Plant Growing Mediums
Because the water is constantly being reclaimed into your fish tank, most aquaponics arrangements avoid using soil for the plants because it just makes the water that much harder to filter and keep clean for the fish.
You have a few options for growing medium. Smaller leafy plants can be grown with no medium at all, with their roots simply submerged in water through the day (lettuce is great for this choice), but larger plants will require more root support, particularly any where the roots are going to be harvested.
This is probably the most popular option because it is very well-suited for plant growth, is easy to use and relatively easy to find. It can be a little more costly compared to other mediums so you'll have to work out a budget yourself to see if it will suit. They are very light and porous pellets made from clay, which makes them particularly good at allowing air though it but the pellets don't hold water. Any system that uses expanded clay will have to ensure that the water cycles fairly frequently because the roots will dry out faster. A deep water system where the roots are never outside of water would work great with clay as well.
You can also go a more simple route and get standard garden gravel. It's very cheap but also very heavy. Large grow beds will need added support because they will weigh a ton. Stay away from common limestone though. It slowly dissolves in water and will wreak havoc on your pH levels and add a lot of calcium. Like expanded clay, you can get lots of lovely air bubbles with a gravel grow bed and the stones do not absorb any water.
Now we get to another option that will absorb water, which makes rockwool better for an ebb/flow type of system where you want less frequent water cycles. The water can be stopped and the medium stays wet for a long time. It's an artificial product made from fine fibers created when certain rock is heated into liquid. You can buy it in pressed packs for easy use. It holds water well, and is a light material for your growing bed.
Lastly, we have another very absorbent option that is a little more organic than the others. It's also very easy to find at any garden store. It will hold water when the flow is stopped, and it can actually hold too much so that aeration is a problem. Don't forget that you need air along with water to keep your plant roots healthy because you can actually drown a plant. You can add some extra air spaces with a little gravel or perlite mixed in.
There are several other options for aquaponics media, but these ones offer the best range of qualities and cost for the beginner to experiment with. How your growing media works with your particular plants and how to built a complete growing table is coming up in the next section.
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