Carol, there are now a few details to finally get the full picture:
<<Alaskan Fish Fertilizer, for example...that's feeding the micro-organisms too?>> Actually both on land or in the ocean, dead tissues decompose into nutrients that are taken up in any food chain. So yes you are feeding the food chain my adding Fish fertilizer. Between earthworms and the final denitrificating bacteria that generate the potassium nitrate there still are quite a few of other organisms doing their part. Some are very tiny invertebrates like millipedes and mites, some are fungi like the Actinomycetes. These are responsible to the wonderful smell of moist rich soil!. Each contribute to the degradation of dead tissue.
Now what is the wonderful contribution of earthworms in particular ?. They eat both organic pieces of dead plant material and soil particles ( i.e sand,perlite, dust etc). Then inside the guts the reduced organic components that are not uptaken by the body of the earthworm are passed on in the format of OM "glued" onto sandy and other inert particles. The earthworm generates thus a slow release product, because the OM is no longer directly or freely available. It has to be broken down still by other minute beings. That generates a time lapse. Then the soil gets in installments its nutritional needs over time. With organic fertilizing you don't need to fertilize every day or every week. It can be done in some cases once in a season, or perhaps two. The reason is that slow release fertilizer was generated by the earthworm poop.
<<So just by reusing the potting soil and adding compost and stuff like that ... that will be good?>> Yes your existing soil, is your support system. The existing particles ( sand, silt, clay) will maintain your bush upright. They serve a mechanical function mainly. They also generate air pockets so that roots have access to air to breathe ( hydroponic systems use areators like those one finds in acquarists hobby fish tanks to make sure that the water carries enough dissolved air to keep the roots alive). The source of soluble nutrients for most garden plants is by getting it from the soil food web. Alternatively, modern research has allowed to use the chemically produced potassium nitrate ( in 15:15:15 the first 15: representrs that) directly. So modern gardeners can choose from both.
Why its better to use earthworm poop instead of 15:15:15?. Because it a NATURAL slow release product. When I stress natural its because there are tons of minicreatures waiting to jump in and do their part, whilst with the highly ( VERY) soluble potassium nitrate, what is not uptaken soon, say days, perhaps a week or so, then the rest is simply washed away and the plant needs replenishing again! Also the soil food chain is damaged. With high doses of available nitrate the final bacteria are no longer needed and dissapear. So by going chemical one ends up with a depleted soil food chain apart from wasting useless extra nitrate and contributing to the pollution of downstream watersheds.
<<bone meal>> This is the best source of Calcium phosphate that has a high soluble phosphate component. Its better than rock phosphate. Second best are seashells ( ground) such as oyster shell. Phosphorus when needed must be soluble. Otherwise the plant can not uptake it. ( it represent the middle number of a fertilizer). Organic phosphorus ( bone meal or seashells) have gone through the phosphorus cycle of the body of animals so that type of phosphorus is much easier to return to a soluble salt. Most of existing phosphorus compounds a very insoluble, thus useless for the life web.
<<I'm thinking that bagged composted manure...wouldn't that already be worked over by micro-organisms already?>>
Not fully. If so you would have just only invisible salt diluted in water. ( what many gardeners call compost tea). A compost is a HALF decomposed organic matter . Its more or less half way in the soil food chain.
I hope that I've covered your questions.Perhaps some readers would want to jump in with a few more. I don't mind having my mind being picked clean if that helps others garden better!