You must have seen the ads. If you've
opened a gardening catalog recently, or clicked through a gardening
Web site, you can hardly have missed the pitches for mycorrhizal
inoculants. "Benefits more than 99 percent of Earth's plants!" "Increases
absorptive surfaces of root systems up to 700 percent!" And
my personal favorite: "Depresses many root diseases, pathogenic
fungi, and pathogenic nematodes!" Just sprinkle the grayish
powders on your soil like fairy dust, and your plants become supercharged.
They bulk up, or so the ads claim, growing visibly healthier and
more vigorous, bearing brighter flowers and more savory fruits.
Inoculated onions taste sweet -— and after harvesting, the bulbs
resist spoilage. Or at least that's what the ads claim.
"Snake oil," you mutter. In fact the new mycorrhizal
inoculants are based on sound science. They are a mixture of naturally
occurring soil-dwelling fungi that partner with plant roots, enhancing
the plants' ability to draw nutrients, minerals, and water from
the surrounding earth. This in turn makes plants better able to
cope with drought, while also enabling them to flourish even in
poor soils. In the latter case, the fungi in the inoculants attack
the cause as well as the symptoms, releasing an organic glue that
causes soil particles to clump together into crumbs, giving the
earth a lighter, looser, and more porous texture.
The ultimate testimonial comes from nature itself: some plants
cannot do without mycorrhizal fungi. According to Dr. Carolyn Scagel
of the USDA Agricultural Research service in Corvallis, Oregon,
mycorrhizal fungi are found in virtually all undisturbed soils.
If there are plants -— trees, shrubs, flowers, or grasses -— growing
there as well.
Then why buy something that nature furnishes for free? Because
the soil in your garden is almost certainly not undisturbed. Our
gardening tradition, after all, revolves around soil disturbance.
My more conscientious friends boast of their double-digging. Even
I routinely turn the vegetable and annual beds a spade-length deep
in spring and fall. In between, I pull and hoe to excise weeds.
Any massive disturbance of the soil can rupture the mycorrhizal
fungi's webs of white, thread-like tentacles. Stripping a site
of topsoil, a common preliminary to the construction of new houses,
also strips the lot of mycorrhizal fungi.
It's not just physical disturbance of the soil, though, that discourages
mycorrhizal growth in gardens. Depletion of soil's organic content
-— the normal fate of gardens nourished only with synthetic fertilizers
-— is fatal to mycorrhizal fungi. Heavy applications of fertilizers,
especially synthetic products rich in phosphates (phosphate content
is indexed by the second number on the fertilizer label; for example,
5-10-5), also hamper mycorrhizal growth. The miracle organism that
lets your plants do more with less does not flourish in a situation
overabundance.
Other sins of insensitive cultivation likewise take a toll. A
poorly calibrated sprinkler system that regularly waterlogs the
soil may drown mycorrhizal fungi. Scagel warns, too, that a number
of common garden fungicides (Benomyl and Captan, for example) attack
not only harmful fungi such as the black spot afflicting your roses
or the brown patch blighting your lawn, but may also cripple the
mycorrhizal fungi in the soil below.
It should not be surprising, then, that the inoculants function
best in situations of the greatest neglect. They have produced
dramatic results in the revegetation of lands devastated by strip
mining, enabling plants to grow on soils that otherwise would remain
sterile. So, too, have they become a standard tool in landscaping
along new highways.
The inoculants' success in the garden, though, has been wildly
unpredictable. Some researchers have reported rapid increase in
fine root development after exposing trees to mycorrhizal inoculants,
while others have seen no measurable response at all. There has
been tantalizing testimony from abroad: the Royal Botanic Gardens
at Kew have made mycorrhizal inoculation of the surrounding soil
a standard part of their program to revitalize declining trees,
and claim to have brought a number of historic specimens back from
the brink of death in this way. The New York Botanical Garden is
also using the inoculants for tree revitalization, though because
it is injecting them into the soil in combination with fertilizer,
it cannot say whether it is the fungi that are responsible for
the trees' subsequent revival.
A clue to the future importance of mycorrhizal inoculants in the
garden can be found, I believe, in a recent British study. It tested
the effect of the inoculants on a variety of different soil types
and found, not surprisingly, that the greatest impact was made
on those soils that were of poor quality. Applying a liquid mycorrhizal
glue to the roots of birch seedlings before planting reduced transplant
loss from 60 to 3 percent.
We will need that kind of assistance, I believe, in the years
to come. Gardening, thanks in part to the rising cost of resources
and labor, is already moving away from the unthinking extravagance
of the past. Over the last couple of decades, an emphasis has arisen
on giving our landscapes only the nutrition and water they need,
rather than all that our spreaders and hoses can deliver. We are
coming to understand that bulimia is not better for our plants
than it is for us. In light of this, how can we fail to appreciate
the partnership that enables a garden to do so much more with so
much less? Our plants have relied on mycorrhizal fungi for more
than 400 million years. Someday soon, I am sure, we will too. |
