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There's more to soil than just dirt

What's in the ground isn't dead -- there's a complex system of checks and balances.

Plants in the wild don't travel far; they're content to put down roots, to live and die in one place. That's largely why Mother Nature doesn't spend her weekends unloading sacks of fertilizer and compost, and why her gardens -- her redwood forests, her prairie grasslands, her oak-studded foothills -- sustain themselves.

In established natural plant communities, little plant material, living or dead, ever leaves the immediate vicinity. Even plant matter consumed by nondomesticated animals isn't generally carried far. As plants die, drop leaves or slough off old roots in Mother Nature's gardens, the carbon the plants have fixed from the atmosphere and converted into organic molecules, and the nutrients the plants have absorbed, feed into the local soil food web.

The soil food web is a complex community of bacteria; fungi; actinomycetes, a very common type of bacteria that grow in colonies and resemble fungi; invertebrates like slugs, millipedes, beetles and sow bugs; several types of worms including earthworms, nematodes and red worms; and protozoa, single-celled organisms important because they consume bacteria and thereby keep the populations of bacteria in check.

In the process of consuming plant residue and each other, and in living and dying, the members of the soil food web slowly release the carbon and nutrients the plants have sequestered. The carbon re-enters the atmosphere in the form of carbon dioxide, and the nutrients become available to new plants. Together, plants and the soil food web continually recycle carbon and nutrients.

Such is not the case in most kitchen gardens, where we interrupt the cycling of carbon and nutrients by harvesting much of the plant material for our own use. What we don't eat we clear away to ready the bed for the next generation of seeds and seedlings.

But when we remove plant matter from the garden, we not only remove a source of nutrients for future crops, but we also starve off the members of the soil food web, which, when well fed, perform a host of functions besides recycling nutrients, helping to ensure good soil and, therefore, good plant health.

Some members of the soil food web excrete substances that glue soil particles together into small aggregates or clods that range from crumb to dime size.

"A good healthy soil will break into little crumbs, not just a fine powder, unless it's been tilled to death. And those little crumbs are held together by sticky substances in the soil," says David Wolfe, author of "Tales From the Underground: A Natural History of Subterranean Life" (2001, Perseus Publishing) and a horticulture professor at Cornell University.

"Most of those sticky substances are literally chemicals: gooey, sugary- type compounds released by microbes, earthworms and fungi. If you don't have a good, healthy, living biomass in your soil, you don't get those little crumbs forming, and you have poor aggregate structure, or soil tilth."

Water and oxygen (the former necessary for plant growth, the latter the breath of life for aerobic microorganisms, the most efficient initiators of decomposition) can more readily penetrate soil clumped into small aggregates, as can plant roots. Soil consisting of fine particles packs together tightly, leaving few pockets for air and water and deterring root spread. Earthworm tunneling also helps oxygen and water enter the soil and creates paths of least resistance for roots to follow to water and nutrient reserves.

As the soil food web breaks organic material down into ever smaller pieces and moves the pieces into the soil, it improves the soil's ability to hang onto nutrients. "The humus-containing fraction of organic matter, the portion of organic matter that has been decomposing for a long time, has features that help hold nutrients that are released by newly dying microbes, keeping the nutrients from leaching out of the soil," says Wolfe. "Humus, just like clay, has an electrostatic surface that helps hold important nutrients to it."

A healthy food web maintains a balance among its various members, helping to keep in check bacteria, fungi and invertebrates that can damage living plants. "There are predator and omnivore organisms in the soil that feed upon other organisms," says Howard Ferris, professor and nematologist at the College of Agriculture and Environmental Sciences at UC Davis.

"If we have an intact food web, a food web with a lot of feeding linkages among the organisms, then we have the top predators -- the lions of the soil, figuratively speaking -- feeding upon the rabbits of the soil, and some of the rabbits of the soil are damaging to plants. With a diverse food web, we have some regulatory effect upon the pest species that interfere with our objectives, and we can reduce our use of pesticides."

However much we might appreciate the good works of the soil food web, we've got to eat our fruits and vegetables, too. But in so doing, we create a gap in the cycling of plant carbon and nutrients, jeopardizing the health of the soil food web and the productivity of the soil.

So the gardener has a choice.

The first option is to follow the route of conventional agriculture: remove nearly all the decaying plant matter from the system, starve the soil food web and use synthetic chemicals and fossil fuels to accomplish the tasks the soil food web formerly managed.

The second option is to enable the natural cycling of carbon and nutrients to continue by replacing the organic material we remove from the garden, adding it back in the form of compost and cover crops such that we can still plant a good many of the vegetables of our choosing each year while also maintaining a healthy soil food web (see article on next page).

"The first step is for people to start thinking of the soil, not as an inert object, a factory floor to use for growing a garden, but as a living, dynamic, ever-changing entire system," says Doug Karlen, soil scientist at the USDA's National Soil Tilth Laboratory. "We're not just dealing with a material of sand, silt and clay; we're dealing with another whole living body."

Deborah K. Rich is a Monterey freelance writer and olive rancher. E-mail her at home@sfchronicle.com.

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