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Organic plant fertilizers

Chapter 6 discussed how even balanced aquaponic systems can experience nutrient deficiencies. Although fish food pellets are a whole feed for fish, they do not necessarily have the right quantities of nutrients for plants. Generally, fish feeds have low iron, calcium and potassium values. Plant deficiencies can also arise in suboptimal growing conditions, such as cold weather and winter months. Thus, supplementary plant fertilizers may be necessary, particularly when growing fruiting vegetables or those with high nutrient demands. Synthetic fertilizers are often too harsh for aquaponics and can upset the balanced ecosystem; instead, aquaponics can rely on compost tea for any nutrient supplementation.

General composting process

Compost is a rich fertilizer that is made from broken down organic matter, including food waste. Compost is extremely useful in soil-based gardening for replenishing organic material, retaining moisture and providing nutrients. In addition, compost can be used to create a liquid fertilizer, called compost tea, which can be added to the aquaponic water to boost the supply of nutrients. Conveniently, high-quality compost can be made from household food waste. Basically, food waste is added to a container, hereafter called the compost unit. Within the compost unit, aerobic bacteria, fungi and other organisms break the organic matter down into simple nutrients for plants to consume. The final substance that is produced is called humus. It consists of about 65 percent organic matter, is free of pathogens and is full of nutrients. The whole process from food waste to humus can take up to six months depending on the temperature inside the compost unit and quality of aeration.

A compost unit is generally a 200-300 litre, barrel-shaped container with a lid and many vents (Figure 9.1). They are usually dark coloured to retain heat, which accelerates the decomposition process. Many types of compost units are available, and they are very easy to build with recycled parts. Compost units that tumble are recommended because they require less space and remain well-aerated and homogenous. Be sure to have enough space to spin the barrel properly. All compost units require adequate airflow.

When making compost, it is important to manage the materials that are going into it. It is best to keep a good ratio of wet and dry organic material layered in equal amounts to reach a moisture content of about 60-70 percent. As the initial 2-3 weeks are a thermal aerobic process with temperatures up to 60-70 °C, it is important to avoid excessive moisture that would reduce the heat. The thermal stage accelerates the composting process and helps to pasteurize the organic wastes from any possible pathogens. The layering is important in order to keep the compost from being too wet and to prevent anaerobic zones. The frequent aeration of the pile is an important task in order to keep bacteria in aerobic conditions and to process the wastes uniformly. The operation consists of simply turning the waste upside down or periodically rotating the drum/container. This helps to aerate the aerobic bacteria.

Good green compost can be obtained from a blend of wet materials, such as vegetable food leftovers, ground coffee, fruits and vegetables, and dry materials such as bread, grass clippings, dry leaves, straw, ash, and wood chips. However, it is important to keep an optimal balance between carbon and nitrogen (C:N ratio at 20-30) as it results in a rapid transformation of the material. In general, it is wise not to use too much straw or wood chips (C:N > 100) but rather use "green" wastes such as grass clippings, preferably slightly dried to reduce their moisture content. It is not recommended to use too much wood ash to avoid excessive pH increases, and to use only ash from wood/vegetable origin, as other sources (i.e. paper) may contain toxic substances. Some material should never be composted, including dairy, meat, citrus fruit, plastic, glass, metal and nylon. Compost is very forgiving, but ideally the compost should have enough moisture and nitrogen to feed all of the beneficial organisms. Water can be added if the compost is too dry. The rise in the temperature of the compost indicates intense microbial activity, indicating that the compost process is occurring. In fact, compost becomes so hot it can be used to heat greenhouses.

Vermicomposting is a special method of composting that uses earthworms in the compost unit (Figure 9.2). There are several benefits to adding worms. First, they accelerate the process of decomposition as they consume organic wastes. Second, their waste (worm castings) is an extremely effective and complete fertilizer. Special vermicompost units can be bought or built, and there is a wealth of information available. It is important to source worms from a reputable source, and to ensure that they have never eaten meat or wastes from animals. Once composted, the worm castings can be used directly in the plant nursery to start seeds as this will introduce the nutrients to the aquaponic system once the seedlings are transplanted. Alternately, the worm castings can be made into a compost tea.

Compost tea and secondary mineralization When the organic waste has finally decomposed into humus, which can take 4-6 months, it is possible to make compost tea. The process is simple. Several large handfuls of compost are tied within a mesh bag, weighted with some stones. This bag is suspended in a bucket of water (20 litres). An air stone connected to a small air pump is positioned underneath the mesh bag so that the bubbles agitate the contents (Figure 9.3). The aeration is very important to prevent anaerobic fermentation from occurring. The mixture is left for several days with constant aeration. The contents should be stirred occasionally to prevent any anoxic areas. After 2-3 days, the compost tea is ready to be used in the unit. The tea should be strained through a fine cloth and then diluted 1:10 with water. Apply to the plants either as a foliar feed in a spraying canister or as liquid fertilizer straight to the plant roots. If adding the diluted tea straight into the unit, begin by using small amounts (50 ml) and patiently document the change in the plant growth. Re-apply when necessary, but be careful not to add too much.

Other nutrient teas

In addition to compost, there are many other nutrient-rich organic materials that can be brewed into nutrient tea in the way explained above. One mentioned above is to use the solid wastes from the fish tank, captured from the mechanical filter. Brewed in the same way, the solid wastes are completely mineralized and available to add back to the aquaponic system. Other sources include seaweeds, nettles and comfrey. Seaweed is a great addition because it is rich in potassium and iron, which are often lacking in aquaponics, but be sure to rinse residual salt from the seaweed. Larger amounts of organic fertilizer teas can also be used to temporarily maintain the aquaponic system without fish. This may be useful in the colder months of the year when fish metabolism is low and the plants need a boost of nutrients.

Compost safety

When using compost make sure it is fully decomposed - making it pathogen-free. Never use organic sources from warm-blooded animals, which increases the risk of introducing pathogens. Moreover, make sure the water is well oxygenated and constantly aerated when producing the tea as this helps in mineralization and prevents some types of pathogenic bacteria from growing. Always avoid placing aquaponic water on the plants leaves, especially when using compost tea. For further information on brewing compost tea, see the section on Further Reading.

Alternative fish feed

Fish feed is one of the most important and expensive inputs for any aquaponic system. It can be purchased or self-made. The authors strongly recommend the use of quality manufactured fish feed pellets because they are a whole food for fish, meaning the pellets fulfill all the nutritional needs of the fish. Even so, below is an example of supplemental fish feed that can be easily produced domestically, which can help save money or used temporarily if manufactured feeds are not available or too expensive. Further information on creating homemade feed pellets is available in Appendix 5.


Duckweed is a fast-growing floating water plant that is rich in protein and can serve as a food source for carp and tilapia (Figure 9.4). Duckweed can double its mass every 1-2 days in optimum conditions, which means that one- half of the duckweed can be harvested every day. Duckweed should be grown in a separate tank from the fish because otherwise the fish would consume the whole stock. Aeration is not necessary and water should flow at a slow rate through the container. Duckweed can be grown in sun-exposed or half-shaded places. Surplus duckweed can be stored and frozen in bags for later use. Duckweed is also a useful feed for poultry.

Duckweed is a useful addition to an aquaponic system, especially if the duckweed- growing container is located along the return line between the plants grow beds and the fish tank. Any nutrients that escape the plant grow beds fertilize the duckweed, thereby ensuring the cleanest water possible returning to the fish. Duckweed does not fix atmospheric nitrogen, and all of the protein in the duckweed ultimately comes from the fish feed or other outside sources.

Azolla, a water fern

Azolla is a genus of fern that grows floating on the surface of the water, much in the manner of duckweed (Figure 9.5). The major difference is that Azolla is able to fix atmospheric nitrogen, essentially creating protein from the air. This occurs because Azolla has a symbiotic relationship with a species of bacteria, Anabaena azollae, which is contained within the leaves. As well as providing a free source of protein, Azolla is an attractive feed source because of its exceptionally high growth rate. Like duckweed, Azolla should be grown in a separate tank with slow water flow. Its growth is often limited by phosphorus, so if Azolla is to be grown intensively an additional source of phosphorous is needed such as compost tea.


Insects are considered undesirable pests in many cultures. However, they have an enormous potential in supporting traditional food chains with more sustainable solutions. In many countries insects are already part of people's diets and sold at the markets. In addition they have been used as animal feed for centuries.

Insects are a healthy nutrient source because they are rich in protein and polyunsaturated fatty acids and full of essential minerals. Their crude protein content ranges between 13 and 77 percent (on average 40 percent) and varies according to the species, the growth stage and the rearing diet. Insects are also rich in essential amino acids, which are a limiting factor in many feed ingredients (Appendix 5). Edible insects are also a good source of lipids, as their quantity of fat can range between 9 and 67 percent. In many species, the content of essential polyunsaturated fatty acids is also high. These characteristics together make insects a healthy and ideal option for both human food and feed for animals or fish.

Given their enormous number and varieties, the choice of the insect to be reared can be tailored to their local availability, climatic conditions/seasonality and type of feed available. The source of food for insects can include staple husks, vegetable leaves, vegetable wastes, manure and even wood or cellulose-rich organic materials, which are suitable for termites. Insects also make a great contribution to waste biodegradation, as they break down organic matter until it is consumed by fungi and bacteria and mineralized into plant nutrients.

The culturing of insects is not as challenging as other animals since the only limiting factor is feed and not rearing space. Sometimes insects are referred to as "micro-livestock". The small space requirement means that insect farms can be created with very limited areas and investment costs. In addition, insect are cold-blooded creatures, this means that their feed conversion efficiency into meat is much higher than terrestrial animals and similar to fish. There are plenty of options possible and additional knowledge on insect farming as feed in the section on Further Reading. Among the many species available, an interesting species to be used as fish feed is the black soldier fly (see below).

Black soldier flies

The larvae of black soldier flies, Hermetia illucens, are extremely high in protein and a valuable protein source for livestock, including fish (Figure 9.6). The lifecycle of this insect makes it a convenient and attractive addition to an integrated homestead farming system in favourable climate conditions. The larvae feed on manure, dead animals and food waste. When culturing black soldier flies, these types of waste are placed in a compost unit that has adequate drainage and airflow. As the larvae reach maturity, they crawl away from their feed source through a ramp installed in the compost unit that leads to a collection bucket. Essentially, the larvae devour wastes, accumulate protein and then harvest themselves. Two-thirds of the larvae can be processed into feed while the remaining one-third should be allowed to develop into adult flies in a separate area. The adult flies are not a vector of disease; adult flies do not have mouthparts, do not eat and are not attracted to any human activities. Adult flies simply mate and then return to the compost unit to lay eggs, dying after a week. Black soldier flies have been shown to prevent houseflies and blowflies in livestock facilities and can actually decrease the pathogen load in the compost. Even so, before feeding the larvae to the fish, the larvae should be processed for safety. Baking in an oven (170 °C for 1 hour) destroys any pathogens, and the resulting dried larvae can be ground and processed into a feed.

Moringa or kalamungay

Moringa oleifera is a species of tropical tree that is very high in nutrients, including proteins and vitamins. Classified by some as a super food and currently being used to combat malnutrition, it is a valuable addition to homemade fish feeds because of these essential nutrients. All parts of the tree are choice edibles suitable for human consumption, but for aquaculture it is typically the leaves that are used. In fact, there has been success in several small-scale aquaponic projects in Africa using leaves of this tree as the only source of feed for tilapia. These trees are fast-growing and drought- resistant and easily propagated through cuttings or seeds. However, they are intolerant of frost or freezing and not appropriate for cold areas. For leaf production, all of the branches are harvested down to the main trunk four times per year in a process called pollarding.

Seed collection

Collecting seeds from growing plants is another important cost-saving and sustainable strategy in many types of small-scale agriculture. It is especially effective for aquaponics because the plants are the primary production goal. Seed collection is a straightforward process, which is discussed here as two major categories, dry seed pods and wet seed pods. In general, only use seeds from mature plants. Young plant seeds will not germinate, and old plants will have already dispersed their seeds. Avoid hybrid plants, which may be sterile. Collecting from many plants helps retain genetic diversity and healthy plants. In addition, consider local seed exchange groups that are available to trade seeds with other small-scale farmers.

Dry seed pods

This subcategory includes basil, lettuce, salad rocket and broccoli. Seeds from some of these plants can be harvested throughout the growing cycle, e.g. basil (Figure 9.7). Other seeds can only be collected after the plant is fully mature and no longer usable as a vegetable, e.g. lettuce and broccoli. The general process is to place the cut dry/mature stems into a large paper bag and store for 3-5 days in a cool, dark place. During this time, it is helpful to lightly shake the sealed paper bag to release the seeds. Next, open the bag and shake the stem or whole plant one final time while still inside the bag. Then, remove the stems and all plant debris and pass them through a sieve to collect the remaining seeds. Gather these seeds and place them back into the paper bag, making sure that only seeds and no plant debris remain.

Wet seed pods

This sub-category includes cucumbers, tomatoes and peppers. The seeds develop inside the actual fruit, usually coated in a gel sac, which prohibits seed germination. When the fruits are ready to harvest, usually indicated by a strong and vibrant colour, remove the fruit from the plant, slice open the fruit with a knife and collect the seeds inside using a spoon. Take the seeds coated with gel and place into a sieve and begin washing off the gel with water and a smooth cloth. Then, take the seeds and lay them out and dry them out in the shade, flipping them occasionally until they are totally dry. Finally, remove any remaining gel or plant debris and store them in a small paper bag.

Seed storage

It is recommended to store seeds inside sealed paper bags or envelopes in a cool, dry and dark place with a minimum of moisture. A small refrigerator is a perfect place to store seeds, best if in an air-tight container with a desiccant bag (i.e. silica gel) to keep moisture below the required levels for fungi to grow. It is vital to make sure that only seeds are present with no other plant or soil debris to remove the risk of disease or premature germination. Plant debris and moisture can also encourage fungus and mould that can damage the seeds. Once placed into the bags, write on bag the date and type of plant. For high percentages of seed germination, the seeds should be used within 2-3 growing seasons.

Rainwater harvesting

Collecting rainwater to resupply aquaponic units is another effective way of reducing running costs. There are several benefits to using rainwater for aquaponics. First and foremost, rain is free. The aquaponic systems described in this publication lose 1-3 percent of their water per day, mostly from transpiration through the plant leaves. Water is a precious resource and can be expensive and unreliable in some areas. Second, most rainwater is high quality. Rainwater is unlikely to have toxins or pathogens. Rainwater does not contain any salts. Rainwater also has low levels of GH and KH, and is typically slightly acidic. This is quite useful, especially in areas where water has a strong alkalinity, because rainwater may offset the need for acid correction of incoming water to keep the aquaponic system within the optimal 6.0-7.0 pH range. However, the lower KH of rainwater means that rainwater is a poor buffer against acid changes in pH. Therefore, if using rainwater as the main source of water, calcium carbonate should be added, as described in Section 3.5.2. Be conscientious about the water collection surface, and try to avoid collecting water from around bird roosts or wherever animal faeces accumulate. A simple method to reduce any risk of pathogen contamination is through slow sand filtration, which can be obtained by simply percolating water into a fine sand filter 50-60 cm high and collecting the filtered water at the bottom opening of the tank.

Rainwater collection can be easily achieved by connecting a large clean container to water drainage pipes surrounding a building or house (Figure 9.8). For example, a catchment area of 36 m2 will collect 11 900 litres of water with as little as 330 mm of rainfall per year. Some of this water is lost, but enough is caught to be sufficient for a small- scale aquaponic unit. The units described here use, on average, 2 000-4 000 litres of water per year. Collecting rainwater is the easy part; storing rainwater is more important and can be more challenging. The water has to be retained until the system needs it, and the water has to be kept clean. The containers should be covered with a screen to prevent mosquitoes and plant debris from entering. It also helps to keep a few small guppies or tilapia fry in the rainwater to eat insects, and a single air stone prevents anoxic bacteria from developing.

Alternative building techniques for aquaponic units

Human ingenuity has provided countless variations on the basic theme of aquaponics. At its most basic sense, aquaponics is simply putting fish and vegetables in different containers with shared oxygenated water. Old water tanks, bathtubs, plastic barrels, tables, wood and metal parts can all be used when building an aquaponic unit (Figure 9.9). Rafts and planting cups for DWC systems can be constructed from bamboo or recycled plastic; and media systems could be filled with locally available gravel. Always be sure that none of the components (fish tank, media beds, grow pipes and plumbing fittings) have been used previously to contain toxic or harmful substances that can hurt the fish, plants or humans. In addition, it is necessary to wash any material thoroughly before using it.

The least expensive aquaponic system consists of one large hole in the ground, lined with cheap 0.6 mm polyethylene plastic pond liner. This pond is separated with wire or mesh to separate the fish from the plants. One side of the pond is the fish tank, stocked with a relatively low density of fish, while the other is a DWC canal covered with polystyrene foam. Aeration and water movement are always required, but can be added either through an airlift with low head height or through human powered pumping. Lifting water up to a header tank and allowing it to cascade back down is one method of adding oxygen without electricity. This approach can be used in places where barrels and IBC containers are too expensive for farmers to consider using, although overall production would be lower.

Appendix 8 shows methods to make aquaponic units using IBCs, which can be easily found all around the world. In addition, the section on Further Reading lists two different guides on do-it-yourself aquaponics.

Alternative energy for aquaponic units

Operation of the unit's electric pumps, both air and water, requires an energy source. Usually, the normal power mains are used, but it is not mandatory. These systems can be operated completely using renewable energy. It is outside the scope of this publication to specify the plans for building renewable energy systems, but useful resources are listed in the section on Further Reading.

Photovoltaic electricity

Solar energy is an alternative and renewable energy that comes from sunlight. Photovoltaic panels convert the electromagnetic radiation from the sun to thermal energy or electricity (Figure 9.10). Water and air pumps for an aquaponic system can be powered with solar energy using photovoltaic solar cells, an AC/DC voltage inverter and large batteries to ensure 24 hour power supply at night or on cloudy days. Although highly sustainable, solar energy entails a large initial investment because of the costs of the extra equipment needed to convert and store the energy from photovoltaic cells. However, in some areas there are incentives to use solar energy which may help off-set these costs.


In winter, it may be necessary to heat the water. There are many methods to achieve this heating by using fossil fuels. However, cheaper and more sustainable options are available such as tank insulation and spiral heating. Insulating the fish tanks with standard insulation during the winter months prevents heat dispersing from fish tank. Significant heat energy is actually dispersed from the activity of the air stones, thus it is best to cover and insulate the biofilter or adopt alternative aeration solutions that avoid air bubbling.

Spiral heating

Spiral heating is a form of passive heat capture from solar energy. Water from the system is circulated through black hose pipe, coiled in a spiral. The black plastic captures the heat from the sun and transfers it to the water. To further heat the system, the spiral heating coil can be contained within a small glass panel house that serves as a mini-greenhouse to further increase the heat. A black background can also help retain heat. For the systems described here, the recommended dimensions are a pipe 25 mm in diameter with a length of 40-80 m (Figure 9.11).

Source: Food and Agriculture Organization of the United Nations, 2014, Christopher Somerville, Moti Cohen, Edoardo Pantanella, Austin Stankus and Alessandro Lovatelli, Small-scale aquaponic food production, Reproduced with permission.

Food and Agriculture Organization of the United Nations

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