Growing Media Used In Hydroponics

Your hydroponic system is soilless, and only the nutrient solution provides the sustenance your plants need to survive. The material they’re grown in is chemically inert but enables the roots to bind to something tangible for the plant’s stability.

Different growing media absorbs varying amounts of nutrient solution, and have pockets of air between the material’s space. How much air depends on the porosity of the media.

There are numerous types of growing media and each have their own properties. Considerations may include the kind of hydroponic system you’re using, which plants you’re growing, or its ease of use.

What is growing media for hydroponics? 

Cation Exchange Capacity

Cation Exchange Capacity (CEC) is the extent to which growing media can hold and exchange cations with the surrounding environment for access by plants. Cations are positively charged ions which will bind negatively charged ions in the growing media and the sum of all of these reactions is referred to at the CEC. Simply put, CEC impacts the media’s potential to absorb nutrient solution and re-release positive ions such as sodium, magnesium, and calcium so plants can absorb them at a consistent rate.

CEC is measured in in milliequivalents per 100 grams (MEQ/100g). The lower the value, the less capacity the media has to hold onto positively charged nutrients.

Water-Holding Capacity

Similarly, water holding capacity (WHC) refers to how much water growing media can hold on to. The sooner a growing medium reaches its saturation point the more chance of nutrients leaching from it, along with the excess water, into the run-off below. If water is readily absorbed by the medium then the air will be forced out to accommodate it resulting in oxygen depletion at the root zone which, if not controlled in the appropriate way, will kill your plants. Conversely, if you don't sufficiently water media which doesn't absorb it well, your plants will be negatively affected.

Factors affecting WHC include the texture, composition, and organic matter present. In a natural setting, the WHC impacts the water available to a plant between periods of rainfall.

Air-Filled Porosity

The air-filled porosity (AFP) is determined by how many pockets between the growing medium there are which ensures a supply essential oxygen to the plant roots, preventing anaerobic bacteria propagating. As we've already stated, without enough oxygen, your plants will die. Before death is reached, however, a lack of oxygen is detrimental to the roots ability to absorb water and subsequently nutrients. This is because the absorption is an active process. There are molecular pumps within the cell walls of the roots which require energy to function and without enough oxygen, they cease to work. Ultimately, yield will decrease.

If the medium doesn’t have much air-filled porosity then caution must be taken not to overwater the roots. This is mediated by spacing the time between waterings.

Types Of Media

Rockwool

Rockwool is manufactured by melting basalt rock and chalk together and spinning it to fibers, which are then compacted down into a mat and sliced into cubes. High temperatures during production mean the material is inert after being in a higher reactivity environment than you would find at room temperature. Rockwool comes in a hydrophilic (water-loving) or hydrophobic (water-hating) granulate format, or as hard blocks or cubes. The hard blocks, termed “bonded”, are primarily used when growing plants.

Rockwool has a CEC of zero, so it doesn't alter or limit the availability of cations in the root zone. There’s a possibility for high optimisation of the nutrient solution but with little room for error, however, if something goes out of balance then it’s easy to remedy. The medium's WHC is also nearly zero and will quickly drain from its saturation point to be filled with air once again. The total percentage of unavailable nutrient solution to your plants grown in Rockwool blocks is 2%.  

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Coco Coir

Coco coir is a reusable mixture of coco fiber, coco pith, and chips which has a pH range of 6-6.7. It's made from the inner pith of coconuts originating from coconut palms all the way from Sri Lanka and India. 

The WHC of coco coir is high at 88% and can hold onto eight to nine times its weight in water, this can make it prone to overwatering though. However, even when saturated, coco coir maintains AFP of 22%. Coco coir has a CEC around 90-100 meq/100g and contains relatively high potassium (K), sodium (Na), and relatively low calcium (Ca) or magnesium (Mg). If all four cations of the elements just referenced were introduced at the same concentrations, Ca2+ and Mg2+ would be absorbed twice as readily at the same sites as Na+ or Kbecause of the extra positive charge. You'll need to wash your coco coir with slightly stronger concentrations of Ca and/or Mg supplements to create a chemical buffer. If you don't the coco coir will remove Ca and Mg from the nutrients solution and it will exchange these cations with the K and Na naturally present, raising their concentrations in the root zone. This unbalancing isn't healthy for your plants and toxicity issues can arise in some species. If you'd like to read more, check this out.

If you choose coco coir as your growing media, be aware you may need to supplement it with calcium-magnesium additives. Alternatively there are pre-mixed nutrient solutions available specifically for coco coir.

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Perlite

Although manufactured throughout the world, perlite, a siliceous volcanic rock heated to extreme temperatures to form volcanic glass, is primarily produced in China today. It consists of relatively large amorphous particle sizes so the WHC is around 20%, an AFP is 41%, and a CEC of close to zero, leaching nutrients easily. It’s lightweight and chemically stable with low density. Disadvantages include floating when submerged but mixing perlite with different types of growing media improve each of their disadvantages for each. For example, improve draining and the ability to hold onto nutrients.

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Vermiculite

Vermiculite is a reusable micaceous mineral from South Africa, China, USA, and Brazil. It’s benefits include a neutral pH, being lightweight, and great at buffering. Vermiculite WHC is high, at 100-150 meq/100g, and if the substrate is used on its own the plants can be starved of oxygen at the root zone. AFP is medium-low, and CEC is medium.

As the WHC is high, this often leads to overwatering, which can be harmful for your roots. It’s inert, and will work well in hydroponic systems.

Mixing 25% vermiculite and 75% perlite in your hydroponic system will improve drainage and at retaining moisture.

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Expanded Clay Balls

These reusable clay balls are heat-expanded to form sizes of 8-16mm. They’re predominantly manufactured in Europe but originate all over the world.

WHC is low, AFP is high, and CEC is low.

The clay ball’s pH is neutral and it’s hard to overwater them, so the ratio between the air and nutrient solution. They are, however, quite bulky and nutrient can precipitate on the outside of the balls.

Before you use the clay balls, you’ll want to wash them to remove any pump-damaging sediment.

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Nutrient Film Technique

Nutrient Film Technique (NFT) is used as an example of a hydroponics system without the use of a growing medium.

As there’s no growing media available to your plants there’s a continuous flow nutrient solution provided to your plants.

WHC is zero as there’s nothing to absorb the water, and if pump failure occurs it won’t take long for your plants to die.

Although it isn’t strictly AFP, aeration of the root system is easy with NFT, however, clogging can happen which could starve the roots of oxygen. Suitable aeration should be worked into the functioning of NFT.

Similarly to WHC, CEC is impossible as there isn’t anything for ions to exchange with.

Nutrient solution isn’t very good at maintaining its pH and constant aeration is necessary to provide suitable concentrations of oxygen. Disinfecting NFT systems is simple and inspecting the roots for disease is straightforward.

Have you seen our other Articles on hydroponics?

The Basics Of Hydroponics  |  The Kratky Method  |  Types Of Hydroponics  |  Deep Water Culture  |  The Drip System  |  The Wick System  |  Lighting In Hydroponics  |  The Ebb And Flow System  |  Nutrient Film Technique  |  Nutrients For Plant Growth  |  Ventilation For Hydroponics  |  Growing Media Used In Hydroponics  |  Hydroponics  |  The Best Way To Cure Root Rot  | How To Choose A Grow Tent | The Best LED Grow Lights | Everything You Need To Know About Nutrient Lockout | The Best Air Pumps For Hydroponics | The Best PH Meters For Hydroponics | What Mom Never Told You About The GH Dual Diaphragm Air Pump |

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