The Mineralogy and Chemical Properties of Soils on Granite ...pertanika2.upm.edu.my/Pertanika PAPERS/JTAS Vol. 18... · of the area, the rock undergoes different degrees of chemical

PertanikaJ. Trop. Agric. Sci. 18(1): 45-56(1995) ISSN: 0126-6128© Universiti Pertanian Malaysia Press

The Mineralogy and Chemical Properties of Soils on GraniteGneiss in Three Climatic Zones in Sri Lanka

J. SHAMSHUDDIN, RUZIAH SALLEH, M.H. AHMAD HUSNI and KAMIS AWANG2

Department of Soil Science,Faculty of Agriculture

Universiti Pertanian Malaysia43400 UPM Serdang, Selangor Darul Ehsan, Malaysia

2MPTS Network Secretariat, Winrock Internationalc/o Faculty of Forestry, Universiti Pertanian Malaysia

43400 UPM Serdang, Selangor Darul Ehsan, Malaysia

Keywords : agro-ecological zone, granite gneiss, mineralogy, soil solution

ABSTRAK

Granit gneis dijumpai wujud di merata-rata di Sri Lanka. Bergantung kepada keadaan iklim dan topografikawasan, bahan ini mengalami luluhawa kimia yang berbeza-beza. Satu kajian mineralogi dan sifat kimiakeatas sifat tanah granit gneis daripada tiga zon agro-ekologi di Sri Lanka telah dijalankan. Tanah dalam zonlembab didapati rendah pH dan kation bertukar ganti, manakala tanah dalam zon kering pula mempunyai pHdan kation bertukar ganti yang tinggi. Warna tanah bertukar daripada coklat kekuningan kepada merahapabila tanah bertambah kering mun^dn disebabkan oleh kehadiran hematit dan/atau goethiL Kaolinit, haloisitdan smektit wujud di dalam semua tanah tanpa mengira iklim; amaun haloisit berkurangan menghala kepermukaan tanah. Boitit wujud dengan banyaknya di dalam tanah zon perantaraan dan kering, tetapi tidakwujud dalam tanah zon lembab. kepekatan Ca dalam larutan tanah bergantung kepada amaun Ca yang wujuddalam bentuk bertukar ganti. Tambahan lagi, kepekatan Ca larutan berkolerasi secara bererti dengan EKPK dannisbah EKPK/KPK tanah.

ABSTRACT

Granite gneiss occurs sporadically throughout Sri Lanka. Depending on the climatic conditions and topographyof the area, the rock undergoes different degrees of chemical weathering. The mineralogy and chemical propertiesof four common granite gneiss soils occurring in three agro-ecological zones in Sri Lanka were studied. The soilin the wet zone is low in pH and basic exchangeable cations, while the soils in the wet zone are high in pH andbasic exchangeable cations. The colour of the soils changes from yellowish-brown to red as the soils get drier,presumably due to the presence of haematite and/or goethite. Kaolinite, halloysite and smectite are present in allthe soils irrespective of the climatic conditions; the amount of halloysite decreases towards the surface. Biotite isabundant in the soil of the intermediate and dry zones, but is absent in the soil of the wet zone. Gibbsite is presentin the highly weathered soil of the wet zone. The concentration of Ca in soil solution is dependent on the amountin the soils existing in the form of exchangeable Ca. Additionally, the solution Ca concentration is significantlycorrelated with the soil ECEC and the ECEC/CEC ratio.

INTRODUCTION receives intermediate rainfall. Soil physico-

Sri Lanka, located near the Equator, is agro- chemical properties and agricultural productionecologically divided into wet, intermediate and *n the country are closely related to the rainfalldry zones (Anon 1979). The wet zone occurs in patterns in the different agro-ecological zones,the southwest, while the dry zone occurs in the The southern part of Sri Lanka is dominatednorth (Fig. 1). The middle part of the country by red yellow podzolic and latosolic soils (Anon

J. SHAMSHUDIN, RUZIAH SALLEH, M.H. AHMAD HUSNI AND KAMIS AWANG

LegendDry zoneIntermediate zoneWet zone

Fig. 1: A map of Sri Lanka showingagro-ecological zones (Anon. 1979)and the sites of soil sampling

1979). Besides red yellow podzolic soils, reddishbrown earth occurs in the intermediate zone.These soils can either be classified as Ultisols orAlfisols in soil taxonomy (Soil Survey Staff 1992).The soils in the middle and northern parts ofthe country are different from those of thesouth due to differences in the rate of leachingand chemical weathering; the rainfall in thenorth is much lower than in the south. Thecolour of these soils is determined primarily bythe type of Fe mineral rather than the absoluteamount of Fe oxide present in the soils.According to Schwertmann (1993), haematite-

containing soils have mostly hues between 5YRand 1OR, whereas goethite-containing soils withno haematite have hues between 7.5YR and2.5Y.

Granite gneiss is an important rock type inSri Lanka, and many soils earmarked for agri-cultural and/or forestry purposes are derivedfrom it. The granite gneiss occurs sporadi-cally throughout the three agro-ecologicalzones of Sri Lanka. Biotite (iron magnesiummica) is one of the major minerals in thegranite gneiss (Paramananthan 1977). Ac-cording to Eswaran and Heng (1976), biotitetransformed to halloysite, kaolinite or goethitedepends on the position of the mineral in aweathering profile. Due to differences in theclimate and topography, the granite gneisssoils in Sri Lanka that have undergone differ-ent degrees of weathering are differentiatedinto Ultisols and Alfisols. Mineralogical trans-formation and soil solution chemical composi-tion in the different agro-ecological zones havenot bean investigated and documented in SriLanka. The objective of this paper is to char-acterize the mineralogy and soil solutionchemical composition of some of the commongranite gneiss soils in the three agro-ecologi-cal zones in Sri Lanka.

MATERIALS AND METHODS

SoilsThe soil profiles for this study were examinedduring a field tour in Sri Lanka in October 1991(Shamshuddin and Ahmad Husni 1992). Thesoils, derived from granite gneiss, were sampledat four localities, representing the three agro-ecological zones in Sri Lanka (Fig. 1). The soils

TABLE 1Location, annual rainfull, agro-ecological zone and

1 1 r C U *l j •

colour classification of the soils under investigation

Location

Meerigama

Badulla

Kandy

Anuradhapura

Rainfallmm/year

4008

1800

1563

1329

Agro-ecologicalzone

Wet

Intermediate

Intermediate

Dry

Subsoilcolour

Yellowishbrown

Red

Red

Red

Subgroup

Typic Hapludults

Typic Hapludults

Typic Rhodustalfs

Typic Rhodustalfs

46 PERTANIKAJ. TROP. AGRIC. SCI. VOL. 18 NO. 1 1995

TABLE 2Relevant chemical properties of the soils under investigation

PE

RT

AN

I

Eiy

GR

IC.

n%O

00

o

s

I /irstion

Meerigama

Badulla

t2-

Kandy

Anuradhapura

Depth(cm)

0-99-3434-6565-8585-120

0-2626-6363-98/10498/104-135135+

0-1414-4949-9797-130130+

0-99-2424-6565-120120-140/150

Hfori7on

ApBwBt,B t 2

B t 3

ApBwB t i

C i

Ap

B CB t 3

A l

B * I

Bt3

B t 4

pH(

5.45.25.25.35.1

5.55.35.95.26.4

6.46.06.36.37.0

6.46.16.36.26.3

1:1)

CaCl2

4.84.13.93.83.8

5.05.15.64.95.7

6.25.55.75.76.1

6.15.65.45.35.2

Ca

1.520.490.530.081.12

3.003.013.034.254.00

12.0010.1011.7010.859.40

10.696.287.237.698.25

Exchangeable Cations

Mg

0.550.400.550.750.54

1.434.096.72

51.2152.60

0.582.433.385.525.45

1.611.652.072.110.32

K

0.040.040.040.040.04

0.450.380.090.180.27

0.290.310.160.270.13

0.540.320.180.130.22

Na- cmolc/kg -

0.130.030.050.080.09

0.080.520.190.320.54

0.570.271.310.480.22

0.950.480.320.240.20

Al

0.200.200.104.005.10

0.100.300.100.0100.10

0.100.100.100.200.10

0.100.100.100.100.20

ECEC

2.441.161.275.676.89

5.788.30

13.1356.0657.51

13.5413.2215.6517.2215.40

13.898.839.90

10.279.15

CEC

3.993.264.438.98

11.92

16.4810.5017.2134.8313.24

14.7114.1216.7717.5016.63

9.579.72

10.7412.5112.65

Fe O

0.40.4o.6 ;1.2 :

i-2 ;

9.4 £8.5

10.43.42.5

3.03.44.04.34.0

1.82.52.93.12.7

Teture (mm)

<2Of

4.8 ;9.8 )

14.331.130.1

14.060.564.134.714.5

27.834.744.846.040.4

16.224.332.335.934.9

2-50 >50

j 8.3 86.6 JT ':

8.6 81.2 "| n.4.8 80.8 ;; ""6.2 62.1 £

12.8 56.8 X ,

56.7 28.8 | Z'14.1 25.117.4 18.218.0 46.5 r £12.9 72.2

5.6 66.5 "6.2 58.85.8 49.15.7 48.0 L7.9 51.7 *

5.8 77.79.1 66.28.4 58.99.3 54.38.1 56.7 :

TH

E N

i

an

g

iS

0

0z

PI

9JEISS


are classified either as Hapludults, Haplustalfs orRhodustalfs (Table 1), based on their chemicalproperties (Table 2) or on profile morphology(data not shown).

MethodspH in water and 0.01 M CaCl2 was determinedafter 1 h of intermittent shaking and an over-night stand. CEC was determined by 1 MNH4OAc buffered at pH7. Basic exchangeablecations were extracted by 1 M NH4OAc; Ca andMg were determined by atomic absorptionspectrophotometry, while K and Na were deter-mined by flame photometry. Exchangeable Alwas extracted by 1 M KC1 and determinedcolorimetrically (Barnhisel and Bertsch 1982).ECEC was calculated as the sum of basic ex-changeable cations and exchangeable Al. Freeiron oxide was determined by the method ofMehra and Jackson (1960). Texture of the soilswas determined by the pipette method of Day(1965). The mineralogy of the clay fractionfrom the topsoil and the subsoil was determinedby XRD analysis and TEM. The XRD analysiswas conducted using a Philips diffractometerequipped with a graphite monochromator, op-erated from 3 to 50 degrees 2 - theta at ascanning speed of '/, degree per minute. Se-lected samples were subjected to further tests inorder to confirm the presence of smectite andchlorite.

Distilled water was added to the air-driedsoils and subsequently incubated for 1 day at amatric suction of 10 kPa (Menzies and Bell1988). This study assumes that a state of equi-librium is reached between the liquid and solidphase of the soils during the incubation period.Soil solutions were extracted by centrifugationat 2000 rpm for 1 h. pH and EC were deter-mined immediately from 2-ml subsamples. Therest of the solutions were stored at 5°C fordetermination of Ca, Mg, Na, K, Al, Mn, Fe, Pand S by inductively coupled plasma atomicemission spectroscopy (ICPAES). Nitrate in thesoil solutions was determined by an autoanalyser.

RESULTS AND DISCUSSIONSoil General PropertiesRainfall distribution pattern in the areas cov-ered by granite gneiss in Sri Lanka is variable,having values ranging from 4008 mm p.a. inMeerigama to 1329 mm p.a. in Anuradhapura

(Table 1). The effects of rainfall on the soils aremanifested clearly by soil pH and the amountsof basic exchangeable cations present. Whererainfall is high (Meerigama), basic exchange-able cations are low and the basic cations arelost via leaching. pH (CaCl2) is < 5, thus the soilis acidic in reaction (Table 2). The topsoil fromthe intermediate agro-ecological zone (Badulla)contains reasonable amounts of basic exchange-able cations. Here the soil pH is higher thanthat at Meerigama. The basic exchangeablecations and the soil pH increase further as therainfall decreases. This is shown clearly by thehigh pH value in the soil at Anuradhapura,where topsoil pH (CaCl2) is higher than 6.

The exchangeable basic cations in the soilssubjected to high rainfall (Meerigama) are low(Table 2) and exchangeable Al in the topsoil isalso 'low, but at depths below 65 cm, the ex-changeable Al is > 4 cmolc/kg soil. However, indrier areas (Kandy and Anuradhapura), the ba-sic exchangeable cations are very high, espe-cially the exchangeable Ca. As pH in these soilsis high, exchangeable Al is only present in traceamounts. Aluminium in the soil solution pre-cipitates as Al(OH);t as pH approaches 5.5.

Exchangeable Mg in the C horizons of thesoil at Badulla is extremely high, having valuesexceeding 50 cmolc/kg soil (Table 2) which ispresumably related to the presence of weath-ered ferromagnesium minerals in the granitegneiss. The typical Mg-bearing minerals in gran-ite gneiss are biotite, pyroxene and amphibole,which on weathering become smectite if condi-tions are favourable for its formation. The highCEC (in terms of cmolc/kg clay) observed in theC horizons points to the presence of this mineral.

The colour of the soils in the B horizonchanges from yellowish brown (10YR 5/6) tored (2.5YR 4/6) and is presumably attributedpartly to the presence of goethite and/or haema-tite which, in turn, depends on the availability ofmoisture in the soil profiles. The red soilsoccurring in the intermediate and dry zones,which are classified as Rhodustalfs, are normallyassociated with the presence of these Fe miner-als. Haematite is coloured 5R to 2.5 YR (Bighamet al 1978), while goethite is 7.5 YR to 10 YR(Kitagawa 1983). Goethite tends to impart yel-lowish brown to brown colours to soils (Allenand Hajek 1989). The soils studied are verydeep, except the soil at Badulla site, which isseriously eroded. The Badulla site is located at


THE MINERALOGY AND CHEMICAL PROPERTIES OF SOILS ON GRANITE GNEISS

Fig. 2: X-ray diffraction pattern of Mg-saturated clay fraction from the Ap horizon of the soil atMeerigama

659 m k.a.s.L, on rugged topography having aslope of 100%. The topsoil is constantly re-moved by geological erosion during rainymonths. Morphologically, the soil is not highlyweathered as rock fragments containingweatherable minerals were observed in the Bhorizon.

Overall, the four soils under study are sandyin nature, an inherent physical property of thesoils derived from granite gneiss, which is essen-tially composed of quartz, feldspar and biotite(black mica), with minor amounts offerromagnesium minerals. The CEC (on thebasis of 1 kg clay) is high, suggesting that someof the soils contain some smectite or mica-smectite mixed-layers. These minerals are thealteration products of biotite weathering underimpeded drainage conditions (Velde 1992).

Mineralqgy

The XRD analysis of the clay fraction from theAp horizon of the soil at Meerigama shows the

presence of smectite (1.47 nm), halloysite (0.74,0.445 nm) kaolinite (0.358 nm) and quartz(0.427, 0.335 nm), with minor amounts ofgibbsite (0.484 nm), feldspar (0.319 nm) andgoethite (0.418, 0.245 nm) (Fig. 2). The subsoilshowed similar mineralogy. Though present inminor amounts, goethite imparts yellowish brown10 YR 5/6 colours to the soil (Kitagawa 1983;Allen and Hajek 1989). There are no biotitereflections in the diffractogram. The biotitepresent in the original rock could have beentransformed completely to halloysite(Paramananthan 1977) and/or goethite underthe strongly leaching environment (Eswaran andHeng 1976). In the humid tropics, biotite canalso be transformed to kaolinite (Eswaran andHeng 1976). The halloysite and quartz XRDpeaks are sharp and intense, showing their domi-nance in the clay fraction. The presence ofsmectite is also reflected in the high CEC (calcu-lated on the basis of 1 kg clay), but the smectiteis not clearly manifested in the TEM micrographsas smectite crystals are very small (Plate 1A, B).

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 18 NO. 1, 1995 49


Plate 1. TEM micrograph of the clay fraction from the Ap (A) and Bt, (B) horizon of the soils at Meerigamaand from the A{ (C) and C{ (D) horizons of the soils at Badulla

The TEM micrographs show clearly thepresence of a mixture of halloysite and kaolinitein the clay fraction of the soil; the halloysitecrystal is tubular while the kaolinite crystal ishexagonal in shape. The amount of halloysitedecreases towards the soil surface, while kaoliniteincreases, which is similar to the observation ofParamananthan (1977) in the granite gneiss soilof Malaysia. This is an indication that halloysitechanges to kaolinite in the course of soil weath-ering. X-ray diffraction pattern of the clay frac-tion from the Bts horizon is similar to that of theAp, showing a similar type of mineralogy in thesoil of both horizons.

The topsoil at the Badulla site has sharpXRD reflections at 0.72 and 0.356 nm, suggest-ing the presence of large amounts of kaolinite(Fig. 3). Biotite is absent in this horizon al-

though the XRD pattern of the clay fractionfrom the C] horizon indicates the presence ofsome biotite, shown by a weak reflection at 1.0nm (data not shown). The biotite in thetopsoil could have been transformed completelyto goethite, halloysite or kaolinite as shown inthe earlier studies (Eswaran and Heng 1976;Paramananthan 1977). The presence ofgoethite is shown by the reflections at 0.416,0.269 and 0.242 nm (Fig. 3). Some smectite ispresent in the topsoil as is shown by a weak1.43 nm reflection. However, the amount ofsmectite in the C} horizon is very high; theXRD reflection at 1.43 nm is very strong andintense (data not shown). Halloysite is presentin small amounts in the topsoil; the amountincreases in the subsoil (Plate 1C, D). Thisphenomenon is similar to the earlier observa-



Fig. 3: X-ray diffraction pattern of Mg-saturated clay fractionfrom the Ap horizon of the soil at Badulla

tion in the TEM micrographs of the soil at theMeerigama site.

Smectite (1.53 nm), biotite (1.01, 0.499,0.334 nm), quartz (0.425, 0.334 nm) andkaolinite (0.72, 0.355 nm) are the dominantminerals in the topsoil at the Kandy site (Fig. 4).Halloysite (0.445 nm) is present in minoramounts. The dominance of kaolinite is furtherconfirmed by TEM observation (Plate 2A). Theclay minerals in the subsoil are coated with Feoxides (Plate 2B) as the amount of Fe2Os in thesoil is moderately high (Table 2). They aremostly present in the form of haematite (0.270,0.251 nm) and goethite (0.270, 0.245 nm). Thedominance of smectite in the soil is reflected bythe high CEC; the CEC values throughout the

profile are > 37 cmolc/kg clay. The amount ofhalloysite in the subsoil is higher than in thetopsoil, as is shown by the appearance of 0.74nm reflection in the XRD diffractogram fromthe Bt4 horizon (data not shown). Some of thishalloysite, which is tubular in morphology, canbe seen in the TEM micrograph in Plate 2B.

The Aj horizon of the soil at Anuradhapurais dominated by biotite (1.02, 0.335 nm), quartz(0.426, 0.335 nm), halloysite (0.73, 0.446 nm),smectite (1.57 nm) and kaolinite (0.357 nm),with some feldspar (0.324 nm) and haematite(0.270, 0.251 nm) (Fig. 5). Plates 2C and Dshow the presence of some of these minerals inthe clay fraction of the soil. The morphology ofthe halloysite and kaolinite is not clearly mani-



Plate 2: TEM micrographs of the clay fraction from the Ap (A) and Bt4 (B) horizons of the soils atKandy and from the A} (C) and Bt4 (D) horizons of the soils at Anuradhapura

fested because the minerals are coated with Feoxides, similar in nature to that of the soil atKandy. The iron minerals are haematite, as isshown by the very clear XRD reflections at 0.270and 0.251 nm. For this reason, the soil atAnuradhapura is red (2.5YR 4/6), and henceclassified as Rhodustalf (Table 1). There is noevidence of the presence of goethite in the top-soil. The soil colour would have been yellowishbrown to yellow if goethite is the dominant formof Fe mineral in the soil (Schwertmann 1993). Inthe Bt4 horizon, however, some goethite waspresent together with haematite (data not shown).Other minerals in the clay fraction are similar tothose found in the A( horizon.

The 0.270 and 0.251 nm reflections aremore intense in the diffractogram of soil atAnuradhapura compared with that at Kandy,

indicating that the amount of haematite is higherin the soil at Anuradhapura. According toBigham et al (1978) haematite is transformed togoethite as soil moisture increases. Therefore,the soils in the intermediate and wet zones donot have haematite and the form of Fe mineralsis goethite. The presence of goethite in the Bt4horizon of the soil at Anuradhapura is a clearmanifestation of the availability of some mois-ture in that horizon.

Gibbsite is absent in the soils at Kandy andAnuradhapura, but present in the soil of thewet zone. This suggests that gibbsite, which isan alteration product of feldspar, can only beformed if the moisture and temperature aresufficiently high. Gibbsite is a common min-eral in the soil at an advanced stage of weath-ering.



Fig. 4: X-ray diffraction pattern of Mg-saturated clay fraction fromAp horizon of the soil at Kandy.

1 -en

42

the

... vCD

47.

The Chemical Properties of Soil Solution

Generally, the soil solution Al concentration islow. High values are only observed in the Bt4horizon of the soils at Kandy and Anuradhapura,having values of 177 and 76 jiM, respectively(Table 3). More Al is present in the soil solu-tion of the drier area. Al is expected to exist asA1(OH)3 which is the normal Al species stable athigh pH. Mineralogical investigation revealsthat the soils are dominated by kaolinite,halloysite and oxides of Fe, which are termedvariable-charge minerals. This means that thenegative charge on the soil surfaces increaseswith increasing pH (Uehara and Gillman 1981).Negative charge is, therefore, high at high pH,when Al is held tightly by the soil surfaces.

Consequently, the soil solution Al concentrationis low.

The soil solution pH (Table 3) is compara-tively higher than the soil pH (Table 2). LowerPHcaci2 v a l u e compared to pHsol is due to greaterhydrolysis of Al in solution of the former; CaCl2

solution can extract more Al than water. Thesoil solution pH is related to the pHCCaO2 by thefollowing equation:

pHso] = 4.56 + 0.52 pHc:aCl2 (r = 0.68, p < 0.05)

Although the basic exchangeable cations inthe soils are high, their concentrations in thesoil solution are low, except in soil solution Naconcentration. It is believed that the cations are


TABLE 3pH, EC and elemental composition of the soil solutions

1NIK

A

ioyhn

SCI. 1'

F00

n). 1 1995

Location

Meerigama

Badulla

Kandy

Anuradhapura

Horizon

Ap

Bt,

Bt,

Ap

Bt,

C . \

Ap

B t i '.

A, ;

Bt,

Bt4

..l. ."ft-*-*

PH

6.9

6.9

6.2

7.0

7.2

7.6

8.4

8.2

i 7.3

7.5

7.0

6.9

EC(mS/cm)

63

53

59

v 44

20

74

207

: 179

: 50

: 175

v ' 71

35

Ca

250

37

18

152

84

207

2478

964

170

1946

170

132

Mg

185

27

6

• 199

122

643

1393

385

158

860

78

76

K

249

27

18

234

27

63

324

55

55

976

39

37

Elemental

Na

470

669

1078

154

160 £

166 ?

444 ;

325

160

511

224

624

composition

AlmM

23

2

10

3

11

7. • • • • •

2

177

9

47

76

of the

Fe

21

1

2

tr

tr

tr

2

1

22

tr

6

10

soil solutions

Mn

8

4

tr

69

1

10

26

14

1

tr

3

tr

P

10

35

3

13

16

81

52

13

21

41

39

26

S

396

92

27

328

43

125

933

111

70

520

88

135

No,

3507

tr

tr

1856

766

1202

1856

549

608

1202

tr

tr

r%

i

1SHU

D

Z

?IZIA

H

IpiX

M.H

.

I33

INSP

AN

D K

AM

^MV

SI

O


.CD

s s6 •?

6

Fig. 4: X-ray diffraction pattern of Mg-saturated clay fraction fromthe Aj horizon of the soil at Anuradhapura

strongly held to the soil surfaces because of thehigh CEC, contributed by smectite and/or be-ing produced by the variable-charge minerals asa result of high pH. Nevertheless, the soilsolution Ca concentration is highly correlatedwith the exchangeable Ca, and the relationshipis represented by this equation:

Ca . = 311 + 147 Ca . (r = 0.74, P < 0.01)sol exch v '

The exchangeable Ca in the Ap and A}

horizons of the soils at Kandy and Anuradhapuraare 12.00 and 10.69 cmol/kg, respectively (Ta-ble 2). The corresponding soil solution Caconcentrations are 2478 and 1946 uM (Table 3).The soil solution Ca concentration is higher inthe dry than in the wet zone (Table 3). The Caconcentrations of soil solutions at the Meerigama,Kandy and Anuradhapura sites increase withdepth. The soil solution Ca concentration is alsohighly correlated with the ECEC (calculated onthe basis of 1 kg clay). The relationship is givenby the following equation:

Ca^, = 1.28 ECEC147 (r = 0.91, P < 0.01)

In addition, the soil solution Ca concentration isrelated to the ECEC/CEC ratio by the equation:

CawI = 342 ECEC/CEC1-74 (r = 0.59, P < 0.05)

In contrast, the soil solution Mg and Alconcentrations are not significantly correlatedwith any of the above-mentioned parameters.

CONCLUSIONThe mineralogy and chemical properties of thesoils in the three agro-ecological zones in SriLanka are different. Where the annual rainfallis high, soil pH and basic exchangeable cationsare low. In the drier areas where leaching isrestricted the pH and basic exchangeable cati-ons are high. Kaolinite, halloysite and smectiteare the major minerals in all the soils irrespec-tive of the climatic conditions. While the amountof haematite increases as the soil get drier, theamount of goethite decreases, and consequently



the colour of the soils becomes redder. In thewet zone, some gibbsite is present. The avail-ability of Ca in the soil is dependent on theexchangeable Ca, ECEC and the ECEC/CECratio. These soil solution attributes can be usedas indices of Ca availability for crop growth inthe soils developed over granite gneiss in SriLanka.

ACKNOWLEDGEMENTS

The authors would like to record their utmostappreciation to Universiti Pertanian Malaysia andWinrock International - F/FRED, Bangkok, forfinancial and technical support to conduct theresearch. The help of Mr. Lek Moncharoen ofDLD, Thailand and the F/FRED project co-ordinators at the various Multipurpose Tree Spe-cies trial sites in Sri Lanka is very much appreci-ated.

REFERENCES

ALLEN, B.L., and B.F. HAJEK. 1989. Mineral occur-rence in soil environments. In Minerals in SoilEnvironments, ed. J.B. Dixon and S.B, Weedp. 199-278. Book series No. 1, Wisconsin, WL:Soil Sci. Soc. Am.

ANON. 1979. Agro-ecological Regions of Sri Lanka.

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(Received 15 February 1994)


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The Mineralogy and Chemical Properties of Soils on Granite ...pertanika2.upm.edu.my/Pertanika PAPERS/JTAS Vol. 18... · of the area, the rock undergoes different degrees of chemical