Article 3-2 Growing MediumIt 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 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: Perlite: Vermiculite: Some Other Beneficial Additions to Soilless Mixes Earth worm castings: Dolomite Lime:
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" 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. Rockwool 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. Synthetics 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. |
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