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PRINCIPLES OF SOIL CHEMISTRY Fo u r t h E d i t i o n  BOOKS IN SOILS, PLANTS, AND THE ENVIRONMENT Editorial Board Agricultural Engineering Robert M. Peart, University of Florida, Gainesville Crops Mohammad Pe...

PRINCIPLES OF SOIL CHEMISTRY Fo u r t h E d i t i o n  BOOKS IN SOILS, PLANTS, AND THE ENVIRONMENT Editorial Board Agricultural Engineering Robert M. Peart, University of Florida, Gainesville Crops Mohammad Pessarakli, University of Arizona, Tucson Environment Kenneth G. Cassman, University of Nebraska, Lincoln Irrigation and Hydrology Donald R. Nielsen, University of California, Davis Microbiology Jan Dirk van Elsas, Research Institute for Plant Protection, Wageningen, The Netherlands Plants L. David Kuykendall, U.S. Department of Agriculture, Beltsville, Maryland Kenneth B. Marcum, Arizona State University, Tempe Soils Jean-Marc Bollag, Pennsylvania State University, University Park Tsuyoshi Miyazaki, University of Tokyo, Japan Soil Biochemistry, Volume 1, edited by A. D. McLaren and G. H. Peterson Soil Biochemistry, Volume 2, edited by A. D. McLaren and J. Skujins Soil Biochemistry, Volume 3, edited by E. A. Paul and A. D. McLaren Soil Biochemistry, Volume 4, edited by E. A. Paul and A. D. McLaren Soil Biochemistry, Volume 5, edited by E. A. Paul and J. N. Ladd Soil Biochemistry, Volume 6, edited by Jean-Marc Bollag and G. Stotzky Soil Biochemistry, Volume 7, edited by G. Stotzky and Jean-Marc Bollag Soil Biochemistry, Volume 8, edited by Jean-Marc Bollag and G. Stotzky Soil Biochemistry, Volume 9, edited by G. Stotzky and Jean-Marc Bollag Organic Chemicals in the Soil Environment, Volumes 1 and 2,edited by C. A. I. Goring and J. W. Hamaker Humic Substances in the Environment, M. Schnitzer and S. U. Khan Microbial Life in the Soil: An Introduction, T. Hattori Principles of Soil Chemistry, Kim H. Tan Soil Analysis: Instrumental Techniques and Related Procedures, edited by Keith A. Smith Soil Reclamation Processes: Microbiological Analyses and Applications, edited by Robert L. Tate III and Donald A. Klein Symbiotic Nitrogen Fixation Technology, edited by Gerald H. Elkan Soil–Water Interactions: Mechanisms and Applications, Shingo Iwata and Toshio Tabuchi with Benno P. Warkentin Soil Analysis: Modern Instrumental Techniques, Second Edition, edited by Keith A. Smith Soil Analysis: Physical Methods, edited by Keith A. Smith and Chris E. Mullins Growth and Mineral Nutrition of Field Crops, N. K. Fageria, V. C. Baligar, and Charles Allan Jones Semiarid Lands and Deserts: Soil Resource and Reclamation, edited by J. Skujins Plant Roots: The Hidden Half, edited by Yoav Waisel, Amram Eshel, and Uzi Kafkafi Plant Biochemical Regulators, edited by Harold W. Gausman Maximizing Crop Yields, N. K. Fageria Transgenic Plants: Fundamentals and Applications, edited by Andrew Hiatt Soil Microbial Ecology: Applications in Agricultural and Environmental Management, edited by F. Blaine Metting, Jr. Principles of Soil Chemistry: Second Edition, Kim H. Tan Water Flow in Soils, edited by Tsuyoshi Miyazaki Handbook of Plant and Crop Stress, edited by Mohammad Pessarakli Genetic Improvement of Field Crops, edited by Gustavo A. Slafer Agricultural Field Experiments: Design and Analysis, Roger G. Petersen Mechanisms of Plant Growth and Improved Productivity: Modern Approaches, edited by Amarjit S. Basra Selenium in the Environment, edited by W. T. Frankenberger, Jr. and Sally Benson Plant–Environment Interactions, edited by Robert E. Wilkinson Handbook of Plant and Crop Physiology, edited by Mohammad Pessarakli Handbook of Phytoalexin Metabolism and Action, edited by M. Daniel and R. P. Purkayastha Soil–Water Interactions: Mechanisms and Applications, Second Edition, Revised and Expanded, Shingo Iwata, Toshio Tabuchi, and Benno P. Warkentin Stored-Grain Ecosystems, edited by Digvir S. Jayas, Noel D. G. White, and William E. Muir Agrochemicals from Natural Products, edited by C. R. A. Godfrey Seed Development and Germination, edited by Jaime Kigel and Gad Galili Nitrogen Fertilization in the Environment, edited by Peter Edward Bacon Phytohormones in Soils: Microbial Production and Function, William T. Frankenberger, Jr., and Muhammad Arshad Handbook of Weed Management Systems, edited by Albert E. Smith Soil Sampling, Preparation, and Analysis, Kim H. Tan Soil Erosion, Conservation, and Rehabilitation, edited by Menachem Agassi Plant Roots: The Hidden Half, Second Edition, Revised and Expanded, edited by Yoav Waisel, Amram Eshel, and Uzi Kafkafi Photoassimilate Distribution in Plants and Crops: Source–Sink Relationships, edited by Eli Zamski and Arthur A. Schaffer Mass Spectrometry of Soils, edited by Thomas W. Boutton and Shinichi Yamasaki Handbook of Photosynthesis, edited by Mohammad Pessarakli Chemical and Isotopic Groundwater Hydrology: The Applied Approach, Second Edition, Revised and Expanded, Emanuel Mazor Fauna in Soil Ecosystems: Recycling Processes, Nutrient Fluxes, and Agricultural Production, edited by Gero Benckiser Soil and Plant Analysis in Sustainable Agriculture and Environment, edited by Teresa Hood and J. Benton Jones, Jr. Seeds Handbook: Biology, Production, Processing, and Storage, B. B. Desai, P. M. Kotecha, and D. K. Salunkhe Modern Soil Microbiology, edited by J. D. van Elsas, J. T. Trevors, and E. M. H. Wellington Growth and Mineral Nutrition of Field Crops: Second Edition, N. K. Fageria, V. C. Baligar, and Charles Allan Jones Fungal Pathogenesis in Plants and Crops: Molecular Biology and Host Defense Mechanisms, P. Vidhyasekaran Plant Pathogen Detection and Disease Diagnosis, P. Narayanasamy Agricultural Systems Modeling and Simulation, edited by Robert M. Peart and R. Bruce Curry Agricultural Biotechnology, edited by Arie Altman Plant–Microbe Interactions and Biological Control, edited by Greg J. Boland and L. David Kuykendall Handbook of Soil Conditioners: Substances That Enhance the Physical Properties of Soil, edited by Arthur Wallace and Richard E. Terry Environmental Chemistry of Selenium, edited by William T. Frankenberger, Jr., and Richard A. Engberg Principles of Soil Chemistry: Third Edition, Revised and Expanded, Kim H. Tan Sulfur in the Environment, edited by Douglas G. Maynard Soil–Machine Interactions: A Finite Element Perspective, edited by Jie Shen and Radhey Lal Kushwaha Mycotoxins in Agriculture and Food Safety, edited by Kaushal K. Sinha and Deepak Bhatnagar Plant Amino Acids: Biochemistry and Biotechnology, edited by Bijay K. Singh Handbook of Functional Plant Ecology, edited by Francisco I. Pugnaire and Fernando Valladares Handbook of Plant and Crop Stress: Second Edition, Revised and Expanded, edited by Mohammad Pessarakli Plant Responses to Environmental Stresses: From Phytohormones to Genome Reorganization, edited by H. R. Lerner Handbook of Pest Management, edited by John R. Ruberson Microbial Endophytes, edited by Charles W. Bacon and James F. White, Jr. Plant–Environment Interactions: Second Edition, edited by Robert E. Wilkinson Microbial Pest Control, Sushil K. Khetan Soil and Environmental Analysis: Physical Methods, Second Edition, Revised and Expanded, edited by Keith A. Smith and Chris E. Mullins The Rhizosphere: Biochemistry and Organic Substances at the Soil–Plant Interface, Roberto Pinton, Zeno Varanini, and Paolo Nannipieri Woody Plants and Woody Plant Management: Ecology, Safety, and Environmental Impact, Rodney W. Bovey Metals in the Environment, M. N. V. Prasad Plant Pathogen Detection and Disease Diagnosis: Second Edition, Revised and Expanded, P. Narayanasamy Handbook of Plant and Crop Physiology: Second Edition, Revised and Expanded, edited by Mohammad Pessarakli Environmental Chemistry of Arsenic, edited by William T. Frankenberger, Jr. Enzymes in the Environment: Activity, Ecology, and Applications, edited by Richard G. Burns and Richard P. Dick Plant Roots: The Hidden Half,Third Edition, Revised and Expanded, edited by Yoav Waisel, Amram Eshel, and Uzi Kafkafi Handbook of Plant Growth: pH as the Master Variable, edited by Zdenko Rengel Biological Control of Major Crop Plant Diseases edited by Samuel S. Gnanamanickam Pesticides in Agriculture and the Environment, edited by Willis B. Wheeler Mathematical Models of Crop Growth and Yield, , Allen R. Overman and Richard Scholtz Plant Biotechnology and Transgenic Plants, edited by Kirsi-Marja Oksman Caldentey and Wolfgang Barz Handbook of Postharvest Technology: Cereals, Fruits, Vegetables,Tea, and Spices, edited by Amalendu Chakraverty, Arun S. Mujumdar, G. S. Vijaya Raghavan, and Hosahalli S. Ramaswamy Handbook of Soil Acidity, edited by Zdenko Rengel Humic Matter in Soil and the Environment: Principles and Controversies, edited by Kim H. Tan Molecular Host Plant Resistance to Pests, edited by S. Sadasivam and B. Thayumanayan Soil and Environmental Analysis: Modern Instrumental Techniques,Third Edition, edited by Keith A. Smith and Malcolm S. Cresser Chemical and Isotopic Groundwater Hydrology,Third Edition, edited by Emanuel Mazor Agricultural Systems Management: Optimizing Efficiency and Performance, edited by Robert M. Peart and W. David Shoup Physiology and Biotechnology Integration for Plant Breeding, edited by Henry T. Nguyen and Abraham Blum Global Water Dynamics: Shallow and Deep Groundwater: Petroleum Hydrology: Hydrothermal Fluids, and Landscaping, edited by Emanuel Mazor Principles of Soil Physics, edited by Rattan Lal Seeds Handbook: Biology, Production, Processing, and Storage, Second Edition, Babasaheb B. Desai Field Sampling: Principles and Practices in Environmental Analysis, edited by Alfred R. Conklin Sustainable Agriculture and the International Rice-Wheat System, edited by Rattan Lal, Peter R. Hobbs, Norman Uphoff, and David O. Hansen Plant Toxicology, Fourth Edition, edited by Bertold Hock and Erich F. Elstner Drought and Water Crises: Science,Technology, and Management Issues, edited by Donald A. Wilhite Soil Sampling, Preparation, and Analysis, Second Edition, Kim H. Tan Climate Change and Global Food Security, edited by Rattan Lal, Norman Uphoff, B. A. Stewart, and David O. Hansen Handbook of Photosynthesis, Second Edition, edited by Mohammad Pessarakli Environmental Soil-Landscape Modeling: Geographic Information Technologies and Pedometrics, edited by Sabine Grunwald Water Flow In Soils, Second Edition, Tsuyoshi Miyazaki Biological Approaches to Sustainable Soil Systems, edited by Norman Uphoff, Andrew S. Ball, Erick Fernandes, Hans Herren, Olivier Husson, Mark Laing, Cheryl Palm, Jules Pretty, Pedro Sanchez, Nteranya Sanginga, and Janice Thies Plant–Environment Interactions,Third Edition, edited by Bingru Huang Biodiversity In Agricultural Production Systems, edited by Gero Benckiser and Sylvia Schnell Organic Production and Use of Alternative Crops, Franc Bavec and Martina Bavec Handbook of Plant Nutrition, edited by Allen V. Barker and David J. Pilbeam Modern Soil Microbiology, Second Edition, edited by Jan Dirk van Elsas, Janet K. Jansson, and Jack T. Trevors Functional Plant Ecology, Second Edition, edited by Francisco I. Pugnaire and Fernando Valladares Fungal Pathogenesis in Plants and Crops: Molecular Biology and Host Defense Mechanisms Second Edition, P. Vidhyasekaran Handbook of Turfgrass Management and Physiology, edited by Mohammad Pessarakli Soils in the Humid Tropics and Monsoon Region of Indonesia, Kim H. Tan Handbook of Agricultural Geophysics, edited by Barry J. Allred, Jeffrey J. Daniels, and M. Reza Ehsani Environmental Soil Science,Third Edition, Kim H. Tan PRINCIPLES OF SOIL CHEMISTRY Fo u r t h E d i t i o n Kim H. Tan University of Georgia Athens, Georgia, U.S.A. Boca Raton London New York CRC Press is an imprint of the Taylor & Francis Group, an informa business CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2011 by Taylor and Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4398-1395-9 (Ebook-PDF) This book contains information obtained from authentic and highly regarded sources. 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Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Contents Preface to the Fourth Edition...........................................................................................................xix Acknowledgments...........................................................................................................................xxv Author...........................................................................................................................................xxvii Chapter 1 Definitions and Concepts of Soil Chemistry.................................................................1 1.1 The Issue of Soil Chemistry...............................................................................1 1.2 Definition of Chemistry......................................................................................2 1.2.1 Analytical Chemistry............................................................................2 1.2.2 Biochemistry.........................................................................................2 1.2.3 Geochemistry........................................................................................3 1.2.4 Inorganic Chemistry..............................................................................3 1.2.5 Organic Chemistry................................................................................3 1.2.6 Physical Chemistry................................................................................3 1.3 The Concept of Soil Physical Chemistry...........................................................3 1.4 The Concept of Geochemistry...........................................................................4 1.5 The Concept of Soil Chemistry..........................................................................5 Chapter 2 Overview of Basic Chemical Principles and Units.......................................................7 2.1 Atom and Atomic Structures..............................................................................7 2.2 Particle Accelerators or Synchrotrons................................................................8 2.2.1 The Stanford Linear Accelerator...........................................................8 2.2.2 The Continuous Electron Beam Accelerator and ELIC........................8 2.2.3 The Fermi National Laboratory Accelerator.........................................9 2.2.4 The Brookhaven Realistic Heavy Ion Collider.....................................9 2.2.5 The CERN Tevatron..............................................................................9 2.3 Elementary Particles......................................................................................... 10 2.3.1 Quarks................................................................................................. 11 2.3.2 Leptons................................................................................................ 11 2.3.2.1 Electrons.............................................................................. 11 2.3.2.2 Neutrinos............................................................................. 11 2.3.3 Force Carriers: Bosons and Mesons.................................................... 13 2.4 String Theory................................................................................................... 13 2.5 The Atomic Model of Rutherford and Bohr..................................................... 14 2.6 Mass and Weight.............................................................................................. 15 2.6.1 Mass.................................................................................................... 15 2.6.2 Weight................................................................................................. 15 2.7 Atomic Mass and Atomic Mass Number......................................................... 15 2.7.1 Atomic Mass Unit or Dalton............................................................... 16 2.8 Atomic Numbers.............................................................................................. 17 2.9 Atomic Weights................................................................................................ 17 2.10 Avogadro’s Number.......................................................................................... 17 2.11 Atomic Orbitals................................................................................................ 18 2.12 Atomic Radius.................................................................................................. 18 2.13 Valence............................................................................................................. 18 ix x Contents 2.14 Equivalent Weight............................................................................................ 19 2.14.1 Acid–Base Titrations........................................................................... 19 2.14.2 Precipitation and Complex Reactions.................................................20 2.14.3 Oxidation–Reduction Reactions..........................................................20 2.15 Chemical Units................................................................................................. 21 2.15.1 Normality............................................................................................ 21 2.15.2 Molarity, Molality, and Formality....................................................... 21 2.15.3 Mole Fractions..................................................................................... 22 2.16 Isotopes............................................................................................................. 22 2.17 Radioactivity..................................................................................................... 23 2.18 Half-Life of Radioactive Material and Carbon Dating....................................24 Chapter 3 Soil Composition and Electrochemical Potentials......................................................25 3.1 Soil Composition..............................................................................................25 3.2 The Concept of Electrochemical Potentials.....................................................26 3.2.1 Electrochemical Cells and Electrode Potentials................................. 27 3.3 The Nernst Equation.........................................................................................28 3.4 Electrode Potentials in Redox Reactions..........................................................28 3.5 Electron Activity.............................................................................................. 31 3.5.1 Electron Activity in the Biochemical Cycle........................................ 32 3.5.2 The Relationship between Electron Activity and Electrode Potential............................................................................................... 33 3.6 The rH Concept................................................................................................34 3.6.1 The Relationship between rH and pe + pH......................................... 35 3.7 Chemical Potential........................................................................................... 36 3.8 Electrochemical Potential................................................................................. 37 3.9 Membrane or Donnan Potential....................................................................... 38 Chapter 4 Soil Gas and Liquid Phases......................................................................................... 41 4.1 The Gaseous Phase........................................................................................... 41 4.2 Composition of Soil Air................................................................................... 41 4.2.1 Aerobic Respiration and Decomposition............................................ 41 4.2.1.1 Aerobic Respiration of Plant Roots and Soil Organisms............................................................................ 42 4.2.1.2 Aerobic Decomposition of Organic Matter......................... 42 4.2.2 Anaerobic Respiration and Decomposition......................................... 42 4.2.2.1 Anaerobic Respiration......................................................... 42 4.2.2.2 Anaerobic Decomposition................................................... 43 4.3 The Oxygen Revolution.................................................................................... 43 4.4 Soil Aeration or Aerification............................................................................44 4.4.1 Mass Flow...........................................................................................44 4.4.2 Diffusion............................................................................................. 45 4.5 Characterization of Soil Air Quality................................................................ 45 4.5.1 Determination of O2 Concentration.................................................... 45 4.5.2 Determination of the Oxygen Diffusion Rate (ODR)......................... 45 4.5.3 Determination of the Redox Potential.................................................46 4.6 Humidity of Soil Air........................................................................................46 4.6.1 Relative Humidity...............................................................................46 4.6.2 Absolute Humidity.............................................................................. 47 Contents xi 4.6.3 Specific Humidity................................................................................ 47 4.7 Hydrotropism.................................................................................................... 48 4.8 The Liquid Phase.............................................................................................. 48 4.8.1 The Concept and Importance of the Soil Solution.............................. 48 4.8.2 Approximations of Soil Solutions....................................................... 48 4.8.3 The Effect of the Gas Phase on the Soil Solution............................... 49 4.8.4 Hypoxia............................................................................................... 50 4.9 Chemistry of Soil Water................................................................................... 50 4.10 Oxygen Demand of Water................................................................................ 52 4.10.1 The Importance of Oxygen Demand in the Environment.................. 53 4.11 Soil Water Energy Concepts............................................................................. 53 4.11.1 Water Potential (Ψw)............................................................................ 54 4.11.2 The Relationship of Water Potential and Chemical Potential of Water................................................................................................... 54 4.11.3 Total Soil Water Potential (Ψt)............................................................ 55 4.11.4 Matric Potential (Ψm).......................................................................... 55 4.11.5 Pressure Potential (Ψp)........................................................................ 56 4.11.6 Osmotic Potential (Ψo)......................................................................... 56 4.11.7 Gravitational Potential (Ψg)................................................................. 56 4.11.8 Units of Soil Water Potential............................................................... 56 4.11.9 The Relationship between Ψw , Water Content, and Relative Humidity............................................................................................. 57 4.12 The Plant–Soil–Water Energy Relation............................................................ 58 4.13 The Law of Mass Action and Equilibrium Constant....................................... 59 4.14 Solubility Products........................................................................................... 61 4.15 The Dissociation of Water................................................................................ 62 4.16 The Dissociation of Strong Electrolytes........................................................... 62 4.17 The Dissociation of Weak Electrolytes............................................................ 63 4.18 The Henderson−Hasselbalch Equation............................................................64 4.18.1 Application of the Henderson–Hasselbalch Concept.......................... 65 4.19 The Equilibrium Constant and Ion Pairs.......................................................... 65 4.20 The Exchange Constant and Ion Exchange...................................................... 67 4.21 The Relationship between the Equilibrium Constant and Cell or Electrode Potential........................................................................................... 67 4.22 The Equilibrium Constant and Free Energy Relationship............................... 68 4.23 The Equilibrium Constant and Electron Activity............................................ 69 4.24 Activity and Standard State.............................................................................. 70 4.25 The Debye–Hückel Theory and Activity Coefficients..................................... 71 4.26 Ionic Strength................................................................................................... 72 Chapter 5 Colloidal Chemistry of Organic Soil Constituents..................................................... 75 5.1 The Colloidal System....................................................................................... 75 5.2 The Organic Components................................................................................. 76 5.3 Soil Humus....................................................................................................... 77 5.3.1 The Issue of Glomalin in Humus........................................................ 78 5.4 Carbohydrates................................................................................................... 78 5.4.1 Properties of Carbohydrates and Their Accumulation in Soils..........80 5.4.2 The Effect of Carbohydrates on Soil Properties................................. 81 5.4.3 The Issue of Carbohydrates in Soil Hydrophobicity or Water Repellency........................................................................................... 81 xii Contents 5.4.4 The Importance of Carbohydrates in Human Nutrition...................... 82 5.4.4.1 The Issue of the Glycemic Index......................................... 82 5.4.5 Carbohydrates for Biofuel Production................................................. 82 5.5 Amino Acids, Peptides, and Proteins............................................................... 83 5.5.1 The Colloidal Chemistry of Amino Acids.......................................... 85 5.5.2 The Issue of Amino Acids in Human Health...................................... 86 5.6 Lipids................................................................................................................ 87 5.6.1 Colloidal Chemistry and the Importance of Lipids in Soils............... 88 5.6.2 The Significance of Fatty Acids in Human Health............................. 89 5.7 Nucleic Acids.................................................................................................... 89 5.7.1 Colloidal Properties and the Importance of Nucleic Acids in Soil......................................................................................................90 5.7.1.1 Cation Exchange Reactions of Nucleic Acids......................90 5.8 Lignins..............................................................................................................90 5.8.1 Colloidal Chemistry and the Importance of Lignins in Soil..............92 5.8.2 The Significance of Lignins in Industry and Pharmacology.............. 93 5.9 Humic Matter...................................................................................................94 5.9.1 The Old and New Look of Humic Matter...........................................94 5.9.1.1 The Issue of the Operational Concept of Humic Substances...........................................................................94 5.9.1.2 The Emergence of the Supramolecular Association Concept................................................................................ 95 5.9.2 Types of Humic Matter.......................................................................97 5.9.3 Extraction and Isolation of Humic Substances from Soils and Water...................................................................................................97 5.9.3.1 Extraction of Soil Humic Matter.........................................97 5.9.3.2 Extraction of Aquatic Humic Matter...................................99 5.9.4 Chemical Characterization and Composition.....................................99 5.9.4.1 Elemental Composition........................................................99 5.9.4.2 Total Acidity...................................................................... 101 5.9.4.3 Carboxyl Groups................................................................ 102 5.9.4.4 Hydroxyl Groups............................................................... 103 5.9.5 Colloidal Chemistry and Reactions.................................................. 103 5.9.6 Colloidal Properties and Potentiometric Titrations of Humic Acid................................................................................................... 104 5.9.7 Chromatography of Humic Substances............................................. 105 5.9.7.1 Gel Chromatography......................................................... 106 5.9.7.2 Gas–Liquid Chromatography............................................ 109 5.9.8 Molecular Weights of Humic Compounds........................................ 110 5.9.9 Spectral Characteristics of Humic Compounds................................ 112 5.9.9.1 Ultraviolet and Visible Light Spectrophotometry............. 112 5.9.9.2 Infrared Spectroscopy....................................................... 114 5.9.9.3 Magnetic Resonance Spectroscopy................................... 115 5.9.10 Electron Microscopy of Humic Matter............................................. 121 5.9.11 Structural Chemistry of Humic Acids.............................................. 124 5.9.11.1 The Hypothesis of Schnitzer and Khan (1972).................. 125 5.9.11.2 The Hypothesis of Kononova (1961).................................. 125 5.9.11.3 The Hypothesis of Flaig (1975)......................................... 126 5.9.11.4 The Dimer Concept of Stevenson (1994)........................... 126 5.9.11.5 The Author’s Hypothesis................................................... 128 5.9.11.6 The Supramolecular Assemblage of Piccolo (2002)......... 129 Contents xiii 5.9.11.7 The Issue of Pseudostructures and Molecular Modeling of Humic Matter Structures.............................. 129 5.9.12 The Agricultural, Industrial, and Environmental Importance of Humic Acids...................................................................................... 130 Chapter 6 The Colloidal Chemistry of Inorganic Soil Constituents.......................................... 133 6.1 The Clay Fraction of Soils.............................................................................. 133 6.2 The Structural Chemistry of Clay Materials.................................................. 135 6.2.1 The Unit Cell and Crystal Lattice..................................................... 135 6.2.2 The Structure of Silicate Clays......................................................... 136 6.2.3 The Kaolinite Group (1:1 Layer Clays)............................................. 138 6.2.4 Halloysite (1:1 Layer Clay)................................................................ 139 6.2.5 The Smectite Group (Expanding 2:1 Layer Clays)........................... 141 6.2.6 Illites (Nonexpanding 2:1 Layer Clays)............................................. 143 6.2.7 Vermiculites...................................................................................... 144 6.2.8 Chlorites (2:2 Layer Clays)................................................................ 145 6.2.9 Mixed-Layer Clays............................................................................ 146 6.2.10 Silica Minerals.................................................................................. 147 6.2.11 Iron and Aluminum Hydrous Oxide Clays....................................... 148 6.2.12 Amorphous Clays, Allophane, and Imogolite................................... 151 6.3 The Identification of Clay Minerals............................................................... 155 6.3.1 Differential Thermal Analysis (DTA)............................................... 155 6.3.2 X-Ray Diffraction Analysis............................................................... 158 6.3.3 Infrared Spectroscopy....................................................................... 161 6.4 The Surface Chemistry of Soil Clays............................................................. 167 6.5 Surface Areas................................................................................................. 169 6.5.1 Total Surface Areas........................................................................... 169 6.5.2 The Specific Surface Area................................................................ 170 6.6 The Origin of Negative Charges in Soil Clays............................................... 171 6.6.1 Isomorphous Substitution.................................................................. 171 6.6.2 The Dissociation of Exposed Hydroxyl Groups................................ 172 6.7 Positive Charges and Zero Point of Charge.................................................... 173 6.8 The Use of ΔpH in the Determination of Negative or Positive Charges........ 174 6.9 Surface Potential............................................................................................. 174 6.10 Electric Double Layer..................................................................................... 175 6.10.1 The Helmholtz Double-Layer Theory............................................... 175 6.10.2 The Gouy–Chapman Double-Layer Theory..................................... 176 6.10.3 The Effect of Electrolytes on the Thickness of the Diffuse Double Layer..................................................................................... 177 6.10.4 The Effect of Valence of Cations on the Thickness of the Diffuse Double Layer........................................................................ 177 6.10.5 Limitations to the Gouy–Chapman Diffuse Double-Layer Theory............................................................................................... 178 6.10.6 The Stern Double-Layer Theory....................................................... 178 6.10.7 The Triple-Layer Theory................................................................... 178 6.11 The Zeta (ζ) Potential..................................................................................... 179 6.11.1 The Effect of Electrolytes on the Zeta (ζ) Potential......................... 180 6.12 The Electric Double Layer and Stability of Clays.......................................... 180 6.13 The Effect of Flocculation and Dispersion on Plant Growth......................... 181 xiv Contents Chapter 7 Adsorption in Soils.................................................................................................... 183 7.1 Types of Adsorption....................................................................................... 183 7.1.1 Positive and Negative Adsorption..................................................... 183 7.1.2 Specific and Nonspecific Adsorption................................................ 184 7.2 Adsorption Characteristics............................................................................. 185 7.3 Forces of Adsorption...................................................................................... 186 7.3.1 Physical Forces.................................................................................. 186 7.3.2 Chemical Forces................................................................................ 186 7.3.3 Hydrogen Bonding............................................................................ 187 7.3.4 Hydrophobic Bonding....................................................................... 187 7.3.5 Electrostatic Bonding........................................................................ 187 7.3.6 Coordination Reactions..................................................................... 187 7.3.7 Ligand Exchanges............................................................................. 188 7.4 Adsorption Isotherms..................................................................................... 188 7.4.1 The Freundlich Equation................................................................... 189 7.4.2 The Langmuir Equation.................................................................... 190 7.4.3 BET and GAB Equations.................................................................. 191 7.4.4 The Gibbs Equation........................................................................... 192 7.5 Adsorption of Water....................................................................................... 193 7.5.1 Adsorption of Water by Silicate and Hydrous Oxide Clays.............. 193 7.5.2 Adsorption of Water by Organic Matter........................................... 195 7.6 The Plant–Soil–Water Energy Relation.......................................................... 197 7.7 Adsorption of Organic Compounds............................................................... 198 7.7.1 Physicochemical Properties of Organic Substances That Influence Adsorption......................................................................... 199 7.7.2 Interlayer Adsorption and Molecular Orientation of Organic Compounds........................................................................................200 7.7.3 The Effect of Molecular Size on Adsorption.................................... 201 7.7.4 The Nature of Adsorption Isotherms................................................202 7.8 Interparticle Attraction...................................................................................203 Chapter 8 Cation Exchange........................................................................................................205 8.1 Adsorption of Cations by Soil Colloids..........................................................205 8.2 Cation Exchange Reactions............................................................................206 8.3 Cation Exchange Capacity..............................................................................208 8.3.1 Types of Cation Exchange Capacity.................................................. 210 8.4 The Exchanging Powers of Cations................................................................ 210 8.5 The Ionic Composition of the Exchange Complex......................................... 210 8.6 Empirical Equations of Cation Exchange...................................................... 212 8.6.1 The Freundlich Equation................................................................... 212 8.6.2 The Langmuir–Vageler Equation...................................................... 212 8.7 Mass Action Law Equations of Cation Exchange.......................................... 213 8.7.1 The Kerr and Gapon Equation.......................................................... 213 8.7.1.1 The Mono-Monovalent Cation Exchange Reaction........... 213 8.7.1.2 The Mono-Divalent Cation Exchange Reaction................ 214 8.7.2 The Vanselow Equation..................................................................... 214 8.8 Kinetic Equations of Cation Exchange.......................................................... 214 8.9 Thermodynamic Equations of Cation Exchange............................................ 215 8.9.1 The Quantity/Intensity (Q/I) Relation............................................... 216 Contents xv 8.10 The Cation Exchange Equation Based on the Donnan Theory...................... 218 8.10.1 The Donnan Equilibrium Law.......................................................... 218 8.10.2 The Donnan Equation of Cation Exchange....................................... 218 8.11 The Cation Exchange Equation of Eriksson.................................................. 220 8.12 The Cation Exchange Equation According to the Diffuse Double-Layer Theory............................................................................................................ 220 8.13 Schofield’s Ratio Law..................................................................................... 220 8.14 Fixation of Cations......................................................................................... 222 8.15 Base Saturation............................................................................................... 223 8.16 Adsorption and Exchange of Cations by Soil Organic Matter.......................224 Chapter 9 Anion Exchange........................................................................................................ 225 9.1 Positive Charges............................................................................................. 225 9.2 Adsorption of Anions by Soil Colloids.......................................................... 226 9.2.1 Nonspecific Adsorption..................................................................... 227 9.2.1.1 Negative Adsorption.......................................................... 227 9.2.1.2 Positive Adsorption............................................................ 228 9.2.2 Specific Adsorption........................................................................... 229 9.3 Phosphate Retention and Fixation.................................................................. 230 9.3.1 Phosphate Retention.......................................................................... 230 9.3.2 Phosphate Fixation in Acid Soils...................................................... 231 9.3.3 Phosphate Fixation in Alkaline Soils................................................ 233 9.4 Biological Fixation of Phosphate.................................................................... 233 9.4.1 Immobilization of Phosphates........................................................... 235 9.4.2 The Interaction of Phosphates with Humic Acids and Other Organic Acids.................................................................................... 236 9.5 Soil Reaction and Availability of Inorganic Phosphates................................ 237 9.6 Prediction of Phosphate Ion Concentration According to Schofield’s Ratio Law....................................................................................................... 239 9.7 The Phosphate Potential.................................................................................240 9.7.1 The Chemical Potential of Phosphate...............................................240 9.7.2 The Electrochemical Potential of Phosphate....................................240 9.7.3 Schofield’s Phosphate Potential......................................................... 241 Chapter 10 Soil Reaction............................................................................................................. 243 10.1 Definition and Importance............................................................................. 243 10.2 Acid–Base Chemistry.....................................................................................244 10.2.1 The Arrhenius Concept.....................................................................244 10.2.2 The Brønsted–Lowry Concept..........................................................244 10.2.3 The Lewis Concept........................................................................... 245 10.2.4 The Usanovich Concept....................................................................246 10.2.5 The Lux and Flood Concept..............................................................246 10.2.6 The Pearson Concept.........................................................................246 10.3 Application in Soils........................................................................................246 10.4 Formulation of Soil Acidity and Alkalinity................................................... 247 10.4.1 Soil pH............................................................................................... 247 10.4.2 Acidity Constant................................................................................248 10.4.3 Acid Strength and Ion Pairs..............................................................248 10.5 Concepts of Soil Acidity................................................................................. 249 xvi Contents 10.5.1 Soil pH Range................................................................................... 249 10.5.2 Active versus Potential Acidity......................................................... 250 10.5.3 Nonselective and Preferential Adsorption of Hydrogen Ions by Soils................................................................................................... 251 10.6 The Role of Aluminum in Soil Acidity.......................................................... 252 10.7 The Role of Fertilizers, Sulfur, Pyrite, N Fixation, and Acid Rain in Soil Acidity............................................................................................................ 254 10.7.1 Ammonium Fertilizers...................................................................... 254 10.7.1.1 Equivalent Acidity and Acidity Index............................... 255 10.7.2 Phosphate Fertilizers......................................................................... 256 10.7.3 Elemental Sulfur............................................................................... 256 10.7.4 Pyrite (FeS2)...................................................................................... 256 10.7.5 Biological Nitrogen Fixation............................................................. 257 10.7.6 Acid Precipitation.............................................................................. 257 10.8 Buffering Capacity of Soils............................................................................ 259 10.9 Electrometric Measurement of Soil pH.......................................................... 259 10.9.1 Measurement of pH in Water Extract................................................ 261 10.9.2 Measurement of pH in Soil Suspension............................................ 261 10.9.3 Measurement of pH in a KCl Solution.............................................. 261 10.9.4 Measurement of pH in a CaCl2 Solution........................................... 261 10.10 Suspension Effect in Soil pH Measurement................................................... 261 10.11 Lime Potential................................................................................................ 262 10.12 The Need for Acidic Soil Reactions............................................................... 262 10.13 Soil Reactions in Saline and Sodic Soils........................................................ 262 10.13.1 Salinization........................................................................................ 262 10.13.2 Sodication and Alkalinization........................................................... 263 10.14 Chemical Characterization of Saline and Sodic Soils................................... 263 10.15 Effect of Salinization and Sodication on Plant Growth.................................264 10.16 Irrigation of Saline and Sodic Soils............................................................... 265 10.16.1 Salinity Hazards................................................................................ 265 10.16.2 Hazards of Sodication....................................................................... 265 10.17 Salt Balance and Leaching Ratio...................................................................266 10.18 Irrigation-Induced Salinization and Sodication.............................................266 Chapter 11 Soil Chemistry and Soil Formation........................................................................... 267 11.1 Concepts of Weathering................................................................................. 267 11.2 Weathering Processes..................................................................................... 268 11.3 Stability and Weathering of Soil Minerals..................................................... 269 11.3.1 Crystal Chemistry and Mineral Properties....................................... 269 11.3.2 Coordination Theory and Pauling’s Rules........................................ 269 11.3.3 Stability of Minerals and Bond Strength.......................................... 272 11.4 Weathering of Feldspars and the Silica Potential........................................... 274 11.5 Weathering of Kaolinite and the Gibbsite Potential....................................... 274 11.6 Stability and Phase Relationships of Soil Minerals....................................... 275 11.7 Biological and Biochemical Weathering........................................................ 276 11.7.1 Biological Weathering....................................................................... 276 11.7.2 Biochemical Weathering................................................................... 278 11.8 Soil Formation Processes or Pedogenesis...................................................... 279 11.8.1 Desilicification.................................................................................. 279 11.8.2 Translocation of Clays....................................................................... 281 Contents xvii 11.8.3 Translocation of Aluminum and Iron................................................ 282 11.9 Oxidation and Reduction Reaction in Soils................................................... 285 11.9.1 Redox Potentials................................................................................ 286 11.9.2 Redox Reaction Limits in Soils and the Eh−pH Relation.................. 286 11.9.3 The pe–pH Relation.......................................................................... 288 11.9.4 The Importance of Eh–pH or pe–pH Relations in Soil and Biological Systems............................................................................ 288 11.9.5 Constancy in Eh–pH or pe–pH Relations.......................................... 289 11.9.6 Application of Redox Potentials in Soils........................................... 289 11.9.7 Stability of Iron Oxides and Hydroxides........................................... 291 11.9.8 Activity of Reduction Products......................................................... 292 Chapter 12 Chemistry of Soil–Organic Matter Interactions....................................................... 293 12.1 Complex Formation and Chelation................................................................. 293 12.1.1 Complex Formation........................................................................... 293 12.1.2 Chelation........................................................................................... 294 12.2 Interactions with Soil Organic Compounds................................................... 295 12.2.1 Effectiveness of Organic Acids in Chelation.................................... 295 12.3 Metal–Organic Complex Reactions............................................................... 296 12.3.1 Stability Constants............................................................................ 296 12.4 Clay–Organic Compound Complexes............................................................ 298 12.4.1 Complex Formation with Organic Anions........................................ 298 12.4.1.1 Positive and Negative Adsorption...................................... 298 12.4.1.2 Metal and Water Bridging................................................. 299 12.4.1.3 Van der Waals Force in the Complexation of Anions....... 299 12.4.2 Complex Formation with Organic Cations....................................... 299 12.4.3 Complex Formation with Amphoteric Organic Compounds............ 301 12.4.4 Complex Formation with Nonionic Organic Compounds................302 12.5 Complex Formation and Mobility of Soil Constituents................................. 303 12.5.1 Metal Mobility.................................................................................. 303 12.6 Stability Diagram of Metal Chelates..............................................................304 12.7 Complex Formation and Clay Mobility.......................................................... 305 12.8 Complex Formation and Soil Fertility............................................................ 305 12.8.1 Complex Formation and Soil Physical Characteristics..................... 305 12.8.2 Complex Formation and Soil Chemical Characteristics...................306 12.8.3 Complex Formation and Soil Biological Characteristics..................307 12.8.4 Chelation and Complexation in Industry and Medicines..................309 Appendix A: Fundamental Constants........................................................................................ 311 Appendix B: Greek Alphabet...................................................................................................... 313 Appendix C: Periodic Classification of Elements...................................................................... 315 Appendix D: X-Ray Diffraction 2θ d-Spacing Conversion....................................................... 317 Appendix E: System International (SI) Units............................................................................ 319 Appendix F: Factors for Converting U.S. Units into System International (SI) Units........... 321 References and Additional Readings.......................................................................................... 323 Index............................................................................................................................................... 343 Preface to the Fourth Edition Since the first edition was printed in 1982 by Marcel Dekker, the Principles of Soil Chemistry has not only been used by the author at the University of Georgia, but was also selected for use at vari- ous universities overseas and in the United States for teaching senior-level undergraduate and grad- uate courses in soil chemistry. It is also a helpful reference for professionals and scientists in need of the basics in soil chemistry. It is the main textbook at the Hebrew University, Israel; the Universiti Pertanian Malaysia, Serdang, Malaysia; the Universidad Nacional del Sur, Bahia Blanca, Argentina; and at major universities in India. In addition, it is a required textbook at the University of Andalas and the University of North Sumatra in Indonesia. The first edition was translated into Bahasa Indonesia, the formal Indonesian language, and published by the University of Gajah Mada Press, Yogyakarta, Indonesia, under contract with Marcel Dekker (New York). It was so well received by southeast Asian students, scientists, and professionals whose main language was not English, that soon after its publication the translated version was sold out. This fourth edition reexamines the entire reach of soil chemistry. By completely revising, updat- ing, and incorporating a decade’s worth of new information, this new edition is an entirely new and improved book. Though some reorganization of the chapters has been necessary, the simple writ- ing style has been maintained to reach most of the audience in need of the basics in soil chemistry, which made the previous editions bestsellers overseas. The author’s attempt is to now address the definitions and concepts of soil chemistry, comparing them with geochemistry and physical chemis- try, and names that have been suggested to replace soil chemistry, which are preferred by a number of U.S. scientists. The comments made by a great number of readers have been gladly accepted and incorporated wherever possible, whereas the suggestions launched by reviewers to pay attention to competing books have been taken seriously by addressing differences in concepts and “bad” science to their full extent. Comments and corrections are suggested by the author for inaccuracies and dis- crepancies found in the competing literature. The author addressed these issues in the third edition as differences of opinions, and it was only a matter of caring by critics to read them. The alleged competing books also differ in style and scope with high-level statistics and, as such, are, in the author’s opinion, “companion” books targeting a higher-level audience that should be well-versed in the basics of soil chemistry. The Chapters A new chapter, Chapter 1 examines the origins of soil chemistry by J. T. Way, Van Helmont, Boussingault, and Moses in biblical times. The different names used throughout history, such as agricultural chemistry, agrogeology, and more are addressed, including the rebirth of the modern name soil chemistry, which occurred from 1970 to 1980, and was underlined by the publication of books by Bolt and Bruggenwert (1976) and Bohn et al. (1976). The concepts and definitions of chemistry, geochemistry, physical chemistry, and soil chemistry are examined, and conclude that identifying soil chemistry as geochemistry is like identifying soil science as geology. Chapter 2 is about basic chemical principles and chemical units. The concepts of atoms, sub- atomic particles, mass, and weight are discussed as the underlying principles of chemistry. Major particle accelerators, especially the Fermi National Accelerator Laboratory and CERN tevatrons, are described for creating elementary particles, quarks, leptons, and neutrinos and for finding the force carriers, bosons, and mesons, and, in particular, the elusive graviton. A new bold theory, the string theory, is presented as the basis of the theory of everything, which scientists believe can be connected with the graviton concept. The atomic model of Rutherford and Bohr, atomic mass unit xix xx Preface to the Fourth Edition or Dalton, valence and equivalent weights, isotopes, radioactivity, and half-life, and more are dis- cussed as still representing the current ideas in soil chemistry. Chapter 3 highlights soil composition, underscoring micropedology with the plasma and soil matrix concepts. The underlying view of soils representing electrochemical cells, similar to a bat- tery, forming the basis of discussions of electrochemical potentials. Several types of potentials, electrode, electrochemical, chemical potentials, membrane, or Donnan, are distinguished with the Nernst equation, which plays a pivotal role in their formulation. The relation with electron activity, pe, and the rH concept are provided. Chapter 4 covers the soil gas and liquid phases. Soil air composition is explained as being depen- dent on aerobic and anaerobic respiration and decomposition that results in an imbalance between CO2 and O2 content. Aeration, also called aerification, the method for restoring O2 content, is described as an exchange of soil air for atmospheric air, with mass flow and diffusion affecting the movement of soil air. Oxygen content, oxygen diffusion rate, and redox potentials are examined as factors for determining soil air quality. The oxygen revolution, a new theory, is introduced to show oxygen as a key element in life today. Oxygen was absent some 3 billion years ago when a different earth atmosphere existed, which would have been highly toxic for life as we know it. Soil air humidity is discussed to introduce another new topic, called hydrotropism, a root response to a gradient in relative humidity in soil air, which is of great importance to gravitropism in space agriculture. The second part of Chapter 4 discusses the chemistry of soil water, oxygen demand, water potentials, and soil–plant–water energy issues. The rules and laws of reaction in soil water are presented, based on the Law of Mass Action or the Law of Equilibrium, in reactions involving solubility products, dissociation of water, strong and weak electrolytes, the Henderson–Hasselbalch equation, ion pairs, exchange constant, activity coefficient, the Debye–Hückel theory, and more. The relationship between equilibrium constants, electrode potentials, thermodynamics, and elec- tron activity is addressed. Chapter 5, concerning colloidal chemistry of organic soil constituents, begins with a study of the concepts and definitions of colloids in terms of linear dimension and mass. A definition of soil humus is given and a division suggested into a nonhumified and humified colloidal fraction. The chemistry, reactions, and implications in environmental quality, industry, and human health of carbohydrates, amino acids, lipids, nucleic acids, and lignins, as the major nonhumified components, are explored. The old and new looks of the humified fraction, represented by the operational and supramolecular association concepts, respectively, are examined. Three major types of humic matter are recog- nized—terrestrial, aquatic, and geologic—and the extraction procedures of humic and fulvic acids and their chemical characterization are provided in some detail. Gel, gas–liquid chromatography, visible light and infrared spectroscopy, in addition to electron paramagnetic, nuclear magnetic reso- nance spectroscopy, and electron microscopy of humic acids are discussed as important tools in the identification and explanation of the chemical reactions of humic acids. The chapter closes with a summary of the agricultural, industrial, and environmental significance of humic acids. Chapter 6 examines the colloidal chemistry of inorganic soil components, the clay fractions of soils. Definitions of clay and how its unit cell and crystal lattice are formulated, and how clays are grouped into crystalline and amorphous, paracrystalline, or short-range-order (SRO) clays. The crystal structure, chemistry, reaction, and characterization by infrared spectroscopy, differential thermal and x-ray diffraction analyses, and electron microscopy are presented for the major crys- talline silicate clays and SRO minerals. The newly discovered minerals akaganeite and schwert- mannite are also discussed. Explanations are provided for surface chemistry; surface charges; positive, negative, and zero point charges (ZPCs); and surface potentials. Corrections are proposed for Sposito’s concept on inner- and outersphere charges. The surface chemistry of soil clays is presented, recognizing three important surfaces: (1) the siloxane surface with innersphere cavities too small for the adsorption of water; (2) the oxyhydroxy surfaces; and (3) the group of silanol, aluminol, and ferrol surfaces of significance as outersphere surfaces. Surface areas, designated as total or specific surfaces, are studied in light of their importance in charge distribution and issues Preface to the Fourth Edition xxi of adsorption. The origin of negative and positive charges, major electric double-layer theories, and the zeta potential are presented. An examination of ZPC, classified by some into ZPC, ZPPC (zero point proton charge), and ZPNC (zero point net charge), finds ZPPC and ZPNC to be insignificant. The importance of the electrochemical behavior of clays is evaluated on dispersion and flocculation of soils in light of soil stability and plant growth. Chapter 7 discusses adsorption in soils as consequences of electrochemical behavior, types of surfaces, surface areas, and surface-charge densities of the organic and inorganic soil colloids. The definition of adsorption is presented as reversible, and equilibrium reactions are character- ized by a positive heat of adsorption in contrast to pseudoadsorption. The concepts of positive and negative adsorption are compared with those of specific and nonspecific adsorption. The use of too many names to replace specific and nonspecific adsorption is discussed and the confusion it is causing for many readers, due also to the fact that the concepts proposed by several authors are totally in disagreement with each other. Physical, chemical, and electrochemical forces responsi- ble for adsorption processes are presented, and some, such as hydrophobic bonding, coordination reaction, and ligand exchange, are handled as special issues. Major adsorption isotherms for char- acterizing the behavior of adsorption are discussed and separated into two groups. The first group is based on identification of shape and curvature of isotherms, and the second group is based on statistical formulation (for example, Freundlich, Langmuir, Brunauer, Emmett, and Teller (BET) and Guggenheim, Anderson, and de Boer (GAB), and Gibbs equations). Changing the names of the isotherms to Van Bemmelen–Freundlich and Langmuir–Freundlich equations, based only on the use of different symbols, is presented as confusing the issue. Adsorption of water is discussed on the basis of water potentials and availability to plants, and on the forces of attraction by sili- cate, hydrous oxide clays, and organic matter. The final sections discuss the adsorption of organic matter in view of concerns of increasing pollution due to pesticides, underscoring physicochemi- cal properties, interlayer adsorption, and molecular orientation of the organics. Chapter 8 discusses cation exchange, beginning with definitions and concepts, lyotropic series, and the law of Coulomb governing differences in preferential adsorption and exchange. The con- version of primary minerals, such as orthoclase into albite and more, as postulated by some, is examined as an aberration of science that sends an improper message about cation exchange reac- tions. The cation exchange capacity (CEC) is defined scientifically on the basis of surface area and charge density and practically by analysis and summation of the determined exchangeable cations. Several types of CECs are recognized—CECp, CECv, CECt, and ECEC—and explained. Statistical models or formulas of exchange reactions are classified into empirical, mass action law, kinetic, and thermodynamic equations, which to some extent differ from the ideas of Sparks. The van Laar, Rothmund–Kornfeld, Freundlich, and Langmuir equations, often used by several scientists, are evaluated to be more applicable to adsorption than to exchange reactions. Considering the van Laar equivalent to the Vanselow equation as postulated by Sposito confuses the basic concept proposed by Vanselow. The Donnan equation and Schofield’s ratio law in cation exchange reactions, seldom used in other books, are addressed, and the differences between the Donnan and Kerr equations are discussed. The fixation of cations is emphasized as a temporary nuisance rather than as a harm- ful effect on crop production. Adsorption and cation exchange reactions by soil organic matter are underlined as a special issue related to complex formation and chelation. Chapter 9 relates anion adsorption and exchange by nonspecific and specific adsorption pro- cesses due to the attraction of positive charges and the displacement of lattice O or exposed OH groups on clay mineral surfaces by anions, respectively. Phosphate ions, the most important anion, are highlighted, and differences in specific adsorption, retention, and fixation of phosphate are outlined. Biological fixation is explained as the immobilization of phosphate by plants and microor- ganisms into their cellular constituents and as an interaction of phosphates with organic substances secreted by roots or humic acids synthesized from decomposition products of soil organic mat- ter. The chemical potential and Schofield’s phosphate potential are formulated, and predictions are made of phosphate ion concentration by the application of Schofield’s ratio law. xxii Preface to the Fourth Edition Chapter 10 addresses the soil reaction that is being used to indicate the acid–base reaction in soils. Acids and bases are defined by the Arrhenius, Brønsted–Lowry, and Lewis theories, and new concepts as postulated by Usanovich, Lux–Flood, and Pearson. The acidity constant, defined by using the BrØnsted–Lowry concept, is applied in distinguishing strong from weak acids, whereas the pH is formulated from the H+ ion concentrations. Based on soil pH ranges, soil acidity classes, such as weakly, moderately, and strongly acidic (or basic) soils, are created as the proper terminol- ogy in indicating the acid–base conditions in soils. The use of terms such as acidic pH, neutral pH, and basic pH by a number of scientists is explained as being inappropriate. Acid strength, potential and active acidity, preferential and nonselective adsorption of H+ ions in soils, and the role of Al and fertilizers are addressed. The magnitude of pH changes due to Al concentration is formulated and expressed in terms of aluminum potential (pAl), whereas the degree of acidity created by fertilizers is formulated in terms of equivalent acidity or acidity index. Pyrite, which is of special importance in coastal areas and soils polluted by mine spoil, elemental S, and acid rain are additional high- lighted topics. Canopy interaction, involving through-fall and stem flow, and the effect of biologi- cal N fixation, not realized by scientists, are highlighted. The determination of soil pH in water extracts, soil suspension, and KCl and CaCl2 solutions are discussed in detail, and the formulation of lime potential is presented. Chemical principles of potentiometric measurement of pH involving the Nernst equation are addressed. Soil reactions in saline and sodic soils, differentiating sodication from alkalinization, are examined using the exchangeable Na+ percentage (ESP) and electrical conductivity (EC). The critical EC value and differences among scientists on the sodium adsorption ratio (SAR) in issues of hazards of salinization and sodication are evaluated. Chapter 11 explains the application of soil chemistry in soil formation. Concepts of weather- ing are examined. Differences between geological weathering producing weathering profiles com- posed of layers of disintegrated rocks, saprolite, saprock, and bedrock underlying a strip of soil are contrasted to pedological weathering forming pedons and exhibiting soil profiles. Crystal chemis- try and other mineral properties and Pauling’s rules are examined in stability and in bond strength affecting the disintegration of cyclo-, ino-, neso-, phyllo-, soro-, and tectosilicates. The silica poten- tial and gibbsite potential are formulated to predict the decomposition of feldspars into kaolinite, and kaolinite into gibbsite, respectively. The stability and phase relations of minerals are expressed in stability diagrams. Biological and biochemical weathering are recognized as two separate but closely related processes. Soil formation processes, called pedogenesis, are discussed, and silicifica- tion and desilicification are examined as main processes in poorly drained and well-drained soils, producing smectite and kaolinite, respectively. Silicification is suggested as the proper term, instead of silication as used by many U.S. scientists. To silicify and not to silicate is to impregnate with silica. The formation of argillic, albic, and spodic horizons is discussed as are the oxidation poten- tials invoked in the translocation of Al and Fe. Oxidation and reduction, called redox reactions, and how they affect the formation of all soils are explained. Redox potential and redox limits in soils are formulated, and the pe and pH relation is examined in biological systems. Applications of redox potentials in soils and the stability of iron oxides and hydroxides are addressed. Chapter 12 discusses the chemistry of interactions between soil and organic matter. The con- cepts of complex formation and chelation are clearly delineated with complexation being a metal– ligand coordination process and chelation being an interaction with the ligand donating more than one of its donor functional groups, yielding a chelate ring. Nonhumified—live or dead—organics and humified substances serving as ligands and their effectiveness in complexation and chelation are examined. Organic substances with acidic properties due to the presence of carboxyl groups, –COOH, are capable of complexing only, in contrast to those possessing both carboxyl and pheno- lic–OH groups, which can exert a variety of interactions, from electrostatic attraction, coadsorption, and complexation to chelation. Stability constants, interpreted by several scientists as solubility constants, are formulated using the law of mass action, and the relative ease of dissolving or degree of solubility of the complexes or chelates are evalutated. Interactions between clay and organic substances are examined in light of positive and negative adsorption, metal and water bridging, Preface to the Fourth Edition xxiii and hydrophobic and van der Waals interactions. The interactions between clays and in particular organic anions, both exhibiting negative charges, are addressed. Metal mobility and clay mobil- ity of the complexes and chelates are highlighted, and the effects of the complexes or chelates on biological and biochemical soil properties are explained. Also new to this edition of the book is a discussion of the beneficial application of complexation and chelation in industry, pharmacology, and medical science. Applications that are examined include their use as water softeners, their application in steam engineering, the use of EDTA in cleaning root canals, and a newly developed chelation therapy for the treatment of clogged arteries. Kim H. Tan Acknowledgments The author wishes to thank and acknowledge students and scientists at various universities and institutions for their comments, suggestions, reviews, and contributions to the development of this fourth edition. Many thanks are due to Dr. Robert L. Wershaw, U.S. Geological Survey, Federal Center, Denver, Colorado, for extending his great cooperation in discussing and supplying the most recent information on his micellar concept of humic substances. Thanks are extended to Dr. Peter Leinweber, Institute of Soil Science, University of Rostock, Rostock, Germany, for providing some insights into the new concepts of molecular structures of humic substances. Grateful appreciation is also due to Elsevier Science, Oxford, United Kingdom, and Springer, Boston, Massachusetts, for providing permissions for the reproduction of figures of molecular structures and magnetic resonance spectra of humic substances. Special recognition goes to John Rema, the University of Georgia, Athens, Georgia, for his assistance in computer programming and literature retrieval. Last but not least, my thanks go to my wife Yelli for her encouragement and support. xxv Author Dr. Kim Howard Tan is Professor Emeritus at the Department of Crops and Soil Science, University of Georgia, Athens, Georgia, where he has taught basic soils, soil chemistry, advanced soil chemistry, and methodology of soil and plant analysis for more than 25 years. Since his retirement in 1996, he has been invited to teach soil chemistry and environmen- tal soil science at various universities overseas. He was a Fulbright visiting professor at the Universidad Nacional del Sur, Argentina, from 2002 to 2003. Dr. Tan is widely known for his research on soil formation from volcanic ash and is considered one of the experts on andosols. His humic acid research has made him one of the authorities on humic acid chemistry and organic–inorganic interaction reactions. He was named Fellow of the American Society of Agronomy and the Soil Science Society of America. He is a member of the International Union of Soil Science. Dr. Tan is the recipient of numerous awards and honors for excellence in teaching, such as the University of Georgia—College of Agriculture Alumni Award and the prestigious D. W. Brooks Award. He was named the Outstanding Teacher of the College of Agriculture for a number of con- secutive years since 1972 in college-wide polls conducted by the horticulture and agronomy clubs. Dr. Tan has also established an audio-tutorial enrichment study center where students can learn at their own pace, and he organized the first soil judging team in 1972. He was elected in 1981, as a junior advisor of Alpha Zeta, an honor fraternity in agriculture, by the students. The author or coauthor of more than 200 articles, abstracts, proceedings, and book chapters, as well as the author, coauthor, or editor of 13 books, Tan has served as the Associate Executive Editor of the Communication of Soil Science and Plant Analysis, Journal of Plant Nutrition, Marcel Dekker, and is an International Panel Member of the Malaysian Journal of Soil Science. His most popular book Principles of Soil Chemistry is now in its fourth edition. His book Environmental Soil Science was published in 2009 in its third edition by Taylor & Francis/CRC Press. Andosols, a Hutchinson Ross benchmark book by Van Nostrand Reinhold Co., was sold out as soon as it was published. Dr. Tan received a master’s degree (1955) in agronomy from the University of Indonesia and graduated with a Ph.D. degree (1958) in pedology under Dr. Ir. J. Van Schuylenborgh, past deacon of the subfaculty of the University of Amsterdam, The Netherlands. After postdoctoral training at the North Carolina State University, Raleigh, North Carolina, and Cornell University, Ithaca, New York, under the tutorship of Dr. Ralph McCracken, Dr. Adolf Mehlich, Dr. Marlin Cline, and Dr. Michael Peech, respectively, he returned to the University of Indonesia, where as the head of the Department of Soil Science, he contributed to the founding of the Agricultural University at Bogor, presently known as IPB (Institut Pertanian Bogor). xxvii  1 Definitions and Concepts of Soil Chemistry 1.1 The Issue of Soil Chemistry Soil chemistry is the youngest branch of soil science. The material is at the same level and as important as that of soil physics, soil genesis and taxonomy, soil microbiology, soil fertility, soil mineralogy, and other branches of soil science. It is essential that each of the subdivisions carries a considerable amount of chemistry for explaining and illustrating their respective topics and issues. This dependency on soil chemistry is greater than many soil scientists would have expected and will be discussed in more detail in Section 1.5. Though many scientists consider J. T. Way as the founder of soil chemistry (Sonon et al., 2001; Thomas, 1977), it was, perhaps, Jan Baptista Van Helmont, with his famous willow tree experi- ments in the early 17th century and J. B. Boussingault in 1834, with the discovery of C, H, and O in plant tissue coming from rain and water (Brady, 1990), who truly started soil chemistry. In many respects, the “old pioneers” are indeed important contributors to the development of some aspects of soil chemistry. But, in line with the idea that soil fertility and plant nutrition began with Phillip Carl Sprengel (1787–1859) and Justus Von Liebig (1803–1873), the argument that the early work on adsorption by J. T. Way around 1850 was the start of the science of soil chemistry may have great merits. However, adsorption and exchange reactions were mentioned long before the 1800s, and an example of these processes was stated earlier in the Bible (Rieman and Walton, 1970). In addition, J. T. Way was allegedly known for the discovery of the nitrification process, hence contributing to the emergence of soil microbiology instead. According to Brady (1990), Way allegedly discovered in 1856 that ammonium substances were converted in soils into nitrates, a reaction that 20 years later was demonstrated by R. Warrington to be a microbial process. Sergei Vinogradskii then isolated in 1890 the two bacteria responsible for what we call today nitrification (Ackert, 2006). In the past, a variety of other names have also been used for soil chemistry, such as agricultural chemistry, chem- istry of soil constituents, and colloid chemistry. Agricultural chemistry was introduced perhaps in the beginning of the 19th century by Humphrey Davy (1778–1829) and is considered by Sparks (2006) to be the forerunner of soil chemistry. At one time, close to the middle of the 20th century, even soil analysis, soil fertility, and geochemistry were identified as soil chemistry. The Dutch school was using at that time the term agrogeology, apparently coined from Agricultural Geology, a book published in 1916 by Rastall. Claiming that soils were not the domains of geologists, Rastall indicated that mineralogical composition and petrography (particle size distribution) were of great value in assessing the agricultural values of soils. As a modern science, soil chemistry started to crystallize in the 1970s, and the real giants in soil chemistry are, therefore, not yet known. The book Soil Chemistry, edited by Bolt and Bruggenwert (1976), and another book with the same title by Bohn et al. (1979) have paved the way for carving the identity of modern soil chemistry. A subsequent publication in 1982 of the first edition of the Principles of Soil Chemistry has, in fact, created an unexpected stir, and several other books have since been published in rapid succession. Though carrying different names, such as The Chemistry of Soils, Soil Physical Chemistry, Environmental Soil Chemistry, Soil and Water Chemistry, Soils and Environmental Quality, and Soils and the Environment, these seemingly opposing books 1 2 Principles of Soil Chemistry, Fourth Edition address only certain specialized chapters of soil chemistry. They are different in approach and style, often emphasizing high-powered statistics, seldom used in everyday soil chemistry and hence of academic importance only. However, the message they carry about the chemistry of soils is similar. These books can serve, perhaps, as filling alleged gaps when or if present in the Principles of Soil Chemistry. Such a dispute is not new and is one way science can further advance. Identifying soil chemistry as geochemistry or physical chemistry and the like is like identifying soil science as geological science or geology. Therefore, it is deemed necessary to start the new edition of this book by addressing first the concept of soil chemistry and its relationship with pure chemistry, geochemistry, soil physical chemistry, and the like. 1.2 Definition of Chemistry The science of pure chemistry is defined as “the study of the composition, structure and proper- ties of matter, chemical processes and phenomena and the changes they are bringing to matter or substances” (Merriam-Webster, 2003; Nebergall et al., 1972). The name chemistry is derived from either the Egyptian term kême (meaning “earth”) or the Arabic al-kîmiyâ, inherited from the Greek and French word al-kemie for alchemie, the ancient chemistry of trying to create gold from base metals. As a science, chemistry allegedly began with Muslim scientists and scholars, the Saracens, who performed controlled and precise analyses that were recorded very carefully. Al-kîmiâ, the most scientific of medieval analyses, has been the source of numerous accidental discoveries and hence has contributed to the development of chemical science. It introduced a distinction between alkalis and acids and produced and refined many chemicals and drugs (Durant, 1980). The basics of pure chemistry are concerned with studying atoms, elements, and molecules and their transformation into matter or substances, ions, and salts. Chemical bonding, reactions, and energy transfer are involved in producing these transformations. All these became an established science after the discovery of the law of conservation of mass by Lavoisier (1743–1794), providing a quantitative basis in chemistry. This law indicates that chemical reactions are balanced reactions and occur in equivalent amounts (Nebergall et al., 1972; Rosenberg et al., 2009). With the rapid advancement in the science of chemistry, today many branches or subdisciplines of pure chemistry are recognized, including analytical chemistry, biochemistry, geochemistry, inorganic chemistry, organic chemistry, and physical chemistry. A description of each subdiscipline mentioned is pro- vided below. 1.2.1 Analytical Chemistry Analytical chemistry deals with analyses of inorganic and organic materials to examine their chem- ical composition and structure. The methods may include physical methods, such as spectroscopy and thermal methods. Called physicochemical analyses, the methods are based on measurement of a physical property, but chemical reactions are essential parts of the procedures. The follow- ing example serves as an illustration. In spectrophotometry, the absorption of light by a colored solution, the absorbance, is measured. Chemical reactions are essential parts of the methods to develop the specific color for identification of the element, but the final step is the measurement of the amount of light absorbed by the colored solution, which is purely a measurement of a physical property. For more detail on physicochemical analyses, refer to Tan (2005). 1.2.2 Biochemistry Biochemistry is the study of the chemistry and reactions of carbohydrates, lipids, amino acids, proteins, nucleic acids, nucleotides, lignin, vitamins, hormones, viruses, and other related biocom- pounds that are artificially produced or present in living cells (Lehninger et al., 2008). Definitions and Concepts of Soil Chemistry 3 1.2.3 Geochemistry Geochemistry involves the study of the chemical composition of the earth, and especially of the chemical characteristics of rocks, minerals, lava, and magma, their mutual interactions, as well as their interactions with the hydrosphere and atmosphere (Krauskopf, 2003; Walther, 2009). The min- eral constitution is the basis for the well-known distinction of acid, intermediate