Chapter 5: Growing Media
Chapter 5: Growing Media
Though your plants will get all of their nutrients from the solutions you provide, they still have to have some sort of physical support for their roots. There are many different options for this, some that are organic and some that are man-made. In many cases, there are minimal difference between one type and the next even though many hydroponics gardeners may swear by one medium over another. You might have to do a little trial-and-error research to see what works best for you.
Before making a choice, you will need to know the basic features that a good growing medium should have. Fundamentally, all materials will be light and porous so that they can hold a mix of air and water within them and still allow roots to grow freely between the particles. They also should be chemically inert so that they do not contribute to the nutrient mix that your solution is providing. That would defeat the main purpose of hydroponics.
A selection of materials is listed in the next few sections but there is no limit to what you can work with. If you have access to similar but unlisted materials, feel free to experiment. Also, there are many benefits to mixing two or more of these materials together to get the perfect conditions for your plants.
You will want to consider the absorbency of your medium and what kind of hydroponics you are going to be using. Some systems rely on the medium to hold water between pump runs, so you should keep that in mind.
Sand
Sand is a very easily acquired material that is also inexpensive. It has been used as a hydroponic medium, but it is not ideal because the particles are so small that there is little space for air once the water has been added. You can mix sand with perlite or another material to improve the aeration qualities. Once water has been added, sand can be very heavy.
Gravel
This works a lot like sand but with larger particles. Smooth gravel will work better than rough, as it is gentler on your tender, growing roots. The pieces will not absorb any water, so you should not use gravel in any system that requires the medium to store water between irrigations. Ebb and flow is one example where this is not the best option. But any static solution or top-drip system would be fine.
Gravel is quite heavy and would not work well in a raft system. As long as the plant container is supported, you should not have much problem.
You also have to know what kind of rocks are in your gravel. Limestone will have an effect on your water’s pH level and should be avoided unless you intend to compensate when you manage your water system.
Vermiculite
This material is a common addition to potting soil as a way of adding extra absorbency. It is made from mica (a natural mineral) that has been heated until the layers within its structure have burst. This creates a sponge-like particle that works well for this purpose. You will get the best results if you mix vermiculite with another loose material such as perlite because alone it absorbs too much water and has no additional space for air. Roots often will drown if grown in a medium of only vermiculite. A 50/50 blend of vermiculite and perlite works very well.
Vermiculite can be a little dirty to work with as well, because the small pieces can create dust more than the other media. This type of breakdown can start to clog small hoses and filters with fine debris if you are using vermiculite for a long period.
Perlite
This is a similar material to vermiculite but is less absorbent. These particles help create additional air spaces in your medium, which is why it is often mixed with vermiculite or coconut fibre for added drainage capabilities. Perlite particles are harder than vermiculite, so you will not have the same debris or clogging issue with it.
One issue with perlite (and vermiculite as well) is that they are very light and may cause you problems in an ebb-and-flow system if your pots are not held in place or otherwise secured in the flooding table. If they are sitting loose, they likely will start to float as the water level rises during a flood cycle. They may tip over, but the main problem is that the water level will not rise up through the root bundle.
Expanded Clay
This is a very popular medium for larger plants, and it goes by many different brand names like Hydroton®, Geolite and Grow Rocks. It is also sometimes just known as Leca, for lightweight expanded clay aggregate. It is also available in different diameters.
In any case, these are all man-made pellets of very porous clay. They make great support for roots and offer excellent aeration qualities, but they do not hold water on their own. If you need a medium that absorbs and holds water, this is not going to be suitable. But any system with a constant flow of water (most drip arrangements or static solution equipment) will do fine with expanded clay. In many ways, it works a lot like gravel, except that these are much larger pieces and much lighter. Small pots may even be too light, similar to the problems mentioned with perlite.
One of the great benefits of expanded clay is that the pieces are reusable. You can just wash them off between growing projects and use them again with a new set of plants. It is also a good idea to sterilize them gently with a dilute solution of bleach to make sure you are not transferring any mold spores from one growing batch to the next. Being reusable does help to offset the fact that they are more expensive to purchase in the first place. They also should be rinsed off well before their first use to remove any clay dust.
Coconut Fibre
Coconut fibre is becoming one of the more popular types of hydroponic media in use right now, partially because it is inexpensive but it holds water well without smothering roots. It is an all-natural and biodegradable material collected from the outside of coconut shells. It also goes by the name coir or coco peat. The fibre is sold in pressed blocks for easier handling. You have to break it apart before use. It works even better if you add some perlite to the mix for a little extra aeration and drainage.
As with other natural fibres, it can break down over time and constant exposure to water. This will lead to some debris in your water lines that will have to be handled.
Rock Wool
Rock wool is light and very fibrous inorganic material. It is made from heated rock spun into fibres when it is molten. It is pressed into various shapes and sizes to accommodate your hydroponics needs, with small plugs working well for seeds to larger bricks for your more mature plants. Though it is primarily made from natural sources, this inorganic material will not biodegrade.
Loose rock wool can be very dusty, and you should wear a mask if you are going to work around it for very long. Once packed into pots and wet, the dust should cease to be a problem. Working with the pressed forms of this product makes this much less of an issue.
Large-scale growers tend to lean toward rock wool as a medium because its preformed shape is easy to work with, and it has excellent absorption qualities as well. Even though it is a processed product, it is also relatively inexpensive.
Sphagnum Moss
If you prefer working with organic materials, this is another very good option. It is easy to buy at most garden stores as a natural soil additive, and it is not expensive. While moss can be used on its own, it does work better when mixed with sand or perlite to add more air spaces between the pieces. Moss alone is very absorbent and can hold too much water on its own.
Like vermiculite, sphagnum moss will come apart and break down with use and that can be a problem with some hoses and valves. An extra filter on your water system can help a great deal with this.
Oasis Cubes
These are used almost exclusively when starting new cuttings or seeds. Oasis is highly absorbent and will hold water well for tiny developing root hairs. Oasis is a specific brand that is very popular. The cubes are made from an artificial material that is a lot like florist’s foam.
They are small squares of foam with a hole for your seed or cutting. You usually buy them in large sheets and just break each cube off as you use it. Once the plants are larger and developing roots, you then transplant the entire piece of foam into a larger portion of whatever main medium you intend on using for the mature plant.