Intercropping and Agroforestry: Their Role in Sustainable Agriculture in the Semi-Arid Tropics

by Ancha Srinivasan

The semi-arid tropics, spread over nearly 20 million square kilometers and covering all or parts of 50 nations of the world, are home to more than 750 million people (Fig. 1). These regions are characterized by low and unstable crop yields due to harsh climate, low-productive soils (low organic matter, nitrogen, and low water infiltration capacity), and high soil erosion. The ever growing demands of food, fuel, and fodder by human and livestock populations, coupled with poor economic infrastructure in these regions, have led to a ruthless exploitation of natural resources, creating imbalances in ecosystems that have already become fragile due to intensive cultivation over centuries. The future of agriculture in these areas depends on sustainable management of land, water, flora, fauna, and the atmosphere.

The concept of sustainable agriculture in not new. Many traditional agricultural systems, including the age old practice of shifting agriculture, were sustainable for centuries. However, given a background of increasing human needs, escalating prices of agricultural inputs, and increasing pollution problems, the concept assumes a greater significance than in the past. In this context, sustainable agriculture may be defined as a system that aims at increasing, stabilizing, and diversifying agricultural production with minimal dependence on high energy inputs (pesticides, fertilizers, etc.) and intensive tillage (using farm machinery), and with the least damage to the environment and biological diversity. Sustainable agriculture thus conveys an idea of a balance between human needs and environmental concerns. New production systems should be modified in order to increase productivity and make full use of locally available, renewable resources.

Intercropping and agroforestry are two practices that have great potential for contributing to sustainability. Both practices are largely associated with resource-poor farmers operating in low rainfall, high-risk situations. Intercropping involves the practice of growing two or more crops simultaneously on the same piece of land. Studies on deferment cropping systems in the semi-arid tropics indicate that intercropping gives a greater increase in stability (less variability in yields over different seasons), productivity, and profitability than relay or sequential systems. This is achieved largely through more efficient use of growth resources (light, water, nutrients, etc.,), better control of weeds, pests, and/or diseases, the provision of physical support and shelter of one crop to another, and the reduction of soil and nutrient losses through reduced soil erosion.

Two Patterns of Intercropping

Intercropping systems may be classified into two types of patterns, temporal or spatial (Table 1). In temporal patterns, crops are selected in such a way that their growth and maturity patterns differ in time significantly. This will mean that the crops make their major demands on soil resources at different times, thus allowing better use of growth resources than if grown separately. The yield advantages in such systems can be substantial, ranging from 20 to 80 percent when the difference in maturity periods is more than 40 days. Intercropping of 80- to 90-day cereals with 180-day pigeonpea is one example of the temporal pattern.

In spatial patterns of intercropping, crops are of similar growth duration, but differ in their use of resources spatially, due to differences in shoot and root zonation, rhizosphere effects, and in peak demands of nutrients at different growth stages. The millet/peanut combination is a commonly practiced spatial pattern of intercropping used in Africa and India. Recently, it has been found that the yield advantages of this millet/peanut system is entirely due to improved efficiency of light conversion because of better distribution of light over more leaves, and the mixture of the short peanut plant with tall millet species. When legumes are chosen as component crops, the yield advantages from intercropping are more substantial, largely through the ability of legumes to fix nitrogen and improve physicochemical properties of soils.

The productivity from intercropping can be improved and stabilized through such practices as staggered sowing, selection of suitable varieties, and use of optimum plant population size. Modifications in plant geometry of the base crop (paired row planting) help in increasing the total productivity by accommodating a higher population of intercrops and minimizing competition.

Benefits of Agroforestry

The impact of agroforestry on sustainability arises primarily through the trees and their regenerative effect on soil fertility, the shelter and fodder they provide for animals, and the range of tree products directly useful to people.

Woody perennials in agroforestry systems serve both productive and protective functions. Trees are productive in their ability to provide fodder for livestock, fuel wood and domestic timber, oils, and fibers. In addition, they can be rich sources of energy (ethanol from fermentation of high carbohydrate fruits, methanol from distillation of oils, latex and other combustible saps and resins) and provide raw materials (wood, fiber, fruit, nuts, tannin, essential oils, and medicinal ingredients) for industries. The trees serve a protective function in their ability to shade and shelter livestock, provide live-fencing, suppress weeds and pests, and improve soil and water conservation. Trees can improve the microclimate and encourage crop growth through increased humidity and decreased soil and air temperature under the tree canopy.

Since soil erosion, low soil fertility, and increased occurrence of weeds, pests, and diseases are major factors responsible for low and unstable yields in the semi-arid tropics, the role of trees in addressing these problems can be very significant.

• Reducing soil erosion: Soil erosion is a serious problem, varying with the soil type and erosive capacity of the rainfall. About 40 percent of total rainfall in these areas is erosive compared with 5 percent in temperate regions. A number of agroforestry techniques can provide more continuous cover, giving better protection against soil erosion, and arresting the loss of nutrients and soil organic matter, which result in reduced soil depth and water holding capacity. Wind breaks and shelterbelts reduce wind velocity and wind erosion, especially in areas exposed to dust storms and desiccating winds. Trees act as physical barriers to rainfall erosivity by decreasing runoff if planted as barriers, and reduce the impact of raindrops on the soil through their leaf cover.

• Soil fertility improvement: The productivity of most low-input farming systems in the semi-arid tropics is limited by nitrogen deficiency as well as lack of water. In addition to the ability of many trees to fix nitrogen, they can improve soil productivity through nutrient retrieval from deep soil layers, and recycling those nutrients back into the soil. The addition of organic matter through fallen leaf litter and decaying roots not only increases the amount of available nutrients in the soil but also improves soil porosity, infiltration, and water holding capacity. By improving soil fertility, an increase in earthworm populations and microbial activity is also achieved. The potential contribution of trees to improving soil fertility will be reduced if not enough leaf litter is allowed to return to the soil however, which can be a problem in areas where virtually all leaf matter is fed to livestock.

• Suppression of weeds, pests, and diseases: Weeds are smothered in most agroforestry systems because of effective ground cover. Pest incidence is also reduced because trees bring about a better biological balance between pests and their enemies by encouraging reproduction of predators and parasites of many pests. For example, in Glyricidia- and Leucaena-based agroforestry systems, reduced pod borer damage on grain legumes is believed to be caused by the fact that the flowers of these trees provide nectar that nourishes the adults of parasites that act against these legume pests.

Species Selection for Agroforestry

Before introducing a new agroforestry practice into a region, there is some need for caution to first determine if that practice will not only be productive and sustainable, but also compatible with conditions in that specific region. For example, the presence of a high termite population in some regions of Kenya would render a variety of species, such as Acacia mangium, Casuarina equisitifolia, and Eucalyptus species inappropriate. In those areas, trees such as Azadirachta indica (Fig. 2), Adhatoda vasica, and Derris indica would be better suited, as these species are supposed to have the ability to repel and discourage termite infestation.

Other potential tree species for agroforestry systems in the semi-arid tropics include: Acacia albida, Acacia arabica, Acacia ferruginea , Acacia nilotica, Acacia senegal, Acacia tortilis, Albizza lebbek, Cajanus cajun, Cassia siamea, Cordeauria edulis, Flemingia congesta, Leucaena leucocephala, Parasponia species, Parkinsonia aculeata, Phoenix dactylifera, Pithacellobiurn dulce, Prosopis chilensis, Prosopis cineraria, Sesbania grandiflora, and Sesbania sesban.

Intercropping and agroforestry are both simple land-use practices involving very little extra costs, yet they have much potential as biological tools to ensure the sustainability of agriculture in the semi-arid tropics. When properly designed and implemented, they can produce a variety of food and by-products while protecting the quality of the land environment.

Dr. Ancha Srinivasan is a VITA Volunteer and currently a postdoctoral fellow at the International Crops Re-search Institute for the Semi-Arid Tropics (ICRISAT) , where he does research in legumes. For further information, write: ICRISAT, Patancheru P.O. , Andhra Pradesh 5O2 324 , India.