Fungi at work

Although we can’t see it, we live in a microbial soup. I just learned that there may be a Milky Way of viruses that floats on air currents around the globe and that trillions fall to earth every day. We know our guts are inhabited by billions of bacteria that we can’t get along without. A teaspoonful of soil contains up to a billion bacteria.

    Some of the most important players in this microscopic world are fungi. This summer has produced a particularly handsome crop of mushrooms and other fungal growths in woods and shady gardens in my area.  Walking woodland paths this month, I can’t resist stopping to snap photos of new, colorful fungal growths that seem to pop up almost every day.

Wet, rotting wood displaying a fungal bonanza

    The mushrooms I’m admiring are just the tip of an underground iceberg, the fruiting bodies of vast networks of thread-like fungal hyphae in the soil below. Compost couldn’t happen without the work of soil fungi. Bacteria, fungi, nematodes, insects and protozoa all work together to break down the organic material we throw on the compost pile into nice black compost.

Microscopic view of fungal cells

    I was amazed to learn a few years ago that experts can customize the balance of fungal and bacterial content in compost by choosing the right mix of ingredients. This is when they make compost on the scale of tons, moving giant mounds of material around with backhoes. 

At large-scale composting operations, piles are turned by machines

High-carbon, tough woody materials are what soil fungi like, so our yard’s endless supply of oak leaves predisposes the compost I make to have high fungal activity. I once had my compost tested for its biological content. The report came back showing excellent fungal presence.

    That’s just as well, because shrubs, trees and perennials—what my garden is mostly made up of—particularly like soil and compost with lots of fungi. By mulching with fall leaves and wood chips, I give the fungi even more to chew on.

Shredded leaf mulch

    Not only are these organisms hard at work in the soil. They actually create an underground architecture in the rhizosphere, the top few inches of soil where there’s the most oxygen and the most biological activity takes place. Roots and soil fungi have evolved to work together, exchanging nutrients. 

A stump in my yard is hosting this impressive mushroom bouquet

     This justifies the lazy woman’s approach to soil amendment: layering compost and good stuff such as composted manure on top of the soil and letting the soil organisms do the work of mixing them in. Tilling the soil, or even turning it with a spade, breaks up those underground networks that fungi have labored to create. That makes it harder, not easier, for plants’ roots to obtain the nutrients they need.

Let soil organisms do the digging

    As the beans and tomatoes in the vegetable garden finish producing and start to wind down, it’s almost time for me to play my part, spreading some compost on top of the bed. By spring, those fungi will have made the soil ready for another season’s planting. I appreciate their beautiful above-ground creations, and I’m even more grateful for the work they’re doing underground.

Soil needs plants

As I become more aware of the “underground herd” of soil organisms, I’m changing my perspective on soil, looking at it as a living system that interacts with plant communities. Two recent paradigm-shifting books shed light on this approach. They are The Living Landscape, by Rick Darke and Doug Tallamy, and Planting in a Post-Wild World, by Thomas Rainer and Claudia West.

    To my surprise, I learned from Rainer and West, “When soil is exposed to sunlight, rain and extreme temperature changes, it is damaged,” and its stored carbon is oxidized and released as carbon dioxide. “The longer a soil is exposed, the harder it is to vegetate later.” 

    Like many suburbanites, I’ve had the fond belief that unplanted areas would be fine for indefinite periods if they’re covered with mulch. 

I thought mulch was enough to protect soil.

Apparently this is better than leaving soil bare to the elements, but it’s not the best approach. 

    It seems that roots and plants’ “underground storage organs” (more on this somewhat risqué phrase in a future post) are crucial for building functioning soil. The channels dug by roots open up space for air and water. When roots decompose in fall and winter, their organic material stays behind as humus, storing carbon and nutrients in the soil. That’s good for plants and good for the climate, too.

Dense groundcover is healthier for soil

    Both author duos describe natural landscapes in terms of plant layers. Among the broad landscape types, my yard is closest to a woodland edge. In this kind of landscape there’s a canopy layer formed by trees, a woody understory of lower trees and shrubs, and an herbaceous layer of perennials and grasses, ranging down to the lowest plants that cover the ground. 
  
    This week I moved some bearded iris that were blocking our view of the fish pond and replaced some tired moss phlox (Phlox subulata) with five pots of bearberry (Arctostaphylos uva-ursi). Their spot is one of the most visible in the yard, within clear view of the kitchen windows. 

The fish pond in August

The bearberry plants join the groundcover layer, and I hope they’ll stay low and neat-looking. I had pulled out some moss phlox a few weeks ago and left the soil bare before covering it with bark mulch last week.

Bearberry and bare soil. I hope it will fill in next spring.

    Both sets of authors emphasize the benefits of a rich and diverse groundcover layer for providing habitat for insects and other small organisms. Native bearberry should help in this regard. I’ve certainly got lots growing at ground level. Over the years I’ve planted something in just about every inch of garden soil. 

    When I squeezed in those little plants I couldn’t resist bringing home, of course, it was to help the environment. As one of this year’s presidential candidates would say, “Believe me!”  And I’m willing to do even more shopping for the sake of the earth.

Banking on living soil

Have you been hearing recently about your gut’s microbiome? All of a sudden we’re recognizing the importance of microorganisms that live in our intestines for maintaining good digestion and general health.

    Similarly, we’ve recently focused on the importance of living things in soil. Instead of noticing just physical properties—sandy versus clay soil—and chemical aspects—acidity and concentrations of minerals—we’re zooming in on the rich community of organisms that live in and maintain soil. Without them, our plants wouldn’t be able to grow.
 

Plants depend on soil organisms to make water and nutrients available

    Bacteria, algae, fungi, nematodes, and larger animals such as earthworms and insects all help create and maintain healthy soil and build the pore spaces needed to hold water and air. Bob Schindelbeck of Cornell’s Department of Soil and Crop Science recently provided a useful overview of what these soil inhabitants are up to and how to encourage them in Fine Gardening. I loved his characterization of soil organisms as “your ‘underground herd.’” This is a new perspective, thinking of soil inhabitants as animals on our team that are worth fostering.

 

A large member of the herd

    Because the top few inches of the soil are full of active organisms breaking down organic material and digging tunnels, cooperating with plant roots to create a useful architecture, it’s better not to till soil or even turn it with a spade when you don’t have to. Instead, to add organic material you can just layer compost and manure on top of the soil and let the “herd” mix it in. 

An interpretive sign along a nature walk tells it all

   
To help them do their job, it’s good to avoid compacting garden soil by walking on it, which can collapse the pore spaces, especially when the ground is wet. I have to admit that I haven’t put enough paths or stepping stones in my garden, wanting every inch for plants. I’m going to work on that in future.

More paths would keep me from damaging soil structure

    Another living component of soil that I didn’t know about until recently is the “seed bank,” the seeds lying in the soil awaiting the right conditions for germination. Although some will germinate as soon as there's enough moisture, others stay dormant for many years.

Lotus seeds can stay viable in pond soil for 1200 years

When I used to turn my vegetable garden’s soil every spring, I was bringing those stored seeds to the surface.

Goldenrod seeds were waiting in the soil for a good growing opportunity

    I’m still seeing the results of a deposit I made to the seed bank. About ten years ago, I planted tall verbena (Verbena bonariensis), which produces a vast harvest of seeds. A few of them sprout every spring, fewer now that I’ve stopped turning the soil.

Tall verbena seeds last several years in the ground

    Another withdrawal from the seed bank is happening without my intervention. For the past few years I’ve seen heart-leaved aster (Symphotrichum cordifolium) sprouting all around the yard, even more this year. 

Dormant seeds of heart-leaved aster have sprouted around the garden

This is a species that grows in dry, sandy, and compacted soil. It’s out-competing other perennials that are more sensitive to drought and my heavy feet. Bees are foraging among the delicate flowers, so I’m glad this native aster banked its seeds.