An Assessment Of Soil Degradation
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Background to the Study
Human activities exert tremendous effects on land cover through a variety of land uses. Human use of land has altered the structure and functioning of the ecosystem (Turner et al., 1994). Land, whether in tropical or temperate regions land is being used for the purpose of growing trees, crops, and animals for food, as well as building sites for houses and roads, or for recreational purposes among others (Hartemink et al., 2008).
Globally, land use/land cover has changed considerably in the past decades – mostly reflecting the enormous growth in human population and their need for food, shelter and other necessities of life. Changing demography and particularly the changing age structure of populations, a high rate of urbanization, and a faster rate of population growth in relation to economic growth (Braimoh, 2003; Ouedraogo, et al., 2010), are major drivers of environmental change with significant negative impacts on the natural resource base. Demographic change therefore constitutes a major driver of land use/land cover change (LULCC): Its primary and most direct impact is through opening up of new land for agriculture, settlement and infrastructural development, although other extractive activities such as logging and mining are also significant (FAO, 2003). The world’s population has doubled smce 1960 (Hartemink, 2006). The developing world accounts for about 95% of the population growth with sub Saharan Africa as the world’s fastest growing region (Hartemink, 2006). The growing population has many implications but most of all it requires an increase in agricultural production to meet food demand. This demand can be met by expansion of agricultural land or by intensification of existing systems. Clearing of new lands, for whatever purpose has long been common in areas with rapidly growing populations (Kates, 2000; Braimoh, 2003 ; Ouedraogo, et al., 2010).
Land resources have been altered by rapid land use and cover changes accelerated by changeable socio-economic factors including high population growth, rapid urbanization (Fabiyi, 2006), agricultural intensification and government policies (Entwisle et al., 1998; Mather et al., 2000; Braimoh, 2003 ; Ouedraogo, et al. , 2010). Human pressure upon land resources and interactions between varying climatic characteristics facilitate changes in land use/land cover (Hartemink et al., 2008). In the absence of alternative livelihood opportunities and proper management of the environment, this rapid population growth and urbanization has resulted in environmental degradation and resource depletion. Between 1990 and 2000, sub-Saharan Africa lost 52 million hectares of forests which amounts to a decrease of 0.8% per year and 56% of the global total. It is estimated that 60% of the tropical forest areas cleared in Africa as a whole between 1990 and 2000 were converted to permanent agricultural smallholdings (FAO, 2003).
Rapid Population growth affects the natural resource base through increased demand for food, water, arable land and other essential materials, such as firewood among others, thereby facilitating encroachment into forests and woodlands as well as increasing demand for fossil fuels and other resources. The poor or low soil quality base arises due to two major factors. With few exceptions, growth in human population have led to a reduction in the per capita land availability and a breakdown of the erstwhile traditional natural fallow system that used to be the means of replenishing soil fertility. The methods used to restore the fertility of soils and to2sustain agricultural productivity under traditional shifting agriculture have become ineffective, and in some cases, have disappeared altogether. As high potential land becomes less available and the rural human population increases, farming is extending into more fragile lands, undermining the natural resource base as well as undermining the continued ability to produce food for the teeming populations. Degradation of the natural resource base in tum impinges on the livelihoods of all, but particularly rural communities. More small farmers are forced to work harder, often on shrinking farms on marginal land, to maintain household incomes.
Soil degradation, and in particular the decline of soil chemical fertility, is a major concern in relation to food production and the sustainable management of land resources. It also affects land use/land cover but the spatial and temporal effects of soil fertility change and its interaction with land use/land cover change remains to be investigated.
Land use involves the manner in which the biophysical attributes ofthe land are manipulated and the intent underlying such manipulation for which the land is used, whereas land cover implies to the biophysical state of the earth’s surface and immediate subsurface including biota, soil, topography, surface and groundwater, human structures etc (Meyer et al., 1994; Lambin et al., 2003). Land use change implies the conversion of land use due to human intervention for various purposes such as agriculture, settlement, transportation, e t c (Williams, 1994; Meyer, 1994; Turner et al., 1995). While land cover change on the other hand, refers to modification of the existing land cover or complete conversion of biophysical cover of the land to a new land cover type (Solomon, 2005).
Though people have been using and modifying land to obtain food and other essentials for thousands of years, current rates, extents and intensities of Land use/land cover change are far greater than ever in history, resulting in unprecedented changes in ecosystems and environmental processes at local, regional and global scales (Lambin et al., 2003).
The area under crop cultivation in the world has increased globally from an estimated 3 00-400 million ha in 1 700 to 1 500-1800 million ha in 1 990, a 4.5- to fivefold increase in three centuries and a 50% net increase just in the twentieth century. The area under pasture increased from around 500 million ha in 1700 to around 3 1 00 million ha in 1990 (Ramankutty et al., 2002b). These increases imply to changes, clearances or conversions of forests cover and the transformation of natural grasslands, steppes, and savannas. Forest area decreased from 5000-6200 million ha in 1 700 to 4300-5300 million ha in 1 990. Steppes, savannas, and grasslands also experienced a rapid decline, from around 3200 million ha in 1700 to 1 800-2700 million ha in 1990 (Ramankutty et al., 2002b)
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