In general, water retention of container media decreases as the height of the water column increases. Media in a tall container characterized by a greater depth drains more readily than the same media in a short container with a shallower media depth. When coarse material is placed at the bottom of a container, the height of the column is shortened, altering the drainage pattern. However, addition of coarse materials to the container bottom aids in drainage by constructing larger pore spaces. Media capacity exists when large pore spaces do not contain any free water after drainage.
Water is retained only in small pore spaces by adhesive and cohesive forces.
After drainage, such a situation exists in the upper portion of container media. As an indication of sufficient water retention, media should absorb two inches of water per hour without runoff. In addition, if one quart of water can flow through media in a one gallon container per minute , adequate drainage exists. Container media should also have the capability to retain sufficient amounts of water for root uptake.