| Growing Media - II
To describe how effective a growing media 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 mediainitially 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.
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.
Soil-less Growing Media.
Typically, growing media is put together from individual soil-less products like vermiculite, composted bark, peat moss sphagnum, perlite, coconut coir, sand, pumice, or any combination of the above.
Soil-less growing media is preferred over garden soil, or even potting soil. The reason is that most soil-less media have significantly higher percentages of total and air porosity. Potting soil and garden soil packs down too much, stays too wet, and limits air porosity. Whereas these products excel in gas exchange and thus root growth.
High porosity mixes, for plants requiring good drainage, may contain 65-75% peat moss sphagnum, and 25-35% perlite (example: "Pro-Mix HP " with 20-25% air porosity; 6 inch pot).
A mix with an all-round average air porosity will contain 75-85% peat moss sphagnum, 5-10% vermiculite and 10-15% perlite (example: "Pro-Mix BX" with 17-22% air porosity; 6 inch pot).
To increase water retention, increase the amount of sphagnum or fine vermiculite. To increase porosity, add more perlite (up to 50% for cuttings). If you mix your own, make sure you adjust the pH to a suitable 5.5-6.0 range using limestone, or consider coconut coir which naturally fall in this range.
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