Composting is the process by which raw organic materials are transformed, primarily by bacteria and fungi, into a stable, nutrient-rich substance known as humus. Humus is chemically complex, spongy, porous, and retains a high concentration of essential nutrients that are readily accessible to the roots of plants. Humus occurs naturally when plant and animal remains in marshes, forests, and grasslands break down over the span of centuries. Humans can hasten this process by constructing piles of various organic materials, and providing adequate moisture and aeration. Replenishing our soils with mature compost (i.e. humus) is the best way of building long-term soil fertility and ensuring an abundant harvest.
Compost improves virtually all physical, chemical, and biological conditions of the soil. High-quality compost helps create healthy, living soil teeming with earthworms, microbes and a vast array of available nutrients that produce robust plants resistant to pests and diseases. Compost-created humus provides plant roots with the proper combination of nutrients without overwhelming them with any particular one; something that frequently occurs with soluble chemical fertilizers. Compost also contains essential trace minerals that plants need. And it is an excellent way of recycling organic materials.
Some of the other advantages offered by composting are:
- Certain elements, particularly nitrogen, can leach out of a plant's root zone. Fortunately, the robust microbial communities present in compost stabilize nitrogen (as well as phosphorus and sulfur) in their bodies. When compost is added to the garden, these organisms die and slowly release nitrogen back to the plants.
- The addition of organic matter through composting increases the soil's ability, particularly those with a sandy texture, to act like a sponge and retain water. Runoff and soil erosion are reduced.
- The structure of clay soils is improved. Compost adds airspaces and loose, crumb-like aggregates, which prevent soil clumping. During the composting process, when temperatures in the pile reach 130 to 140°F, most soil pathogens and viable weed seeds are eliminated.
- Finally, there is nothing else quite as satisfying as holding good compost in your hands, smelling its rich, earthy smell and then turning it into your garden, knowing that you are giving your soil the gift of life.
Composting is somewhat of an alchemical practice. While many techniques exist, there are some basic steps common to all composting situations.
- Organic materials are collected and stacked in layers that alternate brown, carbon-rich matter (straw, leaves, sawdust, pine needles, corn stalks, dried weeds) with green, nitrogen-rich matter (freshly-cut cover crops, kitchen scraps, grass clippings, fresh weeds, manure from herbivores).
- Carbon is the primary energy source for micro-organisms, while nitrogen is the building block for proteins which provide the structure for microbial cells. The most active fermentation occurs when the raw ingredients' carbon to nitrogen ratio is about 25:1.
- Add soil to enhance decomposition and to reduce any unpleasant odors. It is important to turn the pile occasionally with a fork and to keep it moist in order to maintain a high level of biological activity.
- Avoid composting meat, dairy products, and any materials such as sewage sludge, kitty litter and leather wastes that may contain pathogens or toxins.
There are two main phases in the composting process: a "hot" and a "cold" phase. In the hot phase, initial decomposition is aerobic (requiring oxygen). As bacteria begin consuming raw organic material, their population rapidly increases and large amounts of carbon dioxide are released. This generates heat, which destroys harmful pathogens and many weed seeds. In the hot phase, which lasts a few days or a week, steam can often be seen rising off the compost pile on a cool morning.
The compost pile becomes partially anaerobic (without oxygen) and temperatures drop during the cold phase as various bacterial species transform the raw organic material into stable humus. A large portion of the original organic material has been converted to carbon dioxide at this point, while the remaining material has collapsed into a much more compact pile, eliminating most of the air spaces. Further decomposition is now carried out by fungi, earthworms, and actinomycetes—microorganisms with unique antibiotic properties.
A compost pile can be started anytime, even in winter, although materials will decompose much more quickly in warm weather. The amount of time it takes for compost to be ready for the garden varies from two months to two years depending on the type of raw materials and inoculant used, the amount of water and air present, the frequency of turning the pile, and weather conditions. Bio-intensive agriculture advocates adding large amounts of soil to the compost pile to activate decomposition.
Whatever method you adopt, it is important to consistently monitor the pile.
- If it is not heating up, add more nitrogenous material.
- A compost pile may also not heat up because it is too dry.
- If it is too wet, it lacks air and should be turned.
- If the pile smells of ammonia, it has too much nitrogen and needs more carbonaceous material. It may also be too wet.
- Use a Compost Thermometer to keep your pile's temperature at or below 140°F as beneficial microorganisms will begin to be destroyed. Turn your pile before this happens.
If you turn your pile every 7-10 days, your compost will be ready in as few as 6-8 weeks. You can tell that your compost is ready when you can no longer recognize any of the original ingredients that you put in the pile. It should be dark brown, light and crumbly, and smell pleasantly earthy. Congratulations! You have created your own perfectly balanced fertilizer with an ideal ratio of carbon to nitrogen. You can add it to the garden in any amount without burning your plants or poisoning the soil. Our gardens gain the greatest benefit when we apply compost to cover crops. The combination of compost and cover crops produces the very best fertilizer for our plants, an elegantly simple, effective means of enlivening our soils and ensuring a successful garden.