As a result of pressures to increase production: marginal land is being brought under cultivation or grazing; fertilizers and pesticides are widely used by commercial operations (although applications of organic matter in small-scale practices is declining); and fallow periods are being reduced. These activities, although designed to increase productivity, can result in exhaustion of the production capacity of the land, manifested as: declining yields; vegetation and soil degradation and loss; and, in extreme cases, desertification. Climatic variability and change, and inappropriate land use or land tenure policies, add to the pressures and magnify the impacts. The current situation is that approximately 22 per cent of vegetated land in the region (494 million ha) has been classified as degraded, and 66 per cent of this are classified as moderately, severely or extremely degraded (UNU 1998). Africa is not unique in experiencing this problem, as shown in Figure 2f.4, but the effects on food security and the anticipated impacts of climate change make land degradation a priority issue for African leaders.
Figure 2f.4: Regional comparison of land degradation
Source: UNU 1998
Land degradation and reduced productivity can be categorized as: hydrological and chemical degradation; physical degradation; or biological degradation. Hydrological and chemical degradation encompasses: waterlogging; salinization; sodication; and chemical pollution. It is caused by the use of low quality water for irrigation, and environmental pollution. Chemical degradation-resulting from pollution from industrial, household and medical refuse, and mining wastes-also occurs in selected sites. Physical degradation includes deterioration of soil structure and the occurrence of compacted layers, and can be due to: overstocking; inappropriate use of machinery; mining and quarrying activities; frequent waterlogging; and exposure to erosion. Biological degradation refers to the loss of nutrients and micro-organisms vital for maintaining healthy productive crops, and is due to the exhaustion of soil fertility, as a result of: intensive cropping; removal of crop residues; nutrient deficiencies; and insufficient organic matter.
The causes of land degradation in Africa are climatic variability and management practices, in addition to physical factors, such as the slope of the land and soil structure. Although most often associated with cultivation or grazing areas, degradation can also affect: forests, woodlands and savannas; urban and peri-urban areas; and protected areas. For example, clearance of forest vegetation (by fire, drought or overgrazing, or for alternative land uses) leaves soils more susceptible to erosion by wind and rain, especially when on steep slopes, in high rainfall zones or when combined with poor management, such as over-grazing. Draining of wetlands (for cultivation or urban development) disrupts the hydrological cycle and renders the surrounding areas more prone to flooding, and may no longer provide suitable habitat for wildlife, or raw materials for construction and crafts. Conversion of natural habitats to cultivation or grazing in unsuitable areas can also set off negative feedback systems of degradation, as detailed in Box 2f.1
|Box 2f.1 Demographic change and land quality|
|Source: UNU 1998|
Over the past 30 years, soil structure has been damaged, nutrients have been depleted and susceptibility to erosion has been increased, as a result of: increasing application of chemicals; use of inappropriate equipment and technologies; and commercial mono-specific plantations. Irrigation in areas of high evaporation, and by inappropriate technologies, for example, increases the rate of salinization of the soil, because the water is rapidly lost, leaving behind a crust of once-dissolved salts. Likewise, intense grazing, especially in marginal areas and by a single species of livestock, can degrade vegetation, and can lead to soil compaction and accelerated erosion. One study found that most soil degradation in Africa was attributable to overgrazing (50 per cent), followed by: poor agricultural management practices (24 per cent); vegetation removal (14 per cent); and overexploitation (13 per cent) (WRI 1992). Between 1980 and 1995, Africa's permanent pasture declined slightly, indicating either conversion to cultivation, or abandonment due to extreme degradation, or a combination of both (UNU 1998).
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