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nitrogen fixing nodules forming in gritty mix

Posted by alexander3 6 (My Page) on
Sun, Apr 22, 12 at 9:17

In 2008 I planted a silk tree (Albizia julibrissin) from seed in miracle grow potting mix. In 2010 I repotted into the gritty mix, and was surprised to see a few nitrogen fixing nodules in the roots. I knew this was a nitrogen fixing species, but I had made no attempt to inoculate the medium with the necessary bacteria. Somehow they got in there. I assumed there were some in the mix.

Yesterday I repotted again. The root ball was a big mass of mostly fibrous roots, and I saw a lot of nodules, many more than before. I would not have guessed that the gritty mix would support free living rhizobium needed for nodulation, but apparently it does.

Has anyone else seen nodule formation in gritty mix, with or without inoculation?


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RE: nitrogen fixing nodules forming in gritty mix

  • Posted by tapla z5b-6a mid-MI (My Page) on
    Sun, Apr 22, 12 at 11:05

Why would you be surprised?- just curious.


RE: nitrogen fixing nodules forming in gritty mix

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.


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