2004; Kelly, 2006). Mineral fertilizer consumption in Niger is the lowest in the world and amounts to only 0.3 kg of plant nutrients per hectare on average (World Bank, 1997). Limited financial means and the lack of subsidies seem to be the primary reason of the absence or low application of fertilizers and chemicals.

To date, fertilizer use in SSA has not led to increases in agricultural productivity on the scale observed elsewhere. Fertilizer consumption is only 9 kg ha-1 within the region compared with 73 kg ha-1 in Latin America, 100 kg ha-1in South Asia and 135 kg ha-1 in East and Southeast Asia (FAO, 2004a). Such low levels of fertilizer use, combined with shorter fallow periods and insignificant organic fertilizer inputs represent a serious threat to agricultural sustainability. African soils are being steadily depleted of nutrients due to farming without fertilizers (Matlon, 1987; Stoorvogel and Smaling, 1990; Van der Pol, 1992; Cleaver and Schreiber, 1994; Sanders et al., 1996; Steiner, 1996; Buresh et al., 1997; Sanchez et al., 1997; Smaling et al., 1997; Bationo et al., 1998; Eswaran et al., 2001). Fertilizer use is projected to need to increase in SSA from 9 to at least 30 kg ha-1 during the next decade, but increased use may have undesirable environmental impacts such as soil acidification, water pollution and health problems.

No single approach is sufficient to improve soil fertility in SSA. Integrated soil management, combining organic fertilizers (compost, manure, green manure) and reasonable quantities of synthetic fertilizers is an approach adaptable to locally available resources. Recent research on marginal soils in Burkina Faso by ICRISAT has shown that it is possible to increase millet and sorghum yields profitably by using inorganic fertilizer in combination with techniques that conserve and concentrate soil moisture and organic matter (http://www.icrisat.org/gt-aes/IFADPamph.pdf).

Due to different agroecological regions, farmers in SSA use a wide variety of traditional soil and water management techniques. An intensive system of soil and water management was developed over centuries by the Mandara population in the northern uplands of Cameroon and the Dogon people of Mali to restore and maintain soil fertility (Roose, 1994). This system includes terraces, alignment of stones, small dams, drop pipes of irrigation, wells and microdikes combined with agroforestry, compost, mulch
and crop rotations. The Dogon people of Mali developed a series of soil and water management methods. The indigenous agronomic practices of the Kuba and Zande peoples in the Congo involved cassava, cereals and legume rotations; the Sonjo of Tanzania used a sweet potato, cereals and irrigation complex; the Lugbara used a cassava, cereals, legumes and banana complex in Uganda; and in West Africa, rice, tubers and legumes formed the base of an agronomic complex (Kajoba, 1993). These techniques are sustainable at low population pressures. There is impressive historical evidence of the ability of pre-colonial societies in SSA to adapt production systems and livelihood strategies to local ecological conditions resulting in environmental sustainability.

In some SSA countries, programs were imposed during the colonial and post-colonial eras to solve wind and runoff erosion and water problems. This is the case in the region of Machakos in Kenya, where a program of terrace-building

was imposed during the fifties and led to the yearly building of about 5000 km of new terraces (Tiffen et al., 1994; Mortimore and Tiffen, 1995). In Zimbabwe, colonial authorities had imposed the building of more than 7000 km of small breakwaters between 1929 and1938. This practice continued until 1957, totaling more than 200,000 ha of communal lands (Whitlow, 1988).

The same policies were followed in Malawi where 118,000 km of small breakwaters were built between 1945 and 1960 (Stocking, 1985) and in Zambia’s eastern province where many projects were accomplished in the forties and fifties (Mukanda and Mwiinga, 1993). In many cases, measures of sustainable land and water management were rejected by the local population (for example in Zimbabwe) because they were not involved in the process. However, in some regions, techniques and practices had been broadly adopted by farmers anxious for investments to transform their farming systems, as is the case of the Machakos in Kenya. Throughout the region until post-independence, land uses were affected by imposed programs and technologies.

International Institute of Tropical Agriculture (IITA), International Livestock Research Institute (ILRI), International Crops Research Institute for the Semiarid Tropics (ICRISAT), International Centre for Research in Agroforestry (ICRAF) and other research centers, governments, universities and NGOs have been working with farmers and national scientists to identify appropriate solutions to increase productivity in sustainable ways. A number of promising technologies, such as natural resource management (NRM), integrated soil fertility management (ISFM), improved land husbandry (ILH), soil and water conservation (SWC) and conservation agriculture, have been developed and adopted with some success. Mucuna (Mucuna pruriens [velvetbean]) cover cropping is an example of a simple regenerative component for farm systems that can adapted by farmers for local conditions. It has helped many farmers ameliorate soils in maize systems.

Burkina Faso, Mali and Niger have seen remarkable transformations of formerly degraded and abandoned lands with the adoption of traditional water-harvesting techniques, for example, tassas in Niger and zaï in Burkina Faso. The average family in Burkina Faso using the zaï technology has moved from annual cereal deficits of 644 kg (equivalent to 6.5 months of food shortage) to producing a surplus of 153 kg per year (Reij, 1996).
2.1.1.3 Water management
Water scarcity is becoming one of the major limiting factors to economic development and welfare in large parts of sub- Saharan Africa. Unfavorable climatic factors such as erratic rainfall, high evaporative demand, several drought series, etc., contribute to water scarcity. In areas where the climate is hot and dry (e.g., the Sahel region), irrigated lands are subject to substantial water losses through evapotranspiration. Salts contained in precipitation and irrigation water remain in the soil and increase in concentration when the water evaporates from the soil or when the plants take up water for transpiration. If the salt is not leached from the soil, the salt concentration increases constantly, subsequently causing reductions in crop yield.

Sub-Saharan Africa lags far behind the rest of the world