One of the most obvious decisions hydroponic farmers have to make is which medium they should use. Different media are appropriate for different growing techniques.
Growstones are a substrate for growing plants that can be used for soilless purposes or as a soil conditioner.
This substrate is made from recycled glass. It has both more air and water retention space than perlite and peat.
Another property of this medium is that it holds more water than parboiled rice hulls.
Growstones appear to be a comparable alternative to expanded clay aggregate.
Growstones are made in a variety of sizes from a large 1 to 2 inch sized “Lift” product made to supplement drainage in potted plants while retaining superior moisture and nutrients for the plant roots,
To “Gnat Nix” which is a small “5MM” size product to prevent fungus gnats from thriving in potted plants.
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Expanded Clay Aggregate Substrate
It is made by heating clay to over 2,000 degrees Fahrenheit using a rotary kiln, which gives it that signature pebbly form.
This process fills the clay with little air bubbles, making it perfect for holding oxygen as well as moisture around plant roots.
It can be mixed with soil or used alone. The clay is formed into round pellets and fired in rotary kilns at 1,200 °C (2,190 °F).
This causes the clay to expand, like popcorn, and become porous. It is light in weight and does not compact over time.
The shape of an individual pellet can be irregular or uniform depending on the manufacturing process.
Ecologically sustainable, and re-usable growing medium because of its ability to be cleaned and sterilized, typically by washing in solutions of white vinegar, chlorine bleach, or hydrogen peroxide and rinsing completely.
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Coco coir is an increasingly popular type of hydroponic growing medium and for good reason.
There are a whole host of benefits to growing with coconut coir that you can and should take advantage of if you’re new to hydroponics.
Since coco coir comes from large plants grown across a variety of different conditions the actual chemical makeup of the coco can change very substantially.
The table above shows the pH and EC of different coco coir sources.
As you can see we have everything from an EC of 0.1 mS/cm to an EC of 0.9 msS/cm, with pH values that cover anything from 4.9 to 6.8.
This is mainly due to the big variations in the ions contained within the coco and how these ions interact with the plant material.
This media also has a high cation exchange capacity meaning that it can retain large amounts of ions that are only taken out whenever they are replaced by others with a stronger affinity for the media or when strong interactions with chelating agents within the media are possible.
Rice husk is an organic substrate obtained from rice plants. The advantage of this material is that it doesn’t have a fast decomposing, due to its high silicon content.
Nonetheless, it has a high water resistance, although it can provide a great airflow.
A mixture of rice husk and sand is ideal for hydroponic crops, taking into account that the proportions can vary according to the plant’s necessities.
To prevent the rice seed from growing or fermenting and cause a drastic change in the solution’s temperature,
It’s important to wet the rice husk before growing the hydroponic plants.
The idea is to maintain the rice husk for at least a day underwater before using it.
Vermiculite is clay that expands in a limited way with heat. Once expanded, it provides the ideal conditions to be used on hydroponic crops.
Nonetheless, this material also has a high cationic exchange capacity which may cause alteration of cation concentrations in the nutrient solution.
This could be positive or negative, depending on the hydroponic formulation and on the plant.
Vermiculite is a popular hydroponic media favored for its affordability.
It is made from a natural mineral that expands with the application of heat.
The expansion process is called exfoliation which takes place in purpose-built commercial furnaces.
Vermiculite is very lightweight and sterile.
It has excellent water retention and capillary action properties, which allows it to be fully hydrated simply by applying moisture from above or below.
When used in hydroponics, vermiculite is often mixed with perlite if the media becomes too water logged. This improves aeration and drainage.
In hotter climates vermiculite is often used by itself because of its higher water holding capacity.
Vermiculite also tends to break down after a period of time so is favored for more short-term crops such as lettuce.
Growers who use vermiculite usually have a system in place where they manually fill NFT growpots or cell trays with the media.
A seed is then inserted into the top of the media and the pots and trays are placed into a propagation system like an Ebb and Flow table where they remain until the seedlings have emerged and are at a reasonable size for transplanting into the main growing system.
The root system of the seedling holds the media together so individual root bound ‘plugs’ can be removed from the propagating cell trays and placed directly into the plant holes of an NFT gully if required.
Perlite is a type of amorphous volcanic glass with a high content of water.
For this media to be usable in hydroponic crops it has to be heated to 900°C so the water contained in the crystalline structure liberates and therefore the commercial perlite is obtained, also known as expanded perlite.
This type of perlite has a great water-retaining capacity, leaving enough space for airflow.
The size of the particle in this media is also ideal for big plants’ support.
The only problem with perlite is the fact that in most cases it has to be imported, limiting its use.
Why Use Perlite?
The primary reason to use perlite is to help aerate the environment surrounding plants’ root zones.
Plants use oxygen absorbed through their roots to help them metabolize the sugars they make through photosynthesis, part of the process of creating food for themselves.
Perlite holds air and creates pockets of oxygen in the soil or water to keep root function on track so plants can get plenty of food.
Don’t confuse perlite with vermiculite, another soil amendment. Vermiculite attracts and retains water, and is used to help soils hold more moisture very different from the function of perlite.
The fact that perlite doesn’t hold water is why it is so useful in a hydroponic system, as the air held within its pores helps keep the system oxygenated.
Perlite has a neutral pH, so it won’t affect or interact with the water or liquid nutrients used within the system.
You will, however, need to occasionally replace the perlite used for hydroponics because the pores can become clogged with nutrients, algae, and plant roots, reducing its effectiveness.
Because perlite doesn’t hold water, it’s important to use it within a hydroponic system in which the plant roots continually stay wet.
If you’re planning to use perlite as the sole growing medium, drip systems and bucket systems will work better than ebb-and-flow systems.
Never use perlite in aquaponics, though, as the fish can breathe in the small particles, leading to clogged gills.
Sand is a granular material, generally obtained from any mineral that has been finely divided.
This type of material is ideal for hydroponic crops when combined with other materials that can provide a good airflow,
Because sand by its own can’t provide enough space for airflow and therefore the plants could easily die.
Pumice is a cheap and readily available hydroponic media which is mined in New Zealand.
This porous volcanic rock allows for good water retention and aeration, and the use of a combination of fine and coarse materials allows a grower to manipulate the drainage capacity of the media.
Pumice can also be mixed with other types of growing media e.g vermiculite or coir, to improve aeration and drainage.
Pumice is slow to break down and is very lightweight. Its light coloured appearance makes it an ideal media for summer growing as it does not attract heat.
It is popular for tomatoes and capsicums grown in bags, or lettuces and herbs propagated in NFT growpots.
Pumice is graded and kiln dried to 80 Degrees C prior to supply which means it is sterile and ready to use.
Dust is eliminated as part of the screening and washing process.
Gravel. The word gravel refers to any mineral or rock which has particles of a size between 5mm and 2cm.
Gravel provides excellent airflow and drain, but bad water retention.
When mixed with sand it could provide an ideal growing media, although it’s also ideal for NTF systems because it doesn’t block pipes or moves as easily as the rice husk does.
In hydroponics, growers often use gravel for a type of grow media.
That means they fill cups or pots with gravel, and set plant roots inside.
Then they expose that area to nutrient-rich water.
So one of the first things to think about with gravel is how the hydroponics structure will contain it.
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Simply wood. Great and efficient media for hydroponics. If you want to go truly organic with your growing media
You can’t get any better than wood chips!
Additionally, some studies have indicated that wood chips reduce the effect of plant growth regulators, meaning your plants may grow slightly larger.
What are the benefits of wood fiber?
- Holds structure for a long time
What about downsides of wood fiber?
- May not be sterile
- May attract pests
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Rockwool has been around for decades and is well-known in the hydroponic growing community.
It is made by melting rock and spinning it into extremely thin and long fibers, similar to fiberglass.
They take these fibers and press them into cubes of varying sizes.
Rockwool has all of the benefits of most growing media, with some pretty serious downsides.
It’s not easy to dispose of – thin fibers of melted rock will last essentially forever when disposed of.
Additionally, they usually come at a high pH and need soaking.
The fibers and dust created in the spinning and compressing process can be harmful to the eyes, nose, and lungs.
You can prevent the dust by immediately soaking rockwool in water once you take it out of the package.
Because of these downsides, rockwool is rapidly being replaced by starter plugs as a reliable way to get seeds sprouting in your garden.
Name says it all: crushed up bricks. Very similar in effect to gravel.
However, they may affect the pH as they are not pH neutral, and also require extra cleaning to get rid of brick dust.
What are benefits of brick shards?
- Easy to clean
- Drains well
What about downsides?
- May affect pH
- Requires more thorough cleaning
- Plant roots may dry out
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Polystyrene Packing Peanuts
These are the standard packing peanuts used in the shipping industry.
They’re cheap, available everywhere, and drain fairly well.
Tough to use uncovered outdoor due to their light weight…the typical use is in Nutrient Film Technique systems.
There is the possibility that plants will absorb styrene, so these may pose a contamination risk.
Polystyrene packing peanuts are inexpensive, readily available, and have excellent drainage.
However, they can be too lightweight for some uses. They are used mainly in closed-tube systems.
Note that polystyrene peanuts must be used; biodegradable packing peanuts will decompose into a sludge.
Plants may absorb styrene and pass it to their consumers; this is a possible health risk.
Benefits of packing peanuts: Cheap (often free), very lightweight, drain well
Downsides of Packing Peanuts: The only polystyrene will work – biodegradable packing peanuts will turn to slush. Potential for plants to absorb styrene
Wool from shearing sheep is a little-used yet promising renewable growing medium.
In a study comparing wool with peat slabs, coconut fiber slabs, perlite, and rockwool slabs to grow cucumber plants, sheep wool had a greater air capacity of 70%, which decreased with use to a comparable 43%, and water capacity that increased from 23% to 44% with use.
Using sheep wool resulted in the greatest yield out of the tested substrates, while the application of a biostimulator consisting of humic acid, lactic acid, and Bacillus subtilis improved yields in all substrates.
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