What fertilizer should I use in aquaponics ?

In aquaponics, fertilizers are supplements, not mass fertilization solutions. The goal is never to directly feed the plants as one would with conventional fertilizers, but to fill specific deficiencies that harm the system's balance. Each addition must be thought of as a fine correction, carried out with caution, because the slightest error has repercussions on the fish and the bacterial colony. In this context, fertilizer becomes a management tool, allowing productivity to be optimized while preserving the ecosystem.

Iron is probably the most problematic nutrient in aquaponics. Often present, it quickly becomes unavailable when the pH exceeds 7. The plants then show typical symptoms of iron chlorosis : young leaves turn yellow but retain green veins. To correct this, chelated iron is used. EDDHA iron 6% is the most stable even at high pH (up to 9) and remains effective in basic soils or solutions, but it strongly colors the water and contains less active iron. EDTA iron 13% is the most concentrated and economical, but it only works well at pH below 6.5. DTPA iron 11% offers an ideal compromise: it remains stable down to neutral pH, does not alter the water color, and is ideal for aquaponic systems. A few grams are generally enough to restart growth and restore vigorous foliage. Chelated iron is therefore an essential ally, which it is wise to always have available.

Without potassium, plants become fragile, flowering is reduced, and fruit develops poorly. The most visible symptoms are dried leaf edges and brittle stems. To remedy this, aquaponics generally uses potassium bicarbonate, which has the advantage of also regulating pH, or potassium sulfate, which is very effective and easily assimilated. Patentkali is also an interesting option, as it combines potassium and magnesium, a duo particularly appreciated by fruit plants. These contributions must be made with care, because an excess of potassium can hinder the absorption of other nutrients.

Calcium is essential for plant structure and fruit health. Its deficiency causes deformed leaves and the famous blossom-end rot on tomatoes or peppers. Magnesium, at the heart of chlorophyll, is just as vital : its deficiency manifests itself by yellowing between the veins of old leaves. To correct these deficiencies, calcium carbonate or calcium hydroxide, which also remineralize water, and magnesium sulfate , better known as Epsom salt, are generally used. In the right doses, these contributions rebalance the system and quickly restore vigor to crops.

Beyond mineral inputs, certain natural fertilizers can usefully complement an aquaponic system. Marine guano , rich in phosphorus, stimulates flowering and fruiting. Seaweed extracts, rich in trace elements and natural stimulators, strengthen plant resistance. Liquid organic fertilizers can also occasionally support growth. However, their use must be measured, as excess organic matter risks saturating biological filtration. These products therefore have their place as occasional "boosts," and not as a base for fertilization.

While we often talk about iron, potassium, calcium, and magnesium, we must not forget about trace elements : zinc, boron, manganese, copper, etc. These nutrients, required in very small quantities, play a crucial role in the overall health of plants. Their absence sometimes leads to subtle symptoms that are difficult to identify, but which ultimately reduce productivity. To avoid these invisible deficiencies, some producers occasionally add trace element solutions adapted to aquaponics, or opt for natural extracts (seaweed, liquid compost). These micro-supplements complement the work of the fish and ensure balanced growth.

In aquaponics, not all fertilizers are suitable. Chemical fertilizers rich in nitrates or phosphates, which are very common in conventional agriculture, risk seriously unbalancing the water and endangering the fish. Products that are too concentrated or dissolve too quickly should be avoided, as should fertilizers containing heavy metals or chemical additives. The rule is simple : use only aquaponics-compatible products that are natural or specially formulated to be harmless in a living system.

Before adding any fertilizer, it's essential to understand the system's condition. Monitoring pH, conductivity, and water hardness is essential. Regular testing allows you to anticipate deficiencies and adjust your input accurately. It's also a way to ensure that the added supplements aren't disrupting your fish. A good practice is to keep a logbook, noting each addition and its visible effects on your plants. Over time, this monitoring becomes a valuable basis for better understanding your system and avoiding mistakes.

The key to using fertilizers in aquaponics can be summed up in two words : moderation and observation. It's better to split the inputs and introduce them gradually rather than adding a massive dose that would upset the balance. After each addition, you must carefully observe the plants and also monitor the system's reaction (water, fish, bacteria). Aquaponics-compatible fertilizers are designed to limit risks, but their effectiveness depends on careful management.