Composting not only reduces the waste stream and minimizes the production of greenhouse gases, but also it produces nutrient-rich humus, a key ingredient to any sustainable agriculture, be it a single houseplant or next year's organic wheat crop.
All organic matter will biodegrade with or without human intervention, but a little structure and oversight ensures that it happens more efficiently. For example, in a landfill, yard waste breaks down largely by anaerobic decomposition, which produces potent greenhouse gases including methane and nitrous oxide and can be a very slow process, with organic materials surviving undecomposed for decades.
A properly engineered and well-maintained compost pile supports aerobic decomposition with greatly reduced emissions of heat-trapping gases, and it produces a usable product more quickly.
The Environmental Protection Agency estimates that Americans produced more than 250 million tons of garbage in 2007. Although 89 percent of that waste was recyclable – including compostable food waste – only 33 percent was actually recycled. A trip to the local waste transfer station, landfill, or even the alley trash bin will provide firsthand evidence of the EPA’s findings. The rotten odors alone tell us the municipal solid waste stream is largely organic, with compostable yard trimmings and food scraps making up one-quarter of the total. (Recyclable paper products make up the largest portion at 32.7 percent.)
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In practical application, exactly what can and cannot be composted depends on several factors. Composting is not a perfect science, but a few rules of thumb can be applied, whether you live in an apartment or on a farm.
At home you can build a cold or a hot compost. Cold composting requires little maintenance and is best for leaves and grass clippings, which can be piled up and turned once a month. However, the heat produced may not be adequate to kill plants, so weeds should not be included. Also called slow composting, since it can take more than a year to fully break down, cold composting works well when there is no immediate need for the compost.
Hot composting is much quicker, producing usable humus in as little as four weeks. But it requires vigilant stewardship. Location, aeration, high carbon and nitrogen (or brown/green) content, sufficient depth, and moisture are key elements to a successful compost.
CARE AND MAINTENANCE OF YOUR HOT COMPOST PILE
Bin: Should be approximately 3 feet tall, wide and deep, and well ventilated, with easy access to the pile for turning with a pitchfork or shovel. Bins are readily available from many municipal recycling programs and most home and garden stores, or you can build your own.
Location: Keep away from areas that collect water. In cooler climates, avoid constant shade. A spot near the garden will save on labor.
Aeration: In addition to a well-ventilated bin, adding thicker, woody material will keep air flowing throughout the pile. Make sure the compost isn’t soggy or dense.
Carbon and nitrogen: Maintain a 30-to-1 carbon-to-nitrogen ratio. Manure or organic fertilizer can be added to boost the nitrogen level.
Moisture: The pile should always be moist but not soggy. During dry spells the pile can be watered. When it’s rainy, cover to maintain aeration and prevent leaching.
Temperature: Center should reach 130-160 degrees. Turn the pile before it exceeds 160 degrees; the temperature will rise and fall as the material is turned and breaks down. The highs and lows support different kinds of bacteria important to the process of decomposition. Too much nitrogen will make the pile so hot that many of these bacteria will die.
MATERIALS TO COMPOST AND THEIR CARBON/NITROGEN RATIOS
Again, composting is not a perfect science. Use the carbon/nitrogen ratios to help adjust your pile as needed. If the pile is well-aerated and properly moist but is not producing enough heat, add a high-nitrogen ingredient such as horse manure. Conversely, if the pile smells like sulfur and is has a slimy texture (signs of too much nitrogen and anaerobic decomposition), add a high-carbon ingredient such as sawdust.
• Fruit scraps, 35:1
• Vegetable scraps, 15-20:1
• Coffee grounds, 20:1
• Tea leaves, 25:1
• Eggshells, 15:1 (however, the available nitrogen is much lower because they take so long to break down.)
• Paper, 170-200:1
YARD AND GARDEN WASTE
• Deciduous leaves, 20-60:1
• Pine needles, 60-100:1
• Grass clippings, 15-25:1
• Weeds, 25:1
• Horse and cow manure, 20-25:1
• Sawdust, 200-500:1
• Seaweed, 19:1
• Straw, 40-100:1
• Mushroom substrate, 13:1
• Healthy garden soil contains many desirable microorganisms and can be added in small quantities.
• Ground sea shells, egg shells, and lime are low pH and will help make compost or soil less acidic.
Materials to avoid
• Human and pet waste
• Meat and meat byproducts
• Dairy products
• Diseased plants
• Invasive plants gone to seed
• Yard waste treated with pesticides and common fertilizers.
Finally, the past 20 years have seen enormous strides in municipal composting. Much of what can’t be added to an apartment worm box, community garden or backyard compost pile can be taken to a city-run compost. Bulky yard items, grass clippings, even tree limbs are quickly turned into usable humus for neighborhood gardens and landscaping projects.
The next frontier is cutting into the enormous quantity of biomass still going to landfills. Although the EPA, the USDA and many city governments are making inroads in this area, the obstacles remain formidable.
Much of the organic garbage going to landfills is what’s called food residuals and includes last night’s meatloaf and tuna casserole. Food residual pickup is an emerging part of municipal waste management, where separate containers are provided just for these items.