Worm compost, an organic soil improver for the vegetable garden

Passing through the worms' gut enriches the material with micro-organisms: this worm compost contains around fifty types of bacteria and nine beneficial fungi, drawn from the rhizosphere, that is, the zone of soil surrounding the roots. It also contains humic and fulvic acids, as well as enzymes. Once mixed into your soil, these organisms carry on breaking down the organic matter and make its elements available to the roots, which is why an amended soil comes back to life after a growing season.

In pots and planters, vermicompost is mixed into the grow medium at 20% of the volume, that is one bucket of vermicompost to four buckets of soil or compost, which works out at about 100 g per litre of mix. In open ground, use 1 to 2 kg per plant, mixed into the soil from the planting hole. When transplanting, add it together with an organic fertiliser, which takes over as the plant grows. Its fine texture, achieved by sieving, makes it easy to blend into a potting compost.

This worm compost must never be poured into the circuit of an aquaponic system, neither into the fish tank nor into a grow bed through which their water flows. Once submerged, its organic matter breaks down in the water, uses up the oxygen that the fish and the filter bacteria need, releases ammonium and clouds the water, whereas a balanced system receives its nutrition from the fish feed. The FAO manual on aquaponics also recommends keeping the droppings of warm-blooded animals away from the water of a system that produces vegetables eaten raw, and an amendment made from manure falls under that precaution. Your seedlings intended for the system's baskets should therefore be started in an inert medium, such as germination pods or rockwool, and not in an enriched compost whose particles would wash into the water.

Vermicompost, on the other hand, comes into its own around the system. Crops in pots, planters or open ground that you water with the water renewed from the tank benefit from a compost amended at 20%, because this water, rich in nitrate, feeds the plants while the organic matter retains moisture and gives the soil structure. Plants too large for the grow bed, such as squashes or tall tomato plants, thus grow in soil beds next to the aquaponic greenhouse. This way, you keep a clean circuit for the fish, and you put every partial water change to use in the vegetable garden.

The link between worms and aquaponics works the other way round too. The FAO manual describes adding compost worms to grow beds with a grow medium, where they break down the solid waste caught by the clay pebbles, as the article on vermiponics explains in detail. The sludge removed from a settling tank, rich in organic matter, can in turn go into a worm bin, mixed with dry waste such as cardboard, and return to the vegetable garden as a soil conditioner.

Worm compost weighs about 0.5 kg per litre: the 3 kg bag represents 6 litres, the 5 kg bag 10 litres, the 10 kg bag 20 litres and the 20 kg bag 40 litres. At a rate of 20%, the 6 litres in the bag will amend 30 litres of mix, enough to fill about ten 3-litre pots. The 10 kg bag makes up 100 litres of amended compost, and the 20 kg bag covers 10 to 20 plants in open ground, at 1 to 2 kg per plant. The price per kilo falls as the bag gets bigger, which makes the larger sizes worthwhile as soon as you are amending a whole vegetable garden or a series of beds.