Backyard Aquaponics
A place to discuss aquaponics

I've read as much as I could find about different grow bed depths for a flood and drain system, but there is conflicting information out there. I haven't found any scientific studies on grow bed depth or any solid, proven reasons for 12" being the best.

What I've heard:

12" depth - Creates 3 zones. The dry zone at the top, the root growth zone, and the decomposition/sedimentation zone at the bottom. Cleaning the grow bed is unnecessary.

6" depth - Cheap, easy, wash the beds out every once in a while.

Has anyone experimented with this? What are your recommendations?

My opinion is you may have trouble growing anything tall / heavy in a 6" deep bed like tomatoes or corn (well, maybe with scoria but not hydroton). You still need the dry zone at the top, only 4" wet media is limited biofilter, temperature fluctuations are bigger which many plants might not like. But if you are only growing greens and strawberries then 6" deep is probably adequate, unless they are in direct sun and the roots get hot. Then the plants might not grow as well.

I recommend at least 10" deep, but anything more than 12 is a waste. I do have four half-barrel GBs that use scoria media. They are only 10" deep at most and taper to nothing at the sides. They work very well.

From what I've seen so far the recommended 300 mm GB works well. When my first broccoli was pulled from the GB it was interesting to see how the root structure spreads out like a ball in media compared to the skinnier deeper structure of a dirt plant. I'm guessing it's because its easier for the roots to go between the voids in the media than the dirt and as the nutrients and water are evenly distributed throughout the height of the media there is no need for them to go real deep.

.
There are studies regarding grow bed depth. It's been a very long time since I read it, but I'm fairly sure it was Dr Wilson Leonard that came up with the recommended 300mm gravel depth.

A GB with a 300mm gravel depth will still require cleaning out occasionally, but if set up correctly some growers have a achieved 5yrs, and even 7yrs before the beds have required cleaning.

For the average grower, growing a variety of plants, the GB gravel depth has very little to do with the type of plants you will be growing... It is important because it enables a certain amount of fish, and therefore nutrients, per square metre of GB space...

ie: Imagine 2x 1sqm grow beds, one of them with a 150mm (6") gravel depth depth and the other with a 300mm (12") gravel depth, both flooded to 50mm (2") from the surface of the gravel. The shallower GB will be providing 100L of wet gravel bio-filtration, the deeper bed 250L...

Trout for example, require 25L+ of wet gravel bio-filtration per fish to get them safely through to plate size, so the shallower 150mm bed would have 4x Trout providing nutrients for it's 1sqm of plants. The 300mm GB would safely sustain 10x Trout, feeding nutrients to it's 1sqm of plants.

The 300mm gravel depth was found to provide the ideal amount of fish, and therefore nutrients, per square metre for the type of plants commonly grown by most backyard type aquaponic growers.

To take it one step further, if customers are thinking of growing tomatoes, I recommend they run GB gravel depths of 400mm and increase the number of fish accordingly, therefore providing the extra nutrients required for tomatoes... In actual fact, I first recommend people don't grow tomatoes in aquaponics at all, and to use wicking beds instead, but if they want to run them in AP I suggest the 400mm gravel depth.

I found Dr. Lennard's paper and a website (http://www.aquaponic.com.au/) based on his work. The site has all sorts of great information I've been looking for. It will take me a while to read through it all. Thanks for the info!