University of Ghana http://ugspace.ug.edu.gh PHOSPHORUS FRACTIONS OF BIOCHAR-AMENDED PLINTHALQUALF UNDER CLEAN AND WASTE WATER IRRIGATION REGIMES IN NORTHERN GHANA BY MENSAH SAMUEL OBODAI (10551047) THIS THESIS IS SUBMITTED TO THE UNIVERSITY OF GHANA, LEGON IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE AWARD OF M.PHIL DEGREE IN SOIL SCIENCE Department of Soil Science School of Agriculture College of Basic and Applied Science University of Ghana, Legon, Accra, Ghana. JULY, 2018 University of Ghana http://ugspace.ug.edu.gh DECLARATION I, Mensah Samuel Obodai, do hereby declare that this thesis has been written by me and that it is the record of my own research work. It has not been presented for another degree elsewhere. Works of other researchers have been duly cited by references to the authors. All assistance received has also been acknowledged. Sign: …………………………………………………..Date…………………………… Mensah Samuel Obodai (Student) Sign: …………………………………………………..Date…………………………… Porf. M. K. Abekoe (Principal Supervisor) Sign: …………………………………………………..Date……………………………. Dr. E. Nartey (Co-Supervisor) i University of Ghana http://ugspace.ug.edu.gh DEDICATION This thesis is dedicated to my beautiful and supportive wife Mrs. Sylvia Otuo- Acheampong Mensah and to my son Samuel O. Mensah Jnr. It is also dedicated to my caring parents Dr. J. Mensah and Mrs. C. A. Mensah, to my Godmothers, Mama Becky Ofosu and Mrs. R. Nartey-Tokoli, my brothers Emmanuel, Ebenezer and Gideon; my sisters, Vida and Lydia; my cousins and all those who took interest and encouraged me in my academic pursuit. ii University of Ghana http://ugspace.ug.edu.gh ACKNOWLEDGEMENT My foremost acknowledgement goes to Jehovah God, the source of all wisdom and knowledge and the giver of life for bestowing on me this academic status. I wish to express my profound gratitude to my nuclear family, my son Samuel O. Mensah Jnr. and My lovely Silvia Mensah (Mrs.) for their unflinching support and sacrifice throughout the duration of my M.Phil. programme. I sincerely acknowledge the hard work, guidance and encouragement of a perfect gentleman, my principal supervisor Prof. M. K. Abekoe who was very patient with me and never gave up on me during this challenging research work. My profound gratitude also goes to my co-supervisor Dr. E. Nartey for working out the funding and shaping of the study and to Prof. S. Adiku who also assisted me to secure funding. I also wish to sincerely thank the German Federal Ministry of Education and Research (BMBF) for funding the research under the Urban FoodPLUS project. I humbly acknowledge Mr. J. O. Eduah and Prof. G. N. N. Dowuona for their immense contribution to my thesis. Acknowledgement also goes to all the lecturers of the Department Soil Science who in one way or the other contributed to the successful completion of my M.Phil. programme. I am also grateful for the insight on biochar characteristics and soil reaction orally delivered by Dr. D.F.K. Allotey and Dr. Edward Yeboah (SRI). My gratitude also goes to the technical staff of the Department Soil Science, especially, Mr Victor Adusei, Mr Martin Aggrey, Christian of Ecolab and Mr. Moses Ocquaye of Soil Research laboratory for their assistance during the laboratory phase of the work My heartfelt thanks also go to my colleague students in the Department Soil Science especially, Mr. K. Andoh-Payin, Mr. D. Fianko and Miss A. Asamoah-Bediako for their iii University of Ghana http://ugspace.ug.edu.gh friendship as well as every ounce of support and contributions they put forth in making this thesis a reality. I am grateful to each and everyone who contributed directly or indirectly to the successful completion of my thesis; I say thank you all and may Jehovah God richly bless you all. iv University of Ghana http://ugspace.ug.edu.gh ABSTRACT The reactive nature of phosphorus leads to the formation of insoluble Fe, Al and Ca bound phosphate compounds in highly weathered tropical soils, thus limiting P availability for plant uptake. Biochar with its heterogeneous surface properties can influence phosphorus dynamics in tropical soils. To examine phosphorus fractions in a biochar-amended Plinthaqualf, samples were taken from low yielding soils in northern Ghana that had three cycles of vegetable production under clean and waste water irrigation regimes for two seasons. Sampling was done from irrigated soils amended with (i) biochar (B) at application rate of 20 tonnes per hectare, (ii) inorganic fertilizer (Ammonium sulphate-(NH₄)₂SO₄) according to normal agricultural practice (IF-NAP), (iii) a combination of biochar (B) plus (NH₄)₂SO₄) and a control (soil) inclusive. Phosphorus fractionation of the control soil and soil amended plots (B, (NH₄)₂SO₄, and B + (NH₄)₂SO₄) was assessed by a modified Hedley’s method. Water was used as an initial extractant before the resin was applied to ascertain the amount of water leachable P in the soils. The interaction of the amendments [biochar (B), (NH₄)₂SO₄ and (B + (NH₄)₂SO₄)] was assessed, and it showed an increase in the most labile Pi or available Pi (H2O-P, + Resin- P + NaHCO3-P) as well as moderately labile P (NaOH-Pi) fractions in the soils, thus, making P more available for plant uptake. There were significant correlations (p < 0.05; p < 0.01) among P fractions expressing a continuum among the P fractions, The P fractions were also related to the chemical properties of the soil and amended soils. The study showed that the water leachable P highly correlated (p< 0.01) with soil organic carbon, available P and total P at probabilities of 0.43, 0.42 and 0.70, respectively and forms about 24% on the average of total available P fraction of the soil. The study also showed that waste water irrigation regime gave a marginal P increment compared to the v University of Ghana http://ugspace.ug.edu.gh clean water irrigation, especially in the biochar treatment plots. Inference from the results of the study showed that all three amendments; biochar (B), (NH₄)₂SO₄ and (B + (NH₄)₂SO₄) significantly (p < 0.005) improved soil fertility parameters especially, P availability. However, B+(NH₄)₂SO₄ performed much better in the release of most labile P and that it could be used to improve the bioavailability of P in acidic soils such as those used in the study. High amount of water leachable P implies a possible loss of about a fourth of the soil available P if irrigation was done beyond field capacity or water saturation point of the soil. It is therefore, recommended that drip or sprinkle irrigation at reduced amounts is adopted to possibly forestall about a quarter (24%) of the available P that could be lost through the current mode of irrigation. It is also recommended that fortified biochar (B+(NH₄)₂SO₄) should be applied to improve P status of highly weathered soils (e.g. Plinthaqualfs) of northern Ghana. Further studies on application of P sources B, (NH₄)₂SO₄ and B+(NH₄)₂SO₄ should be conducted at different rates to ascertain optimum P application. In addition, the high correlations between water leachable P and the total P and also the available P could be exploited to bypass the use of expensive chemicals through modelling of a mathematical relation. vi University of Ghana http://ugspace.ug.edu.gh TABLE OF CONTENTS TITLE PAGE DECLARATION ............................................................................................................... i DEDICATION ................................................................................................................. ii ACKNOWLEDGEMENT .............................................................................................. iii ABSTRACT ..................................................................................................................... v TABLE OF CONTENTS ............................................................................................... vii LIST OF TABLES ......................................................................................................... xii LIST OF FIGURES ...................................................................................................... xiii CHAPTER ONE ............................................................................................................... 1 1 INTRODUCTION .................................................................................................... 1 1.1 Objectives ............................................................................................................ 5 1.2 Hypothesis ........................................................................................................... 6 CHAPTER TWO .............................................................................................................. 7 2 LITERATURE REVIEW ......................................................................................... 7 2.1 Nature of soils in northern Ghana ........................................................................ 7 2.1.1 Climate ................................................................................................................. 8 2.1.2 Natural soil variability along toposequence in northern Ghana........................... 9 2.1.3 Soil types and properties of semi-arid tropics (SAT) .......................................... 9 2.1.4 Alfisols of northern Ghana ................................................................................. 10 2.1.5 The nature of highly weathered semi-arid tropical soils .................................... 11 2.1.6 Acidic soils and P sorption ................................................................................ 13 2.1.7 Harmattan dust and its significance to soils ....................................................... 16 2.2 Agriculture in northern Ghana ........................................................................... 18 2.2.1 Farming methods and crops grown in northern Ghana ...................................... 20 2.2.2 Vegetable production in Ghana ......................................................................... 21 2.2.2.1 Nature of vegetable production in Ghana .................................................. 22 vii University of Ghana http://ugspace.ug.edu.gh 2.3 Management practices for P availability in soils ............................................... 23 2.3.1 Biochar production and amendment ................................................................. 23 2.3.2 Effect of biochar on soil properties .................................................................... 25 2.3.3 Effect of biochar on phosphorus bioavailability in soil ..................................... 29 2.3.4 Irrigation regiments ............................................................................................ 31 2.3.4.1 Clean water irrigation regiment ................................................................. 32 2.3.4.2 Waste water irrigation regiment ................................................................. 32 2.4 Phosphorus in soil .............................................................................................. 33 2.4.1 Geochemical status of phosphorus..................................................................... 34 2.4.2 Phosphorus status and availability in West African savanna soils .................... 34 2.4.3 Mobility of soil phosphorus ............................................................................... 36 2.4.4 Phosphorus rections in soil ................................................................................ 38 2.4.4.1 Precipitation of P in soil ............................................................................. 39 2.4.4.2 Retention and fixation of P in soil ............................................................. 40 2.4.5 Sesquioxide distribution in soils of northern Ghana .......................................... 42 2.5 Factors affecting P status and limitation in soils of northern Ghana ................. 43 2.5.1 Parent Material ................................................................................................... 43 2.5.2 Extent of P Saturation ........................................................................................ 43 2.5.3 Soil pH ............................................................................................................... 44 2.5.4 Anionic effect..................................................................................................... 45 2.5.5 Type and amount of of clay mineral .................................................................. 46 2.5.6 Leaching and erosion ......................................................................................... 46 2.5.7 Vegetation .......................................................................................................... 47 2.5.8 Sesquioxides ...................................................................................................... 47 2.5.9 Organic matter content of the soil ...................................................................... 48 2.5.10 Lateritic concretions and soil fertility ............................................................... 50 2.5.11 Particle size and soil texture .............................................................................. 50 viii University of Ghana http://ugspace.ug.edu.gh 2.6 Phosphorus fractionation ................................................................................... 52 2.6.1 Inorganic phosphorus pool ................................................................................. 54 2.6.2 Organic phosphorus pool ................................................................................... 56 2.6.3 Residual phosphorus pool .................................................................................. 56 2.6.4 Phosphorus fraction in soils and their functional interpretation ........................ 57 2.6.5 Scientific errors in phosphorus determination during fractionation .................. 58 2.6.5.1 Precipitation of humic acid ........................................................................ 58 2.6.5.2 Hydrolysis of organic matter ...................................................................... 59 CHAPTER THREE ........................................................................................................ 60 3 MATERIALS AND METHODS ............................................................................ 60 3.1 Site characteristics ............................................................................................. 60 3.2 Soils.................................................................................................................... 63 3.2.1 Field experimental design, treatment and soil sampling .................................... 63 3.3 Laboratory analyses ........................................................................................... 64 3.3.1 Particle size distribution ..................................................................................... 64 3.3.2 Soil pH and electrical conductivity .................................................................... 65 3.3.3 Determination of total C, N and S ..................................................................... 66 3.3.4 Exchangeable cations ......................................................................................... 66 3.3.5 Potassium (K) determination ............................................................................. 67 3.3.6 Sodium (Na) determination................................................................................ 67 3.3.7 Calcium (Ca) determination ............................................................................... 68 3.3.8 Magnesium (Mg) determination ........................................................................ 68 3.3.9 Exchangeable acidity (H+ and Al3+)................................................................... 69 3.3.10 Effective cation exchange capacity (ECEC) and base saturation .................. 69 3.3.11 Dithionite-citrate-bicarbonate extractable Fe ................................................. 69 3.3.12 Tamm's oxalate extractable iron, aluminium and manganese ........................ 70 3.3.13 Total phosphorus ............................................................................................ 71 ix University of Ghana http://ugspace.ug.edu.gh 3.3.13.1 Measurement of phosphorus .................................................................... 71 3.3.13.1.1 Reagent A was prepared as follows: ................................................... 71 3.3.13.1.2 Reagent B was prepared as follows: ................................................... 72 3.3.14 Available phosphorus ..................................................................................... 72 3.4 Phosphorus fractionation ................................................................................... 73 3.4.1 Water extractible-P from soil ............................................................................. 75 3.4.2 Resin extractable-P from soil ............................................................................. 75 3.4.3 0.5 M NaHCO3 extractable-P from the resin P extracted soil ........................... 76 3.4.3.1 NaHCO3 extractable-Pi determination ....................................................... 76 3.4.3.2 NaHCO3 extractable-Pt and Po determination ........................................... 77 3.4.4 0.1 M NaOH extractable-P from the 0.5 M NaHCO3 P extracted soil .............. 77 3.4.4.1 NaOH extractable-Pi determination ........................................................... 78 3.4.4.2 NaOH extractable-Pt and Po determination ............................................... 78 3.4.5 HC1 extractable-P from the 0.5 M NaOH P extracted soil ................................ 78 3.4.6 Residual P (occluded P) ..................................................................................... 79 3.5 Statistical analysis .............................................................................................. 79 CHAPTER FOUR .......................................................................................................... 60 4 RESULTS ............................................................................................................... 81 4.1 Physicochemical properties of the soils ............................................................. 81 4.2 Hedley’s phosphorus fractions in soil ................................................................ 91 4.2.1 Effect of soil amendment on P fractions under clean and waste water regimes for depth 0 – 10 cm. ................................................................................................. 92 4.2.2 Effect of soil amendment on P fraction under clean and waste water regimes for depth 10 – 20 cm. ............................................................................................... 96 4.2.3 Effect of mode of irrigation on available P fractions ......................................... 99 4.2.4 Variation among P fractions with depth ............................................................ 99 4.3 Simple correlation coefficients showing relationship among P fractions and with soil properties ............................................................................................................. 102 x University of Ghana http://ugspace.ug.edu.gh CHAPTER FIVE .......................................................................................................... 105 5 DISCUSSION ....................................................................................................... 105 5.1 Physico-chemical properties of the soils.......................................................... 105 5.1.1 Effect of amendment on soil physicochemical properties of the studied soil .. 105 5.2 Hedley’s phosphorus fractions in soil (control) and amended soils (B, IF-NAP and B + IF-NAP) ........................................................................................................ 110 5.3 Effect of irrigation regimes (clean and waste water) on soil P fractions ......... 116 5.3.1 Effect of mode of irrigation on available P fractions ....................................... 117 5.4 Effect of depth on soil P fractions.................................................................... 118 CHAPTER SIX ............................................................................................................ 120 6 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS......................... 120 6.1 Summary .......................................................................................................... 120 6.2 Conclusions ...................................................................................................... 121 6.3 Recommendations ............................................................................................ 122 REFERENCES ............................................................................................................. 123 APPENDICES .............................................................................................................. 148 Appendix 1. Phosphorus standardisation graph (mg P kg-1) ...................................... 148 Appendix 2. Analysis of variance for P fractions at depth 0 - 10 ............................... 148 Appendix 2. Analysis of variance for P fractions at depth 10 - 20 ............................. 152 Appendix 3. Picture presentation of fractionation process ......................................... 157 xi University of Ghana http://ugspace.ug.edu.gh LIST OF TABLES Table 3. 1 Climatic data of the study site……………………………………………....62 Table 4. 1 Physical properties of the soils before treatments were imposed…………..82 Table 4. 2.a. Chemical properties of soils in the treatment plots (fertility)…………......83 Table 4. 2.b. Chemical properties of soils in the treatment plots (fertility)……………...84 Table 4. 3.a. Chemical properties in the soils of treatment plots………………………..87 Table 4. 3.b. Corresponding CV% of chemical properties of soils at Table 4. 3. a…… 88 Table 4. 4.a Chemical properties in the soils of treatment plots .................................... 89 Table 4. 4.b. corresponding CV% of chemical properties of soils at Table 4. 4. a…….90 Table 4. 5 Mean values of the various P fractions for depths (0 – 10 cm) under clean and waste water regime ……………………..…………………………..…93 Table 4. 6 Mean values of the various P fractions for depths (10 – 20 cm) under clean and waste water regime …………………………………………………… 97 Table 4.7. Effect of mode of irrigation on available P fractions ………….………….100 Table 4. 8. Simple correlation coefficients showing relationship among P fractions....103 Table 4. 9. Simple correlation coefficients showing relationship between P fractions and selected soil properties…………………………………………...…..104 xii University of Ghana http://ugspace.ug.edu.gh LIST OF FIGURES 2. 1 Inner sphere formation of P in soil minerals (a) and subsequently, the gradual occlusion of the adsorbed P (b). ………………………………….…………..42 3. 1 Map showing the location of study site. ………………………………………..61 3. 2 Modified Hedley’s P fractionation Method………………………………………..74 4. 1 Relationship between sum fractionated P and total P determined independently via concentrated H2SO4-H2O2 digestion.……………………………...…..………91 4. 2 Variation among P fractions with depth : (A)-sum of most labile P; (B)- NaOH-P; (C) -HCl-P; (D) -Sum of Po; and (E) -residual-P) …………………………….…101 xiii University of Ghana http://ugspace.ug.edu.gh CHAPTER ONE 1 INTRODUCTION Soils of the northern region of Ghana have been known to have high multiple nutritional deficiency and fertility constraints (Abekoe, 1989). These deficiencies are due to the soils being highly weathered and deprived of adequate moisture from the long annual dry season. The soils have low organic matter content due to the harsh climatic conditions, nature of the predominant vegetative cover, the local geology and the consistent annual bush burning. Additionally, the soils have low clay content that is mostly kaolinitic (Nartey et al., 1994). Moreover, these soils are moderately to highly acidic due to the removal of bases, leading to an appreciable amount of sesquioxide minerals of iron and aluminium and also an appreciable amount of lateritic concretions or nodules (Abekoe, 1989). Concretions contain large amounts of iron and aluminium oxides which act as effective phosphorus "sink" for added P (Tiessen et al., 1991b). The soils, thus have a high capacity to fix added P through sorption owing to the highly reactive nature of P (FAO, 2011; Owusu Bennoah et al., 2000). The amount of P assimilated by plants in alkaline and acidic soils is much restrained due to P fixation. Addition of P to acid soils leads to the formation of insoluble Fe and Al phosphate through ligand exchange and precipitation reactions (Galvao and Salcedo, 2009; Schoumans and Chardon, 2015). The severity of the P limitation was further highlighted by Owusu-Bennoah and Acquaye, (1996) who reported that, the deficiency of P in some soils in the savanna region is so acute that plant development terminated as soon as the phosphorus stored in the seed was exhausted. Phosphorus availability has thus become a major factor controlling crop production in tropical weathered soils (Brady 1 University of Ghana http://ugspace.ug.edu.gh and Weil, 2002). The ever increasing population density has made it extremely challenging to sustainably meet the increasing food demands in recent years. The decrease in soil fertility is very high especially with regard to low P content and had made the task very herculean (Gruhn et al., 2000). There is therefore, the need for a considerable P input for optimum plant growth (Erbynn and Asante, 1990; IAEA, 2002). In view of the above, application of P fertilizers such as manure, synthetic inorganic fertilizers (chemical fertilizers), or composts among others, has become necessary to replenish soil P stock (Tisdale et. al., 1990). Also, liming materials would be required to increase the pH to optimum levels convenient for plant production. This liming requirement would not only increase soil pH but also reduce P retention thereby increasing soil P content and availability for plant uptake. The use of chemical fertilizers to increase nutrient availability in soil solution has been reported (Nartey et al., 1997). However, P fertilizers especially in acidic soils have not had the desired impact on soil productivity and crop yields owing to the continuous adsorption of added P by soluble Al and Fe and high level of sesquioxides and kaolinites (Nartey et al., 1997; Abekoe and Tieseen, 1998; Chien et al., 2014). Moreover, excessive application of chemical fertilizer over the past decades has been a huge contributing factor to soil environmental degradation in Africa owing to the residual effects (Bationo et al., 2006; Obiri-Nyarko, 2012). Chemical fertilizers are somewhat expensive for the rural peasant farmers who depend on farm proceeds for their livelihood. The use of cheaper and environmentally friendly 2 University of Ghana http://ugspace.ug.edu.gh sources of P then becomes necessary. Organic sources of P such as organic manures were variedly considered due to their low cost, availability and environmental friendliness. Organic matter is considered a sustainable approach for remediating P deficiency in tropical soils (Hue et al., 1991; Iyamuremye and Dick 1996; Haynes and Mokolobate, 2001) through competitive sorption reaction with mineral oxides for P and also direct discharge of P upon decomposition (SiddiqueandRobinson,2004). However, since decomposition rates of organic matter are high, such an amendment is often short-lived with a concomitant fast release of carbon dioxide, which aggravates global warming leading to a short period of improvement in soil fertility parameters with dire consequences (Barthes and Azontode, 2004). High haulage costs of organic material often lead to high production cost which is a disincentive to many Ghanaian farmers. In northern Ghana where mixed farming is often practised, competition for manureable materials becomes an added challenge to raw material availability, though not severe. The scarcity and high prices of chemical phosphate fertilizers, liming materials and also the short residence time and competition for biomass has necessitated the development and use of more appropriate alternate soil conditioning materials such as biochar. The use of biochar has been suggested to provide an integrated approach to rectifying the challenge of low fertility of tropical soils and improving P contents (Lehmann and Joseph, 2009). Biochar is the pyrogenic end product of biomass under controlled temperatures in the absence of oxygen. Biochar has recently, found increased use in scientific research due to its environmental and agronomic potentials. It is less expensive compared with chemical fertilizers, environmentally friendly and more stable soil 3 University of Ghana http://ugspace.ug.edu.gh conditioner with a high P content (Spokas, 2010); its soil addition improves other soil fertility characteristics (Lehmann et al., 2009; Atkinson et al., 2010). Application of biochar also increases P availability in soils (Chan et al., 2007; Lehmann, 2007; Atkinson et al., 2010). Various mechanisms by which biochar may directly or indirectly control the biotic and abiotic components of the P dynamics have been reported (DeLuca et al., 2009). Biochar contains large proportion of P and, therefore, can directly release soluble P into soil solution to enhance P availability (Chan et al., 2007; Atkinson et al., 2010). Addition of biochar to soil, reduces soil acidity owing to its high alkalinity and consequently reducing P reaction with Fe3+ and Al3+, which are important for evaluating P availability by P sorption and desorption reactions in soils (Wang et al., 2012; Yuan et al., 2011).Joseph et al. (2010). This also indicated that application of biochar to soils culminates into the deposition or precipitation of Fe-(hydro) oxide on biochar surface. The unimodal rainfall in northern Ghana has also been another major challenge to crop production. This leaves the soil environment dry and hot for most part of the farming season. The over dependence on the unimodal rain-fed agriculture as pertained in northern Ghana is gradually shifting to manpower irrigation systems in recent times. It is now evident that vegetable producers are taking advantage of the various irrigants (irrigation water) to propagate their crops. While some use clean water (tap or river), others use wastewater or both for irrigation with different modes of application. The effect of using waste or clean water on soil P fractions under management practices such as soil amendment with biochar is not fully ascertained. 4 University of Ghana http://ugspace.ug.edu.gh An on-going research aimed at developing P management strategies for indigenous vegetable production in the tropical savanna region of northern Ghana under the German sponsored UrbanPLUS Project has been initiated (Werner et. al., 2017). In the study, P management practices such as biochar amendment, inorganic P fertilizer (NH₄)₂SO₄) application as pertaining to normal agricultural practice in northern Ghana and a combination of both management practices were done on a Plinthaqualf. The P amendments were subjected to clean and waste water irrigation regimes over three cycles of vegetable production. The results of the research gave low crop yields with high P leaching, requiring further research. With the background knowledge on phosphorus limitation in soils (Plinthaqualfs) of northern Ghana, it has become imperative to quantify the available P fractions in the soil and to ascertain the influence of the administration of the various types of management practices and irrigation regimes on soil P content and subsequently, its availability. To characterize the different P forms in relation to land use and management (agronomic and fertility) as well as soil development (pedogenesis), the fractionation method proposed by Hedley et al. (1982) was used. This is expected to sequentially separate all soil existing P forms (organic/inorganic) into pools based on lability. The aim of this study is to fractionate phosphorus in the (NH₄)₂SO₄ and biochar-amended soils (Plinthaqualf) from northern Ghana subjected to clean and waste water irrigation regimes. 1.1 Objectives Specific objectives are to: 5 University of Ghana http://ugspace.ug.edu.gh i. Determine the effect of biochar and inorganic (chemical fertilizer) amendments on the physical and chemical properties of soils, ii. Determine the effect of biochar and inorganic (chemical fertilizer) amendments on phosphorus fractions, iii. Ascertain, if any, the influence of type of irrigation water (irrigant) on available P fractions, iv. Ascertain variation of P fractions with depth after two cycles of vegetable production under clean and waste water irrigation. 1.2 Hypothesis HO: Amendment of Plinthaqualf of the Northern Region of Ghana with biochar and subjected to clean and waste water irrigation regiment will not enhance release of available (labile) phosphorus. HA: Amendment of Plinthaqualf of the Northern Region of Ghana with biochar and subjected to clean and waste water irrigation regiment will enhance release of available (labile) Phosphorus. 6 University of Ghana http://ugspace.ug.edu.gh CHAPTER TWO 2 LITERATURE REVIEW 2.1 Nature of soils in northern Ghana The nature of climatic conditions and vegetative cover on the local geology (parent materials) has largely influenced the development of soils of the interior savanna zone of Ghana (Obeng, 1975; Ahenkorah et al. 1993). Contributions by temperature differences, rainfall distribution, geology and relief of the area are important. The prevailing climatic conditions, coupled with the general gentle relief, leads to the formation of very shallow soils overlying impermeable iron pan (Obeng, 1975; Eswaran et al., 1990). Iron pan soils which are common features of the interior savanna zone of Ghana cover about 76, 000 km2 and may occur at different elevations in the landscape (Obeng, 1970; Kanabo et al., 1978; Arshad et. al., 2000). Majority of these soils, are fairly shallow with most of them having profiles less than 150 cm deep (Jones and Wild, 1975). Dominant among the groups of soils in northern Ghana are the lateritic soils with varied concentrations of iron stone concretions or iron-bearing nodules (Abekoe 1996). The soils are highly weathered, rich in sesquioxide, low in organic matter and lower clay content; mainly kaolinitic (Adu, 1957, Obeng, 1975; Arshad et. al., 2000). Clay contents in the surface horizon are generally low owing to the downward movement of clay within the profile. Inherently, these lateritic soils are infertile except for the few, non-gravelly upland soils developed from the rocks and those developed in medium to moderately heavy-textured alluvial materials within the extensive valley flats (Obeng, 1975). 7 University of Ghana http://ugspace.ug.edu.gh 2.1.1 Climate The three sub-divisional ecological zones of the West Africa semi-arid tropics are based on long-term rainfall patterns (Kowal and Kassam, 1978). They include the following: (1) The Sahelian zone in the north, which has less than 600 mm annual rainfall and a rainy season of 2.5 – 4 months; (2) The North Sudanian zone, having a 4 – 5 months rain period at 600 – 1000 mm annual rainfall and (3) the South Sudanian zone with > 1000 mm and 5-6 months of rain. Northern Ghana falls within the North Sudanian ecological zone with a rainfall mostly concentrated within 4 – 5 months. Much as the total annual rainfall received can be adequate, its distribution, variability, and irregularity pose challenges to agricultural activities. Even on these relatively flat lands, the frequency of occurrence of high intensity rain storms results in flash runoff causing severe soil erosion. Northern Ghana is influenced by two dominant air masses of the West African semi-arid zone (Jones and Wild, 1975). These are the Saharan northerly dry harmattan and the South westerly monsoon of humid oceanic air masses. The air masses meet at the Inter- Tropical Convergence Zone which moves north and south once every year. The dominant wind for most of the year in the north is the dry northerly harmattan, whiles the south experiences the south west monsoon. The different wind directions result in the short unimodal wet season in the north, and a longer bimodal rainy season in the south. Climatic constraints are often the most critical determinants of productive agriculture in northern Ghana. Elements of climatic risk include strict rainfall seasonality, with nearly 90% of annual precipitation falling in the rainy season; wide rainfall fluctuations between and within rainy seasons; highly uncertain dates of arrival and withdrawal of seasonal rainfall and high potential evapotranspiration which influences the proportion of available water for crop growth. These climatic constraints are major limitations to 8 University of Ghana http://ugspace.ug.edu.gh productive farming in northern Ghana as pertaining to other semi-arid tropical environments. 2.1.2 Natural soil variability along toposequence in northern Ghana There seems to be the usual drift in West African soil catenas from the relatively few studies undertaken (Nye, 1954; Boulet, 1978; Nartey, 1994). Conspicuously; kaolinite predominate the clay mineralogy with some illite occurring in the topsoils for upland and upper slopes, whereas montmorillonite proportions increased with soil depth and towards the lower slopes (Nye, 1955; Nartey, 1994). The amount and type of clay mineralogy reflected the weathering intensity and exchangeable cation contents of the soils (Boulet, 1978). Illuviation of mainly kaolinitic clays from the topsoil and subsequent formation of argillic subsurface horizons were typical for well-drained soils (Alfisols) of the uplands, upper and mid slopes. This leads to comparatively poor nutritive and sandy topsoils superimposing illuvial silt loam subsoils underneath the natural vegetation. Generally, soil fertility is poor owing to the kaolinitic clay fraction and the relatively low organic matter contents, low cation exchange (< 5 cmol kg-1c ) and low buffering capacities for the upland zones in particular. Besides, the upland profiles are often shallow (0 – 50 cm) containing gravel and hard laterites which mostly get to the surface developing in low moisture holding capacities (Stoop, 1987). Under the wetter conditions of the lower slopes and lowlands, the soils are deeper with high organic matter contents. 2.1.3 Soil types and properties of semi-arid tropics (SAT) Northern Ghana is within the semi-arid tropics (SAT) of West Africa. The distribution and extent of various soils of the semi-arid tropics have been reviewed (El-Swaify et al., 9 University of Ghana http://ugspace.ug.edu.gh 1985; Swindale, 1982). Eight of the twelve orders in the U.S. Soil Taxonomy are represented in the semi-arid tropics. Five of them; Alfisols, Aridisols, Entisols, Oxisols and Vertisols – account for more than two-thirds of the SAT land area (El-Swaify and Caldwell, 1991). Inceptisols are also among the eight Soil Orders found in the SAT and occur in the lower slopes and the lowlands of a catena or a landscape. Within these soil orders, SAT environments are identified at the suborder level order by ustic moisture regimes in which “moisture is present at a time when conditions are suitable for plant growth” (U.S.D.A, 2013). Among the soil Orders on the landscapes, soil-based constraints which affect productivity potentials in SAT regions including northern Ghana are (a) exploitable rooting depth, especially at the upper slopes with high ferruginous nodule content and (b) poor soil aggregation and structural characteristics which are due to low organic matter content, low cation exchange capacity, low water retention and high soil temperatures. These soil properties have resulted in high potential for erosion, vulnerability to sealing and crusting and generally low fertility (Hauffe, 1989; Quansah et al., 1989) 2.1.4 Alfisols of northern Ghana Alfisols occupy the summit and upper slope segment of some landscapes in northern Ghana (Adu, 1952; Hauffe, 1989). The soils have coarse-textured surface horizons, while clay content increases with depth. The clay mineralogy is primarily kaolinitic with varying sesquioxide contents (Van Wambeke, 1989). By definition, Alfisols possess an argillic horizon with high base saturation (> 35%), characteristics which distinguish this soil order from Utisols (U.S.D.A., 2013). However, proposed revisions in the soil Taxonomy may identify some of these argillic horizons as kandic, particularly where clay 10 University of Ghana http://ugspace.ug.edu.gh content undergoes a sharp increase over a short depth (VanWambeke, 1989). The argillic horizons may compound problems of hardpans that are common in some subsoils in the region and may inhibit root development, thus preventing crops from withstanding even moderate droughts. Some soil profiles are characterized by well-developed horizons of gravels or localized concentrations of concretionary materials (e.g. ferruginous nodules). The abundance of such concretionary materials in the surface horizons is due to both clay eluviation and erosion of finer soil particles. These concretionary horizons are often compacted and rigid in the subsoils due to a clay matrix that causes mechanical impedance to root development of many seasonal crops (Babalola and Lal, 1977). Soils with high concretionary contents usually have a high bulk density ranging between 1.5 Mg m-3 and 1. 9 Mg m-3 (Cassel and Lal, 1992). Owing to the high concretionary content of some upland Alfisols in the region, they are marginal in productivity and less frequently cultivated. The Alfisols of northern Ghana are considered to have low to moderate potential for P sorption (Juo and Fox, 1977; Kanabo et al., 1978). 2.1.5 The nature of highly weathered semi-arid tropical soils Highly weathered soils usually occur in moist and hot climates, where weathering proceeds rapidly, but some do occur in the semi-arid tropics as in northern Ghana. Such soils have been subjected to pedogenesis for millions of years resulting in the removal of soluble constituents and minerals, and the relative accumulation of insoluble and resistant minerals which stand out as rock outcrops or small hills of resistant material. The genesis of these rock outcrops is not clear but it is believed that less intense weathering in the 11 University of Ghana http://ugspace.ug.edu.gh semi-arid environment in contrast to the humid tropics results in a preponderance of rock outcrops in the savanna and semi-arid tropics, kaolinite is the dominant clay mineral but also some illite and sesquioxides are present (Obeng, 1970; Nartey, 1994). In soils of the semi-arid tropics, the oxides, hydroxides and oxyhydroxides of Fe and Al are common. The most stable and frequent Fe (III) oxides found in these soils are goethite and hematite (Nartey, 1994). Lepidocrocite, ferrihydrite and maghermite, although less common, also occur in many of these soils. The concentration of Fe oxides in various soils ranges from < 0.1% to > 50% (Schwertmann and Taylor, 1989). They occur evenly dispersed throughout the soil horizons or concentrated in discrete horizons or in particular morphological features such as concretions, nodules and mottles all of which abound in soils of northern Ghana. Iron oxides are small, with crystal size of 10 to 50 nm (Schwertmann and Herbillon, 1992). They, therefore, possess a large specific surface area which effectively contributes to the overall surface area of soils. The best known effect of the Fe-oxide surface is its high affinity towards phosphate retention, demonstrated by a drastic decrease after differential removal of Fe oxide by citrate- bicarbonate-dithionite (Vig and Dev, 1984). Iron oxides also play an important role in determining soil colour. Even at low concentrations in a soil, they have a high pigmenting power that determines the colour of many soils. This particularly applies to tropical catenas where topsoils at upper slopes are typically reddish depicting a well-drained condition and the presence of Fe (III) oxides. As the elevation decreases the colour changes from red to grey at bottom slope. Thus, red and grey soil colours as determined by the type and distribution of Fe oxides 12 University of Ghana http://ugspace.ug.edu.gh within profiles and along landscapes, help explain soil genesis as well as drainage conditions. 2.1.6 Acidic soils P-sorption Poorly formed crystalline Al and Fe oxides compounds are the important soil factors that determine its capacity to fix P (Borling et al., 2001). This is because the poorly ordered crystalline (“amorphous”) Fe and Al hydroxides possess very high specific surface area (SSA) which can be as high as 800 m2 g-1, ten times higher than the SSA of the corresponding crystalline forms. Ferrihydrite and lepidocrocite as amorphous minerals are found to exhibit high P adsorption and adsorption rate (Wang et al., 2013). Aluminium oxides are found to have high sorption affinity for phosphate than that of iron oxides (Borggaard et al., 1990). On per mole basis Al oxides, adsorb almost twice as much phosphate as oxalate-extractable iron oxides. This could be attributed to higher specific surface area (poor crystallinity) of the Al oxides compared to the iron oxides and also a greater charge on the former (Bolan et al., 1985). For synthetic oxides the amounts of phosphate adsorbed per m2 seem to be higher for Al oxides than for iron oxides, although the trend is weak (Bolan et al., 1985). This implies that the variation in crystallinity is the major rationale behind the differences in adsorption capacity (Borggaard et al., 1990). The occurrence of goethite which is mostly promoted by low soil pH and high organic carbon content (Kämpf et al., 2009), usually has a higher sorption affinity for P than hematite (Guzman et al., 1994). Synthetic and natural goethite with varying morphologies have been reported to have a maximum adsorption capacity of 2.50 µmol 13 University of Ghana http://ugspace.ug.edu.gh P m-2 for the binuclear complex on the (110) face (Torrent et al., 1992; Strauss et al., 1997, Wang et al., 2013), whereas it is 0.19 to 3.33 µmol P m-2 for haematites with different morphologies and crystallinity (Barron et al., 1988; Wang et al., 2013). The P sorption capacity of goethite is relatively higher than kaolinite, due to the spread of ≡Fe- OH groups on entire surface of goethite while ≡Al-OH groups on the surface of kaolinite are found solely at the edges of the crystal structure (Wei et al., 2014). The smaller particle size of poorly formed crystalline soil minerals make them more soluble than their crystalline minerals. For instance, the crystalline mineral variscite (AlPO4.2H2O) has a surface area of 1.54 m 2/g (Mattlingly, 1975) and its solubility product (K ) is 10-30.5SP (Bohn et al., 1985). On the other hand, its amorphous Al- phosphate counterpart has a surface area of 10.5 m2/g (Sanchez and Uehara, 1980) and its KSP is 10 -28.1 (Sposito, 2008). Phosphorus is mostly adsorbed by hydroxyl surface groups of Fe and Al oxides, which below pH of 7-9 develop positive charge through protonation (Sparks, 2003; Sposito, 2008). The reaction of water with Fe or Al ions on mineral surfaces and also the completion of it coordination with hydroxyl groups is termed as hydroxylation (Stumm, 1992). Hydroxyl groups may be coordinated by one (type A, ≡Fe-OH–0.5), two (type C, ≡Fe2-OH) or three Fe atoms (type B, ≡Fe3OH +0.5), corresponding to hydroxyls of singly, double or triple coordination respectively (Russel et al., 1974; Essington and Houser, 2003; Sparks, 2003; Sposito, 2008). Type A hydroxyls (≡Fe-OH–0.5) are the most easily protonated (Fontes et al., 2001) as a result of the charge balance in Fe-O bonds, where the electron cloud of oxygen is more electronegative than in doubly or triply coordinated 14 University of Ghana http://ugspace.ug.edu.gh hydroxyls. Protonation weakens the Fe-OH bond by displacing the electron cloud of oxygen to the hydrogen side (Fontes et al., 2001). Two different chemical processes are formed due to the protonation of the hydroxyl groups namely, protonated surfaces develop a positive electric field that attracts phosphate ions and phosphate replaces protonated hydroxyl groups. The phosphate may be adsorbed in mono-dentate or bidentate form based on the number of hydroxyl groups in the phosphate that are bonded to Fe atoms, or in binuclear form when two hydroxyl phosphate groups are adsorbed by two Fe atoms. Spectroscopic and microscopic techniques have been employed to observe and investigate the local bonding environment of phosphate with clays and metal oxide surfaces (Parfitt, 1989; Russell et al., 1974; Goldsberg and Sposito, 1984). With the use of infrared (IR) absorption, Russell et al. (1974) showed that the adsorption of P by goethite surface led to the loss of hydroxyl group. This affirms that ligand exchange is the mechanism by which phosphate is adsorbed by iron oxides leading to the formation of inner-sphere complex (Goldberg and Sposito, 1984). Further studies using spectroscopic techniques have shown that the adsorption of P unto the surface of soil minerals involves; a bidentate, binuclear bridging complex with Fe (III) on iron oxide surfaces and on the edge sites of Al oxides (Luengo et al., 2006; Arai and Sparks, 2007). Wang et al. (2013), studying P adsorption mechanism on ferrihydrite, goethite and hematite also suggested that P adsorption on Fe oxides/hydroxides is principally bidentate binuclear complex. Others have also reported the same P adsorption mechanism on boehmite and goethite (Li et al., 2010; Kim et al., 2011). 15 University of Ghana http://ugspace.ug.edu.gh Diffusion of the adsorbed phosphate into the solid is termed absorption. Adsorbed phosphate may become trapped on the surface of soil minerals if any Fe or Al oxide coating is precipitated on the mineral. The trapped phosphate in the nanopores of Fe/Al oxides is then described as occluded thereby becoming unavailable to the plants (Arai and Sparks, 2007). It is reported that adsorbed P can also be desorbed thermodynamically (Barrow, 1978); however, the rate of P desorption bank on several factors such as the type of clay minerals on which the P is adsorbed (Chintala et al., 2014). According to Parfitt (1989), the binding energy between P and soil minerals increases in the order of monodentate > bidentate > binuclear complexes and the chances of P desorption increases in the reverse order. Sato and Comerford (2006) proposed that disequilibria-desorbable P and ligand- desorbable P are the two main mechanisms for P desorption. 2.1.7 Harmattan dust and its significance to soils Mineral dust movement by the atmospheric wind has been an age long phenomenon, however, efforts to assess the magnitude and significance but have largely come in the last century. Attempts to identify and understand effects of the dust on soils are even more recent, mostly in the last 30 years (Simonson, 1995). The principal sources of mineral dust are in the deserts and semi-arid regions of the world. Dustfalls from the atmosphere have been collected and analysed in different parts of the world. The distribution, frequency, and magnitude of dust storms and their implications to geomorphology were reviewed by Goudie (1978) with extensive coverage of literature. 16 University of Ghana http://ugspace.ug.edu.gh Estimates of atmospheric dust carried from the world’s deserts range from 5 x 1011 kg yr- 1 (Pewe, 1981) to 5x 1012 kg yr-1 (Schutz, 1980), the Sahara Desert being the world’s contributor (Junge, 1979; Schutz, 1980). The harmattan winds are recognized phenomenon of the Sahel region of Africa. They are dust-laden, dry hot winds that blow from a north-easterly direction from the Sahara desert with the dust commonly reaching parts of West Africa, especially the northern and upper regions of Ghana, Nigeria and Niger. Average dustfall in northern Nigeria was reported to be 580 kg ha-1 yr-1 with clay (25%) and silt (57%) as main constituents (Moberg et al., 1991). In Ghana, a dustfall of 150 kg ha-1 yr-1 was reported by Tiessen et al. (1991b) and in Niger, approximately 2000 kg ha-1 yr-1 of dust infall was recorded (Drees et al., 1993). Airborne accessions of dust have a variety of effects on soil development and characteristics. Among others, dust is a primary source of the constituents essentials to some horizons and layers in a number of soils, provides the bulk of the materials for silty surface horizons in many soils, adds minerals like feldspars and micas which serve as long-time K reserves in soils and finally, provides nutrient elements (e.g. P) for plants uptake (Tiessen et al., 1991b; Simonson, 1995). From the identification and documentation of minerals in aerosolic dust from 10 widely scattered sites around the world, Syers et al., (1972) concluded that the dust accessions would be a restoring process for strongly leached and weathered soils. Dust accessions are also sources of carbonate in soils which indirectly influence Phosphorus bio-availability. Carbonate materials may be conveyed in suspension and deposited on the surface as dust or in rain (Doner and Lynn, 1989). Evidence of additions of Ca, Mg and K in airborne dust was attained by Tiessen at al. (1991b) in Ghana. They ascertained the Harmattan dust to have high base saturation of these cations. Accessions during a single year provided bases equivalent to 17 University of Ghana http://ugspace.ug.edu.gh 1% of the effective exchange capacity of soils to a depth of 10cm and the combined amounts of Ca, Mg, K and Na were equivalent to slightly less than 10 kg ha-1. Drees at al. (1993) also found yearly additions of approximately 10 kg ha-1 of exchangeable and water-soluble Ca, Mg, K and Na in dustfalls in Niger. They recounted a considerably greater nutrient status of the dust than the native soil and stated that the dust could serve as a nutrient renewal vector for the soils. In northern Ghana, the effects of weathering or the level of soil fertility may be affected by the continuing accessions of the Harmattan dust from the Sahara. Phosphorus bio-availability is directly or indirectly affected by accessions of harmattan dust. Harmattan dust accession causes the deposition of carbonates that offer competition to P for P binding sites. This allows for the unbound P to be moved into the soil solution for plant uptake. Also, deposition of basic cations Ca2+ and Mg2+ from the Harmattan dust accession, tends to increase pH of northern region soils from strongly acidic to moderately acidic or to near neutral conditions that allow P to be release from the outer sphere of Al3+ and Fe3+ into the soil solution. 2.2 Agriculture in northern Ghana Northern Ghana has been known to have typically large variability in crop stands and low average crop productivity. This is attributed to poor vegetative cover coupled with annual bush fires that lead to a low organic C, total N and available P contents of the soils (Hailu, 1990). The nature of agriculture in this zone is essentially subsistence and is operated small holdings, mostly in the hands of local small scale farmers. This implies that the farmers produce food and other agricultural produce for their nuclear family consumption with the excess sold locally or batter traded. Two distinct forms of agriculture is practiced in northern Ghana. These are crop production and mixed farming. 18 University of Ghana http://ugspace.ug.edu.gh Crop production predominates in most areas of northern Ghana and involves a far greater proportion of the population. However, in some parts of northern Ghana, farmers integrate livestock with crop cultivation (i.e. mixed farming). This has some impact on the maintenance of soil fertility as animal wastes are left in the field to enrich the soil. The farming systems practised are mainly compound farming, rotational bush fallowing and continuous cultivation (Jones and Wild, 1975; Hailu, 1990). These systems are explained as follows: Compound farming: Farmers in northern Ghana have fashioned a pattern of fertile, continually cropped compound fields around the houses through regular dumping of organic manure and waste. Consequently, soil fertility gradually decreases with increasing distance from settlements. Some advantages of compound farming are the proximity to settlements and high productivity of the soil. Rotational bush fallowing: This system involves a deliberate alternation between cropping and bush regeneration. The cyclical fallow maturation of each phase depends on the soil fertility, weed encroachment, and population pressures on the land. The increasing human and livestock populations are steadily increasing the pressure on this system of farming, thus shortening the fallow periods (3-6 years). Marginal lands, generally the upper and mid slopes, are then taken into crop production. Continuous cultivation is thus practised in some areas where high population density defeats the natural fallow system and farmers have to crop season after season on the same field. Intense pressure is then placed on this system thereby increasing the reliance on fertilizer use and on other agrochemicals to sustain high yields. Cash crops are usually grown under this system. 19 University of Ghana http://ugspace.ug.edu.gh 2.2.1 Farming methods and crops grown in northern Ghana According to Jones and Wild (1975) and Hailu (1990), the following features are common to traditional agricultural systems in northern Ghana: 1) Land is cleared by cutting and burning in situ Clearing is rarely complete and trees of economic value are spared. One negative practice that poses serious agricultural challenge in the region stems from bush burning. This action reduces organic matter necessary for cation exchange, nutrient supply and structural aggregate stability of the soils. Fire has some immediate destructive effects which include a sudden rise in soil and air temperatures, volatilization losses of N and S, exposure of soil to climatic hazards and destruction of soil fauna. However, one beneficial effect of fire is that there is a sudden increase is soil ash content which has direct positive effect on the basic cation content of the soils. The use of fire may thus have an important bearing on soil-landscape relationships in the region as soils at some landscape positions may receive more ash which may boost its nutrient content. The unimodal rain-fed and general low input agricultural practice, practically require some P fertilizers or other agrochemicals for better yield. 2) Local farmers use simple tools, the most important are the cutlass and short- handled hoe. 3) Most crops are grown on mounds or ridges with varying dimensions. The main advantages of this farm practice are firstly, to create a relatively deep rooting zone of fertile topsoil and secondly, to improve drainage. 4) Interplanting or mixed cropping predominates over monocropping to reduce the risk of total crop failure within a season. Local agriculture in the region cleverly 20 University of Ghana http://ugspace.ug.edu.gh exploits the soil variability by growing different crops and intercrops on different land types. These are typically cereals and tubers. Cereals such as rice (Oryza spp) is grown on lowlands, sorghum (Sorghum vulgare) and maize (Zea mays) which are slightly more drought resistant than rice are grown on mid and lower slopes. Millet (Pennisetum spp) a drought tolerant crop, cowpea (Vigna sinensis) water melons (Citrullus vulgaris) and groundnuts (Arachis hypogea) are often grown upland. Tubers widely grown in northern Ghana include: yams (Dioscorea rotundata) and cassava (Manihot esculenta) and domestically consumed (indigenous) vegetables in various mixtures such as ‘ayoyo’ (Corchorus spp), ‘borkorborkor’ (Talinum triangulare). Some cash crops are: cotton (Gossypium hirsutum) and tobacco (Nicotiana tabacum). Much as factors affecting crops choice of are complex, however, rainfall and soil are the most important ones (Nyamekye, 1989). The introduction of irrigation in later times had encouraged the production of vegetables on small to medium scale. Vegetables such as tomato, pepper, onions, cabbage, lettuce, cauliflower and carrot as well as indeginous ones like ‘ayoyo’ (Corchorus spp), ‘borkorborkor’ (Talinum triangulare) are now in production in northern Ghana. 2.2.2 Vegetable production in Ghana Vegetable production is one of the global activities practised in every economy since it forms a major component of human diet (Amoah et al., 2014). Vegetable is thus defined as an edible plant or portion of a herbaceous annual or perennial crop that could either be served raw (green/fresh) or after a little processing (Kyei-Baffour et al., 2005). Vegetable provides a source of livelihood for not only the growers but also the traders and the 21 University of Ghana http://ugspace.ug.edu.gh processors. Thus, it provides an excellent source of employment for both rural and urban dwellers as it is grown in many rural areas through truck farming and in the outskirts of town and cities as market gardening and backyard gardening and supplied fresh to the urban markets (Owusu and Amuzu, 2013). The tonnage of production is not well quantified and recorded since consumption is literally immediate. 2.2.2.1 Nature of vegetable production in Ghana Exotic vegetables are the main crops cultivated for commercial purposes in urban areas of Ghana but in recent times indigenous vegetables are being produced. Indigenous vegetables like ‘ayoyo’ (Corchorus spp) and ‘borkorborkor’ (Talinum triangulare) are now being produced alongside the exotic ones. Production of vegetables comes with associated productive activities, such as land preparation and irrigation which are done manually and very laborious (Danso et al.2002; Tallaki 2005). In view of the poor fertility of soils especially those of northern Ghana, fertilizer applications are done to augment plant nutrient requirement. In most cases, inorganic fertilizer is applied. However, due to the expensive nature of these fertilizers, organic alternatives are combined with the inorganic materials. Improvement in scientific research is now introducing new and more effective environmentally friendly but less expensive soil fertility conditioners such as biochar. This pyrolytic product was used in this current study as an amendment. 22 University of Ghana http://ugspace.ug.edu.gh 2.3 Management practices for P availability in soils Soils of Northern Ghana require effective management practices to improve availability of phosphorus for plant uptake (Owusu-Bennoah, and Acquaye, 1996). These management practices include the usual inorganic and organic fertilizer application, mulching, incorporation of manures of heavy P feeders, irrigation regiments among other practices. In recent times, biochar, a pyrogenic product of biomass, is receiving great research attention due to its potential importance in agronomic and environmental applications (Yeboah et al., 2009; Yu and Teruo, 2013; Xu et al., 2014 and Zhang et al., 2016). 2.3.1 Biochar production and amendment Biochar is produced through thermochemic breakdown of biomass that is of both plant and animal sources (Kammann et al., 2015; Schmidt et al., 2015). In accordance with International Biochar Initiative (IBI), biochar is defined as ‘‘a solid material obtained from the carbonization of biomass’’ ( http://www.biochar-intenational.org/biochar; Tang et. al., 2013). It is also defined by Lehmann and Joseph (2009) as a carbonaceous material obtained from the pyrolysis of organic residues at controlled temperatures (<700oC). Apart from carbon (C), hydrogen (H) and oxygen (O) as the major elements of biochar, it also contains nitrogen (N) and sulphur (S) (Laird et al., 2010). Biochar can be produced from a range of biomass including wood (saw dust or shavings, etc.), animal waste (manure) and sewage sludge (Sohi et al. 2010). Biochar is defined by the pyrolytic temperature and type or source of biomass (Yeboah et al., 2009). Thus, the chemical and physical properties of biochar are dependent on the biomass type and 23 University of Ghana http://ugspace.ug.edu.gh pyrolysis conditions such as residence time, heating rate and temperature (Chan et al., 2008; Spokas et al., 2012). For instance, high pyrolytic temperatures (300 to 700oC) refine the carbon content both in quantity and quality but negatively affect the N, O and H concentration through volatilization (Brewer, 2009). The physicochemical properties of biochar differ from the feedstock from which it was produced. It is characterized by its relatively high surface area, high cation exchange capacity, high C and phosphorus content as compared to the biomass (Singh et al., 2010; Lehmann et al., 2011). A number of studies on the use of biochar on highly weathered tropical soil as well as temperate soils has been undertaken (Lehman, 2006; Chan et al., 2010; and Spokas et al., 2012). Most of these studies showed much improvement in soil fertility and high crop yield upon biochar addition to soil (Yeboah et al., 2009). The underlying mechanism for the increase in plant available nutrients in tropical soils upon biochar application has been noted including direct release of soluble nutrients in biochar into soil solution (Sohi et al., 2010), mineralization of the labile carbon contained in biochar (Lehmann et al., 2009), less nutrient leaching due to the physical and chemical characteristics of biochar (Liang et al., 2006) and minimized N losses via denitrification and volatilization (Cayuela et al., 2013). The production of biochar is affordable and also environmentally friendly. Apart from the use of biochar in soil fertility and productivity, it has also been deployed in the area of wastewater treatment, climate change mitigation and energy production (Hassan et al., 2012). 24 University of Ghana http://ugspace.ug.edu.gh 2.3.2 Effect of biochar on soil properties Several studies have reported that biochar has the ability to change the physical properties of tropical and temperate soils such as surface area, bulk density, aggregate stability and water holding capacity (Sohi et al., 2010; Ippolito et al. 2015). The surface area of soil is a key property that controls important functions of soil fertility such as water and nutrient holding capacity, aeration, and microbial activity (Lehman, 2006). Increase in agronomic yield of tropical soils amended with biochar has been attributed to the increased surface area of soil upon biochar application (Lehman, 2006; Yeboah et al., 2009). Although surface area of biochar charred under different temperatures has been reported, not much is known about the surface area of soil-biochar mixture (Turrion et al. 2001). However, an increase in surface area up to 5.1 times more than that of unamended soil has been reported upon biochar addition (Liang et al., 2006). In incubation studies, the incorporation of biochar at a rate of 1% increased the surface area of clay loam soils from 130 to 150 m2 g-1 (Downie et al., 2009). Others have observed that addition of waste-derived biochar to weathered tropical soils increased the amount of mesopores at the expense of macropores (Jones et al., 2010). Evidence from other studies have also proven that the increase in the overall net soil surface area of tropical soils upon biochar amendment is inevitable (Chan et al., 2007) and therefore, may increase soil water and nutrient retention capacity (Downie et al., 2009) as well as soil aeration in fine-textured soils (Kolb, 2007). Several research works on Alfisols and Ultisols have reported significant decrease in bulk density of soils when biochar was applied due to its relatively lower bulk density than mineral soils (Lehmann, 2006; Chen et al., 2011). The addition of rice straw biochar at a 25 University of Ghana http://ugspace.ug.edu.gh rate of 9.4% in annual field work showed a significant reduction in bulk density (Chen et al., 2011). Lehmann (2006) also conducted soil column studies using biochar and reported a decrease in bulk density of the control soils from 1.68 to 1.41 Mg m-3. It is, therefore, obvious that the addition of biochar to soils improves the bulk density optimum for plant growth (Brady and Weil, 2002). Water holding capacity of soil is controlled by other physical properties such as surface area, bulk density and aggregate stability (Brodowski et al., 2006). The application of biochar as low as 0.5% is known to improve the water holding capacity of soils (Jones et al., 2010; Uzoma et al., 2011). The effect of biochar on the water holding capacity of soils also depends on the soil types. In a study of amending biochar to soils of different textural classes, varying responses among the soils were found (Glaser et al., 2002). The moisture content of sandy soil increased by 18% upon the addition of 10% rate of biochar. However, the study recorded a decrease in moisture content in loamy and clay soils. Others have also reported an increase in moisture content in sandy soils (Gaskin et al., 2008). The increase in soil moisture has been attributed to the high surface area of biochar by several authors (Glaser et al., 2002; Gaskin et al., 2008). The same biochar type produced at three charring temperatures was applied to a sandy soil in order to assess soil moisture dynamics. The study reported an increase in soil moisture content by 18.3, 22.3 and 48.2% when biochar pyrolysed at 300oC, 450oC and 550oC were applied, respectively (Uzoma et al., 2011; Shafie et al., 2012). The researchers established that soil moisture content is much improved with biochar produced at high temperatures (>550oC) than low temperatures (300-450oC). The availability of plant nutrients in soil solution have been improved upon biochar 26 University of Ghana http://ugspace.ug.edu.gh application, especially in tropical acidic soils (Stevenson and Cole, 1999; Lehmann et al., 2006; Cheng et al., 2008). This has been attributed to direct release of soluble plant nutrients, increase in cation exchange capacity and soil pH (Lehmann et al., 2003). Mostly, biochars are basic and therefore are used as soil amendment to neutralize acidic soils (Glaser, 2002). The carbonate, bicarbonates and silicates content of biochar makes it a liming agent and therefore have the potential of increasing neutral or acidic soil pH (Van Zweiten et al., 2007). The increase in soil pH is due to the reaction between the acidic functional groups such as carboxylic in biochar and hydrogen ions in soil solution (Lehmann et al., 2003). An increase in soil pH from 4.5 to 6.3 after biochar addition in a short term incubation studies was reported by Mbagwu and Piccolo (1997). Similarly, application of varying biochar types at 1% to some soils of the tropics showed 0.59 to 1.05 units increase in pH after 60 days (Yuan et al., 2011). In a 90 day incubation studies, rice husk biochar raised the pH of tea garden soils by 0.52 units (Wang et al., 2014). Biochar contains soluble basic cations such as Ca2+ and Mg2+, which when applied to soils, are released, exchanging with Al3+ and H+ thereby increasing soil CEC. Liang et al. (2006) reported that increase in soil CEC is proportional to an increase in charge density, surface area of soil as well as soil pH (4.8 to 8.5). The CEC of an Alfisol increased from 4.3% to 18% after the application of rice straw biochar in an incubation studies (Yuan et al., 2012). Similarly, Jien and Wang (2013) observed that Leucaena leucocephala biochar increased the CEC of a highly weathered tropical soil by 6.4 cmolc kg-1. However, other studies have reported that biochar could not improve the CEC of soils with high initial CEC but rather soils of low initial CEC (Peng et al., 2011). The direct release of basic cations into soil solution after biochar amendment is an indication of soil fertility improvement due to the abundance of acidic functional group 27 University of Ghana http://ugspace.ug.edu.gh and also high surface area of biochar (Cheng et al., 2006). The increase in exchangeable plant nutrients including Mg2+, Ca2+ and K+ in soil solution have severally been observed after biochar incorporation. The amount of these exchangeable cations has been shown to increase more than 60% in biochar amended soils (Wang et al., 2014). For instance, Mg2+ and K+ concentration in soils amended with corn cob biochar increased from 26 to 235 mg kg-1 and 18 to 163 mg kg-1, respectively (Wang et al., 2014). The percentage base saturation was raised from 5.4 to 22% (Cheng et al., 2006). The addition of biochar tends to decrease N2O emission from soils of varying physicochemical properties (Stewart et al., 2012). For example, in a study using soybean and soybean biochar, N2O emission was reduced by 50% and 80%, respectively (Wang et al., 2013), On the contrary, most studies have reported that biochar addition did not have any effect on N2O emission in some tropical soils (Clough et al., 2010). Reduction of N2O emission is more sensitive to biochar produced at relatively low charring temperatures (300-400 oC) than at high temperatures possibly due to the high amount of carboxylic and phenolic groups capable of reducing N2O emission (Wang et al., 2013). Steiner et al. (2008) reported that the high CEC of biochar helps in reducing leaching of plant nutrients such as NH +4 making it available for plant uptake. For instance, some researchers observed about 70% reduction of N leaching in a pot experiment (Lehmann et al., 2003; Major et al., 2009). In highly weathered soils, biochar has been affirmed as appropriate soil amendment for reducing N leaching thereby increasing N use efficiency (Steiner et al., 2008). 28 University of Ghana http://ugspace.ug.edu.gh 2.3.3 Effect of biochar on phosphorus bioavailability in soil Results regarding the effect of biochar on the availability of P in soils have inconsistently been reported in literature. Increased P availability has been observed in soils amended with biochar (Lehmann et al., 2006). However, others have reported decreased available P content in a soil column studies involving biochar produced from different feedstock (Novak et al., 2010). The increase in the pH of acid soils upon biochar application due its high alkalinity (Ca2+, Mg2+) can lead to increase P availability by reducing P precipitation with Al3+ and Fe3+ (Steiner et al., 2007; Yuan et al., 2011; Chintala et al., 2013). In contrast, addition of biochar to neutral and alkaline soils were found to decrease P bioavailability as a result of increased P sorption, forming Ca-P and Mg-P compounds (Chintala et al., 2013). Biochar has high native P content and therefore can directly release soluble P and increase its bioavailability in soil solution for plant use (Chan et al., 2007). Biochar can influence P dynamics in soil through its adsorbed chelating organic molecules such as phenolic acids and amino acids (Stevenson and Cole, 1999; Kappler et al., 2014). The adsorbed organic molecules on biochar surface have the potential to reduce the capacity of Al3+, Fe3+ and Ca2+ from precipitating P in soil (Xu et al., 2014). It is reported that biochar can influence P bioavailability by altering the ion exchange property of soil (Cheng et al., 2008). Newly added biochar to soil mostly has high anion exchange capacity and therefore has the tendency of undergoing competitive reaction with poorly crystalline and crystalline Al and Fe oxides for P sorption (Hunt et al., 2007). Decrease in point of zero charge (PZC) and increased negative surface charge potential of soil amended with different biochar types have been observed (Hunt et al., 2007). The amount of negative charge on soil surface at pH 7 is explained by CEC (Jiang et al., 29 University of Ghana http://ugspace.ug.edu.gh 2012). Addition of biochar raises the CEC of soils hence decrease P availability as a result of electrostatic repulsion between the negatively charge soil surface and P (H2PO - 4 or HPO 2-4 ) (Jiang et al., 2015). The carboxylic and phenolic functional groups on biochar surface especially on biochar produced at relatively low temperatures form chemical complexes with Fe and Al oxides and as such serves as a competitor with P for adsorption sites on soil surface (Chen et al., 2011; Cui et al., 2011). The quantity and quality, particularly the size and charge, of biochar determine the level of competition (Weng et al., 2008). Biochar is usually fine in size and has highly charged humic acids and thus, can act potentially as a very effective competitor for P and can bring more P into soil solution (Liang et al., 2006). In this way, it reduces P adsorption by highly weathered tropical soils (Chen et al., 2011). Extracted humic acids from maize straw biochar increased P availability twice as much as soil humic acid (Sohi et al., 2010). Similar observation in Terra Preta (Lehmann et al., 2006) and soil amended with sewage sludge biochar have been reported (Nelson et al., 2011). The biogeochemical cycle of P in soils occurs by the breakdown of organic P and dissolution of inorganically bound P compounds initiated by microbial activities and hydrolytic enzymes released by plant roots. Soil microbes and enzymes are the major determinants of the breakdown of P (Bohme et al., 2005). The addition of biochar to soil increases P mineralization by enhancing microbial activities (Bohme et al., 2005). Approximately 4% increase in microbial biomass in tropical soils was observed after the addition of water hyacinth biochar at a rate of 0.5% (Bohme et al., 2005; Jin et al., 2016). The hydrolysis of P is largely influenced by pH. In high pH soils, alkaline phosphatase activities are enhanced (Du et al., 2014; Jin et al., 2016). In an incubation studies, a 3 to 30 University of Ghana http://ugspace.ug.edu.gh 4 fold increase in phosphatase activity was in sandy clay soils amended with corn cob biochar charred at 360oC (Du et al., 2014). 2.3.4 Irrigation regiments Efficient conservation, management, and use of irrigation water are critical to successful crop production, especially under drought conditions. The frequent, extremely hot and dry conditions can reduce production over large areas of the region, thereby limiting nutrient (phosphorus) uptake by plants. Soil types differ in their ability to retain water following rainfall or irrigation. Water that is held by the soil and can be taken up by plants is called "available soil moisture" (ASM). Sands and coarse sandy loams retain approximately 25 mm of ASM in the crop's rooting zone. Under average conditions, a crop will use this moisture in 7 days. On these soils, a weekly irrigation of 25 mm of water would be required to maintain high yields. Fine sandy loams and silty loams retain 40-60 mm of ASM in the rooting zone. In these soils, heavier and less frequent irrigations (40-60 mm of water every 10-14 days) are required for optimum yields. Application of more water than is required wastes water and can cause leaching of soil nutrients from fertilizers and pesticide applications. Hence mode of irrigation should be critically considered. Drip or sprinkle irrigation is much preferred to manual flooding of crop fields with irrigation water. Different crops have varying response to irrigation. Hence, shallow-rooted crops such as potatoes and onions require frequent, light irrigations whiles deep-rooted crops can use water from a greater volume of the soil profile and would not require irrigation frequently. 31 University of Ghana http://ugspace.ug.edu.gh 2.3.4.1 Clean water irrigation regime Clean water as an irrigant (irrigation liquid), theoretically adds little to no nutrient to the already existing soil nutrient (Eduah, 2012). However, it facilitates easy absorption of soil nutrients since water is a universal solvent and would dissolve nutrient salts and create an interface for nutrients to be made accessible by plant roots. In view of population increase, urbanisation and clean water scarcity, treated or untreated wastewater has gain use as a necessary irrigant. This notwithstanding, had come with its peculiar problems in human and animal health. 2.3.4.2 Waste water irrigation regiment The difficulties of removing phosphorus from treatment of effluents are preferably addressed by controlled irrigation of farmer fields (Pratt, 2012). The reuse of treated wastewater for irrigation is a practical solution to overcome water scarcity, especially in arid and semiarid regions such as Northern Ghana (AL-Jasser, 2011). According to Kiziloglu et al. (2007) and Eduah (2012), wastewater has a high nutritive value that might improve plant growth (Huang et al., 2008). It had been shown that soil irrigated with wastewater contained 4.1% of organic particles by weight, but these particles harboured up to 47.8% of the total soil carbon and 41.7% of nitrogen, and thus represented an important storage of energy and nutrient for microorganisms (Filip et al., 2000). Wastewater (WW) irrigation increases soil pH, electrical conductivity (salinity) and organic matter. However, the results of other field experiments indicate that WW irrigation decreased soil pH but increased soil salinity, soil phosphorus (P), potassium (K), iron (Fe), and manganese (Mn) levels. 32 University of Ghana http://ugspace.ug.edu.gh Average values of Phosphorus were high in soil irrigated with wastewater, (27.33 mg/kg, compared to 6.22 mg/l in soil irrigated with groundwater). These results are consistent with those of Akponikpe et al. (2011) and also Sacks and Bernstein (2011) who noted that irrigation with wastewater increased soil phosphorus. Kiziloglu et al. (2007) showed that wastewater irrigation treatments increased the availability of phosphorus and other micro- and macro-nutrients. 2.4 Phosphorus in soil Phosphorus is one of the major nutrients required by plants along with nitrogen and potassium and hence, very essential for crop production. However, the abundance of phosphorus (P) in soils is not as high as nitrogen (N) and potassium (K). Total P concentration in surface soils varies between 0.02 and 0.1% and this content is much lower in tropical soils including Ghanaian soils. Of the percentage mentioned above, only a fraction is readily available for plant uptake. Soils of the northern Ghana, which fall in the savanna region, have even much lower P content due to a number of factors. Rocks bearing apatites, which are the primary source of P, are few in this area. However, P availability for plant uptake does not commensurate with the total P of a soil because most of the P in the soil are insoluble. Phosphorus in the soil environment is in the form of a phosphate ion and very reactive with other soil components such as Al, Fe and Mn in acidic soils and Ca and Mg in alkaline soils (Jahnke, 1992). This makes P sorption a major problem to the extent that when applications of inorganic P (soluble P) forms are carried out, P might still be unavailable for plant uptake until such a point where the sorption capacity of soil for P is saturated and soil P equilibrium is attained between soil solution and soil solid phase. 33 University of Ghana http://ugspace.ug.edu.gh 2.4.1 Geochemical status of phosphorus It has been estimated that the total amount of phosphorus on earth is of the order of about 1019 t. About 1015 t of this total amount are in the earth's crust, which on the average contains 0.12% phosphorus (van Wazer, 1961). Apatites are the only phosphorus-bearing mineral group found in nature to have been of agronomic significance out of the nearly 200 rock minerals. The apatites have a general chemical formula as M10 (P04)6X2 most frequently the metal ion (M) is calcium and the anion (X) is fluorine. This fluoroapatite rarely occurs as primary mineral in sediments because they are classified among the most easily weathered minerals (Mitchell et. al., 1964). Hence on the average, the P content in igneous rock tends to be higher than that of sediments (Larsen, 1967). During soil formation, marginal loss of P is experienced causing some variation in virgin soils (Larsen, 1967). Soil P is relatively stable. However, soil erosion and leaching may cause illuviation and eluviation of P due to geographical location, relief of landscape and climate. Even though, the primary source of soil P is the parent material, P redistribution could be effected by other biotic and abiotic factors such as actions of wind, water, animals, microorganism and plants resulting in either depletion and or enrichment of soil P (Manu et al., 1991). In modem times, this redistribution has been accelerated by deliberate anthropogenic activities such as manufacturing and application of fertilizers to enrich soil on one hand and agriculture through crop production to deplete on the other hand. 2.4.2 Phosphorus status and availability in West African savanna soils In highly weathered tropical soils, P deficiency is a major constraint to crop production due to the presence of P forms not readily available to plants (Akinola et al.1983). 34 University of Ghana http://ugspace.ug.edu.gh According to Udo and Uzu (1972), phosphorus applied to tropical acid soils has limited availability to plants because of high P-fixing power of the soils. This is because tropical soils contain appreciable amounts of sesquioxides, exchangeable aluminium, iron and other soil factors which are active in P fixation. However, from a review of field experiments carried out in the tropics, Russell (1968) showed that it is wrong to describe tropical soils as generally having high phosphate fixation capacities in the sense that they rapidly convert applied phosphate into unavailable forms since residual P fertilizer effects have been recorded in such soils. Deficiency of phosphate occurs widely in the savanna soils. In some acid soils, deficiency is so acute that plant growth virtually stops as soon as the seed reserves of phosphate have been used up (Jones and Wild 1975; Owusu-Bennoah and Acquaye, 1989). In 181 topsoil samples of ferruginous tropical soils, mostly Alfisols in Nigeria, the total P contents ranged from 13 to 560 ppm with an average of 125 ppm (Baker et al. 1965). Total P contents of 67 topsoils from high-grass savanna of Ghana averaged 134 ppm (Nye and Bertheux, 1957) whereas, Acquaye and Oteng (1972) reported a range of 104 to 270 ppm for topsoils of 48 soil series developed over the principal parent materials in the different ecological zones of Ghana. These average values are far lower than the means reported for some Australian (350 ppm) and American (560 ppm) soils (Jones and Wild 1975). The low values may be due to the advanced weathering process and the low inherent phosphorus content of the parent material. For West African surface soils, available P levels are low. Sobulo (1982) used Bray No.l method to obtain 2.9 to 10.1 ppm P for some soils of the Guinea savanna of Nigeria. Oteng and Acquaye (1971) using various conventional extraction methods on 48 representative Ghanaian soil series obtained mean values ranging between 0.9 to 7.7 ppm 35 University of Ghana http://ugspace.ug.edu.gh available P. Nyamekye (1987) also used the Bray No.l method and reported 3.5 ppm available phosphorus for the Tingoli soil series of the Nyankpala Agricultural station. Kanabo et al. (1978) reported a low value of less than 10 ppm P in some soils of northern Ghana. 2.4.3 Mobility of soil phosphorus Although P is considered to be immobile in soils, considering the long time spans involved in soil profile development, the element can undergo significant changes in form and location. Nevertheless, movement of soil P may occur in three ways namely, (a) by the action of soil organisms, (b) with flowing water (mass flow) and (c) by thermal movement along a concentration gradient (diffusion). For each process, the magnitude of the movement will depend upon the fraction of soil P that is involved and the rate of movement of that fraction. The phosphate anion has a low solubility and strong affinity to soil mineral components and readily moves within the profile and landscape segments even in semi-arid climates (Tiessen et al., 1991b). The fixation of 145 kg ha-1 of fertilizer P into discrete nodules in a fertilizer trial in northern Ghana implies mobility of P within the top soil. It has been suggested that P often moves from the bulk of the soil fines into the nodules (Tiessen et al., 1991b). There is also a growing realisation that P can move within soils in significant amounts particularly in some sandy soils (Frossard et al., 1989). The movement of P in soils along the landscape often produces differences in P status of pedons of the same soil series or association. Smeck and Runge (1971) showed that the 36 University of Ghana http://ugspace.ug.edu.gh P status of some Mollisols varied widely, because of differences in landscape positions. Pedons at the foot of the slopes showed more P contents than soils in upland positions. The redistribution of phosphorus in a landscape has been attributed to surface and subsurface flow of water (Ryden et al.,1973). The extent of run-off and erosion dictates the overland movement and redistribution of P in the landscape. Using a mass balance (pedogenic index) approach, Smeck and Runge (1971) showed that the loss of P from upland soils leads to gains in soils in lower slope positions. Phosphorus is also lost from soils either by surface flow or by leaching (Frossard et al., 1989). Horizontal movement of P by subsurface flow is considered minimal due to sorption of phosphate anions by hydrous oxides of Al and Fe, which have strong affinity for P ions (Larsen, 1967). The mobility of P anions in soils, therefore, tends to depend on the nature of the mineral surfaces and oxide coatings. This is because P anions are strongly adsorbed by mineral constituents such as clays and sesquioxides (Parfitt, 1978; Jones, 1981). Considering the various mechanisms of P movement, the activity of the soil macroorganisms will only cause a random redistribution whereas higher plants will bring about a unidirectional movement (Larsen, 1967). The whole of the labile soil P is involved in this latter process and its rate will also depend on the quantity of P which is taken up by the roots through the plants and released to the topsoil by subsequent decay. This process may result in a very uneven distribution of P in an undisturbed profile. Movement by mass flow may be important in bringing soil P to the plant roots and in causing leaching. The amount of P moved by mass flow is a product of concentration of P in the soil solution and the extent of liquid flow. Since the concentration of P in the soil is generally low, the amount of movement will normally be insignificant. It is noted that phosphorus is not normally considered to be lost by leaching. Nevertheless, some losses 37 University of Ghana http://ugspace.ug.edu.gh may occur over geological time because the total soil P contents are generally low in the parent materials (Larsen, 1967). Diffusion is the process by which materials are transported from one part of a system to another as a result of the thermal movement of molecules or ions (Larsen, 1967). This movement is continuous, but where the system is at equilibrium there is no net transport. However, where differences in concentration exist, transport will occur, which moves the system to equilibrium. The rate of diffusion is related to the degree of saturation of the P adsorption capacity (Gunnary and Sutton, 1965) thus, addition or removal of P or treatments, which bring about changes in the P adsorption capacity of a given system also influence changes in the rate of diffusion. A strong heterogeneity of the distribution of P and sorption potential in concretionary soils often promote P movement by diffusion (Tiessen et al., 1991b). 2.4.4 Phosphorus reaction in soil Soil solution P Labile P Non labile P Labile P is the readily available portion of the quantity factor that exhibits high dissolution rate and rapidly replenishes solution P while the non-labile p is the total P in the soil that is insoluble and unavailable for plant use. The P quantity factor is the inorganic and organic labile P fractions in the soil with the amount of the P in the soil solution being the P intensity factor. Depletion of labile P causes some non-labile P to become labile but at a slow rate. Thus, the quantity factor comprises both labile and non-labile P fractions. Phosphorus anions 38 University of Ghana http://ugspace.ug.edu.gh can be attached to soil mineral constituents with greater bonding that they become insoluble and less available to plants. These can be through three processes namely, retention, precipitation and fixation. These processes are generally referred to as P fixation or retention. 2.4.4.1 Precipitation of P in soil In strongly acid soils, Fe and Al become very soluble and exists as free Fe and Al, especially in highly weathered soils like Oxisols, Ultisols and Alfisols. The P anion exists mainly as H -2PO4 (Smeck, 1985). Under such conditions, the soluble cations chemically precipitate the anion as below: Al3+ +H PO -2 4 + 2H2O 2H + + Al(OH)2H2PO4 (Soluble) Insoluble) The freshly precipitated hydroxy-phosphates are slightly soluble because they have a great deal of surface area exposed to the soil solution. With time, the precipitated hydroxy phosphate matures and becomes less soluble and therefore unavailable to plants. In alkaline soils (e.g. pH >8), Ca and Mg are very soluble (Smeck, 1985). At this pH, HPO 2-4 , predominates in soil solution. The two species react as; Ca2+ + HPO 2-4 CaHPO4 (dicalcium P) The CaHPO4 formed is slightly soluble but further reacts with CaCO3 in the soil as; 2CaHPO4 + CaCO3 Ca3(PO4)2 + CO2 + H2O (tricalcium P) The tricalcium phosphate formed is less soluble and precipitates rendering the P unavailable to plants 39 University of Ghana http://ugspace.ug.edu.gh 2.4.4.2 Retention and fixation of P in soil Phosphorus retention is the adsorption of P ions onto soil mineral surfaces due to electrostatic attraction, nonspecific adsorption or through co-adsorption (Chien, 1980). These reactions are reversible. Phosphorus fixation on the other hand is due to the linkage of P anions to soil minerals through replacement or displacement of structural OH groups. This type of mechanism is more significant in sesquioxides and 1:1 than in 2:1 clay minerals due to the higher contents of OH groups in the former two. Reactions with oxides of Fe and Al and with silicate clays: At low pH, the surfaces of oxides Fe and Al and the broken edges of 1:1 silicate clays become protonated. Al OH + H+ Al OH +2 (Clay edge) (Protonated edge) The protonated surface then adsorbs the H -2PO4 that exists in soil solution. Al OH +2 + H - + - 2PO4 Al OH2 H2PO4 Al (OH)2 H2PO4 Al 3+ + 2OH- + H2PO4 The adsorbed H2PO - 4 could be held as an outer sphere complex (nonspecific adsorption) and is subject to anion exchange with other anions such as OH-, SO 2-4 , MoO 2- 4 and organic acids R-COO- (McDowell, et al., 2013) Al OH + H PO -2 2 4 + COO - Al OH +2 COO - + H - 2PO4 40 University of Ghana http://ugspace.ug.edu.gh Since the reaction is reversible, the H2PO - 4 is made slowly available to crops. This reaction is P retention as the H2PO - 4 is held non-specifically and slowly made available to crops. Availability of the H2PO - 4 may be increased by liming the acidic soil to increase the OH in solution or by adding organic matter to increase the RCOO- concentration in soil solution. The phosphate anion may also replace structural OH of the clay to form an inner sphere complex. There is ligand exchange between the H -2PO4 and the protonated OH - (OH +2 ) of the sesquioxide or1:1 silicate clay. This reaction, while reversible, binds the anion too tightly to the mineral and availability is very low. With time, another structural OH of the mineral and one H of the anion form water which is released into solution. An oxygen of the H2PO - 4 then binds the anion to the mineral. There is thus complex formation or chelation reaction and the H2PO - 4 becomes an integral part of the mineral being bound to two Al atoms. Release of the anion back into solution thus becomes very difficult as the H -2PO4 is buried deep inside the mineral. The P is then termed occluded and is the least available form in most acid soils. This process is termed fixation and exceeds that by retention. 41 University of Ghana http://ugspace.ug.edu.gh Fig. 2. 1 Inner sphere formation of P in soil minerals (a) and subsequently, the gradual occlusion of the adsorbed P (b). Source: Syers, J.K. and Cornforth, I.S. (1993) 2.4.5 Sesquioxide distribution in soils of northern Ghana Sesquioxides are oxides, hydroxides and or oxyhydroxides of Al, Fe and Mn. Their distribution in soils is greatly affected by pedogenesis. The distribution of pedogenic oxides and oxyhydroxides of these elements in soils also characterises the type, direction and extent of pedogenic processes (Schlichting and Blume, 1966). The distribution of oxides of Fe, Al and Mn is also used to interpret soil formation processes in the temperate regions (McKeague and Day, 1966; Blume and Schwertmann, 1969; Nartey, 1994). These oxides and oxyhydroxides of Fe and Al are dominant in highly weathered tropical soils such as the Northern Ghana soils and occur in both poorly crystalline (amorphous) and or crystalline forms. A small portion of Fe and Al is also present in the form of organic complexes (Juo et al., 1974). 42 University of Ghana http://ugspace.ug.edu.gh 2.5 Factors affecting P status and limitation in soils of northern Ghana Factors affecting P status in soils includes the following: the parent material of weathered soil, the pH of the soil, organic matter content of the soil, the type of clay mineral and amount of clay present in the soil, concentration of sesquioxides in the soil, type and biomass density of vegetation supported by, leachability and erosivity of soil, climatic conditions of the area and relief of the landscape. 2.5.1 Parent Material The parent materials of soils of northern Ghana are low in P. Available data on rock samples show an average o f700 mg kg-l total P for 15 granitic rocks and 750 – 2440 mg kg-1 total P for rocks of intermediate and basic composition (Nye and Bertheux, 1957). Information on the age of the soil materials is very scanty but the land surfaces are believed to be as old as mid-Tertiary. Although the soils are not necessarily of the same age, there is evidence that they have been exposed to climatic periods, which were more humid than at present. The extensive occurrence of plinthite implies that the iron was mobile, probably as ferrous iron (Jones and Wild, 1975) resulting in an increased solubility of P. The present low content of soil P may, therefore be a consequence of the age of the parent materials and the pedogenic processes which had taken place in the soils. A high proportion of total P in these soils is often in the occluded form and is not available to plants. 2.5.2 Extent of P saturation In general, P adsorption is greater in soils with little P adsorbed to mineral surfaces. As fertilizer P is added to soil, the quantity of P adsorbed increases and the potential for 43 University of Ghana http://ugspace.ug.edu.gh additional P adsorption decreases (Tiessen et. al., 1984; Osodeke et. al. 1993). When all adsorption sites are saturated with P, further adsorption will not occur and recovery of applied P fertilizer increases (Smeck, 1985). 2.5.3 Soil pH The greatest P fixation occurs in both low and high pH soils. As pH increases from below 5 to about 6.0, the activities of Al and Fe decrease and P becomes more soluble and available. At higher pH, Ca and Mg dominate and P solubility is lowered. However, as pH is lowered from 8 to 6, P solubility increases because the activities of Ca and Mg decrease. P solubility in most soils is at a maximum in the pH range of 5.5 to 6.5 (Nartey, 1994). At low pH values, the retention results largely from the reaction with oxides of Fe and Al and precipitates as FePO4 and AlPO4. Above pH 7, Ca precipitates P as Ca-P compounds. Several investigations have shown the effect of pH on phosphate absorption by soil and synthesis iron oxides (Barrow and Bowden, 1987; Borggaard, 1986). The pH effect on soil oxides absorption seems to be less pronounced than on pure iron oxide adsorption. According to Borggaard et al., (1990), pH affects phosphate adsorption but the effect is limited to adsorption by soils in the pH range of 4- 8 in contrast to adsorption by pure iron oxides. In soils increasing pH has been shown to increase or decrease and have no effect on phosphate adsorption (Barrow, 1985; Haynes and Swift, 1985). Nwoke et al. (2003) found that sorption of P dcereased with increasing soil pH and this was attributed to increase negative charge on variable chargc colloids which cause eletrostatic repulsion of the ionic P species from the surface (Haynes and Swift. 1985). In contrast, Mokwunye 44 University of Ghana http://ugspace.ug.edu.gh (1975) and Agbenin (1996) reported an increase in sorption with inereesmg pH for some savanna soils. He ascribed this trend to the chemistry and retention of Ca2+. Nevertheless, the pH effect on phosphate adsorption should not be exaggerated since this effect is fairly small. Particularly over the pH range covering most soils and ancillary effects may therefore appear relatively important BoIt and Van Riemsdijk 1987). 2.5.4 Anionic effect Both inorganic and organic anions can compete with P for adsorption sites resulting in P being left in solution for plant uptake. Inorganic anions such as OH-, SO 2-4 , MoO 2- 4 and H3SiO - 4 and organic anions e.g. acetate, citrate and oxalate are competitive. (Parfitt, 1978) Anion adsorption follows the sequence PO43->SO42->Cl->NO3-, which accounts for the low availability of phosphorus in many tropical soils (Strahm and Harrison 2007). In contrast to the permanent, negative charge in soils of the temperate zone, tropical soils dominated by oxides and hydrous oxides of iron and aluminium show variable charge, depending on soil pH (Arai and Sparks 2007). Under acid conditions these soils possess positive charge, as a result of the association of H+ with the surface hydroxide groups. With an experimental increase in pH, a soil sample is observed to pass through a zero point of charge (ZPC), where the number of cation and anion exchange sites is equal. Significant anion adsorption capacity (AAC) is present in acid tropical soils in most field situations. Anion adsorption capacity is typically greater on poorly crystalline forms of Fe and Al (oxalate-extractable), which have greater surface area than crystalline forms (dithionate-extractable) (Parfitt and Smart, 1978) 45 University of Ghana http://ugspace.ug.edu.gh 2.5.5 Type and amount of clay mineral Most of the inorganic compounds which P reacts with to become unavailable are clay sized in nature. Thus for soils with similar mineralogy and pH, those with higher clay content will have higher P fixation whereas soils with low clay content will have lower P fixation (Ovalles and Collins, 1986; Day et al., 1987). Certain clay minerals have a higher P fixation levels than others. Clays that possess higher AECs tend to fix larger amounts of P due to their positive charges. This may be allophane and oxides of Fe and Al such as goethite and gibbsite. For the silicate clay minerals, the 1:1 clays have higher fixing capacities than 2:1 because pf their higher exposed OH groups (Smeck, 1985; Nartey, 1994). Soil components responsible for fixation are in the order of 2:1 clays << 1:1 clays 0.05) variation among the amended soils. Amended control soil under wastewater irrigation regime showed significant (p < 0.05) increase in NaHCO3- Pi from 8.02 mg kg-1 (control) to a range of 12.08 to 13.02 mg kg-1. The NaHCO3 Pi content of each treatment under the wastewater was significantly (p < 0.05) higher than the corresponding treatments under clean water. The NaOH extractable Pi expresses soil inorganic P that is bound strongly to soil components such as Fe, Al and Mn oxides by chemisorption. The concentration of NaOH–Pi in the control soil under clean water was statistically similar to the amended soils. Similarly, under waste water treatment, the addition of amendments to the control soil did not show significant (p > 0.05) increase in NaOH-P i with the exception of B+(NH₄)₂SO₄ treatment. Moreover, NaOH-Pi of each of the treatments between 94 University of Ghana http://ugspace.ug.edu.gh wastewater and clean water were statistically the same (p < 0.05). Thus, the water regimes had no effect on the NaOH-Pi, The extractable HCl-Pi is the inorganic P fraction associated with soil calcium as Ca-P in relative stable forms. The control soil had Ca bound P of 6.85 mg kg-1 which increased significantly (p < 0.05) to 15.58 mg kg-1, 14.68 mg kg-1 and 16.95 mg kg-1 for biochar, inorganic fertilizer and B+(NH₄)₂SO₄, respectively under clean water (Table 4.5.). Similar trend was observed under the wastewater regime. In both clean and waste water regime, the increase in Ca bound P was more significant (p < 0.05) in the B+(NH₄)₂SO₄ treatment. The sum of NaHCO3 extractable organic P (NaHCO3- Po) and NaOH extractable organic P (NaOH–Po) constitute the organic P fractions. The easily mineralisable NaHCO3-Po is the organic P adsorbed on the colloidal surfaces of the soil and is therefore considered to be labile organic P pool whiles the NaOH–Po is the organic P bound strongly to Fe and Al by chemical sorption leading to the formation of organic complexes. The NaOH–Po is considered as being moderately labile. The sum of organic P in the control soil (41.26 mg kg-1) under clean water regime was significantly increased upon the addition of biochar (48.39 mg kg-1), inorganic fertilizer (47.68 mg kg-1) and the combined biochar + inorganic fertilizer (B+(NH₄)₂SO₄) (48.73 mg kg-1). The changes in organic P fraction among the amended soils were statistically the same. The same trend was observed under the waste water irrigation regime, showing significant (p < 0.05) increase in the organic P over the control soil but no significant (p > 0.05) difference among the amended soils. The effect of water application on the 95 University of Ghana http://ugspace.ug.edu.gh organic P fraction was only significant (p < 0.05) in the control soil and the B+(NH₄)₂SO₄ treatment. The residual P is considered as the most resistant and insoluble fraction of phosphorus. Consequently, the residual P pool was the dominant P fraction under both irrigation regimes. The addition of amendments did not have any significant (P > 0.05) effect on the residual P pool. The study observed a significant (p < 0.05) increase in residual P pool of the control soil upon amendment (B, (NH₄)₂SO₄ and B+(NH₄)₂SO₄). Comparing residual P pool under both moisture regimes did not show significant difference except for the B+(NH₄)₂SO₄, which recorded a higher amount of residual P in the waste water plot. 4.2.2 Effect of soil amendment on P fraction under clean and waste water regimes for depth 10 – 20 cm. The mean values of the various soil P fractions for the depth 10 – 20 cm are presented in Table 4.6. Comparison was made among P fractions within each water regime (i.e. clean water and waste water) and also between the water regimes under depth 10 -20 cm. The water extractable Pi of the control under clean water regime was 3.09 mg kg-1. The water extractable Pi was significantly (p < 0.05) increased upon application of the soil amendments (biochar, inorganic fertilizer-(NH₄)₂SO₄ and biochar + (NH₄)₂SO₄) to 5.81 mg kg-1, 5.70 mg kg-1, and 5.81 mg kg-1, respectively, but of statistical similarity. The trend was reproduced under the waste water irrigation regime but with marginally higher concentrations. 96 University of Ghana http://ugspace.ug.edu.gh 97 University of Ghana http://ugspace.ug.edu.gh The resin-P pool of the control (3.99 mg kg-1) significantly (p < 0.05) increased after the addition of biochar (7.41 mg kg-1), inorganic fertilizer (7.67 mg kg-1) and the combined biochar + inorganic fertilizer (7.51 mg kg-1) under clean water irrigation regime. However, the resin-Pi of the amended soils was statistically the similar. The same trend was observed when waste water was applied. The amended soils showed more significant (p < 0.05) variation from the control. As observed in the water extractable Pi, resin P was not affected by the type of water used for irrigation. With the exception of the combined application of biochar and inorganic fertilizer (B+(NH₄)₂SO₄), NaHCO3-Pi pool of the other amended soils was statistically (p > 0.05) similar to the control under clean water regime. The B+(NH₄)₂SO₄ treatment significantly (p < 0.05) increased NaHCO3-Pi pool of the control by 4.63 mg kg-1. Similar result was recorded for the amended soils under waste water irrigation regime. The use of either clean or waste water did not also affect the NaHCO3-Pi pool of all the treatments. The moderately labile NaOH-Pi pool gave a mean concentration of 21.60 mg kg-1 for the control under clean water regime. The addition of biochar and combined biochar + inorganic fertilizer did not significantly (p > 0.05) improve the NaOH-Pi pool of the control soil. However, upon the addition of inorganic fertilizer the NaOH-Pi pool of the control soil was significantly (p < 0.05) increased by 1.43 mg kg-1. Under the wastewater regime, the NaOH- Pi pool amended soils did not vary significantly (p > 0.05) from the control. The Student’s t-test value showed no significant difference (p > 0.05) between the effect of clean water and waste water on NaOH-Pi pool for each treatment. The Ca bound P (HCl-Pi) content of the control soil under both clean and waste water was significantly improved upon the addition of amendment. For instance, under clean water irrigation regime, the control Ca bound P increased from 7.31 mg kg-1 to a range of 15.87- 98 University of Ghana http://ugspace.ug.edu.gh 16.87 mg kg-1 whereas under waste water regime, it increased from 7.93 mg kg-1 to a range of 16.56 to 17.22 mg kg-1. The effect of water on Ca bound P pool was sensitive in the amended soils but not in the control soil. The sum of organic P (NaHCO3-Po + NaOH-Po) and residual P of the control soil was statistically (p > 0.05) similar to the amended soils. 4.2.3 Effect of mode of irrigation on available P fractions The modification of the Hedley’s fractionation procedure saw the use of distilled-deionized water as initial extracting solvent. The fractionation method showed that about a fourth (24%) of the available P fraction on the average is water leachable as shown in table 4.7. There was a highly significant (p < 0.01) correlation (r = 0.42 and 0.70) between the water leachable P and soil available P content and also the soil total P content, respectively. 4.2.4 Variation among P fractions with depth The variations in the various P pools for all the treatments under clean and waste water irrigation are shown in Figures 4.2. The sum of labile-P fraction comprises water leachable P (H2O-P), the Resin-P and the NaHCO3-Pi. Although there was a decrease in sum of labile P at increasing depth, significant (p < 0.05) effect only occurred when biochar and the combined biochar + inorganic fertilizer were applied under waste water regime. The NaOH- Pi pool of all the treatments significantly (p < 0.05) decreased with depth except for the inorganic fertilizer treated soil under waste water irrigation. Contrary to the trend observed in the other inorganic P pools i.e. sum of labile P and NaOH-Pi, the Ca bound P marginally increased with depth. The decrease was more significant (p < 0.05) in the combined biochar + inorganic fertilizer treatment irrigated with waste water. 99 University of Ghana http://ugspace.ug.edu.gh 100 University of Ghana http://ugspace.ug.edu.gh 101 University of Ghana http://ugspace.ug.edu.gh The sum of organic P pool i.e. NaHCO3-Po + NaOH-Po of all the treatments significantly (p < 0.05) decreased with increasing depth. The residual P pool also decreased significantly (p < 0.05) as depth increased. This is with exception of control soils under both clean and waste water regime 4.3 Simple correlation coefficients showing relationship among P fractions and with soil properties The correlation coefficient values describing the relationship among P fractions of various treatments are shown in Table 4.8. In the study, high significant (p < 0.01; p < 0.05) correlation coefficient (r) among most of the soil P fractions were observed. This establishes a continuum in the phosphorus fractions. For instance, the most labile inorganic P pool (H2O- Pi + resin-Pi + NaHCO3-Pi pools) positively correlated each other at p < 0.01 (r = 0.48, 0.53 and 0.81). It also correlated positively and significantly with all the sum of organic P. Table 4.9. shows the correlation coefficient describing the relationship between phosphorus fractions and other soil properties. The Bray 1 available P (Bray and Kuntz, 1945) correlated highly with the most labile Pi (r = 0.42, 0.50 and 0.30 for H2O-P, resin-P, NaHCO3-Pi pools respectively), HCl –P (r = 0.33) but not with the moderately labile Pi. The soil organic C content was directly proportional to the sum of Po pool, the sum of labile Pi pool and Total P. There was a positive and significant correlation between Ca2+ and the HCl-P pool (r = 026). The crystalline Fedcb has a negative but significant correlation with H2O-P (r = 0.26) and total P (r = 0.30). There were positive correlations between the amorphous Feox and NaOH-Pi (r = 0.36) and also with sum of Po (r = 0.48) 102 University of Ghana http://ugspace.ug.edu.gh 103 University of Ghana http://ugspace.ug.edu.gh 104 University of Ghana http://ugspace.ug.edu.gh CHAPTER FIVE 5 DISCUSSION 5.1 Physico-chemical properties of the soils The replicates of soil treatments under study are heterogeneous in nature. This outcome had been arrived at due to the great variability observed in the results of the soil sample replicates (Table 4.1.). High coefficient of variations about the means were observed confirming heterogeneity and implying that, soil treatments alone cannot account for changes in P concentrations. This therefore concludes that soil physical and chemical characteristics would have some effects accounting for changes in P concentrations. 5.1.1 Effect of amendment on soil physicochemical properties of the studied soil The soil samples used in the study were soils taken within the biological and productive layer of the profile (0 – 20 cm). The texture of the soil samples was sandy loam with high sand content which makes the soil of the study site a low phosphorus retention soil and easily prone to P leaching (Nartey, 1994; McDowell and Condron, 2001). Studies have shown that the highest total P concentration exist in clay size particles and suggest that P movement is positively correlated with clay movement and texture (Syers et al., 1969; Hanley and Murphy, 1970; Lekwa and Whiteside, 1986; Day et al., 1987). Results of the study showed that total P positively correlated (r = 0.04 - 0.22) with clay content even though not significant. Also, clays are the matrix of soil reaction and their negative poly- functional group surfaces thus compete with other P sorption soil components (Day et al., 1987). This competition frees the soil P allowing for easy plant uptake (Eriksson et al., 2015). This is made possible through a clay-sesquioxide ligands. The obtained results, 105 University of Ghana http://ugspace.ug.edu.gh however, showed lower clay contents suggesting that more P would be bound by the oxides, hydroxides and oxyhydroxides of Al, Fe and Mn making P not immediately accessible by plants (McDowell, et al., 2011). Clays of northern Ghana are mostly dominated by kaolinites which are 1:1clays that bind more P than the limited 2:1 clays due to the more protonated H+ on their surfaces in the acidic soil (Obeng, 1975; Nartey, 1994; Abekoe and Sharawat, 2001). The high proportion of plinthic concretions implies that the soils are at an advance stage of weathering (Daugherty and Arnold, 1982). High concretion content further lowers the chance of P availability for plant up take owing to their being high P sinkers (Abekoe, 1989; Ioannou et al., 1998). The results obtained also show that the soil was strongly acidic which is characteristic of highly weathered soils of northern Ghana as reported by many researchers (Nartey, 1994; Abekoe and Tiessen, 1998 ) The change in pH values, that is, ∆pH = pH KC1 - pH H2O are negative for the fine earth fraction. This negative ∆pH values indicate that the soil colloids possess net negative charge which is characteristic of intensely weathered soils (Juo et al., 1974; Tan, 1982; Abekoe and Sharawat, 2001). This, however, implies that the soils are prone to plant nutrient uptake that may be made available (Abekoe and Sharawat, 2003). The greatest P fixation occurs at very low and very high soil pH with maximum P solubility in most soils at pH range of 5.5 to 6.5 (Nartey, 1994). The control values fall below this range and might largely be due to retention by the action of Fe and Al forming FePO4 and AlPO4 precipitates. Many investigations have shown the effect of pH on phosphate absorption by soil and also iron oxides (Borggaard, 1986; Barrow and Bowden, 1987; Nartey 1994; Abekoe and Tiessen, 1998). However, upon amendment with biochar, the soil became moderately acidic with marginal increase in pH values making it possible for P to be more soluble (Sinclair, et. al., 2010). The pH increase was 106 University of Ghana http://ugspace.ug.edu.gh due to the alkaline nature and high carbonate content of the biochar (Yuan and Xu, 2011; Major, et al., 2012). This has resulted in the significant increment in the labile Pi concentration of the biochar treatment over the control. The residual effect of the inorganic fertilizer has thus resulted in the acidification of the (NH₄)₂SO₄ amended soil. This agrees with findings of Mahler et al., (2008) and Obiri-Nyarko, (2012) whose reports stated that long term application of fertilizer thus increases soil acidity. Organic carbon accumulation in the soils was very low ranging from a mean of 0.59% to 0.98%. The very low organic carbon content is similar to values obtained by Abekoe, (1989) and Tiessen et al. (1991b). This low organic carbon level is due to the low biomass turnover from the predominantly grass vegetative cover, frequent destructive bush fires and high environmental temperature characteristic of the savannah environment as pertained in northern Ghana (Bationo et al, 2003). Hence, organic matter being returned to the soil via the vegetation is low. There was approximately, between 2 mg/kg and 3 mg/kg increment in organic carbon content over the control upon amendment with inorganic fertilizer ((NH₄)₂SO₄) according to normal agricultural practice (NAP), biochar (B) and B+(NH₄)₂SO₄. This increment is due to the great amount of labile organic carbon in the biochar (Aon et al., 2015; Iqbal et al., 2015) and from the biomass residue generated by both the B and (NH₄)₂SO₄ application. However, the mode of irrigation tend to wash-off the carbon inherent in the biochar and or generated by (NH₄)₂SO₄ to partly reduce after three cycles of vegetable production. The high sand content and nutrient mining by plants and other forms of biota could also account for the reduction. 107 University of Ghana http://ugspace.ug.edu.gh Nitrogen content in the soil was generally low since its availability is directly related to soil organic matter content and the extent of mineralization (Abekoe and Sahrawat, 2001). The obvious increment in soil N content upon amendment with especially the (NH₄)₂SO₄ (NAP) is due to the inherent N in the fertilizer applied. Biochar amendment had little change in soil N content owing to the high temperatures used in biochar production that causes N vaporization. The available P content in the soils was low (< 20 mg P/ kg). This low amount of P in the tropical savanna environment could be related to the lower levels of mineral apatite of the parent rock (Halm, 1972). It could also be attributed to the precipitation of phosphate at low pH by Al and Fe oxides (Warren, 1992; Tiessen et al, 1993, Abekoe and Sahrawat, 2001). Amendment of the soil with B, (NH₄)₂SO₄ and B+(NH₄)₂SO₄ saw increments in soil available P concentration (Zhai, et al., 2014). This increase is consistent with other studies that reported similar increments in soils elsewhere (Van Zwieten et. al.,2010; Brewer et al., 2012). The P enrichment in the biochar could explain the P increment in the biochar amended plots; whereas the residual P content in the synthesized inorganic fertilizer could also account for that of the (NH₄)₂SO₄ treatment plots. It could also be due to the synergetic effect of the (NH₄)₂SO₄ that directly or indirectly increase soil P. Generally, the exchangeable bases are very low in the soil resulting in the very low ECEC. This values are consistent with those obtained by Obeng (1975), Tiessen et al., (1987), Owusu-Bennoah and Acquaye. (1989), Tiessen et al., (1991b), and Abekoe and Tiessen, (1998) on similar soils of northern Ghana. The very low ECEC in the soils is due to the near complete removal of weatherable primary minerals and the dominance of 108 University of Ghana http://ugspace.ug.edu.gh the low activity kaolinitic clays under the prevailing tropical environmental conditions (Nartey, 1994). The increase in ECEC of the soils amended with B, (NH₄)₂SO₄ and B+(NH₄)₂SO₄ is due to slight increases in exchangeable bases and organic carbon especially, of biochar origin. Biochar has high amount of calcium in addition to the high organic carbon owing to the ash content (Sohi et al. 2009; Wang et al., 2014). Also the large surface area (graphene surface) and biotic-oxidation of biochar contributes to the increase in ECEC (Cheng et al, 2006). This increment will also boost the negative surface charge potential of the soil to retain more plant essential nutrients thereby improving soil fertility. Organic matter is known to inhibit crystallization of Fe and Al in soils. The low Feox and Alox values in the soils reflected the low build-up of organic carbon contents in the soil. In highly weathered soils such as those of northern Ghana, it is the Feox and Alox that control P saturation (Schwertmann, 1966; Schwertmann et al. 1968; Huang and Violante, 1986). Biochar application saw increase in Feox values, which act as the precursors of Fe crystallization and a decrease in Fedcb. This is as a result of increase in soil organic carbon owing to relatively higher organic matter content of the biochar leading to the inhibition of crystallisation of Al and Fe oxides or hydroxides (Chintala et al., 2014). Also, organic ligands released from biochar can promote the formation of organo-metallic complexes with extractable Al and Fe oxides (Boehm, 2001; Chintala et al., 2014). There was however high crystallinity (i.e. high ratio of Fedcb to Feox- (Table 4. 6.) of the studied soils. This could be attributed to the low organic matter content and the high temperatures existing in northing Ghana (Juo et al., 1974; Nartey, 1994; Abekoe, 1998). The active Fe ratios of the soils are similar to the ratios found in soils of northern Ghana by Tiessen et al. (1993). 109 University of Ghana http://ugspace.ug.edu.gh 5.2 Hedley’s phosphorus fractions in soil (control) and amended soils (B, IF- NAP and B + IF-NAP) The percentage composition of the sum of the readily available or sum of most labile inorganic P fraction (H2O-P, + Resin-P + NaHCO3-P) ranges from 9% to 13% of the total P. This proportion was in agreement with other studies on tropical soils (Abekoe, 1996; Cross and Schlesinger, 2001; Turrion et al. 2001; Asomaning et al., 2015). The plant available P fraction (Hedley et al., 1982b) highly correlated with plant P uptake as noted in similar studies elsewhere (Menon et al., 1989; Sharpley, 1991; Abekoe and Sahrawat, 2003). Amendment of soil with B, (NH₄)₂SO₄ and B+(NH₄)₂SO₄ saw significant improvement in soil fertility parameters. This observed increase in soil P levels upon amendment is consistent with Asomaning et al., (2011); Wang et al. (2014) and Guo et al., (2000) who reported great build-up in the most labile Pi fraction (H2O- P, + Resin-P + NaHCO3-P) of some intensely weathered tropical soils. Similarly, Yeboah et al. (2009) reported increased P uptake by corn (Zea mays) crop grown in sandy loam soil amended with wood biochar. The significant differences in soil labile Pi levels recorded by the other treatments (B, (NH₄)₂SO₄ and B+(NH₄)₂SO₄) compared with the control implies that the soil amendments can be relied on to boost Pi availability for plant nutrition (Yeboah et al., 2009). Contrastingly, other research works have reported no change in P availability levels. Abekoe and Tiessen, (1998) reported no change in P availability when Togo rock phosphate (TRP) was applied due to high P sorption capacity of those concretionary soils. Also, Dormaar and Chang (1995) reported no increase in most labile P pool in manured soils, indicating that P added from the manure was transformed into moderately labile or stable occluded P forms. 110 University of Ghana http://ugspace.ug.edu.gh Soil organic matter has direct and indirect effect on the maintenance of P availability in soils. The direct effect is seen when protonated organic ligand compete with P for binding sites on P sinkers (eg. sesquioxides) and also increasing the soil pH through release of carboxyl ions (Chien and Clayton, 1980). In the indirect sense, protonated organic carbon at lower pKa tends to prevent poorly crystalline Fe and Al oxides from crystallizing. In view of the smaller sizes and larger surface area of these amorphous oxides, more sites for P sorption are thus made available making P limited for plant uptake (Lindsay, 1979; Smeck, 1985; Turrion et al. 2001). The low soil organic carbon content corresponded with the low sum of P in the soils (control). However, the marginal increment in organic carbon led to an increase in the sum of labile Pi (water leachable-P, resin-Pi and NaHCO3-Pi) indicating that, the addition of labile organic carbon (i.e. from especially the biochar) seems to maintain high levels of the most labile P pool of the studied soil. This is affirmed by the high positive correlation between the sum of Po and water-P, resin-Pi as well as NaHCO3-Pi of the soils. This assertion is in agreement with studies conducted by Yuan and Xu. (2012) as well as Schneider and Haderlein (2016) whose research indicated that organic carbon from biochar improves the labile P content of most tropical soils. The increase in the most labile P could be due to mineralization and competitive sorption reaction of labile carbon in biochar with P at the soil exchange sites. This confirmation of increasing organic carbon with concomitant increase in most labile P (water leachable-P, resin-Pi and NaHCO3-Pi) had a decreasing effect on the chemisorbed P to Al and Fe (NaOH-Pi) of the soils (Tiessen et al., 1993). The lower level of NaOH-Pi fraction in the biochar treatment plots attest to the above assertion (Table 4.5 and 4.6). 111 University of Ghana http://ugspace.ug.edu.gh In acidic soils such as those in northern Ghana, a large portion of the soil P is bound by poorly formed crystalline (amorphous) and or crystalline oxides, hydroxides and oxyhydroxides of Al, Fe, and to some extent Mn through chemical sorption (McLaughlin et al., 1977; Abekoe and Tiessen, 1998a; Galvao and Saicedo, 2009). The P fraction that is chemisorbed to the surfaces of these poorly crystalline and crystalline Fe and Al oxides is the NaOH-Pi pool (Hedley et al., 1982a; Tiessen et al., 1984; Wager et al., 1986 and Asomaning, 2011). Williams et al. (1971) have shown that the selective NaOH extractant used in the recovery of the chemisorbed Al and Fe bound P (NaOH- P) actually does recover some portion of the stable Ca-P (HCl-P). This is evident in the positive correlation (r = 0.303 at p<0.05) among the Fe/Al-bound P and Ca bound P pool in the soils of the study site. The NaOH-Pi pool is referred to as moderately labile Pi that replenishes depleting labile Pi stock in soil solution for plant uptake (Abekoe and Tiessen, 1998b). The relatively high levels of Fe oxides in the control soil is expressly shown in the relatively high NaOH-P, suggesting a major P sink for added P from the B, (NH₄)₂SO₄, and B+(NH₄)₂SO₄ treatment plots. Geochemical adsorption of P gradually decreases P lability for plant uptake rendering P to be moderately available and further, to stable and occluded forms as in the Calcium bound P (Ca-P or HCl-P) and the residual-P. This is consistent with reports by Buehler et.al. (2002) where labile P (resin-P) pool was gradually converted to moderately labile P (NaOH-P) pool. Treatment of soil with B, (NH₄)₂SO₄ and B+(NH₄)₂SO₄ indeed raised the levels of chemisorbed moderately labile Pi (NaOH-P) owing to the gradual conversion of the inherent labile P in the applied biochar and the inorganic fertilizer (Wang et al. 2012). In agreement, Kashem et al. (2004) reported an increase in NaOH-Pi concentration in 112 University of Ghana http://ugspace.ug.edu.gh soils amended with corn cob and rice husk biochar. In severely depleted labile P soils, the moderately NaOH-Pi becomes bioavailable to plants in the long term (Shen et. al., 2004). The increase in NaOH-Pi could also be due to high amount of P existing as Fe and Al complexes in the inorganic fertilizer used. This is seen in the marginal increase of the NaOH-P in the (NH₄)₂SO₄ treatment plots relative to the control plots. The result was consistent with that of Abekoe and Tiessen, (1998a) when SSP fertilizer was applied to similar soils of northern Ghana. The Ca bound P (HCl-P) is the P compound in relatively stable forms and associated with the primary mineral materials such as apatites (Hedley et al., 1982b; Tiessen et al., 1994; Asomaning et al. 2011). The stable Ca phosphates may include the following in order of decreasing solubility: monetite (CaHPO4), octacalcium phosphate (Ca4H(PO4)3), tricalcium phosphate (Ca3(PO4)2) and hydroxyapatite (Ca5(PO4)3OH) (Uzu, et. al., 1975; Lindsay 1979; Sato et al. 2005). These Ca bound P are relatively low in highly weathered soils but high in younger soils. Additionally, they are low in acidic soils as pertained in the studied soil. The control recorded very low HCl-P and this is in agreement with other research works (Tiessen et al., 1994; Abekoe and Tiessen, 1998a). The lower HCl-P may be due to intense weathering and transformation of the calcium bearing apatites (Ca-P) to amorphous and crystalline to variscite (Al –P) and strengite (Fe-P) mineral form (Cross and Schlesinger, 1995). Application of the three amendments; which were the biochar (B), inorganic fertilizer according to normal agricultural practice ((NH₄)₂SO₄) and the combination of the two amendments (B+(NH₄)₂SO₄) saw a large increase in the HCl-P pool of the soil treatments relative to the control. The increase is more significant in the biochar 113 University of Ghana http://ugspace.ug.edu.gh treatment plots. This is due to the effect of the appreciable Ca2+ content in the biochar and also the increase in pH of the biochar-amended soil. This observation is similar to that of Ch’ng et al. (2014) who reported a significant increase in Ca-P fractions in Mollisols upon soil amendment with biochar. Mukherjee et al. (2011) also noted that Ca-P formation increased with increasing soil pH after biochar addition. The sum of extractable Po (ΣPo = NaHCO3-Po and NaOH-Po) is usually associated with humic and fulvic acids (Frossard et al., 1995). The organic P fraction, comprising the very labile NaHCO3-Po and the moderately labile NaOH-Po, is dominated by phosphate esters, which are considered to be biologically active. Hence they can mineralize to supply P to crops in the long run (Udo and Ogunwale, 1977; Schmidt et al., 1996). The ΣPo as noted in this study can therefore serve as a source of the moderately labile and also labile Pi through biotic processes (Zheng et al., 2013; Kashem et al., 2004; Negassa and Leinweber 2009). There was relatively high concentration of organic P even in the control (soil). However, upon amendment with B, (NH₄)₂SO₄ and B+(NH₄)₂SO₄, significant increments of similar statistical differences (compared to the control soil) were observed. As usual, the biochar treatment plots gave higher ΣPo content relative to the (NH₄)₂SO₄ treatment plots. This was obviously due to the inherently high organic carbon content of the biochar. There was a positive correlation (r = 0.59; p< 0.01) between the ΣPo and soil organic carbon content (SOC), implying that SOC accumulation in the soil is a necessary requirement for increased organic P pool (Qian et al., 2004). There was also a positive correlation (r = 0.25; p< 0.05) between the ΣPo and Feox confirming the relevance of Fe as a soil P stabilizing agent of the soil. 114 University of Ghana http://ugspace.ug.edu.gh Asomaning (2011) noted a similar trend in tropical sandy soil which received continuous application of manure over a considerable time period. In highly weathered soils, the residual-P pool has the largest proportion of soil P. This fraction comprises both organic and inorganic P that are occluded and hence, considered to be the most resistant and insoluble P fraction (Abekoe and Sahrawat, 2003; Asomaning, 2011). The recalcitrance of the residual P pool is due to the complexity and stable forms of its organic and inorganic P component mixture (Hedley et al., 1982a). The occluded P has been incorporated into the soil matrices either by diffusion or entrapment in the coatings and or concretions of amorphous and crystalline Fe and Al oxides (Abekoe, 1989). Organic P compounds have also been found to form part of the residual P pools since some stable organic matter cannot be extracted using 0.1M NaOH and 0.5M NaHCO3 extractants (Hedley et al., 1982a), thus, ΣPo account for more than 60% of the residual P pool (Tiessen et al., 1993). The very stable Ca bound P that is not easily extracted by the weak acid (i.e. 1M HCl) is thus recovered in the residual P pool (Syers et al., 1968). In the present study, a positive correlation was established between the residual-P pool and the NaOH-Pi, sum of organic P and HCl-Pi pools in both the control and amended plots. This indicated that the residual-P pool could also serve as a reservoir of soil P in the long term when soil conditions are altered from the present condition (Lyngsie et al., 2014). The high average percentage proportion of the residual P pool in the range of 45.51% to 51.76% for both the control and the amended soils is consistent with pervious research results obtained in soils from the same agro- ecological zone in Ghana by Abekoe, (1989); Owusu-Bennoah and Acquaye, (1996). The residual P pool values also highlighted the extent of weathering in the soils. 115 University of Ghana http://ugspace.ug.edu.gh Weathering leads to a progressive change of Ca bound P to residual P (Walker and Syers, 1976). Deducing from the results, the low levels of Ca bound P in the control soil as compared with the corresponding high levels of residual P indicates a high degree of weathering in the soils. 5.3 Effect of irrigation regimes (clean and waste water) on soil P fractions Irrigation is a very necessary management practice when it comes to agricultural practices such as crop production in the semi-arid savanna regions of West Africa. Under the prevailing harsh climatic and soil conditions, irrigation is necessary to make soil nutrient to dissolve and be more accessible or available for easy uptake and assimilation by crops (FAO, 2003). This is on the back drop of water being a universal solvent. Clean water (tap water) with its dissolved trace elements or compounds can only add next to zero nutrient to the soil whereas waste water might add some nutrient to the soil depending on the source. The results obtained have shown a general trend of relatively higher concentrations in soil chemical properties recorded in the waste water irrigation regime as compared to the corresponding clean water irrigation regime. The soil pH, soil organic carbon, N, K, Ca, and the ECEC, base saturation, available and total P contents all went up in the control soil when waste water was used to irrigate. This was in agreement with Kiziloglu et al. (2007); AL-Jasser A. O, (2011), Eduah (2012) who reported increases in these parameters when the soils were irrigated with waste water. These are positive contributions to the soil. However, exchangeable Na went up due to higher Na contents 116 University of Ghana http://ugspace.ug.edu.gh in the waste water as shown in Table 4.4.a (Halliwell, 2001). The above assertion was obvious since waste water has dissolved mineral nutrients in it that can add-up to the already existing (native) soil nutrients (Eduah, 2012). These dissolved nutrients in the waste water may be from domestic, industrial, municipal and other point or non-point sources. Even though application of the three amendments (B, (NH₄)₂SO₄ and B+(NH₄)₂SO₄) did not change the trend, higher concentrations of soil nutrients were realised. The effect of clean and waste water irrigation regimes were also assessed using the student’s t-test (p < 0.05) to compare significant differences in the mean P concentration of the control plots and also the treatment plots. There was generally a relatively high P concentrations recorded for all the P pools with significant differences observed especially in the biochar treatment plots (B and B+(NH₄)₂SO₄) and for the water extractable P pool. This may be due to the presence of water soluble P in the waste water from point and non-point sources (Eduah, 2012). Although this was not comprehensively investigated in this study, the high disparity in results obtained for waste water relative to clean water could attest to the above assertion. The assertion would be supported by the observations of Akponikpe et al. (2011) and Sacks and Bernstein (2011). They noted that irrigation with wastewater increased soil phosphorus. 5.3.1 Effect of mode of irrigation on available P fractions The modification of the Hedley’s fractionation procedure saw the use of distilled- deionized water as initial extracting solvent which extracted about a fourth (24%) of the soil available P as water leachable P on the average as shown in Table. 4.7. This is the 117 University of Ghana http://ugspace.ug.edu.gh quantum of available P the vegetable growers are possibly going to lose should they irrigate beyond fields capacity or water saturation point of the soil. The implication of this loss could partly account for the low yield in vegetable produce (Werner et al., 2017) since a fourth of the primary plant nutrient (P) would be leached beyond root depth of vegetables. In view of the plant nutritional constraints coupled with the prevailing high temperatures under a unimodal seasonal rainfall, daily irrigation becomes a necessity. However, adequate soil moisture and high amounts of phosphorus should be kept in a balance to give the best conditions for proper plant growth. There was a significant (p < 0.01) correlation (r = 0.42 and 0.70) between the water leachable P and soil available P content and also the soil total P content, respectively. This correlation could be exploited as an economic tool, to reduce cost and laborious chemical analyses by establishing a mathematical relation for soils of the study area through modelling. 5.4 Effect of depth on soil P fractions The depth of the soils under study is within the biological and productive layer of the profile (i.e. 0 –10 cm and 10 – 20 cm). This implies that very little variations would be expected within depths due to continuous turning of soil to aerate or control weeds. However, there was a general decrease in P concentration for all P fractions as depth increased irrespective of irrigation water used. This obviously was due to the relatively higher accumulation of soil organic matter in the 0 – 10 cm depth. Since higher levels of soil organic carbon competes with soil P for binding sites on sesquioxides or oxides of Al and Fe, more P are made available. The increase could also be due to increased 118 University of Ghana http://ugspace.ug.edu.gh microbial activities at depth 0 – 10 cm leading to increase digestion of generated biomass and eventual release of organic matter. The stability of soil P owing to it being bound to other soil components might explain the marginal higher P content at the top soil than the subsoil. This is in agreement with the findings by Asomaning et al. (2015) on sandy soils of Keta where several decades of manure application did not affect leaching of soil P down the soil profile and into ground water. 119 University of Ghana http://ugspace.ug.edu.gh CHAPTER SIX 6 SUMMARY, CONCLUSIONS AND RECOMMENDATION 6.1 Summary Phosphorus status of soils of the semi-arid zone of northern Ghana is low. The low P status has been ascribed to the advanced weathering process that had reduced considerably the calcium bearing apatites that primarily hold P. Some of the soils of the region have large proportions of lateritic concretions with low organic matter and low amounts of clay. The soils are mostly acidic and rich in sesquioxides (Al and Fe oxides, hydroxides and oxyhydroxides). This study was conducted on a Plinthaqualf of northern Ghana (control soil) whose crop yield was found to be low after two seasons and three cycles of vegetable production. The main objectives of the study was to fractionate soil phosphorus in a Plinthaqualf amended with biochar (B), inorganic fertilizer according to normal agricultural practice ((NH₄)₂SO₄) and fortified biochar (B+(NH₄)₂SO₄) under clean and waste water irrigation regimes in northern Ghana. The study focused on P reaction and distribution in control soil and amended soil treatments (B, (NH₄)₂SO₄ and B+(NH₄)₂SO₄) and also ascertained the effect of irrigation regimes used. The Hedley’s fractionation procedure was employed and its reliability in accounting for total soil P contents was evaluated by a linear regression with sum of P fractions (i.e., 0.9831) underestimating the total P by < 2%. Modification of the Hedley’s fractionation procedure was adopted. Distilled water was used as initial extracting solvent; this 120 University of Ghana http://ugspace.ug.edu.gh extracted about a fourth (24%) of the soil available P as water leachable form averagely. This water leachable P highly correlated (p < 0.01) with the soil available P and total P. The study confirmed the soil to be nutritionally deficient due to its inherent poorer physical and chemical properties (Tables 4.1.-4.4.b.). This is further depleted through the mode of irrigation which resulted in the gradual leaching of plant available nutrients as shown by the magnitude of water leachable P. Application of soil amendment B, (NH₄)₂SO₄ and B+(NH₄)₂SO₄ however, improved soil fertility parameters greatly and elevated soil P concentrations significantly. Notably is the performance of biochar, which increased P levels substantially. However, the combination of B+(NH₄)₂SO₄ which is actually a fortified biochar fared much better. This notwithstanding, ECEC levels of the soil are below 4 cmol kg-1c implying that rate of amendment application is not up to optimum levels. The study has shown that waste water irrigation was marginally better than clean water due to the observed significant differences (p < 0.05) in the two water application regimes. There was also decrease in soil P and other nutrient levels as depth increased. 6.2 Conclusions The study has shown that all three (3) amendments namely, biochar (B), (NH₄)₂SO₄ and B+(NH₄)₂SO₄ performed much better in improving soil fertility challenges of the control soil. It has also shown that biochar influences P cycling and distribution in tropical soils and can be a better amendment for improving plant available P in soils as (NH₄)₂SO₄ would. However, an ammonium sulphate fortified biochar (B+(NH₄)₂SO₄) at farmer practice rate would do much better in releasing plant available P. The study also showed 121 University of Ghana http://ugspace.ug.edu.gh that waste water irrigation regime improved soil fertility parameters marginally over clean water irrigation regime. The lower yield in vegetable produce observed by Werner et al., (2017) in the preceding study was partly due to low available P for plant uptake owing to low soil pH, low organic carbon and high Fe oxide contents of the studied soils. However, much of nutrient loss could be attributable to heavy leaching of P owing to the mode of irrigation (i.e. the manual bucket irrigation that results in the pummelling of vegetable plots with excess water under gravity). Furthermore, nitrogen contents in the soils were very low and this could also result in the lower vegetable yield. 6.3 Recommendation Based on the results obtained from the study, the following recommendations were made: 1. The P sources should be applied to the soils at different rates to ascertain optimum P application rate. 2. Nitrogen contents of the soils were very low and lower in the biochar amended plots and hence N-fortification of the produced biochar should be done to increase nitrogen levels in the soil. N-fortification of biochar will control nitrogen release and loss. 3. In view of the high amounts of water leachable P in the soils, drip or sprinkle irrigation should be adopted to prevent excessive leaching of phosphorus and other nutrients from the root zones of vegetables. Additionally, irrigation should not be done above soil’s field capacity (water saturation point). 4. 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Roles of biochar in improving phosphorus availability in soils: a phosphate adsorbent and a source of available phosphorus. Geoderma, 276: 1-6. Zheng, H.Z., Deng, X., Zhao, J., Luo, Y., Novak, J., Herbert, S. and Xing, B. (2013). Characteristics and nutrient values of biochars produced from giant reed at different temperatures. Bioresource Technology, 130: 463–471. 147 University of Ghana http://ugspace.ug.edu.gh APPENDICS Appendix 1.0 Phosphorus standardisation graph (mg P kg-1) ABS 0.3 0.25 y = 0.0105x + 0.0023 0.2 R² = 0.9973 0.15 ABS 0.1 Linear (ABS) 0.05 0 0 5 10 15 20 25 30 Appendix 2.0 Analysis of variance for P fractions at depth 0 – 10 cm Variate: SUM_Pi Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 56.4940 18.8313 26.52 <.001 Residual 12 8.5206 0.7100 Total 15 65.0146 Mean CWC0 16.96 a CWB0 25.63 b CWI0 26.45 b CWBI0 27.23 b l.s.d. 1.298 cv% 3.3 148 University of Ghana http://ugspace.ug.edu.gh Variate: SUM_Po Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 149.464 49.821 28.47 <.001 Residual 12 20.997 1.750 Total 15 170.462 Mean CWC0 41.26 a CWB0 48.39 b CWI0 47.67 b CWBI0 48.73 b l.s.d. 2.038 cv% 2.8 Variate: NaOH_Pi Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 10.1407 3.3802 3.97 0.035 Residual 12 10.2174 0.8515 Total 15 20.3581 Mean CWC0 21.91 a CWB0 21.20 ab CWI0 22.23 ab CWBI0 22.86 b l.s.d. 1.422 cv% 4.2 Variate: HCl_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 314.4171 104.8057 759.71 <.001 Residual 12 1.6555 0.1380 Total 15 316.0726 Mean CWC0 6.85 a CWB0 15.58 c CWI0 14.68 b CWBI0 18.95 d l.s.d. 0.5722 cv% 2.7 149 University of Ghana http://ugspace.ug.edu.gh Variate: Residual_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 380.75 126.92 10.72 0.001 Residual 12 142.06 11.84 Total 15 522.81 Mean CWI0 86.17 c CWB0 79.04 a CWC0 83.43 ab CWBI0 82.96 ab l.s.d. 5.301 cv% 4.2 Variate: Total_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 1870.11 623.37 45.67 <.001 Residual 12 163.79 13.65 Total 15 2033.90 Mean CWC0 180.9 a CWB0 193.5 b CWI0 197.5 b CWBI0 211.3 c l.s.d. 5.692 cv% 1.9 Variate: SUM_Pi Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 109.7872 36.5957 107.02 <.001 Residual 12 4.1035 0.3420 Total 15 113.8908 Mean WWC0 18.20 a WWI0 28.20 b WWB0 28.58 b WWBI0 29.44 b l.s.d. 0.901 cv% 2.2 150 University of Ghana http://ugspace.ug.edu.gh Variate: SUM_Po Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 63.958 21.319 20.73 <.001 Residual 12 12.341 1.028 Total 15 76.299 Mean WWC0 41.26 a WWI0 48.39 b WWB0 47.67 b WWBI0 48.73 b l.s.d. 1.562 cv% 2.1 Variate: NaOH_Pi Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 13.3299 4.4433 6.08 0.009 Residual 12 8.7713 0.7309 Total 15 22.1012 Mean WWI0 21.42 a WWC0 21.22 a WWB0 22.33 ab WWBI0 23.56 b l.s.d. 1.317 cv% 3.9 Variate: HCl_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 264.55476 88.18492 1096.12 <.001 Residual 12 0.96542 0.08045 Total 15 265.52018 Mean WWC0 7.92 a WWB0 15.94 b WWI0 15.14 c WWBI0 17.01 d l.s.d. 0.4370 cv% 2.0 151 University of Ghana http://ugspace.ug.edu.gh Variate: Residual_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 39.94 13.31 0.51 0.682 Residual 12 312.42 26.04 Total 15 352.36 Mean WWB0 86.17 b WWI0 80.97 a WWC0 84.48 ab WWBI0 82.85 ab l.s.d. 7.86 cv% 6.4 Variate: Total_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 1302.80 434.27 11.88 <.001 Residual 12 438.54 36.54 Total 15 1741.34 Mean WWC0 183.8 a WWI0 199.3 ab WWB0 202.8 bc WWBI0 208.1 c l.s.d. 9.31 cv% 3.1 Appendix 3.0. Analysis of variance for P fractions at depth 10 – 20 cm Variate: SUM_Pi Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 52.155 17.385 12.35 <.001 Residual 12 16.890 1.408 Total 15 69.046 Mean CWC2 14.05 a CWI2 24.01 b CWBI2 24.56 b CWB2 24.92 b l.s.d. 1.828 cv% 5.2 152 University of Ghana http://ugspace.ug.edu.gh Variate: SUM_Po Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 12.339 4.113 0.78 0.525 Residual 12 62.879 5.240 Total 15 75.218 Mean CWI2 33.57 a CWC2 34.56 a CWB2 34.84 a CWBI2 36.03 a l.s.d. 3.527 cv% 6.6 Variate: NaOH_Pi Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 26.0979 8.6993 9.36 0.002 Residual 12 11.1525 0.9294 Total 15 37.2504 Mean CWB2 21.60 a CWI2 21.22 a CWC2 22.03 ab CWBI2 21.55 b l.s.d. 1.485 cv% 4.3 Variate: HCl_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 357.32128 119.10709 1692.46 <.001 Residual 12 0.84450 0.07038 Total 15 358.16579 Mean CWC2 7.31 a CWB2 16.87 c CWI2 15.87 b CWBI2 17.08 d l.s.d. 0.4087 cv% 1.8 153 University of Ghana http://ugspace.ug.edu.gh Variate: Residual_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 231.49 77.16 3.94 0.036 Residual 12 235.18 19.60 Total 15 466.67 Mean CWB2 83.12 a CWI2 80.05 a CWBI2 81.14 a CWC2 82.85 a l.s.d. 6.82 cv% 5.7 Variate: Total_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 264.17 88.06 3.07 0.069 Residual 12 343.84 28.65 Total 15 608.01 Mean CWC2 170.7 a CWI2 179.1 a CWB2 181.7 a CWBI2 184.0 a l.s.d. 8.25 cv% 3.0 Variate: SUM_Pi Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 20.834 6.945 2.79 0.086 Residual 12 29.905 2.492 Total 15 50.739 Mean WWB2 15.68 a WWC2 22.31 b WWI2 24.32 b WWBI2 24.46 b l.s.d. 2.432 cv% 6.8 154 University of Ghana http://ugspace.ug.edu.gh Variate: SUM_Po Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 2.95 0.98 0.03 0.991 Residual 12 345.71 28.81 Total 15 348.66 Mean WWB2 34.50 a WWC2 35.24 a WWBI2 35.31 a WWI2 35.68 a l.s.d. 8.27 cv% 15.3 Variate: NaOH_Pi Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 8.1988 2.7329 3.06 0.070 Residual 12 10.7309 0.8942 Total 15 18.9297 Mean WWB2 21.73 a WWI2 20.72 a WWC2 22.68 a WWBI2 21.27 a l.s.d. 1.457 cv% 4.4 Variate: HCl_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 366.98845 122.32948 1727.87 <.001 Residual 12 0.84958 0.07080 Total 15 367.83803 Mean WWC2 7.93 a WWB2 17.22 b WWI2 16.56 c WWBI2 18.02 d l.s.d. 0.4099 cv% 1.7 155 University of Ghana http://ugspace.ug.edu.gh Variate: Residual_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 75.30 25.10 1.05 0.404 Residual 12 285.68 23.81 Total 15 360.98 Mean WWB2 85.73 a WWI2 79.31 a WWBI2 81.67 a WWC2 80.67 a l.s.d. 7.52 cv% 6.3 Variate: Total_P Source of variation d.f. s.s. m.s. v.r. F pr. Treatment 3 480.47 160.16 1.77 0.206 Residual 12 1083.34 90.28 Total 15 1563.81 Mean WWC2 171.0 a WWB2 178.8 a WWI2 183.9 a WWBI2 185.9 a l.s.d. 14.64 cv% 5.4 156 University of Ghana http://ugspace.ug.edu.gh Appendix 4.0. Picture presentation of fractionation process 157 University of Ghana http://ugspace.ug.edu.gh 158 University of Ghana http://ugspace.ug.edu.gh 159