8 THEORY: WATER POTENTIAL WP4C
8 Theory: Water Potential
8.1 Water Potential
Water potential is defined as the potential energy per unit volume of
water in a sample. The total water potential of a sample is the sum of
four component potentials: gravitational, matric, osmotic, and pres-
sure. 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 substance in the water. Pressure potential
depends on the hydrostatic or pneumatic pressure on the water.
The WP4C measures the sum of the osmotic and matric potentials in
a sample. Often one or the other of these potentials will be the domi-
nant factor in determining 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).
8.2 Measuring Water Potential
The water potential of a solid or liquid sample can be found by
relating the sample water potential reading to the vapor pressure of
air in equilibrium with the sample. The relation ship between the
sample water potential (Ψ) and the vapor pressure of the air is:
Ψ =
RT
M
∗ ln
p
p
o
(1)
where p is the vapor pressure of the air, p
o
is the saturation vapor
pressure at sample temperature, R is the gas constant (8.31 J/mol
K), T is the Kelvin temperature of the sample, and M is the molec-
ular mass of water. The vapor pressure of the air can be measured
using a chilled mirror, and p
o
is computed from sample temperature.
The WP4C measures water potential by equilibrating the liquid
phase water of the sample with the vapor phase water in the headspace
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