Growing Mediums

Marijuana Horticulture

by Jorge Cervantes

Soilless growing mediums provide support for the root system, as well as hold and make available oxygen, water, and nutrients. Three factors contribute to cannabis’ roots ability to grow in a substrate: texture, pH, and nutrient content, which is measured in EC, electrical conductivity.

The texture of any substrate is governed by the size and physical structure of the particles that constitute it. Proper texture promotes strong root penetration, oxygen retention, nutrient uptake, and drainage. Growing mediums that consist of large particles permit good aeration and drainage. Increased irrigation frequency is necessary to compensate for low water retention. Water and air holding ability and root penetration are a function of texture. The smaller the particles, the closer they pack together and the slower they drain. Larger particles drain faster and retain more air.

Irregular shaped substrates such as perlite and some expanded clays have more surface area and hold more water than round soilless mediums. Avoid crushed gravel with sharp edges that cut into roots if the plant falls or is jostled around. Round pea gravel; smooth, washed gravel; and lava rocks are excellent mediums to grow marijuana in an active recovery system. Thoroughly wash clay and rock growing mediums to get out all the dust that will turn to sediment in your system.

Fibrous materials like vermiculite, peat moss, rockwool, and coconut coir retain large amounts of moisture within their cells. Such substrates are ideal for passive hydroponic systems that operate via capillary action.

Mineral growing mediums are inert and do not react with living organisms or chemicals to change the integrity of the nutrient solution. Coconut cir and peat mosses are also inert.

Non-inert growing mediums cause unforeseen problems. For example, gravel form a limestone quarry is full of calcium carbonate, and old concrete is full of lime. When mixed with water, calcium carbonate will raise the pH, and it is very difficult to make it go down. Growing mediums made from reconstituted concrete bleed out so much lime, they son kill the garden.

Avoid substrates found within a few miles of the ocean or large bodies of salt water. Most likely, such mediums are packed with toxic salts. Rather than washing and leaching salts from the medium, it is easier and more economical to find another source of substrate.

Air is a great medium when it is filled with 100 percent humidity 24 hours a day.

Coconut fiber is an excellent hydroponic medium.

Expanded clay, also called hydroclay, or hydrocorn, is made by many different manufacturers. The clay pellets are coked at high temperatures in a kiln until they expand. Many little catacomb like pockets form inside each pellet that hold air and nutrient solution. It is an excellent medium to mix with Peat-Lite and to grow mothers in large containers. I like the way it drains so well and still retains nutrient solution while holding lots of oxygen. Examples of expanded cay include commercially available Hydroton, Leca, Grorox, Geolite.

Expanded cay can be reused again and again. Once used, pour expanded clay pellets into a container and soak in a sterilizing solution of ten milliliters hydrogen peroxide per four liters of water. Soak for 20-30 minutes. Remove expanded clay and place on a screen of hardware cloth. Wash and separate clay pellets from dead roots and dust. Let dry and reuse.

Expanded mica is similar to expanded clay.

Foam is somewhat popular. It lasts a long time, lends itself to easy sterilization, and holds a lot of water and air.

Gravel is one of the original hydroponic mediums. Although heavy, gravel is inert, holds plenty of air, drains well, and is inexpensive. Still popular today, gravel is difficult to overwater. It holds moisture, nutrient, and oxygen on its outer surfaces. Use pea gravel or washed river gravel with round edges that do not cut roots when jostled about. Grave should be 0.125-0.375 inches in diameter, with more than half the medium about 0.25 inch across. Crushed rock can be packed with many salts. Pre-soak and adjust its pH before use. Gravel has ow water retention and low 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.

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.

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.

Rockwool is an exceptional growing medium and a favorite of many rowers. 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 in the 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 (A), and Silicon (Si). Do not use construction grade vermiculite which is treated with phytotoxic chemicals.

Water alone is a poor medium, because it cannot hold enough oxygen to support plant life. When aerated, water becomes a good growing medium.