Why would you be surprised?- just curious.

Al

Well, while I would never suppose the gritty mix is a sterile environment, I would guess that there would be far fewer bacteria present than there would be in peat based mix or even the 5-1-1 mix. As you have noted, peat based mixes break down much faster than pine bark mixes, presumably due to the action of bacteria and fungi (I have no idea what the relative contribution of each is). The durability of pine bark leads me to believe it is not supporting much of a bacterial population, compared to an equal volume of peat. Perhaps I am underestimating the relative contribution of root exudates in supporting the microherd in containers.

As you have also noted, microbe populations in container culture are not as steady as they are in the ground.

There is an industry based on the production and selling of inoculants for various plants. When I have planted beans and peas without inoculant, I do not see nodules on the roots. Consequently, I have been thinking that the appropriate bacteria are not exactly ubiquitous. Perhaps the bacteria that inoculate Albizia are more common?

In short, my personal experience is that an environment with a diversity of bacteria (garden soil topdressed with compost) did not have sufficient rhizobium to inoculate legume roots. I guessed that an environment with a much smaller population, and probably less diverse population, of bacteria (gritty mix in a pot) would not have sufficient rhizobium to inoculate legume roots.

Clearly, I was wrong :)

Hopefully, my garden soil has the appropriate bacteria to inoculate lupine, since there is no good commercial source of lupine specific inoculant for home garden scale.

Alex