Dry organic fertiliser to mix into compost, 5 L bucket

Seedlings raised in compost enriched with Pre-Mix should not be transplanted as they are into an aquaponic system. Instead, sow them in rock wool cubes or coir fibre cubes, then rinse them in clean water before placing them in the grow bed, so that no trace of compost or fertiliser reaches the fish water. If you are looking to correct the nutrition of the system itself, the most common deficiencies in aquaponics concern iron, potassium and calcium, and they are corrected with fish-safe products: a DTPA chelated iron, applied at around 2 mg/L every three weeks, and a buffering potassium or calcium salt, which also raises a pH that has dropped below 6.4.

Another option is to separate plant cultivation from fish rearing. In a so-called decoupled aquaponic system, the water from the pond feeds an independent growing loop, from which it does not return to the fish. You can then adjust the nutrition there with more freedom, without putting the stock at risk. Pre-Mix remains a grow medium fertiliser, however, and this hydroponic loop calls for fully soluble mineral solutions. Its place in an aquaponic setup is therefore the potted crops around it, the herbs on the terrace and the preparation of the plugs destined for the vegetable garden.

The dose is adjusted to the starting grow medium, to the length of the crop and to its appetite, and the guidelines that follow remain orders of magnitude: a compost that is already fertilised needs less than coco coir or exhausted soil. The low dose, 5 litres for 200 litres of grow medium, suits young plants and undemanding crops such as lettuces, radishes and herbs. The high dose, 5 litres for 100 litres, is for fruiting vegetables set up for a whole season in a large pot, such as tomatoes, peppers or aubergines. The 5-litre tub is thus enough to prepare 100 to 200 litres of grow medium, that is several large containers, or ten 10-litre pots at the high dose and twenty at the low dose, and each 10-litre pot then receives 250 to 500 ml of Pre-Mix.

An organic fertiliser does not dissolve like a mineral fertiliser. Its elements become available as soil life breaks down the organic matter, which explains the presence of fungi in the formula and the value of a grow medium kept moist. For a short crop, this reserve may be enough until harvest. For a tomato kept from May to October, you take over mid-season with a liquid growth fertiliser, the Bio-Grow from the same range or the Pro Organic Grow, when the foliage starts to turn pale.

The 5-litre bucket weighs 2.5 kg and can be resealed between uses, which keeps the powder dry from one potting-on to the next. Pre-Mix is also available in a 25-litre sack, a size designed for those who prepare large volumes of compost. It is certified as an input for organic farming by Control Union and by Soil Association, so you can use it in a vegetable garden run to organic standards. Store the bucket sealed in a dry place, away from damp, which would make the powder cake.