In the three-phase system, solid-liquid-gas, which constitutes a soil, the liquid phase is the phase that feeds the plant. Not only is the solvent—water—a necessity of plant life, the liquid phase is also the medium of transport for the plant’s nutrients and, to a certain extent, of the gases involved in the metabolism of the plant. The availability of the liquid phase to the plant is, therefore, of primary importance. The availability of the moisture of the soil and the dissolved components is governed by the various forces that determine the uptake of the solvent and of the solutes by the plant in competition with the various forces that tend to retain the moisture in the soil. The net driving potential is a function of capillary and gravitational pressure gradients, as well as concentration and electrical potential gradients in the soil system. The rate at which water and dissolved components become available to the plant is also crucial. At a given driving potential, this rate is, to a large extent, affected by the geometry of the solid matrix, which presents a tortuous convection and diffusion path to the liquid-phase components. The analysis of the potential in soil systems and of the transport processes will be discussed in the sections on soil physic and soil chemistry. For this discussion, the knowledge of the properties and components of the liquid phase will be indispensable.