Starting seeds in sterilized soil is fine, but after the sterile soil joins compost it will add goodies to your sterile soil, but won't hurt the compost.

All potting soil gains microorganisms over time. They blow in. If you'd like, you can hasten this by going outside, getting one pinch of good soil, and dropping that one pinch in your pot-plant.
Most people are not aware that there are literally millions of "critters" in each gram of soil, which equates to about a good pinch of soil. These "critters" are bacteria, actinomycetes, fungi, yeasts, protozoa, algae, and nematodes. They are microscopic organisms that live in the soil. It sounds terrible, but we exist today because of them. They are what give the soil life, which is necessary to grow plants, the backbone of our existence.

- Steve Jones, Master Rosarian

Is this potting soil or "garden soil" you are referring to?
Potting soil is not soil but is a growing medium made of peat moss, coir, or finely ground bark, or a combination of those and the Soil Food Web will not be present in that for quite some time. That is one of the reasons many organic growers do not consider container growing to be organic.
Sterilizing garden soil could kill off most of the Soil Food Web which would mean time would be necessary for them to recover, a long time, like years, although putting good, unsterilized compost into that soil might help.

There will be more microbes in fresh compost than in soil, and if you mixed compost into sterile organic matter the microbes will grow and spread. I don't think potting mix would actually be sterile, it looks like the finer bits are already composting so there must be microbes in there.

If you're just transplanting flowers or veggies into pots or containers, you certainly don't need sterile medium. Actually, microbes do a lot of the work of managing nutrients and feeding plants.

kimmsr, I posted a study a little while back ... ah, here we go:
Seedlings were germinated in seven different sand- or soil-based and artificially based growth media. Seedlings grown in the HFC had fewer mycorrhizal short roots than those grown in the open greenhouse atmosphere. Furthermore, the proportion of seedlings from the HFC that were completely non-mycorrhizal was higher than that of seedlings from the greenhouse atmosphere. Seedlings grown in sterilized, artificially based growth media (>50% peat moss, vermiculite, and/or perlite by volume) had fewer mycorrhizal short roots than those grown in sand- or soil-based media.

Leads me to believe it happens fast. (The study is actually about reducing the effect, in order to maintain truly sterile conditions. They have to use "a high-efficiency particulate air-filtered chamber" ti achieve that.)

(The hat tip at the end to "sand- or soil-based media" is a bonus of course.)

Here is a link that might be useful: airborne ectomycorrhizal fungi

Soil is terribly hard to truly sterilize, BTW. I used to work in a lab doing biodegradation studies on pesticides in soil. We were supposed to have 'sterile control' samples to gauge any non-biologically mediated degradation. We'd sterilize the soil in an autoclave, and the samples were incubated in sealed sterile jars, but after a few weeks there were always bacteria in the sterile ones. Fewer of them, but still a lot - maybe 10,000 per gram compared to 10 million in the unsterilized soil. I think you'd have to autoclave it several times, days apart to really kill it.

I've spent a lot of time reading posts on Garden Web and some of the people posting here are pretty informative. Unfortunately, and it's not limited to this forum or these posters, but most of the time people don't answer people's questions.

Your answer: No, sterilized soil will not kill off bacteria added to it.

"should I continue using sterilized soil? "
It is not necessary.

It is actually unclear if the OP is mixing anything, or referring to the organics in their original mix as "compost."

Upon rereading the OP's question I think the issue is actually whether the lack of microbes in sterilized soil (or potting mix, etc., doesn't matter) will result in an inability of that matrix to continue breaking down organic matter (compost) added to it. I would say no, because compost has loads of microbes of its own. I hoped that my statements about potting medium containing compost and it being nearly impossible to really sterilize these materials anyway, would have made that clear but perhaps not.

Hi all, I have a closely related question-- I'd like to experiment with more 'alive' soils for indoor plants and I have lots of materials outdoors to work with.. My concern /question is, can I sterilise the media enough to remove insects without seriously depleting the microbial biota? I don't need to do massive quantities at once, so even more labour intensive /fussy approaches could be possibly used-- eg- could I use water to kill insects/worms etc, or would that be just as damaging to the microbes?

Sorry, no. Sterile means sterile - the process of sterilization is intended to kill off any living organisms, pathogenic and benign.

And I'd have to question the advisability of working with "alive soils" in any container growing situation. 'Alive' would connote the inclusion of your typical soil microorganisms and they need organic matter to survive. Including organic matter - like compost - in any sort of significant percentage as an ingredient in container/potting soil is not recommended. Most commercial potting soils contain little to no organic matter and are comprised almost entirely of relatively inert ingredients.

The reason for this is that organic matter will continue to decay and breakdown over time into smaller and smaller particles. As this happens, your soil loses its porosity and ability to drain freely and plants suffer due to lack of adequate aeration and retained water. Potting soils with a high component of OM eventually (and more rapidly than you would expect) become a mucky, gooey, water-soaked mess.

Not to sound nitpicky but if potting soil is ground bark, coir, peat moss etc. those are all organic matter. The only part that would not be is perlite and vermiculite that is sometimes used. The difference is that bark, etc. used in potting soil decomposes very slowly in the pot (compared to a compost pile with high nitrogen 'greens' added). I completely agree on that last line about muck and goo, but I think that happens when *composted* organic matter is used, i.e. compost.

You are correct, tox. They are organic matter :-) But they are pretty inert organic matter - neither peat nor coir tend to decompose or breakdown any further to any significant degree and as you note, bark decomposes very slowly in a potting mix. Because of this, they offer minimal support to any microbes that may be present.

To encourage the 'live' component the poster was referring to requires undecomposed or only partially decomposed organic matter - compost, manures, leaf mold, whathaveyou. And that's where the problems originate. It's also why organic fertilizers, seldom water soluble, are not a great way of addressing nutrient needs in container growing.

It is tough to reconcile a "good" potting soil with an "alive" potting soil - the two are counterproductive.

I would rather use sterilized soil if I was using that kind of thing, to prevent things like diseases and weed seeds being imported to the garden. All the soil food web will come from my compost already in place. It is silly to panic about the soil food web. Earthworms are not even native to north American by the way, look this up. They don't have to be there to create healthy soil. In fact earth worms have harmed some forests because the ate up all the organic layer and the soil then was damaged. I would rather not have earth worms, but I have tons of them, too many to ever get rid of them.