Intro
Hydroponics means growing plants without soil. However, a growing medium may still be required for some plants for stem support as well as to allow for nutrients and air to provide beneficial water to oxygen ratio to the root system. Hydroponic medium is commonly used in Hydroculture type hydroponic systems or hybrid versions of a hydroculture system. Larger plants and vegetables require the use of some type of medium to support the plant stem base and their root system during growth.
Medium vs. Media
The vernacular of what the difference is between medium and media varies by location and country and can be confusing. In the US they are interchanged regularly with medium being singular and media being plural even though they each have their own singular and plural versions. In Europe and other countries, I have seen the term growing medium to describe the raw material used in a container to grow a plant and media used to describe a formulation of different mediums to achieve a specific balance of air, water, and nutrient holding capacity.
For the most part, and our discussions on GrowWithouitSoil.com, we will define medium as most commonly used in hydroponics and media to be used in potting and plant soils. However, I have seen many gardeners use multiple mediums in some cases. For example, I have seen some gardeners germinate seeds in stone wool and fill the manufacture seed holes and seeds with perlite. I have also seen some use a mixture of hydroton and vermiculite and/or perlite in some systems. Others line either the top or bottom of their DWC containers with river rock before adding grow stone or hydroton to allow for better drainage and avoid stem rot.
Requirements and Considerations When Selecting a Grow Medium
- Cost? This matters more in some areas than others.
- Availability? Is the media you want for your particular plant selection available to you?
- Will it provide good root support and anchorage for the plant?
- Can you use it for germination or will the seedlings need to be transplanted?
- Will it protect the roots if there are any variations in temperature, nutrient, or watering levels?
- Is it free of chemical additives, hormones, pathogens, pests, and weeds?
- Does it provide adequate air spaces for oxygen to get to the roots? This is known in the industry as Air Filled Porosity – AFP. Specifically, the porosity of the medium itself, not necessarily the air spaces between the pieces of medium.
- Will it hold a sufficient amount of water? This is known as Water Holding Capacity -WHC
- Does the medium contain minerals? Will the medium require a presoak to balance the pH?
- Will it hold sufficient available nutrients or will the plants only get nutrients during the water solution delivery times? This is known as Cation Exchange Capacity – CEC
- Is it safe to handle? Some mediums require PPE
- Is it Reusable? Can you sterilize and reuse it?
- Is it Biodegradable? Will it biodegrade when you are done with it?
- Is it Sustainable? Does the manufacturing of it harm the environment in any way?
Popular Types of Medium
Paper Towel
A simple paper towel is a great cheap method for germinating seeds! Use paper towels with no print. Using paper towels instead of traditional mediums saves from wasting your medium on seeds that may not germinate and saves time thinning seedlings from media after sprouting. Not reusable, but biodegradable and sustainable.
Cotton or Cloth Mats and grow plugs
Thick cloth matt, cube or grow plug great for germinating seeds and growing microgreens, wheatgrass, or micro herbs. Saves time on wasting media on older seeds that may not germinate and saves time thinning seedlings from media after sprouting. Great WHC and AFP however some have complained of black mold growth. Most brands are pH neutral. Not reusable, but is biodegradable and sustainable
Stone Wool (Rockwool)
Due to its moderate cost, availability, and superior WHC and AFP, Stone or Rock wool is a very common medium in modern hydroponics. The stone wool is composed primarily of granite and/or limestone which is superheated and melted then spun into small threads like cotton candy. Most brands come with a PH of around 8 so you will need to soak the cubes in a PH solution. It comes in slabs, sheets, blocks, starter cubes, or loose fill. It is not really very reusable, although some have dried it out and reused it again. It is not biodegradable or very sustainable. You must use safety precautions when using this medium. Use a mask and goggles during use as the dry stone wool fibers can easily get in your eyes or lungs. Some growers even use gloves.
Starter Plugs (Rapid Rooter Plugs, Rapid Grow Plugs)
Similar in use to stone wool plugs but these are all-natural soilless grow plugs made of composted tree bark and organic materials. They are expensive, but perhaps one of the best options for seed germination and cloning. Most brands have high WHC and good AFP with a neutral pH, they are not reusable, but they are biodegradable and sustainable.
Oasis Cubes
Inexpensive alternative for germination and seedlings. Similar to Rockwool starter cubes in WHC and AFP properties but don’t hold quite as much water. The material is similar to rigid green or white floral foam used by florists, so it crushes and crumbles easily, especially when saturated. Not reusable, biodegradable, or sustainable.
Polyurethane Foam
Uncommon alternative to Stone wool or other starter cubes/Plugs. Great cheap alternative to make your own starter plugs for germination or for transplanting seedlings.
Phenolic Foam (Floral Foam, Peat Foam)
Phenolic Foam is a synthetic foam similar to oasis starter cubes and with similar properties but crumbles apart easily. Phenolic Foam has very high WHC but low AFP and CEC. Use caution as this Foam holds a lot of water which can waterlog your plant roots or cause stem rot. Particles can accumulate in your reservoir and clog filters. The pH of this foam will slowly increase with hard water usage. OASIS® and Strass foam are designed to maintain acidity in hard water. Most brands are difficult to reuse. Some are biodegradable and most are sustainable.
Coco Coir/Fiber/Chips
Coco Coir is coconut fiber made from ground-up coconut husks. It has a high AFP and WHC capacity. Coco Coir comes with a pH of about 6-6.8, so it may require a pre-soaking pH adjustment. It usually comes as coco fiber particles size similar to potting soil, or in larger sizes like small wood chips. Coir can be purchased in compressed discs which, after soaking in water, expand to about 6-10 times the original size. Because Coco Coir is a hormone-rich and fungus-free medium it will aid in a quick germination and seedling process. It is a waste product so it’s a good sustainable choice.
Clay Pebbles (Hydroton, Hydrocorn)
Lightweight Expanded Clay Aggregate (LECA). Clay pebbles are super-fired heated to create a porous texture. Usually, you will use another method and then plant seedlings, plugs, or cubes into clay pebbles. Superior WHC and AFP balance and capacity. Wash as per manufacture instructions. Clay pebbles release almost no nutrients into the nutrient water and most brands are pH neutral. Some brands require a good soak in a pH solution. They are known to get smelly after a while, so wash in a diluted bleach solution and rinse well before each reuse. They are also biodegradable and sustainable.
Perlite
Composed of minerals that are fired in very high heat that expands them into small, very lightweight, porous, and absorbent aggregate. It has been used to aerate traditional soil and potting soil for years. Most perlite can be purchased sterile and pH neutral. Perlite has excellent AFP. Due to its extremely lightweight, perlite is less common in some hydroponic systems because the medium can float or wash away. If it is used, it is usually mixed with other mediums like vermiculite or coco coir. Wash as per manufacture instructions. Use precaution when handling perlite as Long term inhalation of crystalline silica dust may cause lung cancer (Silicosis). As a precaution, you should always use a mask or respirator when handling any mediums. It is reusable but not biodegradable or sustainable.
Vermiculite
Similar to Perlite but with better WHC and CEC qualities. Vermiculite is a mineral that is superheated and expands into small lightweight pebbles. It comes in other varieties that are used for pool and other construction so make sure you purchase the correct type for hydroponics and gardening. Vermiculite has strong wicking qualities so you will need to be careful of stem rot with some plants in some systems. It is reusable but not biodegradable or sustainable.
Sand
Very plentiful and cheap but not appropriate for all hydroponic systems due to the weight and high maintenance. Sand needs to be sanitized often and generally does not have good WHC, CEC, or AFP capabilities. Sand is reusable, biodegradable, and sustainable.
Peat Moss
Peat Moss has the highest WHC of all hydroponic mediums but one of the lowest AFP’s due to its ability to compress and compact. Best for starting seeds hydroponic Fodder and microgreens. Used in some aquaponic systems. It is usually not reusable or sustainable but is biodegradable.
Gravel
Inexpensive medium most commonly used in aquariums. Gravel is heavy has low WHC, CEC, and AFP capabilities, and can cause pH swings. Not the best choice for most systems but if it’s all you have access to, you can make it work with a proper setup. Gravel is reusable, biodegradable, and sustainable.
Pine shavings
Pine shavings, not sawdust. Common in commercial hydroponics drip irrigation systems. Make sure to use organic, kiln-dried shavings and check to make sure it does not have any chemical additives. Great structure holding capabilities. Studies have shown that wood chips may reduce the effect of plant growth regulators which could aid in overall plant growth and size.
Wood Fiber/Chips
One of the most natural and organic growing mediums available for hydroponics. Wood fiber has excellent WHC, CEC, or AFP properties. Wood fiber is reusable, biodegradable, and sustainable.
Aged Pine Bark
One of the original and oldest mediums used for hydroponics. Pine is preferred as it is slower to decompose and has a more stable pH. Composted or aged bark is best and will have fewer issues with nitrogen loss in your nutrient solution.
Pumice
Lightweight volcanic rock from Iceland very similar medium to perlite. Pumice has a high AFP but a low WHC so it will be good for specific plants or specialty hydroponic applications. It is reusable but not biodegradable or sustainable.
Crushed Brick (Brick Shards)
Very cheap medium with similar properties to gravel and sand but with a little better WHC. Must be very thoroughly cleaned and ph balanced before each use. Crushed Brick is reusable, biodegradable, and sustainable.
Growstones
Cheap alternative to hydroton except they are made from recycled glass instead of clay. They are very lightweight and porous like lava rock and have a high WHC and AFP with a great balanced ratio. This medium has a strong wicking tendency so you may want to use another medium on the bottom or top of your system for better drainage and to avoid stem rot. Very difficult to sterilize and reuse as the roots get stuck in the porous surface. Also difficult to transplant as the roots can get damaged as they can break or rip due to the roots getting stuck into the stones. Not biodegradable but it is very sustainable.
River Rock
Inexpensive and very common in many hydroponic systems for commercial and hobby use. Easy to obtain and comes in a variety of sizes and shapes with smooth or rough edges. If you get it locally sourced make sure you clean well and then soak in a diluted bleach solution overnight and rinse thoroughly before use.
Rice Hulls
Rice hulls are a byproduct of the rice industry and like coco coir, it biodegrades very slowly which makes them suitable for hydroponic use. Rice hulls have a Low WHC and can compact causing a low AFP. Make sure you only use parboiled hulls as the boiling kills contaminants. Rice hulls are usually mixed with other media.
Water absorbing crystals
Not popular in hydroponics but have been used in previous years for indoor plants and floral arrangements. The polymer crystals can hold up an extremely large amount of water for their size, some as much as 50 gallons per pound! The larger size is better in order to have spaces in between for air access to the roots. Some gardeners even add other stone or aggregate type medium to increase the air gaps. Mostly used for Kratky-type systems.
Quick Breakdown
*Best for germinating seeds
*pH varies by manufacture + or – 4
| Hydroponic Mediums Comparison Chart |
| Medium | Cost | WHC | AFP | CEC | pH | Support | Reusable | Biodegradable | Sustainable |
| Paper towel | $ | 3 | 4 | 1 | – | 1 | no | Yes | no |
| Poly Foam | $$ | 5 | 2 | 3 | 7 | 1 | no | no | yes |
| Phenolic Foam | $$$ | 3 | 1 | 1 | 3.7 | 3 | Yes | no | Yes |
| Coco Coir | $$$$ | 4 | 3 | 2 | 6.4 | 3 | Yes | Yes | Yes |
| Stone wool | $$$ | 5 | 2 | 4 | 8 | 3 | no | no | Yes |
| Grow Plugs | $$$$$ | 4 | 3 | 5 | 6.5 | 3 | no | some | Yes |
| Peat Moss | $$ | 5 | 2 | 5 | 4.4 | 3 | no | Yes | no |
| Oasis Cubes | $$$ | 3 | 2 | 2 | 6.5 | 1 | no | no | Yes |
| Hydroton | $$$ | 3 | 3 | 3 | 6 | 4 | Yes | no | Yes |
| Perlite | $$ | 1 | 4 | 1 | 7.2 | 4 | Yes | no | no |
| Vermiculite | $$ | 2 | 3 | 2 | 7.2 | 4 | Yes | no | no |
| Sand | $ | 2 | 1 | 2 | 7 | 2 | Yes | no | no |
| Gravel | $ | 1 | 3 | 1 | 7.5 | 4 | Yes | no | no |
| Pine Shaving | $ | 5 | 2 | 3 | 5 | 4 | no | Yes | yes |
| Wood Fiber | $ | 3 | 3 | 3 | 5 | 4 | no | Yes | Yes |
| Aged Bark | $ | 3 | 3 | 3 | 4.5 | 4 | no | Yes | Yes |
| Pumice | $$ | 2 | 3 | 2 | 7 | 4 | Yes | no | Yes |
| Brick Shards | $ | 2 | 3 | 2 | 7 | 4 | Yes | no | Yes |
| Growstones | $ | 3 | 4 | 1 | 6.5 | 5 | Yes | no | Yes |
| River Rock | $ | 1 | 5 | 1 | 5-7 | 5 | Yes | no | no |
| Rice Hulls | $ | 1 | 2 | 2 | 6.5 | 3 | Yes | Yes | Yes |
| H2O Crystals | $$$$ | 5 | 1 | 4 | 7 | 3 | Yes | no | no |
Most Common Growing Medium Per Hydroponic System Method
Kratky Method – Stone wool, Coco, Grow plugs, Oasis, Floral foam, Polyurethane foam, perlite, and vermiculite. Placed in a net cup and then placed into the system
Wick Method – Requires medium with high WHC. Stone wool, Coco, Grow plugs, Oasis, Floral foam, Polyurethane foam, Peat Moss. Placed in a net cup and then placed into the system
Deep Water Culture – DWC – Requires good drainage. Most Clay, Stones, and Stone Aggregates. Stay away from any medium that floats. Placed in a net cup and then placed into the system.
Raft method – Stone wool, Coco, Grow plugs, Oasis, Floral foam, Polyurethane foam, perlite, vermiculite. Placed directly in the raft or in a net cup and then placed into the raft.
Dutch or Bato Buckets – Stone wool, Coco, Grow plugs, Oasis, Floral foam, Polyurethane foam, Peat Moss, Clay, Stones, Most Aggregates. Placed in a net cup and then placed into the bucket
Ebb and Flow or Flood and Drain – Requires good drainage. Most Clay, Stones, and Stone Aggregates. Stay away from any medium that floats.
Rotary Method – Stone wool, Coco, Grow plugs, Oasis, Floral foam, Polyurethane foam
Nutrient Film Technique – NFT – Stone wool, Coco, Grow plugs, Oasis, Floral foam, Polyurethane foam.
Drip Technique
- Bato, Dutch bucket – Stone wool, Coco, Grow plugs, Oasis, Floral foam, Polyurethane foam, Peat Moss, Clay, Stones, Most Aggregates. Placed in a net cup and then placed into the bucket
- Vertical System – Stone wool, Coco, Grow plugs, Oasis, Floral foam, Polyurethane foam. Placed in a net cup and then placed into the system.
- Run to waste – Most any medium
Final Thoughts
The type and method of system you chose to use will most likely dictate your medium choice. If you have a specific set of plants you wish to grow, make sure you choose the correct type and method of system to grow those plants. Then start with one medium that is best suited for each system and plant and experiment as you learn and grow.
