ABSTRACT
Soil pores are occupied by either water or air. Fluctuations in water content occur with climate controlling the inputs of water and drainage and evaporation from the soil and leaf surfaces controlling losses. The presence of water in soils has a marked effect on their physical properties. The maximum amount of water that a soil can hold is its saturated water content, which depends on the total porosity. This chapter presents the total porosity of soils and typical values for the volumes of transmission, storage and residual pores in soils of different texture. Water retention depends on forces acting between water molecules and hydrophilic particle surfaces these forces include hydrogen bonds, van der Waals' forces and electrical attraction to exchangeable cations. There are two related results. Firstly, water is held in small pores and in the necks between larger air-filled pores, and secondly, thin films of water are held on to particle surfaces enclosing air-filled pores.
