Aquaponics Grow Bed Depth
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 aquaponics grow beds excerpted with permission from Murray Hallam’s Aquaponics Secrets video:
image and copy credit: 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.
I can’t resist putting in a plug for our AquaBundance Aquaponics Grow Bed, which has been designed
with all these principals in mind. It is about 12” deep, with an inside lip at the base of the Dry Zone, and features 7.5 cubic feet of growing area. It is made of ¼” thick food-grade PE plastic and has been designed with enough strength so it will never bow out on the sides. It includes a gravel guard that has space for either flood and drain fittings or a siphon (purchase separately). It is very attractive and comes in 4 color choices. Check it out here and let me know what you think!
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Posted: August 2nd, 2010 under Blog, Growing, The Aquaponic Source.
Tags: aquaponic, aquaponic grow beds, aquaponics, aquaponics grow bed, grow bed, grow table, joel malcom, learn, murray hallam, travis hughey
Comments
Comment from Bob Segraves
Time August 2, 2010 at 5:52 pm
it is interesting that you, Travis, Murray, and Joel all use similar technology in the hydroponic portion of your systems. The fact that you do not use a solids filter is interesting. Other leading growers pump the water from the fish tank to a solids filter/clarifier in conjunction with a nitrification component prior to using it on the grow beds, whether they choose to use media beds for raft method. What are the benefits/shortcoming of using or not using the clarifier?
Comment from slywoman
Time August 2, 2010 at 7:31 pm
Hi Bob. Great question. This is actually one of the key differences between a media based and a raft based (or Deep Water Culture) system. Raft systems are what many of the commercial growers are using, and is the technology developed by the University of the Virgin Islands. Because the plant roots are suspended directly into the water you need to remove the solids so they don’t coat the roots and prevent oxygen from reaching them. In my post from last week I talked about why raft systems are great for commercial operations, but why they have real limitations for home gardeners so I won’t repeat it here. Suffice to say that time spent cleaning solids filters, the disruption of the closed eco-system by the removal of solids, and the lower nutrient levels of a system with the solids removed are among those limitations. Travis, Murray, Joel, and I are all (primarily) media based gardeners.
Comment from garden design sydney
Time August 4, 2010 at 12:27 am
I like your technology .you have tell it wonderful way .it is great.
Comment from Bob Segraves
Time August 7, 2010 at 5:24 pm
Okay, as a prospecive media based grower I suspect at some point I would want my grow bes to have a robust zone 3. How do the worms get there? Do I introduce the worms at some point?
Comment from slywoman
Time August 8, 2010 at 7:26 am
Hi Bob! Yes, you introduce the worms to the system. Although I have heard some say “if you build it they will come”, and I’m sure they are right, that strategy will take so long and the benefits of having worms in your system are so great that I recommend just going ahead and purchase them. I haven’t dialed into any real ratios about lb of worms / sq ft of grow bed but my experience says that somewhere in the 1 lb to every 20ish square ft of grow bed would work well. We sell them on our website shipped to you by a very reputable worm farmer.
Comment from lee
Time August 10, 2010 at 5:26 am
You will find that your worm colony when it gets going in your system will only grow as big as your system allows.
Comment from Paul
Time October 5, 2010 at 2:52 pm
You Guys are awesome. I am sold. I am building a web page for this stuff linked off my page. I cant wait to start setting up a system. I have my hands full for a bit but I am learning tons. I will set up a small system in the garage and need recommendations on grow lights. I use a indoor wood boiler that is located in the Garage what is a good temp in winter for veggies and how much light should I give (hour wise).
Comment from slywoman
Time October 5, 2010 at 2:59 pm
Thanks Paul. The answer to all three of your questions is “it depends on what you are growing”. Think about the outdoor conditions where the plants you want to grow are at their peak. For example, if you are growing tomatoes, which peaks in the mid – end of summer you are going to want warmer temps (although they like a cooler night), long days (about 16 hours of lights) and will need a light that can penetrate through a deep canopy (so probably more of an HID style light). Lettuce, on the other hand, does best in the spring and fall when the days are shorter and cooler, and they have a short canopy so a T5 compact florescent would work. I’ll write a post on indoor lighting soon to explain further – it is getting to be that time of year!
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Time November 2, 2010 at 2:17 am
[...] endorsement of some of the grow bed depth “guidelines” that attend the Speraneo model prompted me to take a fairly narrow line on [...]
Comment from Bill S.
Time December 8, 2010 at 8:35 pm
Excellent info. I’m new to aquaponics and have yet to build my first system, although getting excited as I design and collect the basic materials. I’ve read several blogs and I believe a flood and drain system is best for my project. I have a question however, regarding which drain system to use; a bell siphon, a loop siphon, or a weep hole system. Because of cost and availability, I’m designing a system with two rows of six smaller beds (16.5″ x 24″ x 12″ deep. I’m also using a single 720 GPH pump. Not sure if a timer with weep hole drains is any more reliable than a continuous flow with a bell siphon or loop siphon. Any suggestions?
Comment from slywoman
Time December 9, 2010 at 7:35 am
Hi Bill. First, let me say that plumbing is not my forte, and I’m not sure what “weep hole drains” are except for what I can surmise based on the name. Most everyone I know in aquaponics who has gone away from using a timer and into auto-siphons is using a bell siphon. Is that because they are more widely discussed, or because they are the most reliable? …I honestly don’t know.
Comment from James Smith
Time August 2, 2010 at 4:20 pm
Great article. Thank you.