|Sustaining the Future: Economic, Social, and Environmental Change in Sub-Saharan Africa (UNU, 1996, 365 pages)|
|Part 3: Environment and resource management|
|Agricultural development in the age of sustainability: Crop production|
Sub-Saharan Africa has over 23 million km2 of land with a potential arable area estimated at 643 million hectares and forest at 700 million hectares, which is being cleared at the rate of 3.7 million hectares per year (World Bank 1989). Only 174 million hectares of the land are currently under cultivation. Sub-Saharan Africa is demarcated into five major ecological zones, which are determined mainly by rainfall and relief (table 11.1).
The humid forests of West and Central Africa
In the humid forests of West and Central Africa, tree crops such as oil-palm (Elaeis guineensis), cocoa (Theobroma cacao), rubber (Hevea braziliensis), and protected economic woody plants are grown in plantations or in multistorey associations with root and tuber crops. Rice is a major crop in both swamps and upland areas in the humid forest zone. Compound land (land immediately adjacent to the compound or homestead, often in permanent cultivation) is particularly important in these areas. Household refuse, including ash and plant and animal wastes, is used to maintain a stable multistorey plant production system. Protected trees, perennial herbaceous plants, including plantains and bananas, together with raffia are mixed with vegetables, spices, yams, and some maize in farms around the homesteads. The trees gradually decrease in number or completely disappear from farms as the distance from homesteads increases.
Soil fertility and structural instability are the most important plant production constraints of this zone. Luxurious forest growth soon gives way to eroded land when clearing is followed by intensive cropping (Kang and Juo 1981; Lal 1989). Soil acidity is common, and weeds, which flourish in the heavy rains, compete with tree and other food crops. Another important effect of forest clearing is loss of plant genetic diversity, exposure of soil to wind and water erosion, and the extinction of useful plant resources (Okigbo 1989). This resource waste is further accentuated by high average human (63 persons/km2 in West Africa and 10/km2 in Central Africa) and animal population pressure, intensive farming, overgrazing, construction development, hunting, and burning. A reduction in maize yield in this zone owing to cloudy skies and reduced insolation has been observed (IITA 1983).
The Southern Guinea Savanna and Derived Savanna
The humid forest of the Southern Guinea Savanna, sometimes called the sub-humid zone of West Africa, has been mostly cleared and cropped for a long period and has been overtaken mainly by grasses and shrubs. At an early stage of succession from forest to savanna is the Derived Savanna, which is better known as Guinea Savanna (Ter Kuile 1987). Rainfall in the Derived Savanna may be slightly higher than that in the Southern Guinea Savanna (table 11.1). Sorghum and maize are important cereals, and root crops (cassava and yams) grow and yield highly.
Table 11.1 Major eco-zones and characteristics in Sub-Saharan Africa
|Zone||Number of humid months||Mean annual rainfall||Growing period (days)||Main soils|
|1. Forest: coastal West some Africa and Central Africa||7-9 +||1400-4000 + (mostly unimodal)||270-365||Mostly acidic (ultisols and oxisols); non-acid (inceptisols, entisols, verti sols, alfisols, etc.)|
|2a. Derived Savanna||6-7||1300-1500 (bimodal, some areas)||240-270||Moderately leached soils (alfisols, some ultisols, etc.)|
|2b. Southern Guinea Savanna||5-6||1200-1500 (partially bimodal)||190-240||Mainly alfisols and related soils; acidic ultisols and oxisols in some wetter areas; also entisols and vertisols in some areas|
|3. Northern Guinea Savanna||4-5||880-1300 (unimodal)||140-200||As above, with greater proportion of non-acid alfisols|
|4. Sudan Savanna 2 4||500-880||90-140 (unimodal)||Alfisols and some drier aridisols, etc.|
|5a. Eastern and southern African highlands||7-12||750-1000 (unimodal)||270-365||Ultisols, oxisols, vertisols|
|5b. Eastern and southern African highlands||5-6||750-1000 (bimodal)||190 240||Alfisols, ultisols. oxisols|
Source: Adapted from Papadakis (1966); FAO (1978); Kowal and Kassam (1978); Lawson (1979).
The high infestation of tse-tse fly debars the use of oxen as sources of power; therefore labour at peak growing seasons is a major constraint. This zone is poorly served by roads and marketing systems. Weeds, particularly the parasitic Striga, attack the dominant cereal crops. The soils are relatively rich and are structurally more stable than humid forest soils but are frequently deficient in some major nutrients, whose efficiency may be reduced by negative interaction with minor elements, e.g. phosphorus (P) and zinc (Zn).
The Northern Guinea Savanna
The Northern Guinea Savanna of West Africa, also called the sorghum-millet belt of West Africa, receives lower rainfall than the southern part. It is excellent for maize growth and some of the highest yields in West Africa are obtained from this zone (IITA 1984). Sorghum is also important. However, drought at critical stages of maize development frequently reduces grain yields. High soil temperatures and high evaporation rates are also important constraints (Hullugale 1989). The parasitic weed Striga attacks cereals and grain legumes (cowpea), the two important food-crop groups grown in the Northern Guinea Savanna. The soil is more favourable for cropping and responds to N, P, and S applications. Soil erosion caused by wind and soil crusting and capping has also been reported (Charreau 1970).
The Sudan Savanna
The Sudan Savanna is located to the north of the Northern Guinea Savanna. Rainfall is unimodal, its duration is uncertain, and crop failures are common. Millet and cowpeas are the major food crops in this area. Cereals are grown on about 70 per cent of the total cultivated area of the Sudan Savanna (Matron 1987). Cotton and groundnuts are the major commercial crops and are sometimes grown for export.
The eastern and southern African highlands
The generalizations about the preceding zones are modified by high elevation in Rwanda, eastern Zaire, Burundi, and the mountainous plains of eastern Africa. The monsoon tropical climate and the high incidence of radiation result in extremely high productivity.
In the area of the eastern and southern highlands where rainfall is unimodal, high maize yields are recorded. However, rainfall may limit production. Banana is an important staple and groundnuts are also grown commercially. Coffee and tea, particularly the former, are export crops.
In the area where rainfall is bimodal (March/April to May and November to January), the short duration of the rainfall requires very intensive labour in land preparation and planting, the consequences of which are frequent crop losses. The crops grown are maize, coffee, and bananas. High population pressure on the soils of both highlands, with only 5-7 months of rainfall, causes low productivity. Soil loss is high, particularly in the communal lands of Zimbabwe, where it is reported at 50 tons/ha/yr (Whitlow 1987) and results in reduced yields of crops (Collinson 1987).
There are, therefore, three broad zones: (a) the humid forest zones of West and Central Africa, (b) the savanna zone, demarcated by the level of available rainfall, and (c) the highlands, including plateaux.