How deep should your aquaponics grow bed be? Grow bed depth in a media based aquaponics system is a subject not without a little controversy. The majority of media-based aquaponic growers will say that you should have about 12″ (300 mm) of media, with the top 1 – 2″ being left dry to reduce algae and fungal growth. There are those, however, who claim to be growing successfully in substantially less than that.
Deeper beds are more expensive because of the cost of the extra media, the cost of supporting the weight of the extra media and the need for a larger fish tank to fertigate all that media. Plus, if you stick to a 4 – 6″ depth you can use standard hydroponic flood and drain trays which are widely available.
Here is why you don’t want to do that.
If you pay close attention you will notice that everyone who touts shallow grow beds expresses one or more of the following limitations
- Limitations on the types of plants you can grow in your aquaponics system – shallow grow beds work great for shallow rooted and/or short lived plants such as lettuces and greens, but won’t work for longer lived, deeply rooted plants such as indeterminate tomatoes, cucumbers, corn, melons, etc. This is both because shallow beds don’t provide the base of nutrients and bacteria required for the relatively long life of these plants nor do they provide the space for large root zones. Travis Hughey, Murray Hallam, Joel Malcom (and I) all talk about the almost limitless variety of what you can grow in media based aquaponics. This is because we are all using beds about 12″ deep.
- ‘Dead zones’ and the need to clean out your beds – As the illustration below shows, in deep aquaponics grow beds a layered eco-system establishes that prevents the development of ‘dead zones’ (anaerobic areas) in your grow beds. Because of the thriving environment flush with beneficial bacteria and worms and plenty of space for the roots to grow, you never need to clean out your grow bed – they, along with the plants, do the cleaning for you. When there is enough room for a robust eco-system to establish itself, all the elements of that system thrive and become self-sustaining. That happens at about 12″ of media depth.
Here is an excellent explanation of what happens in aquaponic grow beds, excerpted with permission from Murray Hallam
Surface or dry zone (Zone 1) – The first 2″ (50mm) is the light penetration and dry zone. Evaporation from the bed is minimized by the existence of a dry zone. This dry zone also protects the plant base against collar rot. Additionally, by ensuring that this zone is kept dry, algae is prevented from forming on the surface of the grow bed media and moisture related plant diseases such as powdery mildew are minimized.
Root zone (Zone 2) – Most root growth and plant activity will occur in the next zone of approximately 6″ – 8″ (150 – 200mm). In this zone, during the drain part of the flood and drain cycle, the water drains away completely, allowing for excellent and very efficient delivery of oxygen rich air to the roots, beneficial bacteria, soil microbes, and the resident earth/composting worms.
During the flood part of the cycle, the incoming water distributes moisture, nutrients and incoming solid fish waste particles throughout the growing zone. The worm population does most of its very important work in this zone, breaking down and reducing solid matter and thereby releasing nutrients and minerals to the system. ‘Worm Tea’, as it is commonly known, will be evenly mixed and distributed during each flood and drain cycle. ‘Worm Tea’ and the fish are entirely compatible.
Solid collection and Mineralization Zone (Zone 3) – This is the bottom 2″ (50 mm) of the grow bed. In this zone fish waste solids and worm castings are finally collected.
The solid material has been reduced by up to 60% by volume, by the action of the resident garden/composting worms, and microbial action. During each flood and drain cycle, what is left of the solids percolates down into this zone.Further and final mineralization occurs in this area via bacterial and earth worm activity. Due to the excellent action of the flood and drain cycle, this bottom area is kept ‘fresh’ and vital by the excellent delivery of oxygen rich water during the flood cycle.