Laser diffraction grain-size characteristics of Andisols in perhumid Costa Rica: the aggregate size of allophane

$Page 1: Laser diffraction grain-size characteristics of Andisols in perhumid Costa Rica: the aggregate size of allophane$
Ž .Geoderma 78 1997 71–91

Laser diffraction grain-size characteristics ofAndisols in perhumid Costa Rica: the aggregate size

of allophane

P. Buurman ), K. de Boer, Th. PapeDepartment of Soil Science and Geology, Wageningen Agricultural UniÕersity, P.O. Box 37, 6700 AA

Wageningen, The Netherlands

Received 12 November 1996; accepted 10 February 1997

Abstract

Ž .Grain sizes of eight profiles from a soil catena 640–3160 m on andesitic ash in perhumidŽ .atlantic Costa Rica were investigated by laser diffraction grain-sizing. The samples were 1

Ž . Ž .shaken with water, 2 treated with peroxide, and 3 treated with peroxide–oxalate. Allophane isabsent above 2000 m; it appears around 2000 m and increases downwards. Peroxide treatment andoxalate extraction indicate that the allophane fraction occurs in aggregates of well-defined sizes.The size of the allophane aggregates is between 2 and 20 mm, and it increases with increasingallophane content. Considerable crystalline clay fraction is only obtained upon oxalate extraction.Repeated oxalate extraction does not remove all allophane in high-allophane profiles. The methodfurther allows the recognition of a large number of ash deposits and of recent ash additions, thusfacilitating the recognition of stratified profiles.

Keywords: Andisols; allophane; grain-size; laser grain-sizing; aggregate size; Costa Rica; organic matter

1. Introduction

Grain-size analysis of volcanic soils has always posed problems of dispersion andŽ .definition of the ‘clay’ fraction Colmet Daage et al., 1970; Nanzyo et al., 1993 .

Although in some soils, pipette clay contents after sonication show a good correlationŽ .with oxalate-extracted allophane contents Nanzyo et al., 1993 , allophane contents inŽ .some cases were higher than clay contents Mizota and Van Reeuwijk, 1989 , which

) Corresponding author. Fax: q31 317 482419; E-mail: [email protected]

0016-7061r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.Ž .PII S0016-7061 97 00012-8

( )P. Buurman et al.rGeoderma 78 1997 71–9172

was attributed to incomplete dispersion in clay determination. Incomplete dispersion wasŽ .already mentioned by Maeda et al. 1977 as a reason for low clay contents. The fact

that maximum dispersion is obtained either at low, or at high pH, depending on AlrSicontent in the allophane, and that there is no standardized procedure for ultrasonicationŽ .Gee and Bauder, 1986 , further complicates the issue.

In grain-size determination, it is a standard practice to remove all cementing agentsŽ .organic matter, calcium carbonate, gypsum, ‘free’ iron before determining the grainsize. In volcanic soils, where the fine component that is to be determined also constitutesthe main cementing agent, it is extremely difficult to define the desired pretreatment andthe fractions that are thus obtained. This is a main reason why grain-size classes were

Ž .abandoned in the classification of Andisols Soil Survey Staff, 1994 .The strong resistance against dispersion that is encountered in many Andisols,

indicates that the soil particles are strongly aggregated. Except for a few studies on theŽ .change of aggregation upon drying Warkentin and Maeda, 1980 , however, this

aggregation has not been studied extensively.With particle-size determination through laser diffraction, it has become much easier

to carry out detailed grain-sizing of soil samples. Laser diffraction particle-sizing isbased on the size-dependent light-scattering angle of particles. A simultaneous measure-ment of light scattered at various angles is obtained by an array of detectors around amonochromatic laser beam. The number of measured grain-size fractions depends on thenumber of detectors. Details of the theoretical background are described by Loizeau et

Ž . Ž .al. 1994 and the mathematics are treated in detail by Swithenbank et al. 1977 and DeŽ .Boer et al. 1987 . Some of the problems and possibilities of laser grain-sizing in soil

Ž . Ž .studies have been outlined by Buurman et al. 1997 and Muggler et al. 1997 . Themeasurements by laser diffraction are highly reproducible. The continuous grain-sizedistribution curve obtained by laser diffraction enables one to study details in changes ofgrain-size distribution and aggregation upon various treatments. One of the maindisadvantages of laser diffraction, i.e., that, as in the pipette method, it assumes thepresence of spherical particles in all particle sizes, is not a disadvantage in allophanicsoils because we can assume that aggregates of allophanic material will be more or lessspherical. We have undertaken to study grain size and aggregation of a number ofAndisols from a catena in perhumid atlantic Costa Rica.

2. Materials and methods

Eight profiles on andesitic ashes on the northern slope of the Turrialba volcano,covering a range in altitudes between 640 and 3160 m above sea level, were describedand sampled. Turrialba ashes are quite variable. They contain 18–50% of phenocrystsand the complement of rock fragments and glass. Of the phenocrysts, plagioclaseŽ . Ž . Ž .40–60% anorthoclase is most abundant 14–30% , followed by augite 2–10% , with

Ž .minor amounts of hypersthene, olivine, and hornblende Alvarado Induni, 1989 . On thenorthern slope, where the profiles were sampled, chemical composition of the ashes

Ž .appears to be very homogeneous unpublished results . Earlier descriptions of volcanicŽ .soils in the same area were published by Van Dooremolen et al. 1990 , Jongmans et al.

Ž . Ž .1994 and Nieuwenhuyse and Van Breemen 1996 . The abbreviated profile descrip-

( )P. Buurman et al.rGeoderma 78 1997 71–91 73

Ž .tions, with horizon codes according to FAO 1977 , are given in Table 1. The upperŽ . Ž . Ž . Ž .profiles, T5 3160 m , T7 2650 m , T6 2020 m , and T8 1580 m are strongly layered

Žand exhibit relatively little weathering and low allophane contents unpublished micro-. Ž . Ž .morphological and chemical data , while the lower profiles T2 950 m , T1 800 m , and

Ž . Ž .T3 640 m are both less stratified and more weathered. Profile T4 1250 m isintermediary between the two groups. All profiles are classified as Andisols and includetwo Udivitrands, two Fulvudands, one Placudand, and three Hydrudands.

The differences in weathering are mainly due to three factors.Ž .1 Rainfall. Rainfall increases from 4500 mm at sea level to 7000 mm at 1000 m

Ž .altitude Nieuwenhuyse, 1996 and decreases further upwards. Leaching is thereforestrongest in the lower part of the sequence.

Ž .2 Age. Additions of fresh ash to the profile are more frequent in the upper part ofthe sequence, resulting in more strongly stratified profiles at greater altitudes.

Ž .3 Temperature. With a temperature gradient of 0.68C per 100 m, and a mean annualtemperature of 268C at sea level, the mean temperature is 208C at 1000 m, and 88C at3000 m.

All three factors favour stronger weathering at lower altitudes.Chemical and physical properties were measured according to standard procedures

Ž .Van Reeuwijk, 1995; Buurman et al., 1996 . Allophane was calculated according toŽ . Ž .Parfitt and Kimble 1989 and Mizota and Van Reeuwijk 1989 from oxalate-extracta-

ble Al and Si, and pyrophosphate-extractable Al. Carbon was measured by elementanalyzer and is the mean value of two or three subsamples.

The grain-size analysis of all samples was carried out with a Coulter LS230 lasergrain-sizer. The results are given in 116 grain-size classes between 0.04 and 2000 mm.Each class boundary is 1.098 times the size of the preceding one. Laser grain-sizing isaccurate in both size and quantity at grain sizes larger than 5 mm. Below 5 mm results

Ž .depend strongly on the applied optical model Hoff and Bott, 1990 . For our samples wehave used an optical model containing the following parameters: refractive index ofwater, 1.33; refractive index of minerals, 1.56; light absorption at 450, 600, 750, and900 nm was 0.2, 0.2, 0.15, and 0.2, respectively. The light absorption deviates slightly

Žfrom that measured in suspension mean values were 0.20, 0.14, 0.12, 0.07, respec-.tively , but this does hardly affect measurements at 10% obscuration on which the

present article is based.Ž .Grain-size distributions of all samples were measured three times: 1 after overnight

Ž . Ž .shaking with distilled water ‘untreated’ ; 2 after removal of organic matter by H O ;2 2Ž .3 after removal of organic matter and subsequent extraction of amorphous material by

Ž .acid oxalate 1 : 100 according to Van Lagen 1996 . The combination of the threeanalyses gives an indication of aggregation by organic matter and allophane, and of thedominant grain size of the amorphous material and the remaining primary particles.

Measurements were carried out on 2–5 g of field-moist sample, equivalent to about 1g of air-dry sample, obtained after careful mechanical homogenization of the fine-earthbulk sample. Earlier trials have indicated that measurements on such samples are fully

Ž .reproducible unpublished data . The subsample was suspended in approximately 1 lwater. All measurements were carried out at 10% obscuration, which is the correctsuspension density for this type of measurement.


Tab

le1

Gen

eral

prof

ilede

scri

ptio

ns

Ž.

Alti

tude

asl

,Pr

ofile

Hor

izon

Dep

thC

olou

rFi

eld

Stru

ctur

eT

hix

Mot

tles

Ž.

Coo

rdin

ates

,cm

text

ure

Cla

ssif

icat

ion

3160

mT

5A

h10

–4

YR

2r1

losa

mas

sive

y–

XY

N10

801

12A

B4

–11

10Y

R3r

4lo

sasi

.gr

y–

XY

W83

845

42B

C11

–30

10Y

R5r

4lo

sasi

.gr

yc

5YR

4r6

Udi

vitr

and

R30

q10

m10

YR

6r8

2650

mT

7A

h10

–4r

1010

YR

3r1

salo

mas

sive

y–

XY

N10

801

26C

1y

10r

207.

5YR

2r0

salo

mas

sive

q–

XY

W83

847

062C

y14

r24

2.5Y

R4r

2sa

lom

assi

veq

c7.

5YR

5r8

Ž.

Plac

ic3A

hy

447.

5YR

3r2

lom

.f.s

abq

–U

divi

tran

dir

onpa

n4C

44–

87q

7.5Y

R2r

0sa

qlo

mas

sive

y–

2020

mT

6A

h10

–11

7.5Y

R3r

2lo

mas

sive

y–

XY

N10

803

022A

h1y

20r

255Y

R2.

5r1

lom

assi

vey

–X

YW

8384

657

3Ah1

y33

5YR

2.5r

1lo

mas

sive

y–

Fulv

udan

d4C

y49

7.5Y

R2r

0lo

say

–5B

wy

6910

YR

3r3

lo-c

llom

assi

veq

f10

YR

5r1

6Ah

y80

5YR

2.5r

1lo

mas

sive

q–

6Bw

y10

0m

ixed

lom

assi

veq

–7C

100q

7.5Y

R2r

0sa

–

1580

mT

8A

h10

–5

10Y

R2r

3lo

s.f.

sab

y–

XY

N10

804

092A

hy

12r

1510

YR

2r1

low

.m.s

abq

–X

YW

8384

701

iron

pan

5m

m5Y

R5r

8Pl

acud

and

2Bw

y29

10Y

R3r

4lo

w.s

abq

–2C

By

3010

YR

7r6

3Bw

1y

4910

YR

3r4

low

.sab

q–

3Bw

2y

67r

7010

YR

3r3

lom

assi

veq

–3C

By

69r

795Y

R4r

3sa

lom

assi

veq

q–

4Bw

y10

5q10

YR

3r4

silo

mas

sive

q–


1250

mT

4O

y2–

0X

YN

1080

506

Ah

0–

1710

YR

2r1

silo

m.f

.sab

yX

YW

8384

755

Bw

1y

3510

YR

3r3

silo

w.v

f.sa

by

Fulv

udan

dC

1y

35r

392.

5YR

3r0

losa

si.g

ry

2Bw

1gy

8010

YR

3r3

silo

mas

sive

q2B

w2

y10

810

YR

3r2

cllo

mas

sive

qf

5YR

5r8

3Bw

y13

0q10

YR

4r3

sicl

lom

assi

veq

f5Y

R5r

8

950

mT

2O

y1–

0X

YN

1080

653

Ah1

0–

85Y

R2.

5r1

sicl

los.

f.gr

qX

YW

8384

502

Ah2

y20

5YR

2.5r

2si

cllo

m.f

.sab

qH

ydru

dand

AB

y40

7.5Y

R3r

4si

cllo

mas

sive

q2B

w1

y72

7.5Y

R3r

3si

cllo

mas

sive

q2B

w2

y92

10Y

R3r

4si

cllo

mas

sive

qf

10Y

R4r

23C

g92

q10

YR

5r2

low

.c.p

rq

f5Y

R5r

8

800

mT

1O

y2–

0X

YN

1080

725

Ah1

0–

105Y

R2.

5r2

sicl

los.

vf.s

abq

XY

W83

844

49A

h2y

2710

YR

3r4

sicl

lom

assi

veq

Hyd

ruda

ndA

By

607.

5YR

3r2

sicl

low

.vf.

sab

qB

w1

y93

10Y

R4r

6si

cllo

mas

sive

qB

w2

y12

0r15

010

YR

5r8

sicl

lom

assi

veq

Bq

2R15

0qlo

q

640

mT

3A

h0

–18

10Y

R3r

2si

los.

f.gr

qX

YN

1080

834

AB

y45

10Y

R4r

4si

low

.sab

qX

YW

8384

419

Bw

y60

r80

10Y

R6r

8si

lom

assi

veq

Hyd

ruda

nd2R

80q

q

Abb

revi

atio

ns.

Tex

ture

:cl

los

clay

loam

;lo

slo

am;

losa

slo

amy

sand

;sa

ssa

nd;

salo

ssa

ndy

loam

;si

los

silt

loam

;si

cllo

ssi

ltycl

aylo

am.

Ž.

Ž.

Stru

ctur

e:si

.grs

sing

legr

ain;

m.

vf.

sab

sm

oder

ate

very

fine

suba

ngul

arbl

ocky

;w

.sab

sw

eak

suba

ngul

arbl

ocky

;w

.m.s

abs

wea

km

ediu

msu

bang

ular

bloc

ky;

w.v

f.sa

bs

wea

kve

ryfi

nesu

bang

ular

bloc

ky;

w.c

.prs

wea

kco

arse

pris

mat

ic;

s.f.

grs

stro

ngfi

negr

anul

ar.

Thi

xs

thix

ottr

opic

.


Tab

le2

Sele

cted

chem

ical

char

acte

rist

ics

Prof

ileH

oriz

onD

epth

Sam

ple

No.

CPy

roph

osph

ate

Oxa

late

Allo

phan

epH

pHH

OK

Cl

2

Ž.

Ž.

Ž.

cm%

Al

FeC

Al

FeSi

%Ž

.Ž

.Ž

.Ž

.Ž

.Ž

.%

%%

%%

%

T5

Ah

0–

424

19.1

0.2

0.3

nd0.

30.

50.

00.

0nd

ndA

By

1125

nd0.

30.

5nd

0.3

1.1

0.0

0.3

ndnd

BC

y30

26nd

0.2

0.3

nd0.

31.

10.

10.

5nd

ndR

30q

T7

Ah

0–

1034

4.1

0.3

0.4

nd0.

40.

80.

10.

6nd

ndC

1y

2035

1.9

0.3

0.3

nd0.

40.

60.

10.

7nd

nd2C

y24

36nd

0.1

0.1

nd0.

40.

70.

21.

1nd

nd3A

hy

4437

4.6

0.5

0.5

nd0.

61.

00.

21.

0nd

nd4C

y87

q38

1.1

0.0

0.0

nd0.

30.

40.

21.

4nd

nd

T6

Ah1

0–

1127

13.1

0.6

0.5

ndnd

0.7

0.2

0.7

ndnd

2Ah1

y25

286.

70.

70.

4nd

1.1

1.0

1.3

1.9

ndnd

3Ah1

y33

295.

00.

50.

5nd

1.4

1.2

0.5

3.6

ndnd

4Cy

49nd

nd0.

20.

1nd

1.2

0.9

0.6

ndnd

5Bw

y69

30nd

0.4

1.1

nd0.

91.

70.

31.

8nd

nd6A

hy

8031

nd0.

50.

7nd

2.0

0.6

0.8

5.7

ndnd

6Bw

y10

032

nd0.

30.

1nd

2.2

1.2

1.3

8.1

ndnd

7C10

0q33

1.2

0.1

0.0

nd0.

70.

80.

42.

6nd

nd

T8

Ah1

0–

510

213

.80.

70.

55.

60.

60.

60.

00.

23.

94.

22A

h1y

1510

38.

91.

40.

83.

92.

01.

60.

32.

45.

34.

72B

w1

y29

104

4.6

0.9

1.2

2.8

2.2

2.4

0.4

5.7

5.5

5.1

2CB

y30

105

2.9

0.4

0.3

2.8

1.8

1.9

0.6

5.5

6.9

5.5

3Bw

1y

4910

64.

40.

40.

21.

94.

62.

41.

54.

35.

85.

83B

w2

y70

107

2.4

0.3

0.0

1.5

4.0

1.8

1.7

15.9

5.9

6.0

3CB

y73

108

1.5

0.2

0.0

0.4

2.6

1.1

1.4

14.0

5.9

6.0

4Bw

y10

5q10

93.

60.

30.

11.

24.

32.

11.

99.

66.

06.

1


T4

Ah

0–

1716

10.2

1.7

0.8

nd2.

21.

20.

32.

2nd

ndB

w1

y35

175.

50.

70.

7nd

2.4

1.5

0.7

6.5

ndnd

C1

y39

18nd

0.3

0.1

nd2.

71.

41.

08.

7nd

nd2B

w1g

y80

193.

40.

30.

0nd

4.3

1.3

1.9

14.7

ndnd

2Bw

2y

108

203.

80.

50.

1nd

6.3

2.5

2.8

21.7

ndnd

3Bw

1y

130

214.

00.

50.

1nd

9.8

4.0

4.6

34.7

ndnd

3Bw

213

0q22

nd0.

40.

1nd

10.9

3.9

5.3

39.2

ndnd

T2

Ah1

0–

87

18.0

1.8

0.6

8.9

2.2

3.2

0.2

1.6

4.2

4.1

Ah2

y20

814

.82.

70.

78.

73.

53.

80.

74.

13.

94.

4A

By

409

10.1

2.0

0.4

5.4

6.4

3.9

1.6

17.3

4.6

5.0

2Bw

1y

7210

10.6

1.0

0.5

3.6

10.9

4.0

3.9

37.5

4.9

5.3

2Bw

2y

9211

7.9

0.6

0.0

6.0

9.8

2.4

3.8

34.5

5.1

5.0

3Cg1

92q

126.

00.

50.

02.

112

.61.

25.

545

.45.

05.

3

T1

Ah1

0–

101

21.2

2.3

0.5

9.8

2.8

2.6

0.4

2.3

3.9

4.2

Ah2

y27

212

.32.

60.

56.

55.

33.

41.

310

.46.

34.

4A

By

603

8.1

1.3

0.9

3.5

8.1

4.0

2.5

26.7

6.2

4.7

Bw

1y

934

7.1

0.6

0.1

5.0

8.1

5.3

2.8

28.7

6.5

4.9

Bw

2y

120

54.

30.

40.

23.

16.

24.

72.

122

.75.

35.

3B

q2R

y15

0q6

n.d.

T3

Ah1

0–

1813

7.8

1.3

0.9

3.3

7.8

3.1

2.5

24.9

4.7

4.9

AB

y45

145.

80.

60.

14.

48.

62.

73.

330

.25.

15.

1B

wy

6015

3.7

0.4

0.0

1.4

7.9

1.5

3.2

28.2

4.7

6.2

2Ry

80q

nds

not

dete

rmin

ed.


All samples were subjected to 10 min of sonication outside the Coulter LS230, andinternal sonication was functioning continuously during measurements. This implies thatall macro-aggregates were destroyed before measurement. 10 ml of sodium pyrophos-phate was added as a dispersant to all samples. During the measurement, the suspension

Ž .is transported at high speed 12 lrmin through the sampling cuvette, which means thatthere is very little chance of flocculation.

3. Results

Abbreviated profile descriptions are given in Table 1. Basic chemical data are givenin Table 2. The profiles will be discussed according to altitude, starting with the highestone.

( )3.1. T5 3160 m; not illustrated

Because removal of organic matter resulted in too small amounts of sample, thesamples of this profile were only run without pretreatment. The grain-size distributioncurves show a virtual absence of material finer than 1 mm. All curves have a multimodaldistribution, with peaks around 20, 50, 130, 300, and 700–800 mm, indicating a mixtureof a number of ash deposits.

( )3.2. T7 2650 m; Fig. 1

The untreated samples show a fairly well-sorted ash deposit with two maximabetween 100 and 400 mm. The fraction smaller than 1 mm is virtually lacking, while

Fig. 1. Grain-size analyses of the Ah and 3Ah of profile T7; three treatments.


there is a broad peak between 1 and 20 mm. Peroxide treatment removes this broadpeak, which indicates that it is caused by the presence of organic matter. Extraction ofamorphous material does not change the grain-size distribution, which indicates absenceof allophane and of aggregation due to allophane. Fig. 1 illustrates the change upon

Ž . Ž .removal of organic matter and allophane in the samples 34 Ah and 37 3Ah . Thecurves of all other samples show similar characteristics and maxima. There is hardly any

Ž .effect of oxalate extraction calculated allophane contents are lower than 1.5% .

( )3.3. T6 2020 m; Fig. 2

None of the samples contains an appreciable fraction finer than 1 mm. The untreatedsamples, except for the 5Bw and 7C horizons, exhibit a broad maximum in the fine siltfraction, between 2 and 20 mm, and a number of maxima in the coarser fractions: 50,

Ž .120, 300, 800 mm. The lower horizon 7C has a prominent maximum around 200 mm.Upon removal of organic matter, most of the broad maximum in the fine silt fraction

disappears. In the 6Ah and 6Bw horizons, it is replaced by a distinct maximum around 3mm. Slight changes in the coarser fractions are probably due to disaggregation. In the7C horizon, several minor peaks become more visible.

Oxalate extraction results in only very small changes in the Ah and 2Ah1 horizons. Inthe 3Ah and 5Bw horizons, the small maximum around 2 mm disappears. In the 6Bwhorizon, the maximum around 3 mm decreases upon oxalate extraction and in the 7Chorizon, the slight maximum in the fine silt fraction disappears.

( )3.4. T8 1580 m; Fig. 3

The untreated samples of profile T8 show only minor amounts of fractions smallerthan 1 mm. The materials are predominantly of silt size, with a broad peak between 2and 30 mm. In addition, there are four maxima at coarser sizes: around 50, 120, 350, and700 mm. From the 3Bw2 downwards, the fine-silt peak becomes better defined.

Upon removal of organic matter, the fine-silt peak becomes better defined, whichindicates that the broad shape is due to aggregation by organic matter. The fine-silt peakis less pronounced in the Ah1 and 2Ah1 horizons. The Ah1 appears to lose mainlycoarse fractions, by disaggregation, while in the 2Ah1 the loss is in the 1–30 mmfraction, as in the preceding profiles. In the lower horizons, where organic matter is lessabundant, there is little change in the grain-size distribution upon treatment withperoxide.

Oxalate extraction results in a considerable change in grain-size distribution. Thefine-silt peak virtually disappears in all samples. The change indicates that the allophanein this profile has a well-defined aggregate size of between 2 and 10 mm. The removalof amorphous matter, compared to peroxide treatment, gives slight changes in thecoarsest fraction, which may be due to disaggregation.

( )3.5. T4 1250 m; Fig. 4

In all samples, the fraction smaller than 1 mm is virtually absent. At the top of theprofile, the untreated samples show a broad maximum between 10 and 20 mm. In the


Ž . Ž . Ž .Fig. 2. Grain-size analyses of profile T6: A water-shaken samples; B peroxide-treated samples; Cperoxide- and oxalate-treated samples.

Ž .lower horizons 2Bw1, 2Bw2 , a more well-defined peak with a maximum between 6and 10 mm replaces the broad peak. In the fine fractions, the C1 resembles theunderlying rather than the overlying horizons. In the coarser fractions, maxima are foundaround 60, 120, 300, and 800 mm. The 3Bw1 and 3Bw2 have a very strong maximum


Ž .Fig. 2 continued .

between 5–15, and 10–20 mm, respectively. These horizons lack fractions coarser than200 mm.

Upon removal of organic matter, the broad peaks in the range of 10–20 mm makeplace for better-defined peaks with maxima between 6 and 20 mm, except in the topsoil,which does not have a peak in the fine silt fraction. The peaks in the coarse fractionsremain unchanged. Obviously, the broad maxima between 10 and 20 mm, which arefound in the untreated samples, are due to aggregation by organic matter. The fine-siltmaximum, which is found after removal of organic matter, shifts from 6 mm in the Bw1horizon to 20 mm in the 3Bw2, while the intermediate horizons have their maximumaround 10 mm.

Removal of amorphous material results in virtual disappearance of the maxima in thefine silt fraction and leaves the grain sizes of the coarser fractions unchanged. Thissuggests that the maxima in the fine silt fraction are due to amorphous matter. In the

Ž .3Bw1 and 3Bw2, where the allophane content is very high Table 2 , removal of thisfraction was not complete. When the allophane is completely removed, there is still aclay fraction which consists of well-crystalline low-charge vermiculite and kaoliniteŽ .checked for sample 20 .

( ) ( )3.6. T2 950 m and T1 800 m

Because these profiles exhibit a similar behaviour, which is most strongly illustratedŽ .by T1, we will only illustrate the curves from T1 Fig. 5 .

Both profiles show a considerable fraction smaller than 1 mm, which is smallest in



the topsoil and increases with depth. All samples exhibit a well-defined peak in the finesilt fraction, with a maximum around 6–10 mm. In the Ah1 and Ah2 samples, this peakis asymmetrical, with a secondary maximum around 20 mm. In the coarser fractions,peaks are found around 50, 120, and 300 mm. The coarsest peaks are only found in theAh1 and Ah2 of T1 and in the Ah1-AB of T2.

Upon removal of organic matter, there is little change in the fractions -1 mm. The



maxima in the fine silt fraction become more discrete and shift towards slightly largergrain size. The maximum in the fine silt fraction is around 6 mm in the Ah2, and shiftsto larger sizes in the next three horizons; it is around 20 mm in the 2Bw1 horizon, belowwhich it decreases again. There are no changes in the larger grain sizes, except for aclearer separation of the maximum at 50 mm, probably because of removal of cementingmaterial. In the humic horizons, there is considerable loss in the range between 5 and 30

Ž . Ž .mm T1 or between 2 and 20 mm T2 .Oxalate extraction strongly influences the grain-size distribution. There is a distinct

increase in the fractions -1 mm, especially in the AB, Bw1 and Bq2R horizons,which have a distinct maximum around 0.4 mm. The top horizon appears to contain acoarse fraction, which may have become visible because its concentration was increaseddue to removal of allophane. The maxima above 40 mm in the horizons Ah1 and Ah2are still present. From the AB horizon downwards, there is no coarse fraction left. Thissuggests that the coarse fractions encountered in the untreated and peroxide-treatedfractions consisted of weathered mineral grains, which fall apart when allophane isextracted. Micromorphology indeed shows the presence of weathered grains of primaryminerals.

In the lower horizons, except the Bw2, also most of the fine-silt maximum hasdisappeared, and a fine clay fraction appears. Because of the very high allophanecontent, a single oxalate extraction did not remove all the amorphous material. Upon

Ž .repeated oxalate extraction not shown , however, the fraction smaller than 1 mmincreases, while the peak between 1 and 10 mm decreases further. X-ray diffractionshowed that the fraction -1 mm consists of low-charge vermiculite and kaoliniteŽ .checked for samples 9, 10, 11, and 12 .



( )3.7. T3 640 m; Fig. 6

The untreated samples show a slight maximum around 0.4 mm and a dominance ofthe fine silt fraction with a maximum around 4 mm for the Ah horizon and around 10



mm for the other two horizons. There is no material coarser than 100 mm, and thecoarse fraction has a maximum around 40 mm.

Removal of organic matter results in disappearance of the maximum around 0.4 mm,although some fine material is still present. The maxima in the fine silt fraction becomebetter defined and occur at slightly larger grain sizes. The maximum is now symmetricaland around 11 mm for the Ah horizon, and shifts to slightly larger sizes in the deeperhorizons. The shoulder around 40 mm coincides with the maximum encountered in theuntreated samples. The shifts suggest that oxidation of organic matter removes finematerial in the 1–10 mm range.

Oxalate extraction leaves a slight maximum around 40 mm in the Bw horizon, butappears to disaggregate the fine silt fraction, which results in an increase in the fraction-1 mm and the appearance of a peak around 2 mm. The maxima around 10–12 mmstrongly decrease. The maximum around 40 mm only remains in the Bw horizon,indicating that this fraction consists of weathered mineral grains. The presence ofweathered mineral grains of this size is corroborated by micromorphology.

Also in this profile, the single oxalate extraction did not remove all the amorphousŽ .matter, and upon further extraction more of the fine-silt peaks disappeared not shown .

4. Discussion and conclusions

The grain-size analyses show the aggregating influence of organic matter andallophane. The organic matter is mainly found in the silt-size fractions, between 2 and



Ž .20 40 mm. Removal of organic matter usually brings out a distinct peak in the fine-siltŽ . Žrange. Because this peak disappears low allophane content or decreases high allo-

. Ž .phane content upon oxalate extraction, it is attributed to amorphous material allophane .



The allophane peak is virtually absent in profiles above 2000 m altitude. In profile T6Ž .2020 m , it appears in small amounts in the deeper horizons. From 2000 m downwards,

Ž .allophane content increases and completely dominates the lowest profile T3, 640 m ,where primary material has disappeared from the upper horizons.

The aggregate size of the allophane appears to increase with depth in most profiles.Because in most profiles allophane content also increases with depth, we have plotted

Ž .allophane content against allophane aggregate size for all samples Fig. 7 . It appearsthat the mean aggregate size increases with allophane content in a consistent way for allprofiles studied: larger amounts of allophane result in larger aggregates. The generaltrend is also valid for individual profiles, but may be disrupted by lithological disconti-nuities.

All profiles are formed in more than one ash deposit. This is clearly indicated by thenumber of non-allophane maxima in the silt and sand range. It is common to find 3–5distinct maxima. In the strongly weathered soils of the lower end of the catena, this

Ž .evidence disappears profile T1, subsoil; profile T3 . The presence of well defined sandfractions in the topsoils of T1 and T2 suggests an addition of fresh ash to a preweatheredprofile. Stratification of the profiles is most easily detected in the peroxide-treatedsamples, because removal of organic matter brings out the primary grain sizes withoutdestroying strongly weathered grains.

Fractions smaller than 1 mm are scarce in all untreated and peroxide-treated samples.Appreciable amounts of these fractions appear in high-allophane samples, upon oxalateextraction. A second oxalate extraction removes more of the fine silt fraction, and maycause a stronger peak in the -1 mm fraction. Where investigated, this latter fractionconsisted of well-crystalline kaolinite and vermiculite clays. Under natural circum-


Ž . Ž . Ž .Fig. 6. Grain-size analyses of profile T3: A water-shaken samples; B peroxide-treated samples; Cperoxide- and oxalate-treated samples. N.B. The horizontal scale of these graphs has been expanded.

stances, and even after removal of organic matter and sonication, this clay fraction isvirtually non-existent.

Obviously, allophane in these soils is strongly aggregated and even removal oforganic matter and sonication does not result in an appreciable ‘clay’ fraction. Clay



fraction is only obtained by removal of part of the allophane itself. This illustrates oncemore that it is useless to determine a clay percentage for such Andisols, or to calculatethe CEC of the ‘clay’ fraction. Allophane appears to form aggregates of discrete sizes,

Fig. 7. Relation between allophane content and allophane aggregate size after peroxide treatment.


which depend on the allophane content, and possibly on other environmental factorssuch as burial.

Acknowledgements

The authors thank Dr. A.G. Jongmans for his assistance in the description andsampling of profiles, Mr. J.D.J. van Doesburg for the X-ray diffraction analysis, and MsM.J. Plantinga and Mr. F.J. Lettink for the analyses presented in Table 1.

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Laser diffraction grain-size characteristics of Andisols in perhumid Costa Rica: the aggregate size of allophane