section 5
Research findings and proposals

Chapter 14
The development of alley cropping as a promising agroforestry technology

B.T. Kang and G.F. Wilson
Soil Scientist and Agronomist, respectively
International Institute of Tropical Agriculture (IITA)
P.M.B. 5320, Ibadan, Nigeria

Introduction

Shifting cultivation and related slash-and-burn cultivation systems are still the dominant land-use systems in vast areas of the tropics. These fanning systems extend over 25 percent (360 x lOTia) of the exploitable tropical lands (Saouma, 1974).

These traditional food-crop production systems are based largely on the restorative properties of woody species. A typical example of the system is shifting cultivation involving partial clearing of the forest or bush fallow in the humid zone, or patches of grass and scattered trees in the subhumid zone, followed by flash burning of the vegetation (for seedbed preparation and partial release of nutrients) and short-term intercropping (Allan, 1965). The cropping period is marked by a random spatial arrangement of crops and "regrowth" of woody perennials. This rotational sequence of temporal agroforestry (Nair, 1985), with long fallow periods that allow regeneration of soil productivity and weed suppression, has sustained agricultural production on uplands in many parts of the tropics for many generations.

A recent survey of traditional agriculture in the humid and subhumid zones of southern Nigeria showed that tree-crop-based systems predominate (Getahun et al, 1982). Bene et al. (1977) have pointed out that in most tropical zones food crops and trees do well in combinations. Watson (1983) also stressed the importance of combinations of perennials and food crops in ensuring stable production and satisfactory income for subsistence farmers in the humid tropics. However, due to various socio-economic factors, particularly rapid population growth, these traditional systems have undergone rapid and drastic changes over the past few decades. In tropical Africa, for example, with an annual population growth rate of 3.1 percent (McNamara, 1984), the current population of about 500 million is expected to exceed 900 million by the year 2000. There may not be adequate land to maintain the long fallow that is essential in traditional shifting-cultivation systems. As shown by the examples cited by Prothero (1972), high population pressure has destabilized many traditional production systems: the need for more food has increased deforestation, shortened fallow periods in shifting cultivation cycles, and set in motion a degradative spiral leading to reduced productive capacity of the land and decreased crop yield. In addition, indiscriminate fuelwood gathering, timber harvesting, and grazing have aggravated land degradation in many parts of the tropics (Bene et al, 1977; Poulsen, 1978; Gorse, 1985).

To meet the ever-increasing demand for food in the tropical and subtropical (developing) countries, more land must be brought under cultivation (Dudal, 1980). This is feasible for much of Africa and Latin America where only 18 and 19 percent, respectively, of the potentially-arable lands are under cultivation (IPI, 1986). This will, however, provide only a temporary solution to the food-production problem if it is not followed up by viable and sustainable food-production technologies.

The development of technologies for increasing food production through increase in land productivity thus presents a challenge to scientists. This will involve developing, for the humid and subhumid tropics, highly productive farming techniques that are ecologically sound, economically viable and culturally acceptable.

Soil management problems in the humid and subhumid tropics

Large parts of the humid and subhumid tropics that are currently under shifting cultivation and related traditional fanning systems are covered by "fragile" soils. These are predominantly Ultisols, Oxisols and associated soil types in the humid tropics, and Alfisols and associated soils hi the subhumid tropics (Table 1). Many of these soils are grouped as low-activity clay (LAC) soils because of their limitations, unique management requirements and other distinctive features that adversely affect their potential for crop production (Juo 1980, 1981; Kang and Juo, 1983).

During the past few decades several institutions in the tropics have been actively engaged in determining the constraints and management problems of these upland soils relative to sustainable food-crop production. The results of these investigations (Charreau, 1974; Lal, 1974; IRRI, 1980; Sanchez and Salinas, 1981; Kang and Juo, 1983; Spain, 1983; El-Swaify et al., 1984) and some of the conclusions are highlighted below. Ultisols and Oxisols have problems associated with acidity and Al toxicity, low nutrient reserves, nutrient imbalance and multiple nutrient deficiencies. Ultisols are also prone to erosion, particularly on exposed sloping land. Alfisols and associated soils have major physical limitations. They are extremely susceptible to crusting, compaction and erosion, and their low moisture-retention capacity causes frequent moisture stress to crops. In addition, they acidify rapidly under continuous cropping, particularly when moderate to heavy rates of acidifying fertilizers are used.

Table 1 Geographical distribution of soils in the humid and semi-arid tropics (million hectares)

Where land is abundant, long fallow periods facilitate restoration of soil productivity, resulting in low productivity but biologically stable production systems. The approach for maintaining the desired soil physical conditions is appropriate management of the surface soil through the use of residue mulch and minimum tillage (Lal, 1974). Loss of nutrients during cropping can be compensated for by judicious chemical inputs (Kang and Juo, 1983; Nicholaides et al, 1984), but, due to the inherent low exchange and buffering capacities of LAC soils, maintenance of adequate levels of soil organic matter and judicious crop-residue management play important roles in sustainable crop production (Ofori, 1973; Sedogo et al., 1979; Lal and Kang, 1982; Kang and Juo, 1983). An integrated soil fertility management system, combining the use of chemical soil amendments and biological and organic nutrient sources, will, therefore, be the most desirable nutrient-management system for these LAC soils.

The role of planted fallow in sustainable crop production

Levels of productivity that can be sustained in cropping systems largely reflect the potential and degree of 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 the resources. Evolutionary trends in tropical cropping systems show that management intensities capable of sustaining productivity are usually introduced only after considerable depletion and degradation of resources — especially the non-renewable soil — have taken place. Conservation methods such as use of planted fallow and other agroforestry approaches are seldom practised, and, where they are practised, they have been introduced only after long periods of marginal land management at low levels of energy input.

The important role of the fallow period for soil-productivity regeneration in traditional shifting cultivation is well known (Nye and Greenland, 1960). The fertility of the soil which is depleted during the cropping period is regenerated during the fallow period. The rate and extent of soil-productivity regeneration depend on the length of the fallow period, the nature of the fallow vegetation, soil properties and the management intensity. During the fallow period, plant nutrients are taken up by the fallow vegetation from various soil depths according to the root ranges. While large portions of the nutrients are held in the vegetational biomass, some are returned to the soil surface or lost through leaching, erosion and other processes. In addition, during the fallow period the return of decaying litter and residues greatly add to the improvement of soil organic matter levels.

From the various descriptions of tropical cropping systems (Ruthenberg, 1979; MacDonald, 1982; Benneh, 1972), a framework for a logical evolutionary pathway of traditional crop-production systems in the humid tropics can be developed, as shown in Figure 1. This pathway highlights the major changes and indicates points at which intervention with planted fallows or other agroforestry methods could be introduced, and thus further resource degradation prevented. Raintree and Warner (1986) have also recently described the various agroforestry pathways for the intensification of shifting cultivation.

Figure 1 Stages in the evolution of managed fallow and multistorey cropping complex in the humid tropics.

The pathway depicted in Figure 1 begins with a stage that may be described as a simple rotational sequence of temporal agroforestry. It is characterized by a very short cropping period followed by a very long fallow period. In this fallow period even "inefficient" soil-rejuvenating plant species are able to restore soil productivity. Here the economic return to the input of labour or energy is high; the management input is low and is confined to the cropping period. In the second stage that usually results from population pressure, the cropping period and the area cultivated are expanded. Returns to energy input begin to fall and management intensity increases. At this stage there is an awareness of the contribution of the different species in the fallow system (Benneh, 1972). At the third stage attempts are made to manipulate species in the fallow in order to ensure fertility regeneration in the already shortened fallow period. A good example of this third stage is the retention and use of tree species such as Acioa barterii, Alchomea cordifolia, Dialium guineense and Anthonata macrophyla as efficient soil-fertility restorers (Obi and Tuley, 1973; Okigbo, 1976; Getahun et al., 1982). Farmers near Ibadan, Nigeria have observed that Gliricidia septum, when used as yam stakes, grew and dominated the fallow and restored the land much quicker than did other species. Consequently, they now maintain G. sepium in the fallow even when yam is not included in the cropping cycle. In the fourth stage, mere manipulation of fallow and sole dependence on natural regeneration for the establishment of the desired species are no longer adequate, and a planted fallow of selected species becomes necessary. Though the value and feasibility of planted fallows have been demonstrated experimentally (Webster and Wilson, 1980), the practice has not become widespread. This is the stage at which intervention of techniques such as alley cropping (Kang et al., 1981; Wilson and Kang, 1981) and in-situ mulch (Wilson, 1978) can take place.

At each of these successive stages, length of the cropping period extends progressively and that of the fallow diminishes correspondingly. During these extended cropping periods soil degradation continues, and the damage done cannot be repaired by the shortened fallow. Even when the most efficient soil-rejuvenating species dominate the fallow, they can only sustain yield at a level supportable by the existing resource base.

The fifth (merging of cropping and fallow phases) and sixth (intensive multistorey combinations) stages could evolve from the previous stages, but there is no clear evidence for this. In many areas where multistorey cropping, an intensive agroforestry system with trees and crops (Nair, 1979; Michon, 1983), dominates there is no evidence of stages four and five. The most plausible explanation is that, as population pressures grow, and the area available for stage III shrinks, that of stage VI (which is actually the intensively-managed homegardens where fruit trees are always among the major components) expands. As the two stages merge, the more efficient homegarden undergoes modification which results in the development of the multistorey production system.

If one follows the above evolution pattern, sustainability with high productivity can be achieved when conservation and restoration measures are introduced before resources are badly degraded or depleted. In the humid tropics, the multistorey complex which seems to be the climax of cropping-systems evolution, would be the ideal intervention at stages I or II. However, this may not be possible in all cases. Consequently some other types of agroforestry system, such as the planted fallows, are necessary.

Early attempts to use planted fallow in the tropics were dominated by the use of herbaceous legumes for production of .green manures (Milsum and Bunting, 1928; Vine, 1953; Webster and Wilson, 1980). Though many researchers reported positive responses, the recommendations were never widely adopted. Later studies indicated that green manuring with herbaceous legumes was not compatible with most tropical climates, especially in areas with long dry periods which precede the main planting season (Wilson et al, 1986): most herbaceous species did not survive the dry season and thus did not have green matter to contribute. However, herbaceous legumes such as Pueraria phaseoloides, Centrosema pubescens, Calopogonium mucunoides and C. caeruleum are widely used as ground cover in the tree-crop plantations in the humid regions (Pushparajah, 1982),

Following the introduction of herbicides and no-till crop establishment in the tropics, some of the cover crops such as Mucuna utilis, Pueraria phaseoloides, Centrosema pubescens and Psophocarpus palustris were found capable of producing in-situ mulch for minimum tillage production (Lal, 1974; Wilsori; 1978). Various reports have also shown that trees and shrubs with their deeper root systems are more effective in taking up and recycling plant nutrients from greater depths than herbaceous or grass fallows (Jaiyebo and Moore, 1964; Nye and Greenland, 1960; Lundgren, 1978; Jordan, 1985).

Milsum and Bunting (1928) were among the earlier researchers to suggest that herbaceous legumes were not suitable sources of green manure in the tropics. They believed that shrub legumes, including some perennials such as Crotalaria sp. and Cajanus cajan, were more suitable. They even suggested a cut-and-carry method in which leaves cut from special green manure source plots would be used to manure other plots on which crops would be grown. Cajanus cajan with its deep roots survives most dry seasons, and at the start of the rains has an abundance of litter and leaves to contribute as green manure. Planted fallow of shrub legumes such as Cajanus cajan, already widely used by traditional farmers, was sometimes found to be more efficient than natural regrowth in regenerating fertility and increasing crop yields (Nye, 1958; Webster and Wilson, 1980). With increased use of chemical inputs, serious questions are repeatedly raised as to whether a fallow period is needed and what minimum fallow period will sustain crop production. An objection to the traditional fallow system as illustrated in Figure 1 (phases I and II) is the large land area required for maintaining stable production. On the other hand, modern technologies from the temperate zone introduced to increase food production by continuous cultivation have also not been successful on the LAC soils. Rapid decline in productivity under continuous cultivation continues even with supplementary fertilizer usage (Duthie, 1948; Baldwin, 1957; Allan, 1965; Moormann and Greenland, 1980; FAO, 1985). From the results of a world-wide survey, Young and Wright (1980) concluded that, with available technology, it is still impossible to grow food crops on the soils of tropical regions without either soil degradation or use of inputs at an impracticable or uneconomic level. They further stated, that at all levels of farming with inputs, there may still be a need to fallow, or to put the land temporarily to some other use, depending on soil and climatic conditions. Higgins et al. (1982) have given some estimates of rest periods needed for major tropical soils under various climates with different inputs (Table 2). The rest period needed decreased with increasing input levels.

To overcome the management problems of the upland LAC soils, and to incorporate in them the much-needed fallow component, scientists working at the International Institute of Tropical Agriculture (IITA) in Ibadan, Nigeria, in the 1970s opted for an agroforestry approach which had not been tried before then — the use of woody species for managing these soils. This has led to the development of what is now known as the alley-cropping system (Kang et al., 1981; Wilson and Kang, 1981).

Table 2 Cultivation factors* for some major soils in the tropics depending on input levels

In both planted fallow and alley cropping, the potential for sustainability comes through more intensive management in which the non-crop-producing component (the fallow or woody species) is managed in such a way that a large portion of the energy flowing through that sector is directed towards crop production, and resource degradation and depletion are prevented. When these practices are introduced early they will maintain the resource base at a high level and thus respond more effectively to intensive management.

The potential of alley cropping as a sustainable farming system in the tropics

The concept of alley cropping

In alley cropping, arable crops are grown between hedgerows of planted shrubs and trees, preferably leguminous species, which are periodically pruned to prevent shading the companion crop(s) (Kang etal, 1981,1984). Two field examples of the practice are shown in Figures 2 and 3. This production system is classified by Nair (1985) as a zonal agroforestry system. The shrubs and trees grown in the hedgerows retain the same functions of recycling nutrients, suppressing weeds, and controlling erosion on sloping land as those in the bush fallow (Figure 4). Prunings from the trees and shrubs are a source of mulch and green manure. Leguminous -voody species also add fixed nitrogen to the system. The alley-cropping technique can, therefore, be regarded as an improved bush-fallow system with the following advantages:

1. Cropping and fallow phases are combined;

2. Longer cropping period and increased land-use intensity;

3. Rapid effective soil fertility regeneration with more efficient plant species;

4. Reduced requirements for external inputs; and

5. The system is scale-neutral, being flexible enough for use by small-scale farmers and for large mechanized production.

Figure 2 Alley cropping: Leucaena leucocephala and maize at Machakos, Kenya
(Photo by S. Jackson for ICRAF).

Figure3 Alley cropping: Leucaena and upland rice at IITA, Ibadan, Nigeria

Figure 4 A schematic representation to show the benefits of nutrient cycling and erosion control in an alley-cropping system
(Source: Kang et al., 1986).

By integrating small-ruminant production with alley cropping, the International Livestock Centre for Africa (ILCA) project in Ibadan, Nigeria, has developed the alley-farming concept (Sumberg and Okali, 1983) in which prunings from the hedgerows provide high-quality supplementary fodder. So alley fanning can be defined as the planting of arable crops between hedgerows of woody species that can be used for producing mulch and green manure to improve soil fertility and produce high-quality fodder.

The alley-cropping concept is currently, being evaluated in many parts of the tropics under different names. The International Council for Research in Agroforestry (ICRAF) used the term "hedgerow intercropping" (Torres, 1983), while in Sri Lanka the term "avenue cropping" is used (Wijewardene and Waidyanatha, 1984). Working independently during the late 1960s and the 1970s, the agricultural extension department in Sikka district on the island of Flores in eastern Indonesia also promoted the use of Leucaena for controlling erosion and rehabilitating degraded, slightly acid soils on very steep lands in an approach similar to alley cropping under the "Lamtoronisasi" programme (Metzner, 1976; Parera, 1978; Piggin and Parera, 1985). This extension and development work with Leucaena leucocephala has been very successful (Parera, 1986). According to him, although Leucaena was forced on the farmers in the 1930s for soil rehabilitation, it did not gain early acceptance because it was not accompanied by an appropriate management system. Leucaena came into focus in the region only during the seventies when the "Lamtoronisasi" programme was introduced.

The potential for a sustainable farming system

Various field trials were carried out by IITA scientists over the past ten years on strongly acid soils (Ultisols) and slightly acid soils (Alfisols) in the humid and subhumid regions of Nigeria to test the suitability and benefits of alley cropping. Some of the results of these trials have been published (Kang et al, 1981,1984,1985,1986; Kang and Duguma, 1985; Ngambeki, 1985; Wilson et al., 1986). On Alfisols and associated soils Leucaena leucocephala and Gliricidia sepium were the most promising woody species for alley cropping and alley farming (Atta-Krah et al., 1985). They can be established by direct seeding in association with a growing crop. Once established, the hedgerows can be repeatedly pruned to produce large amounts of biomass that can be used as green manure, mulch or fodder.

Even on degraded land, L. kucocephala and G. sepium prunings had higher nutrient yields than those of some widely used native fallow species such as Acioa barterii or Alchornea cordifolia (Table 3). The high nutrient yields are maintained when prunings are added to the soil. However, under a cut-and-carry system where prunings are continuously removed as fodder, the soil can also become impoverished unless nutrients from other sources are added.

The performance of maize, cassava and cowpea in alley cropping with L. kucocephala and G. sepium has been studied. Higher maize and cassava yields were obtained when alley cropped than in control plots. It is estimated that L. kucocephala can contribute about 40kg N ha"1 to the companion maize crop (Kang and Duguma, 1985). Ngambeki (1985) also reported large savings in nitrogen fertilizer when maize is alley cropped with L. kucocephala. Cowpea yield, however, showed either no increase or reduction in yield when alley cropped with L. kucocephala. Upland rice alley cropped with L. kucocephala does not respond to added fertilizer nitrogen, but the control plot (not alley cropped) responded to 30 kg of applied nitrogen per hectare.

An important aspect of alley cropping is how it affects yield sustainability. Under long-term observations on a sandy soil, maize yields were significantly higher when alley cropped with L. leucocephala than in control plots with or without applied nitrogen (Kang and Duguma, 1985). Similar results were observed in long-term alley cropping trials on degraded Alfisols. With or without applied nitrogen, maize yielded more when alley cropped (Figure 5). This trial also showed that, in addition to nitrogen, improved soil conditions resulting from alley cropping had a positive effect on maize yields.

Table 3 Estimated nutrient yield from hedgerow (4-m interrow spacing) prunings (not including woody material) of four fallow species grown in alley cropping on a degraded Alfisol in southern Nigeria

Figure 5 Grain yield of maize on eroded Alfisol (oxic paleustalf) at Ibadan, Nigeria as affected by alley cropping with woody species (Acioa barterii, Alchoernea cordifolia, Gliricidia sepium and Leucaena leucocephala) and nitrogen rates (kg N/ha)
(Source: B.T. Kang, unpublished data).

Results of long-term studies showed significant improvement in soil properties under alley propping. These soils had higher soil organic matter and nutrient status than in soil receiving no prunings. Prunings added as mulch also substantially increased moisture retention in the topsoil (Kang et al., 1985).

The addition of organic matter and partial shading resulting from alley cropping stimulated increased earthworm activity. Yamoah and Mulongoy (1984) reported higher microbial activity as measured by increased biomass carbon under alley cropping. In addition to improving the soil's chemical, physical and biological condition, hedgerows play an important role in suppressing weeds and reducing runoff and soil erosion. Lundgren and Nair (1985) and Young (1986) have recently illustrated the importance of woody species for soil conservation. Ossewaarde and Wellensiek (1946) had also reported the importance of woody fallow species in soil conservation and weed suppression. Metzner (1982) reported significant results for L. leucocephala in controlling soil erosion and improving and maintaining productivity of degraded and sloping lands on the island of Flores in eastern Indonesia.

Kabeerathumma et al. (1985) and O'Sullivan (1985) reported remarkable reduction in runoff and soil erosion when L. leucocephala was included in the production system. Similarly, observations at IITA showed that, with mechanized alley cropping on sloping land, soil that had been degraded after root-rake clearing and tillage was more stable after L. leucocephala hedegrows were introduced than on adjacent land that was shear-blade cleared and maintained under annual no-tillage planting.

Investigations on acidic Ultisols in the humid tropics showed that for these conditions woody species such as Acioa barterii, Cassia siamea, and Flemingia congesta were suitable for alley cropping. Cassava when alley cropped with Acioa barterii or Cassia siamea yielded more than in the control (Kang et al., 1986). Although most of the research work on alley farming has been carried out in the humid and subhumid tropics, certain aspects of the concept could be applied in other agro-ecological zones in the tropics. Further evaluation of the technology in semi-arid and highland tropics is needed. Such research should include evaluating the suitability of hedgerow species, and hedgerow and crop husbandry methods for local environments and farming systems. Recent results in the semi-arid tropics of India showed that alley farming has good potential, particularly for providing much-needed fodder (Singh et al., 1986). Similarly, trials at the ICRAF Field Station, Machakos, Kenya (700mm of annual bimodal rainfall, '1500 m altitude) have shown the feasibility of alley cropping during years/seasons of "normal" rainfall (Nair, 1987).

Adoption of alley cropping

Spontaneous spread is the most dependable proof of acceptability of a land-use technology. This has happened in the case of alley cropping over the past few years, and, as stated by Vogel (1986), the incentive to adopt alley cropping will increase as pressures on land increase. The practicability and acceptability of alley-cropping technology can be illustrated by its very successful introduction to "critical" areas of eastern Indonesia (Metzner, 1976; Parera, 1978) and in the southern Philippines (O'Sullivan, 1985). Promising developments have been reported from Sri Lanka (Wijewardene and Waidyanatha, 1984) and parts of Africa.

Examples from Nigeria have shown that the concept is readily accepted in certain parts of the country, but land-tenure systems have been a major constraint to adoption in other areas (Francis, 1986). In the yam growing area of Zakibiam, alley cropping was readily adopted as a source of much-needed staking material. Those farmers also realized that alley cropping with L. leucocephala improved soil fertility.

The ILCA project that introduced alley farming at Owa-Ile and Iwo-Ate in Oyo North in southern Nigeria showed high adoption and spontaneous spread of the practice among traditional farmers (Atta-Krah and Francis, 1986). This project will be expanded in the coming years in a joint undertaking by the Nigerian Department of Livestock, the World Bank, and ILCA (L. Reynolds, personal communication).

In introducing the alley cropping/farming technology there are two aspects that have important implications for on-farm research. The first is that alley farming which links several farm enterprises differs from such single-component technologies as improved varieties or fertilizer. The second is that planting and managing the trees implies changes in farmers' behaviour. Since immediate benefits of the system are not directly apparent, introduction and testing of the system in farmers' fields require constant supervision for the first few years. Because of these considerations a group participatory approach appears to be more successful than individual approaches in introducing the technology (Atta-Krah and Francis, 1986; Cashman, 1986). Farmers must be convinced that alley cropping is a long-term investment that will lead to high sustainable productivity.

Outlook

In the traditional system of upland crop production on LAC soils, only a small portion of land is used for food-crop production at any given time. The larger part is under fallow. This extravagant use of land cannot continue, particularly where high population densities prevail. It is also impossible to maintain food production without an adequate fallow period on these LAC soils, unless high inputs are used in combination with short fallow periods. Planted herbaceous fallow, though generally no more efficient than natural regrowth for soil restoration, is useful for reducing adverse effects from cropping. Fallow should be designed to facilitate expansion of production periods. It should arrest degradation, enhance biological recycling, raise labour-use efficiency, and stabilize favourable environmental conditions for crop production. The alley-cropping technology incorporates all the benefits of the fallow period in the food-production period and sustains land productivity for longer periods.

The development of a sustainable production system suitable for large parts of the subhumid -and humid regions, particularly in Africa, will have the additional benefit of reducing the land area needed for food production. Expanded alley cropping could help to arrest rapid deforestation.

Considering the limited input available to traditional farmers in Africa, low-input regenerative production systems like alley cropping deserve attention and promotion. Even in developed countries, as Wittwer (1983) and Blevins (1986) stated, the new trend is towards production technologies involving greater as well as more efficient use of resources. Wittwer (1983) described this variously as regenerative agriculture, sustainable agriculture, organic farming and gardening. The high-input production systems of these countries are considered wasteful, exploitative of natural resources, and environmentally dangerous because of their excessive use of chemical fertilizers and pesticides.

Further research is needed to select more suitable multipurpose woody species for alley cropping, particularly for acid soils and high elevations. Similarly, testing of the alley- cropping and farming concept for the drier areas needs to be carried out. Alley cropping/farming has good potential for rapid dissemination and adoption in suitable areas.

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