Available soil water is retained in the soil by capillary forces and can be extracted by the plant. This soil water is most important for crop production. It is the water held by the soil between field capacity and wilting point. Plants can use approximately 50 percent of the available water without stress. When less than 50 percent of available water remains, stress can occur.
Field capacity is the water content of a soil at the upper limit of the available water range.
It is the amount of water remaining in a soil after it has been saturated and allowed to drain for 24 hours. Permanent wilting point is the lower limit of the available water range. When plants have removed all of the available water from a given soil, they wilt and do not recover. Thus, the water available for plant growth exists between the range of field capacity and wilting point. Available water capacity is all the water that a soil can possibly hold between field capacity and wilting point.