Which Grow Medium - Soil, Coco Coir, Rock Wool, or Hydroton - is Right For You?

Staff Writer Cultivation

When you’re starting an indoor garden, there are multiple grow mediums to choose from -  coco coir, rock wool, and hydroton are some of the most common, while soil is generally not recommended for hydroponic set-ups.  

Aside from choosing the right hydroponic system for your needs, choosing the perfect grow medium is probably the most important factor in your indoor gardening success. 

A good growing media will not only protect the plants’ roots and provide support, but it will provide them with the moisture and oxygen they need, too. It will also help them have maximum exposure to the nutrients they need.

How do you know which one is right for you? It can be a tough decision to make, but with a bit of insight into the science behind indoor hydroponic gardening, you’ll be able to make an educated, informed decision.

To get started, you’re going to need to consider the needs of the plant - but you can’t neglect the needs of your hydroponic system, either. You’ll want to take aeration, air to water ratio, and pH balance into account.

Depending on your set-up, your biggest goal in hydroponic growing is probably to produce massive yields. However, many growers also look to create systems that are environmentally friendly and work to support - rather than hinder - the natural world. After all, if you're growing for profit, this can be an important selling point in your marketing plan!

The best growing media are those that are at least moderately biodegradable, while also maintaining an even ratio of water to air. You’ll want to make sure the media you select is capable of holding nutrients and protecting the plant from abrupt changes in pH.

The Most Common Grow Mediums: Soil, Coco Coir, Rock Wool, and Hydroton

growing in hydroponics

Soil

When it comes to growing plants, soil is the medium with which you are probably the most familiar. After all, it’s the one that you tend to use in traditional cultivation. However, in hydroponics, you aren’t limited solely to soil- and in fact, it's not a wise choice for any kind of hydroponic set-up.

Soil can make things tougher to maintain and measure, since you’ll need to account for changes that the soil can make to your nutrient levels and pH - it is not an inert material. 

Soil makes it easier to overfeed or underfeed certain nutrients. Unless you test the soil before you begin, it’s difficult to know how it will react with added nutrients. It can also introduce insects and mites to your system - even purchased soils sometimes have these pathogens present. 

Remember, what makes a hydroponic setup so effective is that it doesn't have soil. Instead the plant's get fertilized by a nutrient rich solution. That solution contains everything they need. Adding soil to a hydroponic system can cause clogging and other issues. 

Soils soak up and hold nutrients differently than plants in other growing mediums, so it’s important that you take this into consideration. You also need to be aware of the fact that potting mixes are organic - the ingredients included in them will break down gradually and decompose. As they do, they can compact and reduce the availability for water and air. 

Instead of using soil, consider using one of the soilless growing media alternatives we'll tell you about below. These are much better options for hydroponic set-ups.

Coco Coir

Coco coir may not be a grow medium you had ever heard of prior to starting your hydroponic grow room, but rest assured - it’s one of the best growing mediums you’ll find. It's commonly used to grow a wide variety of crops.

Coco Coir is made primarily from ground-up coconut husks and offers many benefits to hydroponic growers.

Coco coir can protect your fragile seeds as they are germinating and a wide range of plants.

Properly produced coco coir is an inert growing medium, so it essentially acts as a blank slate for your hydroponic system. Make sure you get your coco from a reputable source, that has properly soaked and prepared the coco for plants. Depending on where the coconuts are grown, their husks may contain toxic levels of salt. These husks are ground up into coir. So if it's not treated and tested properly, it can lead to disastrous consequences for your crop.

Coco works for a large array of plants. It is also free from fungus, helping you cross this concern off your list of things to worry about that.

Since coco coir is made out of coconut husks, it’s also a renewable and eco-friendly growing medium, too. Coconut husks are typically thrown out when the rest of the coconut is harvested.

There are many positives to using coco coir in your hydroponic garden. It is sustainable, affordable, forgiving, pH neutral, and often organic.

Like other water absorbing media, like rockwool, you do need to be wary of salts building up in your system. There are nutrient lines you can buy for your hydroponic setup that are designed primarily for coco coir. This will make it easier for you to flush your system and remove excess salts. 

How to Use Coco Coir in Your Hydroponic System

You can buy coco as coir (fibers) or chips. The benefits of coco chips are that they are large enough to create air pockets but don't offer good water absorption. Mix the chips with coir to aerate the mix, instead of perlite.

Coco coir are absorbent, and are often a great place to start for beginner growers. This media can settle over time, slightly impacting the roots. Adding an aerating substrate like perlite or coco chips can be very useful.

Remember, too, that coco coir is inert - it does not contain any nutrients. You will need to control the pH and add nutrients while using this growing medium. Typically, plants grown in coco coir tend to be deficient in magnesium and calcium in particular. Consider adding it to your nutrients, or using a coco specific nutrient.

Rockwool

Believe it or not, rockwool is a growing medium that has been around for many decades. It’s a well-known grow medium in the hydroponic community worldwide. 

Rockwool is made from spinning melted rock into long, thin fibers. Kind of like making cotton candy from molten rock. The fibers are then pressed and cut into cubes of different sizes. 

There are several benefits to using rockwool in an indoor growing environment. For starters, it has an excellent air to water ratio. Cubes are self contained, so you don't need additional pots. They hold up well from seedling to harvest. Keep in mind rockwool is often alkaline, so the pH of the solution in the cube will often be higher than what you are irrigating with. To get an idea of what the plant is exposed to, just collect the run of and measure it.

Keep in mind, the dust and fibers that are used to spin and compress the material can irritate your skin, nose, lungs, and eyes if you don’t take proper precautions when using it in your system. Although rockwool is essentially just rock, the problem with using it is that the dust and loose fibers can irritate your respiratory tract in a manner similar to that of asbestos (though of course not as toxic).

Luckily, there are ways to mitigate those concerns.

To prevent the effects of dust, you may want to wear goggles, gloves, and a mask until you have the growing medium in your system. Once it's placed in a hydroponic setup, the irrigation will soak the fibers and reduce the abrasive effects they can have on your respiratory system.

Rockwool plugs can occasionally be high in pH, too, so that’s something else to be aware of. Rinsing and soaking the rockwool before use can help balance this out. 

It can replace peat starter plugs and get seeds going faster in your hydroponic system.  

How to Use Rockwool in Your Hydroponic System

You can find rockwool in all kinds of sizes, shapes, and formulations. At full saturation, rockwool will hold about 70% of its volume in water. The rest of it will produce tiny air pockets to stimulate root growth. 

It’s best used in a top feed or run to waste system. You can buy rockwool in slabs, plugs, cubes, chunks, and soft bales, too, making it convenient for many set-ups. Here are some tips in setting up a grow room with rockwool.

Hydroton

Hydroton is a soilless hydroponic grow medium made out of expanded clay pebbles. These pebbles are great for both aquaponic and hydroponic growing, as they are lightweight, sterile, and easy to harvest. They are also easy to transplant into and easy to dispose of, too.

Their versatility makes them a good option for just about any hydroponic system.

Hydroton is pretty much exactly as it sounds - the grow media consists of small bits of clay that are kiln-fired and expanded to be lightweight and porous. It’s best for nutrient film technique, expandable drip systems, and deep water culture. It’s not ideal for flood and drain systems, though, because it can dry out too quickly. 

Hydroton is eco-friendly since clay is an abundant resource. You can reuse hydroton almost indefinitely - you may just want to clean the pebbles between use with a mixture of hydrogen peroxide. 

To grow healthy plants, their roots need plenty of oxygen. In fact, it is just as important for plants as water, nutrients, and light. Without ample amounts of oxygen, your plant's roots can succumb to root rot. And that can lead low yields and to plant death.

Hydroton is a great grow medium because it provides all the air your roots need. Clay rocks are incredibly loose, so it’s easy to pull your plants out. The medium also rarely becomes blocked so water drains quickly

How to Use Hydroton in Your Hydroponic System

You can leave hydroton pellets whole when you add them to your hydroponic setup, or you can mix the pellets in with the soil. Usually, you’ll want to combine about 30% hydroton with 70% soil to create a growing medium that is excellent at holding water. You’ll put the pellets at the bottom of your growing container (beneath a layer of soil) to improve drainage. 

How to Decide Which Grow Medium (Soil, Coco Coir, Rock Wool, or Hydroton) Is Right For You

Ultimately, any of these growing mediums will work well for you. Just make sure it meets the basic requirements we mentioned earlier in the article for good air to water ratio, pH balance, and usability. 

Of course, your ideal growing media will also probably be affordable and easy to find. It should be easy to store, too, to make your life a little easier when it comes to starting an indoor garden. 

It might be hard for you to find a media that checks off all these boxes. At the end of the day, it may come down to choosing which factors matter most to you personally. All of these grow mediums - soil, coco coir, rock wool, hydroton, and more - are perfectly suitable for most indoor hydroponic systems. Consider your needs and preferences, and choose wisely! 

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