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.
However, the introduction of agricultural machinery into the country has led to increased level of mechanized farming with the aim to ease and hasten the processes of cultivation. Heavy machines are extensively used in land cultivation, from sowing to harvesting. This result in varying degrees of soil compaction that causes profound changes in soil structure. Soil structure is important and must not be damaged because it determines the ability of soil to hold and conduct water, nutrients, and air necessary for plant root activity. Compaction affects not only the physical, chemical and hydraulic properties of the soil, but also seed germination, root growth, water utilization, nutrient uptake by crop (Sataranayana and Ghildyal, 1970) and activities of soil microorganisms. It had been reported that the use of machineries and fertilizers may not preserve productivity if significant soil deterioration occurs (Lal, 1979).
Therefore, there is the need to develop better soil management practices that prevent or reduce the effect of soil compaction on soils and crops. This mainly involves management measures aimed at controlling traffic on soils during and after cultivation. This may also include adoption of agronomic practices that would improve soil physical condition such as conservation farming approaches based on no-tillage or minimum tillage. No-tillage practice refers to zero tillage (zero disturbances on the soil) with direct application of seeds into the soil that aims at 100% ground cover with no plow or disk used. Even with the best soil physical quality, crop productivity depends to a great extent on availability of soil moisture.
Moisture deficiency is one of the most important environmental factors affecting agricultural productivity around the world and may result in considerable yield reductions if unchecked. The need to produce more food with less water poses vast challenges to reassign existing water supplies, encourage more efficient use and promote natural resource protection (Hussain et al., 2007). One of the water conserving irrigation scheduling techniques is deficit irrigation which provides a means of reducing water consumption while minimizing adverse effects on yield and the environment (Ghinassi and Trucchi, 2001; Kirda, 2002; Panda et al., 2003). The main objective of deficit irrigation is to increase the water use efficiency (WUE) of a crop by eliminating irrigations that have little impact on yield. The resulting yield reduction may be small compared with the benefits gained through diverting the saved water to irrigate other crops for which water would normally be insufficient under traditional irrigation practices. This objective will be achieved through improvements in agronomic practice, cultivation of superior legume varieties, and increased efficiency of the nitrogen-fixing process itself by better management of the symbiotic relationship between the legumes and bacteria.
1.1 Problem Statement:
Food production capacity is faced with an ever-growing number of challenges, including a world population expected to grow to nearly 9 billion by 2050 and a falling ratio of arable land to population (PDESAUNS, 2007). Crop production in the northern Guinea Savanna of Nigerian is increasing in scope and intensity and crops are commonly grown under rainfed conditions. The major crops include maize, sorghum, rice, cowpea, groundnut, cotton, and soybean. However, the soils are increasingly being degraded by poor management practices. The soils consequently do not contain sufficient plant nutrients to support vigorous crop growth and high yield (Kowal 1972; Jones and Wild, 1975). There is need to ensure adequate food production using sustainable technologies. This may include application of chemical nitrogen fertilizers. However, chemical nitrogen fertilization is associated with environmental problems such as watershed contamination by nitrogen leaching, volatilization and de-nitrification and all these can be source of environmental pollution (Herridge et al., 2008). Rhizobium inoculants are widely used in agriculture for production as to improve soil fertility because of their ability to fix atmospheric nitrogen in association with legume crops. The products are environmentally friendly and cheaper source of nitrogen.
Successful inoculation and establishment of effective legume-rhizobium symbiosis can only be achieved in the presence of favourable soil physical conditions such as soil porosity, moderate bulk density, moisture content and soil temperature which are all influenced by soil tillage systems. Effects of these soil physical properties and processes can be expressed as changes of soil microbiological activity, soil respiration and consequently changes in plant growth and development
