Building Fertile Soil

Image Gallery Michael Rothman

It's a fundamental axiom of organic gardening and farming, and once you understand what "feeding the soil" means, you'll also understand why organic methods, and no-till techniques in particular, work so well.

Even though you can't see most of it, a complex soil food web lives in your garden; it's teeming with earthworms, mites, bacteria, fungi—all kinds of mostly microscopic, interdependent organisms that release mineral nutrients and create the loose soil structure crops need to thrive. Beneficial mycorrhizal fungi (see "The Magic of Mycorrhizal Fungi," Page 24) grow in and around plant roots, mining subsoil for nutrients and water to share with your crops. Other microorganisms prevent diseases and help plants withstand insect attacks.

Your crops actually help feed all this underground life. Ray Weil, a renowned soil scientist at the University of Maryland, College Park, says that while plants invest a "substantial amount of their photosynthesis in feeding soil microbes, the plants are obviously getting benefits back."

Think of plants, with their green chlorophyll, as little solar-powered engines that pipe a steady flow of carbohydrates out through their root hairs. Between 20 percent and 40 percent of a plant's total carbohydrate production is released into the soil through its roots. In the nutrient-rich area around the root hairs, microscopic bacteria and fungi feed and multiply. Nematodes (tiny worms) and other critters move in to feed on the bacteria; in turn, the root hairs absorb nutrients released by the concentration of microbes.

But this complex, mostly invisible soil ecosystem can be damaged easily. Chemical fertilizers, dehydrated chicken manure or high-nitrogen blood meal can burn tender root hairs, and tilling or plowing destroys soil texture, disturbing the layered web. Leaving the soil bare shuts off the carbohydrate food supply; lack of moisture and ultraviolet rays kill some of the organisms that dwell in the surface layer. (Mother Nature almost never leaves the soil uncovered; only on farms and in gardens do we see naked soil.)

More and more farmers and gardeners are learning new ways to protect and promote the development of this amazing soil food web. They foster the natural fertility that comes from a healthy food web, and you can do the same.

First, minimize plowing, tilling and digging. Second, use compost, grass clippings, leaves and other organic mulches on a regular basis to promote and sustain the soil food web. Third, always keep the soil covered with live crops or, at minimum, an organic mulch. Whenever you are not growing a food crop, sow a cover crop so the carbohydrate pipeline isn't shut off.