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KEY CHARACTERISTICS

There are a few key characteristics to consider when choosing a growing medium for your aquaponics system.  These are:

  • Chemical Neutrality: It is extremely important that your growing medium be chemically inert.  Otherwise, your water will slowly become either too acidic or too basic to the point where your fish can no longer handle the extreme pH.  You could frequently monitor the pH of your water, and buy droplets to adjust it accordingly, but if your system is going to be of any practical size, the cost of this maintenance will be exceedingly impractical.
  • Price/Availability: There are some very fancy aquaponics growing mediums out there, but, frankly, they are not worth the price.  If you can’t find your growing medium within a few miles of home, or for a comparable price on Amazon, it’s not worth purchasing.
  • Porosity: You want your growing medium to retain some moisture so that the roots of your plants don’t dry out, but neither do you want your plants to drown.  Moreover, aeration within your soil medium can speed up plant growth.
  • Size: If your growing  medium particles are too large, there will be less surface area upon which crucial biological interactions can take place.  Too small, however, and your siphons are likely to clog or break because of restricted water flow.
  • Durability: All soil particulates inevitably decay into smaller soil particulates,  especially when constantly submerged in water.  However, you want to avoid changing your growing medium for as long as you can, for each time you do you have to wait for your nutrient and biological cycles to reassert themselves, restricting optimal plant growth.

GRAVEL

Gravel is the most commonly used growing medium in aquaponics.

Gravel is the most commonly used growing medium among beginners.

It’s a bit heavy and usually needs to be rinsed quite thoroughly before being put into a growbed, but it costs next to nothing, is readily available, provides plenty of room for water and root growth, will not throw off your water’s pH or nutrient composition, and is relatively easy to work with.

People often worry that their plants will not grow in gravel.  This worry is reasonable.  It is strange to imagine a soft carrot growing through a mesh of hard gravel.  Believe it or not, however, but even your softest crops will grow uninhibited in gravel and slowly displace whatever is in their way.

If I may offer a few quick words of warning:

  • Smaller gravel particles are much easier to work with than larger ones, as they are lighter and easier to manipulate when planting or removing crops.
  • CERTAIN GRAVELS WILL THROW OFF THE PH BALANCE IN YOUR FISH TANK.  Make sure that the gravel you are using does NOT contain limestone!  Limestone will erode over time and release calcium carbonate ions, which will raise the pH of your water drastically.

CLAY PELLETS

Hydroton clay pellets are probably the easiest to work with of all aquaponic growing mediums.Clay pellets–also known as hydroton or hydrogen pellets–are widely used in aquaponic growbeds for a number of reasons.

In addition to being chemically neutral, hydroton pellets are practically ideal in size, ranging from about 5mm to 20mm.  Water flows quickly through a bed of hydroton pellets, and yet it is still easy to “screen” these pellets using water-permeable weed-blocker, mesh, or chicken wire so that your siphons do not clog.  Hydroton pellets are, in fact, very hard to touch, and so do not decay like one might expect.  They will erode more quickly than gravel, but will still last for many years.  (Even eroded clay pellets can be added to other soil combinations.) Being clay aggregates, they retain a bit of moisture even when the beds are dry, mimicking healthy soil conditions.  Finally, these pellets are freakishly lightweight, making them extremely easy to work with.

Hydroton clay pellets are a bit more expensive than gravel–they typically cost about a dollar per liter.  However, in addition to being extremely easy to work with, they have all the characteristics of an ideal aquaponics growing medium.


COCONUT HUSK

Coconut husk has an incredible capacity for water-retention.Coconut husk is a relatively new growing medium among gardeners, but is gaining rapid popularity and is already widely used.  There are a few reasons for this.

 Despite its seemingly moderate price, it’s actually quite cheap.  This is because when you purchase a block of coconut husk, it is dry and compacted.  Once it is introduced to water, it expands to several times its volume, so it seems that you are getting more for your money.  This ties into perhaps the greatest attraction of coconut husk: it has an enormous water-retaining capacity.  Coconut husk may absorb water an dissolved minerals better than any other medium out there.  However, large aggregates of husk can quickly turn into small pieces and clog your siphons.  If you are going to use coconut husk as a growth medium, you certainly need to install some sort of water-permeable barrier between the husk and your drains.  

Coconut husk might best be used as a component of a hybrid growing medium.  By planting crops in pots filled with coconut husk, and then embedding these pots in gravel-filled growbeds, your growing medium will gain the water-retaining capacity of coconut husk, but without the risk of it clogging your siphons.


ROCKWOOL

Rockwool, or mineral wool, is a chemically-neutral synthetic fiber.  Although it will not erode like other soil mediums, it is better used in limited circumstances.

As a soft, absorptive material which can be very tightly compressed, rockwool is a great material for young or delicate plants, in that it both retains moisture and prevents such plants from slipping through porous gravel.  That being said, these characteristics also preclude it from being a good choice for a primary growing medium.  It would take enormous amounts of rockwool to fill a growbed, and the roots of your plants would not get a strong hold within soft rockwool like they would in a more natural medium.  Moreover, being synthetic, a full bed of rockwool would quickly become rotten, to so speak, and need to be replaced.  Even large pieces of rockwool can sucked up siphons, too.

For young or delicate plants, however, using pots of rockwool within a larger growing medium can be extremely helpful.  Especially with seedlings, a protective rockwool cage can minimize the risk from heavy water flow or gaps in gravel and clay.  Starting seedlings in rockwool cups and then transplanting them once they get roots is a popular and effective technique.


SOIL

Soil is better used as a supplement than as a primary growing medium in aquaponics.I am listing “soil” as an option for a growing medium more to assure you that I didn’t leave it out than to actually recommend it.  If you can pull off using soil, that’s great–good soil contains plentiful salts and minerals that lead to more nutritional plants.  However, working with soil in an aquaponic system is far from easy.  Planting crops in and harvesting crops from soil requires more effort, and filling growbeds with soil is a dirtier and more intensive process.  Perhaps the greatest deterrent from using soil as a growing medium is the fact that it is extremely difficult, time-consuming, and expensive to prevent soil from clogging your siphons.  To keep the tiny particles out of your siphon, you would need to buy some sort of weed blocker that is not only water-permeable and has extremely tight-knit mesh without any holes in it, but is made of some material that won’t decompose under constant moisture and nutrients.  Such meshes are not only difficult to find, but are very expensive and difficult to install.  Furthermore, if some soil does make its way through your weed-blocker into your siphon, you will not only have to temporarily remove and wash out your siphon, but you will have to empty your entire growbed to install a new layer of weed-blocker.  If your weed-blocker decomposes, and your soil finds its way into your fish tank, the increased turbidity may discourage your fish from eating.

Soil, however, does sometimes have a role in aquaponics.  Good soil that is high in salts and essential minerals–purchased commercially or produced from compost–can be sprinkled in thin layers around certain plants, or used in pots, as with coconut husk.  As long as the amount of soil mixed with your medium is kept small, the salts, minerals, and other nutrients within it can be readily absorbed by your plants before clogging up your siphons or muddying up the water in your fish tank.

Another note–don’t be alarmed if you see soil magically appearing in your growbeds.  As microorganisms break down the fish waste dissolved in the water, soil formation will occur naturally.  This soil is nutrient-rich, and, if you decide to replace your growing medium at some point, you can use this soil somewhere else in your garden!

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Essential MacronutrientsThe three most essential macronutrients for any aquaponics system are nitrogen, phosphorous, and potassium.

A shortage of nitrogen should not be a problem.  After all, nitrogen is the primary component of fish waste.

If you have too much nitrogen in your system, it will be a problem for the fish rather than the plants, in which case you will need to reduce your fish stock or increase your plant coverage (But don’t use legumes!  They will INCREASE the amount of nitrogen in your system!)

Unlike nitrogen, phosphorus only exists in a solid form and cannot be absorbed from the air, meaning that it is more likely to be in short supply.  You can add phosphorus to your system by sprinkling small amounts of salty soils into your growbeds.  Once your system is well-established, however, fish waste should provide sufficient phosphorus for your system.

Potassium is the the macronutrient most often deficient in aquaponics systems.  If you need to  add some potassium to your system, try sprinkling bits of banana peel in your growbeds.  Not only will this replenish your system with potassium, but banana peels act as a natural pesticide, too! 


Essential Micronutrients

The three most essential macronutrients for any aquaponics system are nitrogen, phosphorous, and potassium.

A shortage of nitrogen should not be a problem.  After all, nitrogen is the primary component of fish waste.

If you have too much nitrogen in your system, it will be a problem for the fish rather than the plants, in which case you will need to reduce your fish stock or increase your plant coverage (But don’t use legumes!  They will INCREASE the amount of nitrogen in your system!)

Unlike nitrogen, phosphorus only exists in a solid form and cannot be absorbed from the air, meaning that it is more likely to be in short supply.  You can add phosphorus to your system by sprinkling small amounts of salty soils into your growbeds.  Once your system is well-established, however, fish waste should provide sufficient phosphorus for your system.

Potassium is the the macronutrient most often deficient in aquaponics systems.  If you need to  add some potassium to your system, try sprinkling bits of banana peel in your growbeds.  Not only will this replenish your system with potassium, but banana peels act as a natural pesticide, too!