Roblouopp, whereabouts are you? I would think that most Acacias wouldn't do well in a greenhouse environment. Most like direct sun and good air movement around them. They seem to have no problems establishing themselves in disturbed soils, even in subsoils brought to the surface. As Woko said, they always seem to find suitable nitrogen fixing bacteria anywhere.

I am in Cambridge, MA, USA. So not in prime Acacia habitat! I am working in a greenhouse that has had Acacias here for a decade at least so I know that they have been somewhat successful in the past. I think the sickly plants might be due to poor soil-we just use a potting mix. We might just need to change the mix but I don't know what a good mix would be. I was hoping that supplying them with bacteria would help bring them added N, and I looked at some roots and didn't see any nodules so I assumed the correct bacteria was not there..

Which species are they? Some of them are incredibly tolerant of almost any kind of soil. I assume they'd have originated from seed so unlikely their preferred bacteria would have been with them. I'm over run with A. auriculiformis at my place but also have A. holosericea, dimidiata and dunnii.

Mostly A. melanoxylon, A. irrorata, and A. decurrens. We also have an African species I am unaware of its scientific name though.

Can the plants not form associations with bacteria after a certain, critical time in their life?

The medium in which I sued to grow Acacia seedlings through Trees for Life was part sand, part soil & part composted tree bark, if I remember rightly. Osmocote, an Australian plant fertilizer, was used at some stage but I think that might have been discontinued. Once the seeds were sown & tamped into the soil they were covered with a thin layer of fine gravel to provide a mulch & then kept moist by sprinkling. My seedlings were grown in the open & watered once per day, twice if the soil became dry due to extreme heat.

I'm not sure about a critical time for forming associations with bacteria, roblouopp. I expect that soon after roots begin to form they would begin to develop those associations.

Here's a link to a paper on acacias and nitrogen fixation. I haven't read the whole paper but it looks interesting.

Most interesting, tropicbreezent. I read the abstract & will try to read the whole paper later. I particularly liked the idea of planting Acacias for fixing nitrogen in the soil as an alternative to using oil-derived fertilizers, something which would benefit Earth as a whole. My concern for Australia, at least, is that many non-indigenous Acacia species can be quite invasive & damaging of natural ecosystems. No doubt, if Acacia plantations should be established to fix nitrogen then the project managers would have extremely strict management plans in place to deal with escapees.

Roblouopp, the paper is well worth a look &, I think, answers at least some of your questions.