Open Journal of Forestry
2012. Vol.2, No.2, 65-70
Published Online April 2012 in SciRes (
Copyright © 2012 SciRes. 65
Impact of Different Spacings of Cooking Banana Intercropped
with Rubber on Soil Fertility Attributes and Maturity Rate of the
Trees in a Humid Forest Area of South Eastern Nigeria
Timothy U. Esekhade, Ikokwu K. Okore*
Rubber Research Institute of Nigeria, Benin City, Nigeria
Email: *
Received December 28th, 2011; revised February 29th, 2012; accepted March 7th, 2012
The impact of four spacing of cooking banana (CB) within the immature rubber avenues on some soil fer-
tility attributes, maturity rate of rubber trees and dry rubber content (DRC) during the initial six years af-
ter planting (YAP) were evaluated in a humid forest area of South Eastern Nigeria relative to sole rubber.
The CB spacings within immature rubber avenues were 6.7 × 3.4 m; 4.0 × 2.0 m, 3.0 × 3.0 m and 2.0 ×
2.0 m, while the sole rubber was at 6.7 × 3.4 m, all laid out in randomized complete block design with
five replications. Quantities of soil organic C, extractable P, Ca, Mg and earthworm activities were sig-
nificantly higher in the intercrops, with the highest value coming from the 4 × 2 m CB spaced plots.
However a significantly higher value of K stock was observed in the sole rubber plot and declined as the
CB spacing narrowed. While the highest proportion (>90%) of matured hevea tree at six YAP was ob-
served in the 2 × 2 m CB spaced plots; the highest DRC of 1.7 t·ha–1·yr–1 was obtained from CB 4 × 2 m
treatment. Consequently, with some of the observed soil fertility attributes and DRC recorded, 4 × 2 m
CB spacing seems to be a more suitable CB spacings within immature rubber avenues, especially in view
of the levels of K in the 2 × 2 CB plots.
Keywords: Cropping System; Nutrient Stock; Dry Rubber Content; Earth Warm
Majority of the world natural rubber (Hevea brasiliensis
Muel. Arg) is traditionally and economically grown in the low
land humid tropics. This is principally due to its requirement
for uniform rainfall distribution, reduced temperature fluctua-
tion and relative humidity all year round (Von Uexckull &
Mutert, 1995; Rao et al., 1998). Soils of the eco-region are
known to be predominantly of low activity clay minerals char-
acterized by poor mineral nutrient fixation and/or storage ca-
pacity (Fagbani & Fapohund, 1986). Although this inherent
fertility constraint posses a lot of management problem upon
the conversion of the native forest into agriculture use (Von
Uexckull & Mutert, 1995); the soils are known for their sus-
tenance of the natural luxuriant tropical rainforest vegetation
(Novrdwijik, 2002). A number of low external input cropping
systems are traditionally used in the management of the soils
when put to agricultural use. Such management method in-
cludes diversification of crop component of the system through
inter cropping.
Intercropping of rubber with arable and quasi-peremial crops
like banana and pineapples has been found to be a sure way of
ameliorating the problem of zero income phase (immaturity
phase which often spans between 5 - 6 years) in rubber cultiva-
tion (Esekhade & Ojiekpon, 1997; Rodrigo et al., 2004) espe-
cially amongst the small holding who constitute over 75% of
rubber growers (IRSG, 2000). Reports on the effect of various
inter crop components on soil nutrient status (Zainol et al.,
1993; Esekhade et al., 2005) income enhancement; (Lydia et al.,
1999) and resource use efficiency (Rodrigo et al., 1997) and
growth performance of rubber sapling (Rodrigo et al., 2004) are
often limited to the immature period of the main crop (rubber).
There is paucity of information on the effect of the intercrop
components on the subsequent growth and yield and the soil
fertility attributes at the maturity of the main crop.
Consequently, it is the objective of this study to evaluate the
effect of different spacing of a Musa spp in an inter crop with
rubber on some soil fertility indicators such as nutrient stock
and earthworm activities at the maturity and maturity rate
(attainment of tappable girth size) of the main crop under such
cropping system in high rainforest area of south eastern Nige-
Materials and Methods
Site Description
The study herein reported is the extension of an experiment
established in 1993 (Esekhade & Ojiekpon, 1997) on the inter-
cropping of Para rubber (Hevea brasiliensis Mull. Arg) and
Musa Spp (cooking banana) in an acid sand soil of Akwete,
South eastern Nigeria. Akwete lies at Lat. 4.8998˚N and Long.
7.3359˚E. The rainfall pattern is bimodal with peaks in the
months of June, July and September and a mean minimum and
maximum temperature of 23.7˚C and 32˚C, respectively.
The landscape is nearly plain with a maximum elevation of
2% and the soil being classified as Arenic plaudit (USDA) or
Dystric Nitroso (FAO) and sandy alluvium parent material
*Corresponding author.
(Fabgbami & Fapohunda, 1986). At 0 to 30 cm depth the soil
had 84% sand, 8% silt and 8% silt with CEC of 6.40
Cmol·Kg–1, Organic matter of 1.8% and a pH of 4.4.
Experimental Layout
The experiment was laid out in Randomized Complete Block
Design (RCBD). The treatment consist of sole rubber at the
spacing of 6.7 × 3.4 m and four intercrop spacing (6.7 × 3.4 m,
4.0 × 2.0 m, 3.0 × 3.0 m and 2.0 × 2.0 m) of cooking Banana
within immature rubber avenue of 6.7 × 3.4 m. Each of this
treatment was replicated five times. The rubber clones and
cooking banana cultivar used are RRIM 501 and Cadaba, re-
spectively. Details of the field arrangement and management
practices are contained in Esekhade and Ojiekpon (1997).
Maturity Rate Determination
The effect of Banana spacing on maturity period (rate) of
rubber trees was assessed by measuring the girth of 15 trees at
the height of 1.5 m above the bud grafted union at six months
intervals in each treatment from the third year after planting.
However, for ease of handling, data are presented on yearly
Dry Rubber Yield (DRC) Determination
For DRC determination, latex exploitation of the trees be-
tween 1999 and 2001 was used. The S2D2 (half-Spiral, every
third day) tapping method was deployed to obtain the latex. The
dry rubber content (DRC) of the latex was determined by air
drying at room temperature until a constant weight was ob-
Soil Analysis Procedure and Nutrient Stock
Composite soil samples were taken at the depths of 0 - 15
and 15 - 30 cm at the onset of the experiment. At the beginning
of latex exploitation, composite samples were equally taken
across the treatments at similar depth like those of the pretreat-
ment samples. The composite samples consisted of four random
sub samples from each treatment. All the samples were air
dried and sieved through a 2 mm mesh sieve. The sample were
analyzed for organic C, total N, available P and exchangeable
bases (Ca2+, Mg2+ and k+) following Page et al. (1982) procedure.
Stored nutrients (extractable P, K, Ca and Mg and total N and
organic C) in each of the treatment were calculated by multi-
plying the soil concentration by the bulk-destiny and thickness
of the soil horizon considering the soil mass of each of the
treatment (Ellert & Bettary, 1995).
Earthworm Activities Determination
For earth worm activities determination, surface casts were
sampled from five randomly placed 1 × 1 m wooden quadrant
in each experimental unit at the end of rains at the third and
sixth year after planting. The cast were dried at 105˚C for
weight determination. From similar quadrants soil monoliths
(50 × 50 × 20 cm) were obtained using the Tropical soil Biol-
ogy and Fertility (TSBF) approach (Anderson and Ingram,
1993). Earth worm in each monolith were hand sorted and put
into 10% ethanol and later transferred to 40% formaldehyde
before being counted and weight for abundance and biomass
evaluation respectively.
Data generated were subjected to analysis of variance using
RCBD procedure. Where significance differences occurred for
a given variable across the treatment means were separated by
the use of Least Significant Difference (LSD) approach.
Soil Nutrient Stock
As shown in Table 1, the effect of the inter crops (irrespec-
tive of the spacing of the cooking banana) on the stocks of nu-
trients measured except N and K, were significant (higher)
compared to the sole rubber at 0 - 15 and 15 - 30 cm soil depths.
Organic C accumulation across the treatments followed similar
trend. The stock of organic C (30.87 and 28.36 Mg·ha–1) ob-
tained at the inception of tapping from plots where cooking
banana were intercropped at the spacing of 6.7 × 3.4 and 4 × 2
m, respectively within the rubber avenue, differed significantly
from the 24.53 Mg·ha–1 obtained from the sole rubber at 0 - 15
cm. The result at the lower depth 15 - 30 cm followed the same
trend. The effect of the cooking banana spacing within the rub-
ber avenues on total N stock did not differ significantly from
that of the sole plot at 0 - 15 cm, but did at the 15 - 30 cm depth.
The highest value (112 kgN·ha–1) at 15 - 30 cm depth was re-
corded from plots with 4 × 2 m spacing of cooking banana. The
stock of available P and exchangeable bases (Ca and Mg) dif-
fered significantly between the intercrop plots and the sole with
higher values being recorded from the intercrops with exception
of K. The effect of the component crop spacing within the rub-
ber avenue showed that as the spacing narrowed, the soil P
stock decreased with a significantly higher values being ob-
tained from 6.7 × 3.4 and 4 × 2 m compared to 3 × 3 and 2 × 2
m cooking banana spaced plots at the two depths.However, the
values from the intercrop were significantly higher than those
recorded from the sole plots.The stocks of exchangeable Ca and
Mg across the treatments followed similar trend, while the
stock of K+ in the soil under sole rubber was significantly
higher than those of the inter crops with values amongst the
intercrops following the P distribution tend (Table 1) irrespec-
tive of the soil depth.
Earth Worm Activities
Earth worm activities measured as surface cast, biomass and
abundance (Table 2) differed significantly across the treat-
ments irrespective of the sampling period (three or six YAP).
The quantities of the surface cast deposited across the inter
crops at any of the sampling period, were significantly higher
than those of the sole cropping irrespective of the component
crop spacing within the rubber avenue. Amongst the intercrops,
as the spacing of the CB within the main crop narrowed, the
quantity of earth worm cast deposit increased at the third year
after planting. However, this differed at the sixth year after
planting with highest cast deposit (442.06 g·m–2) being obtained
from plots with CB spaced at 4 × 2 m within the rubber avenue.
The effect on earth worm biomass and abundance followed simi-
lar trend with the intercrop system having higher values.
Rubber Tree Maturity Rate
On rates of the attainment of taable girth size amongst rubber pp
Copyright © 2012 SciRes.
Copyright © 2012 SciRes. 67
Table 1.
Effect of cooking banana spacing in an intercrop with rubber on the quantities of nutrient (kg·ha–1) and
organic C (Mg·ha–1) stored in the soil (0 - 30 cm) at 6 years after planting (maturity period).
Org. Total Avail. Exchangeable
(Cooking banana Spacing) C N P Ca Mg K
0 - 15 cm depth
Rubber sole 6.7 × 3.4 m 24.53 125.52 24.62 109.1 107.3 83.01
Rubber + CB (6.7 × 3.4 m) 28.36 128.3 44.66 121.22 113.41 63.42
Rubber + CB (4.0 × 2.0 m) 30.87 128.13 43.68 121.1 113.21 65.66
Rubber + CB (3.0 × 3.0 m) 26.88 128.1 36.95 119.32 117.72 61.62
Rubber + CB (2.0 × 2.0 m) 26.88 126.3 34.12 134.12 118.1 55.32
LSD (0.05) 3.36 NS 6.35 10.15 4.62 15.32
15 - 30 cm depth
Rubber sole 6.7 × 3.4 m 16.42 98.21 30.34 30.24 25.72 19.09
Rubber + CB (6.7 × 3.4 m) 20.92 111.05 36.45 36.45 13.79 22.11
Rubber + CB (4.0 × 2.0 m) 25.35 112.68 34.92 34.92 11.88 16.92
Rubber + CB (3.0 × 3.0 m) 21.36 11.38 22.61 27.61 13.58 18.68
Rubber + CB (2.0 × 2.0 m) 19.67 10.98 25.66 25.66 6.95 37.01
LSD (0.05) 4.13 2.68 3.36 5.51 6.1 2.31
Values in parenthesis are spacing of cooking banana within the main crop (rubber) avenues.
Table 2.
Earth worm cast, biomass (fresh weight) and abundance as affected by cooking banana inter crop at dif-
ferent spacing within rubber avenues at the third and sixth years after plantation establishment.
3rd Year Earth Worm 6th Year Earth Worm
Cast Biomass Abundance Cast Biomass Abundance
(g·m–2) (No·m–2) (g·m–2) (No·m–2)
Rubber sole 6.7 × 3.4 m 195.04 46.57 127.09 350.92 49.34 138.32
Rubber + CB (6.7 ×3.4 m) 262.87 50.13 151.79 430.14 53.52 145.44
Rubber + CB (4.0 × 2.0 m) 269.17 54.64 136.05 442.06 64.08 155.55
Rubber + CB (3.0 × 3.0 m) 270.57 58.56 138.96 414.16 64.64 150.52
Rubber + CB (2.0 × 2.0 m) 275.19 60.50 143.96 410.43 73.62 53.20
LSD (0.05) 8.35 5.12 10.33 21.11 4.92 11.31
Values in parenthesis are spacing of cooking banana within the main crop (rubber) avenues.
trees as shown by girth sizes or range across the treatments
(Table 3); trees in the intercrops showed faster rate as measuerd
in per centages over time with most of the sampled trees attain-
ing 50 cm girth at end of the 5th year compared to the sole.
The attainment of tappability amongst the rubber trees in the
intercrop increased as the spacing of CB within the avenues
narrowed, with approximately 85% of the sampled trees in the
plot with CB at 2 × 2 m spacing attaining > 50 cm girth at the
fifth year after planting. Across the treatments, plots where CB
was spaced 2 × 2 m and 3 × 3 m within the tree avenue, the
proportion of trees attaining the tappable girth size exceeded
the standard 60% often recommended for economic tapping and
this occured at the fifth year after planting.
Dry Rubber Yield
The mean annual yield of rubber (DRC) within the inter
crops were significantly higher than those of the sole plot (Ta-
ble 4). At the onset of tapping (1999), the highest yield of 1.3
t·ha–1 DRC was obtained from plots where CB was inter cropped
at 4 × 2 m spacing. This trend continued all through the data
period. However, the yield from plots with CB being spaced at
3 × 3 m and 2 × 2 m were higher than those from plots where
CB was intercropped at 6.7 × 3.4 m spacing within the rubber
Soil Nutrient and Earthworm Activities
The result of this study showed the potentials of rubber plan-
tation in improving nutrient status of soil as well as organic
matter build up as the years of establishment progress (Table 1);
irrespective of the cropping system. This observation buttressed
the widely reported potentials o rubber plantation is building f
Table 3.
Effects of cooking banana (CB) spacing in an intercrop with rubber on the maturity rate of rubber based
on girth ranges between the 3rd and 6th year after plantation establishment.
Years After Planting
Treatment Girth Range (cm)
3 4 5 6
<30 67 53 0 0
31 - 35 30 42 0 0
36 - 40 3 5 25.5 6
41 - 45 0 0 15.5 7
46 - 50 0 0 28.5 22
Rubber sole 6.7 × 3.4 m
>50 0 0 30.5 65
<30 58 33 0 0
31 - 35 25 38 0 0
36 - 40 17 29 10.5 3.5
41 - 45 0 0 16 4.5
46 - 50 0 0 25 7
Rubber + CB (6.7 × 3.4 m)
>50 0 0 48.5 85
<30 50 28 0 0
31 - 35 36 52 0 0
36 - 40 14 20 0 0
41 - 45 0 0 20.5 0
46 - 50 0 0 30 10
Rubber + CB (4.0 × 2.0 m)
>50 0 0 49.5 90
<30 45 20 0 0
31 - 35 32 50 0 0
36 - 40 23 23.5 0 0
41 - 45 0 6.5 15 0
46 - 50 0 0 23.5 8
Rubber + CB (3.0 × 3.0 m)
>50 0 0 61.5 92
<30 42 13 0 0
31 - 35 27 9.5 0 0
36 - 40 16 30 0 0
41 - 45 14 37 10.5 3
46 - 50 1 10.5 5 3.5
Rubber + CB (2.0 × 2.0 m)
>50 0 0 84.5 93.5
Value in parenthesis are spacing of cooking banana (CB) within the main crop (rubber) avenues.
up soil nutrient and organic C through efficient nutrient cycling
mechanism (Delabarre & Serier, 2001) and enhanced envi-
ronmental conservation facilitated by biodiversity maintenance
and conservation similar to natural forest ecosystem (Beukema
et al., 1997).
On annual basis, the mean (6 years) values of the soil organic
C stock and stocks of N, P, K, Mg and Ca observed across the
treatments in this study are within the range reported by Dela-
barre and Serier (2001) as a mean annual value of 25 year rub-
ber plantation, but above the organic C stock recorded (Nolte et
al., 2001) from a natural fallow land of similar age in Camer-
oon, under a similar soil. The intercropping of cooking banana
within the rubber avenue resulting to a significantly (P > 0.05)
higher organic C stock and stocks of P and exchangeable base
cations except K, relative to the sole rubber (Table 1) could be
due to higher biomass generation and retention from the banana
Similar higher nutrient levels have been reported in banana
Copyright © 2012 SciRes.
Table 4.
Mean yield performance of rubber between 1990 and 2001 as influ-
enced by cooking banana spacing within rubber avenues in an intercrop
Mean annual yield rubber
(kg DRC ha–1·yr–1)
1999 2000 2001 Mean
Rubber sole 6.7 × 3.4 m 920.7 1636.9 1736.6 1431.4
Rubber + CB (6.7 × 3.4 m) 1024 1644.6 1745.8 1471.5
Rubber + CB (4.0 × 2.0 m) 1264.8 1835.8 1937.6 1679.4
Rubber + CB (3.0 × 3.0 m) 1049.8 1782.3 1884.6 1572.2
Rubber + CB (2.0 × 2.0 m) 1015.4 1705.8 1810.8 1510.7
LSD (0.05) 20.8 60.3 43.6
Values in parenthesis are the spacings of cooking banana (CB) within the imma-
ture rubber avenues.
associated cropping system (Bekundal et al., 2000) compared to
other cropping systems. This is attributed among other things to
the reduced rate of nutrient loss ( leaching) resulting from early
shading of the surface soil by the banana plant, applied mulch-
ing materials and retention of a large quantity of the plant resi-
due after harvest. However, a lower level of K Stock was re-
corded from the intercropped plots compared to the sole. This
could be ascribed to higher uptake of K and buttressed by the
observed decline in the nutrient as the spacings of the intercrop
component narrowed. Similar observation was made (Wilson,
1985) who reported the storage of a greater proportion of K at
the fruit portion of the plant compared to the other parts. The
implication is that as the fruit are being harvested and taken
away from the farm for consumption, a large amount of K is
being exported from the field and could have some adverse
effect on the development of the rubber.
At the end of the first three years of the intercrops, the ob-
served significantly higher earthworm activities (surface cast,
biomass and abundance) in plots with cooking banana at closer
spacing (Table 2) could be as a result of shade provided by the
banana plant. Since the banana plant were at closer spacing (2 ×
2 m) within the rubber avenue, their leaves could have been
closely interlocked than those at the wider spacing (6.7 × 3.4 m
or 4 × 2 m); thus providing better shade and moisture conserva-
tion—a mechanism known to affect earthworm activities. The
higher earthworm activities observed in the plots with cooking
banana at 4 × 2 m at the sixth year sampling (maturity of plan-
tation) could be ascribed to higher organic matter content in
that plot (Table 1) resulting from the presence of banana plants
and its residue. Thus the observed higher earthworm activities
in the intercrop compared to the sole rubber is in order and its
implications on nutrient cycling and other ecosystem services
are paramount.
Rubber Tree Maturity Period
As shown in Table 3, inter-cropping shortened the maturity
(tappability) period of the rubber tree with higher percentages
of trees attaining maturity in plots with cooking banana at
closer spacings 2 × 2 m and 3 × 3 m compared to others, at five
years after planting. Theis observation could partly be attrib-
uted to improved management methods resulting from inte-
gration of banana (Esekhade et al., 2005) and the early shading
effect of the banana which may have alleviated the commonly
observed radiation induced photosynthic reduction in young
Hevea plants due to easy penetration of sun rays during the
early growth stage under sole rubber cropping system (Senevi-
rathna et al., 2003). Such shading effect is known to promote
leave production, with an associated leave area increase per
plant. Similarly, increased growth rate and dry matter accumu-
lation have been observed among rubber plants intercropped
with cassava (Esekhade, 2004).
The observed non-impairment of rubber tree growth and
maturity by cooking banana, even at closer spacings is in line
with the work of Rodrigo et al. (1997). Also the higher values
of soil nutrient stocks and biological activities in the intercrop
plots (Tables 1 and 2) may have had some positive effect on
the growth of the tree and consequently its maturity. Hauser et
al. (1997) observed a positive relation between earth worm
activeties and crop performance (growth and yield).
Dry Rubber Yield
The positive effect of the intercrops on rubber did not only
enhance the maturity (tappability) period of rubber, but also
affected the latex yield measured as dry rubber content (DRC),
compared to the sole. However, the pattern of DRC values
across the treatments did not follow the trend observed for the
maturity rate (%) of the plants, rather the highest DRC values
were consistently obtained from plots where the trees were
intercropped with cooking banana at 4 × 2 m (Table 4). This
could be due to high soil nutrient concentration under the
treatment (Table 2) aswell as soil biological activities (earth
worm) which may have influence yield. The effect of soil nu-
trient status on dry rubber content is reported by Delabarre and
serier (2002) and Esekhade et al. (2005).
The problem of overcoming the zero income phrase associ-
ated with rubber cultivation has necessitated the research into
the environmental and economic implication of intercropping
of rubber with other early maturing high value crops. This
study revealed that not only did intercropping of rubber with
cooking banana resulted to improve soil fertility in terms of soil
nutrient and biological activities (earth worm) but it also short-
ened the maturity period of the tree compared to the sole.
However, intercropping banana at closer spacings within the
avenue needs further investigation because of the reduction in
soil K stock. From the information gathered from the study,
intercropping with cooking banana at 4 × 2 m within the rubber
avenue ensures better rubber performance in terms of yield and
soil resources conservation.
Anderson, J. M., & Ingram, J. S. I. (Eds.) (1993). Tropical soil biology
and fertility: A handbook of methods (2nd ed.). Wallingford: CAB
Bekunda, M. A., Wortinann, C. S., Bwamiki, D. P., & Okwakol, M.
(2000). Potentials and challenges of soils fertility management in
Banana based cropping systems of Eastern Africa. In M. P. Gichuru,
A. Bationo, M. A. Bekundia, et al. (Eds.), Soil fertility management
in Africa: A regional perspective (pp. 123-146). Nairobi: Academy
Science Publishers.
Beukema, H., Stolle, F., Van Noordwijk, M., & De Foresta, H. (1997).
Copyright © 2012 SciRes. 69
Copyright © 2012 SciRes.
Biodiversity in rubber agroforestry. Smallholder Rubber Agrofor-
estry Project, ICRAF South East Asian Regional Research Pro-
gramme, Bogor.
Delabarre, M. A., & Serier J. B. (2001) Rubber. Tropical agriculturalist
series. Wageningen: CTA and Macmillan.
Ellert, B. H., & Bettary, J. R. (1995). Calculation of organic matter and
nutrient stored in soils under contrasting management regimes. Ca-
nadian Journal of Soil Science, 75, 529-538.
Esekhade, T. U., & Ojiekpon, I. F. (1997). Effects and economic viabil-
ity of intercropping cooking banana with rubber in Nigeria. Indian
Journal of Natural Rubber Research, 10, 91-96.
Esekhade, T. U. (2003). Effect of phosphorus and selected rubber based
cropping systems on the early development of rubber (Hevea brasil-
iensis (Wild Juss) mueller argoviensis) on acid soil. Ph.D.
Thesis, Ibadan: University of Ibadan.
Esekhade, T. U., Okore, I. K., Ogeh, J., & Idoko, S. O. (2005). Effect of
fertilizer and mulch on the growth and yield of intercropped rub-
ber/cooking banana and soil properties. Journal of Sustainable Agri-
culture and the Environment, 7, 10-20.
Fagbami, A., & Fapohunda A. (1986). Slur imagery for soil mapping
and regional planning in western Nigeria. In M. J. Eden (Ed.), Re-
mote sensing and tropical land management and parry. New York:
John Wiley& Sons Ltd.
Hauser, S., Vanlauwe, B. Asawalam, D. O., & Norgrove, L. (1997)
Role of earthworm in traditional and improved low-input agricultural
systems in West Africa. In L. Brussard, et al. (Eds.), Soil ecology in
sustainable agricultural systems (pp. 113-136). New York: Lewis
IRSG (2000). Rubber statistical bulletin. Wembley: International Rub-
ber Study Group.
Lydia, P. O., Teresita, I. C., & Nelson, T. B. (1999). Natural rubber: A
farming option for Agrarian return communities in Mindango, Phil-
ippines. Proceeding of IRRDB Symposium. Hainan: Hainan Publish-
ing House.
Nolte, C., Koho-Same, J., Moukam, A., Thenkabai, P. S. Weise, S. F.,
Woomer, P. L. & Zapfack L. (2001). Land-use characterization and
estimate of carbon stock in the alternative to slash-and-burn bench
mark areas in Cameroon. IITA Resources and Crop Management
Research Monograph No. 28, 9-21.
Noordwijr, M. V. (2002). Nutrient cycling in ecosystem and nutrient
flows in agro ecosystem. Newsletter on Soil Fertility and Fallow
Management in the Up/and Tropics No. 2.
Page, A. L., Miller, R. H., & Keeney, D. R. (1982). Methods of soil
analysis part 2: Chemical and microbial properties. Madison: Ameri-
can Society of Agronomy, Inc.
Rao, P. S., Saraswathyamma, C. K., & Sethuraji, M. R. (1998). Studies
on the relationship between yield and meteorological parameters of
Para rubber tree (Hevea brasiliensis). Agricultural and Forest Mete-
orology, 90, 235-245.
Rodrigo, V. H. L., Stirling, C. M., Teklehaimanot, Z., & Nugwale, A.
(1997). Effect of planting density on growth and development of
component crops in rubber/banana intercropping system. Field Crop
Research, 52, 95-108 doi:10.1016/S0378-4290(96)01069-6
Rodrigo, V. H. L. Stirling, C. M., Teklehaimanot, Z., & Nugwale, A.
(2001). Intercropping with banana to improve fractional interception
and radiation-use efficiency of immature rubber plantation. Field
Crop Research, 69, 237-249. doi:10.1016/S0378-4290(00)00147-7
Senevirathan, A. M. W. K., Stirling. C. M., & Rodrigo, V. H. L. (2003).
Growth, Photosynthetic performance and shade adaptation of rubber
(Hevea brasiliensis) growth in natural shade. Tree Physiology, 23,
Vonuekull, H. R., & Mutert, E. (1995). Global extent, development and
economic impact of acid soils. Plant and Soil, 171, 1-15.
Wilson, K. C. (1985). Mineral and fertilizer needs. In M. N. Clifford, &
K. C.Wilson (Eds.), Coffee: Botany, biochemistry and production of
beans and Beverage. London: Croom Helm Ltd.
Woomer, D. L. Bekunda, M. A., & Nkalubo, S. T. (1998) Site charac-
terization for organic resource transfer studies: The century model
approach. African Crop Science Journal, 6, 205-214.
Zainol, A. E, Mannual, W., & Suden, M. N ( 1993). Effect of inter-
cropping systems on surface processes in acid ultisols: Changes in
soil chemical properties and their influence on crop performance.
Journal of Natural Rubber Research, 8, 124-136.