Africa, nutrient depletion is widespread, with average an­nual rates of 22 kg N, 2.5 kg P and 15 kg K per ha of arable land (Stoorvogel and Smaling, 1990). Low external input technologies aiming at soil fertility improvement can seldom reduce these rates (Onduru et al., 2006)
.      Protected cultivation systems. Protected cultivation of high value crops has expanded rapidly in the past decades (Castilla et al., 2004), especially in the Mediterranean ba­sin (Box 6-1). At present, however, greenhouse production with limited climate control is ecologically unsustainable as it produces plastic waste and contaminates water due to intensive use of pesticides and fertilizers. Demand for in­novation thus exists with regard to reducing environmental impact, as well as enhancing productivity, product quality and diversity.
     Scope exists to develop affordable plastic films that improve radiation transmission quantitatively and qualita­tively. Multilayer, long-life, thermal polyethylene films can combine desirable characteristics of various materials such as anti-drop and anti-dust effects. Photoselective films have the potential to influence disease and insect pest behavior by blocking certain bands of the solar radiation spectrum (Papadakis et al., 2000) or to limit solar heating without reducing light transmission (Verlodt and Vershaeren, 2000). Protected cultivation has its own, specific pest and disease populations as well as specific challenges related climate and substrate. Plant breeding for these specific conditions has the potential to reduce significantly the amount of pollutants released, while improving productivity. Grafting vegetables to resistant rootstocks is a promising option to control soil-borne pathogens (Oda, 1999; Bletsos, 2005; Edelstein and Ben-Hur, 2006) and may help to address salt and low tem­perature stress (Edelstein, 2004), but needs further research to improve rootstocks. Pest and disease control with the use of antagonists has developed quickly in protected cultures in Northern Europe and Spain (Van Lenteren, 2000, 2003). There are many site and crop specific possibilities for fur­ther development of non-chemical pest control for protected cultivation.
     Production in low-cost greenhouses has the potential to increase productivity and income generation, to improve water use efficiency and reduce pollution of the environ­ment. Variability in climatic and socioeconomic conditions will require the development of location-specific solutions.

Post-harvest loss. Although reduction of post-harvest losses has been an important focus of AKST and development pro­grams in the past, in many cases the technical innovations faced sociocultural or socioeconomic problems such as low profit margins, additional workload or incompatibility with the existing production or post-production system (Bell, 1999). The divergence between technical recommendations and the realities of rural life translated in many cases into low adoption rates.     In specific cases, large shares of food produced are lost after harvest. Yet, the rationale for improvements in the post-harvest systems has been shifting from loss preven­tion (Kader, 2005) to opening new markets opportunities (Hellin and Higman, 2005). Making markets work for the poor (Ferrand et al., 2004) is emerging as the new rationale of development, reflecting a shift away from governmental

operation of post-harvest tasks to enabling frameworks for private sector initiatives in this field (Bell et al., 1999).
     Ecological agricultural systems, which are low external input systems that rely on natural and renewable processes, have the potential to improve environmental and social sus-tainability while maintaining or increasing levels of food production. There is now increasing evidence of the produc­tive potential of ecological agriculture (Pretty, 1999; Pretty, 2003; Pretty et al., 2006; Badgley et al., 2007; Magdoff, 2007).      Some contemporary studies also show the potential of ecological agriculture to promote environmental services such as biodiversity enhancement, carbon sequestration, soil and water protection, and landscape preservation (Cull-iney and Pimentel, 1986; Altieri, 1987; Altieri, 1999; Altieri, 2002; Albrecht and Kandji, 2003).      There is now substantial scientific evidence to show that designing and managing agricultural systems based on the characteristics of the original ecosystem is not a threat to food security. A survey of more than 200 projects from Latin America, Africa, and Asia, all of which addressed the issue of sustainable land use, found a general increase in food production and agricultural sustainability (Pretty et al., 2003). Likewise, low external input crop systems, when properly managed, have demonstrated the potential to in­crease agricultural yield with less impact on the environ­ment (Bunch, 1999; Tiffen and Bunch, 2002; Rasul and Thapa, 2004; Pimentel et al., 2005; Badgley et al., 2007; Scialabba, 2007). A recent investigation comparing organic with conventional farming experiences from different parts of the world indicates that sustainable agriculture can pro­duce enough food for the present global population and, eventually an even larger population, without increasing the area spared for agriculture (Pretty et al., 2003; Badgley et al., 2007).      In spite of the advantages of ecological agriculture in combining poverty reduction, environmental enhancement and food production, few studies address the issues of how to assess the tradeoffs (Scoones, 1998). Tradeoff analysis to assess dynamic relations between the provision of ecosys­tem and economic services can help to harmonize land use options and prevent potential conflict regarding the access to essential ecosystem services (Viglizzo and Frank, 2006).