Soil moisture tension is a measure of the tenacity with which water is retained in
the soil and shows the force per unit area that must be exerted to remove water from a soil.
The tenacity is measured in terms of the potential energy of water in the soil measured, usually with respect to free water. It is usually expressed in atmospheres, the average air pressure at sea level. Other pressure units like cm of water or cm or mm of mercury are also often used (1 atmosphere = 1036 cm of water or 76.39 cm of mercury).
It is also sometimes expressed in bars (1 bar = 106 dynes / cm2 = 1023 cm of water column.
1 millibar = —– bar).
Soil moisture tension is brought about at the smaller dimensions by surface
tension (capillarity), and at the higher dimensions by adhesion. Buckingham (1907)
introduced the concept of ‘capillary potential’ to define the energy with which water is held by soil.
This term, however, does not apply over the entire moisture range. In a wet soil, as long as there is a continuous column of water, it might be called ‘hydrostatic potential’, in the intermediate range the term ‘capillary potential’ is appropriate.
In the dry range the term ‘hygroscopic potential’ would be suitable. However, the term ‘soil moisture potential’, ‘soil moisture suction’ and ‘soil moisture tension’ are often used synonymously to cover the entire range of moisture (Khonke, 1968).