denizens of a typical backyard can transform kitchen scraps
leaf enlarged 5,000 times, with decomposers at work. The
round and oval shapes are bacteria; the threadlike structures
are fungal mycelium.
morning after hes brewed the coffee, my husband, Jeff,
dumps the grounds into a bowl of kitchen scraps and heads
into the backyardbut 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.
removes the "roof" of our compost bin and tosses
the mornings 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.
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.
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 compostdecomposed dead plants.
Decomposition is natures 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.
in the Compost Bin
from the following ingredients, keeping a balance between
green matter and brown matterbread, coffee grounds,
eggshells, garlic skins, grass clippings, dead leaves,
shredded newspaper, spinach, strawberries, chopped-up
twigs, wood chips, and beans and rice. Substitute freely.
Add soil, moisten with water, and mix well. See below
for more information on making compost.
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. "Theres 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.
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 wouldnt see anything thats
terribly excitinglots of little boring rods, spirals,
and spheres. Bacteria all look pretty much the same. Its
what they do thats 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
need nitrogen to make proteins and grow new tissue. Although
nitrogen is plentiful in the air, its in a form that
plants cant 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.
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.
you hoe your garden and notice a rich, earthy smell, youll
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
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
bacteria, and other decomposers break down garbage into fertilizer,
but they dont willingly release those nutrients into
the compost and soil. "Thats 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."
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.
look and move like very tiny eels, thrashing from side to
side to move through their soil environment. These tiny predatorssmaller
than the period at the end of this sentenceseek 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.
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.
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.
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 natures plow. As earthworms burrow and
move soil around, they open up pockets of life-giving oxygen
and make room for plant roots to penetrate.
winter and early spring, when its 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 wont have earthworms unless things are going
Rot into Brown Gold
four years, I have focused on the care and feeding of my population
of microbes, scavengers, and predators, and Im proud
to say that Ive got a really good pile of rot going.
Im 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.
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.
too often, Kate Scow says, conventional farmers and gardeners
overlook the importance of adding carbon to their fields and
gardens. "People dont think about how carbon drives
all the other nutrient cycles in the soil," she points
out." Its the underpinning of the entire system.
The bugs that make nitrogen and phosphorus available to plants
need carbon for fuel."
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 factorschemicals
that promote plant growth.
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 doesnt periodically flood the
plants with excess nutrients that can damage and even inhibit
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 Im 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 Im farming for is the
waste for my compost."
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 youre using
a plastic bag, you should leave it open every other
day to let air in.)
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.)
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-fivetoone
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.
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
materials to avoid are bones, meat, or fat, which often
smell bad and will attract rats and other pests to your
pile. Dont compost weeds or diseased plants, or
you may reintroduce the weed seeds or diseases into
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.
sprinkle the pile with water so that it stays moist
but not soggyit 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
isnt 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.
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
to top of main article
story originally appeared in the Gardening issue of the "Exploratorium
SECTIONS: The Stuff of Life,
Life Needs Energy, Making
More Life, Change Over Time
Exploratorium | The museum of
science, art and human perception| Traits