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    About UTA . . .The International Institute of Tropical Agriculture (IITA) is an autonomous, nonprofit corporationgoverned by a I5-memb er board of trustees headed by Dr . John J. McKelvey Jr. of th e United Statesof America. The Tnstitute's chief executive officer is Director General Dr . Ermond H. Harrmans.

    l ITA seeks to develop alternatives to shifting cultivation that will maintain the productivity of theland under continuous cultivation in the humid and subhumid tropics; to develop higher yieldingpest an d disease resistant varieties of cowpeas, yams an d sweet potatoes worldwide, and of maize, rice,cassava and soybeans in Afril:a, and to strengthen national agricultural research systems by acomprehensive training program and collaborative research.

    l ITA was established in I967 hy the Ford and Rockefeller Foundations, which provided th e initialcapital for buildings an d development, an d the Federal Military Govern ment of Nigeria, who allotted1 ,000 hectan:s of land for a headquarters site seven kilometers north of Ibadan.

    l ITA is one of 13 nonprofit international agricultural research an d training centers supported bythe Consultative Group for International Agricultural Research (CGIAR). The CGIAR is supportedby th e Food and Agriculture Organization of th e United Nations (FAO), the International Bank forRec..:onstruc..:tion and Development (World Bank) and th e United Nations Development Programme(UNDP). The CGIAR consists of about 50 donor c..:ountries, international and regional organizationsan d private foundations.IITA receives support through th e CGIAR from a number of donors including Australia,Belgium, Canada, Ford Foundation, France, Federal Republic of Germany, India, World Bank,International Fund for Agricultural Development (IFAD), Italy, Japan, The Netherlands, Nigeria,Norway, Organization of Petroleum Exporting Countries (OPEC) Fund for AgriculturalDevelopment, United Kingdom and the United Statcs of America. In addition, other donors providefunds to l ITA to support specific research an d training programs.

    Cover: Exper imental field to study the effects of hedgerow heightand time of pruning on crop yield.

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    ALLEY CROPPINGA Stable Alternativeto Shifting Cultivation

    By

    B.T. KangSoil Scientist

    G.F. WilsonAgronomist

    T.L. LawsonAgroc limatoiog isl

    ln tcrnationalln stirute of Tropical AgricultureOyo Road, PMB 5320, Ibadan, Nigeria

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    ForewordIn most parts of he tropics, especially in tropical Africa, thereis a critical need to increase food production to meet thedemand of a rapidly increasing population.

    On e of the challenges presented to the InternationalInstitute of Tropical Agriculture from its beginning hasbeen to develop alternatives to the centuries-old shiftingcultivation and bush fallow production systems predominantin tropical regions. Such traditional systems are effectivegiven an unlimited amount ofland and labor. Th e fertility andproductivity of tropical soils have been maintained by shortperiods (I to 3 years) ofcultivation followed by long periods ofrestorative fallow (bush).Problems arise when the supply of land and labor areno longer unlimited; when rapidly growing and rapidlyurbanizing populations pu t new and heavy demands on thefood production system.

    On e obvious response is to increase the cropping periodan d decrease th e fallow period, keeping more land undercultivation at a given timt!. Bu t this is not as simple as itappearS. The fragile tropical soils do no t respond well totemperate climate farming methods based on the use of heavymachinery and expensive agrochemicals, which often leave

    the land in poorer condition than does a heavily used bushfallow system. In an attempt to incorporate the good featuresof bush fallow into a continuously productive farming system,scientists at !ITA have developed a production system fortropical agriculture called allt!y t.:ropping. This is an agroforestry system that involves growing food crops in alleysformed by hedgerows of leguminous trees or shrubs.

    This exciting new development of integrating the ar t orknoWledge developed over the centuries by the small tropicalfarmer with modern science or technology} we believe, hastremendous potential for feeding the increasing populationsan d simultaneously stabilizing tropical soils for futuregenerations.!ITA scientists B.T. Kang, G.F. Wilson and T.L. Lawson,who pioneered this concept eight years ago and carefullyresearched its many phases, have prepared this technicalbulletin for the information of scientists, teachers, techniciansan d farmers.Ermond H. HartmansDireCtor GeneralInternational Institute of Tropical Agriculture

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    IntroductionIn many parts of the humid and subhumid tropics,particularly in Africa, shifting cu ltivation with therelated bu sh-fallow slash-and-burn cultivation is st illthe dominant food crop production system. In thissystem short (one to two years) cropping periodsalternate with long (six or mo re years) fallow periods.This fallow restores soil ferti lity and rids the land ofmany noxious weeds, pests and diseases. A large area ofthe humid and subhumid region is dominated by lowactivity clay (LA C) soils. The se soils are characterizedby low effective cation exchange capacity, low availablewater and nutrient reserve, and are highly susceptible tosoil erosion (Kang and Iuo, 1981).

    The restorative power of the bu sh fa llow is linked tothe regrowth ofdeep rooted trees and shrubs that recycleplant nutrients and build up soil organic matter (Nyeand Greenland, 1965). During the fallow period plantcover and Ii tter protect the soil from the impact of highintensity raindrops and the roots help to bind the soils,increase water infi ltration and reduce runoff and soilerosion. Moreover, litter mulch and shading by tree andshrub canopies reduce soil temperature and maintainsoi l moisture conditions that are favorab le for thegrowth of beneficial soil macro- and microorganisms .'The humid zone is defined as areaswith precipitationequal to orgreater thanpotential evap otranspiration for six [ 0 eight months of the year. In thesubhumid zone, precipitation is equal to or greater rnan potentialevapotran spiration for fo ur to five months of th e year.

    Figure 1. Traditional farm plot near Onne in southeastern N igeriashowing mixed cropping of maize an d ya m growing onnewly cleared land after 7 years of bush fallow. In-situgrown Am liollala macrophylla stakes are used for stakingyam.This shading also reduces weed infestation .

    In addition to restoring so il fertility, the bush fallowprovides supplementary food , animal feed, stakingmaterial, firewood and herbal medicine (Okigbo, 1983).Where land is abundant the bush fallow has beenfound to be a stable and efficient biological method forsoil productivity restoration. Food crops grow well onnewly cleared land following a long rest period un der

    bush fallow, as illustrated in the example from south-

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    Figure 2. Alley cropping maize with Leucae l/a Iwcocepllala at UT A .

    eastern N igeria shown in Figure 'r.In creasing land pr ess ure, resulting from rapidpopul ation growth in many part s of th e tr opics, hasresu lted in a shortening of th e fallow periods, Over-exploitation of land dominated by highly wea th ered

    kaolinitic soils can easily lead to soil degradation, arapid decline in crop yield and invasion by noxiousweeds, including difficul t to cont rol grass species .Since farmers in many develop ing countri es in the

    tr opics cannot afford cos tly inputs, it is necessar y to2

    develop a low input so il ma nagement technology tbatcan sustain crop product ion . On e promising techniqueis alley c ropping.T his bulletin desc ribes th e basic principles andresults of six years of alley cropping research cond uctedmainly at th e In ternational In stitute of Tropica lAgr icultu re (II TA) in Ibadan, N iger ia .

    The Alley Cropping Food ProductionMethodAlley cropping is essentia lly an agrofores tr y system inwhich food crops are grown in alleys form ed byhedgerows of trees or shrubs (Kang et ai" 1981b;Wilson and Kang, 1981). The hedgerows are cut back atplanti ng and kept pruned during cropping to preventshading and to redu ce comp etition with food crops (F ig.2). When there are no crops , the hedgerows are allowedto grow freely to cover th e land.Alley cropping retains the basic featu res of bu shfa llow. I t can easily be adopted by resour ce-poorfa rmers in the tropics , Trees and shrubs in th e aileysys tem : Provide green manu re or mulch (Fig. 3) fOr"comp anion food crops. In this way plant nutrientsare recycled from deeper soil laye rs, Provide prunings, applied as mu lch, an d shade

    during the fa llow to sup ress weeds.

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    Figur e 3. Mulch cover from Acioa ba rteri; prunings.

    Prov ide favo rable conditions for so il macro- andmicroorganisms. When planted along th e contours of sloping land ,p rov ide a barr ier to contro l so il erosion. Prov ide prunings for b rowse, staking material and

    firewood . Provide biolog ically fixed nitroge n to the compan

    IOn crop .3

    The majo r advan tage of a lley cropping over th etraditional shifting cultivation and bu sh fallow sys tem sis th at th e cropping and fa llow ph ases can take placeconcurr ent ly on the same land, thus allowing the farm erto crop for an ex tend ed period w ithout returning th eland to bu sh fa llow.

    Tree and Shrub SpeciesA number of trees and shrubs are po tentia lly sui ta ble foralley cropping, but onl y a few have been tested . T