Karl-Heinrich Hartge and Rainer Horn

Essential Soil Physics An introduction to soil processes, functions, structure and mechanics

lst edition, based on the 4th, completely revised and extended German edition

With 186 figures and 24 tables

edited by

Robert Horton Rainer Horn Jörg Bachmann Stephan Peth

Schweizerbart Science Publishers · 2016

Contents

Preface

lntroduction Soils: integral part of our environment Soil characteristics . . . . . . . . . .

1 Grain size distribution: texture 1.1 Classification 1.1.1 Grain sizes . . 1.1.2 Grain shapes . 1.1.3 Grain mixtures 1.2 Common soil textures and their origin 1.2.1 Equation of sedimentation . . 1.2.2 Separation processes . . . . . . . . . 1.3 Spatial distribution of textures . . . . 1.4 Modification of grain size distributions in soils 1.5 Grain size distribution and other soil properties 1.6 Methods to measure grain size distributions Problems Chapter 1 . . . . . . . . . . . . . . . . . .

2 Soil structure and structural functions 2.1 Soil structure and intemal morphology . 2.2 Bulk density, particle density . . . . . . . . . 2.3 Pore volume and void ratio . . . . . . . . . . 2.3.1 Theoretical quantities to describe pore volumes 2.3.1.1 lnfluence of grain size and shape on pore volume 2.3.1.2 Effect of particle size . . . . . . . . . . . . . . . 2.3.2 Number of grain contacts . . . . . . . . . . . . . 2.3.2.1 Relationship of contact number and pore volume fraction . 2.3.2.2 Natural grain size distributions and aggregates ... 2.3.3 Influence of grain contact points on soil pedogenesis 2.4 Pore size distributions . . . . . . . . . . . . . . . . 2.4.1 Subdividing pore sizes ............... . 2.4.2 Shapes, sizes of pores, and modes of pore formation 2.4.3 Effects of pore size distribution on soil quality Problems Chapter 2 . . . . . . . . . . . . . . . . . . . .

3 3.1 3.1.1

Mechanical and hydraulic forces in soils Stability and the spatial arrangement of grains . Forces and stresses in soils . . . . . . . . . . . .

9

10 10 11

13 13 14 15 17 20 22 22 24 25 26 29 30

32 32 34 35 36 36 37 38 38 40 40 42 43 45 46 47

48 48 48

3.1.2 Subdivision of the forces and stresses within soils . 3.1.2.1 Particle weight . . . . . . . . . . . . . . . . . . . 3.1.2.2 Loads transmitted by solid phases of soils . . . . . 3.1.2.3 Weight (overburden) stresses transmitted within the solid phase 3.1.2.4 Forces between the surfaces of adjacent particles 3.1.3 Stresses in three-dimensional space 3.2 Soil strength: the balance of forces . . . . . . . . 3.2.1 Shear resistance, a soil property . . . . . . . . . 3.2.1.1 Shear resistance of soils and their determination . 3.3 Stress strain relationship and time-dependent settlement 3.3.1 Stress strain relationship in soils . . . . . . . . 3.3.2 Time dependent settlement behavior of soils . . . . . . . 3.3.3 The meaning of neutral stresses during loading . . . . . 3.4 Stress-, strain-, and deformation processes in three-dimensional space 3.4.1 Stress and strain in three-dimensional space . . . . . . . . . . . 3.4.2 Stress propagation within soils . . . . . . . . . . . . . . . . . . 3.4.3 Base failure as the result of the active and passive Rankine state 3.5 Flow behavior of soils: stresses between individual soil particles 3.6 Influence of soil properties on shear resistance 3.7 Mechanical changes of soil structure . .. . 3.7.l Effects of anthropogenic activities on soils 3.7.2 Effects of animal activity and plant growth 3.7.3 Freezing effects .............. . 3.7.4 Soil compaction in civil engineering and construction . Problems Chapter 3 . . . . . . . . . . . . . . . . . . . . . . .

4 lnteractions between water and soil

52 53 53 53 54 54 55 55 55 61 61 63 64 66 66 66 69 72 77 77 77 79 81 81 83

84 4.1 Adsorption of water in soils . . . . . . . . 84 4.1.1 Adsorption mechanisms . . . . . . . . . . 84 4.1.2 Properties ofwater adsorbed on soil components 86 4.2 Flocculation and peptization of soil particles 87 4.3 Shrinkage of soils . . . . 89 4.3.1 Causes of soil shrinkage 89 4.3.2 Shrinkage in soils . . . . 92 4.4 Swelling of soils . . . . 93 4.4.1 Mechanisms of swelling: swelling pressure 94 4.4.2 Inhibition of swelling . . . . . . . . . 97 4.5 Cracking up: crack formation in soils 98 4.6 Water as a factor of soil stability 101 4.6.1 Static water pressure . . . 101 4.6.2 Flow pressure in soils . . . . . . 103 4. 7 Wetting properties of soils . . . 105 4. 7 .1 Causes and occurrence of inhibited wetting of soil particle surfaces 105 4.7.2 Contact angles and capillarity . . . . . . . . . . . . . . . . . . . . 106 4.7.3 Documenting wetting properties . . . . . . . . . . . . . . . . . . . 109 4.7.4 Impact of wetting properties on the environmental and habitat functions of soils 111 4.8 Electrical flow potentials in soils ......................... 112

4.9 Aggregate shapes and functions ....... . 4.9.1 Natural aggregate-forming processes .... . 4.9.2 Anthropogenie modification of soil aggregates 4.10 Effects of aggregate size, -shape and -age Problems Chapter 4 . . . . . . . . . . . . . . . . . . .

5 Distribution and hydrostatics of soil water

113 114 118 118 120

121 5.1 Distribution and origin of water in soils . . 121 5.2 Forces in soil water . . . . . . . . . . . . . 122 5.3 The groundwater surface as reference plane 124 5.4 Soil water potential . . . . . . . . . . . . . 125 5.4.1 Total water potential and component water potentials 126 5.4.1.1 Matric potential \II m . . . 127 5.4.1.2 Gravitational potential \II z . . . . 127 5.4.1.3 Osmotic potential \II 0 . . • • . • • 128 5.4.1.4 Overburden or load potential \lln . 128 5 .4 .1.5 Pressure potential \II p . . . . . . 129 5.4.2 Combining component potentials 129 5.4.3 Instruments for measuring soil water potentials 130 5.5 Equilibrium water potential . . . . . . . . . . 132 5.6 Relationship between matric potential and water content 133 5.6.1 Effect of grain size distribution on the matric potential / water content relationship 135 5.6.2 Inftuence of structure on the shape of the matric potential/ water content rela-

tionship. . . . . . . . . . . . . . . . . . . . . . . . . . 135 5.6.3 Hysteresis of the matric potential /water content curve . . . . . . . . . . . 137 5.6.4 Measuring matric potential / water content curves . . . . . . . . . . . . . 137 5.6.5 Mathematical description of the matric potential-water content relationship 139 Problems Chapter 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

6 Movement of water within the soil 141 6.1 Water movement in water saturated soil 141 6.1.l Fluid-dynamic phenomena in soils . . . 141 6.1.2 Flow Fields . . . . . . . . . . . . . . . 145 6.1.3 Boundary conditions and spatial limits of ftow fields 145 6.1.4 One-dimensional ftow ....... 146 6.1.5 Two- and three-dimensional ftows 148 6.2 Water movement in unsaturated soil 151 6.3 Transient ftow . . . . . . . . . . . . 153 6.3.1 Hydraulic diffusivity .. ...... 157 6.4 Hydraulic conductivity as a soil property 158 6.5 Vapor transport 166 6.6 Infiltration 167 6.7 Drainage . . 172 6.8 Evaporation . 179 Problems Chapter 6 . 186

7 The gas-phase of soils 7 .1 The energetic state of the gas phase of soils 7.2 Composition of the gas phase in soils ... 7.3 Transport processes in the gas phase of soils . 7.3.1 Gasdiffusion ........... . 7.3.2 Mass fluxes in the gas phase of soils 7 .3.3 Redistribution of gas within the soil Problems Chapter 7 . . . . . . . . . . .

8 Thermal behaviour of soils 8.1 Thermal properties of soils . 8.1.1 Definitions . . . . . . 8.1.2 Heat capacity of soils . 8.1.3 Thermal conductivity . 8.1.4 Thermal diffusivity . . 8.1.5 Mechanisms of heat transport in soils 8.2 Modeling thermal conductivity . . . . 8.3 Techniques for measuring thermal properties 8.4 Phase transitions of H20 and their effects . . 8.4.1 Redistribution of water vapor by thermal flux 8.4.2 Freezing and the formation of ice 8.4.3 Freezing and water movement 8.4.4 Formation of structure Problems Chapter 8 . . . . . . . . . .

9 Combined water-, heat-, and gas budget of soils 9.1 The atmosphere-soil interface ....... . 9.1.1 Radiation components and radiation budgets ... . . . 9.1.2 Energy budget at the soil surface ............ . 9.2 Dynamics and temporal variations of the soil water budget 9.2.1 Ground- and floodwater . .. . .. .. . 9.2.2 Path of matric potentials .. ...... . 9.2.3 Parameters characterizing water budgets . 9.2.3.1 Field capacity ..... . 9.2.3.2 Permanent wilting point .... . 9.3 Heat budget ....... . ... . 9.3.1 Temperature distributions in soils 9.3.2 Heat sources . .. .. . . ... . 9.3.3 Soil temperature variations ... . 9.3.4 Cumulative effects on the heat budget 9.4 Gasbudget of soils .... . .... . 9.4.1 How water content affects gas distribution within the soil profile 9.4.2 Seasonal variations of soil air Problems Chapter 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10 10.1

Plant habitats and their physical modification Plant requirements in terms of water supply . . . . . .

191 191 193 194 195 197 197 200

202 202 202 203 206 208 209 211

. 211

. 212

. 212

. 213

. 214

. 216

. 218

219 . 219

219 222 224 226 229 234 234 237 237 238 239 239 243 245 246 248 251

253 . 253

10.2 Interaction of mechanical and hydraulic processes . . . . . . . . . . . 256 l 0.2. l Mechanical and hydraulic soil deformation . . . . . . . . . . . . . . . 257 10.2.2 Pore function changes resulting from mechanical and hydraulic stresses 259 10.2.3 Interactions between hydraulic pore function and mechanical parameters 262 10.2.4 Effects of soil management on physical parameters 262 10.3 Modification of the hydraulic stress state 264 10.3.1 Drainage . 265 10.3.2 Irrigation . . . . . . . . . . . . . . . . . 268 10.3.3 Percolation . . . . . . . . . . . . . . . . . 270 10.4 Modification of the mechanical stress state . 271 10.4.l Compaction . . . . . . . . 271 10.4.2 Loosening soils . . . . . 272 10.4.3 Material rearrangement . 273 Problems Chapter 10 . . . . . . 275

11 11.1 11.1.l 11.2 11.2.1 11.2.2 11.3

Soil erosion Soil erosion: general principles Delamination of particles or aggregates Approaches to preventing erosion Erodibility of soils . . . . . Erosivity of wind and water Erosion models . . . .

11.3.1 Soil erosion by water . 11.3.2 Soil erosion by wind Problems Chapter 11 . . . . .

12 Solute transport and filter processes in soils 12.l Solute transport: basics ....... . 12.1.1 Breakthrough curves in porous media ..... 12.1.2 Molecular diffusion . . . . . . . . . . . . . . . 12.1.3 Convective flux and hydrodynamic dispersion . 12.1.4 Adsorption . . . .. . ...... . ..... . 12.1.5 Convection-dispersion model of solute transport in soils 12.1.6 Additional factors influencing solute transport . 12.1. 7 Models describing solute transport . 12.2 Filtering processes in soils 12.2.1 Filter types .. . .. 12.2.2 Soils acting as filters .. . 12.2.3 Filter efficiency ..... . 12.2.4 Optimizing filtering processes Problems Chapter 12 ........ .

13 Future perspectives of soil physics

Solutions to the problems for chapters 1-12 Solutions to problems in chapter 1 Solutions to problems in chapter 2 . . . . .

278 . 278

279 283 284 285 285 286 288 288

291 . 292

292 294 296 297 298 300 301 302 302 303 304 306 307

309

317 317

. 318

Solutions to problems in chapter 3 319 Solutions to problems in chapter 4 321 Solutions to problems in chapter 5 323 Solutions to problems in chapter 6 326 Solutions to problems in chapter 7 331 Solutions to problems in chapter 8 332 Solutions to problems in chapter 9 333 Solutions to problems in chapter 10 336 Solutions to problems in chapter 11 341 Solutions to problems in chapter 12 343

14 References 345

15 Commonly used units and conversion factors 372

16 Meaning of abbreviations 372 Basic conversions: density and pore volume . 373 Transport .. . .. . .. .. . .. . . . 374 Derivation of the heat-budget equation . 375 Energy Budget at the soil surface . .. 376 Tensors . . . . ... . .. . ..... . 377 Conversion of data measured in different units 378

Index 379