Hydroponic Growing Media and Substrates Guide

Last Updated on March 15, 2022 by Admin

This is a complete guide to hydroponic growing media and substrates. I’ll show you the different kinds of growing medium you can use for hydroponics, how they differ from one another and what are the best uses for each.

Substrates & Growing Media

In hydroponics, your growing media replaces soil, at least to a degree. That’s because it doesn’t contain any of the nutrients that are found in soil.

A growing medium or substrate is a material that provides support for your plant to grow in. It acts as a foundation to keep your crops standing upright much like soil does in traditional gardening.

That said, if you look around, you’ll notice a ton of different kinds of material used as growing media. And, depending on what kind of hydro system you’re using, one or the other will be work better.

What Makes a Good Growing Medium?

Here are some of the things to consider with growing media.

Durable. Some growing media will decay or break down. While that’s okay after a few cycles, it’s not good if the substrate breaks down too quickly. This will cause the particles to get too small and affect your plants’ growth. It also means you’ll need to keep replacing (and buying) new ones, which increases your costs.
It Shouldn’t React With the Water or Nutrients. Interactions are never a good thing because they end up changing the formulation, adding minerals or emitting other things to your system. When this happens, it messes up the nutrient balance you’ve worked so hard in achieving. Similarly, non pH neutral substrates will affect the overall pH levels of your system. Thus, affecting your crops’ ability to absorb the nutrients in the solution. This is why inert materials as well as those that are pH neutral are good choices.
Sterile. Like potting mixes, the growing media you use must be sterile. Any disease, pests or microorganisms in it will easily be transferred to your plants. That’s why many of these materials go through a sterilization process before being packaged. That said, if you’re not sure about its sterility, you can avoid using it or sterilize it yourself. All you need to do is heat it to 160 degrees Fahrenheit with your oven or stove. Similarly, you can let it soak intense sunlight for about a week.
Retains water. Water is a tricky subject when it comes to growing media. You want something that retains enough water so the roots can draw from them (and not dry out quickly). But, you don’t want it to hold too much water as well. As such, there’s a balance between retention and drainage. Just as importantly, retaining too much water means a lack of oxygenation which can damage your roots as well.
Able to Support Your Plants’ Root System. In addition to all the other features, growing media is meant to support your crops’ roots. As such, it should be able to provide the roots with a surface they can anchor to. And, to do that, the roots need to be able to penetrate through the substrate’s particles. Here’s where too fine or too coarse a material can prevent the roots from taking ground. Similarly, too heavy or too light can make it difficult for your plant to grow in.
Readily Available. It’s important that you can easily purchase it from your garden center or hydroponics shops. In some cases you can find or get them for free. That said, always know where you’re getting your growing media because the source may affect its make up.
Affordable. Since you’ll be needing it as long as you’re hydro gardening, it shouldn’t be expensive. Otherwise, it won’t be practical even if it checks all the other criteria above. The good news is, you won’t be using a ton of it. So, the cost isn’t a huge factor either.

Types of Hydroponic Growing Media

Below is a list of some of the most common (and popular) growing media used by hydroponic gardeners. It’s not a comprehensive list since there are many other substrates available. But, there are among the ones you’ll see or hear of most.

Rock Wool (Stone Wool)

Depending on where you live, it may be called rock or stone wool. Either way, they mean the same thing.

These are small square-shaped substrates that are created from melted rocks which are spun into thin fibers. After this, the fibers are pressed and shaped into cube-like shapes. Although, you’ll also see them in slabs as well.

Rock wool is fairly popular because it is both porous and able to retain water. Both of these are beneficial for growing since you want the medium to retain water and nutrients but drain well enough to keep your plants’ roots from overwatering.

That said, never under any circumstance eat or consume rock wool. They’re bad for your health and harmful to your eyes, nose and lungs.

They also come with a high pH and you need to soak them.

Coconut Coir

Coconut coir is often referred to as coco coir. It’s made from coconut husks and is quite popular with hydro gardeners.

One reason is that coconut husks help protect coconuts as well as provide them a medium to germinate. As such, it does the same for your plants.

That said, they must be washed properly because they can contain a lot of salt. Salt can be destructive to your plants. Similarly, washing helps remove the tannins. This reduces the chance of it staining your growing area.

Perlite

If you grow houseplants or use potting mixes, you’re likely familiar with perlite. They’re made by heating volcanic rock until they “blow up”, much like popcorn. This creates a porous and lightweight material that holds oxygen very well, is organica and inexpensive.

But, just as gravel can be too heavy, perlite can be too light in weight. As such, the water can move the particles of the growing media around or even wash it away. This is why it’s often combined with other substrates like coco coir.

Related

Expanded Clay Pellets (LECA)

Also called hydroton (expanded clay pellets), which is the name of one of its original manufacturers. This was one of the most popular substrates in the past.

These pellets are made by expanding clay to create porous round balls. Their round shape allows for balanced air and water uptake, while the pores in the pellets can retain some water and it drains quite well. Thus, making it hard to overwater which is a common problem with beginner gardeners.

It’s important to rinse them before using them.

Additionally, they’re pH neutral and inert, meaning that they don’t release any chemicals or minerals into the water.

Gravel

You’re likely familiar with gravel. It’s what’s used in aquariums as well as construction. It was also one of the first substrates used in hydroponics.

Using it is simple as you fill the net pots with gravel and set the roots in it.

Like rock wool, it’s inert so there’s no problem of leaking any minerals to your water stream. Plus, it can hold a lot of air which your plants’ roots need to carry water up.

Gravel is likewise cheap and easy to clean. More importantly, it drains very well so it’s almost impossible to overwater your plants.

But, as you would guess, gravel is heavy, and because it drains too well. Thus, there’s a risk of the roots drying out.

Sand

Sand is made from very minuscule rocks. You’re familiar with them since you’ve likely played with it as a child in sandboxes or on the beach.

Sand is often free. And, should you need to buy it, it won’t cost much either.

Unfortunately, it’s heavy. And, its make up doesn’t allow air to flow through it. This makes it difficult for your plants’ roots to get the oxygen they require.

Sawdust

Like sand, sawdust is something you’re likely familiar with. It’s the by-product of wood that you can get from sawmills or whenever there’s carpentry or construction.

One of the biggest benefits of sawdust is that you can get it free. Even if you purchase it, it’s still quite cheap.

Their lightweight nature and water retention qualities also make it a good growing medium.

Unfortunately, sawdust does rot over time.

Also, it’s important to know where it comes from. In some cases, it may be tainted with chemicals or contaminants.

Plus, do make sure to check what kind of wood/tree the sawdust comes from first. Some cedar and pine trees have resins that are toxic to plants. As such, you should avoid them at all costs.

Vermiculite

Vermiculite comes from mica (a kind of silicate material) that’s heated to form spongy pebbles. The heating process sterilizes it so you don’t have to worry about it being dirty or having contaminants.

They’re very similar to perlite and used in potting mixes as well. But, vermiculite retains water better than perlite.

Similarly, its shape allows air to get through so your plants can get oxygen.

However, the problem with vermiculite is that it’s expensive. Plus, it can hold too much water. In some cases, it can also clog up your tubing.