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Many African soils are agriculturally poor, because they are very old, badly leached, and often infertile. Laterites (the oldest soils) are agriculturally unproductive. Laterized red earths are younger and less leached and occur in regions of heavy rainfall, so they are quite agriculturally productive. Nonlaterized red earths, which are found in drier regions (e.g., savannas), are good agricultural soils. Upland red earths are an immature group that occasionally are intensively farmed. In regions of moderate rainfall, the most fertile soils are located in the high veld of southern Africa and parts of west Africa. The black soils-the vertisols-are very fertile but become adhesive during the rainy season and almost rock-like in the drought period. In arid regions, soil humus is very low; moisture often is drawn upward by capillarity and, on evaporation, deposits dissolved minerals in a crust at the surface. In Mediterranean regions, the summer drought, the absence of frost, and the small degree of chemical weathering has led to poorly formed soils.
The ability for soil to support particular natural or agricultural communities is fundamental in any future scenarios of ecosystem development. Soil development is slow and will likely lag climate and vegetation change. In the short term, changes in the soil-water regime and turnover of organic matter and related mineralization or immobilization of nitrogen and other nutrients will have the greatest effect on ecosystem functions. Among the factors that will affect these soil processes, fire and land use probably are the most important. Changes in fire regimes (e.g., frequency, intensity) will directly influence organic-matter processes in the soil and nutrient fluxes-and so will have a significant impact on how soils will function. Land use and land-use history on given sites influence nutrient dynamics and the potential for erosion damage.
Land degradation-defined as "a reduction in the capability of the land to support a particular use" (Blaikie and Brookfield, 1987)-is a major problem in Africa and the whole world. Support by African countries for the Convention on Desertification (United Nations, 1992)-which recognizes that 66% of the continent is desert or dryland, and 73% of the agricultural drylands already are degraded-clearly shows that most African governments are aware of this problem.
Recognized forms of land degradation include soil erosion, salinization, soil contamination, loss of soil organic matter, decline in nutrient levels, acidification, and loss of soil structure. Low rainfall, long dry seasons, recurrent droughts, mobile surface deposits, skeletal soils, and sparse vegetation encourage desertification (Le Houerou, 1989; Dregne, 1983; Kassas, 1995). A combination of climatic variations and human land-management practices can lead to excessive land degradation, eventually leading to desertification. Thus, efforts to reduce vulnerability to climate change must take into account land management and the social and economic factors that drive people's use of the land.
Most studies of soil erosion have looked at soil loss from plot-based measurements and then extrapolated to estimate total soil loss per hectare. Although soil erosion clearly is a major problem in many parts of Africa, simple extrapolations from plots to whole countries and into the future can be misleading. Erosion is a major problem locally, and steps must be taken to combat soil erosion at the farm and catchment levels. Reij et al. (1996) argue for participatory approaches to soil and water conservation, rather than large-scale, top-down interventions that encompass technology alone. The social and economic context is critical for success.
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