cially disaggregated processes and explicit management of tradeoffs among diverse groups. Moreover, a mere set of activities and interventions will not revitalize a system unless its various components are explicitly analyzed for potential tradeoffs and synergies (German et al., 2007).

3.4.1.1 Land and soil degradation
In Africa, the total area of degraded lands is estimated to be 128 million ha. Degradation occurs mostly in the drylands and tropical forest margins where soils have lost their ability to provide ecosystem services (such as nutrient cycling, water filtration, waste absorption and the breakdown of vegetative cover and soil formation) and have become a source of crop yield decline (UNEP, 2002). In SSA, 65% of agricultural cropland and 31% of permanent pasture are estimated to be degraded with 19% of the land seriously degraded (Scherr, 1999).

          Soil nutrients vary greatly across locales and countries. Even in resource-limited small-scale agriculture, not all fields are continuously mined; some fields have positive nutrient balances, usually through use of nutrients concentrated from other parts of the farm (Vanlauwe and Giller, 2006). Yet depletion of soil fertility in SSA is a major cause of low production (Kumwenda et al., 1997; NEPAD, 2002; Ajayi et al., 2006; Henao and Baanante, 2006; Okalebo et al., 2006). The factors contributing to low use include limited access to credit, poor infrastructure in rural areas, weak purchasing power among poor farmers, limited access to fertilizer information, few trained fertilizer stockers, inputs not in affordable sizes, low and irregular supplies and lack of appropriate fertilizers for local conditions (Okalebo et al., 2006).

           There are large variations in fertilizer use among countries (IAC, 2004). Increased fertilizer application is seen by

most scientists as essential, yet availability and costs constrain farmers in SSA and soil moisture stress limits the uptake of nutrients. There is a need to conserve both water and soil organic matter. Phosphate rocks of various origins and agronomic effectiveness are found widely in Africa (Okalebo et al., 2006).

          Research has focused on options for land and soil management that would alleviate biophysical, land-related constraints. These include soil erosion, low levels of major nutrients (organic N in soil organic matter, P and K), loss of vegetative cover, extreme climatic events (see Chapters 1 and 2), and socioeconomic factors such as access to markets and opportunity to develop them and access to land and labor.

          Substantial progress has been made in developing new tools and technologies and applying them to participatory agricultural approaches. These include the integration of geographic information systems and remote sensing, agroecological and farming systems analysis, monitoring and evaluation of ecosystem services, rapid spectroscopy techniques of soil analysis, and molecular tools to study soil biodiversity (Shepherd and Place, 2006). Research has resulted in major innovations in crop–livestock–tree systems, as well as practical options for soil fertility improvement. Although constraints still exist, changes in land use patterns and increased productivity have been noted in several key farming systems (IAC, 2004).

3.4.1.2 Climate change
Climate change, which affects the resilience of farming systems, plant and livestock growth and yields will be increasingly important in SSA (IPCC, 2007). A recent analysis of long-term trends (1900 to 2005) indicates rising temperatures in Africa as a whole, and drying or decreased pre-