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Unit 2 Part (C). Growing Media
Growing Media

There are many types of growing media that have been successfully used for hydroponics and there are probably many more that have never been tried.

Some are:

1. Perlite - a volcanic rock a gray obsidian, that has been heated to 1500 degrees F in a kiln and expanded. It is a light weight porous material that can "wick" water from a bottom container of water.

2. Ceramic grow rock - a clay material also called Geolite, which is often used for aquaculture because the porous material is a good media for growing bacteria to clean water. It does not break down.

3. Rockwool - a material made from rock spun into a fiber like material. A phenol based resin is added as a binder. Rockwool also tends to increase the pH of the water.

4. Pea gravel - this media is just simple gravel but has been graded for size and shape. It is not a porous media so it does not wick water from below and must be used in a system that provides aeration for the water. It can be used to grow bacteria as well as plants.

There are many other types of media used in hydroponic systems. Some have special advantages and disadvantages.

Sand - Many sands, such as beach sand, have salts already in the media, that can cause problems in hydroponics. However, sand is a useful media that retains water. It has to be sterilized between crops.

Sawdust - where there is an extensive timber production, sawdust may be available. The species of tree is important, with softwoods decaying more slowly than hardwoods. Douglas fir and western hemlock work great but red cedar is toxic to plants. Some sawdust is from logs soaked in salt water and is therefore toxic to plants.

Peat - There are three types of peat: peat moss, reed sedge and peat humus. Peat is very acid and can lower the pH of the nutrient water. It breaks down after one or two growing seasons.

Vermiculite - This is a volcanic mica which has been popped in a kiln. It is a magnesium aluminum iron silicon material that can be compressed and lose its porosity.

Pumice - A silicon material of volcanic origin can break down after repeated use.

No media - There are many hydroponic systems that use no media whatsoever. The plant is usually started in a small piece of rockwool, or specially designed plastic collar. The plant is then placed in a growing tube or container that applies nutrient water to the roots.

Choosing a Media

The hydroponic plant roots need to have both nutrient water and air.

If the water is pumped or poured only once or a few times a day, there should be enough media around the plant roots to capture and retain some water. This means the media should be somehow able to capture some air and some water. That is why the "best" hydroponic media are porous materials like pumice, perlite, grow rock and rockwool.

Also, in a system that is pumped or pored a few times a day, there should be some reservoir of extra water for the plant. If the plant runs out of water, first it shuts down or quits working, and then it starts to die.

If the plant roots have a growing area, with both water and air, and a water reservoir, the plant will continue to thrive and grow. If there is a small plant root growing area, or the plant roots run out of water, the plant does not thrive.

If the plant roots "see" extra water below the root growing area, the plant does not have to grow extra root to "find or follow" the water. So the plant can relax about its water supply and use its energy to grow more above ground.

In the simple tub system, there is a root growing area, the water reservoir, and the overflow water container. There is a way to design this to help the plant grow.

The plant root growing area is the area where the roots see air and water. How much area is needed for the plant partly depends on the type of media, how it holds both air and water.

Perlite is an excellent media because it "wicks" water, or draws up water from below. The best mixes of water and sir are in the 50-60% range, and with perlite this area can be up to about 4 inches above the water level.

If a media like perlite is used, less area is required for the root growing area.

Revised: 1 May 2016
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