An evaluation of the effectiveness of coal ash as an amendment for acid soils

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(1)An Evaluation of the Effectiveness of Coal Ash as an Amendment for Acid Soils. by. Ikenna Mbakwe. Submitted in partial fulfilment for the degree Master of Science in Agriculture. at. Stellenbosch University. Department of Soil Science Faculty of Agrisciences Supervisor: Prof M.V. Fey Date: December, 2008.

(2) Declaration By submitting this thesis electronically, I declare that the entirety of the work contained therein is my own, original work, that I am the owner of the copyright thereof (unless to the extent explicitly otherwise stated) and that I have not previously in its entirety or in part submitted it for obtaining any qualification.. Date: 20 November, 2008. Copyright © 2008 Stellenbosch University All rights reserved. i.

(3) Abstract Soil acidity is one of the greatest limitations to crop production in most soils of the world. The increasing high costs of conventional liming materials have made it necessary to explore the possibilities of using cheaper substitutes. In South Africa, 16 million hectares of land are naturally acid while on the other hand, the country’s coalfired power plants generate 28 million tons of mostly alkaline coal ash per year, disposal of which is increasingly becoming difficult. The use of coal ash as an agricultural soil amendment while solving the liming needs of local farmers, may also present a safe and more economical disposal option. This study was carried out to evaluate the effectiveness of coal ash as an agricultural liming material. A greenhouse experiment was conducted using maize as test crop. A field experiment was also established on Beestepan Farm in Middelburg, Mpumalanga Province using dry beans as test crop for the first season. In both experiments, fresh unweathered coal ash from Duvha power station (CCE 10%), dolomitic lime (CCE 77%) and calmasil (calcium silicate slag, CCE 99%) were applied to acidic sandy loam soils in the presence or absence of gypsum. Both calmasil and dolomitic lime were applied at equivalent rates of 0, 1, 2, and 4 tons/ha, and rates of 0, 7, 14 and 28 tons/ha were used for ash. Gypsum was applied at a rate of 4 tons/ha. All treatments were applied in three replications. Results showed that liming increased soil pH, improved soil nutrient status and plant uptake of base cations, and enhanced yield. In the greenhouse, coal ash decreased exchangeable acidity from 13.0 mmolc/kg to 6.67 mmolc/kg, increased Ca levels from 200 mg/kg to 379 mg/kg, and increased Mg levels from 25.9 mg/kg to 42.0 mg/kg. Nitrate levels were also raised from 4.4 mg/kg to 14.8 mg/kg hypothetically as a result of the increase in the activity of nitrifying bacteria following a decrease in soil acidity after ash application. Maize yield in the greenhouse was not significantly affected by ash or by other liming materials, and the sufficient watering and consequent elimination of aluminium-induced drought stress is put forward as having masked crop responses to acidity. In the field, coal ash reduced exchangeable acidity from 10.0 mmolc/kg to 5.88 mmolc/kg, increased Ca levels from 71 mg/kg to 132 mg/kg, and increased Mg levels from 7.3 mg/kg to 17 mg/kg. The increase in bean yield from 958 kg/ha to 1724 kg/ha by ash was similar to that realized by dolomitic lime and calmasil. Gypsum had little effect on soil acidity, but it substantially improved soil Ca and sulfate levels, and enhanced bean yield in the field ii.

(4) experiment. The study demonstrated that coal ash could be effective as a liming material, and underscores the need for a cost-benefit assessment of ash use necessitated by the relatively higher rates of ash required to obtain significant soil and plant responses.. iii.

(5) Uittreksel Grondsuurheid is een van die grootste beperkings met betrekking tot gewasproduksie in die wêreld. Die stygende hoë koste van konvensionele bekalkingsmiddels, het dit nodig gemaak om die moontlikhede van goedkoper opsies te ondersoek. In SuidAfrika is daar 16 miljioen hektaar natuurlike suur grond, terwyl die land se steenkool aangedrewe kragsentrales 28 miljoen ton meestal alkaliese steenkoolas per jaar as byproduk vervaardig, waarvan die wegdoening toenemend moeiliker word. Die gebruik hiervan as landbou grond verbeteringsmiddel sal ’n veilige en ekonomiese opsie vir wegdoening wees, terwyl dit ook die plaaslike landbou bedryf sal bevoordeel. Hierdie studie is onderneem om the effektiwiteit van steenkoolas as landbou kalkmiddel te bepaal. Mielies is as proefplante in ’n kweekhuis eksperiment gebruik terwyl ’n veld eksperiment met boontjies ook gedoen is. Laasgenoemde eksperiment is op die plaas Beestepan in die Middelburg distrik, Mpumalanga, uitgevoer. In albei eksperimente is vars, onverweerde steenkoolas van Duvha kragsentrale (Kalsium Karbonaat Ekwivalent (KKE) 10%), dolomitiese kalk (KKE 77%) en calmasil (kalsium silikaat slik, KKE 99%) op suur sanderige leem-gronde toegedien met of sonder die bykomstige toediening van gips. Calmasil en dolomitiese kalk is in ekwivalente hoeveelhede van 0, 1, 2 en 4 ton/ha toegedien, terwyl steenkoolas in hoeveelhede van. 0, 7, 14 en 28 ton/ha toegedien is. Die gips. toediening was 4ton/ha. Elke behandeling is drie maal herhaal. Resultate het ’n toename in pH, ’n verbetering in grondvrugbaarheid, ’n verhoging in plantopname van basiskatione en ’n toename in opbrengs met die kalk behandelings getoon. In die kweekhuis het die uitruilbare suurheid van 13.0 mmolc/kg tot by 6.67 mmolc/kg gedaal na behandeling met die steenkoolas. Ca het ingelyks van 200 mg/kg tot 379 mg/kg en Mg van 25.9 mg/kg tot 42.0 mg/kg toegeneem. Nitraat het ook toegeneem vanaf 4.4 mg/kg tot 14.8 mg/kg. Die hipotese hiervoor is dat dit as gevolg van ‘n toename in die aktiwiteit van die nitrifiserende grond-organismes is. Die hoër aktiwiteit is as gevolg van die laer grondsuurheid, vanweë die as-behandeling. Mielie opbrengs in die kweekhuis eksperiment het nie noemenswaardig verander met enige van die behandelings nie. As rede hiervoor is voorgestel dat die groeireaksie van die plante hier verbloem is wat verwag kan word van plante wat aan Al-geïnduseerde droogtestres onderworpe is. In die veld eksperiment het toediening van steenkoolas die uitruilbare suurheid van 10.0 mmolc/kg tot 5.88 mmolc/kg laat daal en die Ca- en. iv.

(6) Mg vlakke onderskeidelik van 71 tot 132 mg/kg en 7.3 tot 17 mg/kg laat toeneem. Die toename in opbrengs van bone (958 kg/ha na 1724 kg/ha) was vergelykbaar met die opbrengs van kalk en calmasil behandelings. Gips aanvullings het ‘n minimale effek op die grondsuurheid gehad, maar het wel die grond kalsium en -sulfaat inhoud wesenlik verbeter. Dit het ook die opbrengs van bone in die veld verhoog. Die studie het getoon dat die gebruik van steenkool-as, as ’n bekalkingsmiddel, effektief kan wees. Dit het ook die noodsaaklikheid van ’n koste-voordeel bepaling beklemtoon, omdat daar relatief baie meer steenkoolas benodig word as soortgelyke behandelings om ‘n noemenswaardige reaksie van die grond en plant te verkry.. v.

(7) Acknowledgements This work forms part of a research contract between Eskom and the University of Stellenbosch (Contract number: PR 10277819); and the bursary provided through this collaboration is greatly appreciated. And here’s a salute to individuals who at one time or another made invaluable contributions to the success of this endeavour: First to my supervisor, Prof Martin Fey - for his detailed guidance, prized insights, contagious enthusiasm for scientific discoveries, and amazing style of pointing out areas of improvement while commending laudable efforts at the same time. To Max Kane-Berman and his family at Beestepan - for the use of their farm for trials and for their exceptional hospitality during site visits. To the teaching staff of the Department of Soil Science: Dr Hoffman, J.E., Dr Ellis, F., Dr Rosanov, A., De Clercq, W.P, and Lambrechts, J.J.N. - for their tutorship which in many ways, enhanced a clearer understanding of the processes that took place in this study. To the departmental secretary Mrs French, Annatjie - for her cheerful help with the administrative work necessary for the smooth running of the project. To Julia Harper - for taking care of project logistics, sharing useful thoughts on data analysis and presentation, and for always being happy to be of assistance in many other helpful ways. To the technical staff of the department: Matt Gordon and Herschel Achilles - for help with analytical work; and to Leonard Adams and Nigel Robertson for watering my potted plants while I was away for a workshop. To Nikiwe Shange, Anneline Burger, Christian Ombina, Saskia von Diest, John Chisimkwuo, and Chidi Ofoegbu - for sacrificing their free period to assist in sample vi.

(8) collection, preparation and analyses. Tarina Vermeulen is especially thanked for her assistance in the Afrikaans translation of the abstract of this thesis. And to George Van Zijl, Nicolette Van der Merwe, Nadia Malherbe, Tanya Medinski, Richard Orendo-Smith, Meryl Awkes, Kudzai Kaseke, Jurie and Maggie Goosen and Ulli and Heide Lehmann - friends and colleagues whose companionship made me feel welcome in a land so very far away from home. Finally to the Invisible God - for directing my steps to meet all these wonderful people.. vii.

(9) Table of Contents Declaration......................................................................................................................i Abstract ..........................................................................................................................ii Uittreksel.......................................................................................................................iv Acknowledgements.......................................................................................................vi Table of Contents....................................................................................................... viii List of figures.................................................................................................................x List of Tables ...............................................................................................................xii Chapter 1 Introduction ...................................................................................................1 Chapter 2 Amelioration of soil acidity with special reference to the use of coal ash – a review.............................................................................................................................4 2.1 Introduction..........................................................................................................4 2.2 Causes of soil acidity ...........................................................................................4 2.3 Effects of soil acidity ...........................................................................................6 2.4 Amelioration of soil acidity .................................................................................8 2.5 Fly ash................................................................................................................11 2.6 Agricultural use of fly ash..................................................................................13 2.7 Impact of fly ash on soil organisms ...................................................................15 2.8 Conclusions........................................................................................................17 Chapter 3 Amelioration of an acid soil with coal ash and other amendments – a greenhouse trial with maize .........................................................................................18 3.1 Introduction........................................................................................................18 3.2 Materials and methods .......................................................................................18 3.3 Results and discussion .......................................................................................25 3.3.1 Acidity.........................................................................................................25 3.3.2 Electrical conductivity ................................................................................27 3.3.3 Extractable cations ......................................................................................28 3.3.4 Acid saturation ............................................................................................31 3.3.5 Water-soluble anions ..................................................................................31 3.3.6 Yield............................................................................................................33 3.3.7 Uptake of base cations ................................................................................34 3.3.8 Relationship between uptake and extractability of base cations.................36 3.3.9 Relationship between base cation uptake and exchangeable acidity ..........38 3.4 Conclusions........................................................................................................39 Chapter 4 Amelioration of an acid soil with coal ash and other amendments – a field trial with dry beans.......................................................................................................40 4.1 Introduction........................................................................................................40 4.2 Materials and methods .......................................................................................41 4.3 Results and discussion .......................................................................................44 4.3.1 Acidity.........................................................................................................44 4.3.2 Extractable cations ......................................................................................45 4.3.3 Yield............................................................................................................47 4.4 Conclusions........................................................................................................49 Chapter 5 General discussions and conclusions ..........................................................50. viii.

(10) References....................................................................................................................54 Appendices...................................................................................................................70 Appendix 1: Analytical data of pot experiment.......................................................70 Appendix 2: Analytical data of field experiment.....................................................72 Appendix 3: Graphical representation of analytical error estimates........................73 Appendix 4: Plot layout of field experiment............................................................91. ix.

(11) List of figures Figure 3.1: Scanning electron microscopy images of lime, calmasil and fly ash used in both pot and field experiments Figure 3.2: Photo of maize plants at 4 weeks in the greenhouse, just before termination of the greenhouse experiment Figure 3.3: Effect of lime, calmasil, ash and gypsum on soil pH and exchangeable acidity in the greenhouse experiment Figure 3.4: Effect of lime, calmasil, ash and gypsum on soil electrical conductivity in the greenhouse experiment Figure 3.5: Effect of lime, calmasil, ash and gypsum on extractable cations in the greenhouse experiment Figure 3.6: Effect of gypsum on release of calcium from lime, depicting slower release of calcium from lime in the presence of gypsum Figure 3.7: Effect of lime, calmasil, ash and gypsum on acid saturation in the greenhouse experiment Figure 3.8: Effect of lime, calmasil, ash and gypsum on water-soluble anions in the greenhouse experiment Figure 3.9: Effect of lime, calmasil, ash and gypsum on maize yield in the greenhouse experiment Figure 3.10: Effect of lime, calmasil, ash and gypsum on uptake of base cations in the greenhouse experiment Figure 3.11: Relationship between uptake and extractability of base cations. x.

(12) Figure 3.12: Effect of soil acidity on base cation uptake Figure 4.1: Location of power stations in relation to areas of acid soil in Mpumalanga illustrating the nearness of acid soils to power stations Figure 4.2: Photo of field application of treatments by hand Figure 4.3: Photo of bean plants during pod development in the field Figure 4.4: Effect of lime, calmasil, ash and gypsum on pH and exchangeable acidity in the field experiment Figure 4.5: Effect of lime, calmasil, ash and gypsum on extractable cations in the field experiment Figure 4.6: Effect of lime, calmasil, ash and gypsum on bean yield in the field experiment. xi.

(13) List of Tables Table 3.1: Initial chemical properties of acid soil used for the greenhouse experiment Table 3.2: Chemical properties of soil amendments used in the greenhouse experiment Table 4.1: Selected soil properties of experimental site before the start of the field experiment Table 4.2: Chemical properties of soil amendments used in the field experiment. xii.

(14) Chapter 1 Introduction Soil acidity is one of the greatest limitations to crop production in most of the world (McBride 1994, Naramabuye et al., 2008), and the ubiquitous nature of its natural and anthropogenic causes makes it imperative for periodic applications of lime to be made to most agronomic soils if crop yield is to be sustained. Over the years lime costs have risen and efforts to find cheaper substitutes have intensified. More particularly, research has been directed to the reuse of alkaline waste materials as a lime substitute both to reduce crop production costs and as a panacea for waste disposal problems and resulting environmental pollution. One such waste for which use as an agricultural liming material has been suggested by its alkaline nature is coal ash (also known as fly ash), a waste material resulting from the combustion of coal in coal-fired power stations. Every year, millions of tons of fly ash are produced worldwide. The largest commercial use of fly ash is in the cement industry (ACAA, 2002). However, carbon contents in fly ash above 6% are not suitable for direct use in concrete due to interactions between the carbon and the chemical admixtures used in concrete (Stevens and Dunn, 2004). Consequently, huge quantities of fly ash are left unutilised and are usually piled up in dumps, or emplaced in landfills at substantial financial cost (Heidrich, 2003). The long-term environmental and health hazards posed by landfill disposal of fly ash have long been recognised (Adriano et al., 1980). It is therefore, imperative to explore both the economic and environmental gains of using fly ash as a soil amendment. Since Rees and Sidrak (1956), the use of fly ash as a soil amendment in agriculture has been widely investigated. Encouraging increases in yield have been observed in most of these studies. In some other studies, however (e.g. Hammermeister et al., 1998; Singh et al., 2008), yield decreases and element toxicities have posed serious questions regarding the advisability of using fly ash as a soil amendment for agronomic crops. Low interest in the use of ash in agriculture has been mainly due to the presumption of high levels of heavy metals, uncertainty about the buffering. 1.

(15) capacity of ash to ameliorate acidity, and the costs of transportation (Yunusa et al., 2006). According to Jala and Goyal (2006) the properties of fly ash depend on the nature of parent coal, conditions of combustion, type of emission control devices and storage and handling methods. Accordingly, fly ash generated from different sources may differ in their effects on soil properties. In South Africa, coal-fired power plants are the main sources of energy generation and ash production is estimated to be 28 million tons per annum (Reynolds et al., 2002). Fly ash produced as waste in South Africa’s power plants hold the potential of being a cheap source of alkalinity especially for acidic soils that are in close proximity to these power stations. Although 16 million hectares of soils in South Africa are naturally acid (Wooldridge et al., 1995), the use of South African fly ash as an agricultural soil amendment has not been completely investigated. Perhaps the most extensive work on the use of South African fly ash in crop production was work done on SLASH (Rethman et al., 1999) a material made from mixing fly ash, lime and sewage sludge. Fly ash is only 30% of the components that make up SLASH. The benefits accruing from this product therefore could arise both from the ash and from its organic and calcium carbonate components. The process of production and use of SLASH is undoubtedly more complex than pure fly ash from power stations. There is a need, then, to assess the efficiency of pure, unadulterated South African fly ash as a liming material and compare its potencies with those of more commonly used liming materials. Furthermore, because of the limitations of liming materials in the amelioration of subsoil acidity, it is also relevant to assess the behaviour of fly ash and other liming materials when applied in the presence of gypsum, a material widely used to aid movement of calcium to the subsoil (Shainberg et al., 1989). The overall objective of this work is to evaluate in field and greenhouse experiments, the effect of using coal ash as a substitute for lime and other amendments on acid soils. Key questions addressed are: 1. How effective is fly ash in ameliorating soil acidity and improving plant yield?. 2.

(16) 2. What are the effects of liming on soil nutrient status and plant growth? 3. What are soil and plant responses to gypsum application? It is anticipated that findings from this research will augment current knowledge of the use of fly ash in agriculture, through enhancing a better understanding of how soil fertility is affected by ash application.. 3.

(17) Chapter 2 Amelioration of soil acidity with special reference to the use of coal ash - a review. 2.1 Introduction Research into soil acidity and its amelioration has taken centre stage as farmers, scientists and policy makers seek to gain a clearer understanding of major impediments to crop production in a bid to boost food supply for an ever-increasing population. Recently, there has been an emphasis on the reutilization of industrial wastes as a solution to agricultural and environmental problems. This review examines past and recent research on soil acidity and its amelioration. It summarises the causes, consequences and amelioration of soil acidity, and discusses the effect of the use of coal ash as a soil amendment on the physical, chemical and biological properties of soil. 2.2 Causes of soil acidity A soil is said to be acidic when its pH is below 7. However the major problems encountered in farming with acidic soils begin when soil pH falls below 5.5. Majority of crops prefer optimum pH values of between 6.5 and 7.0, within which the availability of most nutrients is maximised (Yunusa et al., 2006). Soil pH is the single most diagnostic chemical measurement made on soil. It has been called a ‘master variable’ controlling ion exchange, dissolution/precipitation, reduction/oxidation, adsorption, and complexation reactions (McBride, 1994). The reasons why soils are or become acidic are varied and could be natural or anthropogenic. 2.2.1 Leaching of basic cations Rainwater moving down the soil profile is slightly acidified with H2CO3 resulting from the continuous production of CO2 in the soil by microorganisms and root respiration. This acidified water moving down the soil profile provides protons that remove basic cations as bicarbonates in the leachate, thus leaving behind a more acid. 4.

(18) weathered product. If weather conditions are extreme such as found in the humid tropics, parent materials are almost completely weathered. This attests to the high acidity levels of soils such as Ultisols and Oxisols, which have oxides of iron and aluminium as their main components (Sumner, 2001). 2.2.2 Acid sulfate soils In the delta areas of the great rivers of the world where conditions such as dissolved sulfate (usually from sea water), a supply of readily decomposed organic matter e.g. rotting mangrove leaves, a source of iron (mostly from terrestrial sediments) and anaerobic/waterlogged conditions persist, acid sulfate soils are a common occurrence. These are soils or sediments containing a build-up of iron sulfides (most commonly pyrite, FeS2) under waterlogged or highly reducing conditions. When exposed to air due to drainage or disturbance, they become strongly acidic due to the generation of sulfuric acid from the oxidation of pyrite. FeS2 + 15/4O2 + 7/2H2O → Fe(OH)3 + 2SO42- + 4H+ Acid sulfate soils generally have a pH less than 4 and sometimes as low as 2 (Ritsema et al., 2000). It is estimated that ASS occupy about 24 million hectares worldwide (Sumner, 2001). 2.2.3 Acid rain Gases NOx and SO2 emitted mostly from the combustion of fossil fuels acidify rainfall with nitric and sulfuric acids through reactions such as: NO2 + ¼O2 + ½H2O → HNO3 SO2 + ½O2 + H2O → H2SO4 The pH of acid rain is commonly between 4.0 and 4.5, and may be as low as 2.0 (Brady and Weil, 1999). Over a long period of time, acid rain deposition can negatively impact soils especially poorly-buffered, coarse-textured soils with low base status (Sumner, 2001).. 5.

(19) 2.2.4 Crop removal Plants take up large amounts of basic cations such as Ca2+, Mg2+ and K+ from the soil. When plant materials are harvested or removed from a field, there is a loss of alkalinity (representing a gain in acidity) from the system (Sumner, 2001). Rowell (1988) noted that a wheat crop grown on a soil containing 10 tons exchangeable Ca and 0.5 tons exchangeable Mg per hectare to 1m depth may contain a total of 25kg Ca and 8kg Mg per hectare. Thus, crop production and removal leads to a loss of these basic cations from the soil. Losses of Ca could reach up to 40kg per ton for crops such as lucerne (Rowell, 1988). 2.2.5 Use of fertilizers Although for most farmers the initial impulse to increase yield is to apply more fertilizers, use of ammoniacal fertilizers have been shown to reduce soil pH and could in fact, reduce yield (Pierre et al., 1971; Yunusa et al., 2006). Ammonium-based fertilizers such as (NH4)2SO4 and (NH4)2HPO4 are oxidized by soil microbes to produce strong inorganic acids by reactions such as: (NH4)2SO4 + 4O2 → 2HNO3 + H2SO4 + 2H2O Continuous use of these fertilizers without a regular and effective liming program could result to a continuous increase in soil acidity. The effects of ammonium fertilizers on soil pH can be sustained over long periods of time. For example, Malhi et al. (1998) reported a decrease in pH with increasing rate of ammonium nitrate application after 27 years of application. In the 0-5cm layer of plots where N was applied at 168 kg N ha-1 or more, pH was 4.1 or less while it was 6.85 in the plots where no N was applied. 2.3 Effects of soil acidity Soil acidity is a major drawback in crop production because of its effect on nutrient availability and toxicity, and its inhibition of many important soil organisms. 2.3.1 Soluble metal and micronutrient toxicity Al toxicity is probably the most important growth-limiting factor in many acid soils, particularly when pH <5.5 (Tisdale et al., 1993). As pH decreases below 5.5, the 6.

(20) solubility of Al and Mn increase. Excess Al interferes with cell division in plant roots, inhibits nodule initiation, decreases root respiration, interferes with enzymes governing the deposition of polysaccharides in cell walls, increases cell wall rigidity by cross-linking with pectins, and interferes with the uptake, transport and use of nutrients and water by plants (Tisdale et al., 1993). Toxic levels of Al3+ in the soil solution thus, impede root growth and function, and restrict plant uptake of nutrients such as Ca2+ and Mg2+. Moreover, if the pH of the soil decreases by natural or anthropogenic processes, heavy metals percolate through the soil profile and contaminate the ground water beneath (Kumar & Nagendran, 2007). The availability of many micronutrients also increases and reaches toxic levels. 2.3.2 Phosphorus fixation Low P status has been identified as a major problem in highly weathered acid soils (Clark, 1982; Sanchez and Salinas, 1981). This is because such soils contain large quantities of Al and Fe hydrous oxides which have the ability to adsorb P onto their surfaces. At low pH (<4.5-5.0) P anions replace the hydroxyl groups on the surfaces of Al and Fe oxides and hydrous oxides; thus P is fixed and made unavailable for crop use (Haynes and Mokolobate, 2001). In acidic conditions, P forms insoluble Al phosphate compounds, a phenomenon which has been exploited to partially ameliorate Al toxicity in acid soils (for example Utoyo and Sunyoto, 1995). However, the large quantities of P fertilizers needed and the high cost of such fertilizers make them an expensive substitute for lime (Haynes and Mokolobate, 2001). 2.3.3 Inhibition of soil organisms Certain important microorganisms are inhibited by low pH. For example, microorganisms involved with the important process of nitrification (conversion of NH4+ to NO3-) are impeded by pH <7.5 (Kodukula et al., 1988). Moreover, the activity of bacteria (Rhizobia species) which are responsible for nitrogen fixation in legume crops, decreases with a decrease in pH. Aarons and Graham (1991) found that lowering external pH to 4.6-4.7 resulted to an immediate efflux of calcium from the cell of both acid tolerant and sensitive strains of Rhizobium leguminosarum.. 7.

(21) Larger soil organisms could also be affected. For example, Baker and Whitby (2003) found that earthworms showed a strong aversion to soil pH below 4.5. 2.4 Amelioration of soil acidity The most common management practice to ameliorate acid soils is the surface application of lime and other calcareous materials (Bolan et al., 2003; Caires et al., 2008). An agricultural liming material is defined as a material, the Ca and Mg compounds of which are capable of neutralizing soil acidity (Barber, 1967). These materials include quicklime, hydrated lime, limestone, marl, shells, and by-products such as slag. Limestone is the main liming material used (Barber, 1967). Many years ago, calcium and magnesium carbonates were deposited in the bottom of ancient lakes or seas. Under pressure of overlying materials, these carbonates were compressed into layers of limestone rocks. Today these are broken up, ground very finely, and made available for application to soils. Calcite (CaCO3) and dolomite (CaCO3.MgCO3) are the minerals contained in limestones. Apart from costs, especially where limestones are not abundant, one major limitation is that if limestones are not finely ground, they react very slowly with the soil, and give little crop response (Cregan et al., 1989). The cost of limestone increases with its fineness (Tisdale et al., 1993). When lime is added to acid soils, the activity of Al3+ is reduced by precipitation as Al(OH)3. The lime treatment raises soil pH while reducing the level of extractable Al. The increased soil pH counteracts H+ toxicity and reduces the solubility and availability of Mn and micronutrients such as Fe, Zn, Cu, and Co which at low soil pH, are toxic to plants and microorganisms (Brady & Weil, 1999). The addition to the soil of basic cations found in liming materials such as Ca2+ and Mg2+, which are important plant nutrients, is also an extra benefit of liming (Tisdale et al., 1993). 2.4.1 Subsoil acidity and the use of gypsum Approximately 75% of the world’s acid topsoils also have acid subsoils (Sumner, 2001). The high cost of subsoil incorporation of lime coupled with the slow infiltration rate of alkalinity from surface applied lime (Fey, 2001) make amelioration of subsoil acidity a daunting task for most farmers. Caires et al. (2008) reported that 3 8.

(22) years after applying lime at 3 tons/ha to soil, little lime had moved down below 5cm. The downward movement of lime is slow and is affected by timing and rate of liming, soil type, surface pH, weather conditions, management of acidic fertilizers, and cropping systems (Caires et al., 2008). Subsoil acidity has been identified as an important yield-limiting factor especially in regions that suffer from water stress (Caires et al., 2008). The major consequence of subsoil acidity is brought to the fore during periods of drought. As surface soils dry out, plant roots are unable to assess the nutrient and moisture reservoir of the subsoil due to restricted root proliferation resulting from the toxic levels of Al in the subsoil (Sumner, 1995; Tang et al., 2002). Wolf (1975) observed that maize grown on soils with acidic subsoils in the Cerrados of Central Brazil wilts after only 6 days without rain even during the wet season. Although much work has been done to incorporate lime to great depths into the soil (e.g. Perez-Escolar and Lugo-Lopez, 1978; and McKenzie and Nyborg, 1984), and significant yield increases have been recorded, such mechanical incorporation on a large scale is for the most part, uneconomical. The use of gypsum has thus been proposed as an effective way of leaching calcium down the soil profile and ameliorating the effects of subsoil acidity (for example Reeve and Sumner, 1972; Farina and Channon, 1988b). The value of gypsum in soils has been known for a long time. Hilgard (1906) was one of the first to discuss its role in preventing deflocculation. Many research programs on the use of gypsum as a subsoil ameliorant stem from findings originally published by Sumner (1970) and Reeve and Sumner (1972). Gypsum (CaSO4.2H2O) occurs naturally in sedimentary evaporite deposits (Hurlbut and Klein, 1971) and as a by-product of some industrial processes. For example, in the production of phosphoric acid from rock phosphate (apatite) by wet acidulation, gypsum is collected as a waste product from the following reaction: Ca10 (PO4)6F2(s) + 10H2SO4 + 20H2O → 10CaSO4.2H2O + 6H3PO4 + 2HF. 9.

(23) This by-product gypsum is more commonly called phosphogypsum. Another industrial process that yields gypsum is the capture of SO2 from stack gases produced by fossil fuel-fired electric power-generating plants (Shainberg et al., 1989). In this system, environmental legislation stipulates that SO2 be removed from emitted gases in order to reduce atmospheric acidity. Usually, the stack gases are passed through a lime slurry where SO2 gas is oxidised to SO42- and gypsum is produced as follows (Shainberg et al., 1989): 2CaCO3(s) + SO2.2H2O(aq) → CaSO4.2H2O(s) + Ca2+(aq) + 2CO2(g) + 2H2O Mined and industrially produced gypsum have found use in wallboard and as a cement additive. But significant amounts are marketed locally as a soil amendment and used especially for the reclamation of sodic soils (Shainberg et al., 1989). Many soils in semi arid to humid regions have a tendency to disperse when wet and develop a compacted structure especially at the soil surface. This surface crusting reduces water infiltration, increases runoff and erosion, and impedes plant growth and establishment. Clay dispersion which is caused by mutual repulsion between particles is enhanced by presence of highly hydrated monovalent cations such as Na+. Soil permeability also tends to decrease with decreasing electrical conductivity (EC) of the percolating solution (McNeal and Coleman, 1966). When gypsum is added to a sodic soil, permeability is increased both by an increase in EC and by cation-exchange effects (Loveday, 1976). Gypsum dissolves to provide a level of electrolyte in the soil solution that maintains sufficient permeability to allow water entry into and through the soil profile and at the same time, provides Ca for exchange with Na (Shainberg et al., 1989). The ability of gypsum to alter soil pH results from two contrasting reactions between gypsum and soil surfaces in which Ca replaces H and Al (which hydrolyzes to give H+) and SO4 replaces OH by ligand exchange. The change in pH will therefore, depend on the extent of the two reactions in any particular case (Shainberg et al., 1989). In soils high in exchangeable Al, H+ release is likely to exceed OH- release causing a decrease in soil pH (Pavan et al., 1984). For highly weathered soils low in 10.

(24) exchangeable Al, the alternative reaction exceeds the former, in which case SO4 replaces OH and soil pH increases (Ritchey et al., 1980). Liu and Hue (2001) in a simulated soil profile experiment observed that after leaching a soil column with 40 mL deionized water daily at a rate of 10 mL per 15 min for 27 days lime markedly increased pH and reduced exchangeable Al of the surface layer, but had little effect on subsoil pH. Only 7.6% of the applied Ca from lime moved from the applied layer to the next 10-cm layer, while more than 60% of the applied Ca from gypsum moved past the applied layer. They attributed the downward movement of Ca from the gypsum treatment to have been assisted by SO42-. 2.5 Fly ash Electricity generation from coal-fired power plants produces solid wastes collectively called coal combustion by-products (CCPs) (Vom Berg, 1998). These wastes include bottom ash, boiler slag, flue gas desulfurization (FGD) materials and fly ash. Fly ash is the mineral residue consisting of very small particles that are carried up and out of the boiler in the flow of exhaust gases and are collected from stack gases using electrostatic precipitators (ESP). Bottom ash and slag have individual particles much larger than those of fly ash, and fall down through the airflow to the bottom of the boiler. They are therefore, easy to collect and are removed during routine cleaning of the boilers. Fly ash, however, has to be captured by pollution control devices. Jala and Goyal (2006) noted that approximately 70% of CCPs is ESP fly ash. Klein et al. (1975) had observed that up to 90% of the ash in many coal-fired plants is fly ash. In the US alone, over 70 million tons of fly ash are generated each year (Gasiorowski and Bittner, 2006). India generates 65 million tons of fly ash per year (Singh et al., 2008). Reynolds et al. (2002) reported that in South Africa, electricity generation produces about 28 million tons of fly ash per annum and Petrik et al. (2003) estimated that only about 5% of fly ash produced by South African power stations is utilized. 2.5.1 Properties: The mineralogical, physical and chemical properties of fly ash depend on the nature of parent coal, conditions of combustion, type of emission control devices and storage 11.

(25) and handling methods (Jala and Goyal, 2006). Generally, though, fly ash is an amorphous mixture of ferroaluminosilicate minerals occurring as very fine particles having an average diameter of <10 μm and having a low to medium bulk density, high surface area and light texture. The fine particles are aggregated into micron and submicron spherical particles of 0.01–100 mm size (Davison et al., 1974). When viewed under a scanning electron microscope, fly ash particles appear as empty spheres (cenospheres) filled with smaller amorphous particles and crystals (plerospheres). The specific gravity of fly ash ranges from 2.1 to 2.6 g/cm3 and bulk density varies from 1 to 1.8 g/cm3 (Jala & Goyal, 2006). The elemental composition of fly ash varies due to types and sources of used coal (Camberato et al., 1997). In most instances, fly ash consists of plant macro-nutrients, Na, K, P, and Fe and micronutrients Si, Co, B, Zn, Cu and Mn (Mishra et al., 2007). A few potentially toxic elements are also common in many fly ashes. Elements such as Al, Pb, Ni, Cr and Cd have raised concerns over the use of fly ash in crop production. Al and Si are usually the most abundant elements in fly ash (Rees and Sidrak, 1956), but Page et al. (1979) noted that Al in fly ash is mostly bound in insoluble aluminosilicate structures, which considerably limits its biological toxicity. Fly ash is considerably rich in trace elements such as lanthanum, terbium, mercury, cobalt, chromium and boron (Adriano et al., 1980). The pH of fly ash varies from 4.5 to 12.0 depending largely on the sulfur content of the parent coal (Plank and Martens, 1974). 2.5.2 Disposal Fly ash is disposed of either by dry or wet methods. In dry disposal, the fly ash is dumped in landfills and fly ash basins. In the wet method, the fly ash is washed out with water into artificial lagoons and is called pond ash. Most of the fly ash presently produced by electric utilities and industry is landfilled or stored in disposal ponds. Landfilling is not an optimal solution for disposal because of landfill space limitations and tipping costs. Moreover, the fly ash can reach the sub-soil and ultimately cause siltation, clog the natural drainage system, and contaminate the groundwater with heavy metals (Jala and Goyal, 2006). As a result, the use of fly ash as a soil amendment in the reclamation of disturbed areas has become a research topic of growing interest. 12.

(26) 2.6 Agricultural use of fly ash Use of fly ash in agriculture has been gaining impetus in countries such as the United States, India and Australia where coal is used for energy generation, and where research into ash use has intensified over the years. Reported benefits of the use of ash include its improvement of soil physical properties, amelioration of soil acidity and content of plant nutrients. 2.6.1 Improvement of soil physical properties Fly ash has been observed to improve structural characteristics of soils because of properties such as its high soluble Ca contents and high percentage of particles in the 2-200 µm range (Yunusa et al., 2006). The mostly fine sand and silt particles of fly ash could increase the amount of coarse particles in the soil matrix and modify pore structure. It has been shown (Campbell et al., 1983) that addition of fly ash increased the void ration i.e. the ratio of the volume of void space to the volume of solid particles. This will therefore be expected to alter properties such as soil bulk density, hydraulic conductivity, and porosity. Fail and Wochok (1977) and Capp (1978) reported that addition of fly ash at 70 t/ha changed soil texture by increasing silt content. Page et al. (1979) reported that the addition of fly ash decreased the soil bulk density, which in turn, improved soil porosity and workability and enhanced water retention capacity. Chang et al. (1977) observed an increase by 8%, in water holding capacity of sandy and loamy soils due to fly ash amendment and an increase in soil hydraulic conductivity, which helped in reducing surface encrustation. Mishra et al. (2007) reported a gradual increase of water holding capacity of soils from 38.56% to 46% with increasing levels of fly ash amendments. 2.6.2 Amelioration of soil acidity For coal ash to be used in ameliorating soil acidity, it needs to have high Ca and Mg, in the forms of carbonates or oxides, and also a fine texture to enhance dissolution and rapid reaction in soil (Yunusa et al., 2006). Application of alkaline fly ash has been found to increase soil pH and neutralize acidic soils. For example, Phung et al. (1978) demonstrated that alkaline fly ash was chemically equivalent to approximately 20% of reagent grade CaCO3 in increasing 13.

(27) soil pH. Page et al. (1979) observed that fly ash increased the pH of calcareous soils from 8.0 to 10.8, and that of acidic soils from 5.4 to 9.9. In a laboratory experiment involving three acidic soils of varying textures, McCallister et al. (2002) observed that the pH of the soils was equally raised by fly ashes and agricultural lime. Srivastava and Chhonkar (2000) showed the efficacy of fly ash for treating acidic coal mine spoils. They observed that at all levels of application, fly ash and lime were comparable in significantly increasing the soil pH. 2.6.3 Source of plant nutrients and improvement of yield Addition of fly ash to soil has been beneficial to crops not only because of its liming abilities, but also because it could provide needed plant nutrients for crops. Mishra et al. (2007) reported that the availability of plant nutrients (N, P, K, S, Ca, Mg, Cu, Fe, Mn and Zn) in fly ash-amended soil samples were significantly different from the control. Fly ash has been reported to increase the availability of Ca, Mg and S in soil (Fail and Wochok, 1977; Page et al., 1979) with a corresponding increase in crop yield. Fly ash content of trace elements such as B and Mo has been seen to benefit certain crops. For example, Plank and Martens (1974) showed that application of fly ash to a silt loam soil increased dry matter yield of lucerne (Medicago sativa) almost 7-folds due primarily, to the B content of fly ash. Martens (1971), Page et al. (1979), Hill and Lamp (1980), Elseewi et al. (1980a), Elseewi et al. (1980b) and Weinstein et al. (1989) have reported an increase in crop yield of alfalfa, barley, Bermuda grass and white clover after the application of fly ash. Mishra et al. (2007) reported a progressive increase in soil K content with increasing levels of fly ash addition. Greenhouse experiments conducted by Sikka and Kansal (1995) showed that application of 2-4% fly ash significantly increased N, S, Ca, Na and Fe content of rice plants. Lau and Wong (2001) reported that addition of coal fly ash at 5% resulted in higher seed germination rate and greater root length of lettuce. A high resistance to pests and diseases has also been observed among crops grown on soil amended with fly ash. For example, Khan et al. (1997) found that a level of 40% fly ash was useful in the management of root knot disease of tomato caused by Meloidogyne sp. of nematodes. Khan and Singh (2001) reported that tomato cultivars 14.

(28) grown on fly ash-amended soils had higher tolerance to wilt fungus Fusarium oxysporum. It has also been reported that the high boron availability of fly ash could limit crop production (Page et al., 1979; Plank and Martens, 1974). Nevertheless, use of properly weathered fly ash may ensure that B availability remains below toxic levels (Cope, 1962). Weathering of fly ash usually occurs when the ash has been exposed to atmospheric elements for a long period of time (Zevenbergen et al., 1999). 2.7 Impact of fly ash on soil organisms Effect of fly ash on soil will be incomplete without a mention of the impact soil application of ash has on soil organisms. This is because of the many important processes that are carried out by soil organisms and the aversion these organisms show to alteration of their natural habitat. Wong and Wong (1986) studied the effect of fly ash on soil microbial respiration on a sandy and a sandy loam soil. They observed a reduction in microbial respiration with increasing fly ash treatments in the sandy soil, while for the sandy loam, microbial respiration was significantly lowered only at the highest application rate. Schutter and Fuhrmann (2001) in a study of the responses of soil microbial community to fly ash amendment, conducted a field experiment in which plots were amended with fly ash at the rates of 0 or 505 Mg ha-1 and subsequently cropped to a fallow-corn-wheat rotation or continuous fescue. After 20 months, the duo assessed the microbial responses to the fly ash amendment by analyzing the fatty acid composition and carbon substrate utilization potential of microbial communities and aerobic heterotrophic bacteria isolated from the field plots. Fly ash amendment resulted in significantly greater amounts of the fatty acids i15:1 G and 16:1w5c with percentages of the latter nearly doubling and more than tripling in fly ash amended soils cropped to wheat and fescue respectively. Fatty acid 16:1w5c is reported to be an indicator of arbuscular mycorrhizal fungi (Olsson et al., 1997). They also found a higher content of the fatty acid methyl esters (FAMEs) 16:1w7c, 18:1w7c/9t/12t, 17:0cy, and 19:0cy. These FAMEs have been identified as indicative 15.

(29) of Gram-negative bacteria, and their increase in fly ash-amended soils suggested favourable responses of Gram-negative bacteria to the fly ash amendment. Most of the species they studied were not represented by an adequate number of isolates to effectively assess the effects of fly ash amendment. However, numbers of Arthrobacter protophormiae were markedly lower in soils amended with fly ash relative to non-amended soils. Conversely, numbers of A. ilicis increased significantly in soils amended with fly ash. So, generally, from their study, analysis of community fatty acids suggested a shift towards more fungi (16:1w5c and 18:3w6c) and Gramnegative organisms (17:0 cy, 16:1w7c, and 18:1w7c/9t/12t) in response to fly ash. The study concluded that addition of fly ash to soil at a rate of 505 Mg/ha did not appear to be detrimental to soil microbial communities but rather resulted in elevated populations of fungi, including arbuscular mycorrhizal fungi, and Gram-negative bacteria. They proposed that such population increases may have been as a result of the liming and fertilizing effects of fly ash, as well as favourable crop responses to the amendment. Klose et al. (2004) studied the changes in microbial biomass, microbial respiration and activities of soil enzymes involved in C, N, P and S cycling in forest soils in response to high, moderate and low input rates of alkaline fly ash. Their study was conducted on 3 forest sites located along an emission gradient of 3, 6, and 15 km downwind of a coal-fired power plant representing high, moderate and low emission rates. Microbial biomass C and N were significantly lower in the humus layers and mineral topsoil (0-10cm) of forest soils most heavily affected by fly ash depositions compared to less affected sites. Microbial respiration was significantly lower in the Oh horizon at sites subjected to high deposition loads compared to sites with moderate and low deposition loads. They attributed the reduction of the microbial biomass in forest soils that received high loads of fly ash deposition and the decreased respiration rates in their Oh horizons compared to sites with moderate and low deposition loads, to increased concentrations of some heavy metals such as Cd, Cr, Ni, Zn and Co that have accumulated as a result of long-term atmospheric deposition.. 16.

(30) The group postulated that at the beginning, atmospheric deposition of fly ash probably had a positive effect on microbial and biochemical processes due to increases in pH and concentrations of nutrients in soil. However, with increasing deposition loads, selected microbial soil processes were severely inhibited due to the accumulation of metals and almost inert organic carbon compounds derived from lignite and deposited with the fly ash. Muir et al. (2007) studied the responses of 2 species of earthworms in a soil amended with fly ash. In an experiment that lasted 6 weeks, they treated soil cores with fly ash at rates equivalent to 0, 5, or 25 t/ha, and reported that neither survival nor change in weight for the worms was affected by fly ash showing that the worms are tolerant of fly ash application of at least 25 t/ha. Applications of fly ash also had no significant effect on burrow densities. An earlier study did in fact show that earthworms may enhance the fertility of soils treated with fly ash by increasing solubilisation of mineral nutrients such as P in the ash. Bhattacharya and Chattopadyay (2002) found that increases in P solubilisation from cow dung mixed with fly ash in the presence of earthworms is associated with large amounts of solubilising microbes in the gut of earthworms. 2.8 Conclusions The problem of soil acidity is as widespread as the natural and anthropogenic agents that cause it. Its effects on soil properties cause significant yield decreases of agronomic crops. Effective amelioration of soil acidity using lime has been hampered by its high cost and by the slow downward movement of surface applied lime thereby stimulating research into the use of other substitutes such as coal ash. Coal ash, apart from ameliorating soil acidity, is also effective in improving soil physical conditions and soil nutrient status. Literature shows that its use as a soil amendment may not have detrimental effects on soil organisms. However, like many other industrial wastes, its application to agronomic soils at high rates needs to take into consideration the potential risks of constituent toxic elements.. 17.

(31) Chapter 3 Amelioration of an acid soil with coal ash and other amendments - a greenhouse trial with maize 3.1 Introduction Many crops show a marked aversion for acidic conditions. Maize for example easily presents symptoms indicative of aluminium toxicity, manifesting as depressed yield and reduced uptake of plant nutrients. Coal ash may represent a suitable option for acid soil amelioration provided that its positive value on agronomic soils and on crop growth can be demonstrated. A greenhouse experiment was conducted at Stellenbosch University, South Africa to investigate soil and plant responses to acid soil amelioration using coal ash, calcium silicate slag (calmasil) dolomitic lime and gypsum, and employing maize as a test crop. 3.2 Materials and methods 3.2.1 Soil collection and preparation A bulk sample of an acidic soil (pHKCl 4.22) was collected from Beestepan Farm in Middelburg, Mpumalanga Province of South Africa (25° 46' 60"S, 29° 28' 0"E, mean annual precipitation 878mm) in December 2007. The soil is a sandy loam topsoil of a Bainsvlei soil form, Moorfield family (Soil Classification Working Group, 1991), with an orthic A on a red apedal luvic B1 horizon. Chemical characteristics of the soil before the start of the experiment are shown in Table 3.1. (See section 3.2.4 for analytical methods employed). The soil was passed through a 5mm sieve and 1.5g/kg ammonium sulfate was mixed thoroughly with the soil. Apart from acting as a source of nitrogen, this application of ammonium sulfate was intended to stimulate nitrification which would further increase soil acidity so that liming effects might be more apparent. The soil was kept moist, wrapped in a polythene bag and incubated for one month.. 18.

(32) After the period, the soil was air-dried and to meet crop nutrient needs, a basal dressing of 200 mg/kg KCl and 400 mg/kg ammonium phosphate was applied and mixed thoroughly with the soil.. Table 3.1 Chemical properties of topsoil before the start of the experiment Property. Value. pH (KCl). 4.22. pH (H2O). 4.95. Exchangeable acidity. 5.92 mmolc/kg. Ca. 169 mg/kg. Mg. 31 mg/kg. Na. 3 mg/kg. K. 74mg/kg. ECEC. 14.3mmolc/kg. Electrical conductivity 0.0359mS/cm Acid saturation. 37%. 3.2.2 Treatments A sample of fresh unweathered fly ash was collected from Duvha Power Station in Mpumalanga. Duvha Power Station was chosen because of its proximity to many potentially acidic farmlands in the area, thus reducing transport costs for farmers if research outputs encourage ash use. Calmasil was collected from Beestepan farm where it is used as a liming material. Calmasil is an alkaline material produced from milled stainless steel slag. It is produced and marketed by an industry of the same name located downstream from Columbus Stainless Steel in Middelburg, Mpumalanga.. Lime. was. collected. from. Immerpan. Lime,. and. gypsum. (phosphogypsum) was collected from Pistorius Lime. The materials were passed through a 2mm sieve and stored in airtight containers. The following analyses were carried out on these materials: Calcium Carbonate Equivalence (CCE) was determined following the method outlined by AOAC (2005). 1g of the liming materials was placed in a 250ml conical 19.

(33) flask. 50ml of 0.5M HCl was added and the solution was boiled gently in a warm plate for 5min. The solution was cooled and excess acid was titrated with 0.25M NaOH to the pink end point using phenolphthalein as indicator. The percent CaCO3 equivalence was calculated using the formula: 2.5 × (ml HCl – (ml NaOH/2)).. To determine pH, 20g of liming material and 50ml of distilled water were shaken in a bottle in a reciprocating shaker for 15min. The solution was allowed to stand for 5min and the pH of the supernatant liquid was measured using a Metrohm 827 pH meter. Electrical conductivity (EC) was determined in a 1:5 liming material:water mixture. Liming material (10g) was shaken in 50ml distilled water for 10min and the EC of the supernatant liquid was measured using a Cyberscan 510 conductivity meter. Elemental composition was determined using X-ray Fluorescence Spectrometry (XRFS). Samples were repeatedly crushed in a single-action jaw crusher to reduce particle size, thoroughly mixed and reduced in size by means of cone-and-quartering. The sample’s particle size was then further reduced by grinding in a Zibb swing mill. A proportion of the sample, 0.28g sample and 1.5g Spectroflux105, was used to prepare fused glass beads and determine the loss on ignition (LOI) at 1000 °C for major element analysis. Eight grams of sample was mixed with an organic binder, Moviol, to prepare powder briquettes for trace element analysis. Whole chemical analyses were then done by XRFS on a Philips 1404 Wavelength Dispersive spectrometer. Table 3.2 shows chemical properties of the amendments used in the experiment.. 20.

(34) Table 3.2 Chemical properties of soil amendments. pH. Ash. Calmasil Lime Gypsum. 10.0. 11.9. EC (mS/cm) 0.235 2.16. 8.53. 3.45. 0.317 2.09. CCE (%). 10. 99. 77. n.d.. SiO2 (%). 55.1. 15.0. 15.5. 12.1. Al2O3 (%). 28.6. 1.78. 1.44. 0.285. MgO (%). 0.746 7.29. 4.60. 0.527. CaO (%). 2.74. 32.5. 37.9. 50.4. Fe2O3 (%). 5.60. 2.32. 0.779 b.d.l.. MnO (%). 0.047 0.48. 0.042 b.d.l.. TiO2 (%). 1.66. 0.53. 0.125 0.021. Cr2O3 (%). 0.050 1.37. 0.008 b.d.l.. Na2O (%). 0.068 n.d.. 0.045 b.d.l.. K2O (%). 0.655 n.d.. 0.104 b.d.l.. P2O5 (%). 0.438 n.d.. 0.367 1.68. NiO (%). 0.012 0.170. b.d.l.. b.d.l.. n.d. = Not determined; b.d.l. = below detection limit. Scanning electron microscopy (SEM) analysis was carried out on the liming materials using a Leo® 1430VP Scanning Electron Microscope, and images are shown in Figure 3.1.. 21.

(35) a. b. c. Figure 3.1 Scanning electron microscopy images of liming materials. a: dolomitic lime; b: calmasil; c: fly ash. 22.

(36) 3.2.3 Experimental outline Treatments consisted of calmasil at rates of 0, 0.5, 1, and 2g/kg; fly ash at rates of 0, 4, 8, and 16g/kg; and dolomitic lime at rates of 0, 0.5, 1, and 2g/kg, all with 3 replicates. The rates represented control, half the lime requirement, the lime requirement, and double the lime requirement. The whole set of treatments was duplicated on soils amended with 2g/kg gypsum. Treatments were mixed thoroughly with soil and the mixture was placed into pots (1kg dry weight soil mix per pot) ensuring that uniform bulk density was maintained in all the pots. The pots were watered to field capacity and placed in a greenhouse at 25± 3°C under natural light. After 4 days, 7 maize seeds were planted per pot. The pots were watered and covered with paper towels to minimize loss of water by evaporation. The paper towels were removed on the 3rd day after planting when germination had started. Maize plants were thinned down to 4 plants per pot 7 days after planting. The pots were watered daily to field capacity. 3.2.4 Data collection and analyses The maize plants were harvested 4 weeks (28 days) after planting (Figure 3.1). Plants were cut from the base of the shoot, placed in sample bags and dried in an oven (50 °C) for 24 hrs, and subsequently weighed. Furthermore, the dried shoot materials were milled and analysed for basic cations following the method described by Du Preez et al. (1981). One gram of milled plant material was weighed into a crucible and warmed on a hot plate under an extraction fan until the sample smoked without burning. The crucible was placed into a preheated furnace at 500 °C for 5hrs. After heating, the crucible was removed and cooled. 5ml of 1:1 HCl:H2O was added and the crucible was placed in a sand bath and heated until the contents of the crucible turned yellow. The crucible was left to cool after which the contents were washed down into a 50ml volumetric flask with distilled water ensuring that all the sample material was completely washed down into the flask. The flask was made up to the 50ml mark using distilled water. The flask and its contents were shaken by hand and filtered into a 50ml bottle through a Whatman no 1 23.

(37) filter paper. The samples were then analyzed for Ca, Mg and K by atomic absorption spectrophotometry. Cation uptake (mg/pot) was determined by multiplying foliar concentration (mg/g) by dry matter yield (g).. Figure 3.2 Maize plants at 4 weeks. Soil samples were taken from each pot, air-dried and passed through a 2mm sieve. Soil pH was measured in a 1:2.5 soil:1M KCl mixture in which 20g of soil and 50ml of 1M KCl were shaken in a bottle in a reciprocating shaker for 15mins. The solution was allowed to stand for 5min and the pH of the supernatant liquid was measured using a Metrohm 744 pH meter (White, 1997). Soil acidity was determined using a method in which 5g of soil was mixed with 50ml of 1M KCl and shaken in a reciprocating shaker for 4min. The solution was filtered into a 50ml bottle through a Whatman no 1 filter paper. 25ml of this filtrate was pipetted into a 250cm3 conical flask. 6 drops of phenolphthalein was added and the solution was titrated to the pink end point with 0.01M NaOH (White, 1997). Ammonium acetate exchangeable bases were determined by shaking a 1:5 soil:1M ammonium acetate mixture (10g soil : 50ml 1M NH4OAc) in a shaker for 30min. The 24.

(38) solution was filtered through a Whatman no 1 filter paper and analysed for Ca, Mg, K and Na by atomic absorption spectrophotometry. Water soluble anions were determined by shaking a 1:5 soil:water mixture (10g soil : 50ml distilled water) in a shaker for 30min. The solution was filtered through a Whatman no 1 filter paper and analysed for chloride, sulfate and nitrate using a Dionex DX-120 Ion Chromatograph. Electrical conductivity (EC) was determined in a 1:5 soil:water mixture by shaking 10g of soil in 50ml distilled water for 10min. The EC of the supernatant liquid was measured using a Cyberscan 510 conductivity meter. Soil and yield data were analysed by analysis of variance using a randomized complete block design. Means were compared using the Tukey test with significance at P = 0.05.. 3.3 Results and discussion Soil and plant data of analytical results are shown in Appendix 1, and error estimates are represented graphically in Appendix 3a. 3.3.1 Acidity Figure 3.2 shows pH and acidity trends. In general, pH increased as level of liming material increased. However, even at the highest rate of application, which is 2 times the lime requirement, the soil was still very acidic. The main reactions that buffer soil pH include the neutralisation of Al3+ and H+ dissociated from pH dependent cation exchange sites, and the net acid effects of nitrification (Helyar et al., 1995). The relatively high N fertiliser (1.5g/kg ammonium sulfate) added to the soil at the beginning of the experiment could very well have triggered acid-generating nitrification reaction. During nitrification of NH4+ to NO3-, two H+ ions are released (Helyar and Porter, 1989).. 25.

(39) It may also be that at 4 weeks when soil analyses was done, many of the particles of the liming materials where yet to dissolve and react fully with the soil. Soil amended with calmasil had higher pH values than soil amended with lime and ash at comparable levels of added liming material. At all rates of application, the efficiency of increasing pH was calmasil>lime>ash.. With gypsum. No gypsum 4.6. 4.6. Calmasil. 4.2. pH (K C l). pH (K C l). Calmasil Lime Ash. 3.8. 4.2. Ash. 3.8 3.4. 3.4 0. 50. 100. 150. 0. 200. 50. 100. 150. 200. Alkalinity added (% of optimum). Alkalinity added (% of optimum). With gypsum. No gypsum 15. 15. 10. a c idity (m m olc /k g). a c idity (m m olc /k g). Lime. Ash Lime. 5. Calmasil. 0. 10. Ash. 5. Lime Calmasil. 0 0. 50. 100. 150. Alkalinity added (% of optimum). 200. 0. 50. 100. 150. 200. Alkalinity added (% of optimum). Fig 3.3 Effect of lime, calmasil, ash and gypsum on soil pH and exchangeable acidity. Points represent means of replicates. Calmasil had the greatest effect on reduction of acidity, reducing acidity at the highest rate of application, by as much as 89% in the absence of gypsum. Lime caused a 78% decrease and ash decreased acidity from 13.0mmolc/kg to 6.67mmolc/kg in the absence of gypsum, representing a 48% reduction. The supremacy of calmasil to lime and ash in reducing acidity may not be unconnected to calmasil’s higher calcium carbonate equivalence (CCE) of 99%. Rate of application of liming materials was worked out based on preliminary estimates of CCE to give approximately the same. 26.

(40) neutralization for each level applied. The CCE estimate adopted for calmasil was later found to be lower than the CCE reported by its manufacturer, thereby giving the material some advantage at the rates applied. Gypsum did not have any significant effects on pH and acidity. Being a neutral salt, one will not expect it to alter pH (Bastin et al., 1999). However in some instances under field conditions, gypsum has been observed to increase (for example, Farina and Channon, 1988b), decrease (for example Pavan et al., 1982), or have little effect (for example, Pavan et al., 1984) on soil pH. This change in pH if observed, usually has a small magnitude in the order of 0.2-0.3 pH units and is hardly detectable in an electrolyte suspension (Shainberg et al., 1989). Two reactions determine whether gypsum will have an effect on soil pH: the replacement of H and Al by Ca; and the replacement of OH by SO4. The latter reaction is the so-called self-liming effect of Reeve and Sumner (1972) and occurs in highly weathered soils low in exchangeable Al. The extent to which either of these reactions occurs determines the effect that applied gypsum will have on soil pH. Soil pH is likely to increase if the latter reaction is greater than the former. 3.3.2 Electrical conductivity In the absence of gypsum, only the 3rd and 4th levels of calmasil and lime were significantly different (P = 0.05) from the control (Figure 3.4). Ash even at the highest rate of 16g/kg did not increase electrical conductivity (EC) significantly. In the presence of gypsum, none of the treatments differed significantly from each other and from the control. Gypsum however had a highly significant effect on EC, more than doubling the EC values of the no gypsum control (Figure 3.4). EC of water increases as more and more salt is dissolved in it. Thus, the EC of a soil solution provides an indirect measurement of the salt content. The higher EC values in the pots with gypsum are as a result of the elevated levels of sorbed SO4 and Ca which would subtend slightly more electrolyte in the soil solution than the control (Toma et al., 1999).. 27.

(41) No gypsum. 0.70 Calmasil. 0.50. With gypsum. 0.90. E C (m S /c m ). EC (m S/c m ). 0.90. Lime Ash. 0.30. Ash Lime Calmasil. 0.70. 0.50. 0.30 0. 50. 100. 150. 200. 0. Alkalinity added (% of optimum). 50. 100. 150. 200. Alkalinity added (% of optimum). Fig 3.4 Effect of lime, calmasil, ash and gypsum on soil electrical conductivity. Points represent means of replicates. However, the EC levels recorded were well within the limits tolerable by maize. Beltrao and Asher (1997) reported a threshold salinity value of 1 mS/cm for sand and 2 mS/cm for clay and loam. 3.3.3 Extractable cations Figure 3.5 shows the effects of amendments on extractable Ca, Mg, Na and K. Both the treatment and gypsum effects were highly significant for extractable Ca. Calcium was enhanced by all 3 alkaline amendments, ash the least. In the pots without gypsum and at the highest application rate, Ca was raised from 200 mg/kg to 754 mg/kg by lime, 720 mg/kg by calmasil, and 379 mg/kg by ash. Gypsum being the most soluble Ca amendment increased Ca levels by almost 4 times that of the control. Shainberg et al. (1989) noted that gypsum dissolves to an extent of about 2.5g l-1 and that other common Ca salts such as CaCO3 are much less soluble. By-product phosphogypsum used in this experiment, has even been found to be more rapidly soluble that mined gypsum (Keren and Shainberg, 1981).. 28.

(42) With gypsum. No gypsum. 1200. 800. C a (m g/k g). C a (m g/kg). 1200. Lime Calmasil. 400. Ash. Calmasil Lime. 800. Ash. 400. 0. 0 0. 50. 100. 150. 0. 200. 50. 120. 120 Calmasil. 80. M g (m g/k g). M g (m g/k g). 200. With gypsum. No gypsum. Ash. 40. Lime. 0. Calmasil 80 Ash. 40. Lime. 0 0. 50. 100. 150. 200. 0. Alkalinity added (% of optimum). 50. 100. 150. 200. Alkalinity added (% of optimum). With gypsum. No gypsum 60. 60. Ash. Lim e. 40. Calm asil. Na (mg/kg). Na (mg/kg). 150. Alkalinity added (% of optimum). Alkalinity added (% of optimum). Ash. 20. 0 0. 50. 100. 150. 20. 0. 200. 0. 50. Calmasil. K (m g /kg ). 30 20 10. 200. Lime. 40. Lime Ash. 150. With gypsum. 50. 40. 100. Alkalinity added (% of optimum). No gypsum. 50. Lim e Calm asil. 40. Alkalinity added (% of optimum). K (m g/k g). 100. Ash. 30. Calmasil. 20 10. 0. 0 0. 50. 100. 150. Alkalinity added (% of optimum). 200. 0. 50. 100. 150. 200. Alkalinity added (% of optimum). Fig 3.5 Effect of lime, calmasil, ash and gypsum on extractable cations. Points represent means of replicates. 29.

(43) There seems to be a greater response to extractable Ca with increasing levels of liming materials in the pots without gypsum than in those with gypsum (Figure 3.6).. 1200. With gypsum. Ca (m g/kg). 1000. y = 0.9269x + 825.27 R2 = 0.653. 800 600. y = 2.7579x + 225.6 R2 = 0.9815. No gypsum. 400 200 0 0. 50. 100. 150. 200. Alkalinity added (% of optimum). Fig 3.6 Effect of gypsum on release of calcium from lime. Points represent means of replicates. Gypsum seemed to have suppressed the release of Ca from the liming materials. This may not be unconnected with the phenomenon of the common ion effect. The dissolution of liming materials can be represented by equations such as: CaCO3. Ca2+ + CO2 + 2OH. When gypsum is added to the soil, Ca2+ is added to the system thereby moving the reaction to the left and decreasing the dissolution of CaCO3. This might explain the slower release of Ca from liming materials after gypsum application. In both the pots with gypsum and those without, Mg extractability is only enhanced by calmasil, otherwise, it is independent of amendment application (Figure 3.5). The higher content of Mg in calmasil (Table 3.2) may have accounted for the increase in extractable Mg with increasing levels of calmasil. Gypsum did not have any significant effect on extractable Mg. This is in contrast to the observation made by Shainberg et al. (1989) of a reduction in soil Mg content after gypsum application. In the field experiment of Shainberg et al. (1989), Mg had leached down and accumulated in the lower portions. However, in this experiment carried out in partially closed pots and watered only to field capacity, leaching of Mg. 30.

(44) was not to be expected. This might explain why the gypsum had little effect on Mg extractability. Extractable Na was not significantly affected by amendment application, and hovered around a fixed level of about 38 mg/kg in spite of alkaline treatment and rate. Mishra et al. (2007) also observed no significant change in Na contents of the soil after fly ash application of up to 15 metric tons per hectare. This is an indication that at these levels of application, risk of salinity problems is not likely. Extractable K was independent of any amendment application, including gypsum. 3.3.4 Acid saturation All the liming materials and gypsum had highly significant effects on acid saturation (Figure 3.7). Acid saturation decreased consistently with increasing rates of amendments. Although gypsum had little effect on acidity, its contribution to the reduction of acid saturation is highly significant and is mainly as a result of its supply of Ca to the system. No gypsum. With gypsum 60. Acid Sat (%). Acid Sat (%). 60 40 Ash 20 Lim e Calm asil. 0 0. 50. 100. 150. Alkalinity added (% of optimum). 40. 20. Ash Lim e Calm asil. 0. 200. 0. 50. 100. 150. 200. Alkalinity added (% of optimum). Figure 3.7 Effect of lime, calmasil, ash and gypsum on acid saturation. Points represent means of replicates. 3.3.5 Water-soluble anions Liming did not markedly affect water-soluble chloride and sulfate levels (Figure 3.8). However, gypsum application generally caused a slight increase in water soluble Cl, hypothetically as a result of a displacement of less adsorbed Cl ions from the soil exchange sites by sulfate ions. Gypsum also caused more than threefold increase in sulfate levels unsurprisingly because of its sulfate component. 31.

(45) With gypsum. No gypsum 50. 50. 40. Cl (mg/kg). Cl (mg/kg). 40 Calm asil. 30. Lim e. Ash. 20. Ash. 30. Calm asil Lim e. 20 10. 10. 0. 0 0. 50. 100. 150. 0. 200. 50. 100. No gypsum. Ash. 2400 1600. SO4 (mg/kg). SO4 (mg/kg). 3200. Calm asil Lim e. 800. Ash. Lim e Calm asil. 2400 1600 800. 0. 0. 0. 50. 100. 150. 200. 0. Alkalinity added (% of optimum). NO3 (mg/kg). Calm asil. Lim e. 40. 200. 80. Lim e. 40. Ash. Ash 150. Calm asil. 120. 0. 0 100. 150. With gypsum. 120. 50. 100. 160. 160. 80. 50. Alkalinity added (% of optimum). No gypsum. NO3 (mg/kg). 200. With gypsum. 3200. 0. 150. Alkalinity added (% of optimum). Alkalinity added (% of optimum). 200. Alkalinity added (% of optimum). 0. 50. 100. 150. 200. Alkalinity added (% of optimum). Figure 3.8 Effect of lime, calmasil and ash on water-soluble anions. Points represent means of replicates. The trend in nitrate levels was unaffected by gypsum application. However, there was a corresponding increase in nitrate with increasing rate of amendment application, calmasil giving the highest increase and ash the least (Figure 3.8). This trend is especially similar to the effect that treatments had on pH (Figure 3.3) suggesting a correlation between soil pH and nitrate levels. Nitrification is one of the most critical autotrophic processes in soil and is the consecutive oxidation of ammonium to nitrite and subsequently to nitrate by bacteria. 32.

(46) such as Nitrosomonas and Nitrobacter. Autotrophic nitrification can still occur in acid soils (Weier and Gilliam, 1986; Bramley and White, 1990) but the process has been shown to be inhibited by low pH (Burton and Prosser, 2001). Its rate increases with increasing pH up to about pH 8 (Paul and Clark, 1989). The increase in soil nitrate levels with increase in amendment application is therefore, attributed to the liming effects of the treatments. Nitrate levels were increased from 4.4 mg/kg to 14.8 mg/kg by ash, 148 mg/kg by calmasil, and 60.2 mg/kg by lime in the absence of gypsum. Gypsum did not have any significant effect on nitrate hypothetically because of it did not alter soil pH (Figure 3.3). These results also underscore the rapidity with which soil nitrate levels can be raised following small pH increases, and calls attention to the need to manipulate pH changes in limed soils to ensure that nitrate production is matched with plant uptake and that nitrate leaching is minimized. Nitrate leaching into ground water has raised serious health and environmental concerns because of the dangers of nitratecontaminated drinking water (Hansen and Djurhuus, 1997) and the potential of nitrate to stimulate unwanted plant and algae growth in lakes and reservoirs when deposited by runoff (Tisdale et al., 1993). 3.3.6 Yield The maize hybrid used in this experiment grew relatively well in unlimed soil even though the pHKCl was well below 4. Information about the acidity tolerance of this maize hybrid is unknown but results show it could very well have a high tolerance level for acidic conditions. Generally, the amendment applications had very little effect on dry matter yield (Figure 3.9). One possible reason may be suspected low Zn levels of the soil. The plants showed symptoms of Zn deficiency manifesting as light streaking followed by a broad whitish band, starting slightly from the leaf edge and extending to the midrib. The low Zn level in the soil may have reduced the capacity of the maize to respond to treatments.. 33.

No results found