Liquid potassium supplement, chlorine-free

In an aquaponic system, potassium is a limiting element: fish feed supplies too little of it to meet the needs of fruiting vegetables, so it has to be topped up. Fish-safe corrections use simple forms, such as potassium bicarbonate or potash, which management guides alternate with hydrated lime to raise the pH towards 7 while supplying potassium and calcium. The Aquaponic Mix fertiliser, formulated for water that houses fish, is another source. The article on deficiencies in aquaponics helps you recognise a shortage from the leaf.

That said, Mono Potassium should not be poured into the tank or into a bed connected to the circuit: it is dosed for a solution without fish. It does, however, have its place in decoupled aquaponics, where the fish water feeds a separate growing loop that never returns to the pond, and where the plant solution is corrected element by element. You can also use it in an independent hydroponic tank or on fruiting vegetables grown in pots alongside the system.

Fruiting vegetables account for the largest share of demand. On the tomato, potassium supports the colour and flavour of the fruit, and the need peaks during the harvest; strawberries and peppers, which keep swelling their fruit without interruption, follow the rhythm of picking; cucumbers and courgettes produce a lot in a short time and quickly exhaust a solution calibrated to the bare minimum. Lettuces and herbs, harvested for their leaves, need far less and can generally do without this supplement.

With these crops, the balance with calcium deserves particular attention. Blossom end rot in tomatoes, a dry brown patch at the tip of the fruit, signals a calcium deficiency, and an excess of potassium is precisely what slows calcium uptake. It is therefore better to correct in stages, checking the level after each addition, rather than aiming straight for the high dose.

Depending on the dose chosen, a one-litre bottle is enough for 500 to 1,000 litres of solution. For a 50-litre sump tank, allow 50 to 100 ml per preparation, which means between 10 and 20 complete preparations from a single bottle. As the product is used occasionally, for a correction or during fruiting, it generally lasts several seasons for a family vegetable garden or a domestic hydroponic system. Measure each dose with a dosing cup kept for this purpose and rinsed after use, so that the concentrated potassium never comes into contact with a calcium supplement.