NEW FARMING SYSTEMS DEVELOPMENT IN THE WETTER TIOPICSt
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ExplAgric. (1989), volume 25, pp. 145-163 FARMING SYSTEMS SERIES - 19
Printed in Great Britain
NEW FARMING SYSTEMS DEVELOPMENT
IN THE WETTER TIOPICSt
By A. S. R.JUOt
InternationalInstitute of TropicalAgriculture (IITA),
PMB 5320, Ibadan, Nigeria
(Accepted 14 September 1988)
SUMMARY
Farming systems in the wet ter tropics may be divided into three simplified models: the irrigated
paddy-rice multistorey homestead garden complex of Asia, the tree and cash crop plantations
of Latin America, and the mixed root crop-bush fallow systems of Africa. In upland eco
systems, sustainable farming systems development rests on a delicate balance between conserva
tion and utilization. To meet the basic food and nutritional needs of the indigenous population,
there is great potential for the improvement and adaptation of multistorey homestead gardens
and mixed systems which include trees and annual and perennial crops. The principle and
practice of 'alley cropping' provide an ecologically sound basis for future farming systems
research and development in the region.
A. S. R.Juo: Nuevo desarrollo de sistemas agropecuarios en las regiones tropicales mds hdimedas.
RESUMEN
Los sistemas agropecuarios en las regiones tropicales mins h6medas pueden dividirse en tres
modelos simplificados: el complejo ie cultivo (ie arroz con ciscara de regadio/huerta de 'pisos'
mtltiples de Asia, las plantaciones de ;irboles y cultivos comerciales de Amm}rica Latina, y los
sistemas mixtos de cultivo Ie raiceq/barbecho arbustivo deI, frica. En los ecosistemas del alti
piano, el desarrollo de sistemas agropecuarios sostenibles descansa sobre un delicado equilibrio
entre la conservaciim y la utilizaci6n. Para satisfacer las necesidades bisicas alimenticias y nutri
tivas ie la poblaci6n indigena, existe un gran potencial para el mejoramiento y la adaptaci6n de
las huertas de 'pisos' m 6 ltiples y los sistemas mixtos, entre ellos los Arboles y cultivos anuales y
perennes. El principio y la prictica del cultivo en hileras proporcionan una base ecol6gicamente
vilida para la futura investigaciun y desarrollo (ie sistemas agropecuarios en la regi6n.
INTRODUCTION
Agricultural change, for better or worse, is man's response to his physical and
socio-economic environment. Fundamentally, change has always been from
subsistence systems to some form of commercial agriculture, but both the pro
cess and the pace ol this change diff'er considerably in different societies, coun
tries or regions (Bunting, 1970; Anthony et al., 1979; CAB, 1980; Simmonds,
1986).
Historically, many of the agricultural changes in the wetter tropics in the past
century have been ilnfluence(l by the economic and political expansion of
t This paper reviews the state-of-the-art of new farming systems development in the wetter tropics. The
views expressed arc the author's own. They do not necessarily represent tile official views of the Inter
national Institute of Tropical Agriculture.
t Present address: Department of Soil and Crop Sciences, Texas A & M University, College Station, TX,
77843, USA.146 A. S. R. JUO
industrialized societies in temperate regions. Cash and tree crop agriculture
developed during the colonial era to supply industrial raw materials (rubber
and paln oil) and exotic tropical food items (coffee, tea, cocoa, bananas, sugar
and spices) to the technologically advanced societies in temperate regions. In
response to population growth and political independence, food crop produc
tion was emphasized.
More recently, the success of commercial food and cash crop agriculture in a
few countries in the tropics, coupled with huge food surpluses in industrialized
nations, has raised serious political concern about whether tropical and tem
perate regions should continue to produce surpluses of commodities that
compete for the same markets.
The wetter tropics, for the purpose of this paper, refer to humid tropical
regions where mean monthly precipitation equals or exceeds potential evapo
transpiration for more than seven months of the year (T. L. Lawson, personal
communication). This coincides approximately with those areas which have a
rainforest ecology (Golley, 1983a). Thus, geographically, the wetter tropics
include the coastal lowlands of south and southeast Asia, the archipelagoes of
the Philippines, Papua New Guinea and Indonesia, the coastal lowlands of west
and central Africa, the Amazonian and Congo basins, and a major part of'
central America.
Except for some (tensely populated urban areas in the coastal regions of
south Asia and west Africa, the supply of basic foods has not been a real prob
lenm in the wetter tropics. Rice, cassava, sweet potato, cocoyams, small rumi
nants and a wide range of forest products have provided nearly all the basic
needs of the indigenous population. The Amazonian and Congo basins remain
nature's last defenceless frontier. What then should he the future course of agri
cultural development in the wetter tropics? Can researchers and planners in
tropical and temperate regions lay aside their economic, social and cultural bias
and think of' future agricultural development in the tropics in physical, bio
logical and ecological terms? In the first paper of the Farming Systems Series,
Simmonds (1986) stressed the importance of trees and perennial crops in food
and cash crop a-,riculture in the wetter tropics and called for new farming
systems development in the region. This paper highlights some of the unique
features of the tropical forest environment and assesses some pronising tech
nologies that may lead to the development of new farming systems in the wetter
tropics.
NATURAL RESOURCE BASE
In terms of agricultural development, one must recognize fundamental dif
ferences in the natural resourcc base between tropical and temperate regions.
In tropical forest regions, most soils, for example, are inherently less fertile;
the climate is continuously hot and humid; and the average farm worker is
less well educated and less skilled (Waller, 1984).
/Farmingsystems in the wetter tropics 147
Climate and agriculture
'[he two most important climatological factors that affect agriculture are
rain fall and temperature (Griffith, 1976; Biswas, 1984). For annual crops, the
distribution 0)1' rainfiall throughout the year and the soil's rain fall-retention
capacity arc more crucial to crop growth than the total amnt of rain fall.
Omerod (1978) madle an interesting comparison of the rainfall distribution and
soil moisture retention between l.on(lon, England (humid, temperate), Sokoto,
Nigeria (semi-arid, tropical) and Lagos, Nigeria (humid, tropical). Although the
total annual rainfall in Sokoto (688 rm) and Lagos (1820 mm) is 20% and
220% more, respectively, than that of London (568 mm), its distribution in the
two tropical locations is much more erratic. In Sokoto, 97 % of the rain falls
within five months (May to September) and in Lagos, 55% falls during May,
June and July, with a peak in June. A high rate of evaporation, excessive
leaching and water rurt-offf are thereforc common in the tropical locations. In
London, although the total annual rainfall is relatively low by tropical stan
dards, it is uniform and gentle throughout the year. The rainfall retention
capacity of the soils at the three locations is perhaps the most significant
distinguishing factor between the sites (Biswas, 1984). Monthly rainfall re
tainec in the soil varies from 60 to 300 mm in Lagos and from 2 to 107 mm in
Sokoto, whereas in London it ranges from 152 to 290 mm. Thus, the risk of
drought during the cropping season in the two tropical locations is much higher
than in the teriperate location despite the higher total rainfall in the tropics.
The colmlbin lti,)in of high temperaturean( high humidity in the wetter tropics
is another important factir in agricuItral development and planning. Although
SOlle crops from temperate regions can be grown under humid tropical corn
ditions, high pest and disease pressures severely limit their yields. The heat
and humidity also afcCt people;unfortunately, the hottest an..he most humid
pcriol of'ten coincidles with the onset ,of'the rainy season when the ardluous
tasks of lanl preparation an(l planting must be carried out.
Soils and agriculture
Soils in the wetter tropics generally belong to one of three groups: (i) kaoli
nlitic soils, (ii) oxidic and allophanic soils or (iii) hydlromorphic soils (Juo,
1980). The kaolinitic soils are generally acidic and extremely low in inineral
nutrient reserves. They are classified as Oxisols and Ultisols with 'low activity
clays' (USI)A, 1976). Kaolinitic soils comprise approximately 70% of the total
land area of tile low altitide humid tropics (Moormna n and(l van Wambeke,
1978). They cowcr extensive areas ofJ the Amazon and Congo basins and the
coastal regions of west and central Africa. On such so1ils, shifting cultivation
and associated bush falhow systems remain the major means of food production
for the illdigenous people (Rulthllecerg, 1976; I)enevan, 1980). In some (len
sely populated area!; in west Africa and south Asia, cn tinlOus cultivation on
kaolinitic soils has resulted insevere land dfegralation anol mass emigration of148 A. S. R.JUO
rural populations. Southeastern Nigeria and the Kerala state of India are
examples.
Vast areas with kaolinitic soils in tile wetter tropics still remain sparsely
inhabited. Aluminiun toxicity, leaching and multiple nutrient deficiences place
severe constraints on continuous annual food crop cultivation on these soils
(Sanchez et al., 1982; Grimme and Juo, 1985). Experience in recent years has
shown that the tropical forest-kaolinitic soil ecosystem is vulnerable to modern
agricultural technologies developed in the temperate regions, as evidenced by
tile failure of many large-scale projects where forests have been cleared for
mechanized food crop agriculture (Moran, 1983; Weischet, 1984; Lal ct al.,
1985).
The oxidic soils derived from ferromagnesian rocks (e.g. basalt) and lime
stones, and the young allophanic soils derived from volcanic ash comprise a
second group of upland soils in the wetter tropics. ThOugh of restricted distri
bution, they are among tile most productive soils in the world. It is the rich
volcanic soils that support dense populations in Java, western Cameroon, the
Caribbean islands and parts of central America. In addition to intensive food
crop and multi-storey homestead farming, large and small cash-crop plantations
such as banana, cocoa, coffee and sugar cane are commonly found oil such
soils, particularly in Upland areas where the cooler temperatures are more
conducive to intensive farm operations.
The hVdrmnorphic Or alluvial soils in tile wellands form a third group of
important agricultural soils in the wetter tropics. Rice production in tile fertile
river valleys and deltas of south and southeast Asia has for centuries met tile
food needs of the world's most densely populated regions. These wetland
farming systems are !ft for discussiun in another article.
Ecologicaland biologicalconsiderations
From an ecological viewpoint, the central concept behind mankind's success
and failure in agriculture and industry has been that nature's physical and
biological resources can be ceaselessly exploited and manipulated 1)y man to
meet his basic needs ard enjoyment. Our powerful tools are science and tech
nology, and our primary driving force is economics. Only recently ias modern
agricultural and industrial development in temperate regions begun to show
stresses and limitations. Rivers, groundwater and air have become increasingly
polluted, and the genetic diversity of plants and animals is being depleted.
In the wetter tropics, tile increased rate of deforestation by shifting culti
vators, the commercial exploitation of tropical hardwoods and many large
scale agricultural projects have caused worldwide concern. Some speculate that
further destruction of nature's remaining forests may alter the equilibrium of
tile atmosphere, or even cause changes in climate and hydrology (Prance, 1986).
The Amazon forests, the world's largest water catchment area, produce 20%
of tile fresh water that enters the ocean. It may have taken thousands or even
millions of years for the complex nutrient cycling which occurs in tropicall'aroaing systems in the wetter tropics 149
rain forests to develop oi tile predominantly poor saindy and kaolinitic soils.
Illsome areas, such as the Rio Negro region o1 Amazonia and tile 'kernangas'
forests of' Borneo, climax tojiical rainforests exist oii soils where tile nutrient
status is sO Iow that f[irests mav never he replaced by perm ancnit agriculture
(Keller, 1983; Weiscliet, 198-4).
Some stress tile need to 1l)'eSCI'e the ,liversity of tile tropical rainforests as a
future res IIurce f )r I ttui agricultural dC leVec
)pCtII.
n Rubl)ber (Hevea brasiliensis)
and cocoa (Theobroma cacao) are indigeliis tree species in tie -.)th American
raint'orests, and oil palm (Esaeisis"guinvensis) is native to tile rain forests of west
Africa. There mayV be thliOt(Isan Ismore species of' trees, shrubs aitId herbs that
could be used for ')ood, oil, tinier, pulp ai11d meIlicine which have Vet to he
identified (Nleggers 0tal., 1973; Espig, 1979).
Perhaps tile mo)st lt()ughlit-pr(vc king theory f'or agricultural develhpment in
the wetter trci-lics is tile 'ec(develht pin t' theory cif l)asman 0i ai. (1 973) and
,(;(lley (1983b). They believe that mati is a b ih gical-ect dogical Ibeing who
ulti atelv depends Ul))n tile ecosystems in which lie lives. Their system recog
nizes tile physical, bioloical and cultural characteristics (Of tile humid tropical
eCivi() iinen t ad
i 'iCLCcuses
on develo)ment IOr tile basic and real needs off tle
indigenous piCople. Once tile basic needs of' food, clothing and shelter are met,
Other itjpir, vcm en ts in lie's (jual itv depend upoii subjective evalua tio ns.
Godley (I 983b)) itlhIlIr ar-gueS t1fiat inain can aIter and create ecosystems, bLit
that tile link with natura eco svs em, sh(oIld iIver be severed, because they
pro'ide us with ia variet ,Of"powerful but fiee services, such as tile pr)ccssing
oftwastes and the pro\isioi of1' cleani air and water, that are iiot possible wider
mail's direct c()nitr()Il. Moreover, itisnot certain what proportion o(f the earth's
land and water call be altered by Iman he fcre these free services are irreversibly
disrupted. Thus, many ccol,)gists and envir,mncntalists believe that the conver
sion of, nature's remaining ft)rests into managed tree plantations Or pertmatnent
crc p fields slihi ws tile short-sighted ontt lik of'non-Iforest, urban dwellers and
may prove ecologically disastrous ill tile long Iui.
On the cOther hIld, many agricultural scientists believe that it is not necessary
to double the presenlt area 01' agricultural land illorder to double the world's
food pr,)dtictii). Instead, they ibelieve tile emphasis should be On intensi fying
tile use OF existiig crc p landily using nrcv or improved farming methods,
improved seeds and better livestock breeds. The huge food sIturpluses in somn1e
countries illtile temperate regiotns and tle 'greelI revolution' ill the prOductioi
of wheat and rice il some detisely lt)ula tcd tr(opical aniid sub-tr,)Ipical cc untrics
in recent decades attest to,this O)timism. The carth's natutral resources are
u neveiily dist riblited; a fewv Cenun tries (1 soicieties iay have tile capacity to
prIducC enough 'oodIt) :heel the need of* the world's population in the next
few centu ries with It() signit'icact strain ,i nc)-renewable reso)urces. But it
remains f'cim societies and gcivernments to woirk cOut viable systems if' trade and
fOod distribution (Campibell, 1979; Swaminathian, 1983).
Agriculturc may lbe tile basic ccutomic powcr Of a resourcc-rich nation, but150 A. S. R.JUO
to achieve food self-sufficiency in many small, resource-poor countries or
societies in the tropics is another matter. Population control and education are
more fundamental issues than increasing food production if a country or society
wishes to improve the quality of life for its people. Thus, in terms of global
food strategies, an important step toward sustaining growth in food production
may be to concentrate high-yielding grain crop production in ecosystems that
can sustain increase(, intensification, while leaving tile less suitable environments
for the development of alternative but ecologicijlly sound farming systems
(Sioli, 1973; Borlaug, 1983; Simmonds, 1986; Mellor, 1986).
FARMING SYSTEM DEVELOPMENT
State-of-the-art
Historically, new farming systems have evolved when new technologies or
new farming methods became available (Table 1). In early agricultural societies,
modifications or changes in farming systems were primarily driven by the need
for more food and fibre for subsistence. In modern times, the development of
labour-saving farm machinery and the availability of cheap agricultural chemi
cals have made agricultural production in temperate regions a highly com
mercialized enterprise. This is particularly so in societies or countries with a
favourable natural resource base.
Farming systems in the wetter tropics may be illustrated by three simplified
models, the main features of which reflect their natural resource base, their
cultural and social characteristics and, above all, the path of agricultural develop
ment taken during past centuries.
The paddy rice based farming system (Fig. 1) is the system most prevalent in
the wetter regions of tropical Asia. The main features of the system are its high
labour requirement and high output per unit of cultivated area. The high farm
output is achieved by good water control, intensive and systematic multiple
cropp ing and the use of organic manure as well as chemical fertilizer (Harwood
and Price, 1976; Harwood, 1979). However closer examination reveals that this
system is most commonly found in areas with fertile soils, adequate rainfall
Table 1. History of the development of mejor new agriculturaltechnologies
which have led to revolutionary changes in farming systems
Technological development Centre of origin Basis for success
Domestication of plants and animals Centres of early Genetic diversity
civilization
Technology of irrigation and drainage China and Egypt Fertile alluvial soils in large river
valleys and deltas
Green manuring, liming and chemical Western Europe Less weathered soils in prairies
fertilization and loess plains
Engine-powered farm machinery, petroleum- North America and Industrialization and commerciali
based pesticides and herbicides, high yielding western Europe zation
crop cultivars and livestock breedsFarmingsystems in the wetter tropics 151
water- industry
F -wheel Income Labour
powered
inputs n HOS wl and inputs tee
Irigtd ady -74OD 1ntensive
rinrtated bady AND uplands mixed
anuain
relte g s s sl por intercrop
or relay with oINCOME with food and
upland food
crops anda cash crops.
vegetable , in c
region ofdtrOca LD
Multistorey
Lb-rpialAsr
drained fields t homestead
gardens with
tree crops,
vegetables, fish
ponds and small
ruminants.
Fig. 1.Main components of labour-intensive paddy rice based farming systems inthe wetter
regions of tropical and sub-tropical Asia.
and above all, tlecraule air temperature and huidity during thie planting and
harvesting seasons. In less Populated areas with cragile
land (i.e. land eith
steep
slopes, or dominated by acid, kaolinitic or late|'itic soils), shifting cultivation
anlaad bush fallow farming systenms still pre ail.
In tile wetter regions of' tropical Af'rica, the mixed root crop-bush falflow
system is the principal method of food production (Fig. 2). Generally, earm
output is low because of' poor soil resources and the lack of' external nutrient
inputs. Most f'armI Produce is Consumed by the household and any surplus is
sold at local markets (Lageniann, 1977). For sniallholder f'amily f'arms, a high
labour requlil-rment and tile need f'or household f'ood security are f'eatures
common to both the paddy-based system and tihe upland hus-iallow system.
In the
m etter region of Latin America, the dominant f maring systems have
been strongly influenced by tie region's Cultural origin and heritage (Preston,
1980; Possey, 1985). Because of' large land holdings 1)y individuals and corpora
tions, range f'arming of' livestock and tree and cash crop plantations oin fertile
alluvial and volcanic soils remain economically important t'arminlg systems in
tile region (Fig. 3). Such energy- and capital-intensive systemns resemble the
agrohusiness-hased f'arnming systems f'ound in highly industrialized societies in
tile temperate regions in some respects. Although the latter may have a more
(V
I152 A.S. R.JUO
Labour an HOSEOLDL.]u
.Compound farm Distant fields
oil palm Bush fallow
fruit trees (3-7 yrs)
plairtain Cropin
banana AD
FOO (1-2 years)
pineapple ADcassava
cocoyamn INCOME yams
yams maize
maize melon
v.eg ea bles I upland rice _
manure ruminants
MAKE
Fig. 2. Main components of traditional mixed root crop based farming systems in the wetter
regions of sub-Saharan Africa.
Inpts:maciney, HOUSEHOLD OR FAIn us iey
labour, feeds, [- NANAGE1M:NTIN U.. 1| a ees
f e rt ilize r
pesticidesl a tli psicides I
Livestock [ Tree and
range perennial
faringCASH cash crop
INCOME plantations
MAKT
Fig. 3. Main components of plantation and range farming.Farmingsystems in the wetter tropics 15
sophisticated level of farm management, and use more labour-efficient machinery
and more purchased inputs, both systems involve a cash and market oriented
agriculture. On the other hand, there are small family farms in most parts of
the region which continue to practise subsistence agriculture. Most small family
farms in Latin America are found on steep slopes in the densely populated
volcanic areas; the rainforest areas with acid infertile soils are sparsely populated,
Strategies and approaches
Agricultural development In the wetter tropics is faced with a range of strate
gic options. 'Doomsday man', preoccupied by the potential threat of world
population growth, sees an enormous opportunity for converting tropical forest
into farm land. Ecologists and environmentalists on the other hand, forecastinp
long-term ecological and environmental damage as a result of deforestation, call
for conservation of the earth's remaining tropical forest ecosystems.
An overriding fact.,r guiding future agricultural development in the wettez
tropics is probably the fragility of its upland ecosystems, particularly in areas
where acid and kaolinitic or lateritic soils predominate. Agricultural develop.
ment in the region should, therefore, be based on long-term environmental
stability rather than short-term economic returns. Thus, high priority should be
given to developing ecologically sustainable mixed systems involving annual
food crops, trees and perennials on small family farms. Achieving household
security for food and nutrition would remain the most important goal in such
systems; family income would be derived mainly from cash crops and off-farm
activities (Figs I and 2).
However, new farming system development does not explicitly imply invent
ing new systems or component technologies. Human societie- and civilizations
have much to learn from each other if they could only learit selectively. Mankind
has grown increasingly interdependent not only in economic terms, but also in
terms of' natural resources and environmental conservation. Thus, to prevent
further destruction of the earth's remaining tropical forests, there is an urgent
need to develop or introduce new farming systems that are more in harmony
with natural ecosystems and better suited to the cultural heritage of the indi
genous societies in the wetter tropics.
SUSTAINABLE FARMING METHODS
Most agricultural research and development in the wetter tropics during the
past decades has attempted to mimic the enormous success of modern agricul
ture in temperate regions. Considerable effort has been devoted to maximizing
crop yield by mechanization and irrigation and by breeding high yieldiag culti
vary which respond to higher levels of soil management and chemical crop
protcction. Less attention, however, has been giver to developing ecologically
stable and environmentally sound upland farming methods to replace or im
prove the traditional systems of' shifting cultivation. A review of recet litera-
Ii rt' inrf n n t; enni'
c rtnincncr
n nnr-th"t, tc t list - nnr-;e,-Jn
i - tt-t Th,--.."*,154 A. S. R.JUO
described in the following sections. Their economic viability, however, can onl
be judged in relative terms on the basis of existing social and cultural conditior
and the size of the natural resource base of the indigenous societies for whic
they are intended.
Multistorey homestead gardens
Although the homestead garden is among the earliest forms of agriculture, it
subsequent improvement and refinement by some traditional agriculturn
communities in tropical Asia have nearly perfected the system in terms of it
ecological stability and resource use efficiency. The multistorey homestea,
gardens in Sri Lanka and Indonesia provide excellent examples of people'
ability to derive maximum benefits from tropical ecosystems without drasti
cally altering them (Peck, 1982; Michon, 1985; Wiersum, 1983; Marten, 1986)
The multistorey system is essentially a self-sufficient farm unit. It comprise
many cultivated plant species of different heights and architectural type
forming a close cover (Michon, 1985). The upper storey generally consists o
large trees and palns and may produce limber, fruit and nuts as well as pro
viding shade; the middle storey may include coffee, cocoa, fruit trees, plantains
bananas and spice and I'uelwood bushes; and the ground storey may consist o
cassava, yams, taro, sweet potato, maize and vegetables. A simplified model o
the system is shown in Fig. 4.
In such a system, the external inputs are labour, manure, night-soil, anc
Perennials Annual crops
Resource base
* nutrient recycling
* biological N fixation
* soil structural stability
* ecosystem stability
Small ruminants Fish ponds
income household wastes
HOUSEHOLD I
Fig. 4. The multistorey homestead garden: the Asian model.Farmingsystems in the wetter tropics 155
animal feed supplement (household waste) from the farm household. The soil
nutrient pool is maintained by recycling plant residues and manure and through
nitrogen fixation by leguminous annual and perennial crops and trees. Soil
erosion is minimized by maintaining continuous ground cover and good water
infiltration. In Asia, the system is often supplemented by a fish pond and
nearby rice paddy fields. Such a system has for centuries provided the basic
food and nutritional needs of rural households in many densely populated areas
of the wetter tropics.
The system can, however, be modified to give a higher level of output. In
areas where there is a market incentive, purchased inputs such as chemical
fertilizers (e.g. phosphate), line, poultry feed, small farm tools and implements
are used to improve crop yields and labour productivity. Little research and
development effort has been devoted to such mixed small farm systems. Pub
lished work on multistorey hoimestead gardens in Asia is mostly descriptive
and almost exclusively written by geographers, sociologists and anthropologists.
In a large part of the wetter regions of Africa and Latin America, less sophi
sticated multistorey homestead farnm units are common in densely populated
areas such as the high rainfall coastal region of southeastern Nigeria, the volcanic
highlands of central America, western Cameroon and eastern Zaire (Okigbo and
Greenland, 1976; l)enevan, 1980). Farm outputs are generally low.
The traditional African homestead farm, however, may be improved by
introducing a more systematic sequence of planting and spatial arrangement,
and by incorporating small ruminants and fish ponds. The recent introduction
of leguminous fodder trees such as Gliricidiasepium or Leucaena leucocephala
for goats and sheep in homestead farms in southern Nigeria is a good example
of how the system can be improved (ILCA, 1984; Atta-Krah et al., 1986). Much
more research is needed to explore the potential of multistorey farming as a
sustainable component of tropical farming systems.
Alley cropping
In much of the upland agroecosystems in the wetter tropics, shifting cultiva
tion or related bush fallow systems remain the dominant method of food pro
duction. When land is abundant, shifting cultivation, though primitive, is an
ecologically stable form of agriculture. But in areas with increased population
and land pressure, more intensive cropping and shorter periods of fallow have
resulted in severe and degradation due to soil erosion, compaction, loss of soil
organic matter, loss of soil fauna and microbial activity and ultimately, deforesta
tion (Lal et al., 1985;Juo ,nd Kang, 1987). Much effort has been devoted in
recent decades to finding viable alternatives to shifting cultivation. Published
research to date indicates that the recycling of plant residues, judicial use of
fertilizer and systematic inclusion of trees and perennials in food crop systems
are key research issues (Juo and Kang, 1987).
Increasing attention in recent years has been given to the development of
tree and food crop mixed systems in the wetter tropics (Watson, 1983; Rachie,156 A. S. R.JUO
(a) Leucaena alley at end of dry season
I _ 4m --. 1
Leucaena
Maize
Mulch
(b) Pruned Leucaena alley two weeks after
maize emergence
(c) Alley cropped maize at silking stage
Fig. 5. Schematic drawings showing three stages of Leucaena-maize alley cropping.
1983, Okigbo, 1983;Juo, 1985). In west Africa, where non-acid kaolinitic or
lateritic soils (Alfisols and related Inceptisols) are dominant, an 'alley cropping'
system has been shown to allow stable crop yields over a long period of time
(Kang et al., 1984, 1985). In such a system, annual food crops are interplanted
between rows of fast-growing, deep-rooting leguminous trees or shrubs such as
Leucaena leucocephala and Gliricidia sepium. The hedgerows are pruned at the
beginning of the rainy season or before annual food crops are planted. Woody
parts of the hedgerow are saved for firewood or for staking yams and beans in
homestead gardens and leaves and twigs are returned to the soil as green manure
for the food crop (Fig. 5). The recommended spacing between alleys of Leu
caena is 4 m, with 0.5 m within the alley (Kang et al., 1984). The spacings of
the annual crops between the alleys are generally based on standard recommen
dations, or levels of soil fertility and fertilizer input. In the case of maize, four
rows of the crop are usually sown between the 4 m alleys. The leguminous
hedgerow fixes atmospheric nitrogen, recycles sub-soil nutrients, and utilizes
stored sub-soil moisture during the dry season (Kang et al., 1985). Long-term
alley cropping experiments conducted on non-acid soils (Alfisols and associated
Inceptisols) at Ibadan, Nigeria showed that Leucaena produced a total of 7 t ha - '
(dry weight) of green manure which contained approximately 250 kg N, 20 kgFarmingsystems in the wetter tropics 157
P, 185 kg K, 100 kg Ca and 15 kg Mg. These experiments also showed that a
reasonable maize yield of 2 to 3 t ha- 1 could be sustained over a 10 year period
without additional nutrient application. However, for higher maize yields of
4 to 5 t ha - ', improved cultivars and a top dressing of 45 to 60 kg N ha - ' would
be required for each maize crop (Jluo and Kang, 1987). Although Leucaena
yielded 250 kg N ha- , there inay have bcen a significant loss thromigh velatili
zation during decomposition and mineralization because Leucaen, residue is
generally applied to the soil surface as mulch.
The deep-rooted Leucaena facilitates water infiltration. Thus, when estab
lished along the contours on sloping land, Leucaena hedgerows are very effective
at controlling run-off and soil erosion (D. C. Couper, perse aal coin munication).
Alley cropping is now being adopted by farmers in southeast Asia, particu
larlv those cul;ivating land on steep slopes (O'Sullivan, 1985). However, dis
semination of this technology among tile shifting cultivators in the non-acid
soil region of tropical Africa has so far proved a slow and difficult process.
Although favourablc assessment indicates that the m',ize-Lcucaena alley crop
ping system is ready for on-farm adoption, its rapid adoption is mainly preven
ted by unfmaVourable land tenure syst ems in spite of its proven agronomic and
environmental advantages over the traditional bush fallow systems (Ngambeki,
1985; ). S. Ngambki and B. T. Kang, personal communication).
The lcguminotus tree-based alley system may have limited applications on
strongly acidic, calciumldepleted and plhosphorus-deficient soils in high rainfall
regions. Because of their strong acidity (or aluminium toxicity) and lack of
nutrients in the sub-soil most plant roots are concentrated in the surface 10 cm
of the soil, even in the case of woody species tolerant of' aluminium. Thus,
unless inexpensive lime and phosphate sources are readily available, alley
cropping or other more intensive annual food crop poduction systems will
remain uneconomical on acid, upland so ils in the foreseeable future. However,
where lime and phosphates are available, it may be worthwhile in the long run
to invest capital in their use in the first two years to help establish suitable tree
legume species. Subscqucntly, nutrient cycling systems involving hedgerow
p;:,nings, cropping and periodic fallow coulM be established.
Rotation with cover crops
On small fields of less than I ha, where the land is protected from run-off
and erosion by surrounding forests, experimental evidence has shown that
intensive annual food crop systemTs such as maizc-cowpca -1nd upland rice
soyabean rotations involving minimum tillage, residue mulching and the judicial
use of, fertilizers, lime, herhicides and pesticides can sustain crop yields on
highly weathered Alfisols and Ultisols for 10 years or more without the need
to allow the land to revert to bush fallow (lFriessen et al., 1981, Sanchez et al.,
1982;*Jtuo and Kang, 1987). Such intensive faiming technologies could be ex
tended to farmers' fields in many densely populated Ru-al areas, such as Java in
Indonezia, Luzon in the Philippines, Kerala State in India and southeastern158 A.S. R.JUO
Nigeria, providing that the essential inputs were locally available
and their costs
within the reach of the smallholders.
To reduce the cost of inputs and minimize soil degradation
caused by con
tinuous cultivation, systems which include a leguminous
cover crop in the
rotation have been investigated, notably at the International
Institute of
Tropical Agriculture (IITA) in Nigeria and by the Tropical
Soils Research
Programme at Yurimaguas in the Amazonian region of Peru.
On slightly acidic soils, such as kaolinitic Alfisols with pH
values of 5.5 to
6.5, IITA's experiences over a period of 15 years have shown
that systems
involving one year of mucuna (AMucuna utilis) fallow after two
or three years
of cropping with maize, cowpea or cassava are able to sustain
soil productivity
for several cropping cycles on larger fields, from 2 to 20 ha,
depending upon
the slope and soil depth (G. F. Wilson and D. C. Couper, personal
conmunica
tions). Major benefits from mucuna fallow include biological
nitrogen fixation,
the recycling of soil nutrients, the prevention of soil compaction,
the suppres
sion of weed infestation and the maintenance of favourable
soil faunal and
microbial populations. Research is currently being conducted
at IITA to
quantify the contribution of these factors to soil productivity.
Similarly, on acidic soils such as the Ultisols in Yurimaquas,
Peru, a low
input system adopted immediately after 'slash and burn' includes
a one year
kudzu (Puerariaphaseoloides) fallow after three years of continuous
cropping
with upland rice and cowpea. This system was shown to suppress
weeds effec
tively and to supply adequate nutrients to upland rice planted
immediately
after the fallow (Sanchez and Benites, 1987). Both mucuna
and kudzu can
easily be eradicated by slashing and mowing, or by a low application
of a herbi
cide s'.ch as paraquat. The residues can either be burnt or left
on the surface as
mulch.
Some researchers question the feasibility of such systems
on small farms
because farmers do not derive any direct incomc or food from
the cover crop.
But if the long-term benefits of maintaining soil productivity
and sustaining
crop yield are taken into consideration, systems vhich include
a cover crop
fallow are far better than continuous cropping Uuo and Kang,
1987).
Food crop improvement
Research into the improvement of major upland food crops
in the wetter
tropics is relatively recent. In contrast to the highly sophisticated
lowland rice
farming systems in Asia, the cultivation of upland food crops
such as cassava,
yams, sweet potato, taro and upland rice in the wetter tropics
remains a largely
primitive and subsistence form of agriculture. Root crops are
grown in mixed
culture in homestead gardens and surrounding small fields primarily
for home
consumption. Under such systems, the maintenance of yield
stability and
control of pests and diseiir,.:; HC probably far more important
than yield maxi
mization UJuo and Ezumral, 19)88). Notable progress has been
made in recent
years on cassava which is a major staple in the wetter regions
of sub-SaharanFarmingsystems in the wetter tropics 159
Africa. The accidental introduction of two cassava pests from Latin America,
the cassava mealybug (Phenacoccusmanihoti) and the green spider mite (Mono
nychellus spp.), over two decades ago has severely threatened cassava production
in sub-Saharan Africa. However, the problem is now being tackled by research
into the use of two environmentally sound methods: biological control by the
pests' natural enemies and breeding for host plant resistance (Ilahn and Keyser,
1985). Several effective natural enemies of cassava mealybug and green spider
mites have recently been identified in Latin America and these are now the
subject of research into the development of mass-rearing and release methods in
Africa its part of a major effort to improve and sustain cassava production there
(Herren et al., 1987; Neuenschwander et al., 1987).
Another major improvement in cassava in sub-Saharan Africa is the successful
breeding and release of high yielding andi mosiac virus-resistant cultivars at IITA
in collab oration with the region's nati(nal agricultural research centres (Hahn
-1
and Keyser, 1985). The improved cassava cultivars call produce 20 to 30 t ha
Of' fresh tubhers tinder good management and respond to balanced applications
of N and K fertilizer ()n acid soils (juo, 1986). They generally give two to three
times the tuber yield of conmmon local cultivars and are better suited to mono
culture prodluction.
Tree and perennialcrop plantations
In economic terins, there is great potential for the development o0 com-
Inercial productioin of tree and perennial crops on suitable land in the wetter
tropics. Techno logies for the production of tropical tree and perennial crops
such as oil palm, rubber, Coffee, teak, banana, pineapple and cashew are well
known and there has been much progress in plant improvement, but the pro
ductivitv of plantatiln agricultur iin many tropical cotntries has declined
during the post-indepcndence era (Williams et al., 1980). Although a few
nations still maintain viable tree and cash crop production, plantation agri
culture in most tropical countries remains economically unstable. This is
p)rimarily lue to the lack of indigenous management skills, the high costs of
labotr and ptrchased inputs, and an increasingly competitive export market.
Morcover, the rapid and large fluctuations in the international monetary
markets in recent yeais make investment in large tree crop plantations even
less attractive (C. Carlier and J. ter Vrugt, personal communications). The
World Bank's recent emphasis on the development of smallholder tree crop
systems for local use is a plausible alternative to large scale tree crop planta
tions, particularly in rcsoutrcc-poor countries. Thus, the development of tree
and cash crop agricUItre in the wetter tropics is to a large extent an economic
rather than a technical issue and is beyond the scope of this paper.
Livestock rangefarming
Livestock range farming is probably the least desirable farming system for
the wetter tropics both from environmental and economic viewpoints. Stock
are exposed to more severe pest and disease problems, and infertile soils impose160 A. S. R.JUO
nutritional limits to animal production on trropical pastures (Mannet, 1982;
ILCA, 1984). It is also well-known that cattle are less efficient than legumes as
protein converters per unit area of land. Although cattle production is a cul
turally preferred farming system in many societies in the sub-tropical regions
of Latin America, attempts to convert tropical rainforest into range land in
Latin America remain economically unsound and ecologically questionable
(Gliessman et al., 1981). Moreover, even though cattle production in the wetter
tropics can be made economically successful, such development would deprive
millions of nomadic herdsmen and their families in the arid and semi-arid
regions of sub-Saharan Africa of their livelihood.
Some researchers in Latin America have explored the possibilities for inte
grating cattle and tree crop production, and others have indicated the potential
of water buffalo as a source of' meat for the wetter tropics, but sound farming
methods have yet to be developed (NRC, 1981; Payne, 1984). Tree crops and
pastures cannot always be easily incorporated into an existing agroecosystem,
so that homestead production of small ruminants and poultry remains the most
viable livestock option for meeting the nutritional needs of' people living in the
low altitude, wetter tropics.
A farm unit alonga toposequence
Most of the wetter tropics are dominated by rolling or undulating topo
graphy. On such landscapes, the life of the forest dwellers could perhaps be
made more satisfying if' the future development of' farming systems was based
on trees and perennials in a small farm unit situated along a soil toposequence.
The main components of the farm unit might include a farm compound with
small ruminants and a multistorey homestead garden on the mid-slope, a fish
pond with adjacent rice paddies in the valley, and a small tree crop plantation
on the fringe of the protected natural forest. Such farm units would be situated
on the relatively more fertile soils along rivers and major roads so that surplus
produce could easily be collected and transported to local and dbtant markets.
Meanwhile national and international policies could be established to protect
the natural forests occupying the vast inland areas with infertile lateritic soils
from further destruction and exploitation.
The ecology-landscape approach for agricultural development was first
tested in Brazilian Amazonia over three decades ago, but only small scale
schemes founded on a sound ecological basis have succeeded in offering im
proved stability combined with economic value (Sioli, 1973).
CONCLUSIONS
Because of the fragility of the natural resource base, sustainable farming
systems in the wetter tropics rest on a delicate balance between conservation
and utilization. Future agricultural development in the region should give
more emphasis to long-term ecological stability rather than just to short-termFarmingsystems in the wetter tropics 161
economic returns. Much more research is needed to understand the dynamics
of tropical forest ecosystems better and to assess their biological potential for
future agricultural development. Our current knowledge clearly indicates that
predominantly acid and nutrient-depletcd soils in the low altitude wet tropics
are unsuited to energy-intensive and market-oriented food crop agriculture.
However, to meet the food and nutritional needs of the indigenous population,
there is enormous potential for the development of more productive and stable
multistorey homestead gardens and mixed systems including tree, perennial
and annual crops. The wisdom of the various options for future agricultural
development in the wetter tropics may be illuminated by the old Asian proverb:
'If you plan for one year, plant rice; if you plan for ten years, plant trees; if
you plan for it hundred years, educate mankind'.
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