Glossary of Key Composting Terms

This Glossary is reprinted with permission from On-Farm Composting Handbook (NRAES-54). ©1992 by NRAES (Natural Resource, Agriculture, and Engineering Service). All rights reserved. For more information, please contact PALS Publishing, (607) 255-7654, or


  • Actinomycete: A group of microorganisms, intermediate between bacteria and true fungi, that usually produce a characteristic branched mycelium. These organisms are responsible for the earthy smell of compost.
  • Aerated static pile: Forced aeration method of composting in which a freestanding composting pile is aerated by a blower moving air through perforated pipes located beneath the pile.
  • Aeration: The process by which the oxygen-deficient air in compost is replaced by air from the atmosphere. Aeration can be enhanced by turning.
  • Aerobic: An adjective describing an organism or process that requires oxygen (for example, an aerobic organism).
  • Agitated-bed: An in-vessel composting method in which the materials are contained in a bin or reactor and are periodically agitated by a turning machine or by augers. Usually some means of forced aeration is also provided.
  • Agricultural wastes: Wastes normally associated with the production and processing of food and fiber on farms, feedlots, ranches, ranges, and forests. May include animal manure, crop residues, and dead animals. Also agricultural chemicals, fertilizers, and pesticides that may find their way into surface and subsurface water.
  • Air pressure loss (also called static pressure or resistance): The pressure or energy lost as air moves through a system such as the compost pile, pipe, blower, and filter pile of an aerated static pile. The air pressure loss indicates the amount of energy required to move air through the system at the desired flow rate. The pressure loss must be estimated in order to select an appropriate fan or blower.
  • Ambient air temperature: The temperature of the air in the vicinity of the compost pile.
  • Amendment: See composting amendment and soil amendment.
  • Ammonia (NH3) : A gaseous compound comprised of nitrogen and hydrogen. Ammonia, which has a pungent odor, is commonly formed from organic nitrogen compounds during composting.
  • Ammonium (NH4+): An ion comprised of nitrogen and hydrogen. Ammonium is readily converted to and from ammonia depending on conditions in the compost pile.
  • Anaerobic: An adjective describing an organism or process that does not require air or free oxygen.
  • Anion: An atom or molecule with a negative charge (for example, nitrate, NO3-).
  • Aspergillus fumigatus: Species of fungus with spores that cause allergic reactions in some individuals. It can also cause complications for people with certain existing health problems.
  • Availability, nutrient: See nutrient availability.


  • Bacteria: A group of microorganisms having single-celled or noncellular bodies. Bacteria usually appear as spheroid, rodlike, or curved entities but occasionally appear as sheets, chains, or branched filaments.
  • Batch mixer: A type of mixer which blends materials together in distinct loads or batches. The materials are loaded, mixed, and then unloaded in sequence rather than moved through in a continuous flow. Batch mixers for composting are often modified livestock feed mixers using paddles or augers as the mixing mechanisms.
  • Bedded manure pack: A mixture of bedding and manure that accumulate over time in a livestock barn. A bedded pack forms when bedding materials are regularly added to the manure that is deposited by livestock in the barn. The manure-bedding mixture is not frequently removed but gradually builds up and becomes the surface on which the livestock stand and lie. To provide a firm surface, a large amount of bedding is required. Therefore, bedded pack manure usually is dry.
  • Bedding: Dry absorbent materials used to provide a dry lying surface for livestock. Bedding materials such as sawdust and straw absorb moisture from livestock wastes, the soil, and the environment.
  • Bin composting: A composting technique in which mixtures of materials are composted in simple structures (bins) rather than freestanding piles. Bins are considered a form of in-vessel composting, but they are usually not totally enclosed. Many composting bins include a means of forced aeration.
  • Biochemical oxygen demand (BOD): The quantity of oxygen used in the biochemical oxidation of organic matter in a specified time, at a specified temperature, and under specified conditions. Normally five days at 20°C unless otherwise stated. A standard test used in assessing the biodegradable organic matter in municipal wastewater. See also chemical oxygen demand.
  • Biogas: A mixture of gases, including methane and carbon dioxide, which is generated by the anaerobic biological decomposition of organic materials (for example, manure). Biogas can be burned as a fuel.
  • BOD: See biochemical oxygen demand.
  • Buck wall: A relatively short strong wall, often made of concrete or treated wood. It is used primarily as a support to push against when scooping and lifting loose or flowing materials (for example, manure).
  • Bucket loader: A vehicle which employs a hydraulically operated bucket to lift materials. Includes farm tractors with bucket attachments, skid loaders, and large front-end loaders.
  • Bulk density: Weight or mass per unit of volume of a material comprised of many individual particles. For example, the weight of a pile of wood chips divided by the volume of the pile is the bulk density. This is different from the particle density (which, in this case, equals the weight of a single wood chip divided by its volume). See also density.
  • Bulking agent: An ingredient in a mixture of composting raw materials included to improve the structure and porosity of the mix. Bulking agents are usually rigid and dry and often have large particles (for example, straw). The terms "bulking agent" and "amendment" are commonly used interchangeably. See also composting amendment.


  • C: Chemical symbol for carbon.
  • Carbon dioxide (CO2): An inorganic gaseous compound comprised of carbon and oxygen. Carbon dioxide is produced by the oxidation of organic carbon compounds during composting.
  • Carbon-to-nitrogen ratio (C:N ratio): The ratio of the weight of organic carbon (C) to that of total nitrogen (N) in an organic material.
  • Cation: A atom or molecule which has a positive charge (for example, ammonium, NH4+).
  • Cellulose: A long chain of tightly bound sugar molecules that constitutes the chief part of the cell walls of plants.
  • Chemical oxygen demand (COD): A measure of the oxygen-consuming capacity of inorganic and organic matter present in water or wastewater. It is expressed as the amount of oxygen consumed from a chemical oxidant in a specified test. It does not differentiate between stable and unstable organic matter and thus does not necessarily correlate with biochemical oxygen demand. See also biochemical oxygen demand.
  • CO2: Chemical symbol for carbon dioxide.
  • COD: See chemical oxygen demand.
  • Compost: A group of organic residues or a mixture of organic residues and soil that have been piled, moistened, and allowed to undergo aerobic biological decomposition.
  • Composting: Biological degradation of organic matter under aerobic conditions to a relatively stable humus-like material called compost.
  • Composting amendment: An ingredient in a mixture of composting raw materials included to improve the overall characteristics of the mix. Amendments often add carbon, dryness, or porosity to the mix.
  • Compost stability: See stability, of compost.
  • Contamination: Any introduction into the environment (water, air, or soil) of microorganisms, chemicals, wastes, or wastewater in a concentration that makes the environment unfit for its intended use.
  • Cubic yard: A unit of measure equivalent to 27 cubic feet or 22 bushels. A box that is I yard wide, I yard long, and I yard high has a volume of I cubic yard. A cubic yard is often loosely referred to as a "yard" (for example, a one-yard bucket).
  • Curing: Final stage of composting in which stabilization of the compost continues but the rate of decomposition has slowed to a point where turning or forced aeration is no longer necessary. Curing generally occurs at lower, mesophilic temperatures.


  • Damping off disease: The wilting and early death of young seedlings caused by a variety of pathogens.
  • Decomposers: The microorganisms and invertebrates that cause the normal degradation of natural organic materials.
  • Degradability: Term describing the ease and extent that a substance is decomposed by the composting process. Materials which break down quickly and/or completely during the time frame of composting are highly degradable. Materials which resist biological decomposition are poorly or even non-degradable.
  • Denitritication: An anaerobic biological process which converts nitrogen compounds to nitrogen gas or nitrous oxide.
  • Density: The weight or mass of a substance per unit of volume. See also bulk density.
  • Detention basin: See holding pond.
  • Dry matter: The portion of a substance that is not comprised of water. The dry matter content (%) is equal to 100% minus the moisture content (%).


  • Electrical conductivity: A measure of a solution's ability to carry an electrical current; varies both with the number and type of ions contained in the solution.
  • Enzymes: Any of numerous complex proteins produced by living cells to catalyze specific biochemical reactions.
  • Ericaceous: Belonging to the plant family, Ericaceae, the heath family of plants. Characterized by evergreen or deciduous shrubs, trees, and woody plants growing in acid soil and having simple leaves, often showy flowers either solitary or in clusters, and fruit in the form of a berry or capsule.
  • Evaporative cooling: The cooling that occurs when heat from the air or compost pile material is used to evaporate water.
  • Exchange capacity: A measure of the nutrient holding power of a soil or soil amendment, such as compost. Indicates a soil's ability to attract and retain plant nutrients which exist as charged molecules or ions. Cation exchange capacity concerns positively charged ions. Anion exchange capacity refers to negatively charged ions. Cation exchange is usually stressed because most soils have a negative charge and, therefore, attract the positively charged cations typically supplied by fertilizers.
  • Extended pile: A pile form used in the aerated static pile composting technique in which a large pile is constructed of individual cells, each with an aeration system. Cells are added daily and stacked against the previous cell, giving the overall pile a nearly rectangular cross section.


  • Fertilizer value: An estimate of the value of commercial fertilizer elements (N, P, K) that can be replaced by manure or organic waste material. Usually expressed as dollars per ton of manure or quantity of nutrients per ton of manure.
  • Filter press cakes: Residues from filter press operations after filter presses remove liquids.
  • Forced aeration: Means of supplying air to a composting pile or vessel which relies on blowers to move air through the composting materials.
  • Fungus: Plural fungi. A group of simple plants that lack a photosynthetic pigment. The individual cells have a nucleus surrounded by a membrane, and they may be linked together in long filaments called hyphae. The individual hyphae can grow together to form a visible body.


  • Green manure: Plant material incorporated into the soil, while green, to improve the soil.
  • Grinding: Operation which reduces the particle size of materials. Grinding implies that particles are broken apart largely by smashing and crushing rather than tearing or slicing. See also shredding.


  • Heavy metals: A group of metallic elements that include lead, cadmium, zinc, copper, mercury, and nickel. Can be found in considerable concentrations in sewage sludge and several other waste materials. High concentrations in the soil can lead to toxic effects in plants and animals ingesting the plants and soil particles. Federal and many state regulations restrict the land application of materials which contain high concentrations of heavy metals.
  • Herbicides: Agents used to inhibit plant growth or kill specific plant types.
  • Holding pond (also called retention basin or detention basin): An earthen basin to temporarily store precipitation runoff and other water for later use or disposal. Holding ponds can be excavated or formed above grade by constructing earthen embankments.
  • Humic acids: The chemical or biological compounds composed of dark organic substances that are precipitated upon acidification of a basic extract from soil.
  • Humus: The dark or black carbon-rich relatively stable residue resulting from the decomposition of organic matter.
  • Hydrogen sulfide (H2S): A gas with the characteristic odor of rotten eggs, produced by anaerobic decomposition.
  • Hyphae: See fungus.


  • Immobilization, nitrogen: Conversion of nutrient compounds from an inorganic form, available to plants, into the organic tissue of microorganisms (or other plants). The nutrients are unavailable until the microorganisms die and the microbial tissues containing the nutrients decompose. Nitrogen immobilization occurs when materials with a high C:N ratio are land applied. The microorganisms that use the carbon also assimilate the available nitrogen, rendering it unavailable to plants.
  • Infiltration area: An area or strip of land that is vegetated (usually with grass) where water enters the soil in a controlled manner. Infiltration areas can be relatively flat to gently sloping parcels of land or long. narrow, low-sloping channels. Pasture or hay crop land can serve as an infiltration area. Infiltration areas can be used to treat dilute waste water and nutrient-laden runoff.
  • Inoculum: Plural inocula. Living organisms or material containing living organisms (such as bacteria or other microorganisms) which are added to initiate or accelerate a biological process (for example, biological seeding).
  • In-vessel composting: A diverse group of composting methods in which composting materials are contained in a building, reactor, or vessel.


  • K: Chemical symbol for potassium.


  • Land application: Application of manure, sewage sludge, municipal wastewater, and industrial wastes to land either for ultimate disposal or for reuse of the nutrients and organic matter for their fertilizer value.
  • Leachate: The liquid that results when water comes in contact with a solid and extracts material, either dissolved or suspended, from the solid.
  • Lignin: A substance that, together with cellulose, forms the woody cell walls of plants and the cementing material between them. Lignin is resistant to decomposition.
  • Liquid manure (thin slurry): Manure which has had sufficient water added so that it can be pumped easily. Normally fibrous material such as chopped straw or waste hay is not present. See also manure.
  • Litter, poultry: Dry absorbent bedding material such as straw, sawdust, and wood shavings that is spread on the floor of poultry barns to absorb and condition manure. Sometimes the manure-litter combination from the barn is also referred to as litter.


  • Manure: The fecal and urinary excretion of livestock and poultry. Sometimes referred to as livestock waste. This material may also contain bedding, spilled feed, water, or soil. It may also include wastes not associated with livestock excreta, such as milking center wastewater, contaminated milk, hair, feathers, or other debris. See also liquid manure, semi-solid manure, slurry manure, and solid manure.
  • Manure storage: A storage unit to keep manure contained for some period of time prior to its ultimate utilization or disposal. Manure storages are usually classified by type and form of manure stored and/or construction of the storage; for example, above- or below-ground liquid manure tank, earthen storage basin, solid manure storage. See also manure.
  • Mesophilic: Operationally, the temperature range most conducive to the maintenance of optimum digestion by mesophilic bacteria, generally accepted as between 50 and 105°F (10 and 40°C).
  • mho: See mmho.
  • Microbe: See microorganism.
  • Microfauna: Populations of microscopic animals including protozoa and nematodes.
  • Microflora: Populations of microscopic plants including bacteria, actinomycetes, fungi, and algae.
  • Microorganism: An organism requiring magnification for observation.
  • mmho: Plural mmhos. A millimho. One thousandth of a mho (pronounced mo with a long O). A mho is a unit of measurement for electrical conductivity which is the basis for measuring soluble salt concentration (mho is the backward spelling of ohm, the unit of measurement for electrical resistance).
  • Moisture content: The fraction or percentage of a substance comprised of water. Moisture content equals the weight of the water portion divided by the total weight (water plus dry matter portion). Moisture content is sometimes reported on a dry basis. Dry-basis moisture content equals the weight of the water divided by the weight of the dry matter.
  • Mulch: A material spread over the soil surface to conserve moisture and porosity in the soil underneath and to suppress weed growth. Grass clippings, compost, wood chips, barks, sawdust, and straw are common mulch materials.
  • Mycelium: The collective term for fungus filaments or hyphae.


  • N: Chemical symbol for nitrogen.
  • Nitrate-nitrogen: A negatively charged ion comprised of nitrogen and oxygen (N03-). Nitrate is a water soluble and mobile form of nitrogen. Because of its negative charge, it is not strongly held by soil particles (also negative) and can be leached away.
  • Nitrification: The biochemical oxidation of ammonia nitrogen to nitrate.
  • Nutrient availability: The relative proportion of a nutrient in the soil that can be absorbed and assimilated by growing plants.
  • Nutrient-holding capacity: The ability to absorb and retain nutrients so they will be available to the roots of plants. See also exchange capacity.


  • Organic matter: Chemical substances of animal or vegetable origin, consisting of hydrocarbons and their derivatives.


  • P: Chemical symbol for phosphorus.
  • Pad, composting: The surface or area occupied by actively composting windrows and piles.
  • Passive aeration: Air movement through composting windrows and piles which occurs by natural forces including convection, diffusion, wind, and the tendency of warm air to rise (thermal buoyancy).
  • Passive composting: Method of composting in which there is little management and manipulation of the materials after they are mixed and piled. Turning occurs infrequently (for example, monthly). Forced aeration is not provided.
  • Passively aerated windrow composting: A composting method in which windrows are constructed over a series of perforated plastic pipes, which serve as air ducts for passive aeration. Windrows are not turned.
  • Pathogen: Any organism capable of producing disease or infection. Often found in waste material, most pathogens are killed by the high temperatures of the composting process.
  • PCBs: Polychlorinated biphenyls. Persistent, immobile contaminants found in industrial waste and sewage sludge. Federal and many state regulations restrict the land application of materials which contain high concentrations of PCBs.
  • Peat: Unconsolidated soil material consisting largely of organic matter accumulated under conditions of excessive moisture. The organic matter is not decomposed or is only slightly decomposed.
  • Perlite: Volcanic mineral used as an amendment in potting soil.
  • pH: A measure of the concentration of hydrogen ions in a solution. pH is expressed as a negative exponent. Thus, something that has a pH of 8 has ten times fewer hydrogen ions than something with a pH of 7. The lower the pH, the more hydrogen ions present, and the more acidic the material is. The higher the pH, the fewer hydrogen ions present, and the more basic it is. A pH of 7 is considered neutral.
  • Phytophthora: A group of fungal plant pathogens which cause a serious root, crown, and sometimes foliar (leaf) disease on a large number of plants. These fungi are most active under conditions of high soil moisture.
  • Phytophthora root rot: See phytophthora and root rot.
  • Phytotoxic: An adjective describing a substance that has a toxic effect on plants. Immature or anaerobic compost may contain acids or alcohols that can harm seedlings or sensitive plants.
  • Pollution: The presence in a body of water (or soil or air) of a substance (pollutant) in such quantities that it impairs the body's usefulness or renders it offensive to the senses of sight, taste, or smell. In general, a public-health hazard may be created, but in some instances only economic or aesthetics is involved, as when foul odors pollute the air.
  • Polychlorinated biphenyls: See PCBs.
  • Porosity: A measure of the pore space of a material or pile of materials. Porosity is equal to the volume of the pores divided by the total volume. In composting, the term porosity is sometimes used loosely, referring to the volume of the pores occupied by air only (without including the pore space occupied by water).
  • Poultry litter: See litter, poultry.
  • PTO: Power take off. Drive shaft and coupling on a tractor which transmits power from the tractor engine to implements and secondary equipment (for example, pumps, grinders, and windrow turners).
  • Pullet: A young hen, less than one year old.
  • Pythium: A fungal plant pathogen which causes seed, seedling, and root rots on a large number of plants. These fungi are most active under conditions of high moisture.
  • Pythium root rot: See pythium and root rot.


  • Recipe: The ingredients and proportions used in blending together several raw materials for composting.
  • Retention basin: See holding pond.
  • Root rot: A disease of plants characterized by discoloration and decay of the roots.


  • Saturated Paste: A laboratory technique in which solid particles are rendered into a paste in order to measure characteristics such as pH and soluble salt concentration.
  • Semi-solid manure: Manure which has had some bedding added or has received sufficient air drying to raise the solids content such that it will stack but has a lower profile than solid manure and seepage may collect around the outside. It may be pumped with positive displacement jumps or handled with a front-end loader. See also manure.
  • Septage: Waste pumped from septic tanks. Contains human wastes.
  • Setback: A prescribed distance separating the area of a particular activity and a neighboring boundary (for example, the distance between the composting pad and the property line).
  • Sewage sludge: Solid portion of waste from sewage treatment plants. Contains human wastes.
  • Shredding: An operation which reduces the particle size of materials. Shredding implies that the particles are broken apart by tearing and slicing. See also grinding.
  • Slurry manure: Slurry manure has a near liquid consistency. It can be handled with conventional, centrifugal manure pumps and equipment, but the solids content may be too high for irrigation equipment. See also manure.
  • Soil amendment: Any substance (such as lime, sulfur, gypsum, or sawdust) used to alter the properties of a soil (generally, to make it more productive). Fertilizers are one type of soil amendment. However, many soil amendments (such as soil conditioners) do not have significant fertilizer value. See also soil conditioner.
  • Soil conditioner: A soil additive that stabilizes the soil, improves its resistance to erosion, increases its permeability to air and water, improves its texture and the resistance of its surface to crusting, makes it easier to cultivate, or otherwise improves its quality.
  • Soil structure: The combination or arrangement of primary soil particles into secondary particles, units, or peds. Compost helps bind primary soil particles to improve the structure of soil.
  • Soil texture: A characterization of soil type, based on the relative proportions of sand, silt, and clay in a particular soil.
  • Solid manure: Manure which has had sufficient bedding or soil added or has received sufficient air drying to raise the solids content to where it will stack with little or no seepage. It is best handled with a frontend loader. See also manure.
  • Sour compost: Compost which has been produced or stored under anaerobic conditions. It is generally acidic and may contain phytotoxic compounds.
  • Specific conductance: See electrical conductivity.
  • Spontaneous combustion: Self heating and ignition of a combustible substance because of chemical reactions that occur within the substance. Can occur at moisture contents between 25 and 45%.
  • Stability, of compost: The rate of change or decomposition of compost. Usually stability refers to the lack of change or resistance to change. A stable compost continues to decompose at a very slow rate and has a low oxygen demand.
  • Structure, of composting mix or raw material: The ability to resist settling and compaction. Structure is improved by large rigid particles.


  • Texture, of composting mix or raw material: Characteristic which describes the available surface area of particles. A fine texture implies many small particles with a large combined surface area. A course texture implies large particles with less overall surface area.
  • Thermophilic: Heat-loving microorganisms that thrive in and generate temperatures above 105°F (40°C).
  • Thin slurry: See liquid manure.
  • Tipping fees: Fees charged for treating, handling, and/or disposing of waste materials.
  • Top-dressing: Applying a layer of compost, or other material, to the surface of soil.
  • Turning: A composting operation which mixes and agitates material in a windrow pile or vessel. Its main aeration effect is to increase the porosity of the windrow to enhance passive aeration. It can be accomplished with bucket loaders or specially designed turning machines.


  • Vermicomposting: The process by which worms convert organic waste into worm castings -- the dark, fertile, granular excrement of a worm. Castings are rich in plant nutrients.
  • Vermiculite: A natural mineral used as an amendment in potting soil.
  • Vermin: Noxious or objectionable animals, insects, or other pests, especially those of a small size. For example, rats, mice, and flies.
  • Volatile compound: A compound or substance which vaporizes ("evaporates") at relatively low temperatures or is readily converted into a gaseous by-product. Examples include alcohols and ammonia. Volatile compounds are easily lost from the environment of a composting pile.


  • Windrow: A long, relatively narrow, and low pile. Windrows have a large exposed surface area which encourages passive aeration and drying.


  • Yard: See cubic yard.
  • Yard waste: Leaves, grass clippings, yard trimmings, and other organic garden debris.
Last modified: Wednesday, July 2, 2014, 3:12 PM