As the soil moistens, clay particles swell and reduce the size of pores. Soil Water Storage Within the soil system, the storage of water is influenced by several different forces. The strongest force is the molecular force of elements and compounds found on the surface of soil minerals. The water retained by this force is called hygroscopic water and it consists of the water held within 0.0002 millimeters of the surface of soil particles. The maximum limit of this water around a soil particle is known as the hygroscopic coefficient .
Hygroscopic water is essentially non-mobile and can only be removed from the soil through heating.
Matric force holds soil water from 0.0002 to 0.06 millimeters from the surface of soil particles. This force declines in strength with distance from the soil particle. The force becomes nonexistent past 0.06 millimeters. Capillary action moves this water from areas where the matric force is low to areas where it is high. Because this water is primarily moved by capillary action, scientists commonly refer to it as capillary water .