The chemical composition of system water in aquaponics is very complex. Besides a large array of dissolved ions, it contains organic substances resulting from the release of products of fish metabolism and feed digestion, as well as substances excreted by the plants. These substances are largely unknown, and their interactions can further influence the chemical composition and pH of aquaponic nutrient solutions. All this can exert manifold, but mostly yet unknown, effects on the nutrient uptake by plants, on fish health, and on microbial activity.

Nutrients enter an aquaponic system via added water and fish feed (Schmautz et al. 2016). In terms of elemental composition, fish feed contains about 7.5 % nitrogen, 1.3% phosphorus and 46% carbon (Schmautz, unpublished data). In terms of organic compounds, fish feed contains proteins (fishmeal or plant based), fats (fish oil, plant oils) and carbohydrates (Boyd 2015). Herbivorous fish (like Tilapia) need only about 25% protein in their feed, compared to carnivorous fish which require around 55% protein (Boyd 2015). Both fishmeal and soya are unsustainable (for different reasons), so there is intense research towards finding suitable fishmeal replacements and plant-based diets (Boyd 2015; Davidson et al. 2013; Tacon & Metian 2008).

If the feeding ratios are calculated correctly, all the feeds added to the system are eaten, and only whatever is not used for growth and metabolism is excreted (Figure 11). The proportion of excreted nutrients also depends on the quality and digestibility of the diet (Buzby & Lin 2014). The digestibility of the fish feed, the size of the faeces, and the settling ratio are all very important for the system operation (Yavuzcan Yildiz et al. 2017). Therefore, the nutrient composition of aquaponic system water, resulting from the quality of the added water, the added fish feeds, and the entire metabolic reactions in the system, is extremely complex and does not always match the plant requirements. However, the welfare of fish should be of central concern, and fish feed should be chosen to fit the fish species at each development stage. The availability of nutrients that can be assimilated by plants has to be regulated in a second step.

Figure 11: Environmental flow of nitrogen and phosphorus (in %) for (a) Nile Tilapia cage production (after Montanhini Neto & Ostrensky 2015); (b) RAS production (data from Strauch et al. 2018). ‘Unexplained’ denotes the fraction of N and P that could not be attributed to any category

The data in Table 10 show that most plant nutrients, but especially P and Fe, were at significantly lower concentrations in the investigated aquaponic system as compared with the standard hydroponic solutions. This seems to be a typical situation in aquaponic operation; however, the growth rates of aquaponic crops are nevertheless in most cases satisfactory (Schmautz, unpublished data). Let us have a closer look at this phenomenon.

Unfortunately, interpretation of these data is very difficult. The reason is that very recently in plant nutrition the nearly two-century-old ‘Liebig’s law’ (plant growth is controlled by the scarcest resource) has been superseded by complicated mathematical models that take the interactions between the individual nutrient elements, compounds, and ions into account (Baxter 2015). These methods do not allow a simple evaluation of the effects of changes in nutrient levels in a hydroponic or aquaponic system. Also, we must bear in mind that a perfect formulation of nutritional requirements for a particular crop does not exist. The nutritional requirements vary with variety, life cycle stage, day length, and weather conditions (Bittszansky et al. 2016; Resh 2013; Sonneveld & Voogt 2009).

Very generally, for good plant growth in hydroponics, nitrogen concentration should stay above 165 mg/l N, phosphorus above 50 mg/l, and potassium above 210 mg/l (Resh 2013). In aquaponics, such high concentrations are difficult to achieve for several relevant elements because of three reasons:

1. The higher the concentrations in the water, the higher is the loss of nutrients through water exchange or sludge. However, even in closed system, a certain level of water exchange is required, in order to compensate for evapotranspiration losses and to reduce accumulation of unwanted components.

2. With the elevated concentration of nutrients in the water, components like salt or toxins accumulate in the system as well.

3. Phosphorus reacts with calcium if this is present in higher concentrations and precipitates as calcium phosphate.

Plants growing in the hydroponic compartment have specific requirements which depend on the plant variety and the growth stage (Resh 2013). Nutrients can be supplemented either via the system water (Schmautz et al. 2016) or via foliar application (Roosta & Hamidpour 2011).

Table 10: Comparison of concentrations of nutrients in standard hydroponic solution and in water from a closed aquaponic system (Schmautz, unpublished data)

Usually, with appropriate fish stocking rates the levels of nitrogen (N, as nitrate) are sufficient for good plant growth, whereas the levels of several other nutrients, notably iron (Fe), phosphorus (P), potassium (K) and magnesium (Mg) are generally insufficient for maximum plant growth. As seen in the table, other micronutrients could be limiting too. In aquaponic, it is especially important to monitor pH, because at a pH above 7 several nutrients (see Figure 10) may precipitate from water and become thus unavailable for plants.

Potassium (K) is not necessary for fish which leads to a low potassium composition of the fish feed and to even lower levels of potassium available for the plants (Seawright et al. 1998). To supply potassium, KOH pH buffer is often used, as the pH often decreases in aquaponics due to nitrification (Graber & Junge 2009). This has the added benefit of raising the potassium levels, although it can be toxic to fish. The LC50 value of acute fish toxicity was reported to be in the order of 80 mg/l. In aquaponic systems planted with tomato, potassium accumulated mainly in the fruits (Schmautz et al. 2016).

Iron (Fe) is also often a limiting factor in aquaponics, therefore it can be added as a preventive measure before the deficiencies become apparent. High concentrations of iron will not harm an aquaponic system, although it may give a slight red colour to the water. In order to ensure easy uptake by plants, iron has to be added as chelated iron, otherwise known as sequestered iron. There are different types of iron chelates: Fe-EDTA, Fe-DTPA, and Fe-EDDHA. Iron can be added into the system water (for example 2 mg L⁻¹ once every two weeks) or sprayed directly on the leaves (foliar application) of 0.5 g L⁻¹) (Roosta & Hamidpour 2011).

The main source of calcium (Ca), magnesium (Mg), and sulphur (S) is tap water, which facilitates the absorption by the plants as the nutrients are already available (Delaide et al. 2017). Nevertheless, these elements are often at low levels in aquaponic systems (Graber & Junge 2009; Seawright et al. 1998, Schmautz, unpublished data). Especially Ca is often a limiting factor in aquaponics, as it can only be transported through active xylem transpiration. When conditions are too humid, calcium can be available but locked out because the plants are not transpiring. Increasing air flow with vents or fans can prevent this problem. Otherwise, calcium carbonate (CaCO₃) or calcium hydroxide (Ca(OH)₂) ought to be supplemented.

Zinc (Zn) is used as part of the galvanisation process of some metal parts, which may be used in construction of AP (fish tanks, bolts etc.), and it is found in fish waste. While zinc deficiencies are rare, zinc toxicity can pose a problem in aquaponics, because while plants can tolerate an excess, fish cannot. Levels of zinc should be kept between 0.03 - 0.05 mg/l. Most fish will be stressed at 0.1 to 1 mg/l, and will start dying at 4-8 mg/l. The best way to keep zinc levels within harmless range is to avoid galvanised equipment (Storey 2018). Nevertheless, in some systems zinc deficiencies might occur. Zinc deficiency can be alleviated by foliar application of chelated zinc (Treadwell et al. 2010).

The question thus arises whether it is necessary and effective to add nutrients to aquaponic systems (Nozzi et al. 2018). Provided that the system is stocked with enough fish, and the pH is within correct level it is not necessary to add nutrients for plants with a short cropping cycle which do not produce fruits (e.g. leafy greens such as lettuce, Nozzi et al. 2018). In contrast, fruiting vegetables (e.g. tomatoes, aubergines) require nutrient supplementation. The amount of required mineral fertilizers can be calculating by using the HydroBuddy software (Fernandez 2016) (See also the exercise in Module 6). In addition to our experience in supplementing mineral nutrients, in the future commercially available organic hydroponic fertilizers should be tested in order to define which ones do not harm fish life. Recently, the treatment of the fish sludge in a digester, and re-introduction of this digestate into the water system, has been suggested to increase nutrient supply to the plants (Goddek et al. 2016). Another possible benefit of supplying the aquaponic system with organic, instead of mineral, nutrients could be a positive effect on the microbial population.

Copyright © Partners of the Aqu@teach Project. Aqu@teach is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).