Hydroponic Growing Medium

You may or may not need hydroponic growing medium depending on what hydroponic system you’re using. For example, the wick system and ebb and flow systems both require hydroponic grow medium, whereas the NFT system and water culture system do not!

How Does Hydroponic Growing Medium Work?

The purpose of hydroponic grow medium is to provide support for the root system. It serves to provide oxygen, water and nutrients to the roots.

There are three factors which contribute to the roots’ ability to grow in a substrate, these are pH, texture and nutrient content.

The texture of a given substrate is governed by its size and structure. A substrate with good texture will promote a strong root penetration, good nutrient uptake and high oxygen retention. Large particles of substrate will also allow for good aeration and drainage. If the substrate water retention is low then for an ebb and flow system for example, you’ll have to have an increased frequency of irrigation periods.

Smaller particles will of course pack closer together which will cause slower drainage. Whereas larger particles will drain faster and retain more air.

There are two main types of hydroponic growing medium; fibrous and mineral.

Fibrous Hydroponic Growing Medium

Fibrous materials such as peat moss, rockwool, coconut coir and vermiculite hold lots of moisture within their cells. These hydroponic grow media do not alter the composition of the nutrient solution. This makes these hydroponic growing media perfect for passive and ebb and flow hydroponic systems that absorb nutrient solution via capillary action.

Washing built up salt from your fibrous hydroponic growing medium, after you have removed your plants can be tricky and time consuming. Some fibrous mediums also lose much of their air holding capabilities after a crop. Therefore, with fibrous hydroponic growing mediums, you should replace them after each use.

Mineral Hydroponic Grow Mediums

These are mediums such as perlite, pumice, expanded clay, gravel, sand and pottery. The fact that they are mineral means that they do not react with living organisms or chemicals to change the integrity of the hydroponic nutrient solution.

With these hydroponic grow mediums you can wash salt which has built up, root fragments and other debris away for reuse. However be careful about low quality substrates shedding dust which may render it unusable.

You will need to keep this in mind and make a decision on the best hydroponic growing medium depending on what hydroponic system you use and what you’re growing.

Now it’s time to have a look at each of the main types of hydroponic grow mediums below.

The Different Hydroponic Grow Mediums


Rockwool is an inert, porous, sterile and non degradable medium that provides extremely firm root support. It is best suited for seedlings and cuttings.

You can get rockwool in slabs, cubes and granular flock. When rockwool is saturated it contains around 80 percent nutrient solution, 15 percent air space and 5 percent fibre.

Before you use your rockwool you need to make sure that you first soak it overnight in a low pH solution to lower its naturally high pH.

The best rockwool is very lightweight and you can get it with fibers running both horizontally and vertically. This apparant random structure promotes root growth in all directions.

Rockwool also comes wrapped in a thin sheet of plastic to guard against algae growth  and to act as a container.

Coconut Fiber

Coconut fiber, also called palm peat, cocos, coco peat and coir is probably the most popular hydroponic grow medium in the world.

Coconut fiber is actually the coconut pith that comes from just underneath the green husk that has been soaked in water and had all the salts removed as well as gums and natural resins.

Coconut fiber has many advantages over rockwool. It retains much more water and can hold onto oxygen better too. If you’re using an ebb and flow system then this really is better than rockwool. It comes with a pH balance of between 5.5 and 6.8 so unlike with rockwool you don’t have to pre-treat it before use.

Coconut fiber comes in compressed bricks, which when wet expands to about eight times their original size.

It also contains hormones which protects roots from fungus infestation. It really is easy to see why coconut fiber is so popular!

Expanded Clay

Expanded clay pellets are inert, lightweight, pH neutral and reusable. Clay pellets have air pores inside which means this is a hydroponic growing medium with excellent nutrient solution retention ability. Also, since they are round, lots of air is easily able to live between the pellets which stimulates nutrient uptake as well and also allows for fast drainage. You can increase drainage further by mixing the clay pellets with other hydroponic growing mediums such as coconut fiber.

Clay pellets are especially popular in hydroculture hydroponic systems. Also, if your growing plants that require extra good drainage such as roses then this should be the medium you use.

When you’re out shopping for expanded clay pellets, please keep in mind that it’s also called clay pebbles, expanded aggregate or expanded rock. You can also buy it in various sizes.

Soilless Mix

Soilless mix is great to use with wick systems. They hold lots of air and they retain water very well too. There are many different types of soilles mix and some even come with a variety of ingredients. You can of course even mix your soilless mix with other hydroponic growing mediums for good effect. Popular soilless mix includes:

Perlite – This is sand or volcanic glass which has been expanded in heat. It holds water and nutrients very well. You can get perlite in three grades; fine, medium and coarse.

Vermiculite – This is mica which has been processed and expanded by heat. It holds water and nutrients very well. It also holds air great too within the fibers and gives body to fast draining soils. This is best used in wick systems and comes in the same three grades as above.

Peat moss – We all know what peat is, it’s partially decomposed vegetation mined from bogs. Peat is used to amend soil as a hydroponic growing medium. The first time you use peat, you’ll find it difficult to wet. You can get round this by mixing it with other components. Once you do get it wet it will retain up to 30 times its weight in moisture. This makes it great for hydroponic wick systems.

What other hydroponic growing mediums have you used with great success? Please let us know by leaving a comment below!

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