Hey Gilbert, I couldn't help but respond since this is something I have been pondering recently but geared more toward endophytic diazatrophs.
They're basically plant penetrating microbes that fix N. But instead of just living in root nodules these can live on stems, leaves and roots. There's a lot of research available.
Some of the most intriguing are studies that have taken diazatrophs from lodgepole pine needles, cultured them and applied them to the leave s of crops like tomatoes. Apparently the applications increased growth and yield quite a bit.
Just from gleaning the research available it seems that these microbes are about as widespread as mycorhizae and may be just as important for plant health.
So then how to identify and culture them without a lab?
I immediately think of the Korean natural farming methods that utilize inoculum from various areas to make diverse inoculants that can be applied directly or to composts.
Keeping past years residue on the soil may be a good practice too. It probably depends on the specific microbes life cycle, ability to overwinter without a host, presence of other competing microbes etc. So it may be more about a methodology for growing the crops that allows for these to flourish alongside crops that will readily accept them.
All said, without a microscope to identify specific bacteria it may be just as viable to continue selecting the most vigorous plants, which may (or may not) harbor the beneficial microbes.
