March 28, 2022
After a winter that almost seemed like the winters we remember from our childhood, the perennials in the forest gardens are heaving their shoulders through the leaf litter, stretching their leafy arms toward the sun and we can finally see the daffodils blooming outside our kitchen window. Well, a couple days ago that was true. As I write this today, I’m looking out the window not at bright golden daffodils but at 5 inches of fresh snow fallen over the past two days. And that’s a good thing; disconcerting as it may be, the snow will protect those emerging perennials from the 17°F temperatures this morning. It’s the kind of day that has me gleefully embracing my “inner slug” and curling up with a good book and a cup of mint tea.
Okay, well maybe not entirely gleefully. As much as I may aspire to slughood, it just doesn’t seem to be achievable for me, especially at this time of year. There’s just too much to do. We have gardens to clean up, seeds to start, paths to refresh, and 35 cubic yards of mulch to put down. What we don’t have to do, though, is till the gardens.
What’s the problem with tilling? Actually, there are several. The biggest problem with tilling is that it destroys the structure of the soil, a structure that is vitally important to the health of a garden, which makes it all the more amazing that most people aren’t even aware of it. So what is it and why does it matter?
But before we get into that, let’s talk about soil itself. When I was a kid, the words “soil” and “dirt” seemed to be synonyms and I used them more or less interchangeably. It was only when I began to study permaculture that I realized just how far off the truth I was. From an ecological point of view, dirt and soil are two very different things with a slight overlap. An easy, though somewhat oversimplified way to think about it is this: Dirt is composed of variously sized inorganic particles created as friction slowly wears away the underlying bedrock in an area. What those minerals are specifically depends on what type of bedrock is characteristic of the place. Here in northeast Ohio, the bedrock tends to be composed mainly of shale, sandstone and siltstone. A few hundred miles southwest of here, where I went to college in southern Indiana, the bedrock is largely limestone. Although both areas have bedrock composed of sedimentary rock, the distinction between the two is important: the limestone of southern Indiana raises the pH of the dirt so the soil there is “sweeter” than it is here in northern Ohio.
Did you catch that I just used “dirt” and “soil” in the last sentence? It was intentional because of that slight overlap I mentioned above. Soil is not the same thing as dirt but dirt is, in fact, a component of soil. More to the point, dirt is the non-living component of soil. Soil, on the other hand, is a complex mixture of minerals (dirt), organic materials and living beings all working together to create a healthy, fertile eco-system in which complex life can thrive. And there’s the rub: dirt is inert but soil is alive!
Let’s say that again: soil is alive. In fact, healthy soil is absolutely teeming with life in all sizes and shapes from the simplest single-celled organisms to the largest and most ancient living creatures on the planet.
Returning now to the idea of soil structure, it might be helpful to think of soil as the skin of the planet. It’s a remarkably apt analogy. Just as healthy human skin has three distinct layers – the epidermis, the dermis and the hypodermis, healthy soil is made up of four layers – humus, topsoil, subsoil and regolith or parent material.
The humus layer is composed of pure organic material – the decaying leaves and plant material from previous years as well as dead, decomposing animals – and is home to all the many critters who live by eating that organic matter. It’ works much like a compost pile; the bacteria, fungi, insects and other invertebrates that live there eat and digest the raw organic material and excrete the nutrient-rich stuff that makes up the next layer, the topsoil.
The topsoil is what most people think of when they hear the word soil, and for good reason; healthy topsoil is a rich, balanced matrix of decomposed organic material from the layer above (humus) and mineral particles from the layer below (subsoil). It’s where most of the action takes place and is usually the layer that we, as gardeners, are most engaged with. It’s where plants put down roots and take up nutrients and water. And until recently, that’s pretty much all we understood about it. (That understanding, or more accurately, lack of understanding is what is responsible for aberrations like the so-called “Green Revolution” championed by idiots people like Earl Butz in the 1970’s.)
Below the topsoil we find the subsoil. Subsoil is composed primarily of mineral material with only small amounts of organic material present. It is home to earthworms and tree roots and is where most of the mineral nutrients which plants need are found. I tend to think of it as the “opposite number” of the humus layer. The coming together, in a sense, of humus and subsoil is where topsoil comes from.
But wait! Isn’t that mixing together of humus and subsoil pretty much exactly the definition of tilling? Well, yes, it is. And if everything above was the total extent of our knowledge about soil biology then we would probably be in favor of tilling. But we know a lot more about soil now than we did even twenty years ago.
Today, research has shown that healthy topsoil is more than just an apartment complex and en-suite supermarket for living things. It is, in fact, an incredibly complex web of living beings, networked together, sharing nutrients and communicating with each other through symbiotic relationships between plants and fungi and scads of micro-organisms. It is a community.
Ecologists define a community as “a group of interdependent organisms of different species growing or living together in a specified habitat.” There are a couple key ideas encapsulated in that short definition. First of all, a community is made up of organisms of different species; in other words, it is diverse. That’s very different from modern farming models where crops and critters are generally raised in monocultures. Monocultures are generally a bad idea when it comes to ecology; they are more subject to being wiped out by disease, they deplete soil nutrients, and they lack the genetic diversity which enables species to evolve in the face of environmental change.
The other key idea in the definition of community is that all those diverse species are interconnected. It’s important to understand here that the interconnection we’re talking about isn’t metaphorical. It is literal. The roots of plants, the organisms in the soil, all are connected together through fungal networks called mycorrhizae. The full function of these mycorrhizal networks is extremely complex but, in general, one can say that, in a mycorrhizal association, the plant utilizes photosynthesis to make organic compounds like sugars and supplies them to the fungus. The fungus in turn supplies the plant with nutrients, like phosphorus or potassium, taken from the soil. The fungus also plays host to colonies of bacteria which are able to fix nitrogen and make it available to the plant. And as if that’s not amazing enough, recent research has demonstrated that the plants in a healthy community are actually able to communicate with each other through their mutual interconnection with mycorrhizae.
And all of that is what tilling destroys.
When we till, we break down the structure of the soil, mixing the layers, fundamentally changing the soil chemistry and destroying the symbiotic interconnections between plants and animals and fungi. When we then try to plant into that jumbled up mess, we often find that plants grow weakly and more slowly than normal as much of the energy available in the system is being directed towards trying to reestablish the previous structures and biologic relationships.
So, is there ever a time when it’s a good idea to till? I can think of one situation where it could be appropriate. If you wanted to put in a new garden in an area where the soil was depleted and barren of organic material, where the soil was actually just dirt, then tilling a whole bunch of organic material like leaf humus and compost into the top twelve inches or so would probably be a viable option for jumpstarting soil formation. But even in that circumstance, it isn’t your only – or even your best – option. The relatively well-known technique of lasagna gardening would likely be a better choice because there is yet another reason to stay away from tilling.
If all the above talk of fungal networks and plant communication seems a little to woo-woo to believe, you should consider the other immediately obvious result of tilling. When you till the soil and mix all those layers together, you are also bringing seeds that have been lying dormant deep in the soil up to the surface where they suddenly find that conditions are at last just right for germination. So they do. And you get to spend the rest of the gardening season battling the thousands of weeds that are now sprouting everywhere.