Although any inert growing medium could be used for a hydroponic setup, there are a few that are highly used in the hydroponic community because of their ability to retain moisture and oxygen. Or it will at least allow the water to get to the plants, such as clay aggregates, while allowing oxygen when the water level goes down (as in an ebb and flow setup).
Rockwool is a common growing medium that comes in the shape of a cube. You can see our logo at Grow Giant is actually similar to a rockwool cube. These are good because they retain both water and oxygen well. In an ebb and flow system, there are periods of time where the water is completely in the reservoir and so the roots are not bathing in the solution. However, good growing media retains some moisture so that the plants are still happy while they aren’t being directly fed.
NFT systems and aeroponic systems are somewhat risky because typically no growing medium is used. That means that if the system were to experience a power failure, for example, then no more nutrient solution would be reaching the plants. And, unlike other systems, the lack of a growing medium means that no stored nutrients will be available to the plants. Therefore, plants will suffer greatly in these times and it is imperative to return your system to operating condition, or they will die quickly.
Rockwool is one of the most common growing mediums used today and for good reason. It holds oxygen and water very well, providing the perfect environment for your ever important roots. It is made through an interesting process where one cubic yard of ordinary basalt rocks and chalk is expanded into a fine thread that takes up about 37 cubic yards! The process is to heat the rocks and chalk at 1600 degrees Celsius into a lava. Then the lava is blown in a glass chamber into a cotton candy type structure. It also somewhat resembles the insulation in your house. After this is done, they are compressed into blocks, cubes, starter plug mats, and slabs. As you can see, this is a great use of otherwise useless rock, making it also a reusable item.
Since rockwool somewhat resembles insulation in your roof, you should be careful when being around it while it’s dry. It may cause skin irritation and can irritate your lungs because of the dust and particles it gives off.
In order to use rockwool correctly, you must pre-soak it with 5.0-5.5 pH water for about 8 to 24 hours, then you must rinse that out with 7.0 pH water. To soak slabs, simply open the top and pour water in the top, however, to soak cubes, you should fill a bucket of water and place the cubes inside. Drain the slabs by cutting slits in the bottom after about 24 hours.
Rockwool can also be used for a couple of grows after your initial making it a nice reusable source of medium. When it is completely used up, it can still be used like perlite to aerate your garden because it will provide chunks of space in between the soil.
Seedlings can be grown from starter plugs into full grown adults if transplanted into other rockwool cubes, or even another medium such as clay aggregates. These must be in a hydroponic system in order to stay hydrated because they will drain slowly.
Light Expanded Clay Aggregate
These clay rocks are likely the most common found medium in hydroponics because they are fairly inexpensive, can be used again and again, and have lots of spacing for fresh oxygen to circulate. These are good for growers who know that they will be growing repeatedly in the future. However, be careful, because if these fall on a slick surface like tile, they will roll all over the place.
These are great for aerating the roots of plants because they are essentially little rocks with holes in them that allow for circulation of fresh oxygen. These aggregates don’t hold water very well, certainly not like rockwool, therefore, it is imperative to have these in a hydroponic system with nutrient solution being consistently applied to the roots. Also, if you use these in a timed ebb and flow system then you should try to keep the on/off periods 30 minutes each. Since they have a neutral pH it will also be easier to maintain your plant roots pH because you will simply need to check the solution and not the soil or rockwool cube.
It is recommended to wash your clay aggregates off because they are clay and have a lot of dust and potentially harmful particles to your plants. After use and harvest, you can reuse these aggregates by rinsing all the remaining plant material from the rocks, then place them in a 1 to 10 bleach to water solution. Using hydrogen peroxide and water is an alternative way to clean them (using a couple of teaspoons for each gallon of water). These are, however, expensive to ship, so it is likely best to pick them up from your local hydroponics store.
Perlite and Vermiculite
Perlite is the white little balls you typically see in many gardens and pots with soil in them. It serves a big purpose in that it aerates your soil because of its nature. It is siliceous rock that is quickly heated to 1600 degrees Fahrenheit and it pops like popcorn into an inflated airy bubble that has very little to no mass. The pieces of perlite act as little pockets of air in your extremely dense soil. Without this perlite there would be a minimal amount of oxygen getting to the roots leading to below optimal growth.
Vermiculite is on the opposite end of the spectrum of perlite. Although it is light like perlite, it is actually of gold color, and it actually absorbs water. The two actually make a very good combination together and you can probably see why. The vermiculite absorbs the nutrients and the perlite drains the excess water allowing precious oxygen to get to the roots. This is the essence of fine hydroponic growing and this method is common among growers for that very reason.
Soilless Potting Mixes
Soilless mixtures are another good hydroponic route, because they are basically a mix of all the best things you could add to your medium. These mixes could include peat moss, perlite, vermiculite, coconut coir, mycorrhizae, and possibly the micro and macronutrients your plants require.
We currently carry several types of these mixes with the most popular being Pro-Mix HP Mycorrhizae, B’cuzz HydroMix HP, Sunshine Advanced Mix #4. The HP in the B’cuzz and Pro-Mix stands for “high porosity” which means they have good drainage of water, and are, therefore best used with water sensitive plants. The Sunshine Advanced Mix #4 is an excellent combination of peat moss, coir, perlite, and a multispecies blend of endomycorrhizae. These mixes will provide all the drainage and water retention that any plant will benefit from.