Article 3-2 Growing Medium

It is incredible how many different types of growing mediums can be used successfully. Plants have been cultivated by man for tens of thousands of years. Until recently, native soils were the only option.

There are many great substances to stick your plants into. The type you choose depends on your level of experience, time for monitoring, and plant and environmental requirements. Some other factors may even include your growing location: do you really want to transport large bags of growing medium into remote areas or up apartment stairs?

Soilless Mixes

Very popular, and can be as complex as you want to make them. Great results can be achieved in an organic based mix. However, the fastest rates of production usually occur in a finely tuned hydro-system. Some growers, especially herb growers, find that they have greater dry weight and a longer shelf life with a soilless mix as opposed to water culture.

Enough said, as it is not the intention of this article to go into a hydro VS bio debate. Popular commercially prepared soilless mixes such as Sunshine Mix or Pro Mix are mainly composed of sphagnum peat moss. They may include amendments such as perlite, vermiculite, buffers such as lime, and small amounts of inorganic fertilizers. These can be further modified with additional dolomite lime, worm castings, slow release fertilizers such as bone meal and inorganic fertilizers such as magnesium suplhate. They can even be inoculated with beneficial bacteria such as mycorrhizae. Here are a few notes about common ingredients and amendments to peat based soilless mixes:

Sphagnum Peat Moss:

This type of moss is the slowest to break down. Due to it’s acidic nature, it must be buffered if mixing your own soilless medium. Peat holds up to 10 times it’s dry weight in moisture, so let the top 1/2″-1″ of the medium dry out between watering’s. If peat is allowed to completely dry out, a wetting agent will be required so that dry pockets do not develop in the medium.


Excellent for increasing aeration of container grown plants. It helps improve drainage and reduce compaction at the roots of maturing plants. Volcanic in origin, it is extremely slow to break down. Don’t be shy about using lots of perlite.


A clay mineral heated and expanded. It holds a tremendous amount of moisture (up to 4 gal per cubic foot), and provides capillary (“wicking”) action in the root zone. It provides caution exchange capacity (CEC). This means that positively charged ions, such as potassium, are attracted to the surface and are released as the soil solution directly as the roots changes. Use sparingly due to high moisture retention.

Some Other Beneficial Additions to Soilless Mixes

Earth worm castings:

Very rich in slow release nitrogen and organic matter. Some growers feel that it really adds “life” to their mix. This is the product of the worms digesting organic matter. Some brands have a very rich and earthy smell. Make sure to throw a few handfuls in larger planter containers.

Dolomite Lime:

Slow release calcium and magnesium. Excellent for buffering the low pH of peat mixes. It is available in granular and powdered form. The coarser material will buffer for longer, but is initially less effective than finer grades. A mixture of the two is best. Add about five cups per commercially prepared bales of mix. This will help to counteract the build up of fertilizers used for intensive production. It also helps buffer of the acids created due to the decomposition of peat and other organic amendments. It is not within the scope of this article to discuss the following, but they are beneficial to most growing mediums, in particular, organic peat mixes:

  • silicic acid clay i.e. Mineral Magic
  • fulvic acids (liquid and solid)
  • humic acids
  • calcium peroxygen i.e. Oxy-Cal or Soil Blaster
  • granular and powdered organic fertilizers such as bone meal and bat guano

Coconut Fiber

Is a by-product of the coconut husking process. The quality of the coconut fiber as a growing medium is dependent on the conditions in which it has been grown (location), and the processing of the fiber, as it must be separated and is typically leached of salts (from salt water). Sri Lanka is one of the larger exporters of coconut fiber.

Coconut fiber has been used extensively for production in Holland, generally for commercial rose growing operations using “Dutch pot” systems. It allows for excellent lateral root development (as opposed to the roots growing straight down) when used in flat bag systems-typically 3 plants per three’ long sleeve.

It can range from being very coarse to very fine. As a general rule, it tends to be more fibrous than commercial peat. The more fibrous texture tends to stand up to intensive crop production than peat. It retains greater macro pore space, providing the oxygen required at the roots for developing plants, while retaining enough moisture for thirsty plants. It also provides slow release of plant available calcium. It can be purchased in compressed bricks, making transport a snap. Once introduced to moisture, they expand several times in volume. A mid sized brick will fill a three gallon pot. You could fit about six such bricks in a shoebox. The pH of most horticultural grade coconut fiber is near optimum. Remember the coconut fiber is only as good as it’s source and processing treatments.

Lightweight Expanded Clay Aggregate-“LECA” or “Grow Rocks”

The manufacturing process involves heating clay pellets. When heated, the steam brewing inside causes them to expand, providing a sterile, light, and porous structure. They are manufactured in a range of sizes from 2mm up to 24mm diameters. The size you choose depends on the growing application. The smaller sizes have some capillary or “wicking” capability, while the larger sizes are most noted for their rapid drainage. Net pots are best suited to the 8-16mm range, while the larger 24mm sizes are ideal for larger containers. LECA pellets are an excellent growing medium for recirculating hydroponic systems, and can also be used to increase drainage in peat mix containers. Their structure retains significant oxygen and moisture for healthy roots and is relatively neutral in pH. It also provides caution exchange (CEC-see vermiculite).

An additional benefit to clay pellets is that they are extremely slow to break down. They can be reused each crop if sterilized. The best way to do that is remove all leaf and root debris and to soak in a mild bleach solution ( one part bleach: 10 parts water). Rinse very thoroughly and let the remaining chlorine dissipate over a few days (it will turn to a gas and leave the medium). Pre-treat with a balanced nutrient solution before replanting.

Common applications for LECA rocks include: tube systems, bucket systems, flood and drain tables, drip feed containers and troughs, and to improve the drainage and add cuation exchange capacity to organic mediums and rockwool.


Excellent for propagation, including rooting cuttings. Rockwool should not be the medium of choice for beginners or for those without the time or monitoring equipment.

Rockwool is manufactured by spinning molten rock like cotton candy, providing a sterile and fibrous growing medium. The orientation of the strands varies from one rockwool product to another. Those with horizontally orientated strands are slower to drain, but provide more lateral (sideways) root growth (typically slabs). Those with vertical strand orientation are quicker to drain and encourage the downward growth of roots (typically cubes, making them an excellent choice for rooting cuttings). When handling rockwool, particularity when wet, be gentle, as compaction will greatly effect the air holding capacity of the medium.

Cubes and slabs generally hold about 80% moisture, but as mentioned are available with different strand orientations. Granular rockwool (loose) makes an excellent addition to container grown plants. It is available in a water repellent and absorbent form. The repellent is excellent for reducing compaction at the roots, providing greater air space. The absorbent will help to hold moisture and nutrients in reserve. One important aspect of managing rockwool is that it is higher in pH than is optimum. All rockwool should be soaked in a pH adjusted water or mild nutrient solution prior to planting. Soak for at least 24hrs at a pH of 5.5. The pH will still tend to climb higher than desired for the first couple of weeks, but gradually stabilizes. The pH in the reservoir can be significantly lower than in the medium. If the pH in the reservoir is maintained around pH5.5 it should be near pH 6.3 at the roots.

When handling rockwool when dry, wear a protective breathing mask, and cover exposed areas of the skin. Some people are sensitive to skin and lung irritation. It is relatively safe to handle when wet. Spraying your arms with hair spray prior to handling provides relief for some. If rockwool dries out, it can be a little difficult to rewet evenly, so consider a wetting agent, or be very careful not to let it completely dry out. Rockwool is often used in commercial production for up to a year at a time. It must be sterilized and scraped of old roots before reusing. Carpet steamers can effectively sterilize rockwool. The slabs will begin to loose their structure after a couple of crops, but can be chopped into flock (granular form).

Rockwool has a tendency to accumulate salt build-up so there should be at least 15% run-off with each irrigation, or flush with pH adjusted plain water once a week.

The diagram illustrates one of the most effective hydroponic systems the author has had the pleasure experiencing (thanks Bob!). This system can be modified for most growing mediums. If the plants are in individual containers they can be moved throughout the system to optimize lighting and ventilation. In case there is a problem with the growing system, the plants can be moved to another area. Corrugated, opaque (does not allow light to pass through) are available at most building centers. The panels should be supported by sawhorses every 12″ on center. This system is well suited to set out as tiers (different heights per panel). The excess nutrient solution is channeled back to the reservoir, so there is no need for sides. An eavestrough works great for directing nutrient solution to the reservoir, and can catch any debris that may have washed in.


Synthetic growing mediums include Oasis and and polyurethane foams (similar to carpet under padding). Oasis blocks have been used for displaying and preserving cut flower arrangements for quite some time. Oasis has a consistent structure and extremely high porosity, in fact it always floats. In North America growers have found it to be an excellent medium for starting cuttings. For some growers, it has been more effective than peat pellets or rockwool cubes. This is largely due to the fact that it contains extraordinary amounts of air when fully saturated. In Europe, some growers use a granular form of Oasis for potted plants and sleeves. The pieces measure roughly 1″ x 2″. Floral blocks can be easily and inexpensively purchased from florist suppliers. If you have the time, they can be cut down into 1″ X 2″ pieces, and it’s not as time consuming as you may think. Excellent results have been achieved using Oasis as a growing medium. It is slightly acidic, so a pretreatment prior to planting would be in order. It makes an ideal medium or amendment for those who tend to over water or “push” their plants. It should not be reused, and when dry, can be crushed into a fine powder to be easily disposed of.

Polyurethane foams have gained some popularity because they will break down more readily than rockwool, so are thought to be more ecologically sound. Studies suggest that yields can be equal to or greater than rockwool for commercial vegetable crop production. It is generally available as slabs, and the cost tends to be somewhat higher than rockwool. Some Eastern Canadian greenhouse suppliers are familiar with the product.

The above article has been a brief overview of common hydroponic/soilless growing mediums. There are many important management techniques that must be followed with each type for success. They are only as good as how they are managed. True hydroponics-water culture and Aeroponics attempt to by-pass growing mediums altogether, and if used at all, do little more than support the plant. Consider what growing medium will work best for you. Certain strains will prefer one medium over another, so experiment.