Composting is a process of decomposition. It is a natural cycle that has occurred on earth since primitive life existed (Martin, 10). It involves the breakdown of plant and animal waste to a material that resembles soil called humus. Humus is a stable substance made after all the degradable organic matter in compost has broken down. However, it is not quite the final product of compost. Humus does continue to decompose, but very slowly compared to the compost it came from (Campbell, Stu 9). “All humus is brown or black, has a fine, crumbly texture, is very light-weight when dry, and smells like fresh earth” (Solomon, Steve 10).
Compost depends on a complex web of soil microbes and meso/macro fauna to carry out the decomposition process required to form stable humus. The members of the soil community are described in greater detail later in this background information.
O.K. Now that I know what compost does, why is it even important?
History has shown us that people have been composting for millennia. In fact it is the “oldest and most universally practiced form of soil treatment in the world” (Martin 12). Why would people begin to imitate a decomposition process that happens naturally all over the planet?
- Compost builds soil structure: A good soil structure is key to the growth of healthy plants. Compost helps soil hold water, provides space for needed aeration, and maintains moisture needed by plant roots and the soil biota (13).
- Compost aids in aggregate formation: Healthy soil requires the formation of crumb sized and coarse-grained particles. Soil fungi grow in the organic matter of compost. Soil bacteria then come along, eat this fungi, and convert it into a sort of “cement” that holds particles together. Worms live in compost as well and their castings act as a “cement” helping to form the particles needed for healthy soil (16-17).
- Compost protects against drought and erosion: One hundred pounds of humus can hold up to 195 pounds of water. When compost is mixed into soil, it improves the soil’s ability to hold water. In 1986, USDA studies showed that over 6 billion tons of soil eroded in the United States in one year. Compost organic matter and humus help create fertile soil that is more capable of resisting erosion due to good soil structure (18-19).
- Compost improves aeration: Compost builds a good soil structure in which air pockets are present. Soil that is packed down does not allow water to permeate that is needed by the soil biota and plant roots. Compost also provides the space to move oxygen through soil providing the oxygen needed by microorganisms and for the chemical processes that make nutrients available for plants (20-21).
- Compost makes nutrients available to plants: Because the organic matter in compost decomposes at different speeds, compost can provide for a slow release of nutrients like nitrogen and sulfur that plants need over an extended period of time (1). Compost is like a storehouse releasing these nutrients at a pace that matches the growing cycle. As the temperature increases, soil microorganisms work harder releasing more nutrients from organic matter and their own dead bodies (22). Humus particles are called colloids. Colloids have a slightly negative charge that enables them to attract positive elements like potassium, sodium, and calcium (all needed for plant growth and survival). Plant roots can trade positive charged hydrogen ions on their roots for these minerals that they need to survive. High humus soil can maintain its fertility for plants. In fact “one can pour seven times their weight in water through soils with high organic matter in 12 washings, and not lose any appreciable amount of minerals” (23).
- Compost neutralizes toxins: Organic matter in humus can hold heavy metals that are toxic in high quantities to plants. By releasing toxic, heavy metals like aluminum slowly, plants get what they need to survive, but do not get poisoned (24-25).
- Compost buffers plants: When humus is added to soil it helps plants fare better in more acid or alkaline soils that they might not usually survive in. In other words humus “reduces plants’ reliance on specific soil pH levels” (25).
- Compost stimulates plant growth: Humic acids have been shown in studies to stimulate plant growth. Humic acids serve as a nutrient for plants and also help them assimilate oxygen (26).
What ingredients does a compost pile need to break down to humus?
- Appropriate carbon: nitrogen ratio
All compost piles require a balance of nitrogen and carbon to feed the organisms that live inside them to effectively break down materials. In an ideal compost situation the ratio of carbon to nitrogen should be about 25:1 (Martin 32). Carbon compounds “in plant and animal wastes provide food for decomposers in the compost pile” (32) while nitrogen compounds provide proteins for growth (Solomon, 20).
- All the proper ingredients
This ratio can be achieved by assuring that all the necessary components of a compost pile are in balance. These include a balance of brown materials (which contain high amounts of carbon), green materials (which contain high amounts of nitrogen), adequate air, moisture, and a healthy soil biota.
Carbon rich and often brown materials include straw, hay, saw dust, woody materials, paper, autumn leaves and dried plant waste. Nitrogen rich and mostly green materials include manure, grass clippings, weeds, seaweed, and food waste. Since most of the soil microorganisms are aerobic, the compost pile requires adequate aeration for them to survive. To deal with this challenge compost bins can be built with vents or turned during the decomposition process. A compost pile must be moist (in fact a moisture content of 40-60% is good) to break down organic matter. Too much water prevents proper aeration while not enough water leaves the pile of organic matter in the same state for years. To add the soil organisms required for decomposition, simply throw in a handful of soil into the compost bin (Putman, Cynthia 26-50).
- Soil microorganisms:
A healthy compost pile community is an excellent example of biodiversity
What are consumer levels and who works in each level?
It is important to understand how the soil organisms are involved in a complex food web with several levels of consumerism. The biodiversity of these organisms allows for breakdown of the many different kinds of organic waste to the more stable substance, humus. The different consumer levels also keep the soil organism populations in check (Martin, 38). There are two types of decomposition that occur in compost piles, chemical and physical. The bacteria, fungi, protozoa, and actinomycetes are all responsible for the chemical decomposition and they change the chemistry of organic waste (Solomon, 18). The physical decomposers “bite, grind, suck, tear, and chew materials into smaller pieces making them more suitable for the chemical work of the microscopic decomposers” (Martin, 36).
The level one consumers (1°) are the primary decomposers who consume the organic waste directly. These include the bacteria, molds, fungi, and actinomycetes described above. Larger soil fauna like the earthworm, beetle mite, sow bug, enchytraeid, and fly are all members of this consumer level as well (Applehof, 96). The larger soil fauna like earthworms mechanically break down organic matter by chewing on easily digestible substances. Bacteria and fungi can then come along to eat the undigested organic waste found in the excrement of this larger fauna.
Level two consumers (2°) eat the 1°consumers. These organisms include mold mites, feather-winged beetles, protozoa, and nematodes. These soil organisms can be a 1° or 2° depending on what they eat. Level two consumers also break down the tougher parts of organic matter, the lignin and cellulose.
The level three consumers (3°) are the predators who eat the 1° and 2° consumers. This level includes centipedes, rove beetles, ants, and predatory mites (Applehof, 96).
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