Should I add soil with living things, like worms, to a potting mix?

Question

I normally just buy in seedlings from big box stores since I am too disorganised to start my own seedlings in time. However, this year I did manage to get some seeds into the seed raising mix, and they have come up.

I've then pricked out the healthy ones and transplanted them into a flat, which is really just a 9-inch deep square grow bag which I partially filled with potting mix. I used a potting mix since they are used for containers being free draining, I had some bags of the stuff lying around, and it's free of seeds and hopefully other pathogens. But looking at the potting mix today just over a week later, it looks rather sterile and I wonder if I should really be trying to raise seedlings in such a substrate. I suspect it's too late now to change the mix without another shock to the seedlings.

Today in a fevered moment I did grab some wet worm-filled soil from the bottom of my year old compost heap, chucked it into some rain water in a bucket, and then poured the water over the "flat" hoping to inoculate the mix with some microfauna. So, the question is, is potting mix suited at all for raising seedlings, or should I add soil with something living in it to the mix before transplanting into it?

PS: The image is of bok choy seedlings. By life I mean bacteriae, protozoa, microarthropods, worms and optionally mycorrhizal fungi

Answer 1

The trouble with introducing 'life' by inoculating with garden soil is you have no control over which life forms you are introducing, and some of them might be pathogenic when contained inside a pot or grow bag. That is why there are commercially produced potting composts, to avoid that risk. Some people actually grow plants in soil they've dug from the garden and put into a container, often with no ill effect, but there is a risk. Pathogenic or troublesome life forms in open ground are much less of a threat than when contained in a pot or other container.

In particular, seedlings are much more vulnerable to pathogens, which is why its wise to start seeds off in seed and cutting compost, then grow them on in potting compost. Most potting composts contain fertilizer in various ratios which is sufficient to grow plants for up to six weeks healthily, depending on the formulation. After that, some extra fertilization might be necessary, depending on what you're growing in the container. If you'd prefer to allow your plants to have free access to 'microfauna' and fungi, then plant them out in the ground when they are large enough. Over time though, containers full of plants and potting compost do acquire various life forms - visible ones like worms and slugs for instance, and often, various fungi, particularly if the potting compost contains woody material.