Abstract
This chapter defines water content and water potential. The water potential and its components are then described. All water in soil and plants is subjected to force fields originating from four main factors: the presence of the solid phase (the matrix); the gravitational field; dissolved salts; and the action of water pressure. These force fields can be defined in units of potential energy or in head (length) units. The reason why a length unit can represent a potential energy unit is given. Head units are used to show the force fields on water in a column of soil. The direction of movement of water between two tensiometers is determined by calculating the sum of the matric potential and the gravitational potential. A biography of William L. Powers, who is coauthor of a classical book on soil physics from which information in this chapter was taken, is given in the appendix.
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