The Dirt on Soil

May 16, 2022

What comes to mind when you think of healthy soil? Do you envision a dark and loose loam full of nutrients, organic matter and earthworms – something like this ↓ ?

If so, you’re not alone. I’d guess most of us have the same idea. Here’s the plot twist, though: when we’re working with native plants, that image is all wrong.

Big Cabbages vs. Sustainable Plant Communities

All this time, we’ve been following a script laid out for us by the horticulture industry. And, as landscaper Noel Kingsbury puts it, the industry’s focus is on “growing big cabbages.” They take a one-size-fits-all -- or perhaps a cabbage-size-fits-all -- approach to soil and its preparation. This means tilling until the texture is soft and fluffy; adding compost, manure, topsoil and/or fertilizer until the color is dark and the nutrient and organic-matter counts are high; and adding lime until the pH will make nutrients more available to plants.

We’ve also internalized the idea that earthworms in our garden are an indicator of healthy soil. We’re told they “improve” soil structure, water movement, nutrient cycling and plant growth, and that all of those things are desirable. There are whole horticultural and agricultural sub-industries dedicated to selling worms for composting and aeration. Also, there’s that saying about the rewards of being the early bird….

All the tilling, amending, liming and worming (is that a word?) will, indeed, yield big cabbages. They may also give your non-native annuals and perennials a boost.

Native plants, though, do best and reproduce most prolifically when they’re in communities of plants, animals and minerals that have co-evolved together or are otherwise complementary. They're about specialized conditions, not one-size-fits-all. In Planting in a Post-Wild World, landscapers Thomas Rainer and Claudia West say, “Plants have evolved to soils with particular textures and chemistry. They don’t want a generic, ideal soil; they want specific soil.” So the gardener's goal in site preparation should be to preserve the local soil’s unique qualities, not to override them. I suppose we should give nature credit for knowing what it’s doing every now and then?

Where I live, the soil is full of heavy, sticky red clay that looks like this ↓ :

Pretty sure I could make a terra cotta pinch pot out of that, no problem.

The old me was kind of put out by all this clay, considering it an unfortunate mid-Atlantic problem needing to be fixed. I’m just now beginning to understand it’s not a bug, it’s a feature. There are thousands of plants native to the Piedmont ecoregion that think red clay is just…well, the bees’ knees. Their roots know how to push through its clumps, they appreciate the way it holds onto water, and they thrive on the specific nutrients it provides. You could say they’re red clay specialists.

Sure, if you plopped our Maryland state flower, the native black-eyed susan (Rudbeckia fulgida) into a bed of loose and resource-rich black soil, you’d probably get an initial flush of growth. But over the course of a season or multiple years, it would struggle to compete with other species that can capitalize even more efficiently on those resources. Plop it in familiar lean, clumpy clay, though, and that black-eyed susan may not get as showy, but it will have a competitive advantage on its “home turf.” It will cooperate nicely with other native species in its community, go head-to-head with any unwanted weeds and invasives, and survive to disperse its seeds. In short, it will become a constructive member of a sustainable plant community.

"Rudbeckia fulgida 'Goldsturm'" by ptc24 is marked with CC BY 2.0.

Some additional nuances:

What’s wrong with tilling? Untouched soil is typically full of ungerminated weed seeds, quietly biding their time. Once we start tilling, we’re giving those seeds engraved invitations to come to the surface and sprout. Turning the soil also disrupts whatever roots were stabilizing the soil surface, allowing erosion, and it upsets delicately balanced soil ecosystems by destroying beneficial organisms. Best to disturb the soil as little as possible.

What’s wrong with amendments? The problem with chemical fertilizers, compost and manure is that weeds and invasives – which are inherently opportunistic – are better than most native species at taking advantage of those added resources. So you’re basically just feeding the weeds. Also, rich, fertile soils support fewer species, while lean soils support more diversity. High-resource conditions somehow set up a winner-take-all situation, with the victor often being something unwanted. (Any soil scientists out there, please weigh in on how this works; it's not 100% clear to me.) There’s an added concern that any compost, manure or topsoil you import may include seeds and roots from invasive plants.

What’s wrong with adding lime? Those of us who live in areas with acidic soils (e.g., most of the Eastern U.S.) are frequently instructed to raise the soil pH to make beneficial minerals more accessible. But doing so is more helpful to weeds than to native plants. Our resilient indigenous species figured out long ago how to get the nutrients they need from our sour soils.

What’s wrong with worms? Most of the worms we find in our yards hitchhiked here from Europe or Asia. Here in North America, as National Geographic describes it, their “relentless tunneling, feeding and pooping changes the physical, chemical, and biological properties of the soil, reshuffling the ecological deck in ways that native species are not always able to cope with.” Where worms are plentiful, they reduce the diversity of plants and insects. This is most evident in forests. If you already have worms, there’s not much you can do about it. But for purposes of native-plant gardening or protecting forests, we should definitely avoid adding them.

Again, if you’re growing crops, a turfgrass lawn, or other non-native annuals or perennials, then tilling, amending and liming are probably advisable, and worms are perhaps welcome. Those sods, tomatoes and petunias were not bred to thrive in your local conditions and therefore need a lot of extra help. Also, if your soil is significantly compacted or contaminated, remedial measures will be needed. But if you’re focusing on native plants under ordinary conditions, you can abandon those traditional horticultural practices with a clear conscience. Think of all the time, energy and money you’ll save!

Soil Testing

Even if you’re not planning to change your soil, it’s still a good idea to get it professionally tested. Many universities, as well as county cooperative extension services, offer inexpensive soil tests. These services are more accurate and comprehensive than the DIY test kits you find at nurseries and hardware stores.

I’ve used the University of Delaware’s Soil Testing Program. I followed online instructions for collecting a sample, put it in a ziplock baggie, filled out a form, wrote a check for $17, and shipped it all off to Newark. A few weeks later I got back a detailed report. I now know that my soil is too alkaline to grow, say, blueberries, but just right for many plants that like a slightly less sour pH. I learned that I have more than enough organic matter and nutrients. The analysis also scanned for toxins and heavy metals, and I got confirmation that levels of these are low enough that I could safely grow edibles.

Most people use soil tests to tell them how to “improve” their soil. But for the ecological gardener, they’re a tool for determining which plants will thrive in the existing conditions. As landscaper Larry Weaner advises, “match the plant to the site and not the site to the plant.” If you get that part right, no improvements are needed.

Bottom line? Love the soil you’re with.

The Bees’ Keys

In response to a groundswell of popular acclaim (by which I mean one off-hand comment), I'm bringing back the musical portion of this newsletter. I guarantee your mood will brighten listening to this week's selection, a ray of a cappella sunshine from the Soweto-based group The Soil.

The Bees' Knees

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