In hydroponics, we don’t use soil, we use other kinds of growing media.
Hydroponic growing media contains no nutrients. All of the nutrients and minerals growing plants need are supplied through water mixed with hydroponic fertilizer, known as nutrient solution.
In the purest forms of hydroponics, nothing but the nutrient solution is used—the exposed roots hang directly in the solution.
In other cases, a solid substrate is needed to support and protect the plant’s delicate root system.
Solid hydroponic media give the plant stability, provide insulation to the plant roots, and block light from the nutrient solution, preventing algae bloom.
And, if irrigation fails, many grow media can hold enough water to keep the plant alive and fed for hours or days until irrigation comes back online.
Common Grow Media Terms
Bulk density is simply the weight of a medium.
For example, peat moss has very low bulk density and sand has a very high bulk density.
Remember, the weight of a medium may be drastically different when wet compared to dry. Vermiculite is quite light, but it holds so much water that it becomes heavy when wet.
Keep bulk density in mind when building your systems, as your tables, pipes, containers, and many other parts of your build will need to accommodate the weight of your growing medium—both wet and dry.
Water Holding Capacity
Water holding capacity (WHC) is the volume percentage of water retained by a saturated medium after it drains
Vermiculite and coconut coir have very high WHC, while lightweight expanded clay aggregate (LECA) have a low WFC. Sand and gravel have almost no WHC.
Air Filled Porosity
Air filled porosity (AFP) is the measure of the volume of pore space occupied by air after a saturated growing medium drains.
Sand has a very low AFP and clay pellets have a high AFP.
Cation Exchange Capacity
Cation exchange capacity (CEC) is the measure of how a growing medium can hold and exchange cations to its surrounding environment.
CEC is a good indicator of a medium’s fertility because it shows its ability to supply calcium, magnesium, and potassium to a plant.
Coco coir has a high CEC, and rockwool and perlite have a CEC of close to zero.
Main Forms of Hydroponic Grow Media
Hydroponic growing media are used for two basic purposes.
First, they are used to “start” seeds, germinating seeds into seedlings.
Next, they are used to grow seedlings into adult plants.
Starter plugs are little cubes or cylinders of growing media made specifically for starting seeds. Examples include oasis and rockwool cubes (more on these later).
Plugs are great for germination but don’t offer the same characteristics needed for larger, adult plants.
In a normal hydroponic system, once a seed has germinated into a healthy seedling with the help of a starter plug, it is transferred into a new medium that gives it more room to grow and a better chance of success.
Many types of growing media can be used to start seeds, but starting plugs aren’t ever used to grow adult plants.
Here are some of the most popular growing media. We will discuss where they excel, what they lack, and what they are used for
Coconut “Coco” Coir
Coco coir is made from the outer husk of coconuts (coconut fiber).
You can find loose bags of this fluffy, soil-like material, or compacted bricks of it. The product is the same, the only difference is the cost of shipping is a bit cheaper for the bricks.
In traditional gardening, coco coir (coco for short) is becoming a popular alternative to peat moss. Peat moss develops naturally over thousands of years and isn’t a sustainable product. Coconut coir, on the other hand, is sustainable and biodegradable.
Coco is a great hydroponic grow medium because it’s highly absorbent yet it still drains well. This means it will retain enough moisture to keep your plants fed without drowning them or causing problems like root rot and fungal growth.
In fact, coco has natural antifungal properties.
Keep in mind—some coco coir is not inert and may contain phosphorus and potassium. Be sure to tweak your nutrients to ensure your growing plants aren’t getting inundated with these nutrients, or just buy nutrient buffered coco.
Lightweight Expanded Clay Aggregate/Pebbles
Expanded clay pellets, also known as grow rocks or hydroton, are a naturally-derived growing medium.
These are made by heating a special kind of clay to over 2,000° F, which causes gasses in the clay to expand, creating numerous bubbles.
The resulting clay pebbles are inert, highly versatile, and long-lasting.
Because they hold so much air, clay pellets are excellent for drainage and provide lots of aeration to plants’ roots.
However, they don’t hold as much water as some other hydroponic growing media, so frequent watering is necessary.
Rocks and Gravel
First off, it’s important to differentiate naturally occurring rocks (like river rocks) and gravel from processed rocks like perlite and vermiculite, which I will cover below.
Natural rocks aren’t the most popular hydroponic growing medium, despite being cheap, widely available, and draining well.
The problem with rocks and gravel as a growing medium is that they are heavy and don’t hold much water at all. This is why they are undesirable in systems that rely on the grow medium to supply water to the plant roots, like in ebb and flow systems.
However, when other growing media isn’t available, rocks will get the job done.
And here’s a good use for gravel—if you are having drainage problems using another hydroponic medium, you can mix in some gravel or line the bottom of the container with it to greatly improve drainage.
Perlite is an extremely popular growing medium.
Perlite is made when volcanic glass, which contains tiny pockets of water, is heated to a high temperature. In this process, the water rapidly expands, creating thousands of tiny pores.
It’s basically volcanic rock-popcorn.
Because perlite is so porous and light, it’s usually used to add drainage, water retention, and air to a growing medium.
In certain hydroponic systems that use drip irrigation, like Dutch buckets, perlite is used by itself as a growing medium, to great effect.
Rockwool is one of the most commonly used seed starting media.
It’s made by melting together a combination of rocks and then spinning them into a thin fiber. The result is a fluffy, wool-like material.
You usually find rockwool cubes sold in large sheets of starter plugs.
Rockwool is renowned for its water retention and root aeration. It’s inert, sterile, and can be reused several times.
One thing to be aware of—some rockwool will come out of the package with a high pH. This can harm your plants, so be sure to wash or soak it before using.
Believe it or not, sand can be a viable growing medium.
It’s cheap—if not completely free—and widely available. Sand culture, as it’s known, is possibly the oldest hydroponic medium.
There are many downsides to using sand. It gets very hot in the summer and cold in the winter, which can damage plants. Water retention is low, so frequent watering is necessary. Also, natural sand is very salty and alkaline, so it needs to be washed and treated with acid before use.
While sand isn’t the best medium, it can be used for a fun project or experiment.
Soilless mixtures can include a variety of growing media, such as perlite, peat moss, vermiculite, coconut coco, and pine bark.
Instead of combining your own media, trying to find that perfect combination, you can buy a soilless mix that’s balanced and ready to go.
These are especially useful for seed starting. If you were to take potting soil out of a flower container and plant seeds in it, you might run into some trouble. That soil is likely to have unwanted bacteria, fungi, nematodes, weed seeds, and chemicals that can hurt your plants reduce your chances of success.
Instead, you can reach for some soilless mixture that has none of these, holds water well, provides great aeration to the plant roots, and doesn’t compact as much as soil.
However, these mixes tend to break down and compact over time, and can’t be used again and again like other media.
Oasis cubes are similar to rockwool.
They come in sheets of pre-formed plugs that are perfect for seed starting and holding seedlings in another grow medium.
Oasis cubes are really only different from rockwool in their composition. While rockwool is processed stone, oasis cubes are made of a plastic-based foam called phenolic foam.
Phenolic foam is a versatile material used across a variety of industries.
It offers an excellent water absorption and retention to root aeration ratio. In other words, it supplies loads of water to the roots without cutting off their oxygen.
A study in the Korean Journal of Horticultural Science and Technology showed that phenolic foam plugs performed just as well as rockwool plugs when seed starting pepper plants.
Oasis cubes come out of the package sterile and inert, and they can be reused many times.
Vermiculite is very similar to perlite. It is another growing media that is the product of rapidly heating a rock.
It retains slightly more water than perlite and can wick water upwards.
Vermiculite is usually used in combination with one or more other growing media. It adds great drainage and moisture retention to any soilless mix.
The downside is that it can retain too much water for some systems and is fairly expensive.
Rice hulls are a byproduct of rice production. Because they are just a part of the rice plant, they are a natural and compostable growing medium.
Rice hulls are somewhat similar to perlite and vermiculite, they add drainage and aeration. However, unlike vermiculite, rice hulls don’t hold much water.
Many hydroponic growers choose to use this in their soilless mix because it is “green” and sustainable.
If you choose to use rice hulls, one thing to be aware of is that they do decay over time. You shouldn’t reuse rice hulls more than a few times or you could cause problems for your plants.
Most gardeners will be familiar with peat moss.
Peat moss is a fibrous soil-like material formed over thousands of years as moss and other organic materials decompose.
Its biggest advantage is that it can hold lots of water without compacting. For this reason, it is commonly included in soilless mixes.
However, peat moss is slightly acidic and quite expensive. And despite being natural and organic, it isn’t sustainable.
The Best Growing Medium By Plant
The growing medium you choose should mostly come down to the hydroponic system that you use. That being said, certain plants do better in some growing media more than others.
Here is my recommendation of what growing medium to use with these popular fruits and vegetables.
Lettuce can be grown in almost any growing medium.
Coco coir, peat moss, and soilless mixes are often used in a seed tray to germinate lettuce seeds.
While lettuce seedlings could be transplanted into solid growing media, most commonly it goes right into some nutrient mix.
Systems like NFT, the Kratky method, and deep water culture work best for lettuce, so growing media other than plugs and water are unnecessary.
In a Kratky method container, it can grow from seed to harvest in just a single piece of rockwool.
Tomatoes can be grown in a few different types of media. Because they require a large quantity of water and consume lots of nutrients, tomatoes usually don’t fare well in water-based systems like Kratky and deep water culture.
Many hydroponic growers use expanded clay pellets and perlite for drip systems like Dutch buckets and Nutrient Film Technique systems (NFT).
Strawberries are similar to tomatoes in that they don’t do well just sitting in a nutrients. They need a solid substrate to thrive.
Strawberries should be grown in a medium with great drainage, so coco coir is out. You can, however, use coco mixed with something that adds drainage, like perlite.
The most popular choices for growing hydroponic strawberries are expanded clay pellets and soilless mixes.
Basil is another easy-to-grow plant that does well with many hydroponic media.
Basil can thrive in nutrient solution alone, in coco or another soilless mix, or in perlite or vermiculite.
As with any plant, you’d do well to start basil in a soil mix, foam plugs, or rockwool plugs before transplanting it into its final home.
Hydroponic microgreens are different from the other plants we have covered so far. They require no transplanting. Microgreens go from seed to harvest in the same container with the same growing medium.
Coco coir, vermiculite, or a combination of both are great for microgreen propagation.
Some interesting media like burlap and biostrate can also be used. These are just pieces of cloth that plant roots can bind to. They also provide good water retention and aeration.
After starting your onions in soil, oasis cubes, or rockwool, you can transplant them into a more supportive medium.
Green onions, also called scallions, can do quite well in water-based systems like floating rafts and the Kratky system.
For larger-sized onions, perlite, vermiculite, and expanded clay pellets—or some combination of the three—work well.
Grow Medium FAQ
What soil do you use for hydroponics?
In hydroponics, you don’t use any soil. Instead of using soil mixes to support our plants and supply them with nutrients, we use alternative growing media, water, and hydroponic fertilizer.
Soil can be used to start seeds, but all dirt must be washed off the plant roots before it is transplanted into your hydroponic setup.
Is Styrofoam safe for hydroponics?
Yes, Styrofoam is considered safe for use in hydroponics.
Styrene is a compound found in Styrofoam that many people worry about. Styrene is a potential carcinogen, but it isn’t present in high enough levels in Styrofoam to harm us.
In a hydroponic system, there is very little leaking of any toxic materials from Styrofoam under normal temperatures and conditions.
However, once Styrofoam begins to degrade and break down, it should be replaced as small pieces of it may end up clogging pipes or water pumps.
Can you reuse hydroponic water?
Commercial hydroponic growers use sophisticated methods to recycle their nutrients. On a large scale, reverse osmosis filtering, pasteurization, ozonization, and UV disinfection are great solutions.
While these processes may be financially viable when running through thousands of gallons per day, the average grower may find them out of reach.
But, many home growers use a simpler method of recycling or hydroponic water. Try this:
After a grow cycle, remove the leftover solution in your reservoir and give the reservoir a good cleaning. Add new water to your leftover solution.
Test the liquid for nutrients with a simple TDS tester like this one. Now, just add hydroponic fertilizers to the liquid to bring it up to the proper levels for your system.
Can you use potting mix in hydroponics?
Yes, you can use a potting mix in hydroponics, as long as it’s a soilless mix.
Common ingredients in hydroponic soilless mixes are perlite, vermiculite, wood chips, coco coir, peat moss, and other non-soil ingredients.
These mixes work well for starting seeds or even as the main substrate for some vegetables.
Can you reuse growing media?
Yes, most growing media is reusable.
Media like sand, stones, and gravel can be reused indefinitely, as long as you give them a good rinsing after use.
Clay pellets, perlite, vermiculite, and rockwool can be reused for many grow cycles as long as they are maintained and no diseases or pests are present.
Finally, media like coco coir, sawdust, peat moss, and soilless mixes usually need to be changed after each use.
What else can you grow hydroponic plants in?
While your mileage may vary with each, some people like experimenting with unique mediums.
Here are a few worth mentioning:
- floral foam (the green foam blocks florists use)
- recycles glass