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. These products excel in gas exchange and thus root growth. Potting soil and garden soil packs down too much, stays too wet, and limits air porosity.
Pre-mixed growing media can be purchased at garden centers, or online, or you can mix your own. If mixing your own, it is usually best to use 'true-and-tried' combinations which have worked for others, such as peat moss sphagnum, perlite and vermiculite.
| Medium | Aeration | Water Holding | Total |
| Coconut Coir | 36.0% | 50.0% | 86.0% |
| Sphagnum peat moss | 25.4% | 58.8% | 84.2% |
| Vermiculite | 27.5% | 53.0% | 80.5% |
| Perlite | 29.8% | 47.3% | 77.1% |
| Fir Bark | 54.7% | 15.0% | 69.7% |
| Sand | 2.5% | 33.7% | 36.2% |
| Source | |||
A popular choice, which has virtually become the industry standard for growing media, is the so-called "Peat-Lite Mix" formulated by Cornell University, and which consists of Peat moss Sphagnum, Perlite and Vermiculite.
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).
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).
To increase porosity, add more perlite (up to 50% for cuttings). To increase water retention, increase the amount of sphagnum or fine vermiculite. 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.
As mentioned at the beginning of this article, selection of growing media is step one, but in itself not enough to ensure success. Equally important are potting techniques, watering techniques, and the container selection. These processes are discussed in upcoming articles.
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Sources Used:
- "Technical Data Sheets: PRO-MIX HP and PRO-MIX BX", Premier Horticulture.
- "Plug and Bedding Plant - Water, Media, and Nutrition", Excerpts from presentation, Dr. David S. Koranski, Floriculture Fact sheets, Ministry of Agriculture, British Columbia.
- "Importance of Aeration in Container Media", Ministry of Agriculture, British Colombia.
- "Physical Characteristics of Growing Mixes", Publication HYG-1251-97, Dr. Claudio C. Pasian, Ohio State University Extension
- "Growing Media", E.Jay Holcomb, PennState, Department of Horticulture
- "Soil/Root Interactions", Hort408, PennState, Department of Horticulture
- "The Greening of Brown Thumbs, Article #410", T. Niel Davis, Alaska Science Forum
- "Something to Grow On, Nutrient Management: Porosity", Cornell Cooperative Extension
- "Water and Water Quality: Air, Water and Media...Putting Them All Together", Hortipm, Texas A&M University System, et al.
