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What is the Nitrogen Cycle in an Aquaponics System?

nitrification process

Aquaponics and the Aquaponic Nitrogen Cycle...

Aquaponics is a highly efficient method of farming that combines hydroponics and aquaculture to create a closed-loop, man-made ecosystem with the goal of raising both fish and plants whilst addressing the negative impact traditional farming has on the surrounding water, land, and other resources.

The Aquaponic Nitrogen Cycle is crucial in maintaining the health of any aquaponics system. It is responsible for creating the symbiotic relationship between the fish and the plants. Healthy plants then consume the nitrates, which helps to filter and clean the water, biofilters then take over to finish the job so the water can be recycled back into the system.

To achieve this optimal nitrate levels in aquaponics, a naturally-forming beneficial bacteria, known as Nitrosomonas, must first convert the ammonia waste into nitrites. Another beneficial bacteria, called Nitrobacter, then converts these nitrites into nitrates; a nutrient essential for the plants growth. The plants absorb these nitrates to grow, and together with biofilters within the system, effectively clean and filter the water. Once this water has been sufficiently filtered and oxygenated it is then recirculated back into the fish for the whole cycle to start again.

In this article we’ll take a look how the nitrogen cycle works, what it is, and why it’s essential to the proper maintenance and lifespan of your man-made ecosystem.

How Aquaponics Works

In the simplest terms, an aquaponics system cycle can be thought of as starting with the fish.

  • The fish are fed as normal
  • Fish produce excrement that contains ammonia
  • A naturally forming bacteria, from the Nitrobacteraceae family, grows on the insides of the systems and this converts ammonia waste into nirites and nitrates; a form of nitrogen the plants can consume food
  • As the plants grow the roots clean take on the nitrates to aid in filtering the water
  • Solid Separator filters perform further purification before the freshly cleaned water can be reintroduced into the system, starting the cycle over again.

What is the Aquaponic Nitrogen Cycle?

Also known as nitrification, the nitrogen cycle refers to the process in which beneficial bacteria breakdown ammonia waste from the fish, into nitrate nutrients for the plants.

With a little understanding of the aquaponic cycle, it soon becomes clear why the nitrogen cycle is fundamental to aquaponics. Without it, the ammonia waste from the fish would quickly become toxic to the plants, and the fish themselves.

Tasked with converting fish waste into plant food, and partially responsible for keeping the water clean and the aquatic life alive and thriving, the nitrogen cycle truly is the heart of the entire aquaponics process.

Why Nitrogen?

Nitrogen is a macronutrient vital for forming the building blocks of proteins and enzymes essential for the plants growth. It can be found in high concentrations in the ammonia rich waste excreted by the fish.

> In the first stage if the process Nitrosomonas converts ammonia into nitrates.

> In the second stage Nitrococcus converts nitrates into nitriates.

> It does so by utilizing a process known as nitrification. Nitrification transforms the harmful toxic ammonia (NH3) or ammonium (NH4+) waste from the fish, first into nitrites (NO2-) and then into nitrates (NO3-).

In its current form the plants are unable to absorb and process the nitrogen within the excrement, to aid this process a naturally forming bacteria first transforms the ammonia waste into nitrates.

The conversion of ammonia to nitrates is simply the precursor for a second bacteria to transform these nitrites into nitrates; a consumable form of nitrogen that the plants can consume. You won’t see this process in action but it’s crucial to the survival of the system.

what is the nitrification process

How Long To Cycle Aquaponics Prior To Use

The process of balancing the system is by no means a quick one. To ensure the system is stable, one full cycle must be completed so the bacteria can establish themselves and begin to maintain the stability of the ecosystem.

Aquaponics systems replicate natural environments in which things naturally take time to grow and play their part. Mother nature’s habitats typically take months, if not years to form, and so to do the processes that support life in these habits. 

It is possible to start the first cycle with or without fish. The techniques differ when it comes to the production of ammonia; either produced by the fish, or from inoculated grow bed media. Should you choose to use fish, first timers may want to use a sacrificial type of fish such as goldfish as they are both cheap and robust.

Optimal Nitrate Levels in Aquaponics

Fish can generally tolerate nitrates levels between 300-400mg, or 30-40 ppm, however it’s advised to maintain lower levels to prevent excess nitrate concentrations. Experts are yet to reach consensus on what the exact level of nitrates in an aquaponics system should be; however it’s at least agreed that recommended levels should be between 3 to 150 mg p/liter or 150 ppm. 

When starting an aquaponics systems the ecosystem must first complete a full cycle before growing plants. This gives the bacteria time to colonize and mature sufficiently so it can convert enough nitrogen into a consumable form.

This process is possible using only the fish, which can take anywhere from 4-8 weeks for the natural bacteria form. But you can also reduce this time frame to 3-6 weeks by using liquid nitrifying bacteria.

Whatever technique you chose to start with, measuring accurate levels of nitrates will be crucial. So how exactly can we do that?

Testing Nitrate Levels in Aquaponics

Since the process is invisible, by the time visual indicators of health problems appear, it’s often too late. For this reason it’s important to closely monitor the water so you can maintain optimal levels to ensure the longevity of the ecosystem.

The most common way to test nitrate levels in your aquaponics systems is by using a six-in-one test strip, which are readily available from online retailers and local aquarium stores. These test strips are extremely handy and versatile as they can measure pH, and water hardness, as well as nitrites. Once sufficient time has passed for the bacteria colonies to develop and thrive, monitoring should be performed every 2-3 days. 

How to Adjust Nitrate Levels in Aquaponics

Once accurate readings have been taken, it’s common for slight adjustments to be needed. In order to change the levels of nitrates in your aquaponics system there are several approaches you can take, these include:

  • Add more fish: If your nitrate levels are low adding more fish will increase the amount of ammoia waste and therefore the amount of nitrogen that can be converted into nitrates.
  • Add more bacteria: If you have high ammonia levels but still have insufficient nitrate levels then you can add more Nitrosomonas bacteria to aid the conversion of nitrites into nitrates.
  • Add more plants: If nitrate levels are too high, adding more plants will help lower levels.
  • Remove fish: Having too many fish can also lead to excess nitrates, simply removing some can resolve this issue.
  • Change some water: Changing 15-20% of the water with dechlorinated water, which will also lower levels of nitrates.
6 in 1 test strip

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