The concept of using hydroponic substrates and other plant growing media can sometimes be confusing especially for beginner or new hydroponic gardeners. To make this easy to understand, you need to realize that soil is not used in hydroponic farming or gardening. But plants will need to develop good roots to be able to absorb nutrients from hydroponic solutions. That is where growing media or hydroponic substrates come in.

What is Hydroponic Substrate?

Hydroponic substrate can be described as a soilless material or media that can be used to provide support and an anchor for plant roots during the tender early growth stages of plants before transplanting them to their nutrient solution. Hydroponic substrates can be made from various materials and can be broadly classified as organic or inorganic substrates based on the material from which the substrate is made.

Properties of Best Hydroponic Substrates

This is what you need to know if you are asking this question – what is the best hydroponic substrate? There are several factors to consider before deciding which hydroponic substrate may be the best for any situation since there are several factors that affect hydroponic farming or gardening. Some of the properties to consider before deciding which hydroponic substrate is the best include:

1. Porosity: This can be described in simple terms as the amount of air-filled spaces that a material has. Availability of air or oxygen is very important for the growth of new roots and the survival of plants.
2. Water retention: A good substrate must have the ability to retain some volume of water to ensure the plant has some form of moisture for nutrients availability even at low electrical conductivities. Water retention should be at a minimal level to help the roots breathe well (oxygen and dissolved nutrient intake) and prevent susceptibility to root rot and other diseases.
3. Neutral pH: The pH of hydroponic solutions is very important as it can determine whether plants will grow well or die. Extreme pH levels can be detrimental to plants but a good hydroponic substrate creates a good stabilizing effect because of cation exchange capacity that keeps the pH level stable and neutral. Hence, a good hydroponics substrate should be inert without affecting the pH of grow solution.
4. High Thermal Inertia: This is a good property for hydroponic substrates because it helps to keep the water solution temperature stable. Temperature is a very important factor to growing plants hydroponically and a good substrate with high thermal inertia may not change the temperature or get hot or cold easily. This will be helpful in maintaining a stable desired temperature of the hydroponic solution.
5. Relatively Clean: This is important because clean substrates will prevent the hydroponic solution and even the entire system from getting contaminated. Care must be taken to avoid or sterilize any material for disease pathogens, harmful chemical agents before using it as a substrate.
6. Sustainable/Re-useable: It is important to use materials that are environmentally sustainable and reusable to conserve resources and prevent environmental pollution. Some hydroponic substrates may be good but may not be easily renewable. Hence it is important to reuse and conserve them effectively. Other materials may be good but may not be environmentally friendly in terms of decomposition. This can create serious challenges with the disposal and potential environmental pollution after use.

Organic Hydroponic Substrates

Organic hydroponic substrates can be described as grow media that is derived from organic and environmental sources and not from chemical or inorganic sources. Most organic grow substrates can easily decompose and can be reused effectively without any negative impact on the environment.

Example of organic hydroponic substrates include: Coco peat, coco coir, coco chips, perlite, peat moss etc.  

Best Types of Hydroponic Substrates

There are several good hydroponic substrates for growing plant seeds and some of them are very safe for beginners and others may require some experience to effectively utilize them for hydroponic grow media. Some of these best substrates include:

Stone Wool:

This is also known as rock wool and it is made by heating and spinning basaltic rocks into fibers. One of the properties of stone wool that has made it a convenient choice for most hydroponic farmers is that it has a great balance between water retention and porosity.

Advantages of Stone wool

• It has a great balance between porosity and water retention
• It is stable and easy to use for a broad range of plants
• It is light weight
• Can be reused

Disadvantages of stone wool

• Can be an irritant when in contact with skin
• Can become a conducive material for mold growth under certain humid conditions

Where to buy Stone wool hydroponic substrates (Cubes, blocks, slabs etc)

Coconut Coir

This is also commonly referred to as “coco coir” and it is made or derived from the husk of coconuts. It is a very popular, common and good hydroponic grow substrate to use. It is usually well processed to get rid of substances that may be detrimental to plant growth like excess salts and tannins before using it as a hydroponic substrate.

Advantages of Coconut Coir

• Has good optimal water to air ratio
• Can have stable pH when washed or properly processed
• Can be used as an organic substrate
• Can be used without mixing with other substrates
• Can be reused

Disadvantages of Coconut Coir

• Can be detrimental to plants when not washed or processed properly
• Its rich tannins may stain hydroponic reservoir or growing area

Where to buy Coco coir (Home depot: powder, cubes)

Coco Peat

Coco peat is also derived from coconut husks as the leftover powder from the processing of fibrous coconut husks. The powder is then washed, dried, and sieved before use. It has excellent properties that make it one of the best grow substrates to use for hydroponic farming.

Advantages of Coconut Peat

• It has high porosity
• Stable pH when processed well
• Organic and eco-friendly
• It has ideal pH and electrical conductivity that makes it one of the best grow media
• It is anti-fungal and may keep plants free from soil-borne pathogen

Disadvantages of Coconut Peat

• Has high water retention capacity which may be detrimental to some plants

Where to buy Coco Peat (Home depot: powder, cubes)

Coco Chips

Coco chips hydroponic substrate is also derived from coconut husks and it has good features that make it a great option for hydroponic grow substrate. The coconut husk is chipped into small sized chips to create coco chips. It has a good electrical conductivity and a stable pH. The coco coir chips absorbed and retain water but tend to lose that water gradually over time without frequent irrigation. It is available commercially in various sizes.

Advantages of Coconut Chips

• Great water retention capacity
• Stable pH and low salt content makes it a good grow substrate for most plants
• Can be used as a standalone hydroponic substrate or combined with other substrates
• Easy to make and store

Disadvantages of Coconut Chips

• May require frequent irrigation

Where to buy Coco Chips (Home depot: powder, cubes)

Perlite

Perlite is another great hydroponic substrate that is derived from rocks. It is made by heating volcanic rocks until it pops almost in the same manner as making popcorn. It has a lot of commercial applications and mostly used in construction. It is lightweight, cheap, and comes in various sizes. Its porous properties make it a good option for aeration especially when combined with other dense substrates. It can also be used as an organic and standalone hydroponic substrate.

Advantages of Perlite

• It is porous with good aeration properties
• Available in various sizes which creates a variety of options for using it
• It is reusable and eco-friendly
• It is pH neutral which helps in stabilizing nutrient pH.
• It is inexpensive

Disadvantages of Perlite

• Poor water retention
• Perlite dust may cause pumps to malfunction

Where to buy Perlite: Perlite is available (home depot, Walmart, etc.)

Peat

Peat also referred to as Peat Moss is derived from plant matter harvested from bogs that is partly decomposed. It is lightweight when dry and has good water retention properties. Peat typically has a pH that is slightly acidic usually around 3.5 – 4. It is certainly an organic hydroponic substrate

It can either be used alone or mixed with other hydroponic substrates as a combination. It is preferable to use it with a more porous substrate (like perlite) that can complement its water retention ability and low oxygen levels.

Advantages of Peat

• Good water retention properties
• Prevent leaching of nutrients because of its water properties
• Does not compact

Disadvantages of Peat

• Low pH and acidic
• It has low renewable potential because of its decomposition

Where to buy Peat Moss: Home depot, Walmart, etc.

Expanded Clay Pellets

This is also known as Light Expanded Clay Aggregate (LECA) and also as Hydroton (after one of the producers of clay pellets) and sometimes referred to as grow rocks. It is derived or produced from natural clay that is mined, formed into pellets and heated in kiln. The heated pellets expand to form lightweight tough and semi-porous pellets.

LECA has a neutral pH, low water retention, and good aeration potential because of its semi-porous properties. Hydroton or LECA pellets can also be considered as an organic substrate. LECA pellets can be reused but thoroughly washing them after use or before using them again is highly recommended. 

Advantages of Expanded Clay Pellets (LECA)

• Neutral pH
• Good aeration potential
• Maintains good balance in water retention and drainage
• Sterile grow medium
• It can be re-used  

Disadvantages of Expanded Clay Pellets (LECA)

• Lightweight pebbles can float into pumps causing damage
• It can be costly

Where to buy Expanded Clay Pellets (LECA): Home depot, Walmart

Hydroponic Substrates Alternatives

New and better performing alternatives to hydroponic substrates are being discovered on a continuous basis. Some of these new options may have some advantages over the main grow substrates and many of them may be made from new materials or repurposed materials.

Also, cost has been a driving force for some people who want to have cost-effective substrates with more advantages over the typical hydroponic grow medium available.

However, as it is with the main substrates, alternative grow substrates also will certainly have pros and cons depending on how they are used. There are various alternatives to hydroponic substrates and some of these include:

Gravel

Gravel can be described as small stones in irregular shapes and sizes commonly available almost everywhere. The irregular shapes of gravel makes it a good anchor for plant roots as well as some sort of water retention capacity. Though, water retention is relatively low; irrigating at regular intervals will keep plants grown with gravel substrate quite healthy.

One important thing to be mindful of when using gravel as a hydroponic substrate is that it can change the pH of the nutrient solution depending on the parent rock from which the gravel is derived from. Some rocks or gravel from these rocks can dissolve in water under certain environmental conditions and this can affect the pH levels (acidity or alkalinity) of the solution.

Advantages of Gravel

• Provides good anchor for plant roots
• Inexpensive and commonly available
• Can be reused

Disadvantages of Gravel

• It’s heavy and most planting beds may not be able to carry the weight
• It may affect pH of nutrition solution
• It has poor water retention ability

Where to buy Gravel:

Phenolic Foam

Phenolic foam is an inorganic hydroponic growth substrate that can be effectively used as an alternative to other commonly used substrates like rockwool, perlite, LECA/hydroton etc. It has very similar properties to most of the common hydroponic substrates and several scientific studies have shown that it is a good alternative hydroponic grow substrate.

It has good water retention and aeration properties as well as a stable neutral pH that does not interfere or alter the pH of the substrate or nutrient solution. It is a great substrate option for germinating seeds before transplantation.

Advantages of Phenolic Foam

• Neutral pH
• Great water retention capacity
• Excellent for seed germinating and cuttings propagation

Disadvantages of Phenolic Foam

• Not eco-friendly for disposal but can be re-used
• Initial cost may be expensive when using on large scale

Where to buy Phenolic Foam: (Home depot, Walmart)

Rice Hulls

Rice hulls or husks can be described as the outer shells or peels rice that has been harvested, processed, parboiled and milled. Rice hull is usually one of the major byproducts of this process.

It can be used as an alternative grow substrate because it has most of the desirable properties in a hydroponic substrate. Rice husks decompose slowly and it has high and better water retention than other substrates like perlite. It is also less dense and porous which makes it easy for good airflow and oxygenation.

However, it is important to treat parboiled rice husks to remove any weeds or rice seeds that may grow in it before using it as a hydroponic substrate. It may also perform better when used in combination with another substrate. 

Advantages of Rice Hulls as Substrate

• Good water retention
• Porous with good airflow
• It is organic and eco-friendly
• Less expensive

Disadvantages of Phenolic Foam

• Mixing with other substrate can increase water retention significantly

Where to buy Phenolic Foam: (Home depot, Walmart)

River Rock

River rocks just like gravel can also be a good hydroponic substrate alternative. The rocks are pH neutral and does not interfere with the pH of the hydroponic solution. The texture provides a good anchor for plant roots and the porous nature of rocks helps in aerating the substrate.

However, the water retention for river rocks is poor just like gravel and it is often better to use it as a combination with another substrate or to ensure that plants are watered on a regular interval.

Advantages of River Rock

• Porous and good for aeration of grow substrate
• Provide good anchor for plant roots
• Inexpensive

Disadvantages of River Rock

• It has a heavy weight that may be too heavy for most grow beds or containers
• Poor water retention

Where to buy River Rock: (Home depot, Walmart)

Saw Dust

Saw dust is a common byproduct of woodwork and carpentry. It is almost powdery with a fine texture and it can be used as an alternative hydroponic grow substrate. It also has high water retention properties and care should be taken to ensure that plants are not overwatered. It can be used as a substitute for peat or coco dust.

There certain important factors that should be considered when using saw dust as an alternative grow medium. The tannins from the wood can discolor the hydroponic solution and potentially alter the pH. Also, care should be taken to ensure that it does not contain chemicals that may been used in processing or treatment of the wood.   

Advantages of Saw Dust

• Good water retention properties
• Cheap and easy to find

Disadvantages of Saw Dust

• It can decompose which reduces the available nitrogen
• Can be contaminated with mold

Where to buy Saw Dust: (Home depot, Walmart)

Sand

Sand is commonly available almost anywhere and it can be used as an alternative hydroponic substrate. However, sand may not be a very effective grow medium because it has very low water retention capacity and the particles have a tendency to get packed together which may reduce the porous space available for air. The builders’ sand may be more effective because of its coarse nature which make it easier for oxygenation.

Sand is also quite heavy and it may difficult for keeping it in most planting beds. The best way to use sand as an alternative hydroponic substrate is to mix it with other media or substrates especially those with better water retention properties but lower porosity. 

Advantages of Sand as Substrate

• Coarse texture can provide anchor for plant roots
• High porosity is good for aeration and oxygenation
• Cheap and easily available

Disadvantages of Sand as Substrate

• Its heavy and may not be suitable for some grow/planting beds
• Low water retention properties

Wood Bark

Wood bark is the outermost exterior covering of tree stems and branches which serves as a layer of protection among other functions. Wood bark can be used as an alternative hydroponic substrate when properly processed. It’s coarse nature can be a suitable structure or anchor for plant roots. It can also form a porous substrate which makes it easy for oxygenation and air flow.

However, there a couple of things that must be taken into consideration when using wood bark for hydroponic grow substrate. Decomposing bark can use up nitrogen leaving very little for plants use. It’s best to consider using partly decomposed or already decomposed wood bark. It is also important to use wood bark that is free from any chemicals and little or no tannins. Tannins from some plant barks and chemicals from treated woods can leak out and stain or contaminate hydroponic solution.    

Advantages of wood bark as substrate

• Easily available
• High porosity is good for aeration and oxygenation

Disadvantages of wood bark as substrate

• Low water retention
• Can contaminate hydroponic nutrient solution if not properly treated

Pine Shavings and Wood Chips

Pine shavings and wood chips have almost the same conditions and properties as wood bark. One of the precautions to take when using this is to ensure that the shavings or chips are kiln dried before using it. This will help to ensure the substrate is free from pathogens and also slow down the decomposition process.

Advantages of Pine Shavings/Wood Chips

• Its organic
• Good for aeration  

Disadvantages of Pine Shavings/Wood Chips

• It may affect nitrogen availability to plants
• It is biodegradable

Polyurethane Grow Slabs

This material is not commonly used but it has some good potential as an alternative hydroponics substrate. It is used mostly in furniture and home improvement projects but some hydroponic growers are beginning to try it out as a DIY hydroponic substrate.

One of the attributes that make this a good option of grow substrate is its water retention and soft porous texture that may be good for aeration.

However, the biggest drawback for this material is that it is an inorganic material that may not be eco-friendly.

Advantages of Pine polyurethane Grow Slabs

• Good water retention
• Good porosity

Disadvantages of Pine polyurethane Grow Slabs

• Inorganic substrate
• May not be eco-friendly

Vermiculite

Vermiculite is produced by heating hydrated laminar minerals until it expands and form light vermiculite pebbles. It has similar characteristics to perlite but it tends to hold more water than perlite.   

Advantages of Vermiculite

• Good porosity with great aeration
• It is reusable

Disadvantages of vermiculite

• It may interfere with nutrient solution pH
• It can be expensive
• Poor water retention

Polystyrene Packing Peanuts

This is an inexpensive material derived from synthetic polymers. It is often used in packaging of fragile products. It is lightweight and has an excellent drainage.

However, it may not be ecofriendly and its lightweight can be a risk for air pumps which can get damaged when the pump sucks it up.  There is also a potential health risk of polystyrene packing peanuts decomposing into a sludge where plants can absorb styrene and pass it to their consumers.

Advantages of Polystyrene Packing Peanuts

• Inexpensive
• Commonly available
• Good porosity

Disadvantages of Polystyrene Packing Peanuts

• Not ecofriendly
• Poor water retention