2.3.1.3. Soils, Plant Growth, and Land Degradation
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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