The capillary force or tension with which water is held in the soil is most important to plant growth. Also, as films of water around soil separates or aggregates get thinner, water tension increases. As soil dries, the tension of the remaining water increases. Plants can extract water more readily when water tension is small. Soil water tension measures the force with which water is retained by the soil.
Tension is a measure of negative pressure.
The same plant would exert -147 psi if the soil were at 10 bars of tension. Table I illustrates typical soil water tensions for three soil textures. Table I. Soil water tension for three soil textures. In unsaturated soils, water is under tension and it takes energy to remove it from the soil. The negative pressure to remove water from soil at a given water content can be measured.