Plants can use most of this water by way of capillary action until the soil wilting point is reached. Water in excess of capillary and hygroscopic water is called gravitational water . Gravitational water is found beyond 0.06 millimeters from the surface of soil particles and it moves freely under the effect of gravity. When gravitational water has drained away the amount of water that remains is called the soil's field capacity . Figure 6l-1 describes the relationship between the thickness of water film around soil particles and the strength of the force that holds this water.
Force is measured in units called bars .
One bar is equal to a 1000 millibars . The graph also displays the location of hygroscopic water , the hygroscopic coefficient , the wilting point , capillary water , field capacity , and gravitational water along this line. Relationship between soil water film thickness and moisture tension.