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THE CHEMISTRY OF SOIL PROCESSES

Dr. D.J.Greenland, Dr. Michael H.B.Hayes
  • Country of Origin:

  • Imprint:

    NIPA

  • eISBN:

    9789391383817

  • Binding:

    EBook

  • Number Of Pages:

    728

  • Language:

    English

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The book is intended to describe the chemistry of several of the important processes which take place in the soil. These are discussed in detail in the appropriate s which include treatments of precipitation and of ion-exchange reactions, adsorption and the formation of complexes, and of oxidation and reduction.

0 Start Pages

Preface Since the beginning of this century the application of chemical fertilizers has increased many fold, and the production of soil-applied pesticides has become a new major industry during the past twenty five years. Fertilizers and pesticides are not the only chemicals added to the soil, because by accident or design many other compounds which enter the environment end up in the soil. The world population, increasing at a conservative estimate of 2 per cent per annum, doubles in less than thirty five years. The soil has to produce the bulk of the food and fibre to sustain this growing population. If the soil is to continue to be both the source of our foodstuffs and the ‘sink’ for many of our wastes, it is obviously desirable that we understand its complex chemistry. Much scientific endeavour is being directed to this end. This has resulted during the past few decades in a large volume of published literature, including several books concerned with the chemistry of the mineral and organic components of soils, or with certain chemical processes in soils. No recent book has, however, attempted to produce a coherent account of soil chemistry as a whole, suitable for students of both environmental chemistry and soil science, and for the wide range of research and other scientists who are interested in the chemistry of soils. The original intention of the editors of the present book was to attempt to fill this gap. Because of the very diverse topics involved it was felt that it would have to be written by several authors if it was to be appropriately authoritative. The chemistry of soils falls naturally into two parts, the chemistry of the various soil compoents, and the chemistry of soil processes. At first it was intended that these, the static and dynamic aspects of soils, should form two parts of a single volume. However, to deal with the topics involved at the intended level would have produced an unwieldy volume, and so two companion volumes are being issued, The Chemistry of Soil Constituents and The Chemistry of Soil Processes. The opening chapter of the companion volume provides a short historical outline of the development of soil science. This deals briefly with the evolution of soil chemistry and pedology, and with the development of soil mineralogy, soil physics and soil biology—disciplines which are essential for integrated studies of soils and soil processes. It also refers to some of the processes which weather rocks, transform plant and animal residues, and lead to the formation of soil constituents. Several systems exist for classifying the major soil types of the world according to their origins, and the most common of the names used to indicate the same soil types are introduced. The two chapters which follow deal with the inorganic and organic components of soils, respectively. Although the chemical structures of the major inorganic components are now reasonably well known, the same is not true of the organic (or humic) materials in soils. The relevant chapter presents an account of what has been experimentally established regarding the constitution of the peculiarly intractable complex of organic compounds found in soils. Chemical processes in soils are largely determined by reactions at the surfaces of the soil colloids. The final three chapters of the companion volume are therefore concerned with the nature and extent of the surfaces of soil colloids, their electrical characteristics, and the ways in which ions and water are held and arranged at the surfaces. This volume is intended to describe the chemistry of several of the important processes which take place in the soil. These are discussed in detail in the appropriate chapters which include treatments of precipitation and of ion-exchange reactions, adsorption and the formation of complexes, and of oxidation and reduction. The manuscripts do not, however, adhere exclusively to chemical processes. In contexts of soil as a medium for plant growth or as a ‘sink’ for waste products, it is not realistic to separate purely chemical from physical and biological processes. Thus the initial chapter attempts to summarize the properties of soils which are relevant to plant growth, to detoxification, and to transport processes within the soil profile, and it tries to relate the development of specific features of soils to the chemical processes operating on them. Mass flow and diffusion, discussed fully in Chapter 2, controls the transport of molecules and of ions to plant roots, and to adsorption, exchange or deposition sites in the soil profile, or to drainage waters or the atmosphere. Such transport processes are strongly influenced by the extents of aggregation of soil components and by the nature of the aggregates; hence the important concepts of soil structure are summarized in Chapter 1. Similarly, the soil composition and structure determine the aeration and water- holding capacity of the soil, and these properties in turn influence the types, numbers, and activities of the organisms of one kind or another which inhabit it. The diversity and magnitude of the soil flora and fauna are introduced in the first chapter, and frequent mention is made of their activities throughout the volume. Inevitably, in order to discuss the different topics in some depth it has been necessary to assume that the readership will have a significant knowledge of chemistry and an introductory knowledge of soil science. It is hoped that this book, like its companion volume, will be of use to students of soil science and environmental chemistry, as well as to those who are involved in research related to the soil. The editors are grateful for the collaboration and patience of those who have contributed, for the encouragement of the publishers and their tolerance of delays, and for the permission which many authors and publishers have given for material to be reproduced. Special acknowledgements of the latter are made in the text.

 
1 Soil Processes
D. J. Greenland

1.1 INTRODUCTION This volume is concerned with the chemistry of soil processes. These are rather arbitrarily divided and discussed in the first part of this book in terms of mass flow and diffusion, precipitation, cation, anion and ligand exchange, adsorption, translocation of metals, and oxidation and reduction. The way in which these processes operate to determine the fate of different substances added to soils is described in the remaining part of the book. In this initial chapter an attempt is made to show how the individual chemical processes described later combine with physical and biological processes to give soils their ability to support plant life, and to accept the detritus of the world and to convert it to harmless constituents of the soil mass.

1 - 36 (36 Pages)
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2 Mass flow and Diffusion
A. Wild

2.1 INTRODUCTION Mass flow and diffusion are processes which cause the redistribution of many components within the soil profile. They are usually investigated because they cause a redistribution of materials such as fertilizers and pesticides which are added to the soil, or because of the need to reduce salt concentrations to levels safe for crop growth. This has given an economic incentive to the investigations, but the processes are also basic to our understanding of the development of soil properties. For example, diffusion is the main process for the movement of gases into and out of the soil, which in turn affects many soil properties (Chapter 7), and mass flow of water is an important cause of the formation of horizons in soil profiles (Chapter 8). Hence there are both academic and economic reasons for attempting to understand the way the processes operate in soils. Over the last few years our understanding has increased considerably. It is the purpose here to give the principles which have been evolved in this recent work, and to discuss their application. It will also be shown that many problems remain to be solved. Diffusion means a spreading out, and is caused by random thermal motion, as is the Brownian movement of colloidal particles observable under the microscope. In unstirred liquids, which are our main concern in this chapter, all the molecules and ions of the solvent and solute have this random movement (molecules and ions in gases and some solids behave similarly). As a result of this motion, which occurs randomly in all directions, any irregularities in the concentration of a solution or mixture of gases eventually disappear.

37 - 80 (44 Pages)
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3 Precipitation
O. Talibudeen

3.1 INTRODUCTION The chemistry of the soil is much more complex than can be represented simply by precipitation, adsorption, or exchange processes. The complexity arises from the time scale of soil chemical reactions in which the structure and dimensions, the extent and distribution of pores, and the content and mobility of water in them, interact immensely with the reaction chemistry. The fourth process adding to this complexity is the dissolution of native and reprecipitated soil constituents by percolating water containing solutes accumulated by natural processes or added by man. The fifth complicating factor compounding the time scale is that very often the resultants of soil chemical reactions are large complex molecules, occasionally polymeric, made up of several constituent groups or atoms. It is by no means clear which of the four processes participate in a chemical reaction and in which order. That a process reoccurs during a prolonged reaction is very likely. Such complexities explain why no single model related rigorously to the retention and release of cations and anions, can provide a basis for a laboratory procedure to test and compare the chemical properties of soils. In this chapter, some of the soil reactions that are important to plant nutrition and pedological processes are discussed, principally those leading to relatively simple compounds accumulating in soil.

81 - 114 (34 Pages)
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4 Cation exchange in soils
O. Talibudeen

4.1 INTRODUCTION All soil components contribute in some measure to cation exchange. The source, extent, and quality of the contribution of each to the cation exchange capacity is varied and complex. Ocasionally, reactions between components alter the extent and quality of cation exchange. Also, in some this is strongly influenced by the pH of the soil-water complex, and the electrolyte content of the soil solution. Indeed, when ‘simple and rapid’ methods, empirical or allegedly fundamental in origin, appear in the literature, they must be viewed circumspectly. Mostly, they provide an incomplete, or a one-sided, picture of the phenomenon. To justify his invention, the experimenter usually provides correlations with other intrinsic soil properties (e.g. the concentration by weight and the mineralogy of the <2 jxm particles, the organic matter content, the surface area) or with the reaction of soil with its environment (e.g. plant growth and composition). However convincing such statistical parameters might be for the population of soils investigated, they do not help elucidate, and may indeed mask, the true extent and quality of cation exchange. This chapter deals with current views on the sources, extent and quality of cation exchange, and with comprehensive methods for evaluating these. The relationship between the properties thus observed and values obtained from simple and rapid procedures is considered.

115 - 178 (64 Pages)
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5 Anion and ligand Exchange
C. J. B. Mott

5.1 INTRODUCTION One of the generalizations that can be made about soils is that the particles of which they are composed together have a nett negative charge. There are exceptions: for example, some iron- and aluminium-rich tropical soils can attain a positive charge at low enough pH, but such cases are rare. It is nonetheless a fact that most anions are held strongly by the soil complex, and it is the purpose of this chapter to look at some of the reasons why this is so. Historically, the impetus to research in this area has largely been the practical necessity to understand the behaviour of applied phosphate fertilizer, and it is some indication of the complexity of the problems involved that, even after decades of effort by soil chemists and agronomists, this understanding is still far from complete. The reason for this is partly that the soil surfaces themselves are, as has been described by Greenland and Mott (1978) in the companion volume, the product of a ‘dirty’ environment. This generally means that phosphate added to the system has to exchange on sorption with an inadequately characterized suite of anions presorbed on ill-defined surfaces. There are also in the case of phosphate, possibilities of simultaneous precipitation. Faced with these problems, the trend in the literature over the past ten years or so has been to investigate in detail the physical chemistry of the sorption of anions onto simple surfaces which are fairly well understood, in the expectation that this will lead to ultimate comprehension of behaviour in the natural system. The results of many relevant experiments have only recently been published, and so some parts of the account are still at the discussion stage in the literature. This is especially true of phosphate, which looks destined to be a joker in the anion pack for some time to come.

179 - 220 (42 Pages)
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6 Adsorption
S. Burchill, M. H. B. Hayes

6.1 INTRODUCTION This chapter is concerned with the interactions which retain mobile chemical species in the soil environment. Emphasis is centred on the binding of small organic chemicals to soil particles of colloidal dimensions, and on the associations between polymeric materials and inorganic colloids which strongly influence the physical and chemical behaviour of soils. These interactions are also involved in the retention by soil of water, discussed by Farmer (1978) in the companion volume to this book, and in ion and ligand exchange reactions, and in mass flow and diffusion discussed in preceding chapters. The term colloidal refers to particles and discontinuities of matter (e.g. pore spaces or thin films) of dimensions in the range of 1 nm to 1 jxm (IUPAC, 1972). For such finely divided systems the properties of the interfaces are very important because these largely control the behaviour of the colloidal systems. The term phase is used to describe the homogeneous parts of a system and an interface or surface is the boundary region between any two phases; the relevant interfaces are solid/gas, solid/liquid, liquid/liquid, and liquid/vapour. Where none of the solid, liquid, or vapour phases in a system are finely divided the ratios of the interfacial areas to the bulk volumes of the phases are small, and the effects of the interface between neighbouring phases can be neglected. The most extensive surfaces in soils are contained in domains and micro- aggregates of colloidal particles (cf. Greenland and Hayes, 1978; Greenland and Mott, 1978), These surfaces are accessible from the pore spaces between solid particles. Under normal field conditions these spaces remain filled with water. Thus, although modern fertilizers, and chemicals for the control of plant growth, pests, and diseases are applied to soils as solids, liquids, and gases, most of the interactions with the soil colloidal constituents take place at the solid/liquid interface. We shall be primarily concerned here with uptake from the liquid phase by solid soil components, although interactions at the solid/gas interface can sometimes be important.

221 - 400 (180 Pages)
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7 Oxidation and Reduction
D. L. Rowell

7.1 INTRODUCTION An understanding of the processes of oxidation and reduction in soils and the associated changes and practical implications requires a knowledge firstly of soil respiration and the movement of O2 and CO2 in soils, and secondly an appreciation of the significance of the changes which occur when, as a result of the use of all the O2 in a given zone of the soil, respiration changes from aerobic to anaerobic. This results in marked changes in the biochemistry of the system. A basic knowledge is therefore needed of the microbiology and biochemistry of aerobic and anaerobic respiration and of the physics of gas movement, as well as of the chemistry of the system.

401 - 462 (62 Pages)
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8 The Translocation of Metals in Soils
C. Bloomfield

8.1 INTRODUCTION The development of a soil profile involves the mobilization of major and minor elements and their subsequent deposition elsewhere in the profile or removal in the drainage water. For a full understanding of these processes we need to know what are the forms and reactivities of the soil constituents, the nature and mode of action of the mobilizing agents, the forms in which the mobilized material is transported—whether colloidal or in true solution, whether in ionic or complex form, whether alone or in association with other soil constituents—as well as its stability towards oxidation and reduction, microbial action, pH, and sorption and coprecipitation. The availability of micronutrients to plants is presumably governed by factors such as these. The current concern with problems of soil contamination has brought to the attention of soil chemists numerous toxic metals that were previously regarded as essentially the province of the geochemist. It is probably true to say that no one of the factors listed above can be considered in isolation, but so far as any one factor is of dominant importance, this is nature of the mobilizing species; this determines the form in which the metal is transported, and its ultimate fate in the soil or in the drainage water. This chapter is an attempt to summarize our present knowledge of the processes responsible for the mobilization and immobilization of metals in soils.

463 - 504 (42 Pages)
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9 The fate of plant and animal residues in soil
D. S. Jenkinson

9.1 INTRODUCTION The annual production of organic carbon by photosynthesis on land is about 6x 1010 tonnes (Table 9.1), most of which eventually decomposes on or in the top 50 cm of the soil. This organic matter can be in lumps as big as a mature oak tree (say 0.5xl02m3) or as small as a bacterium (0.5 x 10~18m3). It can be in material as resistant to decay as oil palm nuts, which persist for years in soil in the humid tropics, or as transient as mammalian protein. Yet in the long run, no fraction of the organic matter in plants and animals can withstand decomposition to carbon dioxide and water. If this were not so, any completely resistant fraction would by now cover the surface of the earth. Although under normal conditions organic matter does not accumulate indefinitely in soil, exceptions can occur, as, for example, when a soil becomes permanently waterlogged. When a geologically stable surface has been under the same vegetation for a long time, the soil approaches steady state conditions, with the annual losses of organic matter balancing the annual input. Thereafter the organic matter content of the soil remains constant, until the next vegetational change or geological catastrophe. This process, in which losses and gains proceed simultaneously, is described as turnover and may be defined (for carbon) as ‘the flux of carbon through the organic carbon in a given sample of soil’. Turnover time is then the amount of carbon in the soil system (or part of that system) divided by the annual input of carbon into the system (or part of that system). Turnover and turnover time will be discussed more fully in Section 9.2.5.

505 - 562 (58 Pages)
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10 The Fate of Fertilizers
G.W. Cooke

10.1 INTRODUCTION The fate of plant nutrients in fertilizers added to soils is to be: (1) taken up by crops; (2) retained by the soil; (3) removed in solution in drainage water; (4) lost as a gas; or (5) removed by erosion. These five processes are discussed in this chapter. The purpose of using fertilizers is to increase crop growth and they fail in this unless they are taken up by plants. Table 10.1 gives FAO (1972) data which show that large fertilizer use is restricted to the developed countries which use 65 times the amount used per hectare in developing countries. Within the group of ‘developed’ countries most of North-West Europe uses much fertilizer; outside of Europe large use per hectare is confined to a few countries such as Japan and USA. It is in the developed countries where much fertilizer is applied that efficient uptake of plant nutrients is already important to national economies, and where nutrients lost from agriculture cause eutrophication of natural waters and are regarded as pollutants.

563 - 592 (30 Pages)
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11 The Fate of Heavy Metals
L. H. P. Jones, S. C. Jarvis

11.1 INTRODUCTION The heavy metals which, by definition, are elements having a density greater than five in their elemental form, comprise some 38 elements. However, the term usually, and here, refers to twelve metals that are used and discharged by industry, i.e. Cd, Cr, Co, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Sn, Zn. Most of the present discussion will centre on those metals that may represent potential hazards to plants or animals; these are Cd, Cu, Hg, Ni, Pb, and Zn. The heavy metals are widely distributed in the environment, in soils, in plants and animals and in most of their tissues. The concentrations of individual metals in living tissue are ordinarily very low and must be maintained within narrow limits to permit the optimum biological performance of most organisms. Some heavy metals are essential in trace amounts, namely Co, Cu, Fe, Mn, Mo, Zn for plants and, in addition, Cr, Ni, Sn for animals; Cd, Hg, and Pb, have not been shown to be essential for either plants or animals. Deficiencies of the essential metals have been frequently reported and substantial benefits have arisen from correcting these by addition to the soil and subsequent transfer from soil to plant is for most increasing concern about the entry into soil of some heavy metals and the possible adverse effects that they might have on plants and animals and, in turn, on human health. The main sources of these heavy metals are: (i) urban industrial aerosols, created by combustion of fuels, metal ore refining and other industrial processes; (ii) liquid and solid wastes from animals and man; (iii) mining wastes; or (iv) industrial and agricultural chemicals. Although in some circumstances there are direct atmospheric inputs to plants, addition to the soil and subsequent transfer from soil to plant is for most heavy metals the major route of entry into the living tissues of plants, animals, and man. The fate of heavy metals added to soils, including their mobility and reactions in the soil and their subsequent uptake and distribution in plants, is therefore of critical importance in relation to man’s health.

593 - 620 (28 Pages)
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