A commonly uttered nonsense has it that media-based grow beds must be at least 300mm (12 inches) deep and, I have to say, that this is one of my favourite aquaponics myths.
How did this idea come about?
Probably the person most responsible for the current interest in gravel grow beds was aquaponics pioneer Tom Speraneo.
While he wasn’t the first to use media-based grow beds (hydroponicists had been using them for decades) and he wasn’t even the first to use them in aquaponics (UVI had tried and rejected them several years earlier), he made them work for his purposes and he popularised their use.
The early promotion of aquaponics in Australia had a fundamentalist flavour about it so 300mm gravel grow beds became something of a mantra.
Much more recently, local aquaponics kit suppliers built grow bed products that were premised on the notion that they should be 300mm deep and this has further consolidated (indeed intensified) the dogma around grow bed depth. One such supplier states:
“It has been found that grow beds must be 300mm deep as grow beds less than 300mm deep do not support bacteria growth properly resulting in a lack of nutrient processing, poor plant growth and water returning to the fish tank un filtered.”
Utter nonsense!
Let me be clear…..I’m not saying that 300mm grow beds don’t work. My own experience (and that of hundreds of others) evidences that they do work.
What I am saying is that there is nothing magic about the figure of 300mm (12 inches) – they can be deeper or shallower.
Some of the more common arguments in support of 300mm+ grow beds include:
- Greater temperature stability
- More effective nitrification
- Greater plant support
It’s logical that the greater the mass of a grow bed, the more stable the temperature is likely to be.
My response to this perceived benefit, however, is that there are much more cost effective ways to achieve temperature stability.
The claim of more effective nitrification in 300mm+ grow beds ignores the fact that so long as (to ensure the ongoing health of the nitrifying bacteria) the media remains moist and at a consistent temperature between watering cycles, nitrification will occur……regardless of the media depth.
The notion that deeper grow beds offer greater plant root support has seems reasonable enough on the surface. Mature sweetcorn plants, for example, require plenty of root support if they are to resist being blown over in a stiff breeze. Upon closer examination, however, plant support is probably more a question of media type than grow bed depth.
Gravel is certainly the preferred media for growing sweetcorn….and 150mm of gravel will support sweetcorn just as effectively as 300mm. Expanded clay is much less effective for use with sweetcorn – not even 300mm is enough to keep it upright when the wind blows.
Even if there was validity in the plant support argument, operating 300mm+ grow beds simply so that you can grow sweetcorn fails the cost effectiveness test.
The 300mm+ crowd point to the numbers of 300mm grow beds that are in existence as evidence of their superiority. Given the single-mindedness and vigour with which they are promoted by the suppliers (who offer no other option), this is the silliest argument of all.
For practical purposes, I have found that properly managed grow beds can be as little as 75mm – and will work just as well as deeper ones.
Moreover, there are some benefits to be gained from using shallow grow beds and they include:
- Cleaning shallow grow beds is a much easier task than it is with 300mm+ ones. Regardless of what you are told, all media-based grow beds will require cleaning sooner of later. Some of those touted as being examples of how they don’t need cleaning are already long overdue for renovation.
- Shallower grow beds are cheaper to make than deep ones…..and they cost less to buy.
- If you are using expanded clay media, filling a 150mm grow bed will cost half as much as filling a 300mm one.
- Alternatively, you will get twice the growing surface for the same overall volume of media.
- A 150mm grow bed will weigh a bit over half as much as a 300mm one. The support framework for a shallow grow bed can be made of less robust materials because it doesn’t need to support the same weight.
To summarise…….the arguments to support the contention that grow beds must be 300mm+ are not particularly compelling…….particularly in light of the advantages of using shallower ones.
-o0o-
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Gary, I am currently looking at using gravel for the first time over clay media because it is cheaper. That way I can afford to use about 300mm deep in the grow beds. What types of gravel are good to use? Is stuff like 20mm blue metal going to attract too much heat from the sun? My grow beds are not shaded and will run on a flood and drain system.
Thanks, Old Trout
Old Trout……20mm drainage gravel is a good option. If you’re planning to go 300mm deep, consider what solids removal devices you can incorporate into the system before the grow beds. The cleaner you can keep your grow beds, the more effective they will be and the longer they will operate without cleaning (very hard work). If you don’t have a specific reason for going to 300mm, you can save yourself a lot of work and your system can be of lighter construction.
Thanks Gary, I have been contemplating incorporating solids removal devices and have been following your Queenslander thread. I am very interested in building my own devices and eagerly await any designs you offer. The challenge I face is incorporating these devices into my CHOP system. There is only so much verical space between the outlet of the fish tanks (IBC’s)and the sumps below ( bath tubs). I noticed in the Queenslander you had about 4 solid removal devices. How do I fit these type of things into such a small space using only one pump? Reducing the grow bed depth will help to some extent.
Old Trout…….The Queenslander is a Constant Height One Pump (CHOP) system. It features three solids removal devices…..a clarifier fitted with filter pads which catches both sedimentary and suspended solids, a duckweed tank (because we will be stocking one of the systems with jade perch and they like to eat it) and an expanded clay pebble grow bed.
In The Queenslander – Grow Bed Management, I describe how we operate the grow beds to additional solids removal capability. If you were to opt for one 150mm clay pebble grow bed and run all of the water for your other grow beds through it, you would achieve a similar effect. If the water from you fish tank is put through a clarifier of some sort, then through the clay pebble bed,the slow rate of flow coupled with the adhesive effect of the bio-film on the clay pebbles would remove most of the solid material.
Aloha from maui. I am doing aquaponic with telapia. Now i saw a u tube video in philipinnes where in addition they had rabitt cages over the pool so manure would go in directly and pee too.
What do you think? Do the fish eat the rabbit poo? Or it is just to add nutrition to the water?
my other question is if you check “urine fertilizer” you will realise that human urine is sterile and had all the n-p.k needed for grow of plant. See video demonstrating amazing growth compare to chemical!
Now I have been tempted to pee in my pool…should I”?
Please note that when added to a plant it must be diluted 10 times.
In a 5 gallons no more than 300 ml…my tomatoes are groing great that way in hydroponic!
Nader……the farmers of various Asian nations have been undertaking these sorts of integrations for centuries. I have heard of chickens, ducks and rabbits being used in the way you describe. In some places, they’ve even located a human privy over the ponds.
Whether it is the rabbit manure or the urine that is taken up depends on the organisms in the pond…….both contributing to the growth of bacteria, algae and zooplankton. These tiny organisms will then serve as a food source for larger creatures including fish.
nader…….urine from a healthy person may be sterile but I prefer a bit of separation between what I eat and drink and what happens to it after I’ve eaten and drunk it. I take your point about chemicals but I think we’ve got other choices than just using chemicals or peeing in the soup (vegetables).
I suggest that, if you’re already peeing in your hydroponic tomato nutrient tank, it will scarcely matter much if you decide to do the same in the pool.
nice demythification, Gary
like you write, fundamentalists propose 300 mm as the only way to go only on experience, not on argumentation.
Like you, I tend to disagree: the surface exposed to bacteria growth is what determines biofilter capacity and nitrification, not depth.
The only other function is plant support.
Only one question bugs me and might vouch for deeper or even shallower beds: light.
I have not found any conclusive info on how light influences bacterial growth
is it good, is it bad, I don’t know
can you help?
Frank
Frank……the fundamentalists often don’t even rely on experience (not their own anyway). They often just follow the dictates of vested interests.
Nitrifying bacteria are said to be light sensitive but, according to what I’ve read, the offsetting effect of bio-film (where the bacteria can shelter) suggests that the effect of light inhibition on nitrification is probably conservative.
Another consideration is that up to 70% of the bacteria in a system live in the water……which would seem to suggest that it’s more important to cover the fish tank than worry about the depth of the grow beds…..and how much light would penetrate 150mm of grow bed media anyway……particularly if the grow bed was full of plants.
I propose the use of a trickling bio-filter in conjunction with any aquaponic growing system so any light inhibition arising out the grow bed would be more than offset by bacterial colonisation in the bio-filter (not to mention the other benefits associated with using tricking bio-filters).
Like so many of the arguments that aquaponic fundamentalists offer for preserving the status quo, those in support of grow bed depth lack substance.
I’ve watched you deal with them across several forums so I imagine your own efforts to persuade them with logic and rational thinking will have confirmed for you that believing is (for some people) far more important than knowing.
Excuse this lenghthy reply as I don’t want to disturb your nice blog and feel it might not be on topic. Please feel free to move it to another space or to remove it alltogether.
Believers will not accept logic or rational thinking and will always try to deviate from the core of the matter to avoid even the slightest concession.
If that doesn’t work, they (ab)use power, as they do not accept any competition on whatever level. They want the sun to solely shine on themselves.
While to my experience honest competition and open minds works enriching and market-enlarging, certainly in a starting market like Aquaponics.
Only last week was I banned (this time definitively) from the backyard aquaponics forum for posting Bevan Suit’s petition (he is trying to approach the NOSB for possible organic certificability of Aquaponics).
I made the mistake of praising Bevan’s e-book (should have known better).
And of course Joel Malcolm (owner of the backyard aquaponics forum) has his own book which he tries to sell and you have yours which you are trying to sell (so I hope I am not shaking you in any way).
It is my conviction (or belief, grin) that the more books on any matter are published, the more the interest will grow, the more people will look for extra input, the more books will be sold, especially if the merits (and lackings) of one author are accepted, appreciated, praised or contested where needed and of course countered by another author in a renewed version of his own book.
Accepting the skills of another, admitting that you can still learn and are willing and eager to progress to the benefit of your readers, admitting that your own book may have some flaws or lackings, leads to credibility and is much more productive and impressive than trying to denigrate your opposition.
Old Gaius Julius Caesar (Roman Emperor, +/- 100 B.C.) already knew this: praising your opposition enhances your victory over them: he said: The Belgians are the fiercest of the Gauls (I am a Belgian, grin), but he won the battle in the end and by praising his opponents he made that victory bigger than it probably was (grin again, he had to ask for extra financing to finalise his campaign and had to justify this).
The ensuing polite discussion on the backyard aquaponics forum with a moderator because I felt her reaction on the thread denigrating and useless for the thread, (not on line but in private mails, I have learned my lesson) still led to the final ban.
Within minutes of course I have (much to my dislike, as I prefer a straightforward, open faced approach) adopted another Internet Identity and have already been lurking in on the thread, only to find that it is as usual spoiled by the usual crowd…
I have wasted much valuable time in the past and now again on this forum. Will not do it again.
I believe we share a comparable approach to the spreading of knowledge:
you state your much funded views and defend them, but you invite critic contributions and are reading them with an open mind.
So keep up the good work, friend (if I may call you so).
Frank
Frank…….your banning from BYAP is something we have in common…..but don’t be too concerned because, by banning people like you and I, they are performing an important service on behalf of aquaponics.
By banning us, they force us to adopt different vehicles for expressing our opinions and ideas…….and that means more forums, more books, more products, more ideas…….more of everything……and that’s good for aquaponics.
As for books……you’re right about the more books there are, the more books will be sold. I haven’t read Bevan’s book but it was clear from posts that I read on BYAP that its mere existence was offensive to some people over there.
My own book is in its second edition (and selling better than ever) and I’m working on the 3rd Edition…….that’s simply a reflection of the fact that aquaponics is still relatively new and we are all still learning (or should be).
Given some of the other things that I get called by my detractors, friend sounds pretty good to me.