One of the most interesting questions by those who are learning about aquaponics relates to how we can grow food without using soil. Aquaponics takes advantage of nutrient rich water, where plants are rooted in an inert media, and can readily take up all the nutrients they need from the water. Since the plants don’t have to spend lots of energy pushing through heavy soils searching for essential nutrients, they can grow much more efficiently (i.e. faster) in aquaponics. But what exactly would they be grown in again?
There are two main schools of thought for aquaponics growing media. The first is basically a gravel bed and the second uses a floating raft with holes where plants can send their roots into the water below. Each method has it’s advantages so we’ll talk about both.
A media bed uses gravel or similar material which provides enough structure for it to set dwon roots to have a stable plant but also has good spaces between the rocks for air and water movement. For this, gravels which are 1/2″ – 3/4″ work great, not too small like sand where there is little space for air, but not so big that the seeds you’ll plant fall deep between the gravel. Many people use a formed type of expanded clay called hydroton or hygromite which is lightweight, easy to work with, and provides a nearly ideal balance between stability for the plant and air space. Proper media beds are designed to flood and drain so the root systems don’t get too waterlogged and starved of oxygen, leading to root diseases. This is done by having a water pump on a timer or in a more sophisticated way using a device called a ‘bell siphon’, which is a siphon which automatically starts and breaks, letting you flood and drain your media bed on a regular basis, all without any moving parts!
A raft system typically uses thick styrofoam sheets floating on a body of water. Holes are drilled through the styrofoam with plants placed in small net pots in the holes so their roots can grow down and into the water column, where they are bathed in nutrient rich water. A small amount of media is placed in each net pot to allow the plant to set roots but it is the roots descending into the water which gives this method it’s big advantage. Raft systems must have two things to keep the plants from drowning though; they must be high enough above the water that the top is fairly dry and the water must have high oxygen levels so the roots can breathe. Rafts are easy to work with, can be drilled out to whatever spacing your plants need, but don’t provide a lot of support for taller or heavy plants. For these plants which need lots of support, media beds are generally best.
Which type is best? Well that is up to you and what you want to grow. Both methods have a large amount of surface area for nutrient processing bacteria to grow, which is crucial to a stable aquaponics ecosystem. Well designed media beds with a regular flood and drain cycle can host earthworms to help break down solid fish waste. Small invertebrates can also do this same function in raft systems as well but are more difficult to obtain than the ubiquitous earthworm. As we learn more about the benefits of both methods, many feel that a combination of the two offers some of the best of both worlds and lets you easily grow a variety of crops and have a healthy stable aquaponics system.
Santa Barbara Aquaponics 2021 