WP
4
PotentiaMeter
Theory
44
9. Theory: Water potential
Water potential
Water Potential is defined as the potential energy of
water per unit mass of water in the system. The total
water potential of a sample is the sum of four com-
ponent potentials: gravitational, matric, osmotic,
and pressure. Gravitational potential depends on
the position of the water in a gravitational field.
Matric potential depends on the adsorptive forces
binding water to a matrix. Osmotic potential
depends on the concentration of dissolved sub-
stance in the water. Pressure potential depends on
the hydrostatic or pneumatic pressure on the water.
The WP4 measures the sum of the osmotic and mat-
ric potentials in a sample. Often one or the other of
these potentials will be the dominant factor in deter-
mining the total potential. For example, solutions
like the KCl calibration standard have only an
osmotic component. Soils bind water mainly
through matric forces, and therefore have mainly a
matric component (though salt-affected soils can
have a significant osmotic component).
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