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Chapter 2
Potential for sustainable agroforestry and alley farming in tropical Africa

B.T. Kang, Soil Scientist, International Institute of Tropical Agriculture, Ibadan, Nigeria

Trees have commonly formed an important part of Africa's traditional farming systems. In the bush-fallow, slash-and-burn cultivation systems, farmers exploited the potential of trees and shrubs for soil fertility regeneration and weed suppression during the fallow phase and widely used them as source for staking material later during cultivation. Trees and shrubs are also widely grown in cropped areas particularly in compound farms. They are mainly used for soil fertility maintenance and as auxiliary sources of food, fodder, fuelwood and for other domestic needs.

Sub-Saharan Africa is undergoing extensive environmental degradation for various reasons (United Nations Environmental Programme 1985; McNamara 1990; Okigbo, 1989). This is manifesting itself as rapid deforestation, thinning of tree cover in wooded fallow, the creation of grasslands, soil erosion, nutrient depletion and declining soil productivity. These aspects have arisen mainly from short-sighted exploitative farming methods with limited development effort and sustenance perspective. Under pressure of increasing populations, migration and the cultivation of inappropriate lands, traditional farming becomes a major contributor to environmental degradation in developing countries. Because yields in many traditional systems are low, pressure to clear new land for farming continues to increase. In some areas, however, traditional farmers already responded to the need to sustain production by selectively incorporating multipurpose woody species which they consider useful and effective in enriching the production system biologically (Obi and Tuley 1973; Okigbo 1976; Getahun et al. 1982; Kang and Ghuman 1991).

Despite the importance of woody species in traditional farming systems, only limited information is available on the potential contribution, when used in agroforestry systems, of multipurpose woody species to the improvement and sustainance of crop production in the uplands of the humid and sub-humid tropics. To improve traditional systems, alternative farming methods with improved soil management technologies are being investigated by various research institutions worldwide. Emphasis is being given to technologies that promote sustainability and serve as intermediate steps to more permanent cultivation through the use of soil conservation, crop rotation, and intercropping techniques. There has also been increased awareness and research effort during the past two decades in the development of more sustainable and productive land use systems by intercropping selective woody (particularly nitrogen-fixing legume) species with food crops in agroforestry systems. The potential of some of these agroforestry systems for Sub-Saharan West Africa has been highlighted by Kang and VandenBeldt (1990).

Common agroforestry systems in the tropics (Nair 1990)

Humid lowlands Semi-arid lowlands Highlands
Shifting cultivation Silvopastoral systems Soil conservation hedges
Taungya Windbreaks/shelterbelts Silvopastoral combinations
Homegardens Multipurpose trees for fuel/fodder Plantation crop systems
Plantation crop combinations    
Multilayer tree gardens Multipurpose trees for farmlands  
Intercropping systems    

fig2.gif (53562




The concept of agricultural sustainability has received much attention recently. It means, however, different things to different people. Agricultural sustainability and productivity have been a goal of the Consultative Group on International Agriculture Research (CGIAR) from its inception in the 1970s (CGIAR 1990). Since agricultural sustainability is a dynamic concept, CGIAR has defined sustainable agriculture as "The successful management of resources for agriculture to satisfy changing human needs, while maintaining or enhancing the quality of the environment and conserving natural resources". A comprehensive definition reached by the American Society of Agronomy describes sustainable agriculture as agriculture that "over the long-term enhances environmental quality and the resource base on which agricultural depends; provides for basic human food and fiber needs, is economically viable, and enhances the quality of life of farmers and society as a whole" (ASA 1989).

In the simplest sense, sustainability is the ability of a production (cropping) system to produce a stable annual yield of the desired crop over a long period of time. The length of time acceptable as a measure of sustainability is undefined and could be set arbitrarily for each particular environment and set of socioeconomic circumstances.


There is still no general consensus about what agroforestry is. Lundgren and Nair (1985) use agroforestry as a collective name for land use systems and practices where woody perennials are deliberately used on the same land management unit as agricultural crops and/or animals, in some form of spatial arrangement or temporal sequence. Agroforestry thus encompasses many land use systems that have been practised for a long time in the tropics, including shifting cultivation and bush fallow systems. Major types of agroforestry systems are shown in Table 5.

Traditional agroforestry systems have evolved with time in response to population pressure and depletion/degradation of non-renewable resources. The levels of productivity that can be achieved reflect the potential and the degree of the management of the resource base. High productivity comes only from systems where management intensities that are necessary for sustainability are attained without extensive depletion of resources. The possible evolution of traditional agroforestry, from a simple rotational sequence of temporal agroforestry to the intensive and complex multistorey system, was described by Kang and Wilson (1987). A similar description of intensification of shifting cultivation is also given by Raintree and Warner 1986 (see Figure 2).

Although agroforestry is an age-old practice it is a relatively new science. It is very descriptive and lacks suitable experimental methodology and data analysis procedures. Since subsistence agriculture systems in several developing countries are ecologically (and often economically) no longer sustainable, there is increasing interest in using more productive agroforestry techniques as a practical alternative for food crop production.

The sustainability and extent of soil productivity improvement in agroforestry systems depends on many factors including site characteristics, plant species and cultivar, cropping pattern and management factors. However, much of the quantitative data on effect of trees on soil properties is based on natural systems and the effect of agronomic practices are extrapolated from crop production data (Sanchez, 1987). Knowledge of the interactions between trees, crops and environments is still limited. There are a few good examples from the humid and semi-arid tropics that show definite effects of woody species on micro-site soil enrichment, protection and crop yield.

According to Nair (1990) agroforestry has the following characteristics:

The most distinctive elements of agroforestry are:

  1. the inclusion of multipurpose woody species (shrubs and trees). The perception of trees as multipurpose plants is based on their productive attributes (food, fodder, fuelwood, staking material), and their protective and service functions (soil conservation, soil fertility improvement, weed suppression), and
  2. the better use of resources compared with single crop or forest production systems. For example, in a crop production system we are mainly concerned with the upper soil profile, while agroforestry exploits the entire soil profile.

It is now widely believed that agroforestry has considerable potential as a major land-management alternative for conserving the soil and maintaining soil fertility and productivity in the tropics. More work is needed to confirm this belief and to exploit its potential better.


Research in various parts of the humid tropics during the past few decades clearly indicates the important role that soil organic matter and other biotic factors play in maintaining the productivity of fragile uplands dominated by low activity clay soils, which cover large areas of the humid and sub-humid zones of Sub-Saharan Africa. To deal with the unique management problems of these soils, scientists at the International Institute of Tropical Agriculture (IITA) in the 1970s incorporated woody species in crop production systems. This ultimately led to the development of the alley cropping system (Kang et al. 1981). In alley cropping, food crops and woody species are intercropped, food crops are grown in the alleys formed by hedgerows of planted trees and shrubs, preferably legumes. The hedgerows are cut back at planting and periodically pruned during cropping to prevent shading and to reduce competition with the food crops. The prunings are used as green manure or mulch. The hedgerows are allowed to grow freely to cover the land when there are no crops. Alley cropping retains the basic features of the traditional fallow system, integrates "the art and wisdom of traditional farmers" with "the efficiency of current science" and can thus be considered as an improved bush fallow system. One major advantage of alley cropping is that the cropping and fallow phases take place concurrently on the same land, thus allowing the farmer to crop the land for an extended period without a break. Though alley farming was originally designed for use by small farmers, it is believed that it is sufficiently flexible to be adapted for mechanized farming using appropriate machinery. The International Livestock Centre for Africa has extended the concept of alley cropping to include livestock by using a portion of the hedgerow foliage for animal feed, calling the resultant system alley farming (Okali and Sumberg, 1985). The development of alley farming systems has recently been reviewed by Kang and Wilson (1987) and Kang et al. (1990).

Choice of Multipurpose Woody Species

Potential woody species for alley farming in lowland (<750 m above sea level) humid and subhumid tropics

Non-acid soil Acid soil
Acacia auriculiformis

Cajanus cajan*

Gliricidia sepium

Leucaena leucocephala

Acioa baterii

Calliandra calothyrsus

Flemingia macrophylla

Paraserianthes falcataria

* Needs frequent replanting

Success of alley farming system depends on: the right choice of woody species; successful hedgerow establishment, and proper hedgerow and crop husbandry. Desirable multipurpose woody species should be:

  1. easy to establish;
  2. coppiced easily;
  3. able to withstand repeated prunings, and
  4. deep rooting.

In addition species should be suited to local soil conditions and climate. In Table 6 is shown a list of potential multipurpose woody species that can be used for alley farming on acid and non-acid soils in the lowland tropics. Different sets of species will be required for drier areas and the highlands.

Vegetation Management in Alley Cropping

The increasing adverse effects of weeds on crops with continuous cropping under declining soil fertility following bush fallow clearance is well known (Kang et al. 1977). An effective way of controlling weeds is through canopy closure by shading the undergrowth. Traditional farmers have practised this approach effectively in the wooded fallow. The same concept is also utilized in planted fallow and agroforestry systems.

Woody species such as Gliricidia have been used successfully for reclaiming Imperata-infested lands (Wiersum and Dirdjosoemarto 1987). Aken'Ova and Atta-Krah (1986) also showed that uncut Gliricidia hedgerows are effective in controlling Imperata. In the forest savanna transition zone, alley farming with Gliricidia and Leucaena even with annual cropping reduced Imperata infestation as compared to no tree control plot (B.T. Kang and Y. Osinubi, unpublished data).

In the forest zone Yamoah et al. (1986) also reported lower weed yields under two years of uncut hedgerows of Gliricidia, Cassia siamea and Flemingia. In alley farming trials using hedgerow combinations of Leucaena and Acioa, Siaw et al. (1991) observed no difference in weed biomass yield between alley farmed and control treatments. There was however a significant shift in weed composition in plots alley farmed with Leucaena and Acioa compared with the control plot.

Weed suppression in alley farming appears to be due to shading, mulching or allelopathy or a combination of these factors.

Biomass and nutrient yields of woody species from five prunings of hedgerows1, Ibadan, southwestern Nigeria (B.T. Kang, unpublished)

Species Dry matter2
(t ha-1yr-1)
Nutrient yield
    N P K Ca Mg

(kg ha-1yr-1)

Acioa baterii

Alchornea cordifolia

Gliricidia sepium

Leucaena leucocephala

























1Interhedgerow spacing of 4 m
2 Wood harvest not included

Effect of six years of alley farming on properties of surface soil, runoff, soil loss and maize yield on a Luvisol with 7% slope
(Kang and Ghuman 1991)

Treatment pH
Organic carbon
Exchangeable Runoff**
Soil loss**
(t ha-1)
Maize*** yield
(t ha-1)
      K Ca Mg      

(cmol kg-1)

Control (without hedgerows)                  
Alley cropped and tilled                  
2 m - Gliricidia
4 m - Gliricidia
2 m - Leucaena
4 m - Leucaena

* Interhedgerow spacings, 2 and 4 m
**Measured during first season (March-July 1988).
Total amount of rainfall - 704.2 mm
***Fertilizer rate 75 N - 20 P - 30 K in kg ha-1

Nitrogen yield from hedgerow prunings during one maize crop. N uptake by the alley farmed maize and estimated N gain from hedgerows to the system (Kang 1988)

Woody hedgerow N yield from pruning1 N uptake by maize Estimated N gain2 Maize grain yield

------------ kg ha-1 ------------

kg ha-1



Acioa baterii
Alchornea cordifolia
Gliricidia sepium
Leucaena leucocephala

1Not including N removed with wood harvested
2Figures in brackets give percentage N utilization from prunings

Effect of Alley Farming on Soils and Crops

Leguminous species such as Leucaena and Gliricidia grown in hedgerows in alley farming can yield large quantities of biomass and nutrient yield as compared to non legumes such as Acioa or Alchornea cordifolia (Table 7). Repeated additions of prunings can have a profound effect on soil properties. As shown in Table 8, alley-farmed plots with Leucaena and Gliricidia have higher soil organic matter and nutrient status than in tilled control plots. Alley farming also reduces runoff and soil erosion compared with control plots. Runoff and erosion is reduced by the physical barrier of the hedgerows, and also by the better physical condition of the soil under the hedgerows, resulting from higher faunal (earthworm) activity, which increases water infiltration.

Despite the high nitrogen yield from Leucaena and Gliricidia prunings, it is inefficiently used by the associated crop. It is estimated that the N contribution from prunings of these two species is about 40 kg ha-1 (Table 9) to the associated maize crop. Nitrogen utilization by the crop can be improved with

  1. better synchronisation of nitrogen supply and nitrogen use by the crop,
  2. better placement, and
  3. correct incorporation of prunings in the soil. No information is available on the value of the other nutrients in the prunings to associated crops.

Little information is available on the interactions between the hedgerow and crop. Kang et al. (1985) have shown that maize and Leucaena hedgerows do not compete for soil moisture, as the hedgerows use moisture from lower depths in the soil than the crop. On non-acid soils in the humid zone, competition between the woody hedgerows and the crop is mainly for light. In alley cropping with Leucaena, maize grown adjacent to hedgerows shows poorer performance than elsewhere in the plots because of the shading effect, especially where soil fertility is high. Where fertility is low, the shading effect is counteracted by the nutrient contribution from the litter fall near the hedgerows. Thus maize grown adjacent to hedgerows performs better (Fayemelihin 1986).

The below-ground interactions between crops and woody species are gradually receiving more attention. The effects of root turnover, water and nutrient competition in various soil types and climatic conditions need to be further researched.

A key aspect of alley farming is sustainability of yields. As shown in Table 8, under alley farming yields of maize in the seventh cropping year were higher than in the control till and no-till treatments. Results of a long-term alley farming trial with Leucaena carried out on low fertility sandy soil show that maize yields can be sustained at acceptable levels with and without N applications (Table 10). Short fallows of 1-2 years duration in alley farming can also enhance soil fertility (Atta-Krah 1990).

Some progress has been made in adapting the technology for acid and low-base-status soils. The main problem is the selection of suitable woody species that can do well in acid soils with poor nutrient supply. Alley farming studies on acid and low-base-status soils in eastern Nigeria have shown good promise with species such as A. barterii and Flemingia macrophylla. In traditional fallow systems at Mbaise in Imo State of south-eastern Nigeria, farmers have already practised some aspects of alley farming using Acioa on acid soils for a few generations (Kang et al. 1981). The hedgerows in alley farming besides providing prunings for mulching, green manure and fodder also have auxiliary uses as stakes and firewood (Kang et al. 1990).

Grain yields of main season maize (t ha-1) in maize-cowpea rotation grown on a sandy Apomu soil in alley farming with Leucaena at Ibadan, Nigeria, from 1979-1988 (Kang et al. 1990)

(kg h-1)
1979 1980 1981 1982 1983 1984 1986 1987 1988

Hedgerow prunings retained


Hedgerow prunings removed

0 - 1.0 0.5 0.6 0.3 0.7 0.7 0.6 0.4
LSD (5%) 0.4 0.3 0.3 0.4 0.4 0.5 0.2 0.3 0.4

Most of the work on alley farming had been done in the humid and sub-humid zones, its potential for the drier zones has not been fully examined and needs further research.

Despite the intensive research on alley farming by various institutions over the past decade, there are many research gaps for station and on-farm conditions. The research needed is on

  1. selection of multipurpose woody species, particularly for acid soil conditions,
  2. fine tuning of hedgerow and associated crop management practices,
  3. the interactions of crops with hedgerows especially below ground,
  4. the adaptability and adoptability of the system for various agro-ecological and socio-economic conditions.

Part of this work is currently being undertaken by the African Alley Farming Network (AFNETA).

Research and on-farm observations have shown that alley farming can have the following main benefits:


Trees are traditionally part of African farming systems. Where properly integrated they can be an essential element in improved and more productive food crop systems. The potential of alley farming as a sustainable food production system for the humid and sub-humid tropics is described as an example. Other traditional and improved agroforestry systems already exist in Africa, for example, the home gardens and taungya systems in the humid tropics and the use of strips of perennial vegetation, tiered parklands and plantations, and various silvopastoral systems in drier areas. Further research is needed to study the sustainable elements of these and other systems and to assess how to increase their stability and productivity.

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