A fair exchange

An excellent summary by Anne Bikle in Fine Gardening reminded me that there’s no free lunch in the rhizosphere, the top few inches of soil where most of the biological activity happens. True, lots of soil organisms are at work breaking down organic matter into forms that our plants’ roots can use. 

Electron micrograph of soil microbes-photo Pacific Northwest National Lab

But that’s not a coincidence. The roots offer something in return. Every plant exudes proteins, carbohydrates and fats from its roots. These attract the organisms that help plants get what they need and protect them from diseases and pests.

 

Fungal network associated with spruce roots-photo André-Ph. D. Picard

 
    Bikle’s article was a good reminder to support this process by taking good care of soil organisms in the garden. I checked myself against her three recommendations: leaving soil undisturbed when possible, mulching to add organic material to the system, and growing a variety of plants.

    A few years ago I gave up turning the soil in the vegetable bed with my spade before planting in spring. I’d thought this was a necessary step to mix in amendments such as compost and composted manure and turn under any weeds that had sprouted.

     I stopped all this digging when I learned it was counterproductive. I was breaking up soil networks, killing or slowing down soil organisms that were nourishing my plants. I was also churning through organic material by introducing a rush of oxygen into the soil, wasting the compost I added to the bed. 

     I found out it was better to let soil organisms do their work undisturbed. Now I confine the digging to times when I need a planting hole for a seedling such as a young tomato plant. The soil in the vegetable bed has improved. As a side benefit, I’ve got fewer weeds, because when I cut out the digging, I stopped bringing weed seeds to the surface to germinate.

I avoid digging except to plant seedlings

    I’m also making a point of not clearing away leaves that fall on the ground, except on the lawn. I’m still working on striking a balance for fall leaves. I used to chop them up for leaf mulch, until I learned this also chopped up useful insects at various stages of development that were settling into the leaf litter for the winter. For the past couple of years, I’ve mostly let the leaves lie on beds, supplemented by more leaves I drag in from the street that would otherwise go to the city’s composting site. 

 

Letting fall leaves lie

     

     Now I’m missing my leaf shreds. There are places where they’d be especially useful, such as in the newest perennial beds, where the soil could use some quick help. The leaf shreds stay put, not blowing around like whole leaves, and they decompose faster. This fall I think I’ll do some limited shredding to cover those spots.

 
     From Bikle I learned that I’m offering a diverse buffet of root exudates by increasing variety in the garden as I’ve added native plants. That’s because each plant sends out its own recipe to attract organisms to meet its needs. All the better.

 

A mix of native plants: wild ginger, leucothoe, heuchera

 

Working together

Come visit my table at Celebrate Newton, a holiday craft fair, this Sunday December 2 from 10 to 4.

I realize that I’m prone to thinking of the plant world in terms of competition. I suppose this is an over-simplification of Darwinian thought: we picture the garden as a battle for resources where the fittest survive. Experience bears this out when we see aggressive plants-- nonnative invasives but also native plants and imports that happen to be in their best growing environment--crowding out plants that are less well-suited for a particular niche in time and space.

Sweet woodruff ruling the shade garden

    As I learn more about the science of soil and plant communities, though, I find that there’s a lot of cooperation in addition to the competition I tend to notice. The first hint came in junior high, when we were taught the concept of symbiosis through the example of lichens, which, we were told, are a cooperation between fungi and algae. As lichenologist Trevor Goward says, “Lichens are fungi that have discovered agriculture.” The algae provide the fungi with energy through their photosynthesis, and the fungi provide protective structure. It turns out there’s a third partner in this cooperative effort, recently discovered. It’s a kind of yeast.

Lichens are a cooperative effort-photo Alex Proimos

    But there’s more. In The Hidden Life of Trees, forester Peter Wohlleben describes trees as social beings that share nutrients and warn each other of impending danger through their roots. Teaming with Microbes, a fascinating and useful book by Jeff Lowenfels and Wayne Lewis, describes the soil food web, the network of soil organisms that work together to provide plants’ roots with needed nutrients. Each soil organism has something to gain in the complicated transactions going on in the top few inches of soil.

Soil microbes-photo courtesy of Pacific Northwest National Lab

    Fungi called mycorrhizae grow in symbiosis with roots, providing water and nutrients in exchange for carbohydrates made by leaves. Free-living fungi, bacteria, nematodes, protozoa, small insects, and earthworms all seek food for themselves and, as a byproduct of their labor, provide food for others in the network. Each organism has evolved to play its part. It’s interesting that evolution has favored cooperation in this system, not just competitive success.

Fungi play an important role in decomposition

    Above ground there are similar processes at work. When we plant perennials, we tend to think they need plenty of space so they won’t have to compete for water, sunshine, and nutrients as they get established. 

Perennials spaced out for traditional planting

A newer, more natural way of planting recommends closer spacing of different kinds of perennials to form plant communities.

    Some perennials grow upright and tall, others make wide rosettes of leaves or hug the ground and weave their stems between taller growers. Some have taproots that grow straight down, many others create a mat of slender root fibers. Some take off fast in newly open ground, others grow more slowly but persist longer. 

Perennials with different growth habits can grow close together

By working around each other in space and time, the plants get what they need and also help each other by providing shade, edging out intruders, and holding moisture in the soil. In community there is strength.

Fungus among us

Thank goodness some rain fell this fall, although we’re still in a drought. Apparently mushrooms were biding their time, waiting for a bit of moisture before making their move. Once it rained, they popped up around the yard.

    These mushrooms are the fruiting structures of a group of fungi called basidiomycetes. The mushrooms are small parts of these organisms, which include extensive underground networks of hyphae, slender filaments that take up water and carbohydrates. A single individual fungus can send up fruiting bodies over an area as big as a baseball diamond.

    Invisible to me, even after welcome rain, are two huge groups, the saprophytic and mycorrhizal fungi. Saprophytic fungi are decomposers, breaking down dead organic material, including wood, into nutrients plants can use. 

Mycorrhizal fungi join in a symbiotic relationship with plant roots. Thinner and more extensive than root hairs, their hyphae seek out water and nutrients from far and wide. They share these with plants in return for simple sugars.

Fungal hyphae

    Some mycorrhizal fungi excrete a sticky polysaccharide called glomalin that glues soil particles together in small clumps called macroaggregates. These clumps improve growing conditions for roots by creating pore spaces in the soil for water and air. As if that weren’t enough, fungi in the soil also sequester carbon, attack plant pathogens, and cultivate helpful bacteria.

    Luckily the best way to foster these useful partners is to stay out of their way. If I tilled my soil with a rototiller, I’d be breaking up their networks. If I sprinkled chemical fertilizer, I’d turn off their nutrient-collecting efforts. Fortunately there’s no call to do either in my garden. If I need to plant something, I get along fine by digging an appropriately-sized hole. Compost and mulch are providing enough nutrition for my plants.

Offering a home for soil fungi

    It turns out that by mulching with fall leaves and wood chips, I was unknowingly creating a happy environment for soil fungi. I learned about this when I researched the reasons for balancing types of ingredients in the compost pile. Before this I’d thought compost recipes were unnecessarily fussy.

    Compost professionals can titrate ingredients and conditions in their compost piles to determine the proportion of bacteria and fungi in the finished product. By managing the balance of “brown” high-carbon and “green” high-nitrogen compost ingredients, they can cause bacteria or fungi to predominate. Bacteria thrive on simple sugars from green plant material, and fungi prefer high-carbon woody material. 

    Compost high in fungi is best for use around trees, shrubs and perennials, whereas annuals and vegetables prefer more bacteria in the mix. This is because of what soil organisms do with nitrogen from the air. Fungi convert atmospheric nitrogen to ammonium that long-lived plants prefer, whereas bacteria make nitrate that fast-growing short-lived plants particularly need.

    Making compost by recipe probably isn’t in my future, but it’s nice to know that my high-carbon mulches are friendly to soil fungi.