Glossary of Composting Terms
Note: Definitions marked with an asterisk* were obtained and/or adapted from CalRecycle, California Code of Regulations, Title 14, Division 7, Chapter 3.1, Article 1, Section 17852- Definitions, and Chapter 3.2, Article 1, Section 17896.2
Acid pH lower than 7 on a scale of 0 to 14.
Actinomycetes A group of fungi-like bacteria commonly present in composting piles.
Active Compost* Compost feedstock that is in the process of being rapidly decomposed and is unstable. Active compost is generating temperatures of at least 50 degrees Celsius (122 degrees Fahrenheit) during decomposition; or is releasing carbon dioxide at a rate of at least 15 milligrams per gram of compost per day, or the equivalent of oxygen uptake.
ADC* This acronym stands for Alternative Daily Cover, and means cover material other than earthen material placed on the surface of the active face of a municipal solid waste landfill at the end of each operating day to control vectors, fires, odors, blowing litter, and scavenging. CalRecycle currently allows diversion recycling credits for ground green materials used as ADC, but this will change on January 1, 2020, with the passage of AB 1594.
Additives* Material mixed with feedstock or active compost in order to adjust the moisture level, carbon to nitrogen ratio, or porosity to create a favorable condition. Additives include, but are not limited to, fertilizers and urea.
Aerated Static Pile* A composting process that uses an air distribution system to either blow or draw air through the pile. Little or no pile agitation or turning is performed.
Aeration Introducing oxygen into a compost pile through turning or forced air.
Aerobic Decomposition* The biological decomposition of organic substances in the presence of oxygen.
Agricultural By-Product Material* Post-harvest agricultural by-products separated at a processing facility, including, but not limited to, solid or semi-solid materials from fruit, nut, cotton, and vegetable processing facilities such as stems, leaves, seeds, nut hulls and shells, peels, and off-grade, over-ripe, or under-ripe produce.
Agricultural Material* Residual material of plant or animal origin, which results directly from the conduct of agriculture, animal husbandry, horticulture, aquaculture, silviculture, vermiculture, viticulture and similar activities undertaken for the production of food or fiber for human or animal consumption or use, which is separated at the point of generation, and which contains no other solid waste.
Aggregation Clusters of soil particles, allowing flow of air, moisture and root systems. Common in a crumbly, healthy soil.
Alkaline pH above 7 on a scale of 0 to 14.
Alternative Daily Cover* Also known as ADC, alternative daily cover refers to cover material other than earthen material placed on the surface of the active face of a municipal solid waste landfill at the end of each operating day to control vectors, fires, odors, blowing litter, and scavenging. CalRecycle currently allows diversion recycling credits for ground green materials used as ADC, but this will change on January 1, 2020, with the passage of AB 1594.
Ambient temperature Air temperature outside, and in the vicinity of the compost pile.
Amendments* Materials added to stabilized or cured compost to provide attributes for certain compost products, such as product bulk, product nutrient value, product pH, and soils blend.
Anaerobic A windrow, pile or composting material in a vessel that lacks sufficient oxygen to support aerobic microbial activity. A slower rate of decomposition, lower temperatures, and foul odors are often a result of anaerobic conditions. Also refers to organisms that survive without oxygen.
Anaerobic Decomposition* The biological decomposition of organic substances in the absence of oxygen.
Anaerobic digestion The controlled biological decomposition of organic material in the absence of oxygen or in an oxygen-starved environment. Anaerobic digestion produces biogas and a residual digestate.
Anion Opposite of cation, anions are acid forming elements with a negative charge.
Availability Degree to which nutrients, specifically carbon and nitrogen, are available for decomposer microorganisms to consume. In soil, the degree in which a plant can uptake and utilize nutrients.
Bacteria Simple, microscopic organisms that reproduce by splitting apart or forming spores. Of the hundreds of different organisms that may occur in a compost pile, bacteria, especially the thermophilic or heat loving strain, are the most dynamic.
Bedding In vermicomposting, the media put into a worm bin or pile to give worms a working environment. Bedding holds moisture and provides fluff required for aerobic condition.
Biodegradeable An organic, or carbon-based material, capable of being broken down into more simple chemical compounds.
Biosolids* Solid, semi-solid, or liquid residue generated during the treatment of domestic sewage in a treatment works. Biosolids includes, but is not limited to, treated domestic septage and scum or solids removed in primary, secondary, or advanced wastewater treatment processes.
Bulking agent Woody, carbonaceous materials incorporated into the composting mix to provide air spaces necessary for aerobic decomposition. Bulking agents also provide a source of carbon. Leaves, ground brush and woodchips are typical bulking agents.
Buffer zone The amount of area between a compost pile, or facility, and neighboring homes, buildings or areas of human activity.
Bulk density A measure of the ratio of a compost, soil or other material’s weight or mass in relation to its bulk or volume.
Calcium carbonate A common compound found in limestone used for reducing acidity in compost piles or soil.
Carbon A macronutrient that exists in all living or once-living organisms. Microbes in compost piles require it for energy and cell production.
Carbon to nitrogen ratio (C/N ratio) The amount of carbon versus proportion of nitrogen an organic material, or composting pile, possesses. Formulating a blend of various materials with different C/N values to achieve the proper ratio for composting is called developing a “recipe mix.”
Castings A by-product of earthworms, the undigested excretions of organic material, soil, and bacteria.
Cation Positively charged, alkaline, or “base” elements in the soil.
Cation Exchange Capacity (CEC) A measure of the number of base elements a soil can adsorb. Compost improves soil by increases its cation exchange capacity.
Cellulose The primary component of plant cell walls, cellulose is a long chain of glucose molecules.
Chip and Grind, or Chipping and Grinding Facility A facility that does not produce compost, but rather is devoted to receiving, handling, storing and processing, through chipping and grinding, brushy, woody materials such as yard and landscape trimmings, and untreated, unpainted dimensional lumber.
Compostable material* Any organic material that when accumulated will become active compost.
Compost* The product resulting from the controlled biological decomposition of organic solid materials that are source separated or separated at a centralized facility.
Compost maturity A measure of the degree in which a compost has broken down, or stabilized.
Composting A managed biological process used to convert organic materials into beneficial soil amendments.
Composting Facility A facility devoted to receiving, handling, storing and processing, through composting, compostable materials such as yard and landscape trimmings, manure, and food scraps. Receiving, handling, storing and processing, through composting includes controlled biological decomposition, screening, and storage activities related to production of compost, compost feedstocks, and chipped and ground materials.
Cubic yard A common unit of measure in commercial composting for determining facility capacity requirements and selling finished products.
Curing* The final stage of the composting process that occurs after compost has undergone pathogen reduction, and after most of the readily metabolized material has been decomposed and stabilized.
Digestate* The solid and/or liquid residual material remaining after organic material has been processed in an in-vessel digester.
Enclosed composting process* A composting process where the area that is used for the processing, composting, stabilizing, and curing of organic materials, is covered on all exposed sides and rests on a stable surface with environmental controls for moisture and airborne emissions present. See also “In-Vessel Composting”
Feedstock* Any compostable material used in the production of compost or chipped and ground material including, but not limited to, agricultural material, green material, vegetative food material, food material, biosolids, digestate, and mixed material.
Food material* Residual material of plant or animal origin that results from the preparation or processing of food for animal or human consumption and that is separated from the municipal solid waste stream.
Green Material* Any plant material except food material and vegetative food material that is separated at the point of generation.
Friable A term used to describe a crumbly structure characteristic of healthy soils.
Fungi A group of simple plants. Various types of fungi assist in the process of decomposition and composting.
Humus The organic fraction of the soil made up of decomposed organic matter.
Inoculant Dominant strains of microorganisms added to a compost pile to initiate more rapid decomposition or control odors.
In-vessel composting system An enclosed composting operation, or process which is done inside a vessel.
In-vessel Digester* The sealed container(s) or sealed structure in which the entire digestion process occurs.
LEA The Local Enforcement Agency, charged with enforcing CalRecyle rules related to resource management and landfilling. San Diego County has two LEAs: the County of San Diego LEA, which covers the unincorporated area as well as all 17 jurisdictions outside the City of San Diego, and the City of San Diego LEA, which enforces within its own city's boundaries.
Leachate Liquid that runs through garbage or compost, carrying nutrients, and in some cases, potentially toxic compounds such as heavy metals.
Lignin A substance found in plant cells that is resistant to decay.
Macronutrients Nutrients that organisms require in larger amounts. Nitrogen, phosphorus, and potassium are macronutrients.
Macroorganisms Organisms that can be seen without magnification.
Mesophilic In composting, used to describe a group of microorganisms that thrive in temperatures between 40 – 110ºF.
Methane An odorless gas produced by anaerobic bacteria decomposing organic material. Considered a greenhouse gas, most landfills generate a substantial volume as organics are buried and ferment beneath compacted garbage.
Microbe A microscopic organism. Also called “microorganism.”
Mineralization Organic compounds degraded to a point where only basic mineral elements remain.
Mulch Any ground cover material used for suppressing weeds, moderating soil temperature, protecting roots, controlling evaporation and erosion, and enhancing aesthetics.
Nitrogen A common element of living tissue and ambient air, and an essential nutrient in composting. Microbes require it for cell propagation.
Nitrogen draw Nitrogen stolen by soil microorganisms from plants. Occurs when a material high in carbon is incorporated into the soil. Provided with an ample source of carbon, microbes draw nitrogen from the soil to complete their dietary requirements, making it unavailable for plants.
N-P-K The table of element abbreviations for nitrogen, phosphorus, and potassium. Commercial fertilizers are sold by varying ratios of these nutrients, and expressed as, for example, 10-20-10.
Nuisance* Includes anything which:
(A) is injurious to human health or is indecent or offensive to the senses and interferes with the comfortable enjoyment of life or property, and
(B) affects at the same time an entire community, neighborhood or any considerable number of persons. The extent of annoyance or damage inflicted upon an individual may be unequal.
Organic matter Material derived from living or once-living organisms. A critical element of healthy soil.
Oxidation The process of a compound or element chemically combing with oxygen.
Pathogen A organism capable of producing a disease.
PFRP, or Process to Further Reduce Pathogens A required composting process management stage and monitoring/testing protocol regulated by CalRecycle that involves maintaining temperatures of 131ºF for 15 days or longer in open windrow processes and 3 days in an enclosed or within-vessel process.
pH A measure of alkalinity or acidity, from a scale of 0 to 14, with seven being neutral.
Physical contamination or "contaminants” Human-made inert material contained within compostable material, digestate, or compost, including, but not limited to, glass, metal, and plastic.
Phytotoxin An element or compound which is toxic to plants. Immature or partially decayed compost may contain phytotoxins, acids or alcohols that inhibit germination or harm seedlings and sensitive plants.
Psychrophilic In composting, used to describe a group of microorganisms that thrive in cooler temperatures, with an optimum of about 55ºF.
Rendering* All recycling, processing, and conversion of animal and fish materials and carcasses and inedible kitchen grease into fats, oils, proteins, and other products that are used in the animal, poultry, and pet food industries and other industries.
Run-off Water that drains from the surface and runs off-site of a composting facility.
Source reduction A resource management method that involves review of products and processes creating waste and efforts to modify the composition of the product or process to minimize residuals.
Source separation A term used to describe the process of sorting recyclable and compostable materials prior to setting them out for collection.
Soil amendment A common term used for compost, but also any product or material added to soil to change its characteristics.
Soil conditioner A soil amendment applied to improve the quality of soil.
Stable compost Fully broken down and cured compost that has undergone the Process to Further Reduce Pathogens and that no longer has enough nutrients to reheat or cause odors.
Static Pile* A composting process that is similar to the aerated static pile except that the air source may or may not be controlled.
Thermophilic In composting, used to describe a group of microorganisms that thrive in temperatures between 113 -155ºF (with an optimum range from between 120-130ºF.) These microbes are the most efficient decomposers in the compost web.
Tilth Land that has been prepared for planting, and also a measurement of the overall health of the soil.
Top dressing Finely screened compost or similar soil conditioner broadcast on turf or around plants to add nutrients, organic matter, and improve structure.
Vector An organism capable of carrying or transmitting a disease to another organism.
Vegetative Food Material* The fraction of food material that is a plant material and is separated from other food material and the municipal solid waste stream. Vegetative food material may be processed or cooked but must otherwise retain its essential natural character and no salts, preservatives, fats or oils, or adulterants shall have been added.
Vermicomposting A composting system using worms to recycle organic materials. Worms work in tandem with microbes and other macroorganisms.
Vermiculture Breeding earthworms in a bin or under controlled conditions. Sometimes used interchangeably with “vermicomposting.”
Windrow An elongated compost pile, common to commercial composting operations. In an outdoor facility, typically bell shaped, with the height about one third of the width, and as long as space will allow.
Windrow Composting Process* The process in which compostable material is placed in elongated piles. The piles or "windrows" are aerated and/or mechanically turned on a periodic basis.
Within-vessel Composting Process* An aerobic process in which compostable material is enclosed in a drum, silo, bin, tunnel, reactor, or other container for the purpose of producing compost, maintained under uniform conditions of temperature and moisture where air-borne emissions are controlled.
Yard Trimmings* Any residuals generated from the maintenance or alteration of public, commercial or residential landscapes including, but not limited to, yard clippings, leaves, tree trimmings, prunings, brush, and weeds.