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).
