Backyard Composting

Part 1: Composting basics

Introduction

Part 1:
Composting basics

Part 2:
How to make compost

Part 3:
Health and safety questions

Part 4:
Using compost in your yard

Composting coffee grounds

The science of composting

The cycle of growth and decay.

Composting carries out part of the earth's biological cycle of growth and decay. Plants grow by capturing the sun's energy along with carbon dioxide from the air and nutrients and water from the soil. When plants (and the animals that eat them) die, they become raw materials for the composting or decay process. Microorganisms, fungi, insects, worms, mites, and other creatures convert the carbon from dead plants into energy for their own growth, releasing carbon dioxide into the air. Similarly, they recycle the nutrients from the decaying plants into their own bodies and eventually back into the soil. Other plants and microorganisms use the carbon and nutrients released by the composting process, and the cycle begins again.

The material that remains from the decay process is similar to soil organic matter. It holds water and nutrients in the soil, and makes the soil more porous and easier to dig.

Fast or hot composting.

We can manipulate the decay process to make it proceed quickly. We do this by balancing food, water, and air in the compost pile to favor the growth of thermophilic (high-temperature) microorganisms. One byproduct of microbial activity is heat. When conditions are favorable for high-temperature microorganisms, compost piles heat rapidly to 120'F to 150 F. This temperature range kills most weed seeds and pathogens (disease organisms), but does not kill mycorrhizae (beneficial fungi that help plant roots absorb nutrients). Once the hot phase is complete, lower-temperature microorganisms, worms, insects, and other invertebrates complete the decay process.

Slow composting.

If we do not maintain ideal conditions for hot composting, microorganisms will still break down the wastes. Decay will be slower, cooler, and less effective at killing weed seeds and pathogens.

Managing the decay process

You can affect the speed of the composting process and the quality of the compost product by managing the factors described below.

Food (Raw materials).

For fast composting, the initial mix must have the proper moisture and air content, and organic materials that provide a rich food (energy) source for bacteria. A list follows of some materials commonly used in making compost. They are separated into "energy" materials, "bulking agents," and "balanced" materials.

Energy materials provide the nitrogen and high-energy carbon compounds needed for fast microbial growth. If piled without bulking agent, these materials usually are too wet and dense to allow much air into the compost pile. When you open the pile, it will have a foul, "rotten egg" smell.

Bulking agents are dry, porous materials that help aerate the compost pile. They are too low in moisture and nutrients to decay quickly on their own.

Balanced raw materials. Some raw materials contain a balance of energy and bulking agent properties. These materials will compost readily without being blended with other ingredients. Examples include horse manure mixed with bedding, spoiled alfalfa hay, and deciduous leaves. These materials are handy for ensuring the success of hot compost piles.

Mixing bulking agents with energy sources provides the right balance of moisture, air and nutrients for rapid composting. A mixture of one part energy source with two parts bulking agent (by volume) usually gives a reasonable mix for rapid composting.

Particle size. Grinding, cutting, smashing, or chopping raw materials reduces particle size. Small particles have more surface area for microbial activity and are easier to mix. Hot composting requires a relatively uniform particle size of 1/8 to 1/2 inch in diameter. Woody branches that have not been ground often make it difficult to turn a pile. They also decompose very slowly. We suggest grinding or chipping woody branches, or piling them separately.

Mixing. Contrary to advice in many publications, layering is not the best way to build a pile. If all the materials are on hand, mix them thoroughly throughout the pile. If materials accumulate over time, add new materials to the center of the pile. This practice will help aerate the center of the pile, where anaerobic conditions are likely to occur.

Pile size. The pile must be big enough to hold heat. A hot pile decays much faster than a cold pile. Small piles are usually colder, because they have small cores that hold less heat. Small piles also dry out faster. A pile of about one cubic yard is big enough for year-round composting, even in cold-winter areas.

Moisture. All materials in the pile must be moist but not soaking wet. The mixed material should feel moist, but you should not be able to squeeze water out of it with your hands. At dry times of the year, you may need to add water. In rainy winter locations, a pile may not heat up unless you cover it to keep out rainwater. Check moisture when you turn the pile.

Aeration. The microorganisms responsible for fast decomposition need oxygen. In the pile, oxygen is consumed by microbes. The pile needs to be porous to pull outside air into the pile. Use enough bulking agent to create a porous pile. As the pile decomposes, it settles, reducing aeration. Turning the pile or adding bulking agent improves aeration.

Microorganisms. Raw materials used to form a compost pile usually contain all the microorganisms needed to make compost. You do not need to add soil or compost starters with "special" microorganisms. The best source of microorganisms (if needed) is finished compost.

Nutrients. Just like people, microorganisms need nutrients (such as nitrogen, phosphorus, sulfur) to grow and reproduce. These nutrients occur in the raw materials used in the compost mix. Additional fertilizer from any source (organic or inorganic) usually is not needed. Nitrogen fertilizer may be beneficial for mixes consisting mainly of bulking agents. The best way to add fertilizer is to dissolve it in water and wet the pile with a dilute fertilizer solution. Compost additives such as blood meal and bone meal are simply organic fertilizers; they do not contribute anything magic to the compost pile.

Part 2: How to make compost