CHAPTER ONE
1.0 INTRODUCTION
Soybean (Glycine max L.) is the most important and most widely grown of the grain legumes worldwide (Giller and Wilson, 1991) of great nutritional value and enormous uses. It is a source of protein in human food, animal feed and industrial products. Proximate composition of soybean is 40% protein, 20-21 % fat (oil), 32-35% carbohydrate, 5% ash (mineral) and 3% fibre (Anonymous, 2011). Soybean is native to East Asia and first introduced to Africa in late 1800s (Shurtleff and Aoyagi, 2007), and to Nigeria in the year 1904 (Ezedimma, 1964). Nigeria is the largest soybean producing country in Sub-Saharan Africa. A total of about 661,000 ha of soybean were harvested in West Africa out of 99,501,101 ha cultivated worldwide. Nigeria accounts for 95% and the remaining 5% in the rest of the West African countries (FAO, 2009).
Soybean is one of the major legume crops cultivated in northern Guinea Savannah (NGS) of Nigeria. Although its production among legumes requires assimilation of large quantity of nitrogen for maximum yield, soils of this region are poor in nutrient status, especially total N (Machido et al., 2011; Laditi et al., 2012). The situation is further worsened by nutrient depletion by crops and other related processes, such as leaching, denitrification, volatilization and removal of crop residues for alternative uses (Yakubu et al., 2010).
Among the means available to supply and improve soil nitrogen status, fertilizer plays an important role. However, the production and use of chemical nitrogen fertilizers is historically influenced by changing; and often interrelated factors such as increasing populations and economic growth, agricultural production, prices, and government policies (FAO, 2011). Their production requires a great consumption of fossil fuels (1-2 % global fossil fuel) and is subjected to constant variations in prices (Vieira et al., 2010). The comparison, in terms of economic and ecological costs, between chemical and biological nitrogen fertilizers shows that biological nitrogen fertilizers represents an economic, sustainable and environmental friendly resource to guarantee the nitrogen requirement of an agro-ecosystem. It has been reported that significant portion of soybean N (up to 80%) (Salvagiotti, 2008) is derived from biological nitrogen fixation (BNF) when grown in association with effective and compatible soil bacteria known as Bradyrhizobium (Chianu et al., 2009). Although, yields of legumes can be improved by additionof appropriate rhizobium inoculants, this can only be sustained and assured under suitable soil environment. Suitability of the soil environment depends soil management practices. An important soil management practice that influences soil quality is tillage (Mahdi and Hanna, 2004). The traditional tillage practice in this zone involves manual hoe ridging and weeding. These are done with no special attention to conservation measures against soil nutrient depletion (mining), soil erosion and runoff (Kirchhof and Odunze, 2003) and many changes in soil physical qualities.