Africa is rich in both natural and human resources, yet nearly 200 million of its people are undernourished because of inadequate food supplies. Comprehensive strategies are needed across the continent to harness the power of science and technology (S&T) in ways that boost agricultural productivity, profitability, and sustainability -- ultimately ensuring that all Africans have access to enough safe and nutritious food to meet their dietary needs. This report addresses the question of how science and technology can be mobilized to make that promise a reality.
Specific agronomic interventions are needed for the large diversity in soil characteristics that prevail in each locality. Low fertilizer application rates in African countries, reaching only 11 kilograms per hectare of harvested land compared with a world average of 96 kilograms per hectare (FAO, 1999), favour the use of organic fertilizers, such as manure and legumes. (Use of legumes is discussed in Box 4.8.) Current soil nutrient depletion rates, due to increased pressure on the land for food production, warrant an integrated approach to maintain soil health.
Numerous studies reveal a synergistic effect of inorganic and organic fertilization on soil and crop productivity, while neither component in itself shows sustained long-term improvements (Ahmed et al., 2000; Ahmed and Sanders, 1998; Bationo et al., 1998). Also, Giller (2001) points out that the role legumes can play varies between systems, due to strong environmental effects on nitrogen fixation.
Deposits of rock phosphate are useful in eliminating phosphorus deficiencies, which are particularly widespread in East Africa and the Sahel (Van Straaten, 2002). The mild acidity of soils (pH 5-6) in Western Kenya helps to dissolve high-quality rock phosphate, supplying crops with adequate amounts of phosphorus for several years and doubling or tripling maize yields (Sanchez, 2002). Phosphorus deficiency is the most limiting factor for legume productivity in tropical soils (Franco and Munns, 1982). Africa's resources of rock phosphate in combination with zero tillage may be used to break through the low soil organic matter and increase soil productivity (Sisti et al., 2003). Often the availability of high-reactivity rock phosphate is limited, and the effectiveness depends on numerous conditions, such as soil pH and water status. Thus exploitation and application depend on individual circumstances.
The spatial variability of soils requires special attention in integrated nutrient management. Variability is large at regional level and also in farmers' fields. Brouwer and Powell (1998) showed close relations between micro-topographic characteristics of the field and relative wetness and leaching of nitrogen and phosphorus. They indicated that more efficient use can be made of scarce, locally available resources of manure and urine, when application rates are attuned to the variation in the field. Simple procedures such as scoring techniques will capture the variability in yield for guiding spatial application (Gandah et al., 2000). Hence, the principles of precision agriculture can be applied through advanced technologies of satellite-based geo-referenced machines, but also through visual assessment of the micro-topographical characteristics by farmers in their fields.
Integrated approaches bring benefits in the long-term, by preventing both physical and chemical degradation of soils (the typical characteristics of unsustainability) while simultaneously achieving short-term productivity gains. Targeted interventions such as a voucher system for poor farmers to acquire small packs of fertilizers through traders have little distortionary impact on the market, while stimulating fertilizer use (IFDC, 2003).