The interaction between these materials are even more important. For example, fine-particle media added to a coarse mix will result in the finer particles settling in the cavities between the larger particles. This will reduce the overall air holding capabilities of the media while increasing the the water holding capabilities.
To describe how effective a growing medium is in providing air and water to the plants, two terms are commonly used: Total porosity and Air porosity.
Total Porosity
The volume in a container which is not occupied by solid media is referred to as total porosity, e.g. a description of the total pore space available for either water or air.
An ideal medium should have at least 50% total porosity, e.g. water and air pores should comprise more than half of the container volume (solids should comprise less than half).
Air Porosity
Large pore spaces in the growing media initially fill with water, but drain and re-fill with air, also referred to as air porosity. Medium and small pores remain filled with water which is then available for uptake by the roots. A growing media can also contain very small pores which will hold water very tightly making it unavailable to plants (the water molecules will bind closely to the molecules of the solid particles). This is especially true for peat moss sphagnum and coconut coir, for example, which can contain much unavailable water.
For an excellent visual illustration of unavailable water take a look at this science demonstration (will open in a new window).
You can also take a look at the water property primer page.
Note that, besides the media composition, the size of the container and manner in which the pots are filled can also determine the air porosity. Therefore, porosity figures are usually given for a specific container size, such as a 6" pot.
For proper gas exchange, air porosity should represent at least 15% - 35% of the total porosity. Water retaining pore space should make up the remaining pore space. The exact air porosity target depends largely on the plants. Cuttings, Azaleas, and Caladium require relatively high air porosity whereas Pelargonium, Coleus, and Carnations require low air porosity.
Soil-less Growing Media
Typically, growing media is put together from individual soil-less products like peat moss sphagnum, perlite, vermiculite, composted bark, coconut coir, pumice, sand, or any combination of the above.
