Glossary of Composting Terms

Actinomycetes: Also known as actinobacteria, actinomycetes are a form of bacteria existing mainly in soil. They form branches of thread-like filaments much like fungi and are important contributors to the decomposition process. They are especially effective at decomposing wood and hardy materials such as cellulose (tree and plant cell membranes) and chitin (fungi cell membranes). They also perform the valuable task of nitrogen fixing by forming cooperative relationships with plants and breaking down soil nitrogen so that it can be absorbed by the plants’ roots. Many of the nitrogen compounds in soil cannot be used by plants until they are first broken down by actinomycetes. Odors released by actinomycetes are what give compost and soil its “earthy” smell.

Activator: An accelerator that can expedite the natural decomposition process. Their purpose is to increase microbial activity.

Bacteria: Single celled microorganisms present in soil, water, organic material, animals and humans. Bacteria are found pretty much everywhere on earth, from deep sea trenches to human skin to yogurt, and they comprise the majority of organic life on the planet. There are approximately 40 million bacteria per teaspoon of soil and one million per milliliter of fresh water. They come in both beneficial and harmful, pathogenic forms. They help us humans digest our lunch and they help plants absorb nutrients by breaking down organic compounds into useable elemental particles. However, they are also responsible for human diseases such as syphilis and tuberculosis as well as common social faux pas including bad breath and body odor. Bacteria are crucial to the decomposition and nutrient recycling processes in both their aerobic and anaerobic varieties and are a friend and workhorse for every home composter.

Batch Composting: Composting in small amounts at a time using previously segregated materials is called batch composting. It includes composting in separate piles at the same time often with a “hot” pile and a “cold pile. Before batch composting, many gardeners start two separate piles of materials distinguishing one as a “brown” pile and the other as a “green” pile. The brown pile consists of dry materials such as dry leaves or lawn clippings. The green pile is made of kitchen and garden refuse, manures, and fertilizers. When batch composting, combine a larger amount from the “brown” (generally dry carbon-rich) pile with a smaller amount of the “green” (wet nitrogen-rich) pile into a separate pile or container then add some water and a compost activator or starter if needed. The ratio of “brown” to “green” is known as the C/N (carbon-nitrogen) ratio and is crucial in determining whether the pile is “hot” or “cold.” If the pile is aerated often and kept from drying out, you will have fresh, black, nutrient rich compost in as little as four to six weeks. You can also batch compost by simply adding fresh refuse or clippings to a bit of older compost, essentially using the older compost as a starter.

C/N Ratio: The carbon-to-nitrogen ratio in a compost pile. Whether a compost pile is considered “hot” or “cold,” is largely determined by the C/N ratio. The more nitrogen present, generally the “hotter,” “greener,” or more “wet” the pile will be. Nitrogen-rich materials for compost are the kitchen wastes such as food scraps, fruits and vegetables, blood and egg shells as well as fresh lawn clippings, fertilizers, and older compost. Carbon-rich materials are usually dry and “woody” such as dry leaves, sawdust, paper, teabags and coffee filters.

Cold “Slow” Pile: A cold or slow compost pile has a higher carbon-to-nitrogen ratio (C/N ratio) and takes longer to produce finished compost, anywhere from six months to two years. A cold pile is made mainly of carbon-rich dry or “brown” materials such as dry leaves, sawdust, and newspaper. Cold piles are less labor intensive but require larger amounts of both space and time. Because of the slow cool processes involved, cold piles produce compost that is more nitrogen rich. Since cold piles don’t reach the internal temperatures of hot “fast” piles, they tend to be weedier with more uncomposted material.

Compost: The end product of the decomposition of organic matter and biodegradable materials by aerobic and anaerobic microbes is known generally as compost. Compost is nutrient rich since during the decomposition process the larger organic compounds are broken down into their basic elemental particles that are easier for plants and other organisms to absorb. Compost serves as a great growing medium for agricultural and horticultural purposes as well as a soil conditioner. This dark brown to black soil/mulch can be easily made in the kitchen or back yard in a compost container or the more traditional pile form.

Composting: The aerobic or anaerobic decomposition of organic and biodegradable material resulting in a nutrient-rich growing medium and soil amendment. Composting can be performed in the backyard or kitchen and is an important way for people to take charge in recycling food and consumer wastes into a reusable and green energy source for growing food or flowers.

Decomposition: The process by which the organic matter of dead organisms decays or breaks down into constituent particles and elemental compounds. Decomposition is vital to new growth and the continued survival of living organisms since it is the main process by which nutrients are recycled in the natural world.

Enzymes: Protein molecules that act as catalysts in chemical reactions. Enzymes dramatically speed up the rate at which organic compounds decompose. Most enzymes are very specific, acting only on certain molecules. Animal saliva and GI tracts utilize enzymes to breakdown and digest food. Bacteria and fungi in the soil and air use enzymes to dissolve and decompose organic material during the process of decay thereby recycling and dispersing nutrients from dead organisms and reintegrating them into the food chain.

Hot “Active” Pile: A compost pile in which the internal temperature reaches a range between 110 and 160 F. Hot piles are the result of aerobic bacteria which thrive at higher temperatures and also generate heat as a by-product of their metabolism. Frequent aeration keeps the aerobic bacteria working and the temperature up as they consume the decaying organic matter. Hot piles work fast and can produce nutrient-rich compost in as little as 4-6 weeks compared with the 6 months to 2 years it takes a cold pile to achieve the same result.

Humus: Decayed organic matter usually seen as dark brown or black soil. Humus refers to organic material that has reached a point of stability in which it can decay no further. Fully-finished compost is humus. It is rich in nutrients and makes for a very fertile growing medium. Humus comes in different varieties depending upon the decomposed material that makes it. These varieties are very commonly used as soil amendments. Topsoil is an example of humus found nearly everywhere.

Inoculant: Beneficial fungi or bacteria added to soil to either destroy plant pathogens, form cooperative relationships with plant roots so they can absorb more nutrients, or to break down soil compounds into elemental particles that are more easily used by plants. Common soil inoculants are microbes called rhizo-bacteria which act as nitrogen-fixers by capturing and breaking down nitrogen in soils and air.

Leachate: A liquid solution resulting from bodies of decomposing matter such as compost piles and landfills when water is added. Bacteria, fungi, nutrients and pollutants can all be suspended in a leachate solution depending on the original ingredients. In composting, leachate is commonly referred to as compost tea. This is due to the process of brewing or steeping compost in water in order to create a leachate that is higher in nutrients and microbes than would naturally occur. Leachate or compost tea is often applied to soils in order to increase the microbial presence so that decomposition of organic matter such as leaves is sped up.

Lime: Generally refers to inorganic materials made up predominantly of calcium such as gypsum. Specifically, lime refers to calcium oxide (CaO), also known as quick lime, which is highly caustic. Lime made from burning shells such as oyster shell lime are predominantly made of calcium carbonate and is commonly used as a soil amendment. Lime as a soil amendment is used to raise soil pH in order to make the soil more alkaline and is sometimes used to improve soil structure. When composting, it is important not to throw fresh ashes on the compost as they will act as quick lime destroying all microbial life within the pile.

Mesophiles: Organisms which thrive in moderate temperature environments ranging from 77 to 104 F. Mesophiles are generally bacteria and other microbes living in soil, food, and animals. Beer, yogurt, and cheese are examples of foods that rely on mesophilic microorganisms for their preparation.

Microorganism: Microscopic organisms, also called microbes, not visible to the naked eye. Bacteria and fungi are common microorganisms. Microscopic plants such as algae as well as microscopic worms and parasites are also considered to be microorganisms.

Psychrophiles: Organisms that survive, grow and thrive in cold temperatures generally described as below 70F. Obviously these organisms are fairly ubiquitous as the majority of the earth’s climates experience temperatures in this range. Psychrophilic microorganisms exist in alpine and arctic soils as well as in trenches miles below the surface of the ocean.

Sheet Composting: Composting by spreading a thin layer of organic material across a large area such as a garden bed. With sheet composting, there is no need to create a compost pile. Grass and garden clippings make great materials for sheet composting and since they compost slowly they offer many of the same benefits as mulch by protecting soil from weather and erosion. Shredding or chopping up the materials can speed up the process when sheet composting.

Thermophiles: Organisms that grow and thrive at high temperatures generally described as above 110F. Thermophiles are commonly found in decomposing organic material such as compost and peat moss as well as in natural hot spots such as geothermal hot springs and deep sea vents. Thermophiles can withstand intense heat and actually utilize this heat to help break down compounds which they metabolize. Many thermophiles require elemental sulfur to survive.

Vermicomposting: Using worms to compost organic material. Vermicompost is generally richer in nutrients than regular compost due to a higher presence of microbes and the nutrient-retaining properties of worm castings. Red wiggler worms are the most common worms used for composting though European earthworms are sometimes used. Worms are not the only organisms involved in vermicomposting; bacteria and fungi, as in regular composting, also contribute to the process. Red wiggler worms are commonly found in nature in soil, manure and rotting vegetation. These worms adapt easily to living in worm bins since they naturally survive off of decaying organic matter and live close to the surface. Vermicomposting requires less physical labor than normal composting since the worms do most of the aerating. In large scale operations, a windrow system is often used. Bins are the most common method for vermicomposting on a small scale.

Windrow System: Long, open-air piles, windrows consist of organic matter or compost that is aerated manually or mechanically. The windrow system is most commonly used for composting on a large scale such as on a farm. Windrows are built from a mixture of brown (carbon-rich) and green (nitrogen-rich) materials in ratios that optimize air and water infiltration and microbial life just like a regular compost pile. On a large scale, windrow compost turners or backhoes are used to turn over and aerate the windrows. On a small-garden scale, a shovel or pitchfork and some physical work suffices.

Worm Casting: Worm excrement that is nutrient-rich with excellent moisture retention often used as a natural fertilizer and soil amendment. Worm castings contain mucus that helps retain nutrients so they are not washed away during waterings or rain. They also stimulate microbial life within the soil, which aids decomposition and overall soil health. As a soil amendment, worm castings are often used not only for nutrients but also to improve soil structure and oxygen/water penetration.