Soil Chemistry

DANIEL G. STRAWN, PHD, is a professor at the University of Idaho. HINRICH L. BOHN, PHD, is Professor Emeritus, at the University of Arizona. GEORGE A. O'CONNOR, PHD, is Professor Emeritus, at the University of Florida.

• Preface to Fifth Edition xiiPreface to Fourth Edition xiiiAcknowledgments xiv1 Introduction to Soil Chemistry 11.1 The soil chemistry discipline 11.2 Historical background 31.3 The soil environment 61.3.1 Soil chemical and biological interfaces 61.3.2 Soil solids 101.3.3 Soil interaction with the hydrosphere 111.3.4 Interaction of soil and the atmosphere 121.4 Chemical reactions in soils 151.4.1 Flow of chemical energy in soils 171.4.2 Soil chemical speciation 181.4.3 Chemical reaction types in soils 191.5 Soil biogeochemical cycling 221.6 Soil chemical influences on food production 221.7 Soils and environmental health 231.7.1 Soil chemistry and environmental toxicology 241.8 Units in soil chemistry 261.8.1 Converting units 261.9 Summary of important concepts in soil chemistry 26Questions 29Bibliography 292 Properties of Elements and Molecules 312.1 Introduction 312.2 Ionization and ionic charge 332.3 Ionic radius 332.4 Molecular bonds 362.5 Nature of water and hydration of ions 372.6 Ligands and metal bonds 402.7 Summary of important concepts of elemental and molecular properties 42Questions 42Bibliography 423 Characteristics of Chemicals in Soils 433.1 Introduction 433.2 Occurrence of elements in soils 433.3 Essential elements 473.3.1 Plant deficiency 493.4 Inorganic contaminants in the environment 493.4.1 Assessing contamination status of soils 513.5 Anthropogenic organic chemicals in the soil environment 533.5.1 Pesticides in the environment 543.5.2 Chemicals of emerging concern in the environment 543.5.3 Chemical factors affecting organic chemical reactions in soil 573.6 Properties of the elements in soils 583.6.1 Alkali and alkaline earth cations 593.6.2 Major soluble anions in soils 613.6.3 Poorly soluble anions 633.6.4 Poorly soluble metal cations 673.6.5 Common toxic elements in soils 693.6.6 Major biogeochemical elements: carbon, nitrogen, and sulfur 713.7 Summary of important concepts for chemicals in the soil environment 75Questions 75Bibliography 764 Soil Water Chemistry 774.1 Introduction 774.2 Thermodynamic approach to aqueous soil chemistry 784.2.1 Example using thermodynamics to calculate gypsum solubility in soils 794.2.2 Types of equilibrium constants 824.3 Calculation of ion activity 834.3.1 Use of ionic strength to calculate activity coefficients 844.3.2 Example calculation of activity coefficient 864.4 Acids and bases 864.4.1 Bases 874.4.2 Weak acids 874.5 Gas dissolution 894.5.1 Predicting dissolution of ammonia in water 904.5.2 Predicting pH of water due to CO 2 dissolution 914.6 Precipitation and dissolution reactions 914.6.1 Solubility of minerals 924.6.2 Iron and aluminum dissolution from oxides and hydroxides 934.6.3 Calcite and carbon dioxide in soils 954.6.4 Solubility of minerals in soils 974.6.5 Solubility of contaminant metals from minerals 1004.7 Cation hydrolysis 1024.8 Complexation 1054.8.1 Predicting equilibrium for complexation reactions 1064.8.2 Chelate reactions with metals 1064.8.3 Trends in cation ligand affinity 1094.8.4 Predicting complexation using the hard and soft acid‐base (HASB) concept 1104.9 Using software to predict soil solution equilibrium 1104.10 Kinetics of chemical reaction in soil solution 1114.11 Summary of important concepts for soil solution chemistry 116Questions 116Bibliography 1175 Redox Reactions in Soils 1195.1 Introduction 1195.2 Redox reactions in nature 1215.2.1 Photosynthesis redox reactions 1215.2.2 Electron donors in nature 1225.2.3 Electron acceptors in nature 1225.3 Basic approaches for characterizing soil redox processes 1265.3.1 Using chemical species in soils to monitor redox status of soils 1275.3.2 Predicting redox processes in soil using chemical reactions 1285.3.3 Quantifying redox potential with a redox electrode 1305.3.4 Relating Eh to pe 1325.4 The role of protons in redox reactions 1335.5 Redox potential limits in natural systems 1335.6 pe–pH diagrams 1355.7 Prediction of oxidation and reduction reactions in soils 1375.7.1 Reduction reactions on the redox ladder 1395.7.2 Oxidation reactions on the redox ladder 1405.8 Redox measurement in soils 1415.8.1 Other methods to assess redox status of soils 1415.9 Soil redoximorphic features and iron reduction in wetlands 1425.10 Nitrogen redox reactions in soils 1445.10.1 Nitrogen assimilation 1455.10.2 Ammonification 1455.10.3 Nitrification 1455.10.4 Denitrification 1465.10.5 Biological nitrogen fixation 1465.10.6 Anammox and dissimilatory nitrogen reduction to ammonium 1475.10.7 Limitations to theoretical nitrogen redox reaction predictions 1475.11 Summary of important concepts in soil redox reactions 147Questions 148Bibliography 1486 Mineralogy and Weathering Processes in Soils 1506.1 Introduction 1506.2 Common soil minerals 1526.3 Crystal chemistry of minerals 1536.3.1 Bonds in minerals 1546.3.2 Rules for assembling minerals 1546.3.3 Isomorphic substitution 1596.3.4 Mineral formulas 1606.4 Common primary mineral silicates in soils 1616.4.1 Nesosilicates 1626.4.2 Inosilicates 1626.4.3 Phyllosilicates 1626.4.4 Tectosilicates 1636.4.5 Cations in primary silicates 1636.5 Minerals and elements in rocks 1646.5.1 Elemental composition of rocks 1646.6 Stability of silicates to weathering 1656.7 Chemistry of soil weathering and mineral formation 1676.7.1 Initial breakdown of primary minerals 1676.7.2 Formation of soil minerals 1676.7.3 Weathering effects on element composition in soils 1696.8 Formation of secondary minerals in soils 1706.8.1 Prediction of secondary mineral formation 1726.9 Soil carbonates 1746.10 Evaporites 1766.11 Soil phosphate minerals 1776.12 Sulfur minerals 1776.13 Time sequence of mineral formation in soils 1786.14 Measurement of soil mineralogy 1806.14.1 Principles of X‐Ray Diffraction (XRD) for clay mineralogy 1806.14.2 Example calculation of d‐spacing from a diffractogram 1836.14.3 Selective extraction of iron oxides and amorphous aluminosilicates from soils 1846.15 Important concepts in soil mineralogy 184Questions 184Bibliography 1857 Chemistry of Soil Clays 1867.1 Introduction 1867.2 Structural characteristics of phyllosilicates 1877.2.1 1:1 phyllosilicates 1897.2.2 2:1 phyllosilicates 1917.3 Relation of phyllosilicate structure to physical and chemical properties 1937.3.1 Interlayer bond 1937.3.2 Surface area 1937.3.3 c‐spacing 1947.3.4 Cation adsorption and layer charge 1947.3.5 Shrink and swell behavior and interlayer collapse 1957.4 Detailed properties of phyllosilicates 1997.4.1 Kaolins 1997.4.2 Smectite 2007.4.3 Vermiculite 2007.4.4 Mica and Illite 2017.4.5 Chlorite 2047.5 Allophane and imogolite 2047.6 Zeolite 2057.7 Oxide minerals 2057.7.1 Aluminum oxides 2067.7.2 Iron oxides 2077.7.3 Titanium oxides 2087.7.4 Manganese oxides 2107.8 Summary of soil clays 210Questions 211Bibliography 2128 Production and Chemistry of Soil Organic Matter 2148.1 Introduction 2148.1.1 Components in SOM 2158.1.2 Studying SOM 2168.2 Ecosystem carbon storage and fluxes 2178.3 Soil organic matter formation factors 2198.3.1 Residence time of SOM 2198.3.2 Climate effects on SOM 2208.3.3 SOM in wetlands 2208.3.4 Soil mineral effects on SOM 2218.4 Organic chemistry of SOM 2218.5 Plant and microbial compounds input into soil 2238.6 SOM decay processes 2258.7 SOM composition and structure 2298.8 NaOH extraction of SOM 2318.9 Function of organic matter in soil 2338.9.1 Organic nitrogen, sulfur, and phosphorus 2358.9.2 SOM influences on chemical processes 2368.9.3 SOM influences on physical properties 2378.9.4 Organic chemical partitioning 2378.10 Summary of SOM 237Questions 238Bibliography 2389 Surface Properties of Soil Colloids 2409.1 Introduction 2409.2 Permanent charge 2409.3 pH‐dependent charge 2419.3.1 Balancing surface charge 2439.3.2 Variable charge on phyllosilicates 2439.3.3 pH‐dependent charge on iron and aluminum oxides 2459.4 Point of zero charge of variable charged surfaces 2479.5 pH‐dependent charge of SOM 2509.6 Hydrophobic regions of soil organic matter 2529.7 Summary of important concepts in soil surface charge 252Questions 253Bibliography 25310 Adsorption Processes in Soils 25510.1 Introduction 25510.1.1 Outer‐sphere adsorption 25610.1.2 Inner‐sphere adsorption 25710.1.3 Adsorption of non‐charged chemicals to soil particles 25810.1.4 Desorption 25810.2 Physical model of charged soil particle surfaces 26010.2.1 Force of ion attraction to charged surfaces 26010.2.2 The diffuse double layer 26110.2.3 Surface potential on variable charged surfaces 26310.2.4 Stern modification of the Gouy‐Chapman DDL theory 26410.2.5 Interacting diffuse double layers from adjacent particles 26410.3 Cation exchange on soils 26610.3.1 Cation exchange selectivity 26710.3.2 Cation exchange equations 27010.3.3 Measuring CEC 27310.4 Inner‐sphere adsorbed cations 27310.4.1 Inner‐sphere adsorption of cations on minerals 27610.4.2 Metal adsorption selectivity on minerals 27710.4.3 Inner‐sphere metal adsorption on soil organic matter 27810.4.4 Inner‐sphere metal adsorption in soils 27810.5 Anion adsorption 28110.5.1 Outer‐sphere adsorbed anions 28210.5.2 Inner‐sphere adsorption of anions 28310.6 Adsorption of anthropogenic organic chemicals in soils 28610.6.1 Mechanisms of organic chemical retention 28610.6.2 Adsorption of charged pesticides 28710.6.3 Retention of nonionic organic chemicals 28910.6.4 Predicting organic chemical retention in soil 29010.6.5 Aging effects on organic chemical adsorption 29110.7 Summary of important concepts for adsorption and desorption reactions in soils 292Questions 293Bibliography 29411 Measuring and Predicting Sorption Processes in Soils 29611.1 Introduction 29611.2 Adsorption experiments 29611.3 Predicting adsorption using empirical models 29711.3.1 Linear adsorption isotherms 29811.3.2 Nonlinear adsorption isotherms 29811.4 Predicting adsorption using mechanistic models 30011.5 Rates of adsorption 30411.5.1 Modeling adsorption kinetics 30511.6 Reactive transport 30611.7 Surface precipitation 30911.8 Analytical methods for determining adsorption mechanisms 31011.9 Summary of important concepts for modeling surface reactions in soils 311Questions 312Bibliography 31212 Soil Acidity 31412.1 Introduction 31412.1.1 Measurement of soil acidity 31412.2 History of soil acidity 31712.3 The role of aluminum in soil pH 31812.3.1 Creation of exchangeable aluminum 31812.4 Base cations in soil solutions 31912.4.1 Aqueous chemistry of base cations 31912.4.2 Exchangeable base cations 32012.4.3 Total exchangeable acidity 32112.5 Soil acidification processes 32112.5.1 Organic matter influences on pH 32212.5.2 Acidity from the nitrogen cycle 32312.5.3 Phosphate and sulfate fertilizer additions to soil acidity 32512.5.4 Plant root influences on soil acidity 32512.5.5 Protonation and deprotonation of mineral surfaces 32512.5.6 Pollution sources of soil acidity 32512.5.7 Redox reaction influence on soil acidity 32612.6 Aluminum and manganese toxicity 32612.7 Plant nutrients in acid soils 32712.8 Managing acidic soils 32712.8.1 Predicting lime requirement 32712.8.2 Optimal management of soil pH 32812.9 Summary of important concepts in soil acidity 329Questions 329Bibliography 33013 Salt‐affected Soils 33113.1 Introduction 33113.2 Distribution and origin of salt‐affected soils 33113.2.1 Mineral weathering sources of salts 33213.2.2 Salinity from fossil salts 33213.2.3 Atmospheric salt sources 33213.2.4 Topographic influence on soil salt concentrations 33213.2.5 Human sources of soil salinity 33313.3 Characterization of salinity in soil and water 33413.3.1 Total dissolved solids 33413.3.2 Electrical conductivity 33413.3.3 Sodium hazard 33613.3.4 Exchangeable sodium percentage 33713.3.5 Bicarbonate hazard 33913.3.6 Other problematic solutes in irrigation water 34013.4 Describing salt‐affected soils 34013.4.1 Saline soils 34013.4.2 Saline‐sodic soils 34013.4.3 Sodic soils 34113.5 Effects of salts on soils and plants 34113.6 Salt balance and leaching requirement 34313.7 Reclamation 34413.8 Summary of important concepts in soil salinity 345Questions 345Bibliography 346Index 347