Organic seedling compost for pots and soil blocks

The compost provides water and air, but it is temperature that decides the speed of germination. Each vegetable has its own range: lettuce germinates from just a few degrees and reaches its optimum around 24 °C, whereas tomato and pepper prefer a grow medium close to 29 °C and pepper hardly emerges at all below 16 °C. A tray of pots left on a cold windowsill can therefore stay dormant for two weeks, whereas the same seedlings emerge within a few days on a heated mat or in a heated room. Spinach and cabbages, which are more tolerant, can be sown earlier in spring, when the soil is still cool.

As soon as the seedlings break through the surface, their needs change. They then require plenty of light, otherwise the stem lengthens and flops over, which is known as etiolation: place the tray near a very bright window or under a grow light switched on for 14 to 16 hours a day. Water preferably from below, leaving the pots to soak for a few minutes in a saucer, then empty out the remaining water, because compost that is permanently soggy encourages damping off of seedlings. Finally, sow at a depth of two to three times the size of the seed, and leave very fine seeds on the surface, simply pressed onto the moist compost.

In an aquaponic system, this potting compost is used to raise your plants before they are installed, not to fill the grow beds. A plant sown in a pot can be transplanted into open ground, a pot or a planter without any special precautions. If it is going into a bed of clay pebbles, gently rinse the root ball in a bucket of room-temperature water to remove most of the compost, because peat particles cloud the water, end up in the pump, and the fertiliser they carry has no place in the fish circuit. For plants intended exclusively for the beds, rockwool cubes avoid this step. The guide to getting spring seedlings off to a good start in aquaponics sets out this calendar.

Raising the seedlings separately has another advantage, which lies in how the harvests are organised. While the lettuces in place finish growing in the system, the next batch grows in small pots and gains three to four weeks. You then replace each harvested plant with one that is already well developed, so the tanks are never left empty and the fish always find roots to absorb the nitrates they produce. The nursery is also the only place where you can feed the young plants freely, since their water does not reach the pond.

SeedMix is sold in bags of 2.5, 5 and 10 litres. The small size fills a seedling tray or a few seed trays, which is enough for a batch of herbs on a balcony. The 10-litre bag covers a season of pots, since a 7 cm pot holds about a quarter of a litre, so it fills around forty of them. If you sow several staggered batches for an aquaponic system, the large size saves you opening a fresh bag for every sowing, provided you keep it closed between uses so that it does not dry out.