South Pacific Study Vol. 17, No. 1, 1996

1

Changes in Physical and Chemical Properties of Surface
Soil in a Swidden and Subsequent Fallow in a
Northwestern Region of Malaita Island,
Solomon Islands＊

Kazutaka NAKANO ＊＊ and Nobufumi MIYAUCHI ＊＊＊

Abstract

Changes in the physical and chemical properties of the surface layers of soil of a slash-and-burn swidden(garden) in a tropical and ever-humid climate region in northwest Malaita Island, Solomon Islands, were investigated. Soil samples were obtained from the same site nearly spanning the entire swidden cycle and from an almost
adjacent long fallow forest. The fallow period in the swidden cycle was slightly more than ten years. The soil there
was found to be acidic. The changes in physical properties suggest that slight soil compaction of the surface layers
occurred during the fallow, and that the soil became softer, and more permeable with increasing fallow length. Thechanges in the content of organic carbon seem to parallel this change. Both organic carbon and total nitrogen in
the topsoil had the lowest value in the middle of the fallow period. The surface layers accumulated much organic
matter during the long fallow period. The levels of available phosphorus, exchangeable calcium, magnesium,
potassium, and sodium, base saturation percentage and pH of the topsoil were lowest during the fallow period. On
the other hand, effective cation exchange capacity was high in the middle of the fallow period owing to a highvalue of exchange acidity. Exchangeable calcium was noticeably abundant in both the topsoil and the 15 20cm soil
layer in the long fallow forest. After burning the plant debris, no rise of pH value measured in de-ionized water
was recognized.
From the preceding results, it can be concluded that some properties of the topsoil in fallows far less than ten
years after garden abandonment are less favorable for swiddening even when the beneficial effects of heating andadding ash are taken into account. Furthermore, the present investigation suggests that the ecological characteristics of swiddening in an ever-humid region are substantially different from those in a region with humid anddistinctly dry seasons.
Key words: Soil properties, Swidden cycle, Malaita Island (Solomon Islands).

＊

This study was partially funded by Fukutake Science and Culture Foundation in Japan. The receipt of thisgrant-in-aid 1988 is gratefully acknowledged.
＊＊
鹿児島大学南太平洋海域研究センター
中野和敬
Kagoshima University Research Center for the South Pacific, 1-21-24, Korimoto, Kagoshima, JAPAN 890.
＊＊＊
鹿児島大学農学部生物環境学科生産制御化学講座土壌科学研究室
宮内信文
Laboratory of Soil Science, Chair of Agronomical and Food Chemistry, Department of Environmental
Sciences and Technology, Faculty of Agriculture, Kagoshima University, 1-21-24, Korimoto, Kagoshima,JAPAN 890.

2South Pacific Study Vol. 17, No. 1, 1996

Introduction
Determining the changes in soil properties of an actual swidden and its subsequent fallow
during a swidden cycle has some aspects of great significance for relevant fields of scienceand technology. The two main aspects are as follows. Determining these changes is a basic
step for understanding mineral cycling in the whole ecosystem of a swidden and the subsequent fallow; understanding these changes can indicate the factors responsible for thechanges in the fertility and other growing conditions for the crops during the respective
stages of the cycle (namely, clearing a forest, burning the debris of felled or slashed plants,
growing crops, abandoning the swidden, and fallowing till the next cycle). For the latter
purpose, quantitatively detailed data of the properties of surface soils should be emphasized,
since most roots of the majority of the herbaceous crops in the region where this investigation was pursued do not penetrate very deeply into the soil, and the growth of those crops
is considered to be related mainly to the fertility and the physical conditions of the shallow
soil layers directly in contact with their roots.
So far, a considerable, though not satisfactory, amount of quantitative data concerning theproperties of the soils in swiddens and the subsequent fallows all over the world has been
accumulated. However, such data covering one entire cycle are not numerous. For the Asianand the Pacific regions, those have been obtained only from East Malaysia (ANDRIESSE,
1977), Northern Thailand (SABHASRI, 1978; ZINKE et al., 1978; NAKANO, 1978), andNorth-Eastern India (RAMAKRISHNAN, 1992). Many investigations which provide data onchanges in soil properties in a swidden are based on the analyses of soil samples fromvarious stages of the swidden cycle (from forest selection to abandonment or a few yearsof the fallows).
Most of these studies have a serious defect: that is, the comparisons of the results ofanalyses are not made from identical sampling sites at various stages. According to
ANDRIESSE (1977, p. 10), "the natural variability in soil properties within one field, or
between fields of one soil series is a serious limitation in studying the effect of one or more
external factors upon such properties". In order to surmount this problem,
NAKANO (1978) adopted a measure of combining data on quantitative changes of thechemical components of soil obtained over a year or so in some sites. Despite improvement,
this measure does not provide a highly satisfactory model of change in soil properties
during a swidden cycle.
STR MGAARD (1992) studied, over nine years since the stage shortly before burning
piled slash, changes in soil properties of a swidden and the subsequent fallow stages in
Miombo woodlands in Zambia. The samples for analyses from any Plot for his investigation
were obtained from an identical site. Even if the data of his Plots A and B at the stage
before burning the plant debris are considered to indicate the soil properties at the final
stage of the preceding swidden cycle with a long fallow period, however, his investigation
did not cover the entire swidden cycle, because his last sampling at either Plot A or B was
conducted at the several years fallow stage which is the early phase in the whole fallow

NAKANO and MIYAUCHI : Changes in Properties of Surface Soil during a Swidden Cycle

3

period there.
This paper presents quantitative data, by means of physical and chemical analyses of thesamples from the same site, on the change in the properties of surface soil in an actual
swidden or garden and the subsequent fallow in a northwestern region of Malaita Island,
Solomon Islands. Samples represent a nearly complete swidden cycle. Quantitative data on
vegetational change of the site during the fallow have been already published (NAKANO,
1992). Thus, this paper should be regarded as one of a pair of papers which describe anddiscuss the dynamic relationship between vegetation and soil of a complete swidden cycle(garden and fallow) in an ever-humid island of Melanesia.
We are deeply indebted to the staff of Soil Section (represented formerly by Mr. Rodney
J. CHEATLE and successively, till 1994, Mr. Morgan WAIRIU) at Dodo Creek Research
Station, the Ministry of Agriculture and Fisheries of Solomon Islands, for providing the firstauthor of this paper with the opportunity for pursuing the field surveys to complete thepresent paper. We owe much to the inhabitants of the Aitea Hamlet for their willingness andcooperative hospitality.

Survey Procedure
The location of the survey site for the present investigation near Aitea Hamlet has beenshown in the previous paper (NAKANO, 1992). According to the FAO-UNESCO system ofsoil taxonomy, the soils in the region where this site is situated are Haplic Phaeozems(Anonymous, 1978), although some data shown below do not appear to satisfy fully therevised condition necessary for this soil group (Anonymous, 1988). Furthermore, this siteis also in one of the regions characterized by Tamba’a Land System (WALL and HANSELL,
1974).
As mentioned in the previous paper (NAKANO, 1992), this site has a typical ever-humid
tropical climate; and usual monthly precipitation there is more than 100mm. Table 1 showsthat this region had only nine months with less than 100mm of precipitation during the 74
months from January 1989 until February 1995. Table 1 also shows that none of those ninemonths appear twice or more. This means that this region has no definite, distinctly recognized dry season. Moreover, it should be also noted that all the region’s inhabitants agree
that Aitea Hamlet has, on account of its higher altitude, much more precipitation thancoastal Auki where the meteorological data shown in Table 1 were obtained.
The soil samples were collected from a site previously designated as "Old Garden 1" and"Sanctuary Forest" (NAKANO, 1992). A soil profile was sampled by horizons and is presented in Table 2. In addition, surface soils at the two depth layers (0 5cm and 15 20cm)
were sampled five times in the "Old Garden 1", namely, at these stages: 1) after clearing
the vegetation of slightly more than ten years fallow; 2) one week after burning the debrisof felled and slashed plants; 3) eight months after the abandonment of the sweet potato (4South Pacific Study Vol. 17, No. 1, 1996

Table 1. Months (from January 1989 to February 1995), when the actual precipitaion wasless than 100mm at the meteorological station in Auki, Malaita Island, Solomon
Islands.
Year

Month

Precipitation (mm)1990

February

98.0

1991

April

60.4

1992
1992

May
June

60.0
29.0

1993
1993

January
October

96.6
73.3

1994
1994
1994

September
Novemer
December

27.4
89.6
53.6

Note 1 : The year 1989 had no months when the precipitation was less than 100mm.
Note 2 : Auki is located 10km to the south-south-west of the survey site.
Data source : Solomon Islands Meteorological Service.

Ipomoea batatas (L.) LAM.) garden; 4) two years and a half fallow; and, 5) five years
fallow. It was impossible to separate the ash components from the top soil sampled one
week after burning. Each sampling occasion is shown in Tables 3 and 4. With respect toTable 1, the sampling site had no very dry period during at least one month before eachsampling occasion. When the last sampling was conducted in March 1995, the owner of the"Old Garden 1" told the first author that the fallow vegetation would be cleared again for
gardening in two years. This means that, in the swidden cycle since 1989, the fallow periodof the "Old Garden 1" was to be seven years, which is several years less than the previous
cycle. Taking these processes into account, the sampling occasions in the "Old Garden 1"
cover nearly an entire swidden cycle, although the present investigation cannot be referred
to as a monitoring study. In addition to the "Old Garden 1", surface soil in the "Sanctuary
Forest" was sampled in November, 1990, shortly after the vegetational survey reported in
the previous paper (NAKANO, 1992).
The land which includes both the "Old Garden 1" and the "Sanctuary Forest" consists ofa ridge extending roughly in a north south direction and having an east facing side slope.
The maximal angle of inclination is approximately 15 degrees. As mentioned previously (NAKANO, 1992), the "Sanctuary Forest" lies on an inconspicuous almost flat peak. The areasof the "Old Garden 1" and the "Sanctuary Forest" were slightly more than 0.1ha and lessthan 0.03ha respectively. The distance between respective centers of the two is somewhat
more than 50m. Precisely speaking, the boundaries of the two are not adjacent to each other,
but are interrupted by another swidden and its subsequent fallow. This intervening site, too,
was gardened successively after planting sweet potatoes in the "Old Garden 1". The samples

NAKANO and MIYAUCHI : Changes in Properties of Surface Soil during a Swidden Cycle

5

from the "Sanctuary Forest" were obtained only from the plot (0.01ha) for the vegetational
survey (NAKANO, 1992).
A soil profile was dug in a remnant fallow forest of the "Old Garden 1" shortly before
clearing it. This fallow forest was the stage slightly more than ten years in age. The hardness at each horizon of soil profile was measured using a Yamanaka Type hardness meter
and was determined by averaging the more than five values actually measured there when
the sample pit was dug. The soil samples for the bulk density analysis were obtained fromeach horizon using three 100ml metallic cylindrical soil samplers. Samples for the analysesof particle size distribution and chemical analyses were collected using a stainless steel
knife at the corresponding depths.
In both the "Old Garden 1" and the "Sanctuary Forest", most samples for physical andchemical analyses of the two surface layers were obtained using the preceding cylindricalsamplers. With regard to a few bulk density samples from the 15 20cm depth layer, however, suitable sizes of rectangular parallelopipedons of soil were, at stony spots, manually
cut out with a stainless steel knife. The sampling spots were always determined randomly.
At respective sampling occasions, the samples were usually collected from ten spots in the"Old Garden 1" or the "Sanctuary Forest". Except for the samples from the five years
fallow, in most cases, one sample for physical analyses and two for chemical analyses weretaken at one depth at each spot. At the first two sampling occasions shortly before and after
burning the plant debris, however, the samples for measuring water permeability were notcollected, because no plan to collect data on this physical property had been arranged during
the initial phase of the present investigation. The samples for physical analyses were sealed
and kept separately until analyses in Japan. All of the samples for chemical analyses obtained from one depth of ten spots were thoroughly mixed, air-dried locally and sent toJapan to be analyzed.
With reference to the five years fallow stage, the samples were collected from four or five
points distributed in a square less than 1,000cm2 area. The samples for physical analyses
from one or two points and the samples for chemical analyses from three or four points
were taken at one depth at each sampling spot. Those for physical analyses were treatedsimilarly as stated above. On the other hand, those for chemical analyses from three or four
points at one depth of each spot were thoroughly mixed. The respective mixed and air-driedones from two depth layers from each of the ten spots were kept separate. That is, thesetwenty batches in total were sent to Japan for chemical analyses.

Soil Analyses1) Physical analyses
Except for bulk density and water permeability, analyses were made on air-dried soil
samples sieved through a 2mm mesh. Particle size distribution of the samples was6South Pacific Study Vol. 17, No. 1, 1996

determined using the procedure described in NAKANO and Syahbuddin (1989). Water
permeability was measured using a DIK-4000 apparatus manufactured by Daiki Rika
Kogyo Co. Ltd., Japan. Mineralogical analysis of the clay fraction by x-ray diffraction was
conducted as follows: After a small amount of soil sample was treated with hydrogen
peroxide to oxidize the organic matter in it, it was suspended in de-ionized water, and only
the clay fraction with less than 2μ diameter was collected. One half of this clay fraction
was Mg-saturated, and the other half was K-saturated. The Mg- or K-saturated material was
dropped on a glass slide, dried, and x-rayed by means of a RAD-rB type apparatus manufactured by Rigaku-denki Co. Ltd., Japan, using CuKα.
2) Chemical analyses
Besides the preceding analysis for particle size distribution, chemical analyses werepartly conducted in cooperation with the technical staff of Palynosurvey Co. Ltd. in Japan.
All analyses were carried out on air-dried soil samples sieved through a 2mm mesh. pH was
measured by glass electrode and using a suspension of 10 g soil in 25 ml of de-ionizedwater or 1N KCl solution. Organic carbon was measured by ALLISON’s (1960) method.
Total nitrogen was estimated by a semi-micro KJELDAHL technique using K2SO4 and CuS
O4 as catalysts. "Available" phosphorus was measured by the TRUOG and the BRAY II
methods according to, respectively, ALLEN et al. (1974) and Commission for Determining
the methods of Plant-nutrient Analyses of Soil (1970). Exchangeable cations were extracted from soil samples with 1N ammonium acetate at pH 7.0 and analyzed using atomic
absorption spectroscopy for calcium and magnesium, and flame photometry for potassium
and sodium. Exchange acidity was determined by the titration of 0.02N NaOH solution to
the extract from 10g soil with 25 ml of 1N KCl solution for five days. With respect to cationexchange capacity (CEC), LANDON(1984, p.128) remarks that "in acid soils the exchange
capacity equals the sum of the exchangeable bases plus the exchange acidity". Thus, thissuggestion is followed and the above-stated values are designated here as "effective cationexchange capacity".

Results and Discussion
1) Soil profile
The description of the soil profile in the remnant fallow forest in the "Old Garden 1" isas follows:
HorizonA1

Depth (cm)
0 10 (on the average)

Description
Brownish black (5YR2 1, moist) friable heavyclay, fine angular blocky structure with abundantroots of various sizes, mixed with small and angularstones and gravel (3％ in areal ratio), gradual andnoticeably wavy boundary to the following deeper
horizon.

NAKANO and MIYAUCHI : Changes in Properties of Surface Soil during a Swidden Cycle

A3

10 13 (on the average)

B1

13 18 (on the average)

B21

18 60

B22

60 70

B23

70 85

B24

85 100 & 105

B3

100 & 105

7

Brownish black (5YR3 1, moist) friable heavyclay, fine angular blocky structure with small
amount of roots of various sizes, mixed with small
and angular stones and gravel (5％ in areal ratio),
gradual and noticeably wavy boundary to the following deeper horizon.
Bright brown (7.5YR5 6, moist) friable heavyclay, moderately fine angular blocky structure withvery small amount of fine roots, mixed with angularstones (5％ in areal ratio), gradual and noticeably
wavy boundary to the following deeper horizon.
Reddish brown (5YR4 8, moist) friable heavyclay, medium-sized angular blocky structure withfine roots scarcely found (down to 50cm depth),
mixed with angular stones (7 10％ in areal ratio),
clear and smooth boundary to the following deeper
horizon.
Dull reddish brown (5YR4 3, moist) friable heavyclay, medium-sized angular blocky structure withnoroots,
mixedwithangularstones(7 10％ in areal ratio), clear and smooth boundary
to the following deeper horizon.
Brown (7.5YR4 6, moist) friable heavy clay, medium-sized angular blocky structure with no roots,
mixed with angular stones (10 20％ in areal ratio),
gradual and wavy boundary to the following deeper
horizon.
Red (10R4 8, moist) friable heavy clay, medium
sized angular blocky structure with no roots, mixed
with angular stones (nearly 20％ in areal ratio)
gradual and wavy boundary to the following deeper
horizon.
Bright reddish brown (5YR5 8, moist), mixed withangular stones and white gravel (40 50％ in areal
ratio).

Most stones in the upper horizons were found to be tuff-breccia. The result of hardness
measurement at the respective horizons was as follows: A1 , 8.5mm; A3 , 15.7mm; B1 ,17.2mm; B21, 18.0mm; B22, 19.1mm; B23, 18.4mm; B24, 18.2mm; B3, 18.0mm. These values
indicate that the roots have little difficulty in penetrating down to 50cm depth.
Table 2 shows the data of soil texture, bulk density, and the results of chemical analysesof the soil samples from the respective horizons except for very thin A3. The data of soil

8South Pacific Study Vol. 17, No. 1, 1996

Table 2. Some properties of the soil sampled from a profile at a fallow near Aitea
TextureHorizon of Depth ofSampled Soil Sampled Soil

Organic C Total N
Coarse Sand Fine Sand

Silt

Clay

＊

Mean±S.D. (n )g 100cm3 soil

％

cm

＊

Bulk Density

A1

0 5

23.9

11.6 17.8 46.6 39.7±1.0(3)

B1

15 20

13.7

11.4 14.2 60.7 57.2±6.5(3)

B21

40 45

8.8

6.8 10.7 73.7 70.2±3.7(3)

B22

63 68

15.8

18.6 20.4 45.1 81.9±6.8(3)

B23

75 80

7.4

14.0 18.2 60.4 72.4±5.5(3)B24

95 100

3.7

9.6 13.5 73.2 67.5±1.7(3)％％C N Ratio
11.43 1.00
11.43
2.83 0.28
10.11
0.92 0.19
4.84
0.70 0.12
5.83
0.63 0.14
4.50
0.62 0.11
5.64"Available" P
TRUOG
BRAY
MethodMethod Ⅱ
mg 100g soil mg 100g soil
3.36

5.55

0.61

0.83

0.52

1.92

0.52

0.83

0.44

0.31

0.48

0.26

Number of samples.

texture precisely confirm the observed texture in the preceding profile description.
Fundamentally, the soil in the survey site is acidic at every horizon. The difference of thepH values (between those measured in de-ionized water and 1N KCl solution) is considerable. By the latter method, pH values of the B horizon show noticeably high acidity. The
contents of both organic carbon and total nitrogen indicate richness in organic matter in the
top layer. The decrease in C N ratio between 20cm and 40cm depths is marked. Basesaturation percentage in the top layer is very high mainly owing to the high content of
exchangeable calcium. The corresponding values of exchangeable calcium in the B horizon
are rather low although the values of effective cation exchange capacity are high becauseof noticeably high values of exchange acidity. Exchangeable potassium is higher in theB24 horizon than in the B22 and B23. This fact may reflect the great mobility of potassiumwith leaching (ANDRIESSE and SCHELHAAS, 1987b; STR MGAARD, 1992). If so, the higher
content of exchangeable potassium at the bottom layer of soil can be concluded to be aconsequence of its accumulation owing to the leaching process during the fallow period.
The data in Table 2 seem to support this inference. As has been mentioned above, given thebase saturation data in Table 2, this soil does not appear to fulfill the requirements for
Phaeozems as indicated by the revised legend of FAO-UNESCO system of soil taxonomy(Anonymous, 1988).
Fig. 1 displays a result of x-ray diffraction analysis on the composition of clay minerals
in a sample from the B horizon of the profile. This and other figures strongly suggest thatthe soil contains no 1 : 1 type mineral, such as kaolinite and halloysite. More complex clays
are indicated by the mixture of vermiculite- and illite-structured clay minerals including

9

NAKANO and MIYAUCHI : Changes in Properties of Surface Soil during a Swidden Cycle

Hamlet, northwestern Malaita Island, Solomon Islands.
Content of

Exchangeable Main
Cations＋ Exchange
AcidityCa
Na(Effective CEC)
mg-eq. 100g soil mg-eq. 100g soil mg-eq. 100g soil mg-eq. 100g soil mg-eq. 100g soil mg-eq. 100g soil
Base Saturation, ％Exchangeable
Mg
K

pH

Exchange
Acidity

13.5

5.3

1.1

1.1

2.9

5.3

1.3

0.5

0.3

35.2

2.4

1.1

0.5

0.4

79.5

1.1

0.4

0.2

0.2

31.5

1.4

0.6

0.3

0.3

48.6

1.6

0.8

0.5

0.3

65.7

23.9
87.9
42.6
17.4
83.9
5.2
33.4
5.7
51.2
5.1
68.9
4.6

in H2Oin 1N KCl

5.3

4.4

5.1

3.8

5.4

3.7

5.3

3.9

5.3

3.8

5.2

3.7

Fig. 1. X-ray diffraction patterns of oriented clay specimens from B21 horizon of the profile.

10South Pacific Study Vol. 17, No. 1, 1996

Table 3. Change in some physical and chemical properties of the soil (0 5cm depth)
properties of the soil (0 5cm depth) of a sanctuary forest beside the swidden
Physical Properties

Stage

Organic C
Total N
Bulk Density log10 Permeability Mean±S.D.(n＊) Mean±S.D.(n＊)
Mean±S.D.(n＊) Mean±S.D.(n＊)％％
g 100cm3 soil log10(ml cm2 sec.)(g m2)(g m2)(g soil m2)
C N Ratio

1 week after 41.9±5.4(4)burning(20,950)
8 months
fallow44.6±5.9(5)(22,300)

2½ years
fallow44.8±3.4(6)(22,400)

5 years
fallow45.0±7.6(6)(22,500)

Slightly more 40.6±3.1(6)than 10 years(20,300)
fallow＊＊＊
Long fallow 34.3±5.3(5)("Sanctuary(17,150)
Forest")7.44±n.a.＊＊(10) 0.70±n.a.＊＊(10)(1,560)(147)
10.57
0.83±0.52(3) 6.93±n.a.＊＊(10) 0.63±n.a.＊＊(10)(1,550)(140)
11.00
1.03±0.49(5) 7.20±n.a.＊＊(10) 0.67±n.a.＊＊(10)(1,610)(150)
10.75
1.64±0.24(5) 4.58±0.90 (10) 0.49±0.090 (10)
(1,030)(110)9.34±0.87 (10)no data6.34±n.a.＊＊(10) 0.66±n.a.＊＊(10)(1,280)(134)9.54
0.86±0.64(5) 10.56±n.a.＊＊(10) 1.00±n.a.＊＊(10)(1,810)(171)
10.56
no dataContent of"Available" P
TRUOG Method BRAY Method Ⅱ
Ca
Mean±S.D.(n＊)
Mean±S.D.(n＊) Mean±S.D.(n＊)
mg 100g soil
mg 100g soil
mg-eq. 100g soil
(g m2)(g m2)(g-eq. m2)
2.58±n.a.＊＊(10)(0.54)

3.45±n.a.＊＊(10)(0.72)

14.2±n.a.＊＊(10)(2.97)

1.31±n.a.＊＊(10)(0.29)

0.87±n.a.＊＊(10)(0.19)

5.1±n.a.＊＊(10)(1.14)

1.05±n.a.＊＊(10)(0.24)

1.62±n.a.＊＊(10)(0.36)

5.6±n.a.＊＊(10)(1.25)

0.95±0.28 (10)(0.21)

1.34±0.80 (10)(0.30)

3.0±2.00 (10)(0.68)

1.79±n.a.＊＊(10)(0.36)

2.27±n.a.＊＊(10)(0.46)

11.5±n.a.＊＊(10)(2.33)

3.19±n.a.＊＊(10)(0.55)

1.66±n.a.＊＊(10)(0.28)

24.2±n.a.＊＊(10)(4.15)＊

Number of sampling spots.
Not available.
＊＊＊
The spots where soil sampling was conducted had already been cleared but not burned.
＊＊

Al-vermiculite which shows properties different from those of typical vermiculite.
Furthermore, smectite is not readily found in this figure. These conclusions support theabove-referred passage in Booker Tropical Soil Manual (LANDON, 1984) concerning CEC,
and are compatible with the high values of exchange acidity (Table 2).
2) Change in Soil Properties during the Swidden Cycle
Tables 3 and 4 show the detailed data of the changes in some physical and chemical
properties of 0 5cm and 15 20cm soil layers, respectively, in the swidden and subsequent
fallow for the swidden cycle and the corresponding data of the "Sanctuary Forest". Both themeans and standard deviations are presented for physical properties. On the other hand, thestandard deviations for chemical analysis data at most stages cannot be calculated, because
all the samples for chemical analyses (except for those at the stage of five years fallow)
were thoroughly mixed. Nevertheless, a rough approximation of the standard deviations is
possible if the standard deviations of the five years fallow are applied to those of the otherstages. In the following discussion, the data of samples collected shortly before burning thefelled or slashed and dried plants are considered to be equivalent to those at slightly more

NAKANO and MIYAUCHI : Changes in Properties of Surface Soil during a Swidden Cycle

11

in a swidden and the subsequent fallow during a swidden cycle and the corresponding
near Aitea Hamlet, northwestern Malaita Island, Solomon Islands.
pH
Exchangeable Main
Exchange
Cations＋Exchange
Acidity
Exchangeablein H2Oin 1N KCl
Sampling Time
Acidity
Mg
K
Na(Effective CEC)
＊
＊
＊
＊
＊
＊
＊
Mean±S.D.(n ) Mean±S.D.(n ) Mean±S.D.(n ) Mean±S.D.(n ) Mean±S.D.(n ) Mean±S.D.(n ) Mean±S.D.(n ) Month, Yearmg-eq. 100g soil mg-eq. 100g soil mg-eq. 100g soil mg-eq. 100g soil mg-eq. 100g2 soil
(g-eq. m )(g-eq. m2)(g-eq. m2)(g-eq. m2)(g-eq. m2)
Base Saturation, ％5.6 ±n.a.＊＊(10) 1.5 ±n.a.＊＊(10) 0.6 ±n.a.＊＊(10) 1.7 ±n.a.＊＊(10) 23.6±n.a.＊＊(10)5.6±n.a.＊＊(10) 4.8±n.a.＊＊(10)May, 1989(1.17)(0.31)(0.13)(0.36)(4.94)
92.8
2.0 ±n.a.＊＊(10) 0.2 ±n.a.＊＊(10) 0.2 ±n.a.＊＊(10) 4.8 ±n.a.＊＊(10) 12.3±n.a.＊＊(10)4.6±n.a.＊＊(10) 3.9±n.a.＊＊(10) December, 1990
(0.45)(0.04)(0.04)(1.07)(2.74)
61.0
2.9 ±n.a.＊＊(10) 0.3 ±n.a.＊＊(10) 0.3 ±n.a.＊＊(10) 1.9 ±n.a.＊＊(10) 12.0±n.a.＊＊(10)5.1±n.a.＊＊(10) 4.2±n.a.＊＊(10)August, 1992
(0.65)(0.07)(0.07)(0.65)(2.69)
75.8
1.44±0.53 (10) 0.18±0.04 (10) 0.26±0.05 (10) 19.1±6.8 (10)24.1±6.3 (10)4.5±0.13 (10) 4.1±0.07 (10)March, 1995
(0.32)(0.04)(0.06)(4.30)(5.42)
22.3±15.2 (10)4.1 ±n.a.＊＊(10) 0.5 ±n.a.＊＊(10) 0.7 ±n.a.＊＊(10) 2.0 ±n.a.＊＊(10) 18.9±n.a.＊＊(10)5.6±n.a.＊＊(10) 4.4±n.a.＊＊(10)May, 1989(0.83)(0.12)(0.14)(0.41)(3.84)
89.4
4.1 ±n.a.＊＊(10) 0.5 ±n.a.＊＊(10) 0.9 ±n.a.＊＊(10) 1.7 ±n.a.＊＊(10) 31.4±n.a.＊＊(10)5.8±n.a.＊＊(10) 5.3±n.a.＊＊(10) November, 1990
(0.70)(0.09)(0.15)(0.29)(5.39)
94.6

than ten years fallow stage, even though the samples were taken after clearing the fallow
vegetation. Some changes in soil properties during, at the majority of sampling spots, one
or more months between the clearing and burning times cannot be completely ruled out as
the values of a few components in the profile sample (Table 2) are noticeably higher thanthe corresponding ones in Table 3. At any rate, however, it ought to be remembered that thedata in Table 2 are based on the samples from only one spot.
a. Physical properties, organic carbon, and total nitrogen
None of the differences in bulk density values are statistically significant at 5％ level(two-sided) using the STUDENT’s t-tests (TAKEUCHI, 1963). At 10％ level, however, thetopsoil bulk density value is significantly lower in the slightly more than ten years fallow
in comparison with the two years and a half fallow, and will be further lowered as thefallow period is increased. This coincides with the conclusion from AWETO’s (1981b) datafrom southwestern Nigeria. As for the 15 20cm layer, the standard deviation at the stage of
one week after burning plant debris is so high that the mean value is somewhat unreliable.
Nevertheless, at 10％ level, the bulk density which shows an increase up to the stage of five
years fallow seems to be lowered when the fallow period becomes very long. The resultsof water permeability, expressed in log10 values, seem to parallel those of bulk density. At

12South Pacific Study Vol. 17, No. 1, 1996

Table 4. Change in some physical and chemical properties of the soil (15 20cm depth)properties of the soil (15 20cm depth) of a sanctuary forest beside the swidden
Physical Properties

Stage

Organic C
Total N
Bulk Density log10 Permeability Mean±S.D.(n＊) Mean±S.D.(n＊)
Mean±S.D.(n＊) Mean±S.D.(n＊)％％
g 100cm3 soil log10 (ml cm2 sec.)(g m2)(g m2)(g soil m2)
C N Ratio

1 week after 71.5±25.1(4)burning(35,750)
8 months
fallow

52.7± 9.1(5)(26,350)

2½ years
fallow59.4± 5.5(5)(29,700)

5 years
fallow

65.1± 6.3(6)(32,550)Slightly more 63.7± 8.7(6)than 10 years(31,850)
fallow＊＊＊＊
Long fallow 53.0± 9.2(5)("Sanctuary(26,500)
Forest")Content of"Available" P
TRUOG Method BRAY Method Ⅱ
Ca
Mean±S.D.(n＊)
Mean±S.D.(n＊) Mean±S.D.(n＊)
mg 100g soil
mg 100g soil
mg-eq. 100g soil
(g m2)(g m2)(g-eq. m2)

2.26±n.a.＊＊(10) 0.27±n.a.＊＊(10) 0.70±n.a.＊＊(10)
(810)
(97)
(0.25)
8.37
1.20±0.62(3) 1.93±n.a.＊＊(10) 0.22±n.a.＊＊(10) 0.48±n.a.＊＊(10)
(510)
(58)
(0.13)
8.77
2.11±1.35(5) 2.03±n.a.＊＊(10) 0.22±n.a.＊＊(10) 0.31±n.a.＊＊(10)
(600)
(65)
(0.09)
9.23
2.49±0.58(5) 1.45±0.24 (10) 0.20±0.030 (10) 0.50±0.32 (10)
(470)
(65)
(0.16)
7.20±0.92 (10)no data2.20±n.a.＊＊(10) 0.27±n.a.＊＊(10) 0.66±n.a.＊＊(10)
(700)
(86)
(0.21)
8.15
0.62±0.14(4) 3.69±n.a.＊＊(10) 0.39±n.a.＊＊(10) 12.15±n.a.＊＊(10)
(980)
(103)
(3.22)
9.46
no data

1.05±n.a.＊＊(10)(0.38)

4.8 ±n.a.＊＊(10)(1.72)

0.09±n.a.＊＊(10)(0.02)

1.5 ±n.a.＊＊(10)(0.40)

0.57±n.a.＊＊(10)(0.17)

1.2 ±n.a.＊＊(10)(0.36)

0.57±0.26 (10)(0.19)

1.05±0.40 (10)(0.34)

0.44±n.a.＊＊(10)(0.14)

5.3 ±n.a.＊＊(10)(1.69)

4.46±n.a.＊＊(10)(1.18)

19.6 ±n.a.＊＊(10)(5.19)＊

Number of sampling spots.
Not available.
＊＊＊
One sample with a suspect value of exchange acidity, 150.4mg-eq. 100g soil, is excluded. If included, the means and the＊＊＊＊
The spots where soil sampling was conducted had already been cleared but not burned.
＊＊

10％ level, the permeability of the topsoil at the five years fallow stage has significantly
decreased from the two years and a half fallow. On the other hand, it is markedly higher inthe "Sanctuary Forest". The results of physical properties suggest that slight soil compaction
at the surface layer occurred during the course of fallow, and that the soil became softer andmore permeable as the fallow period became longer than ten years. The data of organiccarbon content may support this inference. In both the topsoil and the deeper layers, therespective contents of organic carbon at the five years fallow stage have the lowest (significant at 0.1％ level) values during the swidden cycle, while those at the "Sanctuary Forest"
are respectively much higher. AWETO’s (1981b) results, which were based on the samples
from different sites (AWETO, 1981a), should be noted here. His results showed a slightdecline in topsoil organic matter during the first three years following the cessation ofcultivation and its recovery after the early fallow stage.
The effects of burning on the organic or total carbon of topsoil have been presented bymany authors. In the paper based on the soil samples collected in Northern Thailand, N
AKANO (1978) reported that carbon content in the topsoil following a distinct dry season
was slightly lowered after burning, although the difference was not significant. R

13

NAKANO and MIYAUCHI : Changes in Properties of Surface Soil during a Swidden Cycle

in a swidden and the subsequent fallow during a swidden cycle and the corresponding
near Aitea Hamlet, northwestern Malaita Island, Solomon Islands.
Exchangeable Main
Exchange
Cations＋Exchange
Acidity
Exchangeable
Acidity
Mg
K
Na(Effective CEC)＊＊＊＊
Mean±S.D.(n ) Mean±S.D.(n ) Mean±S.D.(n ) Mean±S.D.(n ) Mean±S.D.(n＊)
mg-eq. 100g soil mg-eq. 100g soil mg-eq. 100g soil mg-eq. 100g soil mg-eq. 100g2 soil
(g-eq. m )(g-eq. m2)(g-eq. m2)(g-eq. m2)(g-eq. m2)
Base Saturation, ％1.4 ±n.a.＊＊(10) 0.4 ±n.a.＊＊(10) 0.3 ±n.a.＊＊(10) 4.2±n.a.＊＊(10) 11.1±n.a.＊＊(10)(0.50)(0.14)(0.11)( 1.50)( 3.97)62.2
0.4 ±n.a.＊＊(10) 0.1 ±n.a.＊＊(10) 0.1 ±n.a.＊＊(10) 20.1±n.a.＊＊(10) 22.2±n.a.＊＊(10)(0.11)(0.03)(0.03)( 5.30)( 5.85)9.5
0.4 ±n.a.＊＊(10) 0.1 ±n.a.＊＊(10) 0.1 ±n.a.＊＊(10) 20.4±n.a.＊＊(10) 22.2±n.a.＊＊(10)(0.12)(0.03)(0.03)( 6.06)( 6.59)8.1
0.55±0.28 (10) 0.10±0.09 (10) 0.18±0.06 (10) 32.2±16.1 (9)＊＊＊ 34.0±16.4 (9)＊＊＊(0.18)(0.03)(0.06)(10.48)(11.07)5.9±2.5 (9)1.8 ±n.a.＊＊(10) 0.4 ±n.a.＊＊(10) 0.3 ±n.a.＊＊(10) 28.2±n.a.＊＊(10) 36.0±n.a.＊＊(10)(0.57)(0.13)(0.10)( 8.98)(11.47)21.7
0.8 ±n.a.＊＊(10) 0.1 ±n.a.＊＊(10) 0.3 ±n.a.＊＊(10) 0.5±n.a.＊＊(10) 21.3±n.a.＊＊(10)(0.21)(0.03)(0.08)( 0.13)( 5.64)
97.7

pHin H2Oin 1N KCl＊

Sampling Time＊

Mean±S.D.(n ) Mean±S.D.(n )Month, Year5.2±n.a.＊＊(10) 4.1±n.a.＊＊(10)May, 1989

4.7±n.a.＊＊(10) 3.7±n.a.＊＊(10)December, 1990

5.1±n.a.＊＊(10) 4.0±n.a.＊＊(10)August, 1992

4.9±0.14 (10)4.1±0.16 (10)March, 1995

5.0±n.a.＊＊(10) 3.9±n.a.＊＊(10)May, 1989

6.4±n.a.＊＊(10) 5.7±n.a.＊＊(10) November, 1990

standard deviations of exchange acidity and effective CEC are respectively 44.0±40.4(10＊) and 45.9±40.8(10＊).

AMAKRISHNAN and TOKY (1981) reported similar results for North-Eastern India. On theother hand, SEUBERT et al. (1977) found an increase in organic carbon in the low jungleof Peruvian Amazon where there was no pronounced dry season. In Table 3 of this paper,
the content of organic carbon increased significantly, if the standard deviation at the five
years fallow stage is applied to all the other stages, at 5％ level after burning the plantdebris. We agree with STR MGAARD’s (1992, p. 23) view that "... the concentration andburning of woody matter resulted in the addition of considerable amounts of partly burned
and unburned biomass to the surface soil." The quantity of such additions may be larger forever-humid regions like this one. In contrast to regions where the thoroughly dried plantdebris is easily burned towards the end of a distinct dry season, burning wettish or, sometimes, thoroughly wet organic matter in the ever-humid Malaita Island is laborious.
Furthermore, in this case, the temperature at the topsoil does not rise very much during
burning. ANDRIESSE and SCHELHAAS (1987b) have a similar conclusion.
Total nitrogen in the topsoil is the lowest (significant at 1％ level) in the five years
fallow stage. In the 15 20cm layer, however, the differences between the five years andeight months or two years and a half fallow stages are not found to be significant. In

14South Pacific Study Vol. 17, No. 1, 1996

contrast to organic carbon, the increase in total nitrogen in the topsoil at the burning stage
is not statistically significant, and, in this respect, the result here is not similar to that of S
EUBERT et al. (1977) which shows a marked increase in total nitrogen as well as organiccarbon. Both organic carbon and total nitrogen in both soil layers in the "Sanctuary Forest"
are very high. This implies that, as the fallow period increases, the layer rich in organic
matter becomes thicker. The results stated so far seem to support the popular view that "...
soil humus increases in amount under the fallow" (NYE and GREENLAND, 1960, p. 47). As
well as organic carbon, the C N ratio in the topsoil increases significantly at the burning
stage. These increases are probably related to the amount of partly burned and unburned
biomass. The C N ratio of the topsoil is also the lowest at the stage of five years fallow.
The difference between this and the previous stages is significant. On the other hand, theincrease during the later phase of fallow (until the slightly more than ten years fallow
stage) does not appear to be significant. The C N value of the "Sanctuary Forest" marks
again significantly higher one than that at the five years fallow stage. With reference to the
15 20cm layer, this ratio at the five years fallow stage is significantly the lowest (Table 4).
b. "Available" phosphorus
The increase in "available" phosphorus as a result of burning the plant debris is obvious
in the topsoil and is consistent with the results reported in many other papers. The percentage increase in "available" phosphorus here, however, is not as noticeable as that reported
in Northern Thailand (NAKANO, 1978) where the plant debris was burned during the latestage of a distinct dry season. ANDRIESSE and KOOPMANS (1984) found an increase of"available" phosphorus to be due partly to soil heating and possible mineralization of organic phosphorus. The difference in the percentage increase of "available" phosphorus
between this case and that in Northern Thailand may be due to the difference in the degreeof temperature rise at the topsoil layer. This inference is based on the difference in thecircumstances of the two cases. Whereas thoroughly dried plant debris were burned and,accordingly, the topsoil temperature ought to have risen very much in Northern Thailand (ZINKE et al., 1978), burning wet organic matter ought not to have raised it much in the caseof the present investigation.
In the topsoil, the content of "available" phosphorus, measured by the TRUOG and the B
RAY II methods, significantly decreases during the cropping stage and the subsequent initialstage of fallow, and significantly increases during the later phase of fallow. Whether its
decrease during the earlier phase of fallow is statistically significant or not depends on theanalysis method. LANDON (1984) has a cautious attitude towards the TRUOG method. If weregard the BRAY method as more reliable in this case, the decrease during the earlier phaseof fallow is not significant. The results by this method indicate that its increase in the 15
20cm layer at the burning stage is significant, and the lowest value appears at the stage of
8 months fallow (Table 4). According to both methods, "available" phosphorus in the 15
20cm soil layer in the "Sanctuary Forest" are much higher than the topsoil. STR MGAARD(1992) reported another somewhat similar phenomenon. His data show an increase in"available" phosphorus at subsoil-level at the several years fallow stage. At present, weNAKANO and MIYAUCHI : Changes in Properties of Surface Soil during a Swidden Cycle

15

have no idea whether or not our preceding result is basically caused by the same procedureas the phenomenon shown in his data. Nevertheless, it is reasonable to assume that the long
fallow may ensure the supply of "available" phosphorus at a deeper cropping layer.
c. Exchangeable cations, effective CEC, and pH
Although the increase in exchangeable calcium in the topsoil at the burning stage isstatistically significant, it is not very noticeable. The decline of exchangeable calcium
during the cropping stage and the subsequent initial stage of fallow is marked. The lowest
value of exchangeable calcium is found at the five years fallow stage. It seems to increaseduring the later phase of fallow. The content of exchangeable calcium in the soil in the"Sanctuary Forest" is abundant. In the 15 20cm layer, the changes are roughly parallel withthose of the topsoil, although no increase at the burning stage is found (Table 4). These
data indicate, in contrast to those by RAMAKRISHNAN and TOKY (1981), the importance of
fallow in the establishment of high levels of exchangeable calcium.
The pattern of exchangeable magnesium in both soil layers during the swidden cycle isvery similar to that of exchangeable calcium, although the corresponding absolute values

Fig 2. Graphical representation of the selected data in Table 3. Note that, for the two stages(shortly before burning the felled or slashed and dried plants and slightly more than 10
years fallow), the respectively identical data shown by the ordinates of the graphs areplotted, because the very soil sample obtained at the former stage is also that at the latterstage.

16

South Pacific Study Vol. 17, No. 1, 1996

are much lower. In the "Sanctuary Forest", however, the amount of exchangeable magnesium in each soil layer is not more than that found in the slightly more than ten years fallow.
In the case of exchangeable potassium, there is a marked increase in the topsoil at theburning stage. This component was probably supplied by the ash. The decline in the topsoil
during the cropping stage and the subsequent initial fallow stage is sharp. A recovery,
however, from the lowest value at the five years fallow stage is significantly recognized in
the later phase of fallow (Table 3 and Fig. 2). During a long fallow period, the exchangeable potassium content in the topsoil does not seem to increase at all. Exchangeable potassium in the 15 20cm layer is very low throughout the earlier phase of fallow. As well as
the topsoil, its increase at this layer during the late phase of fallow is noticeable (Table 4).
In the "Sanctuary Forest", however, exchangeable potassium is low in the 15 20cm layer.
The change pattern of exchangeable sodium parallels that of exchangeable potassium.
The former, however, does not show an increase at the burning stage at all. The "Sanctuary
Forest" topsoil shows some accumulation of sodium (Table 3).
JORDAN’s (1987) experimental study showed little decrease in exchangeable calcium andpotassium during the cropping stage, but a noticeable decline at the initial fallow stage. On
the other hand, NAKANO (1978) indicated declines in both components in the topsoil during
the cropping stage as well as the fallowing stage. The data in this paper concerning exchangeable calcium and potassium are inconclusive as to which pattern is applicable. NYE
and FOSTER (1961) presented the idea of "pumping of nutrients" during the fallow phase.
The results of this investigation suggest exchangeable cations accumulate to some degree atthe surface layers during the late phase of fallow in the swidden cycle.
The nutrient stock in soil is determined by the balance between its supply and loss. As
shown in the previous paper (NAKANO, 1992), the recovery growth of vegetation in thissurvey site during the earlier phase of fallow is very vigorous. Therefore, it is reasonable tosuggest that large amounts of some nutrients are depleted from the surface layer of soil,
besides the losses from leaching and soil erosion, partly by absorption through the roots ofseedlings. Further factor at the initial fallow stage may be low litter production. These
factors will bring about the decline in the stock of those components at the surface layer ofsoil. This popular theory seems to be reflected by the results of some components in Tables
3 and 4, and is displayed in Fig. 2.
The values of effective cation exchange capacity (CEC) in the topsoil decrease markedly
during the cropping stage and the initial fallow stage owing to the decline in exchangeable
cations. During the middle and later stages of fallow in the swidden cycle, however, effective CEC recovers, at first, owing mainly to the high value of exchange acidity (Table 3).
Accordingly, the base saturation percentage at the five years fallow stage is lowest during
the swidden cycle. The topsoil of the "Sanctuary Forest" has high values of both cationexchange capacity and base saturation percentage. The change patterns of the two indicators
are also displayed in Fig. 2. ANDRIESSE and SCHELHAAS (1987b) showed a noticeable
increase in CEC of the topsoil with burning in their experimental site in Sarawak with no

NAKANO and MIYAUCHI : Changes in Properties of Surface Soil during a Swidden Cycle17

dry season. They reported that this rise in CEC was closely related to the increase in carbon
because of the incomplete burning of wet organic matter. The circumstances at Aitea,
Malaita Island, are similar to those in Sarawak. Table 3 showed a slight increase in CEC
after burning, although the statistical significance of the difference cannot be confirmed. In
the case of the present investigation (Table 3), however, the CEC increase is due mainly
to the increase in exchangeable calcium and potassium, probably derived from the ash. We
are unsure whether or not this fact supports ANDRIESSE and SCHELHAAS’s (1987b) assertion. In reference to the 15 20cm layer, on the other hand, its sharp decline at the burning
stage is marked mainly because of the abrupt decline, the reason for which is unknown atpresent, in exchange acidity. However, it increases during the cropping stage and the initialstage of fallow, and has almost recovered in the middle of the fallow period due mainly tothe rise of the values of exchange acidity. The change pattern of base saturation percentage
at the 15 20cm layer is opposite to that of cation exchange capacity. The exchange acidityof the soil in the "Sanctuary Forest" shows a very low value, and, accordingly, base saturation percentage is extremely high. The effective CEC value at this deeper soil layer of the"Sanctuary Forest" is similar to or much lower than that of the fallow, and lower than thatof the topsoil there (Tables 3 and 4).
A rise of pH at the burning plant debris stage is confirmed (for instance, NAKANO, 1978;
JORDAN, 1987). Its rise is sometimes discussed in relation to the availability of phosphorus
components in acid soils (JORDAN, 1987). In this investigation, such a rise was not measured in pure water for topsoil layer, although it seems fully significant in 1N KCl. The pH
measurements of the sample from the 15 20cm layer show also only slight rises during theburning time for both methods. Throughout the earlier phase of fallow up to the five years
fallow stage, all the pH values of topsoil samples are noticeably lower, by means of bothmethods, than the stage of one week after burning. During the later phase of fallow, itsvalue seems to rise significantly. The sample from the "Sanctuary Forest" showed higher
values using both methods than the corresponding sample from the fallow. These results arealso displayed in Fig. 2. According to the both methods, the pH values of the samples fromthe 15 20cm layer did not change much throughout the swidden cycle. Those of the 15
20cm layer sample from the "Sanctuary Forest" showed much higher values than any fromthe fallow. Particularly, the pH value of the "Sanctuary Forest" in pure water is remarkably
high. On the basis of the discussion mentioned above, this result may be related to the highcontent of "available" phosphorus in this deeper soil layer.

Concluding Remarks
From Table 3 and Fig. 2, it can be concluded that some properties of the topsoil in thefallows far less than ten years after garden abandonment are less favorable for the18

South Pacific Study Vol. 17, No. 1, 1996

reutilization of the investigation site as a swidden. This conclusion does not seem to changeeven when the beneficial effects of heating and adding ash are taken into account. After thestage of five years fallow, the soil condition seems to improve gradually. In this investigation, the significance of long fallow for swiddening is considered to have been scientificallypointed out, at least, to some degree. As was discussed in the previous paper (NAKANO,
1992), the results in Fig. 2 strongly suggests the appearance and spread of degraded andirreversible grasslands are brought about mainly by soil conditions unfavorable for forest
recovery, particularly if the fallow periods of most swiddens in a region are extremely
shortened.
LAMBERT’s (1986) data based on his three years investigation in seasonally wet and dry
Belize show considerable monthly fluctuations of the values concerning soil properties. Not
a few fluctuations, however, appear to be related to these seasons. Because the northwesternregion of Malaita Island where this investigation is sited is ever-humid (Table 1), thefluctuations owing to dry periods may have been alleviated by our sampling schedule.
Nevertheless, future investigations will involve precise monitoring over a few decades ofswidden cycles.
We suggest that, on the basis of this study, ecology of a swidden cycle in an ever-humid
region ought to be considered substantially different from those in a region with humid anddistinctly dry seasons because "The precise action of bush-fallow on soil fertility under
different climatic conditions is still not completely understood" (ANDRIESSE and S
CHELHAAS, 1987a, p. 286).

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(Acceped July 31, 1996)