Mycorrhizae--What Is It?


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May 25, 2005
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Bill Robinson

Mycorrhizae are symbiotic soil fungi, present in most soils, that attach themselves directly onto the roots of most plants. They help the host plants absorb more water and nutrients while the host plants provide food for the fungi. Because the surface area of the hyphae, the feeding structures of the mycorrhizae, may be several hundred times the surface area of the roots, the mycorrhizae can feed on a larger soil mass than the roots and they do so more thoroughly. Mycorrhizae have the potential to be a central nutritional source for the host plant. Mycorrhiza also seem to cause the host to increase the production of certain metabolites but those studies are incomplete.

One study compared the fresh weight of a group of different roses whose roots had been inoculated with two different mycorrhizae against the weight of the same varieties grown without mycorrhizae. The inoculation was done by dusting the rose roots with spores of the mycorrhizae at the time of planting. The mycorrhizae employed were Glomus deserticola and G. intraradices. After several months the roses were dug up, washed to remove any foreign material, and then weighed roots and all. The inoculated roses showed significant increases in weight in every instance. Some of the roses did better with G. deserticola while others did better with G. intraradices. In one instance a miniature rose showed a 75% increase in fresh weight with both of the test mycorrhizae.

If the conditions are right, a viable mycorrrhizae colony can help the plant become more vigorous, more salt tolerant, less fertilizer dependent and more tolerant of chronic low moisture - but the key word is, "if" and there are qualifiers. First, the mycorrhizae are relatively fragile. They thrive when the organic content of the soil is high but languish when the organic content is low. Mycorrhizae also resent having the soil disturbed. Rather than digging in organic material and disturbing the soil, it is best to simply put down a 2" layer of compost in the winter and another 2" layer in the summer. The mycorrhizae sends hyphae to this rich organic layer as a prime nutritional source.

Mycorrhizae are destroyed when the soil is fumigated, sterilized, solarized or drenched with most of the pesticides used to control nematodes, grubs, or weeds. They are also sensitive to high levels of chemical fertilizer. There is a difference of opinion as to the mechanics. Some research workers say that excessive chemical fertilizer burns the hyphae just as it burns roots and the performance of the mycorrhizae falls off because they have been injured. Others research workers say that the high levels of chemical fertilizer, especially phosphorus, cause the plant roots to shift feeding patterns away from the mycorrhizae. There are many unresolved issues about the way mycorrhizae take up mineral nutrients and particularly phosphorus. There is, however, general agreement that avoiding high levels of chemical fertilizer, particularly phosphorus, promotes a more vigorous feeding through the mycorrhizae. The best management of mycorrhizae is to simply use composts with added composted manures to feed the plants while, at the same time, feeding the mycorrhizae.

Note, because mycorrhizae are fungi, they are sensitive to fungicides. There are a few scattered reports showing adverse effects on the mycorrhizae as a result of the use of garden pesticides but the studies seemed incomplete.

Finally, mycorrhizae are relatively plant specific. A mycorrhiza that gives a good response with one rose may give only a fair response with another. One way around this is to inoculate the rose at the time of planting with a "cocktail" containing a number of different mycorrhizae. A small amount of the mix is simply applied directly onto the root of the plant host. In time the various mycorrhizae will battle it out to establish the dominate colony. Commercial inoculum mixes containing a variety of viable spores are starting to become available.

Mycorrhizae may be of particular value here in Southern California where our tap water is alkaline. Iron, one of the essential nutrients, reacts with certain organic components in the soil to form complex chemical structures. The roots can not absorb the iron from these structures when the conditions are alkaline. In essence, the iron is "locked up" within the chemical structure and is no longer available to the root. The mycorrhizae, however, seem to be able absorb this "locked up" iron. Studies on the way mycorrhizae are able to do this are on-going. BioOrganics Supply Center in Camarillo sells a "cocktail" containing spores of Glomus intraradices, G. deserticola, G. etunicatum, G. clarum, and G. mosseae. This seems a good mix for my garden which contains a collection of various Old Garden Roses, English Roses, a few modern roses, a variety of perennials, shrubs, and ground covers. I am adding some additional Old Garden Roses and English Roses. My plan is to dust the roots of these new roses with this "cocktail" and to allow the various mycorrhizae to spread through the garden on their own. I have high hopes for a way around my problems with my alkaline tap water and the iron nutrients.

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