Gravity will pull a certain amount of water out of saturated soils if there is somewhere for it to go. The water remaining after gravity removes what it can is called the field capacity. Water also evaporates or is taken up by plants. A certain amount of water is unavailable to plants because it adheres to the surface of soil particles. Wilting point is the point at which plants cannot absorb any more water from soil to remain turgid.
Any water remaining in soil is unavailable because it is strongly adhering to the soil particles.
Soils with high available water normally have a high organic matter content. Any organic matter increases the water-holding capacity of soil. Peat moss, for instance, retains more than 100 percent of its weight in water. Peat moss must be worked into the soil to be effective. Gardeners can alter soil to improve drainage.