Soil Chemistry

Dr. Daniel G. Strawn is a Professor of Soil Chemistry in the Plant, Soil, and Entomological Sciences Department at the University of Idaho in Moscow. Dr. Hinrich L. Bohn is a Professor Emeritus in the Department of Soil, Water, and Environmental Science at the University of Arizona, Tucson. Dr. George A. O Connor is a Professor in the Soil and Water Science Department at the University of Florida in Gainesville.

Preface xi Acknowledgments xiii 1 Introduction to Soil Chemistry 1 1.1 Historical background 2 1.2 The soil environment 5 1.2.1 Soil chemical and biological interfaces 6 1.2.2 Soil solids 8 1.2.3 Soil interaction with the hydrosphere 10 1.2.4 Interaction of soil and the atmosphere 12 1.3 Chemical reactions in soils 16 1.3.1 Flow of chemical energy in soils 17 1.3.2 Soil chemical speciation 18 1.3.3 Chemical reaction types in soils 18 1.4 Soil biogeochemical cycling 21 1.5 Soil chemical influences on food production 22 1.6 Soils and environmental health 22 1.6.1 Soil chemistry and environmental toxicology 24 1.7 Units in soil chemistry 25 1.7.1 Converting units 25 2 Properties of Elements and Molecules 29 2.1 Introduction 29 2.2 Ionization and ionic charge 29 2.3 Ionic radius 32 2.4 Molecular bonds 33 2.5 The nature of water and hydration of ions 36 3 Characteristics of Chemicals in Soils 41 3.1 Introduction 41 3.2 Occurrence of elements in soils 41 3.3 Essential elements 45 3.3.1 Plant deficiency 46 3.4 Inorganic contaminants in the environment 46 3.4.1 Assessing contamination status of soils 49 3.5 Anthropogenic organic chemicals in the soil environment 49 3.5.1 Pesticides in the environment 51 3.5.2 Chemicals of emerging concern in the environment 53 3.5.3 Chemical factors affecting organic chemical reactions in soil 54 3.6 Properties of the elements in soils 55 3.6.1 Alkali and alkaline earth cations 56 3.6.2 Major soluble anions in soils 58 3.6.3 Weakly soluble anions 60 3.6.4 Poorly soluble metal cations 64 3.6.5 Common toxic elements in soils 65 3.6.6 Major biogeochemical elements carbon, nitrogen, and sulfur 68 4 Soil Water Chemistry 73 4.1 Introduction 73 4.2 Thermodynamic approach to aqueous soil chemistry 74 4.2.1 Types of equilibrium constants 79 4.3 Calculation of ion activity 80 4.3.1 Example calculation of activity coefficient 82 4.4 Acids and bases 83 4.5 Gas dissolution 87 4.6 Precipitation and dissolution reactions 88 4.6.1 Solubility of oxides and hydroxides 89 4.6.2 Calcite and carbon dioxide in soils 90 4.6.3 Solubility of soil minerals 93 4.6.4 Using equilibrium reactions to model soil mineral precipitation and dissolution reactions 95 4.7 Cation hydrolysis 95 4.8 Complexation 98 4.8.1 Predicting equilibrium for complexation reactions 99 4.8.2 Chelate reactions with metals 101 4.8.3 Trends in cation ligand affinity 104 4.8.4 Predicting complexation using the hard and soft acid-base (HASB) concept 105 4.9 Using software to predict soil solution equilibrium 106 4.10 Kinetics of chemical reaction in soil solution 107 5 Redox Reactions in Soils 113 5.1 Introduction 113 5.2 Redox reactions in nature 115 5.2.1 Photosynthesis redox reactions 115 5.2.2 Electron donors in nature 116 5.2.3 Electron acceptors in nature 117 5.3 Basic approaches for characterizing soil redox processes 119 5.3.1 Measuring redox-induced changes in chemical species in soils 119 5.3.2 Predicting redox processes in soil using redox reactions 121 5.4 Quantifying redox reactions 121 5.4.1 The redox electrode 122 5.4.2 Thermodynamic models of redox reactions 123 5.4.3 Predicting redox reactions using equilibrium constants 124 5.4.4 Theoretical Eh of redox reactions 126 5.5 Redox potential limits in natural systems 128 5.6 The role of protons in redox reactions 129 5.6.1 pe-pH diagrams 129 5.6.2 Adjusting Eh for pH and species activities 130 5.7 Prediction of oxidation and reduction reactions in soils 132 5.7.1 Reduction reactions on the redox ladder 133 5.7.2 Oxidation reactions on the redox ladder 135 5.8 Redox measurement in soils 135 5.8.1 Other methods to assess redox status of soils 136 5.9 Soil redoximorphic features and iron reduction in wetlands 136 5.10 Nitrogen redox reactions in soils 138 5.11 Important concepts for understanding redox in soils 141 6 Mineralogy and Weathering Processes in Soils 145 6.1 Introduction 145 6.2 Common soil minerals 146 6.3 Crystal chemistry of minerals 147 6.3.1 Bonds in minerals 148 6.3.2 Rules for assembling minerals 150 6.3.3 Isomorphic substitution 156 6.3.4 Mineral formulas 157 6.4 Common primary mineral silicates in soils 159 6.4.1 Nesosilicates 159 6.4.2 Inosilicates 159 6.4.3 Phyllosilicates 159 6.4.4 Tectosilicates 159 6.4.5 Cations in primary silicates 161 6.5 Minerals and elements in rocks 162 6.5.1 Elemental composition of rocks 163 6.6 Stability of silicates to weathering 164 6.7 Chemistry of soil weathering and mineral formation 166 6.7.1 Initial breakdown of primary minerals 166 6.7.2 Formation of soil minerals 167 6.7.3 Weathering effects on ion composition in soils 168 6.8 Formation of secondary minerals in soils 170 6.8.1 Prediction of secondary mineral formation 172 6.9 Soil carbonates 174 6.10 Evaporites 175 6.11 Soil phosphate minerals 176 6.12 Sulfur minerals 177 6.13 Time sequence of mineral formation in soils 178 6.14 Measurement of soil mineralogy 180 6.14.1 Principles of x-ray diffraction (XRD) for clay mineralogy 181 7 Chemistry of Soil Clays 185 7.1 Introduction 185 7.2 Structural characteristics of phyllosilicates 185 7.2.1 1:1 Phyllosilicates 189 7.2.2 2:1 Phyllosilicates 189 7.3 Relation of phyllosilicate structure to physical and chemical properties 193 7.3.1 Interlayer bond 193 7.3.2 Surface area 195 7.3.3 c-spacing 195 7.3.4 Cation adsorption and layer charge 195 7.3.5 Shrink and swell behavior and interlayer collapse 198 7.4 Detailed properties of phyllosilicates 198 7.4.1 Kaolinite 198 7.4.2 Smectite 199 7.4.3 Vermiculite 201 7.4.4 Mica and illite 201 7.4.5 Chlorite 202 7.5 Allophane and Imogolite 202 7.6 Zeolite 203 7.7 Oxide minerals 204 7.7.1 Aluminium oxides 204 7.7.2 Iron oxides 204 7.7.3 Titanium oxides 208 7.7.4 Manganese oxides 209 7.8 Soil-mineral mixtures 210 8 Production and Chemistry of Soil Organic Matter 213 8.1 Introduction 213 8.1.1 Components in SOM 213 8.1.2 Studying SOM 214 8.2 Ecosystem carbon storage and fluxes 215 8.3 Soil organic matter formation factors 217 8.3.1 Residence time of SOM 217 8.3.2 Climate effects on SOM 218 8.3.3 SOM in wetlands 218 8.3.4 Soil mineral effects on SOM 219 8.4 Organic chemistry of SOM 219 8.5 Plant and microbial compounds input into soil 219 8.6 SOM decay processes 224 8.7 Humus composition and structure 227 8.8 Classical SOM fractionation 229 8.9 Function of organic matter in soil 231 8.9.1 Organic nitrogen, sulfur and phosphorus 233 8.9.2 SOM influences on chemical processes 233 8.9.3 SOM influences on physical properties 234 8.9.4 Organic chemical partitioning 234 9 Surface Properties of Soil Colloids 237 9.1 Introduction 237 9.2 Permanent charge 237 9.3 pH-dependent charge 238 9.3.1 Balancing surface charge 239 9.3.2 Variable charge on phyllosilicates 240 9.3.3 pH-dependent charge on Fe and Al oxides 242 9.4 Point of zero charge of variable charged surfaces 243 9.5 pH-dependent charge of SOM 245 9.6 Hydrophobic regions of soil organic matter 248 10 Adsorption Processes in Soils 251 10.1 Introduction 251 10.1.1 Outer-sphere adsorption 251 10.1.2 Inner-sphere adsorption 253 10.1.3 Adsorption of non-charged chemicals to soil particles 254 10.1.4 Desorption 254 10.2 Cation exchange on soils 256 10.2.1 Force of ion attraction to charged surfaces 258 10.2.2 Cation exchange selectivity 258 10.2.3 Description of the diffuse double Layer 261 10.2.4 Interacting diffuse double layers from adjacent particles 265 10.2.5 Stern modification of the Gouy-Chapman DDL theory 267 10.2.6 Cation exchange equations 267 10.2.7 Measuring CEC 272 10.3 Inner-sphere adsorbed cations 273 10.3.1 Inner-sphere adsorption on minerals 274 10.3.2 Metal adsorption selectivity 276 10.3.3 Inner-sphere metal adsorption on soil organic matter 277 10.3.4 Inner-sphere metal adsorption in soils 277 10.4 Anion Adsorption 280 10.4.1 Outer-sphere adsorbed anions 281 10.4.2 Inner-sphere adsorption of anions 282 10.5 Adsorption of anthropogenic organic chemicals in soils 286 10.5.1 Mechanisms of organic chemical retention 287 10.5.2 Adsorption of charged pesticides 287 10.5.3 Retention of non-ionic organic chemicals 289 10.5.4 Predicting organic chemical retention in soil 290 11 Measuring and Predicting Sorption Processes in Soils 295 11.1 Introduction 295 11.2 Sorption experiments 295 11.3 Predicting sorption using empirical models 297 11.4 Predicting adsorption using mechanistic models 301 11.5 Rates of adsorption 305 11.5.1 Modeling adsorption kinetics 306 11.6 Reactive transport 308 11.7 Surface precipitation 311 11.8 Analytical methods for measuring adsorption mechanisms 312 12 Soil Acidity 315 12.1 Introduction 315 12.1.1 Measurement of soil acidity 315 12.2 History of soil acidity 317 12.3 The Role of aluminum in soil pH 319 12.3.1 Creation of exchangeable aluminum 319 12.4 Base cations in soil solutions 320 12.4.1 Aqueous chemistry of base cations 320 12.4.2 Exchangeable base cations 321 12.5 Soil acidification processes 322 12.5.1 Organic matter influences on pH 322 12.5.2 Acidity from the nitrogen cycle 324 12.5.3 Phosphate and sulfate fertilizer additions to soil acidity 327 12.5.4 Plant root influences on soil acidity 327 12.5.5 Protonation and deprotonation of mineral surfaces 327 12.5.6 Pollution sources of soil acidity 327 12.5.7 Redox reactions 328 12.6 Aluminum and Manganese toxicity 329 12.7 Plant nutrients in acid soils 329 12.8 Managing acidic soils 330 12.8.1 Predicting lime requirement 330 12.8.2 Optimal management of soil pH 331 13 Salt-Affected Soils 333 13.1 Introduction 333 13.2 Distribution and origin of salt-affected soils 333 13.2.1 Mineral weathering sources of salts 334 13.2.2 Sources of fossil salt 334 13.2.3 Atmospheric salt sources 334 13.2.4 Topographic influence on soil salt concentrations 334 13.2.5 Human sources of soil salinity 335 13.3 Characterization of salinity in soil and water 336 13.3.1 Total dissolved solids 337 13.3.2 Electrical conductivity 337 13.3.3 Sodium hazard 338 13.3.4 Exchangeable sodium percentage 340 13.3.5 Bicarbonate hazard 341 13.3.6 Other problematic solutes in irrigation water 342 13.4 Describing salt-affected soils 343 13.4.1 Saline soils 343 13.4.2 Saline-sodic soils 344 13.4.3 Sodic soils 344 13.5 Effects of salts on soils and plants 345 13.6 Salt balance and leaching requirement 346 13.7 Reclamation 347 Bibliography 351 Index 357