[Homestead] Soil 101, microbes
genegerue at ruralize.com
Mon Sep 13 13:13:25 EDT 2004
To feed plants, we feed the soil. The soil community then converts the food
we give it into a form the tiny plant root hairs can assimilate. To keep
the process going, just keep feeding groceries to the gang underground.
Lots of mulch does the trick.
There are sound reasons why informed gardeners do not plow or till, why
they create and maintain garden paths, and shun chemical fertilizers. The
living soil is complex and well-ordered; everything is in its optimal
place, and to till it is to turn things inside out, if not upside down.
Imagine Zeus tilling a human subdivision with a hundred-foot RotoCruncher
and you get the idea. Give the soil kids a break and sell the tiller.
The Secret Life of Microbes
By Barbara Damrosch
Special to The Washington Post
Thursday, July 8, 2004; Page H07
When I was in college, I marched into the woods one day to do a biology
project on life in the soil. Brushing aside dead leaves and the leaf mold
under them, I dug happily, encountering worms, ants, grubs, beetles --
sleeping pupae of insects that were airborne in summer.
Alas, much of that leaf layer was poison ivy. My professor hastily rerouted
me into work on goldfish parasites in the campus pond. But had I stayed on
track, I could have explored a world far more complex than what had first
met my eyes.
The top six inches of the soil teem with microscopic organisms -- bacteria,
fungi acetomycetes, algae, single-celled protozoa such as amoebas. Larger,
but still unseen, are tiny animals such as nematodes and mites. The numbers
are staggering -- millions of organisms per gram of soil, where conditions
favor them. Their total weight, in a fertile acre of land, is as great as
the total weight of the above-ground creatures that acre can support.
Counting soil microbes might seem like an odd corner of knowledge, a bit
like those theological debates about how many angels can dance on the head
of a pin. But these unseen workers quite literally make life on Earth
possible. Of all the species, these -- the tiniest -- are the least
expendable. Without them no plants would grow. Without plants no animals
Plants feed on organic wastes -- decomposed plant and animal matter -- but
not directly. Soil microbes must break them down into compounds that plant
roots can absorb, just as intestinal microbes break down our food for us.
The end result of decomposition in the soil is humus, which has other
virtues besides plant nutrition. It gives the soil good structure and
tilth, with plenty of air spaces between soil particles, so that plant
roots can breathe, and a spongy texture that allows the soil to retain
moisture and resist erosion. It helps moderate the temperature shifts that
make up the soil's "weather." Certain soil fungi produce a gummy substance
called glomalin that causes soil particles to clump up and produce the good
crumb texture that gardeners prize.
In the wild, plant residues decay on the soil surface through the action of
bacteria and fungi, and their remains are gradually worked down into the
soil with the help of worms and other burrowers. Worms and plant roots also
bring up minerals from below, so that a lot of trafficking in materials
goes on. As long as the materials are there, microbial workers will show up
to process them, so that the cycle of growth and decay can continue.
It's very busy down there. Entire civilizations are doing business,
cooperating, battling, building structures, even herding one another. These
populations are not only numerous but also varied, doing work under
specific conditions and in different soil strata. In your compost pile,
which seethes with life, heat-seeking bacteria work the center, while
others perform best at the cooler edges. In alkaline soils you'll find
mostly bacteria; in acid soils the fungi predominate.
If all these workers are so important, how can you make sure they're there
-- and active -- in your garden? How do you become the guardian of a living
First, it is very important to make sure that it contains plenty of organic
matter in the form of compost, manure and other amendments. This will
activate the microbes that are already there and cause them to multiply.
Then try hard not to disrupt the delicate structure beneath your feet.
Stick to the paths and avoid walking on your beds. Dig and till as little
as possible. Keep things growing. The principle "use it or lose it" applies
to garden soil, because the absence of plants and their roots -- where
microbes congregate -- means less soil life. Use cover crops, green manures
I avoid chemical fertilizers because I fear these can kill soil organisms
of any size. They may deliver the final product in a crude sense, but they
bypass the process. You can deliver a molecule of nitrogen to a plant
without microbial aid, but it will do nothing for the soil's texture and
its resilience. You will have to keep your plants on life support, because
a lifeless soil is not able to replenish what is used up.
Don't treat unseen microbes as hidden dangers. Though a few can be harmful
to plants, animals and each other, poisoning them does more permanent harm
to the helpful organisms that hold them in check. There is never anything
to be gained from making your soil a sterile place.
Don't fear what seems dark, unknown or dirty. The sky may seem more bright
and beautiful -- a perfect metaphor for infinite life, but the further you
climb into it the less life you will find. The Earth's fragile skin is
where the action is. If we keep it hospitable to life, it will support an
infinity of helpers, all of them dancing.
© 2004 The Washington Post Company
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