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Hydroponic media (Substrates)
 

There are important pro’s and con’s to consider when choosing a hydroponic medium. Some offer excellent drainage, are sterile when new and simplify root inspections. As a negative however, some can quickly dry out in the event of nutrient pump failure, or are non biodegradable when disposed. This chart is a guide to 5 of the most popular hydroponic media.

NFT Rockwool Coco fibre Perlite Expanded clay

Nutrient Film Technique (NFT) provides no medium for roots. Tall plants are held up by overhead support.

Rockwool is a fibrous material made from molten rock. It is available as compressed slabs, as propagation cubes, or in a granulated form.

* Rockwool is a trade name. Use of this term refers to all equivalent brands.

Coco coir (‘coco’) is derived from the husk of the coconut. It is available in a loose, hydrated form, or in compressed dehydrated blocks.

 

Perlite is a soft and absorbent, granular material. It is produced by heating volcanic rock.

It is available in bags of loose fill.

Expanded clay is a hard, granular medium. It is produced by heating clay.

It is available in bags of loose fill.

Water holding capacity

Water holding capacity defines the amount of nutrient solution retained in the medium after watering, once drainage has occurred. It is this capacity which largely determines whether a medium is more suitable for ‘run-to-waste’ or ‘recirculating’ systems.

When the water holding capacity is ‘high’, the medium retains a large proportion of the nutrient solution after each watering. This nutrient “source” is exploited by the run-to-waste concept, and is the reason why only intermittent watering is needed. Hence, the watering frequency is influenced by the water holding capacity.

A high water holding capacity generally comes at the expense of “air filled porosity” (see below), arguably a more important characteristic.

NFT

NFT requires no medium and therefore the nutrient pump is run either frequently or continuously.

A variation of pure NFT is regular waterings followed by complete drainage (i.e. “pulsed” NFT or “flood and drain”). Drainage draws air into the root mat which improves oxygenation of the roots.

Because a high proportion of nutrient from each watering is unused, recirculating prevents the wastage which would otherwise occur with run-to-waste.

Although NFT offers many potential benefits over media based systems, pump failure can result in plant death within a few hours – especially in hot weather.

 

Rockwool

Although Rockwool is sometimes used in recirculating systems, its high water holding capacity makes it ideal for use in run-to-waste systems. It only requires watering at a low frequency. It is prone to water-logging and therefore accurate feed regimes are required.

Small blocks of Rockwool are commonly used for propagation of cuttings and seedlings which can be subsequently installed in larger systems.

 

Coco fibre

Coco’s high water holding capacity makes it ideal for use in run-to-waste systems. It requires only low frequency watering.

It is prone to water-logging therefore accurate feed regimes are required.

 

Perlite

Perlite’s moderate water holding capacity makes it suitable for either recirculating or run-to-waste systems.

It is available in various ‘crop specific’ grades – finer grades offer higher water holding capacity, whilst coarser grades offer superior air filled porosity.

 

Expanded clay

Expanded clay’s poor water holding capacity makes it most appropriate for recirculating systems.

Depending on system design, pump failure can result in plant death within a few hours, especially in hot weather.

Air filled porosity

Air filled porosity (i.e. air filled pores) enables the medium to accommodate oxygen for the process of ‘respiration’, and also aids the removal of the gaseous products of root respiration. Adequate air filled porosity is vital for root development.

Approximately 98% of the oxygen that a plant uses is absorbed through its root system. Care needs to be taken not to over-water those media that have poor air filled porosity. This is done by allowing sufficient time between waterings for oxygen to penetrate to the root mat.

Although a 30% air, 70% moisture ratio is typically regarded as optimum for media, higher levels of air are sometimes desirable.

NFT

Optimum aeration is easier to achieve in NFT systems.

However, if channels are too narrow they can become clogged with roots. This causes drowning of roots and inhibits flushing.

The working nutrient solution should be aerated to ensure its oxygen content is adequate.

 

Rockwool

Provided it is not over watered, Rockwool offers adequate air filled porosity.

Feed volumes and frequency must be adjusted as weather changes.

As Rockwool ages however, it can tend to become compressed from the effect of root mat expansion. This reduces the air filled porosity and increases the risk of water-logging.

 

Coco fibre

Coco offers adequate air filled porosity provided it is not over watered.

Feed volumes and frequency must be adjusted as weather changes.

As coco ages it can tend to become compressed from the effect of root-mat expansion. This will reduce the air filled porosity and increase the risk of water-logging.

 

Perlite

Perlite provides good air filled porosity. As discussed above, the coarser grades of perlite provide better aeration than finer grades.

Aeration is enhanced when employed in ‘flood & drain’ systems because the oxygen depleted air is expelled each time the medium is flooded and then replaced with oxygenated air when drained.

Recirculating nutrient should be aerated to ensure its oxygen content is adequate.

 

Expanded clay

Expanded clay provides good air filled porosity.

Aeration is enhanced when employed in ‘flood & drain’ systems because the oxygen depleted air is expelled each time the medium is flooded and then replaced with oxygenated air when drained.

The working nutrient solution should be aerated to ensure its oxygen content is adequate.

Ability to disinfect

Unlike soil grown plants, in hydroponically grown plants the roots and nutrient solution are exposed to the atmosphere and are therefore more prone to disease ingress. To minimize this problem, the nutrient solution, medium and roots should be regularly disinfected.

‘Inert’ media (i.e. those that exert zero demand on oxidizing agents) are preferable in hydroponics because they improve the effectiveness of using oxidizing agents as disinfectants.

For maximum effectiveness, oxidizing agents must yield a non toxic and residual chemical when dissolved in the working nutrient so that complete system disinfection occurs each time plants are watered.

Low air filled porosity of media and root mat decreases the ability of oxidizing agents to contact roots. This results in lower disinfection efficiency.

NFT

NFT systems are the easiest to disinfect. The absence of any medium also permits routine inspection of roots for signs of disease.

Wider channels are best. In a wider channel the roots are able to spread out which allows the oxidizing agent to penetrate more of the roots.

However, with wider channels, ensure the full width of the channel is evenly covered by flowing nutrient.

 

Rockwool

Rockwool is inert and therefore facilitates the effective use of oxidizing agents.

Its physical structure however, prevents inspection of roots.

Ability to disinfect the root mat decreases as the size and density of the root mat increases.

 

Coco fibre

Coco is not inert and can only be treated using certain oxidizing agents.

As with soil, it is difficult to conduct routine disease inspections of roots.

Ability to disinfect the root mat decreases as the size and density of the root mat increases.

 

Perlite

Perlite is inert and therefore permits the effective use of oxidizing agents.

To ensure thorough disinfection of roots and medium, ‘flood & drain’ systems offer optimum delivery of oxidizing agents.

Perlite is clean and light which does permit some degree of inspection of near surface roots for signs of disease.

Ability to effectively disinfect the whole root mat decreases as the size and density of the root mat increases.

 

Expanded clay

Expanded clay is inert and therefore permits the effective use of oxidizing agents.

To ensure thorough disinfection of roots and medium, ‘flood & drain’ systems offer optimum delivery of oxidizing agents.

It is clean and light which does permit some degree of root inspection of near surface roots to help confirm or otherwise, the presence of diseased roots.

Pre-treatment of new medium

Do not presume new media are sterile and ready to use without any pre-treatment. Media often contain high levels of alkalinity, sodium, chloride or dust and therefore may need to be disinfected, soaked and/ or flushed with water or nutrient solution prior to use.

NFT

No medium - not applicable.

 

Rockwool

Rockwool is usually sterile and clean when new.

It can be relatively alkaline due to the binding agent used during manufacturing. This can be removed prior to planting by soaking and flushing with 20% bloom nutrient solution*. Note, ensure to use ‘horticultural’ grade Rockwool. ‘Insulation’ grade Rockwool can incorporate toxic binding agents.

* Lower pH to ~5.0 prior to use.

 

Coco fibre

Coco can vary widely in quality - check with your retailer. It is unsterile when purchased and can harbour disease.

It can also contain high levels of sodium, chloride and alkalinity. This can be removed prior to planting by soaking / flushing with slightly acidic* water. Monitor the EC of the run-off water. Flush until the EC of the run-off equals (approximately) that of the input water.

* Lower pH to ~5.0 prior to use.

 

Perlite

Perlite is usually sterile and relatively pH stable when new.

It can however contain excessive perlite ‘dust’. This must be removed by flushing with water prior to use. Dust contributes to nutrient solution turbidity which in turn facilitates unwanted bacterial growth.

 

Expanded clay

Expanded clay is usually sterile and relatively pH stable when new.

It can however contain clay ‘dust’. This can be removed by flushing with water prior to use.

Re-useable

At the end of each crop remove all plant matter from the medium, and then disinfect if possible**. This minimizes the threat of disease.
Complete removal of old roots is necessary as these will eventually decay and provide an ideal environment for diseases and pests.

**Sodium hypochlorite solution (bleach) is adequate for this role. See “Disease Prevention”.

NFT

NFT avoids the on-going expense of medium replacement.

Compared to a medium based system, NFT is simple to clean-up between crops. Only hardware (channels, etc) needs to be cleaned by thorough wet brushing, hosing down and disinfection** prior to replanting.

 

Rockwool

Rockwool is generally not re-useable.

Hardware needs to be cleaned and disinfected** prior to replanting.

 

Coco fibre

Coco is generally not re-useable.

The difficulty of removing roots plus the non inert nature of coco means effective disinfection is not possible.

Hardware needs to be cleaned and disinfected** prior to replanting.

 

Perlite

With care, the majority of roots may be able to be removed from perlite to enable its reuse. Screening the loose material may further assist this process. Subsequent flushing with water followed by disinfection with a strong dose of sodium hypochlorite** is then required. However, to remove traces of toxic hypochlorite, rinse repeatedly until rinse water shows <1ppm with a swimming pool chlorine test kit. Spreading perlite out in the sun will further assist this process.

Hardware must also be cleaned and disinfected** prior to replanting.

 

Expanded clay

With care, the majority of roots can be removed from expanded clay to enable its reuse. Tumbling followed by screening may further assist this process. Disinfection can then be effectively employed using a strong dose of sodium hypochlorite**. The problem of removing traces of toxic hypochlorite is less than with perlite. Some large scale commercial operations use rotary kilns to disinfect clay between crops.

Hardware must also be cleaned and disinfected** prior to replanting.

Disposal

Biodegradable media can be placed in the garden after use. However, disposal is a significant problem with commercial size systems.

NFT

Perfect - not applicable. NFT avoids the disposal problems associated with medium based systems.

 

Rockwool

Rockwool is non biodegradable and therefore must be disposed of in an environmentally friendly manner as per local regulations.

 

Coco fibre

Coco is biodegradable and effective as a garden mulch.

 

Perlite

Perlite is non biodegradable and therefore must be disposed of in an environmentally friendly manner as per local regulations.

 

Expanded clay

Expanded clay is non biodegradable and therefore must be disposed of in an environmentally friendly manner as per local regulations.

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Related topics

"System components":

Nutrient reservoirs | Growing containers (trays / channels / pots) |

Feed circuits (plumbing) | Feed outlets | Blockages)

Nutrient pumps | Timers | Media (substrates)