Saturated soil has no oxygen in its pore spaces. This dramatically reduces root growth because roots require oxygen for growth. If saturation continues for extended periods, roots will die. 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.