How to compost

If you can do only one thing to improve your garden this year, build a compost pile. No matter your soil type, your climatic zone, or the type of garden, composting will enhance your soil, resulting in stronger and healthier plants.

Compost in the soil improves soil structure, aids in water retention (important in sandy soils, improves aeration and drainage (important in clay soils), aids in nutrient retention, contains a low level of nutrients slowly released to plants, increases the number of microorganisms in the soil, increases heat absorption of soil, and helps neutralize soil pH, acting as a pH "buffer."

All organic materials—whether leaves, bones, coffee grounds, or heaps of dead weeds—will eventually rot. However, a random stacking of organic materials won't necessarily result in great compost, so compost is perhaps best defined as a pile of organic materials deliberately assembled for fast decomposition. A simple mound works, but many gardeners prefer to use wire or plastic bins, especially in urban or suburban settings.

For efficient decomposition, a compost pile needs a good balance of the Basic Four: Greens, Browns, Moisture, and Air.

"Greens" are the nitrogen materials (grass clippings, vegetable and fruit trimmings, green weeds) and "Browns" are the carbon materials (fallen leaves, straw, sawdust, or shredded newspaper). Fast decomposition requires both a good moisture content and ample oxygen for the decomposer organisms in the pile.

Hot vs. cold

All organic materials will decompose eventually, whether carefully layered and combined or not. "Hot" composting literally generates heat in the center of a pile that combines layers of green and brown material, with adequate moisture and air. "Cold" composting is gradual decomposition. A pile of fall leaves make take a year (or even more in the case of whole oak leaves) to turn crumbly, but it will eventually. Take care not to place mature weed seeds or tenacious weeds (such as Witch Grass or Nut Sedge) in a cold pile, because they may come back to haunt you.

What NOT to use:

Meat and meat bones, dairy products, and greasy foods are likely to attract pests. Cat, dog, and human feces can contain harmful pathogens. Pernicious weeds, especially those with rhizomatous root systems, may not be killed in the composting process. Diseased plants, bug-infested plants, and weeds with mature seeds should be kept out of slow, cool piles and should be added with discretion to the center of hot piles (when it doubt, keep it out). Needles from conifers are very slow to break down and can often be quite acidic. Leaves from eucalyptus, walnuts, and laurel trees contain tannins. (A few are okay.)

Making a compost pile

Beginners can use this general guideline: layer 50% green to 50% brown by volume. Layers can be 2-8 inches thick depending on the particle size and moisture of the materials. (An exception is grass clippings, whose high moisture content and small particle size make them mat and compact; use no more than a 2-inch layer at a time.)

Build a Compost Layer Cake

Layering is a good way of estimating equal proportions. Loosen soil under area where pile will be built. Layer brown materials (2-8 inches thick) covering at least a 3' x 3' area, or the full width of a bin. Water as needed to make material "moist as a wrung-out sponge." Layer green materials (2-8 inches thick), pulling material out to corners and edges. Add a sprinkling of soil or finished compost. Repeat layers, watering brown layers as needed, and keeping the shape cubical until pile is 3-4 feet tall. Finish pile with a brown layer and cap it with a 2-inch layer of soil if possible.

A large pile with a good balance of brown and green materials should decompose quickly, generate heat, and can give off a very slight odor of ammonia (one form of nitrogen gas). If a pile doesn't have enough nitrogen, it won't get very hot and won't decompose quickly. To speed it up, turn the pile and layer in high nitrogen materials. A pile with too much nitrogen will get very hot and will have a very strong odor of ammonia. A pile with excess nitrogen can sometimes become anaerobic, generating a "rotten egg" odor, especially if the materials are very wet. To remedy this problem, turn the pile to aerate it, and layer in some carbon materials, such as straw or shredded newspaper as you rebuild the pile.

Moisture: The Wrung-out Sponge Test

A compost pile should ideally be 40-60% moisture or as "moist as a wrung-out sponge." For best results, add water as needed as you build the pile, sprinkling the dry brown layer, especially around the outside edges. In warm, dry climates, water is most often the missing ingredient in piles that fail to heat up. Meanwhile, in cold, wet climates, excess moisture can lead to soggy, compacted piles. Decomposer organisms, from the smallest bacteria to the longest worm, need a consistent supply of moisture for peak efficiency.

If you are slowly building a pile over time, remember that much of the moisture can evaporate from the top foot of the pile, so re-wet the top of the pile as you build it.

Too much moisture can result in compaction and a loss of oxygen in the pile. When oxygen is squished out, anaerobic bacteria come into the scene, generating an unsavory "rotten eggs" smell.

Air: Building it into a Pile

The best decomposers for composting are aerobic (oxygen-requiring) microbes. If a pile lacks adequate oxygen—because it is too wet, too dense, or too big—anaerobic bacteria will take over, producing their characteristic "rotten eggs" smell. Air can be "built into" a large pile in a combination of ways:

1. Be sure to first loosen the soil that will lie under the pile;
2. add bulky materials, such as straw or cut-up brush, to the bottom of the pile;
3. if using layers of wet, finely-textured materials such as grass clippings, layer them with bulky materials such as straw to avoid compaction;
4. turn the pile at least once.

Oxygen is often the limiting factor in later stages of decomposition. Watch how much a large pile condenses in the first few weeks. Much of the oxygen in the pile can get used up or compressed out in this initial stage. As oxygen becomes less available, the microbial activity slows down. Turning the pile will reintroduce oxygen which can help the pile heat up again. If a pile becomes compacted due to excess moisture, aerate it by turning it and adding some bulky materials.

Particle size of materials

The size of organic materials affects how fast they compost. Smaller particles have more overall surface area exposed for bacteria and other decomposers to munch on. To speed up the composting process, chop large rough materials (especially woody stalks) with a sharp spade; use a lawnmower to shred leaves. Warning: If the particle size of all the materials is very small, the layers can compact and become matted.

Size of the compost pile

When the conditions are right in a compost pile, it becomes a small ecosystem teeming with decomposer organisms of different sizes and shapes. To achieve the benefits of hot composting, a pile must have a volume of three feet cubed or greater. Some sources say that piles should be closer to four feet cubed, but not much larger than five feet tall and five feet wide (and any length). A large, properly built pile is self-insulating and can reach temperatures from 140° to 160° sustained for 10 days to two weeks. These high temperatures will kill most weed seed and plant pathogens. Hot composting also yields a high-quality end product by rapidly reducing organic matter while conserving nutrients in a stable form. A smaller pile may also heat up, but it won't sustain high temperatures long enough to kill a significant number of weed seeds and pathogens.

Turning and maintenance

Turning a compost pile speeds the composting process and produces a better end product for several reasons:

1. turning brings oxygen into the pile;
2. the brown and green materials are remixed;
3. outside materials are put in the middle of the reconstructed pile, resulting in more thorough decomposition;
4. as you turn you can troubleshoot any problems and remedy them in the rebuilding process.

With hot composting, it's best to turn the pile about 3 weeks to 1 month after initial construction. At this point, oxygen becomes the limiting factor and the pile cools down. When turned, the pile can reheat with a new surge of microbial activity. Be sure to try to turn the pile at least once if not more often.

For most efficient maintenance, locate your pile close to the garden, within reach of a garden hose, and preferably on level ground with space for stockpiling ingredients. Locate piles under deciduous trees to shade them in the summer when the foliage is thick and to expose them to the warming sun when the tree is bare in the winter. In warm, dry climates, maintenance should include sprinkling the outside of piles that sit out in the sun. General maintenance for piles includes covering them with sheet plastic or a tarp to retain moisture in the summer and to avoid leaching in the winter.