Growing Media




Growing Media 101

Don't ask: "Which growing media is the best?" Rather you should ask: "How do I get the best out of my chosen growing media?" Haven't chosen a growing media yet? Then read on!

Plants grow in soil right? Yes-but not always! Plants don't actually need soil-as far as the roots are concerned, they need water, oxygen, nutrients and physical stability. 

The general term we use for the "stuff" that plants grow in is "growing media."

If it seems a little obtuse to want to grow plants in anything but soil, please read on! It turns out that there are some amazing (and often eco-friendly) alternatives out there that confer a whole host of additional benefits to your plants, including:

  • a cleaner environment, less prone to pests and diseases
  • more control over the water, oxygen and nutrients available to your plants
  • faster growth rates
  • bigger yields

Don't Skip This Bit!

Please take a few seconds to be sure you understand these key terms so that you get the most out of this article:

Cation Exchange Capacity - CEC

When growing hydroponically we are adding the all-important nutritional elements via a nutrient solution. The ability, therefore, of a growing media to hold on to and release these positive charged elements (cations) is really crucial. Growing media with a low CEC allows cations to be easily leached away whereas growing media with a high CEC withhold cations and act as a longer-term cache.

Air Filled Porosity - AFP

The amount of air space in the growing media.

Water Holding Capacity - WHC

The ability of a growing media to hold and store water.

Coco Fiber (Coir)

Coco coir is extremely absorbent while also providing a home for beneficial biology.

The shredded inner pith of the coconut husk
Origin
: Mostly from coconut palms in Sri Lanka and India.
Cost: $13-$50 (3 cu. ft.)
Reusable: Yes
pH: 6.0
CEC: Medium AFP: Medium WHC: High
Pros: Naturally contains the beneficial fungus Trichoderma, slowly releases potassium.
Cons: Draws down calcium, easily over-watered.
Irrigation: Manual top-fed, ebb/flow, drip.
Nutrient Requirements: Many growers choose coco coir specific nutrients, others add calcium-magnesium additives.
Usage notes: Coco coir comes in various compressed forms: bricks, bales and slabs. Also available in ready to use loose fill bags. It has the ability to hold on to nutrients for an extended period of time. Incredibly it can hold up to eight-ten times its weight in water!

Coco Chips (Croutons)

Coco chips are often added to coco coir potting mixes to increase drainage and aeration.

Cube-shaped coconut husk chips
Origin
: Mostly from coconut palms in Sri Lanka and India.
How much: Around $65 (3 cu. ft.)
Reusable: Yes
pH: 6.0
CEC: Medium AFP: High WHC: Very low
Pros: Naturally contains the beneficial fungus Trichoderma, slowly releases potassium, natural alternative to clay pebbles.
Cons: Tends to float when flooded, needs frequent irrigation.
Irrigation: Ebb/flow, drip
Nutrient Requirements: When using on their own consider incorporating a calcium-magnesium additive to your nutrient regimen.
Usage notes: Excellent for mixing with coco coir fiber to lower the WHC, ideal for using as a mulch on the top of other growing media; excellent for growing orchids.

Perlite

Perlite can be used on its own but it is often added to other loose fix hydroponic growing mixes such as coco coir to aid in aeration and drainage.

Superheated volcanic silicous rock
Origin
: Produced worldwide but now mostly in China.
How much: $45 3 cu. ft.)
Reusable: Yes
pH: Neutral
CEC: Low AFP: High-medium WHC: Medium
Pros: Lightweight, readily-available, great for rooting cuttings, inert, chemically stable.
Cons: Has no buffering qualities, leaches nutrient easily and tends to float when flooded
Irrigation: Manual top-feed, Drip, ebb/flow and aeroponics?Nutrient Requirements: Naturally Inert medium, suits most hydroponic nutrient solutions.
Usage notes: Perlite is available in many grades. 4-12mm is most common for horticulture. Perlite can be used alone or amended into coir, vermiculite, peat moss, or soil mixes to improve aeration/drainage. A 50/50 mix of perlite and vermiculite is ideal for rooting most cuttings.

Vermiculite

Vermiculite can be used on its own as a hydroponics growing media or added to coco coir and soilless mixes.

A natural micaceous mineral that expands when heated
Origin
: South Africa, China, USA, or Brazil
How much: $40 (3 cu. ft.)
Reusable: Yes
pH: Neutral
CEC: Medium AFP: Medium-Low WHC: High
Pros: Lightweight, excellent buffering qualities
Cons: Easily over-watered.
Irrigation: Drip, ebb/flow, and manual top-feed
Nutrient Requirements: Naturally Inert medium, suits most hydroponic nutrient solutions.
Usage notes: Most growers mix vermiculite with perlite to give better results. A mix of 75% perlite 25% vermiculite will provide adequate drainage and moisture retention.

Diatomaceous Earth

Diatomite provides natural silica and root pest control

A sedimentary rock made from fossilized remains of diatoms
Origin
: Worldwide
How much: $55 (40 litres)
Reusable: Yes
pH: Neutral
CEC: Medium AFP: High WHC: Medium-low
Pros: Does not roll, contains silica, sterile (but harbors beneficials well), holds more water than clay pebbles
Cons: Heavy weight; releases sediment
Irrigation: Ebb/flow, drip, DWC, aeroponics
Nutrient Requirements: No special requirements. Diatomite contains silica, which is absorbed into plant tissue and helps improve plant structure and resistance to pests / diseases
Usage notes: Prewash, as sediment may clog drippers. Many growers mix it with hydroton; this makes for improved air / water ratio. Also acts as a good killer of soil dwelling pests.

Rockwool

Rockwool (aka Stonewool) is commonly available as loose fill, slabs, blocks and cubes. It is very popular in commercial farming applications.

Heated basalt rock and chalk spun into a fibrous, lightweight material
Origin
: Mainly Europe
How much: $80 (9 4"x4" blocks + 3 slabs)
Reusable: Completely biodegradeable and reuseable for one or two crops
pH: 8.0 (pre-soaking at pH 5.0 - 5.5 absolutely required!)
CEC: Low AFP: Medium WHC: High
Pros: Lightweight, sterile, recyclable
Cons: Skin irritant, needs pre-treating before use.
Irrigation: Manual top-fed, ebb/flow, drip, DWC, NFT
Nutrient Requirements: Inert medium; requires pre-soaking, suits most hydroponic nutrient solutions.
Usage notes: Presoak with a water and pH Down solution - 5.0 - 5.5 pH for 24 hours. After soaking, allow to drain and irrigate with a suitable nutrient solution before planting. Rockwool comes in starter cubes, plugs, blocks, slabs, mats, and loose-fill (absorbent or repellent granulate).

Clay Pebbles / Clay Balls

Clay balls / pebbles. This non-restrictive growing media is easy to work with.


Heat-expanded, round-shaped clay pebbles of mixed sizes (8-16mm most commonly used)
Origin
: Worldwide, mainly Europe
How much? $70 (100 liters)
Reusable: Yes
pH: Neutral
CEC: Low AFP: High WHC: Low
Pros: Difficult to over-water, maintains an excellent air to water ratio when irrigated correctly.
Cons: Bulky, nutrient precipitation on outer surface is common, needs washing before use?
Irrigation: ebb/flow, drip, DWC, aeroponics
Nutrient Requirements: Inert medium, suits most hydroponic nutrient solutions.
Usage notes: Spills can be messy. Wash thoroughly before use to remove the small clay particles, this messy sediment may clog pumps and drippers.

Growstones

Growstones are growing in popularity - very easy to use and easy to set up irrigation cycles.


Porous rocks made from recycled glass beverage containers received from either the landfill or another source collecting and processing waste glass.
Origins
: Santa Fe, NM (USA)
How much: $80 (3.75 cu. ft.)
Reusable: Yes
pH: Neutral
CEC: Low AFP: High WHC: Medium-low
Pros: Lightweight; 35% water-holding capacity while maintaining an 85% air-filled porosity; capillary action up to 6" (15 cm).?Cons: Bulky, needs frequent irrigations.
Irrigation: ebb/flow, DWC, aeroponics.
Nutrient Requirements: Inert medium, suits most hydroponic nutrient solutions.
Usage notes: Wash thoroughly before use to remove small particles. Ideal for using neat or for mixing into coco coir, peat and other growing media.

Peat

Peat is used in soilless potting mixes.


Origin: Peat forms in wetland areas of North America, Ireland, Russia and Northern Europe?
How much: Varies
Reusable: Yes
pH: 3.4 to 4.8
CEC: Medium - High AFP: Medium WHC: Medium-high
Pros: Readily available,supports beneficials, excellent at holding nutrients and has a good air to water ratio.?
Cons: Limited natural resource, extaraction is harmful to the environment, does not re-wet well if left to dry out, naturally acidic.?Irrigation: Manual top-fed, drip?
Nutrient Requirements: 'Soil' specific nutrients are recommended.?Usage notes: Peat is found in many grow stores in pre-mixed bags or bales. It usually has perlite added for improved drainage, a wetting agent for good re-wetting, and dolomite lime to raise the pH.

SteadyGRO

SteadyGro is another up and coming hydroponics growing media.


Origin: USA and Canada, out of materials from India
How much: $41 (9 4"x4" blocks and 3 slabs.)
Reusable: Not recommended by manufacturer.
pH: 6.0
CEC: Low-medium AFP: Medium WHC: High.
Pros: No algae growth, sterile.
Cons: Reports of phenolic resin's carcinogenicity by NTP, IARC, and OSHA
Irrigation: Manual top-fed, ebb/flow, DWC, and NFT.
Nutrient Requirements: inert medium, suits most hydroponic nutrient solutions.
Usage notes: Comes in two varieties: SteadyGroPro (low water retention) and SteadyGroPro H+ (high water retention)

Water (DWC, NFT)

Pure hydroponics! Aerated, nutrient rich water can be a home for your roots too! Just be sure to regular temperature and dissolved oxygen levels.

Origin: Good question! Obtain a water analysis.
How much: 0.1 cent per liter for domestic volumes, and 0.03 cents per liter for industrial volumes.
Reusable: Yes (in recalculating systems)?
pH: varies; distilled water is 7.0 pH?
CEC: N/A AFP: % of dissolved O2 increases as temperature drops WHC: N/A (It is water! Doh!)
Pros: Readily-available, roots love it when properly aerated and at the correct temperature (64-70F.)
Cons: Poor buffering capacity, pH-fickle, may harbor pathogens
Irrigation: DWC, NFT
Nutrient Requirements: inert medium, suits most hydroponic nutrient solutions.?
Usage notes: Requires constant aeration to maintain dissolved oxygen levels necessary for healthy roots.

Sand

Sand is cheap and plentiful. It drains very quickly but it has a low CEC.

Origin
: Varies?
How much: $10-$15 (3 cu/ ft.)
Reusable: Yes?
pH: Varies according to its mineral content
CEC: Low?AFP: Low?WHC: Medium
Pros: Cheap, excellent drainage
Cons: heavy, must be irrigated on a schedule for optimal results.
Irrigation: manual top-fed, ebb/flow, drip
Nutrient Requirements: Most are inert, some may contain lime. Suits most hydroponic nutrient solutions.
Usage notes: Wash thoroughly before use. Check and correct pH of runoff prior to planting.

Sawdust

Sawdust is also cheap if you know where to find it - but watch out for pH issues.

Origin: Varies, usually as the byproduct of sawmills and retail hardware stores.
How much: If you ask nicely, they may give it to you for free!
Reusable: Not recommended
pH: 6.1
CEC: Medium AFP: Medium-High WHC: Medium
Pros: Inexpensive, lightweight, biodegradable, harbors beneficials
Cons: pH-fickle, needs frequent irrigations
Irrigation: Manual top-fed, ebb/flow, DWC
Nutrient Requirements: Diligent pH monitoring and adjusting is of the essence. Suits most hydroponic nutrient solutions.
Usage notes: Best used for cycle crops and annuals.

Gravel

Gravel is a cheap and free draining hydroponic growing media.

Origin: Worldwide, mostly USA
How much: $12-$15 (3 cu. ft.)
Reusable: Yes
pH: neutral
CEC: Low AFP: Medium-high WHC: Low
Pros: Inexpensive, easily-available
Cons: Heavy weight, bulky
Irrigation: Ebb/flow, DWC, Aeroponics
Nutrient Requirements: Mostly inert medium, may contain soluble minerals. Suits most hydroponic nutrient solutions.
Usage notes: Wash thoroughly before use. Gravel is an old school hydroponic substrate.

Written by E. Lozada