Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

buffering ability.
    Pumice is a naturally occurring, porous, lightweight, volcanic rock that holds moisture and air in catacomb-like surfaces. Light and easy to work with, some lava rock is so light it floats. Be careful that sharp edges on the rocks do not damage roots. Lava rock is still a good medium and acts similarly to expanded clay. See “Pumice” under “Soil Amendments.”
    Peat moss is partially decomposed vegetation. Decomposition has been slow in the northern regions where it is found in bogs. There are three common kinds of peat moss–Sphagnum, Hypnum, and Reed/Sedge. Sphagnum peat is about 75 percent fiber with a pH of 3 to 4. Hypnum peat is about 50 percent fiber with a pH of about 6. Reed/sedge peat is about 35 percent fiber with a pH of six or more. For more information see “Soil Amendments” in Chapter Ten.
    Perlite drains fast, but it’s very light and tends to float when flooded with water. Perlite has no buffering capacity and is best used to aerate soil or soilless mix. See “Soil Amendments” in Chapter Ten.
    Rockwool is an exceptional growing medium and a favorite of many growers. It is an inert, sterile, porous, non-degradable growing medium that provides firm root support. Rockwool has the ability to hold water and air for the roots. The roots are able to draw in most of the water stored in the rockwool, but it has no buffering capacity and a high pH. Rockwool is probably the most popular hydroponic growing medium inthe world. Popular brand names include Grodan, HydroGro and Vacrok.
    Sand is heavy and has no buffering ability. Some sand has a high pH. Make sure to use sharp river sand. Do not use ocean or salty beach sand. Sand drains quickly but still retains moisture. Sand is best used as a soil amendment in volumes of less than ten percent.
    Sawdust holds too much water for marijuana growth and is usually too acidic. Be wary of soils with too much wood matter. Such mediums use available nitrogen to decompose the leglin in the wood.
    Vermiculite holds a lot of water and is best suited for rooting cuttings when it is mixed with sand or perlite. With excellent buffering qualities, vermiculite holds lots of water and has traces of magnesium (Mg), phosphorus (P), aluminum (Al), and Silicon (Si). Do not use construction grade vermiculite which is treated with phytotoxic chemicals. See “Soil Amendments” in Chapter Ten for more information.
    Water alone is a poor medium, because it cannot hold enough oxygen to support plant life. When aerated, water becomes a good growing medium.
Sterilizing
    To reuse a growing medium, it must be sterilized to remove destructive pests and diseases. Sterilizing is less expensive and often easier than replacing the growing medium. Sterilizing works best on rigid growing mediums that do not lose their shape such as gravel, expanded clay, and mica. Avoid sterilizing and reusing substrates that compact and lose structure such as rockwool, coconut coir, peat moss, perlite, and vermiculite. Avoid problems caused by compaction and dead roots by replacing used growing mediums. Once sterilized, the medium is free of harmful microorganisms including bacteria and fungi, plus pests and their eggs.
    Remove roots from the growing medium before sterilizing. A three- to four-month-old marijuana plant has a root mass about the size of an old desk telephone. Separate the medium by shaking and pulling roots away. Bounce the medium on a screen so roots come to the top. Scoop up and remove roots by hand. Fewer decaying roots cause fewer pest and disease problems, and decrease incidence of clogged feeder tubes.

Rockwool cubes hold plenty of air and nutrient solution within their fiber; plus, they are dean and easy to use.

Soilless grow medium.

Vermiculite holds a lot of nutrient solution.

Remove harvested plant root balls from the growing medium, and remove roots and stems.

Wash substrate with plenty of fresh water to remove built-up salts and dead roots.
    Substrate can also be washed in a large container such as a barrel or bathtub. Washing works best with lighter substrates such as expanded clay or mica. Roots float to the top and are readily skimmed off with a screen or by hand.
    Once roots are removed, soak the substrate in a sterilant such as a five percent laundry bleach (calcium or sodium hypochlorite) solution for at least an hour. Or mix hydrochloric acid, the kind used in hot tubs and swimming pools. Pour, drain, or pump off the sterilant, and flush the