Chapter 11

      The growing medium holds the roots firmly so that they can support the plant, and holds the water-nutrient solution and air so that they are available to the plants. It is obvious that the roots are used by the plant to obtain water and nutrients, but they need oxygen too. Roots not obtaining sufficient oxygen become sickly and are attacked by mildews and rots.
      Planting mediums range the spectrum from totally organic to artificial materials. Organic materials such as compost, topsoil, humus, worm castings and steer manure have nutrients tied up in complex molecules.
      Almost everyone has grown a house plant After the plant was in the container for a while, its growth slowed for 2 reasons: the roots were pot bound and the nutrients in the planting medium were used up. When some fertilizer was added to the water, the plant showed renewed vigor.
      The inert soil held the water-nutrient solution but did not supply nutrients of its own. The container became a simple hydroponic unit The nutrients in the fertilizer were in soluble form and immediately available to the plant through the water.
      Most books take a vehement stand on the advantages of hydroponics vs. soil for indoor cultivation. We are courageously sticking to a non judgmental middle ground. Almost everyone growing marijuana indoors delivers at least part of the nutrients mixed in water. This is, of course, hydroponic.
      Many growers make a nutrient rich "planting mix" or use top soil. These mediums support growth for some time without additional fertilization, but they are not "natural". Frankly, it is impossible to get a mini-ecosystem going in each container or tray. It does not matter to the plant. As long as its needs are met, it thrives.
      Some growers maintain that earth based systems produce tastier crops, but I have experienced some appetizing hydroponic product, and in fact the best tasting tomato I ever ate was grown in a hydroponic store’s Sunlit growing unit. Various books and magazines advise that hydroponic growing is more exacting and less forgiving than organic growing methods. In fact, hydroponic growing takes no more expertise or skill than growing in soil based mediums.

Here is a list of ingredients for planting mixes:

      TOPSOIL - is a rich mixture of decaying organic matter and minerals which is the uppermost and richest layer of soil. It is sold in nurseries for use outdoors. It is looks dark brown, almost black and smells earthy. It is about as organic as you can get. However, it is not sterilized or pasteurized, so it may contain pests or pest eggs as well as fungi and diseases. This is usually not a problem though. Although topsoil works well in the ground, it is heavy in containers and clumps or packs unless used with other ingredients which lighten it. Packed soil prevents water from being distributed evenly. Part of the medium becomes soaked, while the other part remains dry.

      COMPOST - is an earthy smelling almost black crumbly mixture containing decayed plant matter. it is teeming with life and although not necessarily high in nutrients, it provides a rich environment for the roots. It is acidic unless limed. Some commercial composts are nothing more than chopped up dried plant matter. This material may add some organic matter to the soil, but is not the same as real compost.

      WORM CASTINGS - is compost digested by worms. As they digest the ingredients they concentrate them so that the nutrients are readily available to the plants. It is a excellent ingredient in mixes.

      HUMUS - is a compost produced in a very moist environment. It is very fine textured and rich in nutrients, but is quite acidic.

      COMMERCIAL POTTING MIXES - are not soils at all, but mixes containing ingredients such as tree bark, peat moss, wood by-products, as well as artificial ingredients. These mixes have virtually no nutrient value unless fertilizers have been added. Usually mixes with organic ingredients are long on carbon compounds and short on nitrogen, which means they need fertilization. In a controlled experiment, researchers with the California Dept of Agriculture found that commercial planting mixes vary in their ability to support plant growth, even with fertilizers added. There was no way of telling which mediums were best without testing them by growing plants in them.

      VERMICULITE - is puffed mica which has been "popped" with heat. It is inert, and holds water like a sponge. It is often mixed with other ingredients to loosen the mix and aid in both its water and air retention. It comes in various sizes. The coarse and medium sizes are preferred because they allow more air to form between the particles than the fine. Vermiculite is very light weight

CAUTION: Dry vermiculite produces a lot of dust which is harmful to breathe. It contains minute amounts of asbestos. Before using the material wet it down with water. This prevents the dust from forming. It comes in 4 cubic foot bags at nurseries and grow stores.


      PERLITE - is puffed volcanic pumice. It does not absorb water, but holds it on/in its pitted surface. It is used to loosen planting mixes and stabilize their water holding properties. It is so light weight it floats in water. It comes in various sizes. Coarse perlite allows the most air to mix with the medium.

CAUTION: Dry perlite produces an obnoxious dust. Wet it down before using it.


      SAND - both construction or horticultural - was much more popular as a soil ingredient before vermiculite and perlite were available. It performs many of the same duties in the planting mix; stabilizing water retention and loosening the structure. The problem with sand is its weight. Even a cupful of sand adds considerable weight to a container.

      GRAVEL - holds a little water on its surface and loosens soil. It is heavy and tends to sink in the medium. It is sometimes used alone in hydroponic mixes.

      LAVA - holds water on its irregular surface and holes in its structure. It is lighter weight than gravel. It is sometimes used as a hydroponic medium. Clay pellets are sometimes used in place of lava because they are lighter weight Pea size pieces are the best to use.

      STYROFOAM - is hydrophobic, and is used to keep mediums dryer. It is extremely lightweight and tends to float to the surface of the medium. Usually the little balls are used but sometimes irregular chips are.

      PEAT MOSS - is chopped and decayed moss. It performs many tasks in planting mixes. It helps to retain water and holds nutrients and is a nutrient buffer which holds excess nutrients rather than letting them remain too concentrated in the water. For this reason most commercial mixes contain peat moss. It is very acidic and will lower the pH of the medium so that it should compose no more than 20% of the mix.

      STEER MANURE - is fairly rich in nitrogen and other nutrients including trace elements. It holds water well. Many growers swear by it. Unless it is pasteurized, it may contain insect eggs and other pests.

      BARK - is lightweight, absorbs water and holds air in its pores. As it comes in contact with fertilized water it slowly deteriorates, becoming more of a compost. It is used extensively by commercial greenhouse growers. It can be substituted for lava and it weighs much less.


      Substrates have recently become the hot end of the medium market These are materials which come in a solid form, usually a block, and need no pre-preparation. They are just placed in the growing chamber and watered. They are inert; sterile and hold water and air well. Most experiments show that plants do better in these mediums than in most mixes. Transplanting substrates is very easy. The smaller used piece is placed on top of the larger new piece. The roots grow into the new block. All of the substrates support fast vigorous growth.

      ROCKWOOL - is the most popular substrate. Originally it was used as an insulating material in home construction. Then commercial greenhouse growers in Europe started to use it for their crops. Rockwool looks a lot like fiberglass. It is made by heating rock and extruding it into thin threads. Rockwool comes in pre-pressed blocks and filled bags. It is lightweight and it holds a tremendous amount of water, more than soil, but allows plenty of air in. It comes in several forms, blocks and cubes of various sizes, bags filled with loose fiber and bales of fiber to be placed in containers. It is reuseable for several crops.

CAUTION: Rockwool releases noxious fibers when it is dry. Before growers use rockwool, the material should be wetted. A face mask and rubber or leather gloves, should be used. Body should be covered with a face mask in place.


Rockwool provides a uniform consistency and holds both water and air.


      FLORAL FOAM - is used to make flower arrangements. It is very lightweight when dry, but holds a tremendous amount of water. It is inert and easy to use. It releases no deleterious fibers into the environment. The problem with floral foam is that horticultural grades come only in small cubes. The larger blocks which are used for floral arrangements have been treated with a preservative which is not good for growing plants. Before these blocks are used they should be well rinsed with water to remove the chemicals.

      FOAM RUBBER - (such as the stuff used for mattresses) is lightweight and holds a lot of water and air. It is inert and easy to use in either the block form or as chips in a container. It can also be added to planting mixes if chopped to pea size.

      UPHOLSTERER’S FOAM - is a thin structured foam used for furniture. It comes in rolls and is about ½-¾ inch thick, although it is easily compressed. It holds ample quantities of both water and air. Since it does not come in block form it can be used by rolling it up firmly and placing the cylinder in a container or by holding it together using a rubber band or tape. Growers have reported fantastic results using it.

What Growers Do In The Confusion

      All of this information might seem a little confusing. An interested Party might ask, "Can’t someone just throw some dirt in a pot and plant the Seed? What’s with all of this complex stuff?" Selecting the right medium is very important to the plant, and the mixes are easy to prepare.
      Successful houseplant growers often choose their favorite house plant mix. Here are some adaptions of popular mixes. The mixes with soil, compost or worm castings contain some nutrients for plants and help to "buffer" the nutrients supplied through the water. Buffering means holding nutrients within the chemical structure so that they are temporarily unavailable. This helps prevent over fertilization.

Organic Mixes

      These mixes contain organic ingredients which help to support plant growth and act as a buffer.

  1. 4 parts topsoil, 1 part peat moss, 1 part vermiculite, 1 part perlite. Moist. Contains medium high amounts of nutrients. Best for hand watering systems.

  2. 1 part worm castings, 2 parts vermiculite, 1 part perlite. Light weight, high in nutrients.

  3. 1 part worm castings, 1 part compost, 1 part topsoil, 2 parts vermiculite, 2 parts perlite, 3 parts styrofoam. Holds high amounts of water and air.

  4. 1 part worm castings, 1 part peat moss, 1 part lava, 1 part vermiculite, 1 part perlite, 1 part styrofoam. Good buffering capabilities.

Inorganic Mixes

      These mixes contain only sterile, inert ingredients and have no nutrient value.

  1. 1 part vermiculite, 1 part perlite.

  2. 3 parts vermiculite, 3 parts perlite, 2 parts styrofoam.

  3. 1 part vermiculite, 1 part perlite, 2 parts styrofoam, 1 part sand, I part lava, 1 part peat moss.

  4. Lava, pea sized gravel or small ceramic beads alone or mixed with a little vermiculite.

      All of the mixes listed will support a vigorous, fast growing crop. Some growers try several different mixes to see which they like working with.
      If I had to choose one medium for cultivation, I would use one of the substrates. I feel they have many advantages: They are easy to prepare (no preparation), distribute water and air well, are easily disposable and promote rapid growth. Their main disadvantage is that they have no buffering abilities so that the plants are more sensitive to over fertilization. First time growers usually feel more confident with a mix.

Step by Step

  1. Several different mixes are sometimes tried at the same time.

    1. Mixes with nutrients supply some of the nutrients required by the plant.

    2. Mixes with organic ingredients "buffer" or chemically bind with fertilizers. They allow the grower a little leeway.

    3. Substrates are convenient to set up. They often require no other containers. They require a little more care than other systems.

  2. Enough mix is prepared to fill the containers.


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