Swelling is the process of volume expansion, usually brought about by the intake of water. One mechanism by which water can be drawn into a material is if the material has less water than it would like to have, a dry sponge is a typical example. The stress which a moisture deficient material can exert on an adjacent body of water is known as the suction. As water is absorbed into the material the affinity which the material has for the water decreases as does the suction in the material. Suction is very important in the characterization of materials (particularly soils) with a moisture deficiency, but its measurement is not easy to perform. In agriculture, the ability of crops to extract water from the soil is governed by the suction of the soil. In particular the plant has to overcome the affinity of the soil for water in order to feed. The suction at which the plant can no longer do this is known as the wilting point for obvious reasons. In engineering, the strength and volume of an unsaturated soil (one in which the spaces between adjacent soil particles are filled with a mixture of water and a gas) is characterized by a combination of the external stresses applied to the soil and the suction within the soil. It is principally these two fields which have been concerned with the measurement of the suction. However only a relatively few techniques have evolved which successfully measure the quantity and only one exists which is capable of making a direct measurement under atmospheric conditions. As a consequence of the lack of direct techniques it has been necessary to use indirect methods. Indirect measurements are classified as those which actually measure another quantity such as humidity, absorbtion or conductivity, which can then be converted to a measure of suction. Ridley (1993) has reviewed the available methods of suction measurement.

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