In the twenty-first century, food security in many African countries remains a major concern. A recent United Nations report estimates that in sub-Saharan Africa (SSA-this region is the focus of the chapter, but some reference is made to the rest of the continent), 218 million people, some 30% of the population, suffer from chronic malnutrition (FAO 2009). Average cereal yields in the region of 1.2 Mg/ha

15.1 Introduction .................................................................................................. 401 15.1.1 Soils in Africa ...................................................................................403 15.1.2 Crop Intensification ..........................................................................404

15.2 Rates of Nutrient Depletion ..........................................................................406 15.2.1 Methodology .....................................................................................406 15.2.2 Nutrient Balances at International and Regional Scales ..................408 15.2.3 Nutrient Balances at National Scale .................................................409 15.2.4 Nutrient Balances at Farm and Plot Scales ....................................... 411

15.3 Drivers of Nutrient Mining ........................................................................... 412 15.3.1 Population Pressure .......................................................................... 412 15.3.2 Low External Nutrient Inputs ........................................................... 413 15.3.3 Climatic Risk .................................................................................... 414

15.4 Addressing the Challenge of Nutrient Mining ............................................. 414 15.4.1 Recycling Nutrients in Peri-Urban and Urban Agriculture .............. 414 15.4.2 Conservation Agriculture ................................................................. 415 15.4.3 Integrated Soil Fertility Management............................................... 416 15.4.4 Policy Issues ..................................................................................... 416

15.5 Conclusions ................................................................................................... 417 Abbreviations ......................................................................................................... 418 References .............................................................................................................. 418

contrast with average yields of 3 Mg/ha in the developing world as a whole. Many economic and technological factors have been cited to explain these low crop yields, including the fact that 80% of the total farm area in SSA is made up of the 33 million farms of less than 2 ha. Whereas smallholder farms also dominate agriculture in most areas of Asia, the high yields there were built on intensification utilizing the Green Revolution packages of irrigation, modern crop varieties, and external inputs. Apart from the lack of irrigation in SSA, the most obvious technological factor contributing directly to the low yields is the inadequate use of external inputs. The Food and Agriculture Organization (FAO 2009) estimates that fertilizer use on arable land in SSA in 2002 amounted to only 13 kg/ha compared with 190 kg/ ha in Asia. Continuing low yields will not meet the demand for food as the population increases from 770 million in 2005 to between 1.5 and 2 billion in 2050. The consequent intensification through continuous cropping to produce more food will exacerbate land degradation, which is already widespread (Vlek et al. 2010).