As a result, this water is more available to the plant and also drains at a faster rate than water occupying smaller pores. The length of the soil column also influences the rate of drainage. The taller the container, the greater the force of gravity on the water occupying the pore space. This results in increased drainage. Shorter columns of an equal volume of medium hold more water, drain more slowly and contain less air.
Compaction is another factor that affects drainage.
Packing growing medium into a container can significantly reduce the number of large pores. When this occurs, it creates less available water to the plant, reduces aeration and gas exchange, increases water holding and decreases drainage. In smaller containers the effect of compaction can be even greater. Establishing a balance between the water holding capacity of a medium, aeration and drainage is a key to optimum plant growth. The cost of irrigation often forces growers to utilize a medium that holds excessive amounts of water.