That last leaf falls from the now-naked tree and the gardener's heart sinks. But for the creatures of the underworld, as biologist James B. Nardi calls them, the leaf arrives like the fatted calf. From bacteria to insects to worms, snails and slugs, the soil is home to an entire universe of living organisms. Illustrates GARDENING (category l), by Adrian Higgins (c) 2007, The Washington Post. Moved Wednesday, Dec. 19, 2007. (MUST CREDIT: Illustration by James B. Nardi.)

Dynamic display of diversity lives in your dirt

That last leaf falls from the now-naked tree and the gardener's heart sinks. Winter is here. But for the creatures of the underworld, as biologist James B. Nardi calls them, the leaf arrives like the fatted calf. As it settles on the soil surface, organisms that can be seen by us, such as wood lice, and those that cannot, such as bacteria, set into motion a hidden, primal banquet featuring hordes of revelers and many courses.

First, wood lice, millipedes and critters called springtails eat little holes in the leaf, providing openings for hungry bacteria and fungal threads. Their feeding, in turn, makes the leaf carcass more palatable to the larvae of crane flies and midges. Next to feast are fly maggots. Then snails, mites, crickets, earwigs and something called a bristletail have their fill. The countless jaw marks now provide a foothold for more bacteria. As they hasten the rot, mites, springtails and tiny worms called potworms finish off the meal. Almost. The remaining leaf skeleton, the least-digestible element of the structure, is pulled downward by earthworms, which use bacteria in their gut to complete the leaf's transformation into the humus that gives soil its dark richness and life. And what life.

Nardi, a scientist at the University of Illinois, writes in his newly published book, "Life in the Soil," that a square meter of healthy garden soil is home to 10 trillion bacteria, 10 billion protozoa, 5 million nematodes, 100,000 mites, 50,000 springtails, 10,000 creatures called rotifers and tardigrades, 5,000 insects and arachnids, 3,000 worms and 100 snails and slugs. Throw in the occasional mammal and a salamander or two, and you get the idea that you don't have to travel to the Brazilian rain forest to luxuriate in the biodiversity at our feet.

Thin leaves that have a higher nitrogen content, such as elm, will break down in a year. Maple leaves, with more carbon, rot in two years. Oak and beech, tougher and with a greater ratio of carbon to nitrogen, take roughly three years to decay. But the gardener can vastly accelerate that process by chopping up the leaves, giving a far greater surface area to the microbes.

For really speedy decay, you can start a compost pile, where the correct ratio of brown to green matter, constant moisture and a certain mass of material soon will invite the sugar fungi. They secrete antibiotics to ward off rival fungi and bacteria, and they survive the high heat generated by their own metabolism: as much as 165 degrees in the heart of a new compost pile.

Nardi writes that "the billions upon billions of soil bacteria exert an extraordinary influence on the health of the earth." For example, if you were to measure the metabolic activity of bacteria in the top 6 inches of an acre of fertile soil, he writes, you would find that it exceeds the metabolic energy of 50,000 people.

Nardi quotes Leonardo da Vinci as saying that "we know more about the movement of celestial bodies than about the soil underfoot."

Organic gardeners and farmers have always felt intuitively that you ignore the soil at your peril, but not until the recent past has the scientific basis for that become more broadly understood beyond the realm of soil scientists and microbiologists. We know now that long-lived trees and shrubs rely heavily on beneficial fungi and bacteria, which essentially extend the size of the root zone immeasurably.

In support of that, I recall a professional gardener at an arboretum telling me years ago that beech trees don't show meaningful top growth for seven years after planting, for it is during that period that beech-specific fungi are growing in the root zone.

Nardi explains that phenomenon in detail. I'd say that his book, which is published by the University of Chicago Press and costs $25, is a must-read for anyone who wants a better understanding of this world and how to protect it.

As he points out, organic matter not only improves the structure of the soil, it also makes nutrients available to plants that otherwise would be chemically locked. It also feeds this vast world of vertebrates, insects and microbes.

It is ironic that the growth and health of plants are dependent on decay, but perhaps it isn't so much an oddity as the natural order of life on our planet.

But decay can run its course, with disastrous consequences for the soil world. "Over time," Nardi writes, "soil that loses more plant and animal material than it gains eventually loses its crumbs, its structure, its pores, and its air spaces." In warm climates, the rate of decay is far greater than in cooler ones. Once temperatures exceed 80 degrees, nature alone cannot replenish the amount of material consumed. So think of these microbes as workers in the factory, and you're the boss, and you have to keep the raw materials coming. Crank up that compost pile.

Nardi gives instructions in compost making, but so do a lot of other authors. More valuably, perhaps, he shows you how to capture some of these mysterious creatures and how to observe them, with a simple microscope. Dig up a small piece of beech or oak root and see for yourself what few others have observed: stubby little feeder roots joined by necklaces of beneficial root fungi.

It is surely as thrilling as seeing the rings of Saturn.

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