An e-publication by the World Agroforestry Centre
AGROFORESTRY EXTENSION MANUAL FOR KENYA
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An e-publication by the World Agroforestry Centre |
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AGROFORESTRY EXTENSION MANUAL FOR KENYA
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6. THE CROP COMPONENT 6.2 Some important crops in relation to agroforestry Maize (lea mays) Origin: Maize originated in Mexico and was introduced into East Africa as early as 1643 (Ngugi, Karau and Nguyo, 1990). The early varieties came from the Caribbean and were only suited to the coastal strip. The spread of maize into the highlands and medium-altitude areas, with a consequent decline in most places of the indigenous cereals, was largely due to the introduction by European settlers of varieties from South Africa (Acland, 1971).
Ecology: Maize has a higher yielding potential than the indigenous cereals where water and drainage are favourable. The young maize plant is moderately drought resistant, but from five weeks onwards it is less so. Maize requires well-drained soil and a good supply of nutrients. Features in agroforestry: Maize is light demanding, so shade may reduce crop yields. It is sensitive to competition for moisture, so intercropped trees must be of a non-competitive type in drier areas. For example, Leucaena in hedgerows is not recommended in areas with less than 800 mm rain. Wind damage can be a problem, so trees are useful as windbreaks.
Origin: South America. Ecology: Beans are annual legumes and have the capacity to fix atmospheric nitrogen. The degree of nodulation is very variable, however: in some areas large nodules are formed while in others none are produced. Beans are not drought resistant and ideally they need moist soil throughout the growing period. High temperatures cause a poor fruit set. Beans demand free-draining soils with a reasonably high nutrient content.
Features in agroforestry: Beans are shade tolerant, and since they prefer lower temperatures they normally perform well under maize, bananas or other trees.
Origin: Indigenous to Africa. Ecology: Cassava is widespread in areas below 1,500 m, occasionally higher. It is very drought-resistant and is able to give good yields on poor soils. Cassava is suitable for areas with erratic rainfall where other crops may fail in bad years. Its main requirement is well-drained soil. As a result, cassava is one of the few crops that fits into a definite rotational pattern on small holdings and owing to its undemanding nature is often planted as the last crop in the arable period before the land reverts to fallow. Weeding is only required during the first year; later weeds are tolerated. Cassava has been demonstrated to be a soil improver. Features in agroforestry: Cassava appears to be sensitive to shade but not to other forms of competition from trees.
Origin: Indigenous to Africa. It has been cultivated for more than 4,000 years in Egypt. Ecology: Sorghum is drought-resistant and often out-yields maize under dry conditions. It is reasonably tolerant of waterlogging and yields comparatively well on infertile soils. Sorghum has twice as many roots as maize in a given volume of soil. The roots do not die back during drought and the leaves roll up to minimize transpiration. In this way the plant can remain dormant when other crops would be killed, and when the rains start again it recovers rapidly. Features in agroforestry: Sorghum is sensitive to shade, as are all cereals. Birds, particularly Quelea, are one of the main causes of crop loss in sorghum: bird-susceptible varieties sometimes give no yield at all. Weavers, starlings, bishop birds and many other kinds of bird also attack sorghum but are less devastating than quelea. The exact role trees may play in attracting such birds has not been determined, and neither do we know if increasing perching sites for birds of prey, by pollarding trees for example, can reduce the damage by grain-eating birds.
Origin: Some millets are believed to be indigenous to eastern Africa, but they are now widely grown in both Africa and Asia. Ecology: Finger millet tolerates dry spells in the early stages of growth, but after the first month it requires a steady supply of moisture if good yields are to be obtained. It is usually grown between 900 and 2,400 m in areas with at least 900 mm rainfall. It only yields well on fertile soils with good drainage. Features in agroforestry: Similar to sorghum with regard to shade and bird problems.
Origin: Southeast Asia is regarded as the centre of origin for rice from where it has spread to many parts of the world. Ecology: Rice is an unusual crop in that it can grow not only under waterlogged conditions but also in standing water. It prefers high temperatures and grows best below 1,00 m. Sandy soils can support rice, but only if there is a permanent high watertable. Otherwise heavy soils are needed to retain irrigation water. Features in agroforestry: There are practically no trees on the irrigation schemes in Kenya where rice is grown. Paddy fields are special environments and few trees can thrive there. Rice is also a shade-sensitive crop and wood supplies for irrigation schemes are best met from woodlots planted nearby.
Origin: From Burma tea has been taken to many other parts of the world. The first tea was planted in Kenya in 1903 near Limuru, but most tea plantations were established after 1945. Ecology: Tea tolerates short dry spells but only produces well with adequate rainfall. The lowest altitude at which it will grow well, 1,200 m, is set by scarcity of rainfall, whereas the upper limit, 2,200 m, is set by the incidence of frost. For good growth, soils should be deep, well drained and slightly acidic. Features in agroforestry: Tea growing areas frequently experience strong winds and windbreaks are recommended. A tree species particularly well suited to use in tree growing areas is Hakea saligna. Armillaria root rot, Armillaria mellea, is a serious disease of tea in east Africa. Armillaria is promoted by the presence of old stumps and roots of trees which have been infected, so such stumps should always be carefully removed before new tea is established.
In other parts of the world shade trees are interplanted with tea, but this is not normally done in Kenya. Shade trees in tea plantations are only beneficial in hot and humid areas, e.g. Assam, but due to the higher altitudes of the tea plantations in Kenya the soil temperatures are sub-optimal for the growth of tea. Shade trees serve to moderate the soil-temperature differences between day and night, but there is no need for this in the cool tea-growing areas in Kenya. The overall impact of shade trees on the micro-environment has been found to result in lower tea yields. Drying of tea leaves requires a lot of energy. Currently the energy source for this purpose in Kenya is largely imported oil, and it has been argued that this could be substituted with fuelwood produced near the tea estates. Increased wood production for this purpose would best be achieved through establishment of more woodlots.
Origin: Ethiopia. Coffee was introduced into Kenya in 1897 by missionaries. Coffee is one of the most valuable commodities in international world trade and is very important for Kenya's foreign exchange earnings. Ecology: The best coffee areas in Kenya are between 1,400 and 1,900 m altitude, with rainfall of 1,500-2,250 mm. The best soils are well-drained volcanic soils, deep, fertile and slightly acidic. Features in agroforestry: Coffee is shade tolerant, which enables farmers to grow trees in many spatial arrangements with the crop. Grevillea was originally introduced as a shade tree for coffee and is still the most popular tree in coffee-growing areas. Other common trees with coffee are Cordia abyssinica and Albizia spp. Armillaria root rot attacks coffee, so all old roots of trees that may harbour Armillaria should be removed when new coffee is established. In large plantations where there are no shade trees windbreaks should be considered.
Origin: Bananas originally came from Southeast Asia, but were introduced to Africa many hundreds of years ago. Ecology: For good yields bananas require a constant supply of moisture. They are thus most important in areas with well-distributed rainfall, e.g. Kisii and Kaka-mega. Bananas also require a warm climate, so the crop performs best at altitudes below 1,800 m. Bananas also require rich, deep and well-drained soils. Features in agroforestry: Bananas are often used as a middle- or upper-storey crop with other crops being grown underneath. Sometimes they are mixed with taller trees. Occasionally bananas are also grown in monoculture. Bananas are shallow rooted, most roots being found in the top 15 cm of the soil. Therefore they are sensitive to wind damage and windbreaks or support stakes to prevent the banana plants from falling over are essential. Bananas are sensitive to high nematode populations in the soil. If the nematode population is too high banana cultivation must be halted temporarily and other crops grown until the nematode population has decreased.
Origin: South-western Asia. Wheat was introduced into Kenya early this century by the white settlers. Ecology: The best wheat-growing areas are above 1,800 m with at least 750 mm rainfall. The soils should be well drained and have a high nutrient content. Features in agroforestry: In some wheat-growing areas Acacia trees have been left in the fields, apparently with little or no reduction in yields. Strong winds may cause lodging, hence trees are useful for windbreaks. Some bird damage may occur, but it is not as serious as for sorghum and millet. Wheat is a light-demanding crop, so trees in fields must either have a natural light shade or be managed to reduce the intensity of the shade.
Origin: South America, but widespread in the tropics. Ecology: Sweet potato is a crop which grows under a very wide range of ecological conditions from sea level up to 2,400 m, in a wide variety of soils, and in areas receiving more than 750 mm rainfall. Little weeding is required since the vines cover the soil very effectively and suppress most weeds. Features in agroforestry: Sweet potatoes are not normally grown with an upper storey of trees, and in general trees do not have a big role to play in association with sweet potatoes.
Origin: South America, but now widespread in temperate regions and in the tropical and subtropical highlands. Ecology: The Irish potato grows best in cool climates with a rainfall of 25 mm per week during the growing season and at altitudes ranging from 1,500 m to 2,900 m. The soils should be well drained and rich in nutrients. Sandy soils are better than heavy soils. Features in agroforestry: Irish potatoes are relatively shade tolerant and can sometimes be seen growing under bananas with good results. Irish potatoes are badly damaged by high nematode badly damaged by high nematode populations, and if nematode infestation is severe potato cultivation may have to be discontinued.
Origin: Different species have different origins. Now widespread all over the tropics. Ecology: Tropical areas which have a long rainy season are best for this crop. Most types of yams require at least six months of rain, and the average annual rainfall should be at least 1,500 mm. Temperatures of 25-30°C are best for yam cultivation, and the soils should be fertile, loose and well drained. Intercropping and crop rotation are recommended since yams are sensitive to nematode build-up. In Kenya, yams are common in Central Province and in Eastern Province around Mount Kenya. Features in agroforestry: In Kenya, yams are often grown in association with Commiphora eminii, subsp. zimmermanni. A cutting of Commiphora is planted as a stake for the yam, and since Commiphora cuttings take root easily eventually a Commiphora tree provides support to the yams. Both grow well together without any signs of competition and there are several other benefits from the Commiphora beside being a support to the yams, e.g. emergency fodder, timber, making of mole traps and firewood.
Origin: South America from where it was brought to Europe and later by the Portuguese to Africa. Tobacco growing was initiated by the British American Tobacco Company (BAT) in the late 1930s, but it was in the 1970s that the cultivation of tobacco expanded considerably. Ecology: Tobacco requires about 750 mm of rain and the altitude range is from sea level up to 1,260 m. A high light intensity is required for good production and the relatively low light intensities in Kenya result in low yields compared to those in some other countries. Crop rotation is required to avoid build-up of nematode populations. Features in agroforestry: Curing tobacco requires fuelwood and BAT has long promoted tree growing among small-scale tobacco producers in Kenya. Recommended tree species are Grevillea robusta, Cassia siamea, Markhamia lutea, Eucalyptus spp., and to a lesser extent Terminalia brownii, Gmelina arborea, Acacia albida, Cordia abyssinica, Sesbania sesban and Azadirachta indica. Eucalyptus spp. cannot be used for fire-curing tobacco because of the odour the wood imparts to the smoke. Woodlots with a spacing of 2 x 2 m should be established, preferably on marginal land where little or no other production takes place. Planting of trees is also recommended on farm lands, e.g. on boundaries and terrace risers. In dry areas, water-harvesting methods are adopted. The light-demanding nature of tobacco limits its scope for intercropping with trees.
Origin: New Guinea. Sugar-cane growing expanded in Kenya during the 1920s with establishment of the first sugar factories. Ecology: Sugar cane is a perennial grass which grows well at altitudes from sea level up to 1,600 m in areas with not less than 1,500 mm rainfall. It prefers heavy fertile soils. Features in agroforestry: Sugar cane is strongly light demanding, so intercropping trees and sugar cane cannot be recommended when sugar cane is commercially grown. Sugar-cane growing areas are often those where severe shortages of wood are experienced, so establishment of woodlots and intensified tree growing around homesteads are recommended to meet the needs for wood. Under small-scale irrigation in the Kerio Valley sugar cane is sometimes grown with Ficus sycomorus and Acacia tortilis.
Section 6.2-is based on Ngugi, Karau and Nguyo, 1990; Acland, 1971; and the author's experience. |
