EFFECTS OF SOIL AMENDMENTS AND RHIZOBIUM INOCULATION ON SOYBEAN NODULATION, GROWTH AND YIELD IN THE SEMI-DECIDUOUS FOREST AGRO-ECOLOGICAL ZONE OF GHANA BY ALHASSAN MOHAMMED (10703451) THIS THESIS IS SUBMITTED TO THE UNIVERSITY OF GHANA, LEGON IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF MASTER OF PHILOSOPHY IN CROP SCIENCE DEPARTMENT OF CROP SCIENCE COLLEGE OF BASIC AND APPLIED SCIENCES UNIVERSITY OF GHANA, LEGON JANUARY, 2023. University of Ghana http://ugspace.ug.edu.gh i DECLARATION I, Mohammed Alhassan hereby declare that except for the duly cited references of other researchers, this thesis I am submitting to the University of Ghana for Master of Philosophy Degree is my original research work carried out under supervision. Its entirety or part has not been previously presented elsewhere to another institution for the award of a degree. Mohammed Alhassan Signature……………………. (Student) Date: 16/10/2023 Prof. Samuel Adjei-Nsiah Signature…………………. (Principal Supervisor) Date…16/10/2023……… Dr. Jacob Ulzen Signature………………….. (Co-Supervisor) Date: 16/10/2023 University of Ghana http://ugspace.ug.edu.gh ii DEDICATION I dedicate this thesis to Almighty Allah for His protection and guidance throughout my education. Special dedication also goes to my mother, Adama Mohammed of blessed memory. May her soul continue to rest in perfect peace. University of Ghana http://ugspace.ug.edu.gh iii ACKNOWLEDGEMENTS Utmost thanks go to the Almighty Allah for the knowledge, wisdom and understanding given to me and also for His guidance and protection throughout this study. I also take this opportunity to express my profound gratitude to my distinguished supervisors, Prof. Samuel Adjei-Nsiah and Dr. Jacob Ulzen for their unflinching support, patience, and guidance as well as the tutelage throughout this research. I want to acknowledge Mr. Adams Yakubu and Mr. David Martey of FOHCREC for their support during the fieldwork. My appreciation to Mr. Samuel Osabutey, and my former course mates Mr. Jacob Agbemadzi and Michael Zogli, all of Department of Crop Science, for their inspirational words. Finally, I am indebted to my wife Mrs. Zalia Yussif and Alhaji Musah Gariba, for their words of encouragement and support during the study. University of Ghana http://ugspace.ug.edu.gh iv TABLE OF CONTENTS DECLARATION ........................................................................................................................ i DEDICATION ........................................................................................................................... ii ACKNOWLEDGEMENTS ......................................................................................................iii TABLE OF CONTENTS .......................................................................................................... iv LIST OF TABLES ..................................................................................................................... x LIST OF FIGURES .................................................................................................................. xi ABSTRACT ............................................................................................................................. xii CHAPTER ONE ........................................................................................................................ 1 INTRODUCTION ..................................................................................................................... 1 CHAPTER TWO ....................................................................................................................... 5 LITERATURE REVIEW .......................................................................................................... 5 2.1 Origin and distribution of soybean ...................................................................................... 5 2.2 Morphological description of soybean ............................................................................... 6 2.3 Soil requirement .................................................................................................................. 7 2.3.1 Soil pH ...................................................................................................... 8 2.3.2 Causes of soil acidity ................................................................................ 9 2.3.3 Effect of soil acidity on nutrient availability and plant growth .............. 11 2.3.4 Control of soil acidity ............................................................................. 13 2.3.5 Soil acidity and its effects on soybean production .................................. 14 University of Ghana http://ugspace.ug.edu.gh v 2.4 Factors that affect the growth and yield of soybean ........................................................ 15 2.5 Factors influencing nitrogen fixation in legumes ............................................................ 16 2.5.1 Soil moisture content ................................................................................................... 17 2.5.2 Soil temperature ........................................................................................................... 17 2.5.3 Availability of phosphorus .......................................................................................... 18 2.5.4 Population of rhizobia strain in the soil ....................................................................... 19 2.5.5 Soil nutrient ………………………………………………………………………….19 2.6 Lime requirement .............................................................................................................. 20 2.6.1 Types of liming materials ............................................................................................. 21 2.6.2 Advantages of liming in acidic soils............................................................................. 21 2.7 Biochar……….. ........................................................................................................... 23 2.7.1 Biochar production.................................................................................. 24 2.7.2 Biochar stability in the soil ..................................................................... 25 2.7.3 Structural composition of biochar ........................................................... 25 2.7.4 Biochar as liming material and mode of application .............................. 26 2.7.5 Agronomic importance of biochar .......................................................... 27 2.7.6 Importance of biochar in the environment .............................................. 28 2.7.7 Biochar impact on soil performance and resource implications ............. 29 2.7.8 Biochar and nitrogen fertilizer interactions ............................................ 30 2.7.9 Negative effect of biochar on soil ........................................................... 31 2.7.10 Effects of biochar on soybean growth ................................................... 31 University of Ghana http://ugspace.ug.edu.gh vi 2.8 Phosphorus ....................................................................................................................... 32 2.9 Soil available nitrogen ..................................................................................................... 35 2.10 Rhizobium Inoculation.................................................................................................... 36 2.11Soybean nodula tion, biological nitrogen fixation (BNF) and BNF-related factors ......... 37 2.12 Benefits of soybean .......................................................................................................... 40 CHAPTER THREE ................................................................................................................. 42 MATERIALS AND METHODS ............................................................................................. 42 3.1 Site description ................................................................................................................... 42 3.1.1 Soil characteristics .................................................................................. 43 3.2 Experimental design and field layout ...................................................... 43 3.3 Soil liming ............................................................................................... 43 3.4 Biochar and Phosphorus amendment ...................................................... 44 3.5 Rhizobium inoculation ............................................................................ 45 3.6 Soybean planting and cultural practices .................................................. 45 3.7 Measurement of grain yield and growth parameters ............................... 46 3.7.1 Nodule number and effectiveness ........................................................... 46 3.7.2 Nodule fresh weight ................................................................................ 46 3.7.3 Shoot biomass ............................................................................................. 46 3.7.3 One hundred seed weight ........................................................................ 47 3.7.4 Harvest index .......................................................................................... 47 3.7.5 Grain yield .............................................................................................. 47 University of Ghana http://ugspace.ug.edu.gh vii 3.7.6 Haulm weight .......................................................................................... 47 3.8 Agronomic P-use efficiency .................................................................... 48 3.9 Rainwater-use efficiency ........................................................................ 48 3.10 Final soil chemical analysis .................................................................... 48 3.10.1 Soil pH ................................................................................................... 49 3.10.2 Total Nitrogen by Kjeldahl Method ....................................................... 49 3.10.3 Available P determination ...................................................................... 49 3.10.4 Organic carbon determination ................................................................ 50 3.10.5 Soil exchangeable bases ........................................................................... 51 3.11 Statistical analysis ................................................................................... 51 CHAPTER FOUR .................................................................................................................... 52 RESULTS ................................................................................................................................ 52 4.1 Physico-chemical characteristics of the soil and rice husk biochar ........ 52 4.2 Effect of rhizobium inoculation, soil amendments, and phosphorus on soil chemical properties 120 days after treatment application. ........................... 53 4.3 Effect of rhizobium inoculation, soil amendments, and phosphorus on nodule number, nodule effectiveness and nodule fresh weight. .......................... 56 4.4 Effects of Rhizobium inoculation, soil amendments, and phosphorus on dry shoot biomass ................................................................................................ 57 4.5 Effects of rhizobium inoculation, soil amendments, and phosphorus on haulm weight and one hundred seed weight ........................................................ 59 4.5 Effects of rhizobium inoculation, soil amendments, and phosphorus on grain yield of soybean .......................................................................................... 59 4.6 Effects of rhizobium inoculation, soil amendments and phosphorus on harvest index (HI), P –use efficiency, and rainwater-use efficiency ................... 61 University of Ghana http://ugspace.ug.edu.gh viii CHAPTER FIVE ..................................................................................................................... 63 DISCUSSION .......................................................................................................................... 63 Soil and biochar characterizations ........................................................................................... 63 5.1 Effect of rhizobium inoculation, soil amendments and phosphorus on soil chemical properties. ...................................................................................... 64 5.2 Effects of rhizobium inoculation, soil amendments, and phosphorus on grain yield ............................................................................................................ 66 5.3 Effects of rhizobium inoculation, soil amendments, and phosphorus on nodule number, nodule effectiveness, nodule fresh weight, one hundred seed weight and dry shoot biomass.............................................................................. 67 5.4 Effects of rhizobium inoculation, soil amendments and phosphorus on harvest index (HI), P-use efficiency (PUE) and rainwater-use efficiency (RWUE) ............................................................................................................... 68 CHAPTER SIX ........................................................................................................................ 69 CONCLUSIONS AND RECOMMENDATIONS .................................................................. 69 6.1 Conclusions ............................................................................................. 69 6.2 Recommendations/ Suggestions ............................................................. 69 REFERENCES ........................................................................................................................ 70 APPENDICES ....................................................................................................................... 106 Appendix 1. ANOVA Table for Average nodule number ..................................................... 106 Appendix 3. ANOVA Table for Grain Yield ......................................................................... 108 Appendix 5. ANOVA Table for Haulms weight ................................................................... 110 Appendix 7. ANOVA Table for Shoot biomass .................................................................... 112 Appendix 9. ANOVA Table for Total Seed weight ............................................................... 114 Appendix 11. ANOVA Table for P-Use use efficiency ......................................................... 116 University of Ghana http://ugspace.ug.edu.gh ix Appendix 12. ANOVA Table soil pH .................................................................................... 117 Appendix 14. ANOVA Table Soil Organic Carbon .............................................................. 118 Appendix 16. ANOVA Table Soil Exchangeable Mg ........................................................... 119 Appendix 18. ANOVA Table Soil Available P ..................................................................... 120 University of Ghana http://ugspace.ug.edu.gh x LIST OF TABLES Table 3.1: Chemical properties of rice husk biochar used in the study. .................................. 44 Table 4.1: Initial physical and chemical properties of 0-20 cm layer of soil and rice husk biochar used for the experiment. ........................................................................................................... 53 Table 4.2: Effects of rhizobium inoculation, soil amendment, and phosphorus on soil chemical properties.................................................................................................................................. 55 Table 4.3: Effects of rhizobium inoculation, soil amendment, and phosphorus on nodule number, nodule effectiveness, nodule fresh weight, and dry shoot biomass ........................... 58 Table 4.4: Effects of rhizobium inoculation, soil amendments, and phosphorus on total seed weight, one hundred seed weight, haulm weight and grain yield. ........................................... 60 Table 4.5: Effects of rhizobium inoculation, soil amendment and phosphorus on harvest index (HI), P - use Efficiency and Rainwater-Use efficiency………………………………............62 University of Ghana http://ugspace.ug.edu.gh xi LIST OF FIGURES Figure 1: Daily and cumulative rainfall during the growing season in Kade, Ghana. ............. 42 University of Ghana http://ugspace.ug.edu.gh xii ABSTRACT Ghana's soybean cultivation is primarily restricted to the Guinea savanna and the forest/savanna transitional agro-ecological zones. Although soybean can be grown in the semi- deciduous forest zone, its productivity is limited due to low soil pH and limited nodulation. The study was conducted at the University of Ghana Forest and Horticultural Crops Research Centre at Kade in the semi-deciduous forest agro-ecological zone of Ghana between August and December, 2021.The objective of the study was to assess the combined effects of soil amendments, phosphorus fertilizer and rhizobium inoculation on soil chemical properties, nodulation, growth and yield of soybean. The experiment was laid in a split-split plot design with four (4) replications with main plot being soil amendments (No amendment, 2 tons/ha lime and 5 tons/ha rice husk biochar), subplot being P fertilizer at 0 and 20 kg P ha-1 and sub subplot with or without Rhizobium inoculation. Data on nodule number and effectiveness, shoot biomass, one hundred seed weight and grain yield were taken. Results from the study indicated that phosphorus application significantly influenced grain yield as grain yield was increased by about 60% due to P application. There was increase in soil pH from the initial 5.09 to 5.52 and 5.54 on plots that received biochar and lime respectively, 17 weeks after treatment application. The effect of inoculation on pH was also significant (p < 0.05). Rhizobium inoculation had significant effects on exchangeable K and Mg. The inoculated plots had exchangeable K and Mg values of 0.37 and 2.62 cmol (+)/kg soil, respectively, while the values for the uninoculated plots were 0.33 and 3.31 cmol (+)/kg soil for K and Mg respectively. Inoculation significantly influenced nodulation parameters such as nodule number, nodule effectiveness and nodule fresh weight of soybean. The application of rhizobium inoculant significantly (p < 0.001) increased nodule number and nodule effectiveness by 44 % and 45 % respectively, over plants that received no inoculants. The sole application of P fertilizer increased the number of nodules by 44 % compared to the plots that received no P fertilizer. University of Ghana http://ugspace.ug.edu.gh xiii However, this did not translate into increased grain yield. The interaction between Rhizobium inoculation and Phosphorus fertilizer significantly affected dry shoot biomass of soybean. Treatment interaction between soil amendments and P fertilizer significantly influenced P-use efficiency. The results show that the three factors that were studied did not interact to significantly influence nodulation, growth and yield of soybean However, the three factors interacted to significantly enhance nodulation and improve P-use efficiency and some soil chemical properties (OC, Total N and exchangeable Ca and Mg). However, it is recommended that farmers can apply phosphorus fertilizer at the rate of 20 kg P/ha for increased grain yield of soybean on acid soil. University of Ghana http://ugspace.ug.edu.gh 1 CHAPTER ONE INTRODUCTION Soybean [Glycine max (L.) Merrill] is an annual legume in the Fabaceae family and the genus Glycine Willd (Karnwal & Singh, 2009). The UN Food and Agriculture Organization classifies soybean as an oil seed rather than a pulse (Saxena & Vyas, 2016). Chinese traders along Africa's east coast introduced soybean to the continent in the nineteenth century (Ibrahim et al., 2018). South Africa was the largest soybean producer in SSA in 2018/19 (1.17 million MT), followed by Nigeria and Zambia (Engelbrecht et al., 2020). For the year 2019, total global soybean production was around 350 million tonnes, with Africa accounting for 3.1 million tonnes. Almost one-third of the latter (920,000 tonnes) was produced in West Africa (FAOSTATS, 2020). During the past ten years, production of soybean has increased steadily from 74,800 MT in 2008 to 176,670 MT in 2018 in Ghana (MoFA, 2019). Soybean is a key component of the predominantly cereal-based farming systems of the Guinea savannah agro- ecological zone of West Africa. It is an important component in poultry feed preparation. Soybean is also a source of household income and nutritional security, particularly among underprivileged households (Sanginga and Bergvinson, 2015). Because soybean contains high- quality protein, it can be used in place of expensive animal proteins to fight malnutrition. It contains all the necessary amino acids in addition to having a high protein level (FAOSTAT, 2009); explains why it has recently been advocated for large-scale production and usage as a meat alternative in Ghana (FAOSTAT, 2009). When grown in rotation with cereals, soybean reduces the amount of mineral nitrogen fertilizer required for cereal crops due to its symbiosis with nitrogen-fixing bacteria (NFB) (Sanginga 2003; Giller et al., 2011). In Ghana, soybean is mostly cultivated in the five northern regions and the northern part of Volta region where about 90 % of the crop is produced (MoFA, 2010). In recent times, soybean production is being expanded to the semi deciduous agro-ecological zone of Ghana where the soils are University of Ghana http://ugspace.ug.edu.gh 2 relatively better because of its increasing demand. However, in this part of Ghana, soybean production is limited by low soil pH and limited nodulation (Adjei-Nsiah et al., 2022). Recent research by Adjei-Nsiah et al. (2022) suggests that soybean could yield as high as 2.4 t ha-1 in the semi-deciduous agro-ecological zone with application of P fertilizer and rhizobium inoculation which is comparable to if not higher than yields obtained in the Guinea savanna agro-ecological zone under similar management practices (Adjei-Nsiah 2018, Adjei-Nsiah 2021; Ahiabor et al., 2014). It is anticipated that the yield could even be higher if the pH is increased to slightly acidic to neutral since soybean performs well within that pH range. P availability, biological nitrogen fixation and microbial activity are all decreased in acidic soils, which has an impact on the growth and yield of soybeans (Ahiabor et al., 2014). By adding rice husk biochar and conventional lime as soil amendments acidic soils can be corrected. Agricultural lime decreases Al and Mn toxicity that enhance plant roots system and promotes Ca, Mg, Mo, and P uptake (Brady and Weil, 2002). Biochar could be a panacea for the low pH and low P availability problem of Ghanaian soils. There is abundance of farm waste such as rice husk, cocoa pod husk and corncob which could be charred anaerobically to produce biochar. However, the use of these feedstock has received little attention by soybean farmers. Biochar enhances the organic matter content of the soil which has an impact on ion exchange capacity, plant nutrient retention, water holding capacity, bulk density and soil structure (Gaskin et al., 2007). Biochar made from rice husk and cocoa pod husk burned at 450 oC had accessible P concentrations of 531 mg/kg and 3897 mg/kg (Sam, 2014). As a result, biochar could be used as a liming material with the added benefit of being a P source. While several studies by Adjei-Nsiah et al., (2018); (2021); and (2022), Ahiabor et al., (2014), Ulzen et al., (2016) and Ronner et al., (2016), have demonstrated the beneficial effect of P- fertilizer application and rhizobium inoculation on the yield of soybean, information on the combined effect of liming, P-fertilizer application and rhizobium inoculation in the semi- deciduous forest agro-ecological zone of Ghana is scanty. University of Ghana http://ugspace.ug.edu.gh 3 Increasing soybean yields in the semi -deciduous forest zone starts by reducing the soil acidity to a level at which the crop can produce its potential, followed by the increase and maintenance of soil fertility through application of P fertilizer and introducing external rhizobial population through inoculation. Phosphorus often regulates nitrogen fixation by legumes (Pérez- Fernández et al., 2019), and this macronutrient is limited in Ghanaian soils (Buri et al., 2010; Masso et al., 2016). Seed inoculation with rhizobium strains improve nitrogen fixation in grain legumes (van Heerwaarden et al., 2018; Adjei-Nsiah et al., 2018; Ulzen et al., 2018; Osei et al., 2020; Ahiabor et al., 2014; Asei et al., 2015; Masso et al., 2016; Ronner et al., 2016). These inputs in addition to Bradyrhizobium inoculants will improve soil microbial activity, biological nitrogen fixation and chemical characteristics of acid soils. The aim of the present research was to evaluate the combined effects of lime (CaCO3), rice husk biochar, phosphorus fertilizer and Bradyrhizobium inoculants on soybean production in the forest agro-ecological zone of Ghana. The specific objectives of the study were to: 1. Assess the effects of agricultural lime (CaCO3), rice husk biochar, and Phosphorus fertilizer on the chemical characteristics of acidic soils. 2. Assess the effects of Bradyrhizobium inoculation, lime, rice husk biochar, and P fertilizer on soybean nodulation and P-use efficiency of soybean on acid soils. 3. Assess the effects of Bradyrhizobium inoculation, lime, rice husk biochar, and P fertilizer on growth and yield of soybean grown on acid soils. University of Ghana http://ugspace.ug.edu.gh 4 The objectives were based on the hypothesis that; 1. The application of agricultural lime (CaCO3) and rice husk biochar will improve soil chemical characteristics. 2. The addition of CaCO3 and rice husk biochar will increase soil pH and hence enhance nodulation, and P-use efficiency of soybean on an acid soil. 3. The addition of Bradyrhizobium inoculant, lime, rice husk biochar and P fertilizer will increase growth and yield of soybean. University of Ghana http://ugspace.ug.edu.gh 5 CHAPTER TWO LITERATURE REVIEW 2.1 Origin and distribution of soybean In general, researchers from all around the world stated that China is where soybeans originated. The annual wild soybean (Glycine soja) is a close relative of the soybean (Glycine max) that is currently cultivated in China. Glycine soja is only found in China, Japan, Korea and far Eastern Russia in East Asia, but China has the widest distribution, the highest numbers and the greatest variety of kinds (Qui and Chang, 2010). Based on observations that semi-wild soybeans are widely dispersed in northeast China but not in other areas, Fukuda (1933) theorized that northeast China is where the soybean originated. He added that different approaches to research and material collection may well have an impact on the fact that these wild soybeans are widely distributed in northeast China but are scarce in other areas. While Wang (1985) studied the origin of soybean using ancient Chinese literature, inscriptions on bones and tortoise shells of the Shang dynasty based on which he also concluded that the earliest region for cultivating soybean was around the central or downstream of the Yellow Valley which was seconded by Chang (1989) based on his study of the relationship between the origin of agriculture and the origin of soybean. Hymowitz (1970) also believed that the origin of the soybean was the eastern part of northern China, which he referred to as winter wheat (T. aestivum). Literature related to soybean in past dynasties of China was collected by Guo (1993) who analyzed the arguments related to the origin of soybean and concluded that the origin of cultivated soybean is northeast China, but the exact origin of soybean remains unknown and therefore thought that these arguments are not conclusive. Asia has the longest history of growing soybean, with China having the largest cultivated area of the crop. Japan, North and South Korea and Indonesia are some of the countries that cultivate soybean. In Japan, most soybean varieties are large-seed types and are used as vegetable soybean which is called University of Ghana http://ugspace.ug.edu.gh 6 ‘edamame’. A 100 fresh seed weight is greater than 70 g and a dry weight greater than 30 g, whereas in South Korea, varieties are small seed types; thus 100 seed weight is less than 15g (Qui and Chang, 2010). According to reports by Hymowitz (1984), soybean cultivation began in the United States as early as 1765, when Samuel Bowen, an east India Company sailor imported soybean from China to Savenna (Georgia). The United States dispatched scientists to China, Korea, and Japan to gather soybean germplasm. Thousands of accessions of soybean were brought back from these nations and are now the main sources of soybean breeding in the United States. In Africa, it was first introduced in 1857 (Shurtleff and Aoyagi, 2012) and later introduced by the Portuguese missionaries into Ghana in 1910, with major growing areas being, Bawku, Nakpanduri, Bimbilla, and Karaga. The main problem facing soybean farmers at that time was the loss of seed viability during storage (Plahar, 2006). 2.2 Morphological description of soybean As an annual crop, soybean plants have a substantial taproot system, the majority of which is in the soil. The hypocotyls typically produce adventitious roots, while the taproots typically grow into the soil. The alternating trifoliate leaves are oblong and lanceolate in shape with a mucronate tip. They have long petioles and tiny stipules (Chaturvedi et al., 2011). Soybean flowers emerge from auxiliary buds on the main stem and branches. A raceme of 2-35 papilionaceous flowers forms an inflorescence at each axil (Smith, 1995). Flowers typically self- pollinate, but in about 1% of cases, insects will cross-pollinate them (Chaturvedi et al., 2011). Short-stemmed, 13–15 in clusters, 3–7 cm long, hairy, light brown in colour, and somewhat constrictive between seeds are the characteristics of mature pods (Chaturvedi et al., 2011). Soybean cultivars can be divided into three categories based on their growth habits: determinate, semi-determinate, and indeterminate. In contrast to the indeterminate, which University of Ghana http://ugspace.ug.edu.gh 7 experiences simultaneous vegetative and reproductive growth, the determinate plant's vegetative growth is almost finished when it starts to flower. After flowering periods, indeterminate stems on semi-determinates abruptly stop growing vegetatively (OECD, 2000). A thicket of hairs, which are typically fine in nature, mostly conceals the stems. The stem carries nutrients and water while also supporting the flowers and leaves. Depending on the variety and planting date, soybeans can grow to heights of 60–140 cm. They have erect, bushy growth patterns (Belfield et al., 2011) The stem, pods and leaves are covered in tiny, brown or gray-like hairs. Each leaf of a trifoliate has three to four unique structures. The seeds might vary in appearance, size, colour and form, but they are typically round or oval and have a cream seed coat. Flowers are normally pea- shaped, 5-6 mm long, and come in a range of colours, such as white, purple, and pink. Pods can be black, yellowish, or brown in colour. The fruit of the soybean is a hairy pod that contains three to four seeds (Rienke and Joke, 2005; Kumudini, 2010; Chaturvedi et al., 2011). There are two stages in the growth and development of soybeans: the vegetative and reproductive phases. The fully unrolled leaf at the unifoliate node (V1), the first node above the unifoliate node (V2), three nodes on the main stem beginning with the unifoliate node (V3), and N nodes on the main stem beginning with the unifoliate nodes (V4) are all indicators of vegetative growth (Nand et al., 2010). 2.3 Soil requirement Soybean tolerates a variety of soil conditions, but thrives in warm, moist, well drained fertile loamy soils with adequate nutrients and good seed-to-soil contact for rapid germination and growth (Hans et al., 1997; Addo-Quaye et al., 1993). According to Ngeze (1993), soybean grows well in fertile sandy soils with pH between 5.5 and 7.0, and the crop can tolerate acidic soils better than other legumes, but it does not grow well in waterlogged, alkaline or saline soils. Keeping soil pH between 5.5 and 7.0 improves nutrient availability such as nitrogen and University of Ghana http://ugspace.ug.edu.gh 8 phosphorus, microbes' ability to breakdown crop residues and symbiotic nitrogen fixation (Ferguson et al., 2006). Rienke and Joke (2005) found that loamy textured soil produces higher yields, and that if the seeds germinate, they grow better in clayey soils. 2.3.1 Soil pH The pH of agricultural soils determines the availability of nutrients, and thus the fertility and productivity of the soil. The concentration or activity of hydrogen ions in the soil is measured by pH. It determines how acidic or alkaline the soil is. The pH value decreases as the H+ ion concentration rises, and thus soil acidity rises (USDA, 1999). The pH range between 6.1 and 7.3 on the pH scale is considered neutral in soil, unlike the pure system where neutrality is at seven (USDA, 1999). According to the USDA system (Table 2.1) provides descriptive terms for pH ranges in soils. Table 2. 1: Descriptive Terms Associated with Soil pH pH ranges Descriptive Term < 4.0 extremely acid 4.1 – 5.0 very strongly acid 5.1 – 5.5 strongly acid 5.6 – 6.0 moderately acid 6.1 – 7.3 neutral 7.4 – 8.0 slightly alkaline 8.1 – 9.0 alkaline > 9.0 strongly alkaline Source: USDA (1999). Low soil pH causes conditions that stifle plant growth and development, resulting in stunted University of Ghana http://ugspace.ug.edu.gh 9 growth. Soil acidity has the potential to harm legume growth by reducing nodule development and nitrogen fixation. The tolerance of different rhizobia strains to soil acidity varies more than that of the host plant (Mohammadi et al., 2012). 2.3.2 Causes of soil acidity Soil acidity problems harm about a quarter of the world's farmland (Graham et al., 2000). The effective treatment actions are necessary to understand the sources of acidity. Several natural and man-made causes contribute to soil acidity. Leaching of bases because of heavy rainfall, plant uptake of basic nutrients and decomposition of organic waste are examples of natural sources, whereas anthropogenic causes include the use of inorganic fertilizers, particularly ammonium-based fertilizers. Soil acidity rises in general when rainfall rises. Soil acidity is determined by the concentration of H+ ions in the soil solution; hence, a high H+ ion concentration will result in a lower pH value and, as a result, a higher acidity. H+ can be found in soils from a variety of sources, including: i) Carbon dioxide released from plant roots and microbial respiration which combines with soil water to produce carbonic acid. This acid then dissociates to release H+. CO2 + H2O → H2CO3 ⇌ HCO3 - + H+ [2.1] (ii) Decomposition of organic matter with concomitant releases of H+. (iii) Roots of plants also release H+ and organic acids to lower the pH of soils. (iv) Nitrification of ammonium NH4 + + 2O2- ⇌ NO3 - +2H+ + H2O [2.2] (v) Hydrolysis of Al in soils releases large quantities of H+ into soil solution as shown in the equations below. Al3+ + H2O→ Al(OH)2+ + H+ [2.3] University of Ghana http://ugspace.ug.edu.gh 10 Al(OH)2+ + H2O→ Al(OH) + + H+ [2.4] Al(OH)+ + H2O → Al(OH)3 + H+ [2.5] A summary of the three reactions is thus Al3+ + 3H2O→ Al(OH)3 + 3H+ [2.6] (vi) Oxidation of Sulphur compounds in soils leads to acidification as depicted in equation S0 + 3/2 O2 + H2O → H2SO4 [2.7] S0 is elemental Sulphur There are three general pools, or types of acidity: active, exchangeable and residual. (i) Active Acidity Active acidity is the concentration of hydrogen ions present in the soil solution, which can be measured by determining the pH value of a water suspension or soil extract. This concentration is influenced by carbonic acid (H2CO3), soluble organic acids, and acid salts formed through hydrolysis. The active acidity level plays a crucial role in the growth and development of plants and soil microorganisms (Pankova et al., 2009). (ii) Exchangeable Acidity The hydrogen (H) and aluminum (Al) ions get adsorbed onto soil colloids. There is an equilibrium between the adsorbed and soil solution ions, which is known as exchangeable acidity. This allows for easy interchangeability between the two forms. In other words, it is the acidity caused by the exchangeable hydrogen and aluminum ions that can be easily dissolved in a simple salt solution like KCl (Getachew et al., 2019). University of Ghana http://ugspace.ug.edu.gh 11 (iii) Residual Acidity It is the concentration of hydrogen ions attached to clay and organic matter and is measured as buffer pH in a buffer solution. The adsorbed H and Al ions pass into the soil solution and its acidity is also known as potential or adsorbed or reserve acidity. In an acid soil, most of the H+ present is absorbed by the soil (Thomas, 1996). 2.3.3 Effect of soil acidity on nutrient availability and plant growth Soil pH has a primarily indirect effect on plant growth because it affects chemical reactions and biological processes (Neina, 2019). Most crops absorb nutrients effectively when the soil pH is in the neutral range, according to Giller and Wilson (1991). Plant growth and development might be harmed if soil acidity is not appropriately regulated (Nduwumuremyi, 2013). The availability of nutrients such as nitrogen, molybdenum, phosphorus, and potassium are often affected as soil acidity rises (Brady and Weil, 2002). Solubilized rhizotoxic aluminum species in highly acidic soils, according to Kochian (1995), can inhibit root growth and function in most plants. Al toxicity, according to Pineros et al., (2005), limits plant growth primarily through its negative effects on root growth and development. Aluminum toxicity also makes plants more susceptible to drought and limits their access to subsoil nutrients, preventing them from reaching their full genetic potential (Ownby and Popham, 1990). Aluminum toxicity, according to Giller et al., (1998), reduces nutrient agronomic and recovery efficiencies. Plant development is hampered by high soil acidity, which inhibits the growth and proliferation of soil microorganisms. The biological nitrogen fixation by bacteria that dwell in the nodules of legumes such as cowpea, peanut, and soybean is adversely affected by low pH (below 6) soils. According to Brockwell et al. (1995), the quantity of S. meliloti in soils with a pH less than 6 has fallen by roughly 10-3 S. meliloti in soils with a pH more than 7.0. Under low soil pH conditions, microbial growth and multiplication are inhibited, resulting in delayed organic matter decomposition (Zhao et al., 2022). University of Ghana http://ugspace.ug.edu.gh 12 Phosphorus availability is affected in very acidic soils. There is a large concentration of soluble Al3+ and Fe3+ in the soil in very severely acid conditions, such as in Oxisols (Hu et al., 2022). Any additional P is precipitated as shown in equation [2.8] Al3+ + H2PO4 - + 2H2O →2H+ + Al(OH)2H2PO4 [2.8] Due to their large surface area exposed to the soil solution, the newly precipitated hydroxy phosphates are marginally soluble. The precipitated hydroxy phosphate matures over time, becoming less soluble and so unavailable to plants (Tisdale et al., 1993). The effectiveness of P fertilizer use is reduced as a result of this. Kaolinites and aluminum and iron oxides are the principal clay minerals of Ghana's Oxisols, Ultisols, and Alfisols, which are the most common soil types (Nartey, 1998). These clay minerals exposed OH groups have a significant affinity to P. (Tan, 2010). The exposed OH groups in clay minerals become protonated at low pH and so retain additional P, as shown in equation [2.9]. Al(OH)2 + + H2PO4 - → Al(OH)2H2PO4 [2.9] The phosphate anion may also produce an inner sphere complex by replacing the clay's structural OH. Between the H2PO4 - and the protonated OH, there is ligand exchange. While reversible, this reaction attaches the anion to the mineral too tightly, resulting in poor availability (Brady and Weil, 2002). Low soil pH also lowers the availability of Mo and B, both of which are important for nodulation (Tisdale et al., 1993). Basic nutrients including K, Ca, Mg, and Na become insufficient in plants due to their low solubility in acid soils. Because of the high solubility of these micronutrients in soil solution, Fe, Mn, Zn, and Cu are hazardous to plants in acidic soil conditions (Tisdale et al., 1993). Particularly in mining areas and on soils where agrochemicals are used, high soil acidity causes considerable accumulation of hazardous heavy metals such as Hg, As, Pb, and Cd in crops (Sparks, 2003). University of Ghana http://ugspace.ug.edu.gh 13 2.3.4 Control of soil acidity As a solution, it has been suggested that soil acidity-tolerant crops be produced and planted. It is, however, time-consuming, and as a result, certain crop features are usually lost or suppressed. Breeding and the use of tolerant crops are primarily coping and/or adaptive strategies, not corrective ones (Curtin and Trolove, 2013). This has not proven very efficient due to the relatively slow decomposition of organic matter to release bases and the fact that organic matter functions as a buffer and thus resists changes in pH (Curtin and Trolove, 2013). As a result, liming has long been the most feasible method for correcting soil acidity and increasing soil productivity (Curtin and Trolove, 2013). According to Brady and Weil (2002), agricultural lime is any material that contains calcium and magnesium as cations in combination with anions such as carbonates, hydroxides, oxides, and silicates, and is used to elevate pH in acid soils. Liming components include calcite, dolomite, slaked lime, quick lime, and basic slag (Brady and Weil, 2002). In the series of events below, the methods by which calcite, the most often used agricultural lime, elevates soil pH when applied to wet soil are depicted. (1) Water dissolves the material in the soil to produce Ca2+ and hydroxide (OH–) according to the equation [2.10]: CaCO3 + H2O →Ca2+ + 2OH– + CO2 [2.10] (2) The released Ca2+ substitutes for Al3+ and H+ at the exchange sites of the soil. (3) The OH– produced from equation 2.10 reacts with Al3+ to form Al(OH)3, and/or H+ to form H2O: 3OH– + Al3+ →Al(OH)3 [2.11] OH– + H+ →H2O [2.12] The Al(OH)3 formed precipitates out of the solution raising the soil pH. University of Ghana http://ugspace.ug.edu.gh 14 High levels of Al3+ and H+ are lowered as a result of the combination of liming and OH–. (Tisdale et al.1993). The release of excess OH– from lime, which elevates the soil pH, is the most evident effect of liming. Liming, depending on the substance, nourishes the soil with two macronutrients: calcium and magnesium. The response rates of liming materials in soil are governed by particle size and surface area. The pH of the soil, the degree of mixing with the soil, and the chemical nature and content of the material all influence the reaction rate (Brady and Weil, 2002). Oxides and hydroxides, for example, react faster than carbonates due to their larger solubilities. There must be adequate moisture in the soil for the reaction to occur (Brady and Weil, 2002). 2.3.5 Soil acidity and its effects on soybean production Phosphorus is precipitated or surface-adsorbed with Al and Fe as insoluble compounds, it is insufficient in soil solutions with low pH soils with high amounts of Al and Fe oxides (Kanyanjua et al., 2002). The soil solution lacks a number of other vital cation-containing plant nutrients. Acidic soils affect soybeans both directly and indirectly. These effects include plant root damage, which decreases water and nutrient uptake, a reduction in the availability of essential plant nutrients, the toxicity of aluminum and manganese (Mn), and soil microbe survival (Crawford et al., 2008; Onwonga et al., 2008). Several methods for correcting nutrient deficiencies can be used to enhance crop production in acid soils. Liming, organic matter addition, and mineral fertilizer application are some of these options (Onwonga et al., 2010; Masarirambi et al., 2012). Liming decreases Al3+ and H+ ions by reacting with water to produce OH- ions, which then react with Al3+ and H+ in acid soil to produce Al(OH)3 and H2O. Lime increases pH by precipitating Al3+ and H+, which boosts microbial activity and nutrient availability (Onwonga et al., 2008). As a leguminous crop, soybeans rely on microbial nitrogen fixation as a source of nitrogen. However, in acid soils, the population of rhizobia bacteria decreases, impairing nodulation University of Ghana http://ugspace.ug.edu.gh 15 and nitrogen fixation. This has an adverse effect on crop nutrition and productivity. As a result, liming acid soils for soybean production increases the soil's microorganism development conditions. Because mineral fertilizers are easily available, they boost nutrient availability in the soil solution, while organic matter acts as a source of food for microorganisms, increasing their number and hence mineralization (Crawford et al., 2008). 2.4 Factors that affect the growth and yield of soybean Both biotic and abiotic variables influence the growth, development, and yield of the soybean plant. Soil microorganisms, soil water properties, and the availability of plant nutrients, as well as climate, insect and disease infestation, and cultivation and management strategies, are all elements to consider (Frempong-Manso et al., 2019). Average temperatures of 20 to 30oC are ideal for growing conditions. Temperatures below 20 °C and above 40°C are unsuitable for the crop. Likewise high temperatures can also have an impact on soybean yield. Changes in temperature affect the physiological, biochemical, metabolic and molecular functions of plants, according to Guy et al. (2018) which can hinder the growth and yield of soybean crops (Kotak et al., 2017). The optimal rainfall is between 350 and 750 mm, evenly distributed throughout the growing season (Ngeze, 1993). The over-reliance on rainfall, which has become increasingly erratic (Dankwa et al., 2021) has led to drought spells that negatively affect the growth and yield of soybean crops (MacCarthy et al., 2017). Soybeans may grow in a variety of soil types, but they thrive in warm, moist, fertile loamy soils with good drainage and sufficient nutrients (Hans et al., 1997). Soybean thrives in fertile soils with a pH of 5.5 to 7.0 by Ngeze (1993). It is an acid-tolerant crop that outperforms all other legumes. It, on the other hand, does not grow well in alkaline, saline, or reduced soils. Soy beans can only endure a small amount of waterlogging (Norman et al., 1995). The availability of nutrients such as nitrogen and phosphorus, microbial decomposition of crop wastes, and symbiotic nitrogen fixation are all improved by keeping soil pH between 5.5 and 7.0 (Ferguson University of Ghana http://ugspace.ug.edu.gh 16 et al., 2006). Seeds of soya beans require enough moisture to germinate and grow. 2.5 Factors influencing nitrogen fixation in legumes According to reports, grain legumes are capable of biologically fixing nitrogen in the range of 15-210 kg N/ha per year in Africa, and therefore play an important role in subsistence farmers' cropping systems (Dakora & Keya, 1997). They are also well-known for their capacity to establish symbiotic associations with N2 fixing rhizobia bacteria (Zhang et al., 2020). The overall N fixation process depends on numerous environmental factors that affect nodules formation, bacterial metabolism (Santachiara et al., 2019) and plant growth (Aranjuelo et al., 2014). Many environmental factors influence biological nitrogen fixation. Environmental factors such as saline and sodic soils, extreme soil pH (Acquino-Alves et al., (2021), low nutrient availability (Divito and Sadras, 2014), mineral toxicity, extreme temperature (Alexander and Oliveira, 2013), soil water content (Munjonji et al., 2018), soil mineral N (Torabian et al., 2019), affect N fixation. Soil pH extremes, either low or high, can reduce rhizobial colonisation in the legume rhizosphere. Acidic soils, according to van Jaarsveld (2002), can reduce the amount of N2 fixed. Under low soil pH conditions, nodulation and the amount of N2 fixed are more strongly influenced than plant growth. According to Panchali (2011), even the most effective rhizobium strains cannot form effective associations with the host plant in nodulation and N fixation under these conditions. Agricultural management factors, in addition to environmental conditions, influence the percentage of N2 obtained from the atmosphere. Some management factors that can influence plant growth and development include inoculation, phosphorus fertilizer application, genotype selection, and plant population selection (Ronner & Franke, 2012). The purpose of inoculation is largely determined by the presence and effectiveness of compatible rhizobia in the soil (Giller, 2001). University of Ghana http://ugspace.ug.edu.gh 17 2.5.1 Soil moisture content Soil moisture/water influences the growth of soil macro and microorganisms by mass flow, diffusion and nutrient content. Soil texture is significant because it determines how much water a particular soil can retain within its pore space; hence, soils with wide pores and pore spaces hold less water. As a result, soil aggregates with fewer inner pores, such as clayey loam and loamy sand, support the growth of rhizobia and other soil bacteria better (Turco & Sadowsky, 1995). In general, the growth and establishment of rhizosphere microorganisms such as rhizobia are directly affected by soil water supply due to decreasing activity below critical tolerance limits. Soil water also has an indirect effect on plants by causing changes in plant growth, root exudates, and root architecture. According to Boscari et al. (2002), low rhizobial population levels in dry seasons are most likely to blame for little or no nodulation of legumes in tropical soils. As a result, the impact of soil moisture/water on microorganisms, plant growth, vigour, and nodulation should not be underestimated. Rhizobia adapts to osmotic pressure in a variety of ways, primarily through the accumulation of organic and inorganic solutes in the intracellular space. R. meliloti is a typical example of a rhizobium that overcomes osmotic stress by accumulating compatible solutes such as K+, trehalose, glycine, betaine, glutamate, dipeptide, N-acetyl glutminyl glutamine amide, proline, and proline betaine. 2.5.2 Soil temperature The survival and proliferation of rhizobial strains in soils are both influenced by temperature. Temperature appears to have a different impact on rhizobia depending on the soil type and strain. Rhizobium strain for instance bacillus thuringiensis var. leguminosarum Bradyrhizobium sp. was outperformed by trifolii under temperature of the soil (Mohammadi University of Ghana http://ugspace.ug.edu.gh 18 et al., 2012). High soil temperatures, according to Triplett and Sadowsky (1992), cause nodulation to be delayed or limited to subsurface soils. Drought and high temperatures both reduced plant dry mass and leaf area, especially when the two stresses were combined. The inhibitory effect of high temperatures on plant growth was caused by lower CO2 and N2 fixation rates as reported by Aranjuelo et al. (2007). High-root temperatures also lowered infection, N2-fixation ability, and legume development, according to Hungria and Franco (1993), but this is also reliant on rhizobia strain type and strain-cultivar relationships (Arayankoon et al., 1990). The ideal temperature correlations for various legumes and Rhizobia combinations are between 35 and 40°C for soybeans, peanut, and cowpea. 2.5.3 Availability of phosphorus Phosphorus availability in the soil during seedling development is critical for legume growth, N2 fixation, and grain production, and a lack of it can prevent nodulation (Giller, 2001). Nodule development and function are major P sinks, with nodules containing the most P in the plant (Sinclair and Vadez, 2002). P deficiency reduces nodule development, but P fertilizer application increases nodule number and dry mass, as well as N fixation (Sinclair and Vadez, 2002). Low soil phosphorus levels reported by Yakubu et al., (2010), inhibited rhizobia population and root development in legumes, lowering N2 fixation potential. Increased soil P input from 20 to 40 kg P2O5 ha-1 resulted in an increase in total plant growth and plant nitrogen content (Sinclair and Vadez, 2002; Magani and Kuchinda, 2009). African farmers’ limited access to P fertilizers have contributed to low legume productivity (Sinclair and Vadez, 2002). All the cowpea types evaluated in the Sudan Savanna zone of Nigeria showed a strong response to applied P up to 60 kg P2O5/ha, according to Singh et al. (2011). Higher availability of P has been attributed to higher grain yield in cowpea as reported by Singh et al. (2011). According to Assuming-Brempong et al. (2013), growing legumes in Ghana's coastal savanna zone necessitates 90 to 120 kg P2O5/ha of phosphorus. According to Vesterager et al., (2008), the amount of N fixed in a cowpea monocrop increased from 58 to 77 kg N/ha and in a cowpea- University of Ghana http://ugspace.ug.edu.gh 19 intercrop increased from 30 to 43 kg N/ha as a result of P treatment in Tanzania's semi-arid zone. Nodulation has been reported to diminish as the amount of P and N applied increases, implying that readily available N has an antagonistic influence on the functions of Phosphorus (Vesterager et al., 2008). This finding suggests that in the presence of easily available nitrogen in the soil, P treatment may not maximize biological nitrogen fixation. 2.5.4 Population of rhizobia strain in the soil Rhizobia associate symbiotically with legume roots to fix atmospheric nitrogen. The more rhizobia populations grow, the more likely nodule infection becomes. The symbiotic properties of soil rhizobia populations and soil composition can differ (Martins et al., 2003). On the root of the same plant, nodules formed by multiple strains and species can appear (Moreira and Siqueira, 2006). The legume-rhizobia symbiosis can sustain agriculture in the tropics at moderate output levels if all environmental barriers to the symbiosis' proper functioning are removed (TSB-CIAT, 2004). Singleton et al. (1992) found that less than 60 % of tropical soils studied from Africa had less than 1,000 rhizobia/g soil and 47 % had less than 100 rhizobia/g soil, indicating that cowpea cross-inoculation was prevalent. Fening and Danso (2002) reported that 68 % of rhizobia samples collected from 20 Ghanaian soils were moderately effective in nodulating soybean. This suggested that Ghanaian soils are rich in the soil rhizobia which could boost the yield of leguminous crops with relatively low supplement of fertilizer. 2.5.5 Soil nutrient The symbiosis, as well as the independent growth and survival of legume crops, are significantly influenced by the nutritional quality of the soil. With increasing legume age, nitrogen fixation declines, owing to an increase in soil nitrogen content hence lower yield. The University of Ghana http://ugspace.ug.edu.gh 20 rate of N fertilizer and N2 fixation have been found to have a negative exponential connection (Ledgard and Steele, 1992). The attachment of rhizobia to root hairs, as well as nodulation and nodule formation, are all affected by calcium deficiency, with or without the confusing influence of low pH (Alva et al., 1990). At the molecular level, calcium plays a critical role in symbiotic connections. An Al- induced Ca deficit has been blamed for poor nodulation of soybeans in acid soil (Biswas et al., 2003). Several other nutritional variables influence the growth and survival of rhizobia in soils, in addition to macronutrients (Brockwell et al., 1995). Glutamate, glycerol, and organic matter supplementation of soil and inoculants has been proven to boost rhizobia survival and populations in soils, as well as early nodulation and N2 fixation (Rynne et al., 1994). This finding reveals that, while rhizobia may certainly exist in soils, their efficacy can be improved by adding carbon, implying that they are C constrained in their natural condition. 2.6 Lime requirement The amount of agricultural liming material needed to neutralize the undissociated and dissociated acidity in the range from the initial acid condition to a desirable neutral or less acid condition is referred to as a soil's lime need (McLean, 1971). Lime requirement is defined as the amount of liming material required to generate the highest economic yield of crops grown on low pH soils (McLean, 1971). There are a variety of practical methods for predicting the amount of lime to apply in order to achieve a sufficient level that eliminates Al toxicity to plant growth and development. Monitoring the concentration of exchangeable Al is one of the most frequent approaches for calculating the lime demand. Bell and Bessho (1993) claim that adding basic ions to soil, particularly Ca2+ ions, neutralizes exchangeable Al and promotes root growth. According to Hakim et al. (1989), the ideal lime rate for improving food crop yield on Ultisol is around 6 Mg CaCO3/ha. Although numerous extraction solutions have been University of Ghana http://ugspace.ug.edu.gh 21 developed to estimate the extractable Al, the KCl extraction method is often utilized (Oates and Kamprath, 1983). 2.6.1 Types of liming materials Most limestone is mined and then processed into tiny particle sizes to improve the surface area and thus reactivity. Calcium carbonate and other impurities are common in limestone. Most of the lime put into the soil is made up of limestone. Calcitic limestone is ground limestone that has less than 6 % magnesium, while dolomitic limestone contains more than 6 % magnesium (Carey et al., 2009). Ground limestone is subjected to high temperatures to remove carbon dioxide, resulting in burnt lime (quicklime). After the heating process, calcium oxide is obtained. Magnesium oxide will only be present if it was part of the ground limestone before it was heated. Burnt lime reacts quickly with water to form hydrated lime (Ca (OH)2) and releases a lot of heat, therefore it needs to be handled carefully (Carey et al., 2009). Because of its high solubility, calcium hydroxide is extremely reactive, and too much of it in the soil can quickly raise the pH above the desired level. Because of its caustic nature, it harms already- established plants in the field. Marls are made up of sea shells and calcium carbonate. Farmers in coastal locations use this type of equipment. Ground limestone and marls have a comparable reactivity (Carey et al., 2009). 2.6.2 Advantages of liming in acidic soils Lime is a type of substance that contains carbonates, oxides, or hydroxides and is used to increase the pH of acidic soils while also neutralizing hazardous components. The pH of a soil is used to assess whether or not it should be limed (TSO, 2010). CaCO3, Ca, Mg (CaCO3)2, Ca (OH)2, CaO, and other liming minerals exist, and their neutralizing value and degree of fineness vary (TSO, 2010). Ca2+ and Mg2+ ions displace H+, Fe2+, Al3+, Mn4+, and Cu2+ ions from the soil adsorption site when lime is applied, resulting in an increase in soil pH. In University of Ghana http://ugspace.ug.edu.gh 22 addition to boosting soil pH, lime also delivers substantial amounts of Ca and Mg, depending on the type of lime used. Increased availability of P, Mo, and B, as well as more favorable conditions for microbially mediated reactions like nitrogen fixation and nitrification, and, in some situations, enhanced soil structure, are all indirect impacts of lime (Nekesa et al., 2005). For instance, lime application enhanced soybean root and shoot yields in Nigeria, according to Anetor and Akinrende (2006), as well as soybean grain yields in Brazil (Anetor & Akinrinde, 2006). In Croatia, Andric et al. (2012) found a 44 percent increase in soybean yield as a result of due to lime application. Furthermore, Nekesa et al. (2011) reported a good response of soybean grain yield to lime application, either alone or in combination with TSP fertilizer or Di-ammonium phosphate (DAP) in Western Kenya. Liming is extensively used in the treatment or correction of soil acidity according to Kaitibie et al. (2002). Liming ensures maximum yields from numerous food crops grown on low pH soils. When lime is applied to the soil at the proper rate, it causes a variety of chemical and biological changes in the soil that are advantageous to assuring optimal productivity in acid soils. Liming reduces the quantities of soluble aluminum and manganese to levels that are non- toxic. The availability of Mn reduces as pH rises, which becomes a severe issue in many plants below pH 5.0 (Nduwumuremyi, 2013). Acid soils benefit from liming because it raises the calcium and magnesium levels. Because a large part of applied P fertilizer is biologically fixed to oxides of Fe, Al, and clay minerals, acid soils are often low in total and accessible plant phosphorus. Liming increases the amount of accessible P for plant uptake and use. The release of P from Al and Fe oxides increases soil accessible P concentration at pH ranges between 5.0 and 6.5 (Tan, 2010). Soil microbial characteristics can be utilized to determine soil quality (Brady and Weil, 2002). Soil acidity limits the activities of helpful bacteria, except for fungi, which can thrive across a wide range of pH. Liming boosts the multiplication and activity of most microorganisms, which speed up soil processes including organic matter decomposition and nutrient release (Brady and Weil, 2002). Liming also improves legume nitrogen fixation University of Ghana http://ugspace.ug.edu.gh 23 in acid soils. It aids in the formation of phytohormones, increases root surface area, and improves the absorption of less mobile nutrients like P and micronutrients like Mo and B. (Brady and Weil, 2002). Increasing soil pH enhances heavy metal complexation in soils (McBride, 1994). The key determinants of heavy metals bioavailability in soil are soil qualities such as nature and type of clay, organic matter status, redox potential, and soil pH, and so liming helps to reduce heavy metals availability to crops. Haynes (1983) found that calcium released from lime added to the soil increased plant resistance to a variety of pathogens, including Erwinia sp., phytophthora spp., Ralstonia solani, Sclerotium rolfsii, and Fusarium oxysporum. According to Haynes (1984), calcium combines with pectic chains to form rigid bonds that help plant cell walls resist enzymatic destruction by pathogens. Liming has been reported to lower soil N2O emissions when soil moisture content is kept at field capacity, and it has been advocated as a mitigation method (Stevens et al., 1998). Soil pH has a potential impact on N2O emission pathways and the conversion of N2O to N2, and it is suggested that liming could be a viable option for reducing N2O emissions from farmland (Stevens et al., 1998). 2.7 Biochar Biochar is a porous, high-carbon substance produced by thermal burning of biomass under restricted oxygen conditions and at quite high temperatures, often between 300 and 1000 degrees Celsius (Lehmann et al., 2003). Biochar is made by charring feedstocks including sawdust, animal manure, and crop leftovers to help recycle forestry and agricultural wastes (Lehmann et al., 2003). Biochar is gaining popularity in agriculture as an environmentally beneficial amendment aimed primarily at mitigating climate change (Lehmann et al., 2003). Biochar, often referred to black carbon, is a carbon-rich substance with a significant specific surface area that has been shown to increase soil water and nutrient retention. Biochar, as soil amendment, reduces climate University of Ghana http://ugspace.ug.edu.gh 24 change by capturing carbon from the atmosphere (Lehmann and Joseph, 2009). By increasing nutrient adsorption, water holding capacity and microbial activity, it also raises soil productivity, leading to increased crop yields. 2.7.1 Biochar production Biochar is obtained by burning biomass with little or no oxygen. This makes it different from actual burning of biomass which involves naked flame and oxygen to oxidize the carbon in the biomass completely to carbon dioxide leading to the production of ashes and small amounts of carbon. Limiting oxygen accessibility leads to high carbon retention in the biomass. The yield of carbon in biochar is usually 50 % or less because the pyrolysis process also produces combustible gases and volatile compounds from the pyrolyzed biomass (Lehmann, 2007). Heating of biomass under ambient temperatures results in dehydration. Moisture in the biomass is first driven off and this involves the provision of great energy due to high heat capacity of water and large quantity of energy needed to vapourize the water content (Taylor and Mason, 2010). Thus, fresh feed stocks are not ideal for biochar production. The moisture content of biomass should be between 10 and 15 % prior to pyrolysis. The torre faction phase in the thermal decomposition process starts when the biomass is dry the biomass is “roasted”, turning dark in colour due to chemical changes. Gases and other volatile compounds are released from the biomass. True pyrolysis starts when the temperature reaches 3000o C, resulting in exothermic reactions. The feedstock fully readjusts itself to form solid biochar releasing volatile compounds and combustible gases (Taylor and Mason, 2010). The features of the final products are greatly influenced by the rate of pyrolysis. The quality of the product is also dependent on the feed stock type (Taylor and Mason, 2010). University of Ghana http://ugspace.ug.edu.gh 25 2.7.2 Biochar stability in the soil The stability of a system influences how long it can maintain its soil and water quality (Lehmann, 2007). Biochar can last much longer in the soil environment than any other carbon- containing organic additive. Despite the rapid rates of mineralization common to organic matter in those conditions, traces of biochar have been identified in the soils of humid tropical climates such as the Amazon several years after application (Sombroek, et al., 2003). 2.7.3 Structural composition of biochar Thermal decomposition of cellulose in organic biomass between 250o C and 350o C leads to considerable loss of volatile compounds with a concomitant increase in aromatic C concentration. According to Demirbas (2004), water evaporates first, followed by hydrocarbons, tarry vapour, hydrogen gas, carbon monoxide and then carbon dioxide. Thereafter, there is the transformation of alkyl and O-alkyl aryl carbon (Baldock and Smernik, 2002). Consequently, a large mass of amorphous carbon matrix is formed. At a temperature of about 330o C, there is lateral growth of graphene sheet, at the expense of the amorphous carbon phase and finally coalesce. At temperature above 600o C, there is the elimination of most of the remaining non-carbon atoms and carbonization becomes the dominant process; a consequence of which is relative increase in carbon content. Carbonization can reach 90 % by weight in biochar produced from woody feed stocks (Demirbas, 2004). According to Chan and Xu (2009), carbon content of biochar irrespective of type is between 172 and 905 g/kg, although organic carbon usually accounts for less than 500 g/kg for different materials. Total nitrogen content of biochar ranges from 1.8 to 56.4 g/kg depending on the type of biomass. The high total nitrogen content may not be available to crops as a result of complexation reactions with mineral nitrogen content less than 2 mg/kg. Studies have shown that the carbon-nitrogen ratio varies widely from 7 to 500. Total phosphorus and potassium University of Ghana http://ugspace.ug.edu.gh 26 content of biochar fall between the ranges of 2.7 to 480 and 1.0 to 58.0 g/kg, respectively (Chan and Xu, 2009). Because of its oxidation and H+ accretion from the soil solution in the first few weeks after the amendment to the soil result different properties of biochar vary as it ages in the soil. The feedstock used to make the biochar, the soil, and the current climate all affect how much its qualities change with time ((Cheng et al., 2008: Heitkotter and Marschner, 2015). 2.7.4 Biochar as liming material and mode of application Many studies on the pH of biochar have revealed that it is typically neutral to basic in soil reactivity. As a result, their use has been discovered to raise soil pH (Joseph et al., 2010). The liming characteristic of biochar, according to Verheijen et al. (2010), is one of the most plausible processes driving increases in crop output when it is employed as a soil supplement. The addition of biochar to tropical soils reduced aluminum toxicity by lowering acidity levels (Verheijen et al., 2010). Noble et al. (1996), reported the liming impact of agricultural wastes and other biomass when burned and applied to the soil. Farrell et al. (2013) and Masto et al. (2013) showed an increase in soil pH after application of biochar on different types of soils, which is attributable to the temperature during pyrolysis and type of feedstock. Decarboxylation of organic anions, as shown by excess cations in biochar, consumes H+ and thereby raises soil pH, according to Yuan et al. (2011). Alternatively, negatively charged functional groups on biochar surfaces, such as phenol, carboxyl, and hydroxyl, adsorb H+ from soil solution, lowering its concentration and raising soil pH as a result (Brewer and Brown, 2012; Chintala et al., 2014). Biochar's silicates, carbonates, and bicarbonates form complexes with H+ ions, rendering the proton unavailable to the soil solution (Brewer and Brown, 2012; Chintala et al., 2014). In acidic soils and soils with low organic matter levels, biochar has a greater impact on raising soil pH. (Stewart et al., 2013). Due to the buffering potential of organic matter, soils with high organic matter content resist changes in pH when biochar is applied (Curtin and Trolove, University of Ghana http://ugspace.ug.edu.gh 27 2013). This is because leaching of base cations is reduced as adsorption of H+ ions to negatively charged functional groups of biochar, organic matter, and organo-mineral complexes is promoted (Chan et al., 2007; Nelissen et al., 2012; Taketani et al., 2013). However, the intensity of this impact may be governed by the soil organic matter content, which is the principal determinant of soil cation exchange capacity (Brady and Weil, 2008). The behaviour and fate of biochar materials in the soil and the environment is influenced by the method of its application to soils (Verheijen et al., 2010). There are three main methods of biochar application viz; topsoil application, depth application and top-dressing. 2.7.5 Agronomic importance of biochar The addition of biochar enhances plant growth and development, and crop yields, and increases the production of food and sustainability in marginal soils with low organic matter, inadequate water, and poor nutrient status (Lehman et al., 2006). Due to the fact different soils react differently to the application of biochar, it takes some time to compare the responses of soils (Lehman et al., 2006). The ideal biochar application rate depends on the soil type and the crop management system (Verheijen et al., 2010). A unique strategy for creating a sink for atmospheric carbon dioxide in terrestrial ecosystems is the application of biochar (Lehman et al., 2006). In addition to having a positive impact on emissions and greenhouse gas sequestration, the manufacture of biochar and its application to the soil has immediate advantages through improved soil fertility and greater crop output (Lehman et al., 2006). According to Southavong et al. (2012) biochar might offer a quick fix for handling agricultural waste. The positive plant responses to the application of biochar have been attributed mainly to the direct supply of nutrients with very little consideration given to other biochemical factors that may affect nutrient availability (Lehmann et al., 2003; Chan et al., 2007; Van Zwieten et al., 2007). The positive responses as a result of the application of biochar were attributed to either nutrient retention as in fertilizers or enhanced fertilizer-use efficiency and therefore can University of Ghana http://ugspace.ug.edu.gh 28 be viewed as an indirect nutrient value of biochars. Rondon et al. (2007) and Van Zwieten et al. (2007) reported on how plants responded to biochar application-induced increases in pH or pH stability. According to Hoshi (2001), the capacity of biochar to regulate the pH of the soil was a contributing factor in the 20 % rise in height and 40 % increase in the volume of tea trees. The liming value of biochar is correlated with its capacity to preserve pH. When biochar made from paper mill sludge was treated at a rate of 10 t ha-1 to acidic soil, Van Zwieten et al. (2007) saw a nearly 30 % to 40 % increase in wheat height. By neutralizing the deleterious effects of the exchangeable aluminum's presence, they came to the conclusion that the carbonates in the biochar enhanced wheat development. In addition to toxin neutralization (Wardle et al., 2008), improved soil physical properties, such as an increase in water-holding capacity (Eswaran et al., 1980), and decreased soil strength, there have been other explanations for the positive responses to the application of biochar that are unrelated to plant nutrition (Chan et al., 2007). Additionally, compared to a control that got the same amount of N without biochar, dry matter yield increased by 26 % when N fertilizer was also supplied at a rate of 100 kg N/ha in addition to biochar. The increased N fertilizer usage efficiency of radish following the application of biochar was related to the improved physical characteristics of the soil, which included decreased soil strength and increased water holding capacity. Lehmann et al. (2003) also observed that biochar has the capacity to retain fertilizer that had been applied and reduced leaching. 2.7.6 Importance of biochar in the environment The process of capturing carbon that would otherwise be released into or remain in the atmosphere and storing it thereafter in plants and soils is known as carbon sequestration (FAO, 2008). Large quantities of carbon in biochar can be sequestered in the soil for thousands of University of Ghana http://ugspace.ug.edu.gh 29 years (Lehmann et al., 2006). Marris (2006) stated that about 250 ha farms could sequester approximately 1900 tons of carbon dioxide per annum. The ability to store carbon in plant and soil systems means that there is a greater chance of reducing the greenhouse effect (Lal, 2004). According to Schmidt and Noack, (2000) biochar has recalcitrant carbon which can resist degradation. This characteristic of biochar makes it essential for major carbon sink. With respect to other terrestrial sequestration techniques, biochar has a higher potential to increase carbon storage time than afforestation (Ogawa et al., 2006). Biochar application resulted in decreased emission of nitrous oxide and methane (Duku et al., 2011). 2.7.7 Biochar impact on soil performance and resource implications Interactions among soils, biochar, micro-organisms and root of plants start occurring within a short time after biochar application (Lehmann and Joseph, 2009). Glaser et al. (2002) indicated that water retention in biochar amended soil is 18 % higher than in adjacent soils with little or no biochar amendment. The stable macro pore structure of biochar is in part responsible for improving a soil’s water holding capacity (Brodowski et al., 2007). Biochar has a very high capacity for cation sorption due to its huge specific surface area and strong cation exchange capacity (Gaskin et al., 2007). The specific surface area of biochar increases as temperature increases due to the creation of more micropores (Bird et al., 2008), and great quantities of carboxyl groups on the surfaces. Cheng et al. (2006) suggested that increases of carboxyl groups on char surfaces with time are in part due to either partial oxidation of open surfaces by biological and non-biological processes and/or chemisorption. The cation exchange capacity that biochar offers differs from that of soil organic matter due to the stability that it possesses. Given the incremental improvement in cation exchange capacity, there does not appear to be a cap on the amount of benefit that may be obtained through repeated addition. Water can be purified with biochar by removing nitrate and phosphate University of Ghana http://ugspace.ug.edu.gh 30 (Mizuta et al., 2004; Eduah, 2009). Having an established affinity for organic compounds, biochar may be able to loosely hold nutrients in a bio-available form, which is important for crop growth. It may also be able to bind hazardous substances in the soil (Yu et al., 2006). The indirect influence of biochar on the chemistry of the soil seems to arise from the amendment of soil pH. Studies show that terra preta sites have higher pH and phosphorus than surrounding soils. The ash component of biochar has more available forms of nutrients than uncharred biomass. The indirect effect of biochar on phosphorus availability in the soil and the mineral ash of its matrix containing phosphorus, potassium, and other potentially important micronutrients are essential in explaining its short- term influence on crop growth (Lehmann and Joseph, 2009). According to Steiner et al. (2008), microbial activity in soil is enhanced on the addition of biochar. Sam (2014) also found that degradative abilities of heterotrophs were enhanced when cocoa pod husk biochar was amended to atrazine and paraquat contaminated soils. There is a sizable body of research that supports biochar's ability to stimulate local arbuscular mycorrhizal fungus, which has been linked to improved plant growth (Rondon et al., 2007). The microbial structure in soil that contains aged biochar is distinctively different from those in which fresh biochar has been amended (Kim et al., 2007). Microbial populations react initially with labile components of biochar on its amendment to soil and pyrolysis condensates seem to promote microbial activity in the soil (Steiner et al., 2008). 2.7.8 Biochar and nitrogen fertilizer interactions One approach to improve the efficiency of fertilizer use is integrated crop management which uses organic manure and other organic resources (Fageria and Baligar, 2005). Organic matter added to soil decomposed faster in tropical environments compared to charred biomass. When added to soil, biochar is more recalcitrant yet enhances the use of nitrogen from added inorganic fertilizers (Steiner et al., 2007; Widowati and Asnah, 2014). This is because the University of Ghana http://ugspace.ug.edu.gh 31 application of biochar boosted the soil’s cation exchange capacity, which decreased nitrogen loss (Chan et al., 2008) and also its capacity to inhibit ammonium transformation to nitrate released from fertilizer (Widowati and Asnah, 2014). 2.7.9 Negative effect of biochar on soil McClellan et al. (2007) reported several instances of reduced plant growth as a result of biochar application due to the temporary high levels of pH and volatile nutrient imbalances associated with it when applied fresh. Mostly, biochar has an initial alkaline pH which is favoured for application to soils with low pH. When applied to alkaline soils, however, plants experience nutrient deficiency particularly basic cations and P. High pH of soils due to biochar addition may also cause NH3 volatilization when ammonium-based fertilizers or organic manures are applied. Tars, resins and other transient compounds that are left on the surface of biochar just after production can impede plant growth (McClellan et al., 2007) Inaccessible to microbial and enzymatic degradation, biochar adsorbs substances like insecticides and organic waste (Kookana et al., 2011; Zimmerman et al., 2011). Some biochar products may be hazardous to plants and soil microfauna, according to Kookana et al. (2011). 2.7.10 Effects of biochar on soybean growth Biochar has recently been the subject of in-depth study and is highly suggested for crop productivity. One of the crops on which the effects of biochar have been fairly assessed is soybean. For instance, Wang et al. (2016) found that soybean growth has improved during their investigation. Biochar might change the way soils were structured, as well as how well they could store water and absorb nitrogen. In comparison to areas lacking biochar, it was concluded that biochar increased soybean growth and gave the plants a more uniform growth during the reproductive periods. Following the application of biochar, Suppadit et al. (2012); Yooyen et al. (2015); Egamberdieva et al. (2016) reported an increase in nutrient uptake, University of Ghana http://ugspace.ug.edu.gh 32 growth, dry matter, nodulation, and yield. Some soybean types respond favourably to rice straw biochar in terms of nodulation, growth, dry matter buildup, yield, and plant uptake of nutrients from the soil (Agbanu, 2017). Thies and Rillig (2009) have suggested that the application of biochar can improve the activity of microorganisms in acidic soils by increasing the pH levels, which is conducive for soybean production. It has been observed that the combination of biochar with lime and other amendments significantly enhances soil water content, organic carbon (OC), nitrogen (N), phosphorus (P) and cation exchange capacity (CEC) (Agegnehu et al., 2016). Laird et al. (2010) reported that biochar is more effective than manure in reducing soil bulk density and enhancing water holding capacity. One of the main practical benefits of biochar is that it enhances grain yield while reducing the leaching of soil nutrients (Biederman and Harpole, 2012). According to a study conducted by Arabi et al. (2018), soybean yield increased by 51% after the application of biochar. The study attributed this increase to the improvement in soil acidity caused by the biochar, resulting in an increase in the number of pods per plant and ultimately leading to higher grain yield. Another study conducted by Bayan (2013) reported a 62 % increase in the number of soybean pods per plant under the 2 % wheat straw biochar treatment compared to the biochar-free treatment. The study highlighted the positive effect of biochar on the growth and nodulation of soybeans. 2.8 Phosphorus A vital plant nutrient that has been widely distributed in nature is phosphorus. For plant and animal life, phosphorus, nitrogen (N), and potassium (K) are crucial nutrients. Since phosphorus is a finite, non-renewable resource, its effective utilization is crucial, and its University of Ghana http://ugspace.ug.edu.gh 33 shortage in soils significantly lowers crop yields. An essential component of plant bioenergetics is phosphorus. Phosphorus, a component of ATP, is necessary for the conversion of light energy to chemical energy during photosynthesis (ATP). P is employed in the phosphorylation process to change various enzymes' activities as well as for cell signaling. Many plant biomolecules could utilize ATP for biosynthesis, phosphorus is crucial for plant development and flower/seed formation. Phosphate esters are the raw materials used to make DNA, RNA, and phospholipids. P is most frequently found in soil as the insoluble polyprotic phosphoric acid (H3PO4), but plants prefer to absorb it in the monovalent and divalent forms of H2PO4 - and HPO4 2- at pHs of 6-7, where each form accounts for 50 % of the total P. At a pH of 4-6, H2PO4 - contains approximately 100 % of the total amount of P in solution (Black, 1968). Additionally, only 20 % of total P is represented by H2PO4 - and 80 % is represented by HPO4 2- at pH 8. Because of its low concentration in soil and high demand by plants and microbes, it is typically the limiting element. When mycorrhiza is present, phosphorus uptake by plants may increase. The deep purple colouring of plant leaves indicates phosphorus deficiency. The division of photosynthates among the source, leaves, and reproductive organs is controlled by the quantity of P supplied throughout the reproductive growth phase, according to Marschner (1995), and this effect is critical for N-fixing legumes. Studies have revealed that a significant phosphorus shortage during early growth results in overall plant stunting, which often manifests as an unnatural coloring. The plants are often a dark bluish-green tint, with purple-tinged stems and leaves. Older leaves will exhibit the first signs of phosphorus deficiency since it is a very mobile nutrient in plants, and it may be transferred from older tissues to actively growing parts (McBride, 1994). Perennial crops will benefit from receiving fertilizer with a high phosphorus concentration because it may promote the development of strong roots. For the best agricultural yields, the phosphorus (P) content of the soil is essential. Additionally, phosphorus enables a plant's root development, energy storage, and transfer, University of Ghana http://ugspace.ug.edu.gh 34 flower and fruit development, and early maturity. The majority of it, however, is in insoluble compounds that are not usually available to plants due to physico-chemical properties in most soils. The P- soluble chemicals have low solubility indices, are stationary, and are extremely reactive. Important activities including mineralization and immobilization of organic P molecules are a part of the phosphorus cycling in soils. According to research, 0.2 μg/ml P was sufficient for ultimate growth, implying that a low P level in the soil solution is often suitable for normal plant growth (FAO, 1984: Barber, 1995). One way to address soil fertility constraints for sustainable agriculture in West Africa's Savanna regions is to create soil nutrient management technologies, based on a sufficient supply and viable share of inorganic and organic fertilizers. According to Tisdale et al. (1985); Gupta (2011), phosphorus is a crucial plant nutrient for initial root development, energy transmission, photosynthesis, water use efficiency, nodulation, seed formation, size, and number, all of which contribute to high soybean grain yield. However, ongoing agriculture practices reduce soil P availability due to plant removal and Al3+ and Fe2+ ion fixation in acidic soils. P is supplied by the use of P fertilizers and organic sources to boost P availability. While organic sources of fertilizer take longer to break down and release nutrients, mineral fertilizers are easily accessible. For smallholder farmers, access to and the cost of mineral P fertilizers are significant obstacles (Buresh & Smithson, 1997). In Chuka and Muthambi (Meru South District), Mugendi et al. (2010) discovered a notable rise in the weight of 1000 fresh soybean seeds and pods after applying 50 kg ha-1 P2O5. Also in Nigeria, the yield of soybean grains on acid soil was significantly boosted by applying P fertilizer at a rate of 30 kg P2O5 ha-1 (Mahamood et al., 2009). Following the application of 60 kg P2O5 ha-1, Mabapa et al. (2010) found an increase in above-ground biomass and grain yields of soybean in South Africa. Following the application of P fertilizers in Nigeria, increased soybean grain yield and component levels have also been documented (Kamara et al., 2007; 2008; 2011). Numerous studies have shown how P affects N fixation and helps it to become University of Ghana http://ugspace.ug.edu.gh 35 more stable. For instance, in the central Kenyan highlands, Mugendi et al. (2010) found that adding more P fertilizer up to 25 kg P2O5 ha-1 produced greater nodule fresh weight. According to Ogoke et al. (2004), application of P fertilizer to Nigeria at rates of 30 kilogram P2O5 ha-1 and 60 kg P2O5 ha-1 greatly enhanced the number of nodules, whereas the largest quantity of N fixed was found at 26.4 kg P2O5 ha-1 (Amba et al., 2011). Similarly, P application greatly influenced N fixation in Nigeria, according to Chiezey and Odunze (2009). Meanwhile, Lapinskas and Piaulokait-Motuzien (2006) discovered that lime added to inoculated seed fixed 106 kg N ha-1 when operating under acid soil in Lithuania. 2.9 Soil available nitrogen The amount of N fixed in low-mineral-N soils is often high, but only when there is sufficient water and plant-growth-supporting nutrients (Unkovich et al., 2008). The creation, establishment, and activity of nitrogenase are also inhibited by soils with high levels of nitrogen in the root area, according to a number of studies (Abdel-Wahab et al., 1996; Peoples et al., 1995) because legumes absorb soil N with less energy than biologically fixing N from the atmosphere (Cannell and Thornley, 2000). N fixation is inhibited as soil mineral content N increases (Macduff et al., 1996). According to Gan et al. (2004), applying starter N can sometimes boost nodule establishment and N fixation compared to not applying any mineral N. Depending on the crop type and environmental factors that affect crop growth, a starter N application that enhances BNF normally uses less than 4mM for NH4 + and 2mM for NO3 -. This, according to Keyser and Li (1992), is due to the possibility that symbiotic legume-rhizobium does not create enough nitrogen during the early stages of growth to satisfy the N need for mineral nitrogen that is beneficial to early growth. N fertilizer can boost crop biomass and pod size by 16 % and 44 %, respectively, depending on whether it is administered during the vegetative or reproductive growth stages (Katulanda, 2011). Yinbo et al. (1997) found that timing and application rates University of Ghana http://ugspace.ug.edu.gh 36 have a significant impact on legume response to N fertilizer application. The amount of plant N acquired from fixation is increased when N fertilizer is applied at the R5 (pod filling) stage (Yinbo et al., 1997). 2.10 Rhizobium Inoculation Due to a smaller population of efficient and compactible rhizobium in the soil, nodules produced on the roots of legumes may not develop when they are first introduced to the soil. Therefore, it is crucial to add a suitable strain of rhizobium in areas where legumes have not been sown or where there are no local rhizobia populations (Ledgard and Steele, 1992). The process of inoculating legume seeds with enough of the right strain of rhizobia results in a quick and successful nodulation of that legume in the field (Sinha, 1997). Depending on the application method, inoculant formulations are available in a wide range of forms. Examples include concentrates that are liquid or frozen, inoculated granules, porous gypsum granules, and natural peat granules. The most popular kind of inoculant is peat-based, which is administered directly to seeds or in liquid form (Sham et al., 2005). Sedge peat and rhizobia broth culture are combined to create peat inoculant. A sufficient amount of moisture is added to promote rhizobia development and multiplication (Ledgard and Steele, 1992; FAO, 1984). The main objective of inoculation is to increase the quantity of the desired strain of rhizobia in the rhizosphere because this is necessary for optimizing nodulation and N fixation, which enhances BNF and grain yields of soybean (Lupwayi et al., 2000). In addition to preventing crop failure, inoculation also helps prevent issues that are less directly related to production, such as growing crops that are N deficient (Deaker et al., 2004). The most advantageous agronomic technique for maximizing legume output was found to be inoculation with a desired rhizobium (Gudni and Graig, 2003). However, inadequate competitiveness with native strains, unfavourable environmental circumstances, and other related factors limits their quantity and viability (Batilan and Johnson, 1995). Under ideal conditions, legumes can fix 200 kg N/ha per University of Ghana http://ugspace.ug.edu.gh 37 year (Giller, 2001). Only the existence of productive and efficient rhizobial strains—which may be native or introduced—allows for the benefit of legume-N. Prior to planting, it is important to inoculate seeds since this can increase nodulation and N fixation by establishing a significant rhizobial population in the rhizosphere. The most important of these was a deficiency in bacteria necessary for soybean nodulation (Khidir, 1997). According to reports, the crop should be infected with the right rhizobium strain for higher yields (Hardson and Atkins, 2003). An effective substitute for nitrogen fertilization has been found as soybean inoculation. Due to its lower cost and advantages in terms of plant growth and seed quality, rhizobium inoculation has also been discovered to offer enormous potential as a fertilizer substitute. Although adding a small amount of N fertilizer to the soil improves nodulation, nodule development and N fixation are greatly reduced when large amounts of N are added to the plant (Ahmed, 2013). Only 6 % of natural rhizobia populations in Ghanaian soils, according to research by Fening et al. (2002), are highly successful, with the rest 68 % and 26 % being only moderately and ineffectively effective. In Yamgambi Congo, infected soybeans produced 80–300 % more yields in another field study (Shurleff and Aoyagi, 2009). 2.11 Soybean nodulation, biological nitrogen fixation (BNF) and BNF-related factors Like other legumes, soybeans collaborate with soil microbes to fix nitrogen through root nodules. In both natural and agricultural systems, the rhizobium-legume connection is significant. The formation of root nodules, which act as both a habitat and a reliable source of food for bacterial symbionts that also provide anaerobic conditions (such as low oxygen) for nitrogen fixation, is caused by the infection of legume roots with nitrogen-fixing bacteria of the Rhizobiaceae family in low nitrogen soils (Eckardt, 2006). The three (3) key stages of the root nodule's formation are pre-infection, nodule initiation, and diffe