Land degradation was a significant global issue during the
20th century and remains of high importance in the 21st
century as it affects the environment, agronomic productivity,
food security, and quality of life (Eswaran et al. 2001). Soil
degradative processes include the loss of topsoil by the action
of water or wind, chemical deterioration such as nutrient
depletion, physical degradation such as compaction, and
biological deterioration of natural resources including the
reduction of soil biodiversity (Lal 2001).
In Nigeria, West Africa, human-induced soil degradation is
a common phenomenon. Its severity is light for 37.5% of the
area (342,917 km2
), moderate for 4.3% (39,440 km2
), high
for 26.3% (240,495 km2
), and very high for 27.9% (255,167
km2
) (UN Food and Agriculture Organization 2005). Soil
erosion is the most widespread type of soil degradation in the
country and has been recognized for a long time as a serious
problem (Stamp 1938). In 1989, 693,000 km2
were already
characterized by runoff-induced soil loss in the south and
231,000 km2
were degraded, mainly by wind erosion, in the
north. Sheet erosion dominates all over the country, whereas
rill and gully erosion are common in the eastern part and
along rivers in northern Nigeria (Ologe 1988; Igbozurike
1989).
Redistribution of soil by erosion and deposition is the
result of perturbation and a natural landscape-forming
process. However, it has been greatly accelerated by human
activities in recent decades as the traditional shifting
cultivation system has been replaced by more intensive but
generally unstable cropping systems (Lal 1993a). The main
reason for the land use intensification was and still is the
increase in food production required to feed the rapidly
growing population. For example, the Nigerian population
has increased from 115 million in 1991 to 140 million in
2006 (Federal Republic of Nigeria 2007).
The expansion of agriculture into marginal areas,
deforestation, the shortening or elimination of fallows,
inappropriate farming practices, and low input inevitably
have several environmental and economic impacts, especially
in sub-Saharan Africa where the resilience ability of the soil is
limited (Lal 1995a). This expansion of agriculture causes onsite
degradation of natural resources and productivity decline.
For example, Mbagwu et al. (1984) observed that soil erosion
causes a yield reduction of about 30% to 90% in some
root-restrictive shallow lands of southern Nigeria. Off-site
problems, such as the siltation of reservoirs, are also common
consequences of soil loss. Hence, low agricultural production,
food insecurity, low income of the rural population, and
poverty are some consequences of soil erosion. Avoidance
of soil loss by improved management and the conservation
of natural resources is therefore important to maintain the
functions of the soil and contribute to food security today
and for future generations (Ehui and Pender 2005).
Research on soil conservation has been conducted for
many years in Sub-Saharan Africa (e.g., Fournier 1967;
Greenland and Lal 1977; Quansah 1990; Kayombo and
Mrema 1998; Ehrenstein 2002) and in Nigeria (Lal 1976a,
1990). Initiatives have resulted in various so-called on-farm
strategies including agronomic measures, soil management,
and mechanical methods, as well as off-farm strategies,
including mechanical or biological soil conservation
technologies.
This work provides an extensive literature review to
obtain information on past and present initiatives focusing
on on-farm soil conservation strategies and their application
in Nigeria (figure 1). Based on this, the most promising soil
conservation technologies for the savanna are identified to
improve the management and conservation of soil resources
in the country