Fwd: Mycorrizal "fertilizers"?

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This is from the San Diego CNPS listserv.  I thought you all might find it interesting.... the discussion about mycorrhizae continues...

Begin forwarded message:

From: Frank Landis <f*@yahoo.com>

Date: May 24, 2011 8:37:00 PM PDT

To: c*@ucsd.edu

Subject: [Cnpssd-L] Re: Mycorrizal "fertilizers"?

Hi All,

Speaking as someone who did his PhD on mycorrhizae...

For native plants, the best inoculum for native plants is to go out into nature with a bucket and shovel, find a healthy specimen of the plant, get a bucketful of soil (and leaf litter), bring it back, and put it in the planting hole.  You actually don't need more than a couple of handsful.

I don't recommend buying commercial mycorrhizal preparations for use on native plants.  The reason is that the fungi they use are ones that produce huge numbers of spores in the greenhouse.  I should know, because I cultured the same species for the same reason.  They don't necessarily work well in your garden, in part because fungi that invest all their resources in producing spores don't always produce a big body to gather lots of nutrients for the plant.  Conversely, species that produce big fungal bodies (mycelia) can be really hard to grow from spores.  That's why I recommend the bucket of soil approach.  Additionally, you get all the other good bacteria, mites, etc. that aren't mycorrhizal fungi.  There's a whole complex ecosystem in the soil, and the mycorrhizal fungi are only one part.

How mycorrhizal fungi work (massively simplified): basically, the fungi are better than the plants at getting phosphorus (and other nutrients) out of the soil than most plants are, while the plants are better at producing sugar than the fungi are.  The plants contract out phosphorus acquisition to the fungus, for up to 20% of the sugar the plants get from photosynthesis.  As with any contracting relationship, there are complexities.  If the plant isn't short of phosphorus, having mycorrhizae can actually hurt it--this is why you don't want to inoculate plants in a well-fertilized garden.  In fact, well-fertilized plants will boot the fungi out of the roots, which is why mixing mycorrhizal spores in with phosphate fertilizer is a waste of time.  

In general, native plants are very efficient with their nutrients, meaning they don't need much fertilizer.  I think having native mycorrhizal fungi on the roots is a great way to go, in that you probably won't need to fertilize them much if at all.  However, if you're planting natives in an area that's already been heavily fertilized, the mycorrhizal fungi are unlikely to help.  In any case, I don't recommend buying the commercial preparations, simply because I don't think they're worth the cost.

My 0.002 cents,

Frank