Composting

Compost is a dark, crumbly mixture of decomposed organic matter, such as grass clippings, leaves, and thin twigs.

How Does Composting Work?

Even the first-time composter can make good quality compost. Like good cooking, composting is part science, part art. Attention to the following parameters will help you get started.

Materials

Anything that was once alive will naturally decompose. However, some organic wastes should not be composted at home.

What does compost actually do?

• It helps fertilize the garden
• It improves soil structure by increasing the aeration process and adding much needed texture
• It improves soil's water-holding capacity, especially important to new plants or in arid climates
• It helps sandy soils retain moisture
• It loosens clay soil by adding components that keep the soil broken apart
• It provides food for micro organisms
• It acts as a mulch

DO compost these items:

• grass clippings
• leaves
• small or flexible plant stalks
• hedge trimmings
• old potting soil
• very thin twigs
• annual weeds without seed heads
• vegetable scraps
• coffee filters
• tea bags
• paper and cardboard

Do NOT compost these items:

• diseased plants
• weeds with seed heads
• invasive weeds
• pet droppings
• dead animals
• bread and grains
• meat or fish parts
• dairy products
• grease and cooking oil, or oily foods.

Although all of these can be composted, they are also likely to attract unwelcome visitors such as rats. Although most plant diseases and weeds will be killed by the heat in the middle of a heap, they may be able to survive near the cooler outer edges and still be present when the compost is used.

Making It Work

To prepare compost, organic material, microorganisms, air, water, and a small amount of nitrogen are needed.

Organic materials are such things as leaves, grass clippings, etc. that you are trying to decompose. Microorganisms are small forms of plant and animal life, which break down the organic material. A small amount of garden soil or manure provides sufficient microorganisms.

The nitrogen, air, and water provide a favorable environment for the microorganisms to make the compost. A small amount of nitrogen fertilizer can add sufficient nitrogen to the compost. Nitrogen fertilizers can be purchased at many hardware stores, feed stores, or nurseries.

Air is the only part which cannot be added in excess. Too much nitrogen fertiliser can kill microbes; too much water causes insufficient aeration in the pile. Turning the pile helps with aeration. Aeration is vital because the organisms in the compost are aerobic which means they need air in order to do their jobs.

Biology

Bacteria are the first to break down plant tissue and are the most numerous and effective compost makers in your compost pile. Fungi and protozoans soon join the bacteria and, somewhat later in the cycle, centipedes, millipedes, beetles, and worms complete the composting process.

Surface Area

If the microorganisms have more surface area to feed on, the materials will break down faster. Chopping your garden debris with a machete, or using a chipper, shredder, or lawnmower to shred materials will help them decompose faster. Composting will also work better if soft items (e.g. grass clippings) and hard items (e.g. twigs, wood chips) are mixed together, rather than added separately.

Volume

Compost piles produce heat generated by the activity of millions of microorganisms. A compost pile one metre by one metre by one metre is considered a minimum size for hot, fast composting. Piles wider or taller than 1.5 m don't allow enough air to reach the microorganisms at the center.

Moisture and Aeration

The microorganisms in the compost pile function best when the materials are as damp as a wrung-out sponge and have many air passages. Extremes of sun or rain can adversely affect the balance of air and moisture in your pile. The air in the pile is usually used up faster than the moisture, so the materials must be turned or mixed up occasionallly to add air that will sustain high temperatures and control odor. Materials can be turned with a pitchfork, rake, or other garden tool.

Time and Temperature

The most efficient decomposing bacteria thrive in temperatures between 40°C to 55°C. Thus, the hotter the pile, the faster the composting. If you achieve a good balance of carbon and nitrogen, provide lots of surface area within a large volume of material, and maintain adequate moisture and aeration, the temperature will rise over several days.

Uses for Compost

Compost contains nutrients, but it is not a substitute for fertilizers. Compost holds nutrients in the soil until plants can use them, loosens and aerates clay soils, and retains water in sandy soils.

To use as a soil amendment, mix 5–10 cm of compost into vegetable and flower gardens each year before planting.

In a potting mixture, add one part compost to two parts commercial potting soil, or make your own mixture by using equal parts of compost and sand or perlite.

As a mulch, spread 5 cm of compost around annual flowers and vegetables, and up to 10 cm around trees and shrubs.

As a top dressing, mix finely sifted compost with sand and sprinkle evenly over lawns.

See also

• Organic Gardener's Composting

Sources

• USDA
• Soil and Compost - Hints and Tips