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.
Adding organic matter may improve drainage by improving soil structure. In low areas, drainage ditches, drain tile, swales or dry wells help drain overly wet areas. Soil depth has a direct effect on drainage. A deep, well-structured soil provides plenty of space for vertical and horizontal drainage. Deep soils with a large capacity for water and nutrients have a positive effect on crop yields.