Like a good chicken or vegetable stock that gently simmers away on a back burner, compost isn’t something you make exactly—it’s a process that, with the right ingredients and a little care, just happens. And like a supply of homemade stock in the freezer, compost pays untold dividends. It’s not only a simple way to help reduce your household waste, but is a (free) amendment, or improver, to less-than-perfect soil. It gradually releases nutrients to plants; makes clay soil easier to work; decreases erosion; and enables sandy soil to retain water, thus increasing your plants’ drought resistance. The EPA paints a bigger picture, noting that among composting’s benefits is the reduction of methane (a potent greenhouse gas with 21 times the global warming potential of carbon dioxide) from landfills, which are a major source of human-related methane in the United States; they account for more than 20 percent of all methane emissions.
One sign that composting is becoming more popular is the ready availability of stylish yet practical small composting pails or crocks that serve as a countertop or under-the-sink waystation for kitchen scraps. Once full, they’re emptied into a more permanent composting pile or bin outside—or toted to a designated drop-off point, such as those provided by the New York City Greenmarket. You’ll find a range of choices at retailers from Gardener’s Supply and Williams-Sonoma to Target and Bed Bath & Beyond. Before buying one, it’s worth reading what compostjunkie has to say about factors like capacity, cleanability, and weight. And if the last thing you need is to go buy one more dratted thing, then repurpose a large yogurt or other container.
The Science Behind Composting
First, a quick word about soil. In the textbook Essential Soil Science, M.R. Ashman and G. Puri write that it’s formed “when mineral material from rocks and organic matter from plants and animals are combined together.... Together they make up approximately 50 percent of the soil volume; the remaining 50 percent is pore space, filled with either air or water depending on how wet the soil is.” Just to clarify, in this sort of scientific context, the word organic doesn’t mean USDA-certified organic, but instead simply carbon-based. All living things, even when they’ve decayed into soil, are made of compounds that contain mostly carbon. Compost is basically the biological decomposition of organic wastes into a dark, moist, rich, crumbly mix that helps plants thrive. Think of compost as a stellar example of the whole being greater than the sum of its parts.
Decomposition is accomplished by the organisms that occur naturally in organic matter. While fungi, worms, and various invertebrates (centipedes, beetles, and other creepy-crawlies) play a role, the real heavy lifters are several different kinds of bacteria, which, according to Cornell Composting, make up 80 to 90 percent of the billions of microorganisms typically found in one gram of compost.
Like all living things, those teeming bacteria in a compost pile need water and air to flourish. Water allows them to grow into vast communities and travel around within the compost, decomposing organic materials as they go. Organic wastes contain water in varying degrees, but moisture may also come from rain or the garden hose. According to conventional wisdom, compost should feel like a wrung-out sponge.
What You Can Compost and What You Can’t
The optimal composting blend is a roughly 50:50 ratio of nitrogen-rich “greens” and carbon-rich “browns.” Most kitchen scraps, as well as coffee grounds and used tea leaves, are considered greens—a nutrient and moisture source for the microorganisms hard at work breaking down that organic matter. Other greens are fresh grass clippings; seaweed; over-the-hill crops (bolted lettuce or herbs, for instance, or spent pansies) and farm-animal manure.
Browns provide carbon, thus energy, for the microorganisms. They add bulk, help keep the pile porous, and facilitate airflow. But browns break down far more gradually than greens. Fallen autumn leaves are considered a brown but should be added to compost with restraint, especially flat ones such as maple leaves. They tend to mat and thereby exclude air, explains Barbara Damrosch in The Garden Primer. The best solution is to have a separate heap for leaves, one that can break down over a few years to yield leaf mold—another time-honored soil amendment. Other examples of browns include dried grasses; straw; twigs and bark; shredded cardboard, such as paper towel or toilet paper tubes; and shredded noncoated or colored paper. As far as egg shells go, they take a while to break down as well, so let them dry and crush them thoroughly. The Michigan State University Extension has more tips on how best to deal with egg shells in compost, and they will allay any fears about spreading salmonella as well.
Materials to avoid composting include meat, dairy, bread and other baked goods, and cooking oils (they all attract vermin, dogs, raccoons, and other scavengers); diseased plants; weeds gone to seed; any plant parts from poison ivy, oak, or sumac; coal or charcoal ash (they contain toxic residues); pet poop; newspapers or magazines with color photographs or coated paper; and plastic.
You can find links to more detailed lists of what to compost (dryer lint! Who knew?) and what not to (and why) on the EPA’s composting page.
Assembling a Compost Pile
According to the composting experts at Cornell, a minimum volume of one cubic yard (3’ by 3’ by 3’) is required for a pile to become sufficiently self-insulating to retain heat, which helps reduce pathogens and allows the process to occur more quickly. Strictly speaking, you don’t need a bin; you can simply build a compost pile on the ground. What a bin does do, though, is keep the pile corralled, retain heat and moisture, and help deter pests. If you decide to use a bin, you can either make one yourself—if you, like me, lack rudimentary carpentry skills, using bales of straw is an easy out—or buy one. You can see a few options, including rotating bins, on the Cornell site. I’ve also heard great things about the very unobtrusive trap-wire bin from Johnny’s Seeds. Greg Seaman’s EarthEasy blog has a good overview of the pros and cons of commercial bins and tumblers.
Start off your pile by spreading a layer of browns a few inches thick. Top with a few inches of greens and then a thin layer of soil. Add another layer of browns and repeat layering, gradually working your way up to a pile that’s about three feet high. Aside from keeping it moistened if the rain doesn’t cooperate, just leave it be; cover it with a tarp to protect it from excessive rain or drought if necessary. Every few weeks, it’s a good idea to turn the pile with a pitchfork or shovel, moving the matter in the center of the pile to the outside and moving the stuff on the outside to the center. Turning helps aerate the materials, which allows decomposition to occur at a faster rate. It also gives you a chance to moisten the blend if it’s too dry or add more browns if it’s too damp.
When you first turn a compost pile, you may see steam rising from it. This is a happy sign that the organic wastes are decomposing; at the center, the temperature can climb to 160 degrees Fahrenheit. If you turn the pile every few weeks and keep it moist, you will begin to see earthworms in the compost, and the center of the pile will eventually turn into rich, crumbly “black gold” that smells sweet and earthy. When you have enough finished compost in the pile to use in your garden, shovel it out and begin your next pile with any remnants that haven’t fully decomposed. Finished compost will take up only 25 to 40 percent of the space occupied by the original materials. Depending on what went into the pile, how often it’s turned, etc., the whole process can take from about two months to two years; a compost tumbler takes about six to eight weeks.
No Backyard or Time to Spare? No Problem
The lack of an outdoor space shouldn’t stop you from composting. The Japanese bokashi system takes place entirely in a special sealed bin and simply requires sprinkling each layer of waste with bran dust that’s been inoculated with microorganisms that thrive without oxygen—that is, they decompose organic wastes through an anaerobic process. Strictly speaking, bokashi is fermentation, not composting; the end result smells sweet and pickled rather than sweet and earthy. Among its advantages, though, is that you can compost cooked foods, dairy, meat, and fish, and the process takes about two weeks instead of months. DIYers should check out the Compost Guy’s how-to advice. You can also buy a convenient kit at sources such as Bokashicycle.
Worm composting, or vermiposting, which can be done indoors or outdoors, is another option, and if you have kids, a worm bin is a wonderful way to introduce them to gardening—not to mention a “waste not, want not” philosophy. Most commercial worm bins come with a spigot for the “compost tea,” the liquid that sinks to the bottom tray. Dilute it to one-fifth strength with water, and spray it on the leaves of the plants to increase yields and reduce pests.