Plant Roots, And Grow Bed Construction.

When considering aquaponic grow bed construction, there are basically two ways to go:

  • Aquaponic shallow raceways filled with water from the fish tanks, and no growing media. These use floating rafts to support the plants.  This deep water culture floating raft method is what is commonly used in large commercial aquaponics systems. Your plant roots here are suspended in fish waste water alone.
  • Aquaponic shallow raceways filled with growing media. These are commonly used in backyard and hobby systems, where clogging of the pipes can be fixed on a small scale with few problems. The growing media are inert, chemically stable, and pH neutral. They are  not like soil, in that growing media are sterile when put in the raceways. There are no pests in the growing media to snack on your plant roots. They contain no humus or bioactive constituents, and can be one of the following materials:

Growing Media.

  1. Rockwool. One meter long rockwool slabs are available. These usually come wrapped in a white plastic cover. Before introducing the 3 or 4 inch rockwool propagation blocks into them, the base of the white covering must be slit to allow water to flow through the block, and a hole cut in the top for the base of the block to sit on.  To use these, you simply site them in the long hydroponic gullies sold for this purpose, and cover the tops of the blocks in PVC to stop algae build-up. Make sure the fish water you are using for this is properly filtered of all fish poo particles, or these rockwool slabs will eventually clog up, and need extensive high pressure rinsing to get clean. Using rockwool for grow beds is very expensive, and like other growing media, they clog up very easily.
  2. Perlite/Vermiculite.  Perlite is a soft sterile white inert growing medium.  It is very light and has high water retention properties.  It is chemically stable, drains well, absorbs water, but does not allow great permeability of minerals, which is an advantage. This is the best medium for filling grow beds intended to grow root vegetables such as potatoes, carrots, and celery. Perlite can also be sterilised and re-used after harvest.  Beware, however, of the fine dust that comes in bags of perlite.  If you breathe it in, it will irritate your throat and lungs.  A proper industrial face filter mask is in order here. To be on the safe side, run a hose through the bag and rinse out the contents before you start digging around in the growing media.  This will wash out most of the dust, and settle the rest. Vermiculite has first-class properties as a growing medium, with high water absorbtion capacity and a reasonably high cation exchange capacity (CEC). This simply means it has the ability to hold nutrients and release at a later stage.  When mixed with perlite it provides a very good all-round growing medium.
  3. Expanded clay balls, (usually Hydroton or Hydroleca brands).  These balls are produced by baking clay in a revolving kiln, producing balls of between 4 and 12 millimetres in size. The shape of the balls allows for good drainage, and space for air to reach the plant roots. The pitted surface of the balls provides an ideal home for nitrifying bacteria, which transform dissolved ammonia from the fish into plant food.  Like perlite, they can be sterilised and used repeatedly.  As with perlite, ensure that the new clay balls are washed thoroughly.  This will prevent clay dust from getting in your eyes and throat and causing health problems.  I usually get the hose and slit the top and bottom of the bag and wash the whole lot through for ten minutes or so until the water emerging at the bottom runs clear. As a growing medium the balls are usually used to fill a grow bed or raceway and then rockwool or other medium seed plugs with seedlings in, are pushed into them.
  4. Coir (coconut fiber).  This is very moisture retentive, but contains no available nutrients. Will Allen of Growing Power uses this extensively in his famous aquaponic system, where he grows a million pounds in weight of food a year on only three acres in downtown Milwaukee.  However, it is a biodegradable medium and rots down.  As such, it can cause clogging of pipes and pumps as it becomes mobile in the water stream.  Extreme vigilance is advised when using this as a growing medium, and extra filter sieves should be put at the exits of all raceways, grow beds, etc., and regularly checked and cleaned.
  5. Sand.  This was one of the first growing media tried for aquaponic grow beds.  However, it does tend to get everywhere, and get into pumps and filters. Very heavy, which is a disadvantage if you want to put your grow beds on raised tables or shelving.
  6. Pumice.  Similar to volcanic scoria, below.  Watch out for pumice dust!  A breathing mask is in order here when unpacking it from the bags. Use the rinsing methods as for perlite and expanded clay balls. A very lightweight aquaponic growing medium, make sure your grow beds are not exposed to wind, this stuff can easily blow away.
  7. Volcanic scoria (volcanic gravel). Like expanded clay balls, volcanic scoria is lightweight, and full of little air bubbles. It is an aerolite as such. As a growing medium, it has similar properties to expanded clay balls, in that its rough surfaces provide an ideal home for nitrifying bacteria, and it drains well, allowing plenty of spaces for air to circulate past the plant roots. In addition, it is a very efficient insulator, and can protect the plant roots of your plants from extremes of temperature, provided you have calculated adequate depth of the grow bed to allow for this possibility. A three foot deep grow bed (1 metre) can help protect the plant roots from extremes of heat and cold. However, these should really not occur in the vicinity of your aquaponic system, since what is above ground will still suffer. A very inexpensive option for filling your grow beds.  But if you do not want extremes of temperature to affect your crops, get a greenhouse!

Grow Bed Construction:

The main thing to keep in mind when constructing grow beds is that they should be at least 12 inches deep, to allow for sufficient biological activity in the growing media. Four feet wide is good, since you can reach everything across four feet for maintenance, planting and harvesting.  Grow beds should also be in proportion in volume to the volume of your fish tanks.  So if you have 250 gallons of fish tanks, you should have at least 250 gallons of grow beds.  In fact, you can go as high as 500 gallons of grow beds to your 250 gallons of fish tanks, as long as you are stocking your fish tanks fairly intensively, and have densely planted plants at some stage of growth in all your grow beds for that volume.  This is becaus you need the plant roots to take up the nitrates from the water and clean it for the fish.

Remember that in a backyard aquaponics system, the grow beds are the biofilter for the fish tanks.  Without adequate biofiltration of fish solids and the toxins they contain, your fish will sicken and die quite rapidly. Maintenance of your growing media involves daily inspections to make sure that they are still permeable to water, and that the pipework is still clear and not leaking anywhere.  Also check all the standpipes in the grow beds and tanks are properly seated and clear of obstructions and gunge.  Water should only come to a couple of inches below the actual surface of the growing media.  This is to discourage algae from growing on the surface of your grow beds.  An aquaponic growbed should not have any algae visible on the surface. Algae can soak up oxygen needed by plant roots and clog up the media for you royally.

You also have to make sure that your water flow is going around at around 3000 liters an hour. This is to make sure that for the above volumes, the plant roots are getting adequate air and nutrients at all times.  Keep checking your pump to make sure that it is not clogged with growing media dust and gunge.  Basically, you are reproducing the conditions in a riverine ecosystem, where plant roots penetrate gravel full of flowing water at the river’s edge.

When constructing grow beds, therefore, make sure you pipe diameters are at least 3 inches, to allow for this continuous flow.  DO NOT USE THE SAME PIPE DIAMETERS AS FOR HYDROPONICSAquaponics is not quite hydroponics, the oxygen and flow requirements are much greater, and wider pipes have less tendency to clog with escaped gunge from the fish, should this occur.

Thee are all sorts of large 12 inch deep plastic tanks on the market that make suitable containers for grow beds.  One of my old tutors used plastic dog baths as his basic tanks!  You can also make frameworks out of cedar or other non-decomposing materials and line them with LDPE pond liner to form the raceways that you then fill with expanded clay balls, vermiculite, volcanic scoria, etc.  Media based aquaponics systems use the media to support the plant roots in the water and keep the green parts of plants in the sunlight.

Media based aquaponics is really only for the smaller scale systems, however, because all the evidence suggests that media based aquaponic systems are much more susceptible to the scourge of clogging.  The addition of composting worms to your growing media, when the aquaponic systems are up and running, will help to discourage this, however, since composting worms love to eat the gunge and plant detritus that so loves to get into your pipework and stop the water circulating. Plant roots rot in stagnant water!

As mentioned above, for large seriously commercial systems, the way to go is not with media based aquaponics at all, but with simple raceways for the flowing water, at least two feet deep.  Oxygen is supplied through flexible vinyl pipes equipped with air lines and airstones every three feet or so along the raceway bottoms. These water-filled raceways support floating rafts which are made of closed cell polystyrene which has been painted with a white non-toxic roof paint such as Cool Cote.  Holes at the appropriate plant spacing distances (5 by 6 inches for lettuce, for instance) are drilled in these rafts, and small plastic net pots inserted.  The plant plugs containing the seedlings are inserted into these net pots when ready and left on the floating raft to mature until harvest time some 29 days later, for each raft planted.

These floating raft systems are not strictly speaking grow beds, but aquaponic systems raceways.  Floating raft systems, filled only with water, are much easier to maintain and clean, do not clog up with fish waste, and have greater biofilter properties than the growing media in the long term. As far as construction goes, the retaining walls of these raceways can be made of a variety of materials, including fence posts and heavy duty welded fence wire mesh, concrete berms, and planks supported by fence posts.  The interior is lined with LDPE pond liner which is cemented down with aquarium sealant and allowed to overlap the sides, where it is secured with glue.  The floating rafts are cut to the exact 4 foot interior width of the raceways, and should cover the entire surface, in order to limit water evaporation and take advantage of the growing space to the maximum.

Aquaponic systems also have, of course, the fish tank and piping element, not to mention pumps and air blowers. These all connect up to the grow beds and/or floating raft systems of your aquaponic systems. To get into the finer detail of grow bed construction and maintenance, for which there is no space here, I recommend a couple of reasonably priced and deeply fascinating expert books on aquaponics systems construction, see below:

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