The maximum amount of water that a soil can hold against the pull of gravity is called the field capacity. Generally when a soil is at field capacity, half the pores are filled with water. Plants cannot use all of the water in soil. As water evaporates and is drawn out of soil by plants, the water content gradually declines until plants can no longer extract the small amount of remaining water held tightly to soil particles. This remaining amount of soil water is called the wilting point.
Clays have a high wilting point.
They hold water more tightly than coarser-textured soils and so less of the water is available to plants. Salts prevent roots from absorbing water and thus increase the wilting point. Many roots and soil organisms will die if the excess water does not quickly percolate out and allow air into the pores again. Changes in Soil Water . Field capacity minus wilting point is the amount of water available to plants.