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There are probably hundreds of different kinds of growing medium, anything that a plant can grow in is considered a growing medium. There are manmade as well as organic (natural) mediums. Even plain old AIR can be an effective growing environment for roots.|
I have been asked many times what growing medium is the best. This is like asking what is the best color? Or what is the best kind of vehicle to own. Sometimes the answer depends on the job you need it to do. You wouldn't try to use a soiless mix in an Aeroponic system and you don't plow a field with a Rolls Royce limousine. However, if you want to build a Non-Recovery Drip hydroponic system then the soiless mix would be an excellent choice, and a John Deere tractor can handle the field (save the Rolls for a night on the town, pick me up at 8). What I'm trying to say, is that the best growing medium for your purpose depends on many variables. The type of system you are using, what kind of crop you are growing and local environment are just some of the many determining factors involved when choosing a growing medium. There may be several mediums that will work equally well for your particular needs. Many times it boils down to availability, price or personal preference.
I have listed the most popular types of growing mediums below, click on the name to view details about the general use, advantages and disadvantages, and particular characteristics of the specified growing medium.
Growing Medium Index
These lightweight pre-formed cubes are designed for propagation. A very popular medium for use when growing from seed or from cuttings. This product has a neutral pH and retains water very well.
The cubes are meant to be a starter medium and come in three sizes up to 2" x 2". They can be easily transplanted into practically any kind of hydroponic system or growing medium (or into soil).
Coconut fiber is rapidly becoming one of the most popular growing mediums in the world. In fact it may soon be THE most popular. It is the first totally "organic" growing medium that offers top performance in hydroponic systems. Coconut fiber is essentially a waste product of the coconut industry, it is the powdered husks of the coconut itself.
There are many advantages - it maintains a larger oxygen capacity than rockwool, yet also has superior water holding ability than rockwool which is a real advantage for hydroponic systems that have intermittent watering cycles.
Coconut fiber is also high in root stimulating hormones and offers some protection against root diseases including fungus infestation. Dutch growers have found that a mixture of 50% coconut fiber and 50% expanded clay pellets is the perfect growing medium.
One word of caution about coconut fiber, you must be careful when you purchase coconut fiber. There is a commonly available, lower grade of coconut fiber that is high in sea-salt and is very fine grained. This lower grade coconut fiber will lead to disappointing results when used in a hydroponic system.
Good old perlite! It's been around for years, mainly for use as a soil additive to increase aeration and draining of the soil. Perlite is a mined material, a form of volcanic glass that when rapidly heated to more than 1600 deg. f. it pops much like popcorn as the water vaporizes and makes countless tiny bubbles.
Perlite is one of the best hydroponic growing mediums around. Used by itself or as a mixture with other mediums. Perlite is commonly used with vermiculite ( a 50 - 50 mix is a very popular medium), and is also one of the major ingredients of soiless mix's. perlite has good wicking action which makes it a good choice for wick-type hydroponic systems. Perlite is also relatively inexpensive.
The biggest drawback to perlite is that it doesn't retain water well which means that it will dry out quickly between waterings. The dust from perlite is bad for your health so you should wear a dust mask when handling it.
Vermiculite is another mined material. In it's natural state it resembles mica rock, but when quickly heated it expands due to the generation of interlaminar steam.
Vermiculite is most frequently used in conjunction with perlite as the two complement each other well. Vermiculite retains moisture (about 200% - 300% by weight), and perlite doesn't so you can balance your growing medium so that it retains water and nutrients well but still supplies the roots with plenty of oxygen. A 50/50 mix of vermiculite and perlite is a very popular medium for drip type hydroponic systems as well as ebb and flow systems. Vermiculite is inexpensive.
The major drawback of vermiculite is that it retains too much water to be used by itself. It can suffocate the roots of plants if used straight.
There are many kinds of soiless mix's containing a vast assortment of ingredients. Most contain things like Spaghnam moss, Perlite and Vermiculite.
These kind of growing medium are usually considered organic and are frequently used for container gardening wick systems and on-recovery drip systems. They can be used in recovery systems, however most of these mixes have some very fine particles that can clog pumps and drip emitters if you don't use a good filtration system (NOTE: The professor says that you can use panty hose as a filter, use on the return line and on the pump inlet to filter out the fine particles).
Most soiless mixes retain water well and have great wicking action while still holding a good amount of air, making them a good growing medium for a variety of hydroponic and organic gardens.
Expanded Clay Pellets
This man-made product is often called grow rocks and is an extremely good growing medium. It is made by baking clay in a kiln. The inside of the clay pellets is full of tiny air pockets (much like lava rock) which makes this a light weight medium (some of the pellets even float).
The pellets are great for ebb & flow systems or other systems that have frequent watering cycles (clay pellets do not retain much water so they need to be watered often so that the roots of your plants do not dry out). The rocks are often mixed with other growing medium(s) to increase oxygen retention.
Expanded clay pellets are rather expensive but they are one of the few kinds of growing medium that is easily reusable, which makes them a good choice for the long term. After you harvest your crop you can wash the clay rocks to remove all the old roots and then sterilize them with a 10% bleach and water mix (one part bleach to 9 parts water). The grow rocks can also be sterilized by using a mixture of Hydrogen Peroxide and water (use 1 or 2 teaspoons of 35% food grade hydrogen peroxide per gallon of water).
A completely natural medium that is used as a major ingredient in most soiless mixes. Sphagnum moss can also be used by itself in a hydroponic system. Sphagnum moss makes a good fluffy growing medium that retains a high percentage of air and retains water well also.
The major problem with this growing medium is that it can decompose over time and you can get small particles that can plug up your pump and (or) drip emitters if you are using a recovery type hydroponic system.
Lava rock has been used successfully for years, it is light-weight and retains a fair amount of water in it's holes and pores. It is used most often in ebb & flow (fill & drain) systems with frequent watering cycles.
With a good selection of first rate growing mediums available lava rock is used much less often these days. The drop in popularity is due mostly to the fact that the sharp edges of the rock can cause root damage to the plants, and in most areas of the world it can be hard to find lava rock that is not chemically treated.
This growing medium has had limited success. There are many variables that determine how well saw dust will work, most predominantly is the KIND of wood that the dust is made from. Some kinds of wood can give off chemicals that are detrimental to the health of the plant(s). Another problem with saw dust is that it will decompose, which can cause problems. Saw dust also retains a lot of moisture so care must be taken not to over water.
The best thing about saw dust is that it's usually free. I don't recommend using saw dust unless you are into experimenting.
When most people think of hydroponics they think of plants with their roots suspended in water with the nutrients dissolved in it. This is a very popular method of growing hydroponically and there are several types of systems that use water as the growing medium (deep flow N.F.T., shallow flow N.F.T. and water culture are among the most popular).
Water is a critical element in the growth of plants anyway, so using it as the growing medium makes a lot of sense. Care must be taken when selecting a system that uses water as the only growing medium, to ensure that the plant(s) are compatible. For example: Water-loving plants like "Bibb" type lettuce does best in a water-culture system where the plants float directly on the surface with their roots hanging into the water, but the same system will not work as well for most other plants because there is too much water and not enough oxygen. These other plants will do much better in a N.F.T. system where more oxygen is available to the roots because the plants are suspended above the level of the water.
This growing medium has been used for years and works well. Many or the earlier hydroponic systems that were commercially available to the public were gravel based ebb / flow (flood and drain) type systems.
Gravel supplies plenty of air to the roots, but doesn't retain water, which means that the plants roots can dry out quickly if they are not watered enough. Another drawback to gravel is its weight, it's very heavy, and toting it around is difficult.
Gravel is usually fairly cheap (depending on where you live) and easy to find. You can easily reuse gravel as long as you wash and sterilize well between crops. After you harvest your crop you can wash the gravel to remove all the old roots and then sterilize them with a 10% bleach and water mix (one part bleach to 9 parts water). The gravel can also be sterilized by using a mixture of Hydrogen Peroxide and water (use 1 or 2 teaspoons of 35% food grade hydrogen peroxide per gallon of water).
People talk about hydroponic systems that do not use any growing medium at all. As far as I know, that would be impossible. The plants roots would have to growing in a complete vacuum. This would instantly kill the plant.
Air on the other hand is frequently used as a growing medium. Aeroponic systems have the plants roots hanging in air and are periodically sprayed with a nutrient solution.
The biggest advantage to growing in air is the roots get all the oxygen they could possibly need (roots need plenty of oxygen). Another major advantage to air is it's cost (Free is hard to beat!). There is no disposal problems as with some other mediums either.
The biggest problem associated with aeroponics is it's susceptibility to power failures and pump or timer failures. There is NO buffer.The roots could start to dry out within minutes and the loss of the total crop can come very quickly.
I have never personally used fiberglass insulation but I have known several people that have used it in their hydroponic systems. There were mixed reviews, and none of those people are still using it.
The most common complaint that I have heard is that it retains too much water, not leaving room for enough air around the roots, which can cause problems with the plant. I have heard that sometimes the insulation is treated with chemicals for fireproofing, etc., so unless you want to experiment, the Professor would not recommend using fiberglass insulation.
The Hanging Gardens of Babylon were thought to be sand-based hydroponic systems (what else would you use if you're stuck in the middle of the desert?). This is probably the first hydroponic growing medium ever used, and it is still being used successfully.
Sand has a tendency to pack tightly together reducing the amount of air available to the roots. So you should use a coarse builders sand or mix the sand with perlite or other material that will increase aeration.