Skip to main content
U.S. flag

An official website of the United States government

Soil Health Glossary

Below are two glossaries that can be helpful:

  • Soil Health Terms (including soil properties, soil health indicators, and assessment terminology)
  • Soil Ecology Terms (including soil organisms, biological processes, and ecology terminology)

The Soil Science Society of America, Madison WI maintains an Internet Glossary of Soil Science Terms.

Soil Health Terms

aggregate stability: The ability of soil aggregates to resist degradation. An aggregate is many soil particles held together in a small mass. In a “well-aggregated soil” the aggregates and pores between them hold up well to forces such as rain, wind, and compaction. (Compare to slake test.)

amoozemeter: A tool that uses a constant head of water to measure the rate of water movement in a saturated soil, and thus estimates saturated hydraulic conductivity.

anthropogenic: Generated by humans. Used to indicate soil conditions, disturbances, or stresses that are created by people.

assessing soil health: Estimating the functional capacity of soil by comparing a soil to a standard such as an ecological site description, a similar soil under native vegetation, a reference soil condition, or quality criteria. The objective of the assessment dictates the standard to be used. (Compare to monitoring.)

attributes of soil change: Quantifiable properties used to describe the nature of soil change, including drivers, types, rates, reversibility, and pathways of change.

available water capacity: Loosely, the amount of water available for plants to use. Specifically, the volume of water released from soil between the time the soil is at field capacity (the maximum water held in soil against the pull of gravity) until the time it is at the wilting point (the amount of water held too tightly in soil for commonly grown crops to extract). Loamy soils and soils high in organic matter have the highest AWC.

baseline: The initial soil condition before monitoring soil quality over time. Subsequent measurements on the same soil are compared to the baseline measurement.

benchmark soil: A benchmark soil is one of large extent, holds a key position in the soil classification system, or is of special significance to farming, engineering, forestry, livestock production, or other uses. The purpose of benchmark soils is to focus data collection and research efforts on soils that have the greatest potential for expansion of data and interpretations.

bulk density (Db or BD): The density of soil, i.e., the weight of soil divided by its volume. The BD of agricultural soils normally ranges from 1.0 to 1.6 g/cm3.

cation exchange capacity (CEC): The capacity of soil to hold nutrients for plant use. Specifically, CEC is the amount of negative charges available on clay and humus to hold positively charged ions. Effective cation exchange capacity (ECEC) is reported for acid soils (pH<5). Expressed as centimoles of charge per kilogram of soil (cmolc/kg).

cotton strip assay:  Measures the amount of biological activity as determined by the degree of degradation of a standardized strip of cotton buried in the soil.

disturbance: An event or its change in intensity or frequency which alters the structure or functional status of an ecosystem. Examples of disturbances that can affect soil include drought, fire, harvest, tillage, compaction, overgrazing, or addition of pesticides.

dynamic soil properties: Soil properties that change over the human time scale in response to anthropogenic (management, land use) and non-anthropogenic (natural disturbances and cycles) factors. Many are important for characterizing soil functions and ecological processes and for predicting soil behavior on human time scales. (Compare to use-dependent soil properties.)

electrical conductivity (EC): How well the soil conducts an electrical charge. EC is a measure of salinity, generally expressed as decisiemens per meter at 25ºC (dS/m).

fatty acid analysis: Examination of the fatty acid methyl esters (FAMEs) in the soil using gas chromatography. Fatty acids are within the cell walls of soil organisms, so the types of fatty acids found in soil are an indicator of the structure and diversity of the soil community.

function: A service, role, or task that meets objectives for sustaining life and fulfilling humanity’s needs and is performed by soil or an ecosystem. (Compare to soil function.)

functional capacity: The quantified or estimated measure of physical and biophysical mechanisms or processes selected to represent the soil’s ability to carry out the function.

human time scale: That portion of the pedogenic time scale that covers periods of centuries, decades, or less.

hydraulic conductivity (Ksat): A quantitative measure of how easily water flows through soil. (Compare to infiltration and permeability.)

indicator of soil quality: A quantitative or qualitative measure used to estimate soil functional capacity. Indicators should be adequately sensitive to change, accurately reflect the processes or biophysical mechanisms relevant to the function of interest,, and be cost effective and relatively easy and practical to measure. Soil quality indicators are often categorized into biological, chemical, and physical indicators.

indicators of soil quality, biological: Measures of living organisms or their activity used as indicators of soil quality. Measuring soil organisms can be done in three general ways: 1) counting soil organisms or measuring microbial biomass, 2) measuring their activity (e.g. soil basal respiration, cotton strip assay, or potentially mineralizable nitrogen), or 3) measuring diversity, such as diversity of functions (e.g., biolog plates) or diversity of chemical structure (e.g. cell components, fatty acids, or DNA). Each approach provides different information.

indicators of soil quality, chemical: These include tests of organic matter, pH, electrical conductivity, heavy metals, cation exchange capacity, and others.

indicators of soil quality, physical: Physical characteristics that vary with management include bulk density, aggregate stability, infiltration, hydraulic conductivity, and penetration resistance.

infiltration rate: The rate at which water enters soil. (Compare to hydraulic conductivity.)

inventory: The systematic acquisition of resource information needed for planning and management.

microbial biomass: The total amount of organisms in the soil, excluding macrofauna and plant roots. Microbial biomass is typically determined through substrate-induced respiration, or fumigation-extraction methods.

minimum data set (MDS): The smallest set of soil properties that can be used to characterize or measure soil quality. The MDS will vary based on the intended land use, soil type, and climate. The first MDS was suggested by Larson and Pierce and included the following: nutrient availability, total organic C, particle size or texture, labile organic C, plant-available water capacity, soil structure, soil strength, maximum rooting depth, pH, and electrical conductivity.

monitoring soil quality: Tracking trends in quantitative indicators or the functional capacity of the soil in order to determine the success of management practices or the need for additional management changes. Monitoring involves the orderly collection, analysis, and interpretation of data from the same locations over time. (Compare to assessing.)

organic matter: Any material that is part of or originated from living organisms. Includes soil organic matter, plant residue, mulch, compost, and other materials.

organic matter, active fraction: The highly dynamic or labile portion of soil organic matter that is readily available to soil organisms. May also include the living biomass. Particulate organic matter (POM) and light fraction (LF) are measurable indicators of the active fraction. POM particles are larger than other SOM and can be separated from soil by sieving. LF particles are lighter than other SOM and can be separated from soil by centrifugation.

organic matter, stabilized organic matter: The pool of soil organic matter that is resistant to biological degradation because it is either physically or chemically inaccessible to microbial activity. These compounds are created through a combination of biological activity and chemical reactions in the soil. Humus is usually a synonym for stabilized organic matter, but is sometimes used to refer to all soil organic matter.

pedotransfer function (PTF): A mathematical relationship between two or more soil properties that shows a reasonably high level of statistical confidence. PTF’s are used to predict difficult-to-measure soil properties from readily obtained properties of the same soil.

penetration resistance or penetrability: The ease with which a probe can be pushed into the soil.

permeability: The qualitative estimate of the ease with which fluids, gases, or plant roots pass through soil.

pitfall trap: A small container (trap) buried so the rim is at the level of the soil surface. It is used to catch soil arthropods that move across the ground surface.

porosity: The volume of pores in a soil sample divided by the bulk volume of the sample. Air-filled porosity is the fraction of the bulk volume of soil that is filled with air at any given time or under a given condition, such as a specified soil-water content.

potentially mineralizable nitrogen (PMN): A test measuring the amount of soil organic nitrogen converted to plant available forms under specific conditions of temperature, moisture, aeration, and time. It is a measure of biological activity and indicates the amount of N that is relatively rapidly available.

primary ecological processes: Ecological processes including the water cycle (the capture, storage and redistribution of precipitation), energy flow (conversion of sunlight to plant and animal matter), and the nutrient cycle (the cycle of nutrients such as nitrogen and phosphorus through the physical and biotic components of the environment).

processes: Physical, chemical and biological mechanisms that follow fundamental scientific laws. Examples include pedogenic processes, geomorphic processes, and ecological processes.

reference soil condition: The condition of the soil to which functional capacity is compared. . Soil quality is usually assessed by comparing a soil to a reference condition. The reference condition may be data from a comparable benchmark soil, baseline measurements taken previously on the same soil, or measurements from a similar soil under undisturbed vegetation, or under similar management.

scoring function:A standardization procedure used to convert measured values or subjective ratings to unitless values usually between 0 and 1. This allows all soil property measurements to be integrated into one value or index for soil quality. The four general types of scoring functions used in soil quality assessments are: more is better (higher measurements mean higher soil quality, e.g. SOM); less is better (lower measurements mean higher soil quality, e.g. salinity); optimum range (a moderate range of values is desirable, e.g. pH); undesirable range (a specific range of values is undesirable)

slake test: A measure of disintegration of soil aggregates when exposed to rapid wetting.

soil change: Temporal variation in soil at various time scales at a specific location.

soil function: Any service, role, or task that soil performs, especially: 1) sustaining biological activity, diversity, and productivity; 2) regulating and partitioning water and solute flow; 3) filtering, buffering, degrading, and detoxifying potential pollutants; 4) storing and cycling nutrients; and 5) providing support for buildings and other structures and to protect archaeological treasures. (Compare to function, functional capacity.)

soil health or soil quality: The capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation. In short, the capacity of the soil to function. There are two aspects of the definition: inherent soil quality and dynamic soil quality. (Compare to functional capacity.)

soil health, dynamic: That aspect of soil quality relating to soil properties that change as a result of soil use and management or over the human time scale.

soil health, inherent: That aspect of soil quality relating to a soil’s natural composition and properties as influenced by the factors and processes of soil formation, in the absence of human impacts.

soil organic matter: The total organic matter in the soil. It can be divided into three general pools: living biomass of microorganisms, fresh and partially decomposed residues (the active fraction), and the well-decomposed and highly stable organic material. Surface litter is generally not included as part of soil organic matter.

soil resilience: The capacity of a soil to recover its functional capacity after a disturbance.

soil resistance: The capacity of the soil to maintain its functional capacity through a disturbance.

soil respiration: The amount of carbon dioxide given off by living organisms and roots in the soil.

soil respiration, basal: The level of carbon dioxide given off by a soil sample. Basal respiration is a measure of the total biological activity of microorganisms, macroorganisms, and roots.

soil respiration, substrate-induced: A measure of the carbon dioxide given off by a soil sample after adding sugar or other food. It is used to estimate microbial biomass in the sample.

soil structure: The arrangement of soil particles into aggregates which form structural units. Size, shape, and distinctness are used to describe soil structure. Farmers often describe soil structure with words such as crumbly or cloddy.

tilth: The overall physical character of soil with regard to its suitability for crop production.

use-dependant or management-dependent properties: Soil properties that show change and respond to use and management of the soil, such as soil organic matter levels and aggregate stability. This is a narrower term than dynamic soil properties which encompasses all changes on the human time scale including those induced by natural disturbances or cycles.

use-invariant properties: Soil properties that show little change over time and are not affected by use and management of the soil, such as mineralogy and particle size distribution.

water holding capacity: The amount of water that can be held in soil against the pull of gravity.


Soil Ecology Terms

actinomycetes: A large group of bacteria that grow in long filaments that are too small to see without magnification. Actinomycetes generate the smell of “healthy soil,” and are important in decomposing cellulose, chitin, and other hard-to-decompose compounds, especially at higher pH levels. Many produce antibiotics.

aerobic: With oxygen. Aerobic organisms, including animals and most soil organisms, require environments with oxygen. See anaerobic.

algae: Non-vascular photosynthetic plant-like organisms, some of which live in or on the soil. They are informally divided into groups by their dominant pigments (i.e., green, blue-green, etc.)

anaerobic: Without oxygen. Anaerobic organisms, including some soil bacteria, need oxygen-free environments such as saturated soils. Facultative anaerobes can function as either aerobes or anaerobes depending on environmental conditions. See aerobic.

arbuscular mycorrhizae: The group of endomycorrhizal fungi important in non-woody plants, including many agricultural crops. Sometimes called vesicular-arbuscular mycorrhizae (VAM).

arthropods: Invertebrate animals with jointed legs. They include insects, crustaceans, sowbugs, springtails, arachnids (spiders), and others.

bacteria: Microscopic, single-celled organisms. They include the photosynthetic cyanobacteria (formerly called blue-green algae), and actinomycetes (filamentous bacteria that give healthy soil its characteristic smell).

bacterial-dominated food web: A soil food web in which the ratio of fungal biomass to bacterial biomass is less than one.

biological soil crust: Also called microbiotic, microphytic, cryptobiotic or cryptogamic crusts. A living community of bacteria, microfungi, cyanobacteria, green algae, mosses, liverworts, and lichens that grow on or just below the soil surface. Biological crusts can heavily influence the morphology of the soil surface, stabilize soil, fix carbon and nitrogen, and can either increase or decrease infiltration. The percent cover and the components of the crust can vary across short distances. Identification of biological crust organisms is simplified through the use of three broad morphological groups: The cyanobacteria group includes cyanobacteria and green algae. The moss group includes short and tall mosses, but not club moss mats, such as those in northern latitudes, or spike moss. The lichen group includes crustose, gelatinous, squamulose, foliose, and fruiticose lichen, as well as liverworts.

comminuters: Organisms that shred organic material into smaller pieces.

compost tea: An infusion made by leaching water through compost, sometimes with nutrients added, such as molasses and kelp, to encourage certain organisms. Soluble organic matter and the organisms in the compost are rinsed out of the solid phase and left suspended in the water. This "liquid compost" is easier to apply than solid compost.

cyanobacteria: Filamentous or single-celled bacteria that fix carbon and nitrogen (formerly called blue-green algae). Only the filamentous species can be seen without a microscope. Cyanobacterial crusts with low biomass are generally the color of the soil and those with high biomass and diversity are dark (brown to black).

decomposition: The biochemical breakdown of organic matter into organic compounds and nutrients, and ultimately into its original components.

denitrification: A process performed by a few species of anaerobic soil bacteria in which nitrite or nitrate is converted to nitrogen gas (N2) or nitrous oxide (N2O). Both N2 and N2O are volatile and lost to the atmosphere.

detritivores: Organisms that eat detritus, that is, dead plants and animals.

diversity: Biological diversity can refer to the number of species in an area, the number of types of species (e.g. microbial functional groups, or plant structural types), the degree of genetic variability within a species, or the distribution of species within an area.

ectomycorrhizal fungi: A type of mycorrhizal fungi that grows between root cells and forms a sheath around roots, but does not actually invade cells. They are important to many woody plants.

emergent properties: Properties of a whole system that are not apparent from examining properties of the components of the system.

endomycorrhizal fungi: A type of mycorrhizal fungi that invades the cells of plant roots.

exudates: Soluble sugars, amino acids and other compounds secreted by roots.

food web, soil: The interconnected community of organisms living all or part of their lives in the soil.

functional redundancy: The presence of several species that serve similar functions (e.g. nitrification).

fungal-dominated food web: A soil food web in which the ratio of fungal biomass to bacterial biomass is greater than one.

fungi: Multi-celled, non-photosynthetic organisms that are neither plants nor animals. Fungal cells form long chains called hyphae and may form fruiting bodies such as mold or mushrooms to disperse spores. Some fungi such as yeast are single-celled.

fungivores: Organisms that eat fungi.

generalist: A species that will eat or prey on a wide variety of food resources. (See specialist)

grazers: Organisms such as protozoa and nematodes that eat bacteria and fungi.

habitat: The environment where an animal, plant, or microbe lives and grows.

hyphae: Long chains of cells formed by fungi usually occurring between aggregates rather than within micropores. (Compare to mycelium.)

immobilization: The conversion by soil organisms of inorganic nutrients such as ammonium or nitrate into organic compounds that are part of their cells. This makes the nutrients temporarily immobile in the soil and unavailable to plants. (See mineralization.)

keystone species: A species which, if removed from an ecosystem, causes a dramatic change in the system, and which has been proposed as an indicator of the functional capacity of the system.

lichen: A composite of fungi and algae or cyanobacteria. The fungi capture and cultivate photosynthetic organisms which together provide themselves needed water and nutrients. Lichen species occur in many colors including black, brown, dark olive green, red, yellow and white.

lignin: A hard-to-degrade compound that is part of the structure of older or woody plants. The carbon rings in lignin can be degraded by a few fungi.

liverworts: Small non-vascular plants.

metabolic quotient (qCO2): The ratio of microbial activity to microbial biomass.

microbe or microorganism: An imprecise term referring to any organism too small to see with the naked eye. Generally, “microbes” refers to bacteria, fungi, and sometimes protozoa.

mineralization: The conversion of organic compounds into inorganic, plant-available compounds such as ammonium. This is accomplished by soil organisms as they consume organic matter and excrete wastes. (See immobilization.)

moss: Photosynthetic plants with small leaves that unfurl when moistened (thus the moss appears to swell). When dry, mosses are dark and dull-colored; when moistened, the color changes markedly to a bright, light green to brown. This makes them easy to distinguish from lichens.

mutualists: Two species that have evolved together into a mutually beneficial relationship. For example, mycorrhizal fungi get carbon compounds from plant roots and help deliver water and nutrients to the root.

mycelium: A bundle of fungal hyphae that form the vegetative body of many fungal organisms.

mycorrhizal associations: A symbiotic association of certain fungi with roots. The fungi receive energy and nutrients from the plant. The plant receives improved access to water and some nutrients. Except for brassicas (mustard, broccoli, canola) and chenopods (beets, lamb’s-quarters, chard, spinach), most plants form mycorrhizal associations.

nematodes: Tiny, usually microscopic, unsegmented worms. Some are parasites of animals or plants. Most live free in the soil.

nitrification: A process accomplished by a few groups of aerobic organisms in which ammonia is converted to nitrite and then nitrate.

population: All the individuals of a species in a given area.

protozoa: Single-celled organisms with animal-like cells, including amoeba, ciliates, and flaggelates.

rhizine: Root-like structure of lichen and other organisms.

rhizoid: Root-like structure of mosses and ferns used to attach to a substrate.

rhizosphere: The narrow region around roots where most soil biological activity occurs. Soil organisms take advantage of the sloughed and dead root cells and the root exudates found in this region.

saprophytic fungi: Fungi that decompose dead organic matter.

sheath: Tubular structure formed around a chain of cells or around a bundle of filaments. The fine, polysaccharide sheaths formed by some filamentous cyanobacteria help bind soil particles together and can be seen dangling from soil surface fragments. Ectomycorrhizal fungi form a sheath of hyphae around plant roots.

soil ecology: The study of interrelations among soil organisms and between organisms and the soil environment.

specialist: A species that consumes only one or a few types of food sources or forms associations with a narrow range of hosts. For example, certain collembola (tiny insects called springtails) specialize in eating specific species of fungi. (See generalist)

trophic levels: Levels of the food chain. The first trophic level includes photosynthesizers that get energy from the sun. Organisms that eat photosynthesizers make up the second trophic level. Third trophic level organisms eat those in the second level, and so on. It is a simplified way of thinking of the food web. In fact, some organisms eat members of several trophic levels.