The ability of water to do work is determined by its chemical potential. For water in the soil, plants, and the atmosphere, this ability to do work is called the water potential. The water potential of pure water is arbitrarily considered to be zero bars. Most of the water in plant cells, soil, and the atmosphere has a lower water potential than pure water. In water potential terminology, the more negative the value, the lower the ability of the water to do work.
Water molecules tend to move from a less negative potential towards a more negative potential.
Plant roots generally have water potential values of ca. Plant leaves often have midday water potential values of ca. The lower atmosphere on a summer day often has a midday water potential value of ca. In the example above, there is a gradation of water potential values, with the least negative in the soil, and the most negative in the atmosphere. It is this difference in water potential values that provides the drawing force for moving water from the soil, into plants, and out of leaves into the atmosphere.