Book excerpt: Soils are the porous skin of the Earth with variable and complex structures composed of solid, liquid and gaseous phases. This textbook (based on the 4th, German language edition) introduces the reader gently but comprehensively to soil physical processes. The authors discuss both the origin and dynamics of soil physical properties and functions -- including volume-mass relations of the solid, water and gas phases, grain and pore size distributions, permeability and storage capacity for water, gases and heat -- and finally soil deformation and strength in relation to mechanical and hydraulic stresses resulting in structural changes through compaction, kneading, slaking and soil crusting.