In place of soil, hydroponic growers around the world use a variety of grow media to provide a substrate that anchors the plants and assists in delivering water, nutrients, and oxygen to the plant roots. Here is a comparison of some of the most common growing medium options, with additional information on maintenance considerations such as sterilization and flushing, to help you choose a suitable medium for your hydroponic operation.

The medium you choose will depend upon the design, scale, and location of your hydroponic system, as well as the crops you grow. Here are some features to check for in your growing media, as you ponder your options:

• Is it easy to handle and pot-up?
• Is it affordable and available in your region?
• If used for seed-starting, does it provide good seed-to-medium contact?
• Does it afford growing plants some physical stability?
• If temperature swings are a concern, does it provide a measure of thermal insulation to the roots?
• Is it free of toxins, contaminants, and pathogens that could inhibit germination, stunt growth, or introduce disease?
• What is its cation exchange capacity (CEC)? This is the ability of a growing medium to retain and release positively-charged elements (cations) from the nutrient solution, while not upsetting the balance of the nutrient solution. Most fertilizer micronutrients, as well as a few macronutrients, have a positive charge of +1 or +2.
• What is its water holding capacity (WHC)? If you rely on your system to hold water between irrigation cycles, look for a medium with high water retention.
• Inversely to the above, what is its air-filled porosity (AFP)? This is the extent of voids (air space) in the grow medium at any point of time that are not filled with water. The medium should be able to maintain good aeration to plant roots, or, at minimum, have no negative bearing on aeration.
• Do you plan to grow using integrated pest management (IPM) and/or organically? If so, look for an organic medium, such as coco coir, that can be inoculated with beneficial microbes.
• What is the material's reusability and recyclability?

All of these characteristics are important to your success. Before you scale up, plan to experiment with different media to find the best solution for your growing needs. Different crops may thrive best within different systems, conditions, and media configurations.

Baby leaf salad greens, started in plugs then potted up into expanded clay pellets

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Photo courtesy of Nagualma Hydroponic Farm, Cannes, FRANCE

Some hydroponic media come in bulk, loose form that you can use to fill trays, bags, or buckets. Other media come in the form of discrete plugs, blocks, discs, or slabs.

The key advantage of using plugs is that you can seed directly into small-sized propagation plugs marked with dibbles that hold each seed, and as the plants develop, bump them up into larger growing blocks, without moving them from the propagation plugs. Potting seedlings up in this fashion helps to prevent or minimize transplant shock. Many commercial hydroponic growers start plants in plugs, pot-up the starts into larger blocks, then pot-up a third time into the final medium, which could be pots, trays, slabs, bags, or buckets filled with a soilless mix.

Of the many options for hydroponic media, these are some of the most common.

COCO COIR

Made from the outer husk of coconuts, coco coir is a byproduct of the coconut industry. It is usually sold as plugs, blocks, or slabs. It is highly absorptive and retains water well between irrigation cycles while also providing good root aeration. It has natural antifungal properties and can be inoculated with beneficial microbes. The physical properties should last for several years of use before needing to be replaced, as long as disease infestation is managed. Depending upon production techniques, coir may contain high salt levels at the time of purchase; it is therefore important to buy coir that is labeled as having low EC (electroconductivity) or low salt levels. If high EC is a concern, it is recommended to reduce the initial coir EC by leaching the medium with water before planting. Coir is not inert; it contains potassium. As a result, you may want to use a coir-specific nutrient solution formulated to provide nutrients at a ratio appropriate for growing in coir. Unlike rockwool, coir is biodegradable and renewable.

EXPANDED CLAY PELLETS or PEBBLES

Also called grow rocks, expanded clay pellets are available in sizes ranging from 1–18mm in diameter. They contain tiny air pockets that provide for good drainage and root aeration, but do not retain water well and thus require frequent irrigation. Clay pellets last for many crop cycles before they begin to break down, but diseases must be managed if the pellets are reused. They are bulky, however, and nutrients may precipitate out of the solution and coat the pellet surface, requiring washing before reuse. To cut down on clay particulates that can clog filters and drippers, thoroughly rinse clay pellets before using them.

GRAVEL

Gravel is inexpensive, readily obtainable, easy to clean, and drains well. Because of its weight and inability to absorb water, however, it is not a commonly used medium. It can be useful when other media are unavailable, is isolated locales, or for limited applications. It is not suited for use in systems that rely on the medium to keep the plant roots moist, such as ebb-and-flow systems, but can be used in drip or flood systems. The gravel used should be noncalcareous (avoid limestone) and chemically inert. Your system materials and methods must be able to accommodate the considerable weight associated with this medium.

HEMP FIBER

Grow mats made from decorticated and cleaned industrial hemp fibers are a pH-neutral (6.7), 100% biodegradable and compostable option. Hemp's high water-holding capacity provides a moist environment favorable to germination and supports strong root growth, making it an especially well-suited substrate for hydroponic microgreens production.

PERLITE

A volcanic rock mined from lava flows, horticultural grades of perlite are available in fine grades suitable for starting seeds, coarse grades suitable for propagation, or mixes of sizes for crop production. It can be used on its own or as part of a soilless mix. It is extremely lightweight, has good wicking, is highly absorptive, and provides good root aeration, making it very well suited to wick systems. It does not retain water well, however, and can dry out quickly between irrigation cycles. It has no cation exchange capacity, so it provides no buffering. Because it floats in water it may not be ideal for ebb-and-flow systems. Be sure to use eye protection and a dust mask when handling dry perlite. It is safer to pre-wet the perlite before handling the product.

PHENOLIC FOAM

'Rex' lettuce starts, seeded in peat and phenolic foam-based plugs, for use in a floating raft system

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Photo courtesy of Paul Gallione, Johnny's Territory Sales Representative (retired);  owner of  Moosehead Trail Farm, Waldo, Maine

Also known as floral foam or peat foam, phenolic foam is a synthetic product available as plugs and commonly used for starting seeds. It is designed to offer an optimal water absorption/retention to root aeration ratio. It is generally reusable for more than one growing season; be sure to check the manufacturer's documentation for the expected lifespan of specific brand name foams, as well as requesting specifics on compostability/biodegradability. Some are biodegradable in about 18–24 months.

ROCKWOOL

Rockwool is made from volcanic rock, limestone, and coke, melted together at high temperature. The molten rock is then spun into cotton-candy like fibers, cooled, and woven into plugs, blocks, or slabs. It is a sterile medium, and one of the most commonly used hydroponic substrates worldwide, providing good water retention and root aeration. With thorough rinsing and no disease issues, it can be reused, and should last for several years before needing to be replaced. Rockwool is nonbiodegradable, so the user needs to arrange for proper disposal. Rockwool products can have a high pH from bonding agents used to shape the material. For best results, presoak rockwool in a slightly acidic solution (pH 5.5) before use and make a practice of monitoring system pH closely. This fibrous material is highly irritating to the skin, eyes, and lungs, and wearing a dust mask, goggles, and gloves whenever handling rockwool is strongly recommended.

SAND

Sand is inexpensive and easy to source. Sand of adequately large particle size can provide good water retention and root aeration; the ideal sand particle size is at least 2mm in diameter. If water drains freely and doesn't puddle, then your sand is probably of the right size. Sometimes perlite is added to sand for increased aeration. Avoid limestone sand, as it will raise the pH of your nutrient solution. Also note that wet sand is very heavy; plan your system accordingly.

SAWDUST

Sawdust can have excellent water absorption and retention. Choose sawdust with a moderate particle size; moisture spreads more easily through fine sawdust than coarse sawdust, but if the particles are too fine it can limit root aeration. Avoid chemically treated source material and avoid Western red cedar, which is toxic to plants. Source wood can sometimes become contaminated with salt during transport of logs to the mill; be sure to test initial salt levels with an electroconductivity (EC) meter and if salt levels exceed 10ppm, flush thoroughly with fresh water before use. Sawdust breaks down quickly, and at least a portion of the media will need to be replaced between each crop cycle.

SOILLESS MIXTURES

There are many kinds of soilless mixtures available. Commonly used components include vermiculite (a micaceous volcanic mineral), peat moss, pine bark, sand, and/or perlite. The well-known Peat-Lite mixtures developed at Cornell University, for example, are a 50-50 mix of peat and vermiculite (Mix A); a 50-50 mix of peat and perlite (Mix B); and a 50-25-25 mix of peat, vermiculite, and perlite (Mix C). Soilless mixtures tend to break down during the growing period, eventually resulting in compaction, and therefore need to be replaced periodically. The environmental impact of draining peat bogs for peat moss extraction is also a concern with mixtures containing peat.