The driest, dullest entry in almost any garden how-to book is the advice on soil. It's a shame: The tale of soil is full of weird characters and fascinating processes we understand barely, if at all. It's also unfortunate because soil is the single most important factor determining success in your garden. Right now, this autumn minute, is the time to improve your soil. Right now, conveniently enough, is when you have a free supply of the ideal soil conditioner—fallen leaves.
Most books include that same old dreary litany about finding out whether your soil is clay or sand, with the added burdensome assignment of getting a soil test. The soil test, something I've never done, seems darkly reminiscent of a biopsy; you can learn a fair bit without a test. Just look around the neighborhood and note what plants are doing well in local conditions.
It's worth knowing what Mother Earth has dealt you in the dirt department. But there isn't much you can do about the type of soil you have—clay or sand. You can sit around despairingly running sand through your fingers or squishing sticky clay into balls, and, of course, you can complain forever and say how much nicer it would be to have a garden in Shropshire, England; Champaign-Urbana, Ill.; or Tallahassee, Fla.
What any gardener possesses is basically flour ground from rocks with organic frosting on top. What you can change pretty easily is the health and texture of that frosting—the structure of your topsoil. Structure refers to how the soil sticks together or doesn't. How clumps of soil cohere determines the availability of pore spaces for air and water to enter, and how easily roots can penetrate.
A major source of misery for me when I worked in New York City parks was seeing my fellow gardeners using a leaf blower to blast dead leaves off the planted beds. The useful foliage was then bagged and trucked to a landfill. First, the poor plants were getting Hurricane Ike-style winds—but without moisture. Second, it's fine to simply leave leaves on perennials. By spring, they'll have broken down into humus. The ideal would have been to run a lawnmower over leaf piles and put the chopped-up leaf litter around the plants. Lacking a working lawnmower, what I did was collect leaves in hidden piles to break down on their own over the winter and use in the spring or the following fall.
The roots, even of big trees, take in most of their moisture and nutrients from an amazingly thin layer of soil—less than a foot down and often only the first few inches. We used to think of soil as that brown substance useful for holding plants upright, into which we could inject fertilizer. Aside from a beneficial earthworm or troublesome mole, most gardeners didn't care much about what was living underground.
In fact, the fertility of those top inches depends on an incredibly complex society of small creatures—mites, beetles, sow bugs, water bears, nematodes, millipedes, springtails, fungi, and bacteria—that we're only beginning to understand. As Leonardo da Vinci observed, "We know more about the movement of celestial bodies than about the soil underfoot." It's only in the past couple of decades that soil ecologists have systematically applied microbiological techniques to learning more.