Hygroscopic Water

Water that has been absorbed from a humid atmosphere and which is held very tightly onto soil particles by adhesion, and which is not generally available for use by a plant. If a soil has dried and / or a plant has used all available water and this is the only water remaining within a soil then unless water is readily made available to a plant, or it is able to become effectively dormant whilst the dry conditions persist, then it will have reached permanent wilting point and will die.