Gravitational water is not considered available to plants because it is in the soil only a short time and reduces oxygen levels to the point where the plant will not be absorbing water anyway. Eventually a point is reached where the plant can no longer remove water. This is called the wilt point , and occurs at -15 bars water potential for most plants. If the soil dries to an air dry state, the potential is -31 bars. Plants cannot exert enough tension to pull water away from the soil.
Tension is used to express water potential with positive numbers.
Additional drying requires putting the soil in an oven to drive off the tightly held water. The investigation in the laboratory will be using the soil placed in the drying cans last week. You let the sample air dry, and later it was placed in the oven and dried, then weighed to obtain the oven dry weight. Sample Water Problems are available in Chapter 5. A tension is built up between the soil and the gauge.