Mycorrhizal Fungi: The Amazing Underground Secret to a Better Garden

Nurture the ancient, symbiotic relationship between mycorrhizal fungi and plants’ roots for increased garden harvests and healthier soil.

| August/September 2014

Fungal Network and Hyphae Illustration

A white fungal network called hyphae, not plant roots, is the principal structure for the uptake of many important nutrients in the plant kingdom.

Illustration by Michael Rothman

Throughout history, people’s explanations of life involved all kinds of wonderful stories and complex philosophies. Facts just weren’t really in the mix. That began to change with the rise of science. Scientific momentum picked up sharply during the 16th and 17th centuries. As scholars scrambled to collect and categorize exotic beasts and botanical wonders, they dreamed of piecing together a full portrait of nature. Then, eyeglass lens-makers in the Netherlands assembled the first high-powered microscopes, and scientists looked closer at a few items that were right in front of them: soil, old bread and drops of muddy water. The world they had been trying to make sense of for so long suddenly seemed ten, a hundred, a thousand times more intricate, strange and beautiful — alive in more ways than anybody could have ever imagined.

The Microscopic World of Beneficial Microbes

We still define natural habitats primarily in terms of plants and animals, the two kingdoms of life we can see with unaided eyes. The greatest amount of biological activity and the largest diversity of species and genes, however, come from the other four kingdoms science now recognizes: bacteria, archaea (a less-studied division of life-forms formerly considered bacteria), protists (mostly single-celled algae and protozoans), and fungi. The vast majority of these members are microscopic in size. They cannot be seen with the naked eye, but we now know they permeate soils and suffuse waters. They drift en masse through air. They thrive not only on the surface of every plant and animal, but within them as well. From the upper reaches of the atmosphere to the bottom of the seas, down into the rock layers and outnumbering the stars in the known universe, microbes are literally the creatures that make Earth a living planet.

Microbes remain mostly in the “out of sight, out of mind” category of nature for a lot of folks. Others, chemical spray in hand, can hardly stop thinking about them, envisioning “germs,” mold spores and other unseen swarmers poised to unleash disease and rot. Either way, a broader understanding of the life-forms that truly put the “bio” in “biosphere” has been slow to emerge. Interest is building, though, as the public learns more about the positive roles microorganisms play, including how some types can boost yields in gardens. These mycorrhizae — extraordinary fungi that interact with our garden crops — are what we’ll be zooming in on.

Behind the Scenes With Beneficial Fungi

I’m a wildlife biologist. Decades ago, I visited a team working to restore streamsides churned to bare gravel by placer mining. They were planting willow and alder in hopes of stabilizing the banks and preventing further erosion. Other vegetation could then move in and once again shade the passing waters, cooling them for native trout and spawning salmon. I was already picturing songbirds returning to nest in the lush foliage while mink, otters, and bears patrolled the shores, except the normally hardy willow and alder wouldn’t grow. They withered instead, and the banks stayed empty — until the team prepared the next batch to be planted by first soaking their roots in a broth containing certain fungi. This is common practice today. It wasn’t then. Besides changing the way I’ve planted trees at home ever since, the visit made me realize that my view of the most important wildlife in ecosystems might be upside-down. 

What is called a mushroom is merely the temporary structure some fungi grow to produce spores. The main body of a fungus typically consists of a network of fine-branching threads known as “hyphae.” While you’ll sometimes see them massed together, spread like a web across a decomposing log, they’re usually hidden underground and essentially invisible to us; the individual filaments are only a single cell wide. The network of fungal hyphae is called a “mycelium.” As it turns out, the largest known creature on Earth is neither a blue whale nor a redwood tree; it’s the several-hundred-ton mycelium of one humongous fungus that’s between 2,000 and 8,000 years old. Spread across 4 square miles of Oregon’s Blue Mountains, the fungal network grows at an average depth of only a few feet. By contrast, the mycelia of most species are small, but they’re as common as, well, dirt. If you pick up a pinch of soil almost anywhere, you’ll have miles of hyphae in your hand.

Estimates for the number of fungi species run in the millions. Mycologists have identified close to 100,000 so far. Of those, nearly 6,000 interact with plants’ roots. These are roughly divided into two types: those in which the fungus remains outside the root’s cells (ectomycorrhizal fungi) and those that penetrate the root’s cells (endomycorrhizal fungi, illustrated in the Slideshow). The outcome in both cases is a continual exchange of goods. Ten to 20 percent of the sugars a plant produces through photosynthesis are absorbed by the mycorrhizae. In return, the fungus delivers many essential nutrients to the plant and increases drought resistance. Higher crop yields can be the result for gardeners. As the ends of the hyphae weave among soil particles via cracks and crannies too small for even the narrowest root hair, the mycelium becomes an auxiliary root system that’s in contact with a subterranean volume of soil from several hundred to 2,500 times greater than what the plant could reach alone.

Crowd at Seven Springs MOTHER EARTH NEWS FAIR


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