One way to make the most of the space in a polytunnel, whether is is domestic or commercial, is to set up an aquaponics system.
What is Aquaponics?
Aquaponics is a system for growing food that combines traditional fish-keeping (aquaculture) with the practice of growing food in water (hydroponics). There are a number of different systems that all come under the umbrella of aquaponics and aquaponics systems can vary greatly in scope and scale.
All aquaponics systems, however, will have certain characteristics in common, including the basic nutrient cycle which allowing the two food production systems to work holistically together. Speaking simply, aquaponics systems are low-waste systems.
The excretions of the fish or other aquatic creatures being kept in a traditional aquaculture system will gradually build up over time and have to be removed to prevent the water environment of tanks from becoming toxic to their inhabitants. In aquaponics, the excrement laden water is fed to a hydroponic system. Bacteria will break down the waste in the water into nitrates, which can provide nutrition to plants whose roots dangle into the water from the aquaculture tanks. This water, cleansed by the plants roots, can then be fed back into the tanks that it came from.
How Does an Aquaponics System Work?
In an aquaponic system, a beneficial relationship is created, as in natural systems, between fish and plant life. An aquaponic system simply takes advantage of this natural symbiosis to benefit human beings through sustainable and low-waste food production. This symbiotic relationship is all about how nitrogen is transferred throughout a natural system. Understanding the nitrogen cycle is important to successful food production, both in soil and in aquaponics systems. It is important to our understanding of ecology and that understanding of ecology can help us to form food systems that are increasingly using natural processes in more and more productive and efficient ways.
On our planet, nitrogen is a common element. It makes up around 78 percent of the air we breathe. Yet in a form which is useable by plants, it has become scarce in many ecosystems. The nitrogen cycle and the ways it has changed due to human actions can tell us a lot about ecology and the state of our planet. In aquaponics, we are using a part of the nitrogen cycle to our advantage.
In aquaponics, we join the cycle at the point at which ammonification has taken place and waste from an animal (in this case from a fish or other aquatic creature) has raised the levels of NH3 (ammonia) in the water of the rearing tank or tanks. Nitrifying bacteria then go to work, turning that NH3 into NO2 (nitrite) and then NO3 (nitrate). This NO3 can then be taken up by the roots of plants. When these plants go on to be eaten/ decompose the cycle continues.
Aquaponic systems allow the fish to aid the plants and the plants to aid the fish. The fish cannot thrive in an environment with too many nitrates (which is why there is a limit to how many fish can be kept in an aquaponic or aquaculture system of a given size) and so they benefit when the plants remove these for their own use. The plants, of course, benefit from the nutrition that comes from the fish excrement in the water.
It is crucial that an aquaponics system gets the balance between the fish and plants correct. Any imbalance, as in natural systems, can throw off the whole and result in a breakdown in the natural order of things.
It is important to remember, however, that like the whole of the nitrogen cycle, this symbiotic relationship would not function without the various bacteria that make the whole process possible. Like any natural ecosystem, an aquaponics system is a web of interactions that cannot be boiled down to a simple two-way street between fish and plants. Setting up a successful aquaponics system involves having an understanding of the nitrogen cycle and of the many beneficial interactions between all the elements of the artificial ecosystem you are creating.
Why Choose an Aquaponics System for Your Polytunnel?
A well designed aquaponics system can minimise the use of polluting energy sources, water and land and can create a system run on renewables which is far more sustainable than many other forms of food production. For home growers or commercial businesses with polytunnels, some form of aquaponics can be a sustainable and sensible solution.
An aquaponics system can have a number of advantages over other food production techniques that you might use in your garden. These include:
- The chance to gain a dual yield of both plant matter and fish
- Enhanced productivity and faster plant growth.
- Reduced water usage. (Traditional gardening can use 20x as much water as a recirculating system.)
- Reduced land use and optimal use of space. (Aquaponics systems can be suitable even for very small spaces.)
What Do You Need For an Aquaponics System?
Though different aquaponics systems will operate somewhat differently, the following components are usually required: firstly, a tank in which to rear the fish or other water creatures, a settling basin which will catch uneaten food and larger particulates that cannot be filtered out by plants, a biofilter where nitrification bacteria can grow, the hydroponics section where plant growth and plant water-filtering occurs and a sump, to and from which water is pumped en route back to the rearing tanks.
In rudimentary aquaponics systems, some of the above components may be combined. At their simplest, aquaponics systems can be as simple as a fish tank or pond with floating rafts supporting growing plants on top.
While the systems can differ greatly, they do all share the three main living components
- fish or other water creatures,
- plant life
- bacteria that allow for the conversion of ammonia in waste to nutritional nitrates that can be made use of by the plants.
Some systems also have additional living components, such as worms, or larvae, bred to feed the fish.
The living components in an aquaponics system will also require water (or course), light and oxygen and any systems must take all environmental needs into account – including the temperature and control of pests and disease.
Types of Aquaponic System
There are various different types of aquaponic system suitable for home growers. These include:
Media Bed – Constant Flow, or Ebb and Flow:
The most simple form of aquaponics involves the use of beds filled with a growing medium. The best media for these beds and the depth required for optimal plant growth are hotly debated and will depend on your requirements, location and the other elements of your system. Some systems keep beds constantly flooded, while others flood and then drain again in a cyclical way.
Nutrient Film Technique:
With this technique, crops are grown in lightweight drain piping/ guttering through which a thin trickle of water from aquaculture tanks runs. This form of aquaponics is only suitable for a certain range of delicate, leafy plants as other plants will be too heavy or have root systems that become too strong and invasive. For growing salads, however, this can be a truly efficient and sustainable solution.
Deep Water Culture:
Plants in these systems float on top of the water on rafts, with their roots dangling into it. This system has the benefit that, due to the tanks or channels beneath the plants, more water is held within the system. This means that the nutrient solution will be more stable and the whole system will require less monitoring than systems where far less water is used in the loop.
The final growing method used within aquaponics systems is the wicking bed. Wicking beds are traditional raised beds filled with dirt that sit on top of a reservoir of water – this reservoir of water can be part of an aquaponic system. This type of bed can allow other vegetables to be grown as part of an aquaponics setup that could not be grown in other media or using the other methods above, such as root crops.
Choosing Fish for an Aquaponics System
Tilapia are the best choice for warmer climates while trout are usually regarded as the best choice for those living in cooler temperate zones. However, there are a number of other fish species and other aquatic creatures that could be considered.
One of the benefits of choosing a fish like trout is that these fish are better suited to the climate here in the UK. You could also consider other freshwater fish such as carp, which, although not as good to eat as trout, can adapt well to a wide range of environments, making them easier to keep for novices. As well as edible freshwater fish, you could also consider keeping carp species such as goldfish or koi. Though you will not have the additional edible yield, these fish can still be used to feed plants in aquaponics systems.
However, while trout is better suited to our climate, UK gardeners, especially those with polytunnels, could also consider growing warmth-loving fish like tilapia in a heated undercover structure. A polytunnel heated with renewable energy could still be an excellent sustainable option.
In addition to keeping fish in your aquaponics system, you could also consider choosing other aquatic creatures such as freshwater mussels, crayfish or freshwater prawns in addition to the fish.
One general rule of thumb sometimes cited for those planning an aquaponics system is that 1/2kg fish requires 20-26 litres of water, and that 1/2kg of fish is sufficient to fertilise 0.1sq m of grow bed surface area. Of course the exact requirements will vary depending on the exact set up, on which type of fish you use, and on a number of other factors.
Plants for Aquaponic Systems
A wide range of plants can be grown in different aquaponic systems, though all are extremely well suited to growing leafy, nitrogen-hungry plants such as lettuce, brassicas and herbs. If you want to grow root vegetables and other such plants, wicking beds are usually the system you would have to go for. Of course the infrastructure of your aquaponics system and its size will dictate the number and size of plants that you are able to grow.
Nitrogen hungry plants should be placed closer to the outflow from your fish tank or tanks, while those that require less nitrogen would be happier further round the system, once a quantity of the nitrogen in the water has already been absorbed.
You can experiment with growing most of your favourite fruits and vegetables using an aquaponics system and make more out of the space you have available to you for growing food. Share your aquaponics experiences in the comments below.
Elizabeth Waddington is a writer and green living consultant living in Scotland. Permaculture and sustainability are at the heart of everything she does, from designing gardens and farms around the world, to inspiring and facilitating positive change for small companies and individuals.
She also works on her own property, where she grows fruit and vegetables, keeps chickens and is working on the eco-renovation of an old stone barn.