ADVANCES IN INTEGRATED SOIL FERTILITY MANAGEMENT IN SUB SAHARAN AFRICA: CHALLENGES AND OPPORTUNITIES

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ADVANCES IN INTEGRATED SOIL FERTILITY MANAGEMENT IN SUB SAHARAN AFRICA: CHALLENGES AND OPPORTUNITIES

Since the 1970s, research throughout West Africa showed that low soil organic matter and limited availability of plant nutrients, in particular phosphorus and nitrogen, are major bottlenecks to agricultural productivity, which is further hampered by substantial topsoil losses through wind and water erosion. A few widely recognized publications pointing to massive nutrient mining of the existing crop–livestock production systems triggered numerous studies on a wide array of management strategies and policies suited to improve soil fertility. Throughout Sudano-Sahelian West Africa, the application of crop residue mulch, animal manure, rockphosphates and soluble mineral fertilizers have been shown to enhance crop yields, whereby yield increases varied with the agro-ecological setting and the rates of amendments applied. In more humid areas of Western Africa, the intercropping of cereals with herbaceous or ligneous leguminous species, the installation of fodder banks for increased livestock and manure production, and composting of organic material also proved beneficial to crop production. However, there is evidence that the low adoption of improved management strategies and the lack of long-term investments in soil fertility can be ascribed to low product prices for agricultural commodities, immediate cash needs, risk aversion and labour shortage of small-scale farmers across the region. The wealth of knowledge gathered during several decades of on-station and on-farm experimentation calls for an integration of these data into a database to serve as input variables for models geared towards ex-ante assessment of the suitability of technologies and policies at the scale of farms, communities and regions. Several modelling approaches exist that can be exploited in this sense. Yet, they have to be improved in their ability to account for agro-ecological and socio-economic differences at various geographical scales and for residual effects of management options, thereby allowing scenario analysis and guiding further fundamental and participatory research, extension and political counselling. Soil fertility – the perpetual issue Owing greatly to the two major Sahelian droughts in the early 1970s and 1980s, the poor productivity of agropastoral systems in Sudano-Sahelian West Africa (SSWA) has raised worldwide concern and subsequently stimulated numerous research and development projects dealing with issues of soil This article has been previously published in the journal “Nutrient Cycling in Agroecosystems” Volume 76 Issues 2–3. A. Bationo (eds.), Advances in Integrated Soil Fertility Management in Sub-Saharan Africa: Challenges and Opportunities, 1–28. © 2007 Springer. fertility, land degradation and desertification (Schlecht and Hiernaux 2004). For the West African Sahel in particular, Dutch scientists demonstrated that above an average annual precipitation of 250 mm, low soil organic matter (SOM) and limited availability of plant nutrients, in particular phosphorus (P) and nitrogen (N), are major bottlenecks to plant production (Penning de Vries and Djitèye 1982; Breman and De Wit 1983). Even if one has to be careful to generalize across SSWA given the widespread occurrence of erosion-related micro-habitats (such as depressions with clay and nutrient accumulations), there is evidence of a clear eco-regional gradient in the physico-chemical factors that limit plant growth. While at an average annual rainfall <300 mm and soil pH values between 5.5–6.5 water availability is most limiting, from 300 to 600 mm of rainfall and soil pH from 4.1 to 4.5, low P and N availability are primarily hampering biomass production. Finally, at <600 mm annual precipitation and soil pH>4.5, N, sulphur (S) and potassium (K) deficiencies prevail. Across the same rainfall gradient, the soils’ texture successively increases in clay content from below 5% to more than 15% (Buerkert et al. 2000). Another issue of major concern throughout the Sahelian and Sudanian zones were the topsoil loss and redistribution processes caused by wind and water erosion and their effects on the soils’ nutrient status and productivity. Both topics received new momentum in the early 1990s with a series of widely discussed nutrient budget assessments pointing to massive nutrient mining of the existing crop–livestock production systems (Stoorvogel and Smaling 1990; Van der Pol 1992; Krogh 1997). The controversial discussion about the validity of these budgets at different scales triggered a new series of studies on nutrient and natural resource management strategies and policies which aimed at improving soil fertility and consequently crop and livestock performance in SSWA (Bationo et al. 1998). Erosion control by mulch application or ridging, a better integration of livestock and crop production and targeted use of organic amendments were all seen as prerequisites to maintain soil fertility (McIntire et al. 1992; Powell and Unger 1998). Despite controversy about the necessary technical and political approaches, there was widespread consensus that any increase in the region’s agricultural production level would require substantial inputs of mineral fertilizers (Van Keulen and Breman 1990; Buerkert and Hiernaux 1998; Breman et al. 2001). Concurrently with these more fundamental studies, a considerable number of technologies were generated to improve the productivity of African soils, but farmers have not or only partly implemented many of these (Bationo et al. 1998; Haigis 2004). Increasingly, social scientists, economists and agronomists started to acknowledge the efficiency of farmers’ low external input strategies to maintain the productivity of selected parts of their fields (Brouwer et al. 1993; Lamers and Feil 1995; Lamers et al. 1995; Sterk and Haigis 1998), and provided contrasting evidence to previous predictions of imminent doomsday-scenarios (Scoones and Toulmin 1998; Mazzucato and Niemeijer 2001; De Ridder et al. 2004). The existence of such gaps between scientific findings and farmers’ reality, however, only partly explains the low adoption rates observed for recommendations to enhance the soil productivity of farmers’ fields. Aiming at conclusions about how to better match our present knowledge with farmers’ shortand long-term needs of increased productivity, the achievements and shortcomings of relevant studies on soil fertility maintenance in the crop–livestock systems of SSWA were critically reviewed. Hereby, the analysis concentrated on the comparability of results of similar studies across locations and climatic gradients, and the quantification of possible residual effects of technologies on soil fertility and productivity. Particular emphasis was put on the understanding of the physical and social buffering capacity of the land use systems under study. It was also evaluated whether the studies accounted for the requirement and availability of amendments, labour and capital needed for specific technologies at the farmers’ level. In addition, the usefulness of existing bio-economic and land use models for ex-ante analysis of the biophysical and socioeconomic benefits of technologies from the plot to the regional scale was examined. In the present context, the term Sahelian zone is used synonymously for the semi-arid zone (average rainfall 250– 600 mm a ) and the term Sudanian zone for the sub-humid zone (600–900 mm a ).

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