Microscopic denizens of a typical backyard can transform kitchen scraps into paydirt.

by Mary Miller

A leaf enlarged 5,000 times, with decomposers at work. The round and oval shapes are bacteria; the threadlike structures are fungal mycelium.

Every morning after he’s brewed the coffee, my husband, Jeff, dumps the grounds into a bowl of kitchen scraps and heads into the backyard—but not toward the trash can. Instead, the vegetable scraps, coffee grounds, and eggshells go into a green plastic structure about the size and shape of a doghouse in a corner of the yard near my vegetable garden.

Jeff removes the "roof" of our compost bin and tosses the morning’s refuse on top of a pile of grass clippings and shredded twigs. Once a week, he mixes and turns the pile with a pitchfork and spritzes it with water. Sometimes on cool mornings, I can see steam rising out of the little green house, a sign that my pile of rot is alive and well. In a month or so, the decomposed debris will be ready for working into my raised vegetable beds, where it will perform its magic on my tomatoes and peppers.

Before we started gardening, the compact, bricklike clay soil in our backyard supported only crabgrass and a few stray weeds. Plants grow best in soil that has a balance of clay and sand, so one of the first things we did was order a truckload of sand and fine topsoil to mix into our garden soil.

Because plants must have nutrients, I knew that we also needed to feed the soil with humus, or organic material. We bought bags and bags of steer manure and mushroom compost and spread it on top of the soil, but I soon tired of lugging stinky cow poop home from the garden store. So I began making my own humus in the form of compost—decomposed dead plants. Decomposition is nature’s way of recycling nutrients; I help the process along by providing food and shelter for a balanced ecosystem of compost critters, including scavengers, predators, and decomposers.

Life in the Compost Bin

Much of the heavy decomposition of kitchen scraps and yard trimmings is performed by microscopic actors: actinomycetes and other bacteria, protozoa, and fungi. I asked Kate Scow, associate professor of soil science and microbial ecology at the University of California at Davis, to describe the action. "You start out with these big chunks of plant material," she says. "There’s no way plants can use them in that form." She explains that the fungi and actinomycetes break up the big pieces of garbage into smaller chunks. Simple bacteria finish the job by digesting these small pieces into their chemical components: nitrogen, carbon, phosphate, and other plant nutrients.

Of the soil decomposers, bacteria are among the most important and abundant. A pea-sized clump of garden soil can contain a billion bacteria. If you looked at them under a good microscope, Scow says, "you wouldn’t see anything that’s terribly exciting—lots of little boring rods, spirals, and spheres. Bacteria all look pretty much the same. It’s what they do that’s fantastic. They can make a living on just about everything on the planet. You can have the same little rods, and one of them will live off inorganic iron, another one will break down sugar, and another one will fix nitrogen."

Plants need nitrogen to make proteins and grow new tissue. Although nitrogen is plentiful in the air, it’s in a form that plants can’t use. Certain bacteria fix, or convert, nitrogen into a form that plants can take up in their roots. This is a metabolically expensive procedure for the microbes, requiring lots of carbon for fuel, and most of the nitrogen-fixing bacteria live in a symbiotic partnership with certain plants such as beans and other legumes.

The bacteria live inside special plant structures called nodules on the roots of legumes. The plants provide carbon for the bacteria, and the bacteria release excess nitrogen for the plants. To bring up the nitrogen levels in their soil, farmers and home gardeners often rotate bean crops in different places every year. Extra nitrogen, leaked from the legumes, helps nourish the following crop.

When you hoe your garden and notice a rich, earthy smell, you’ll know that actinomycetes are at work. Chemicals given off by the actinomycetes are largely responsible for this characteristic odor. They live deep in the soil near the plant roots, where they convert dead organic matter into a peatlike substance. Actinomycetes, which compete for resources with other bacteria in the soil, conduct a kind of biological warfare: the actinomycetes produce their own antibiotics, chemical substances that inhibit bacterial growth.

Other prominent microscopic denizens of the soil are the fungi. Soil fungi are related to the mushrooms that grace our dinner table and the molds that invade our refrigerator. Fungi, once considered primitive plants, are now classified in their own kingdom. They lack chlorophyll and cannot manufacture carbohydrates from photosynthesis as plants do. Most of the life of a fungus is spent underground, where it forms a bundle of strands known as mycelium, a string of simple cells that produce digestive enzymes and acids. These chemicals, which can break down the toughest wood fibers, give fungi the ability to eat just about anything.

Fungi, bacteria, and other decomposers break down garbage into fertilizer, but they don’t willingly release those nutrients into the compost and soil. "That’s where the importance of the microbe predators, the arthropods and nematode worms, comes in," Scow says. "They essentially bust open the guts of the microorganisms."

Only recently have experts come to appreciate the critical role predators play by liberating the pool of nutrients stored inside the bodies of microbes and making those nutrients available for plants, Scow adds. Important predators include mites, sow bugs, springtails, and nematodes. Soil mites, arthropod cousins of the tick, live their entire lives underground feeding on plant matter, nematodes, fly larvae, other mites, and springtails. Sow bugs, which have a roly-poly shape and an armor-plated body, eat decaying vegetation.

Nematodes look and move like very tiny eels, thrashing from side to side to move through their soil environment. These tiny predators—smaller than the period at the end of this sentence—seek out living bacteria, fungi, protozoa, and other nematodes in the compost. In the soil, a few species of parasitic nematodes can also wreak havoc on the roots of living plants by sucking all the juice from them.

But as in all healthy ecosystems, the compost pile has checks and balances. Living among the rotting banana peels and liquefied veggies are certain fungi that are natural-born killers of the sometimes harmful nematodes. These predatory fungi snare the nematodes by casting a net of sticky fibers. When the wiggly worm blunders into the net, it sticks like a fly to flypaper, and the fungus grows new tendrils to completely envelop and eventually dissolve and absorb the nematode.

My all-time favorite compost critter is the earthworm. I love earthworms so much that I bought a box of red worms (a relative of the garden-variety field worm) to add to my compost bin. More than 1,800 different earthworm species exist worldwide, including a ten-foot Australian specimen with a three-inch girth that weighs a couple of pounds.

Darwin himself took a fancy to earthworms and wrote an entire volume on the benefits of earthworms to the soil. The earthworm, little more than a digestive tube surrounded by skin, is a veritable humus factory, taking in garbage and leaving its droppings, called castings, which are rich in nutrients. The earthworm is nature’s plow. As earthworms burrow and move soil around, they open up pockets of life-giving oxygen and make room for plant roots to penetrate.

In winter and early spring, when it’s wet and cool, the earthworm population explodes, and I see thousands of little pink bodies wiggling through the compost. According to Kate Scow, earthworms are a good sign. "As long as you can see the big guys, your compost is doing okay," she says. "You won’t have earthworms unless things are going well."

Turning Rot into Brown Gold

For four years, I have focused on the care and feeding of my population of microbes, scavengers, and predators, and I’m proud to say that I’ve got a really good pile of rot going. I’m continually amazed at the work they do. Into the compost bin go slimy gobs of goo that once were salad greens, along with dead leaves and fresh grass clippings. Out comes a crumbly, brown material that smells like a walk in a forest. I used to feel guilty about abandoning leftovers in the back of the refrigerator while I ate take-out Chinese food, but I now have an easy out. The zucchini that grew to frightening proportions in the garden and the mold-covered bowl of rice go back into the soil instead of to the local dump. But the benefits go far beyond simple guilt relief.

Compost is like a gift I give to my garden. In return, it produces beautiful peppers, tomatoes, and herbs every year. Compost improves soil structure by breaking up clay particles and binding together particles of sand. It also aerates, improves drainage, and prevents soil erosion. When added to soil, compost neutralizes toxins, holds precious moisture, and feeds beneficial soil microbes with a steady diet of carbon, the precious secret ingredient of compost.

All too often, Kate Scow says, conventional farmers and gardeners overlook the importance of adding carbon to their fields and gardens. "People don’t think about how carbon drives all the other nutrient cycles in the soil," she points out." It’s the underpinning of the entire system. The bugs that make nitrogen and phosphorus available to plants need carbon for fuel."

There’s even some evidence that compost promotes plant health and helps prevent disease, Scow adds. "There are some studies showing that a really active microbial biomass will shut out detrimental organisms, possibly just through competition for nutrients," she says. Some microbes also secrete toxins to prevent disease organisms from spreading in the soil. Scow notes, too, that some microbes may produce growth factors—chemicals that promote plant growth.

As the carbon in compost is broken down by soil microbes, nutrients are slowly leaked into the soil and taken up by plants. This steady trickle of nutrients is preferable to the boom-and-bust approach of applying chemical fertilizers because it lasts all growing season and doesn’t periodically flood the plants with excess nutrients that can damage and even inhibit plant growth.

Beyond those practical applications, however, having a compost pile can become its own reward. I started out thinking that it was just a neat way of recycling kitchen scraps and dead plants. But somewhere along the way, things got turned around, and now I’m focused as much on feeding my compost pile as I am on feeding my garden. Kate Scow expresses a similar sentiment: "Sometimes I feel that what I’m farming for is the waste for my compost."

Making Compost First, you need a compost bin. We bought a prefabricated bin from the local garden supply store, but you can also build one out of plywood and chicken wire, dig a pit in the ground, or even use a plastic garbage bag or a garbage can with holes poked in the bottom, top, and sides for drainage and aeration. (If you’re using a plastic bag, you should leave it open every other day to let air in.) A simple recipe for starting compost is to layer one-third dry (brown) material, one-third green vegetation, and one-third soil. (Soil provides a starter supply of microbes to get the compost going.) After that, almost anything goes: kitchen waste, grass clippings, dry leaves, dead plants, coffee grounds, shredded newspaper, and even clothes-dryer lint and pet hair. When adding ingredients, you should strive for a twenty-five–to–one ratio of carbon sources (brown stuff like dead leaves and newspaper) to nitrogen sources (green stuff like grass clippings and foliage). Green materials contain carbon as well as nitrogen, but dead leaves and other brown materials are largely devoid of nitrogen. Manure is an excellent compost ingredient if you can get ahold of it. Cattle, horse, and chicken manure work well, but avoid pet and human wastes, as they may spread human diseases. Other materials to avoid are bones, meat, or fat, which often smell bad and will attract rats and other pests to your pile. Don’t compost weeds or diseased plants, or you may reintroduce the weed seeds or diseases into your garden. To give your compost a head start, chop up or shred materials before adding them to the bin. You might consider buying or renting a mechanical chipper/shredder if you have large branches to compost. Periodically sprinkle the pile with water so that it stays moist but not soggy—it should be about as damp as a moist sponge. With a pitchfork or spade, turn or stir the compost to introduce air. A pile that is too wet or isn’t exposed to enough oxygen can turn stinky. An easy cure for this problem is to open the container and let the compost dry out for a couple of days. For more information, "The Rodale Book of Composting," edited by Deborah L. Martin and Grace Gershuny and published by Rodale Press, is an excellent and detailed how-to manual. <<Back to top of main article

--------------------------------

This story originally appeared in the Gardening issue of the "Exploratorium Magazine."

EXHIBIT SECTIONS : The Stuff of Life , Life Needs Energy , Making More Life , Change Over Time

© Exploratorium | The museum of science, art and human perception| Traits of Life