Life Down Under: Microbes in Your Soil

—by Roxanne Bergeron

I have a problem with death.

Last April the queen wasp I had knocked down with a blast from my hose and had under my heel could sense my weakness. Undeterred after repeated hits with that jet force of water, she kept crawling up out of the wet grass, sodden and helpless yet exhibiting a heartbreaking tenacity. I finally threw down my dreadful hose in defeat, abandoning my intention to eradicate wasps from my garden this year. When I had the chance, I simply could not take her out.

In a fit of cognitive dissonance, I read up on wasps to remind myself how they are actually quite beneficial, the way they eat aphids and such, and how big things dine on smaller things, and those smaller things eat even smaller things. Beneath the skin of the soil, multitudes of infinitesimally small beings play out their own unseen and unseeable sagas of life and death, just as we surface beings are entangled in dramas of survival on the flip side of their underworld.

Life underground

Ninety percent of all organisms on the seven continents live underground,1 with anywhere from 10 to 50 thousand different species hanging out together in a single teaspoon of the soil that supports and nourishes my floral brethren. Bacteria, archaeons, fungi, algae, protozoa, nematodes, arthropods, earthworms, and gastropods live collectively in and on the ground.2,3 All those tiny life forms join forces by developing what are called “soil food webs”— with plants driving the dramas.

Complex aggregations of interdependent microscopic lifestyles and foodways are critical to the health of plants. You see, plants cannot eat without microbes working the soil,4 and the microbes depend on the plants to survive.

Plant meets soil

Much of the drama takes place in the rhizosphere—that area where a plant’s roots meet the dirt. The plant uses sun for energy and more or less sweats out an exudate of carbohydrates and protein which arouses and attracts certain bacteria and fungi. The bacteria, one of a host of single-cell life forms, act as primary decomposers, soaking up food molecules such as carbon, sulfur, and nitrogen with the help of tiny protein waiters in their cell walls. These nutritional tidbits get locked inside, or “immobilized.” When the bacteria are subsequently eaten or die and decay, these bits of nutrition are released, or “mineralized,” and the plant can eat.

Fungi are not limited to the tasty mushrooms dotting your pasta sauce. These fragile tubular beings grow by virtue of making hyphae cells, which are interconnected and can be yards long, coiled among the granules in a teaspoon of good soil. They, too, lock up nutrients that are released when they die or are eaten. Fungi develop helpful symbiotic relationships with the plant root called “mycorrhizae” and act as food transports for the plant. It’s a practice of sharing and communal living that began some 450 million years ago. Some mycorrhizal fungi make webs around roots and some burrow and grow inside as well as outside.

Archaeons and algae

Archaeons are a recent discovery, whose existence as distinct life forms has been teased out of the bacterial cadre through the magic of 21st-century genetic analysis. These microscopic players in the food soil web fix, or make available to the atmosphere, not only nitrogen but methane—everyone’s favorite greenhouse gas component. Archaeons are “extremophiles,” living in places no self-respecting bacteria would be, such as hot springs, volcanoes, and two miles under the ice. They are that mysterious riverboat gambler puffing on a stogie, telling stories about life in the dead zone at the mouth of the Mississippi.

Terrestrial algae are single-cell varieties living on or near the soil surface. A teaspoon of good garden soil can have from ten thousand to 100 thousand cells of diatoms and green and green- yellow algae. They make tasty salad for nematodes, tiny, blind microscopic roundworms about two millimeters long that do that mineralizing trick when they eat bacteria, fungi, and algae.

Nematodes and other animals

Nematodes constitute the second most dominant form of animal life, next to arthropods. A teaspoon of garden soil has about twenty soil nematodes that eat bacteria, twenty more that will eat fungi, and about ten that are nasty and predatory. Nematodes eat other nematodes (yummy!), which helps with population control and balance.

Protozoa are yet another kind of single-cell creatures. They mostly live in moist soil and can number in the thousands in that ever-growing menagerie in a teaspoon of soil. They eat mostly bacteria along with the occasional fungus and other protozoa.

Enter the ubiquitous arthropods! Insects and bugs—spiders, flies, beetles, mites, butterflies, bees, wasps, ants, dragonflies, and some blood-sucking varieties that we’ll not name here—shred their food and aerate the soil, to the benefit of other web members.
Burrowing up and down and from side to side are earthworms, the favorite life form of many a gardener. These night crawlers and red wigglers shred, aerate, and aggregate soil particles and move matter and microbes through the soil. Their actions help increase microbial populations. They mostly eat bacteria, along with fungi, nematodes, protozoa, and the surrounding organic matter these tiny soil web dwellers call home.

Slugs and snails, collectively known as gastropods, slime along eating fungi, algae, lichens, and rotting organic matter. Their shredding opens up that organic material, making it easier for fungi and bacteria to have at it. Their passages below the surface create pathways for air, water, and roots, and their slimy essence helps hold soil particles together. They are food for birds, snakes, lizards, spiders, and, yes, even nematodes, which are very clever when it comes to consuming things way bigger than they are.

A balanced web

With balanced, rich soil food webs in place, there are edibles enough to satisfy all the birds, squirrels, mice, groundhogs, rabbits, lizards, snakes, and the other reptiles and mammals attracted to a vigorous garden plot. They contribute their dung as food, transport microbes around on their feet, and are food for all takers in death.

We inhabit a planet where something must have died in order for something else to have lived. I take this law as seriously as the
mother wasp with which I am bonded in our mutual struggle to survive. We are in step with one another, a pirouette of connection engaged in the common mission of all living things—to live.

One afternoon I watched as one of the mother wasp’s many progeny engaged in a turf war with a Monarch butterfly on the petal landing field of a giant zinnia. In a small garden bed a few feet away, a score of sparrows were pecking away at the seeds and bugs and picking up bits of mulch straw.

Their presence at Roxy’s Back Porch Cafe confirms the vigor of my soil food webs. If they have bugs to eat, the bugs have food enough due to the diverse existence of tinier, then tinier, then even tinier creatures engaged in their various underworld enterprises. I was enchanted to see them so chirpy and busy. So was my cat.

Article References

  3. Teaming With Microbes: the Organic Gardener’s Guide to the Soil Food Web by Jeff Lowenfels and Wayne Lewis. Timber Press, 2010.

Additional References

  1. A basic table is part of this reference and shows who’s who, who eats what, and how they are naughty or nice to soil food web systems: .
  2. “Soil Biology Primer,” from the United States Department of Agriculture: .

[Roxanne Bergeron is a volunteer cashier at HPC. She contributes regularly to the newsletter.]