The high value of the soil solution wasconsistent with the high exchangeable Cain that horizon; the exchangeable Ca in theBt2 horizon of the Alfisol was 29.7 cmole/kg(Table 2). The relationship between the soilsolution Ca concentration and exchange­able Ca in the soils is given by thisequation:

Casal = 0.79 + 41.40 Caexeh,(r = 0.75, p < 0.05)

Soil solution Ca concentration was re­lated to the ECEC (calculated on the basisof 1 kg clay) by this equation:

Casal = -2.70 + 11.00 ECEC,(r = 0.78, p < 0.01)

Although the exchangeable Ca in thesoils was high, not all the Ca was releasedinto the soil solution (for Philippine soils).Most of these cations were held tightly bythe soil surfaces because of the high ECEC.The high ECEC is generated by themoderately high pH (Table 1). Thisphenomenon is illustrated in Fig. 4. Thedominance of variable-charge minerals inthe soils is consistent with the high CEC athigh pH. The results showed that a higheramount of Ca was available in the Entisoland Alfisol than in the Oxisol.

CONCLUSION

The soils studied are classified as Entisol,Alfisol, Andisol or Oxisols. The clayfraction of the Entisol contains smectite,mica and quartz, while the Alfisol isdominated by smectite and kaolinite, withminor amounts of halloysite and goethite.

PERTANlKA J. TRap. AGRIC. SCI. VOL. 19 O. I, 1996 51

J. SHAMSHUDDIN, RUZIAH SALLEH, M.H.A. HUSNI AND KAMIS AWANG

Halloysite is abundant in the Andisol, whilethe Oxisol contains a mixture of kaoliniteand halloysite with significant amounts ofsesquioxides.

The mineralogy of the soils significantlyaffects their charge characteristics. TheEntisol and Alfisol have a high negativecharge, but the Andisol has a high positivecharge. Generally, the charge characteris­tics of the Andisol are similar to those of theOxisol. The soil solution Ca is correlated tothe exchangeable Ca and the chargecharacteristics. The availability of Ca istherefore governed by the amount ofexchangeable Ca and the ECEC of thesoils.

ACKNOWLEDGEMENTS

The authors would like to record theirappreciation to U niversiti Pertanian Ma­laysia, National Council for ScientificResearch and Development and WinrockInternational, Bangkok for financial andtechnical support.

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(Received 13 December 1994)( Accepted 27 June 1996)

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