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Do wattles (roughly Australian Acacia species) have the same ability as other members of the Fabaceae (legume) family to fix nitrogen from air via symbiosis with soil bacteria? For instance, it's well known that peas and beans do, but what about the more tree-like legumes?

It's not a completely academic question for me. I'm wondering if planting wattles would be a useful way to gradually improve the surrounding soil by adding nitrogen, while being a chance to plant native species.

Wattle: Acacia buxifolia

(Acacia buxifolia image source)

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  • Do the roots have small tubers like on this image? http://commons.wikimedia.org/wiki/File:Faboideae_root_nodules_Taub42.png If they do, then I would think they have soil bacterias (Rhizobium) for nitrogen enrichment. Nov 30 '11 at 9:14
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    At least some acacia's are allelopathic - they inhibit germination of other plants and may otherwise discourage growth. Raking up any leaf litter may keep this from happening. I can't find a good hands-on reference on this, but it's described extensively in academic literature. And I'm in the wrong climate, so I've no experience myself.
    – Ed Staub
    Dec 1 '11 at 16:28
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My understanding is that this is one of the defining points of the Fabaceae. Trees in this group definitely act like legumes - other examples include mesquite and Texas red bud. As it happens, only a couple of weeks ago I was talking about plant diversity with MrsWinwaed - one of the ones I mentioned were the Fabaceae which include clover, beans, and mesquite - three quite different looking plants!

Another feature of them is they usually have high protein levels (eg. beans) due to the extra nitrogen made available by the nodules. Acacia are known for having high protein, so I think it is safe to say they do fix nitrogen in reasonable quantities.

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  • Excellent explanation and thanks! Doubt was introduced into my mind because I read somewhere (Wikipedia I think) that most Fabaceae fix nitrogren in this way, but couldn't find out much about what exclusions there are or why.
    – Lisa
    Dec 1 '11 at 0:08

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