Glossary of terms.
Click one of the letters above to go to the page of all terms beginning with that letter.


a horizon

The upper layer of soil is usually higher in organic matter and darker in color than the layers below. This upper layer is called the A horizon, or topsoil. The A horizon is the portion of the soil most exposed to the weathering action of the sun, rain, wind, and ice and to the action of living things. The more easily decomposed materials weather away first, thus leaving the more resistant minerals concentrated in the topsoil.




the natural separation of an organ, as a leaf, bud or flower, from the plant.

abscisic acid

(ABA), a major plant hormone involved in stress protection. It acts in induction of bud and seed dormancy, and also in inducing stomatal closure under water deficit. ABA is synthesized in roots, seeds and overwintering buds.

absolute zero

Temperature at which all molecular motion is theorized to stop: -273.15° C or 0 K.


See adsorption.


A pesticide that kills mites and ticks.

accelerated erosion

Erosion resulting from human involvement is called accelerated erosion because it is typically many times as fast as geologic erosion.

Acceptable Daily Intake (ADI)

the level of a residue (of, for example, a pesticide) to which a daily exposure over a lifetime will not cause appreciable risk of chronic poisoning.

accessory characteristic

See differentiating characteristic.


temporary adjustments in plant growth, development, and/or physiology in response to temporary or short-term changes in growing conditions.

acclimatization, or acclimation

The reversible physiological adaptation to changes in climate or environment, such as light, temperature, or moisture availability.


a cushion-shaped type of fruiting structure of some fungi in which conidia are formed.


A synaptic neurotransmitter, CH3COOCH2CH2N(CH3)3.


An enzyme that catalyzes the breakdown of acetylcholine.


Acids are water solutions that contain more H+ cations than OH- anions (other ions are also present in amounts that make the total numbers of + and - charges equal). The concentration of active H+ ions in solution is evaluated by pH.

Acid Detergent Fiber (ADF)

ADF consists primarily of cellulose, lignin, and acid detergent fiber crude protein. It is closely related to indigestibility of forages and is the major factor in calculating energy content of feeds. The lower the ADF the more energy the feed contains and the more digestible it will be..

acid soil

Soil with a pH value \< 7.0.

acidic rock

The terms acidic and basic as applied to rocks have nothing to do with pH. This terminology was developed under early theories of rock crystallization by which quartz was supposed to be a remnant of silicic acids. A rock that contained quartz crystals was therefore called an acid rock, and one without quartz crystals was called a basic rock. The theories have been replaced, but the terminology is still used.

acquired resistance

A non-inherited resistance response developed by a normally susceptible host following a predisposing treatment, such as inoculation with a virus, fungus, bacterium, or treatment with certain chemicals.


developing from the base toward the apex of the plant or the inflorescence.


Actinomycetes are threadlike organisms that appear to be single celled. The threads (hyphae with no visible cell walls) are about the diameter of bacteria (and of coarse clay) and are often branched and tangled. The pleasant odor of freshly plowed ground comes from actinomycetes in the soil.

Action potential

A brief electrical depolarization that propagates along an axon or muscle fiber.

activated complex

transition state. An intermediate structure formed in the conversion of reactants to products. The activated complex is the structure at the maximum energy point along the reaction path; the activation energy is the difference between the energies of the activated complex and the reactants.

activation energy

(Ea)--The minimum energy required to convert reactants into products; the difference between the energies of the activated complex and the reactants.

active ingredient

active ingredient--(ai) is the chemical in an herbicide formulation primarily responsible for its phytotoxicity and which is identified as the active ingredient on the product label.


The activity of an ion is what the concentration appears to be rather than what it actually is. The relationship between the two is called the activity coefficient and is usually represented as g. The value of g is near 1.0 for very dilute solutions but tends to decrease with increasing concentrations. That is, the activity generally does not increase as fast as the concentration increases &#151; some of the ions appear to be inactive.

acute poisoning

Illness or death from a single dose of a toxicant.

acute toxicity

acute toxicity--is the potential of a substance to cause injury or illness shortly after exposure.


The process of adjustment to change. Physiological adaptation involves rapid changes in metabolism or morphology (e.g., stomatal closure) within an individual that allows better performance in a specific environment. Genetic adaptation is evolutionary adjustment within a population that allows fitness to environmental change over time.

adenosine diphosphate

(ADP) See adenosine triphosphate.

adenosine triphosphate

(ATP) Chemical energy carrier. In the electron transport systems of respiration and photosynthesis, and in certain other metabolic reactions (substrate-level phosphorylation), chemical energy is trapped, as opposed to escaping as heat, by addition of a phosphate ion to ADP. The ATP thus formed can participate in energy-requiring reactions, giving such reactions forward momentum.


refers to a change in temperature without an exchange of heat. Usually occurs by expansion or contraction.


a material that is mixed with a spray solution or suspension to improve the performance, handling, or application of herbicides. They are inert chemicals that are classified according to their use, rather than chemical or physical properties.

adsorbed cation

The cations held by the soil micelles are often called adsorbed cations. They are held in place by the attraction between the positive charge of the cation and the negative charge of the micelle.


Adherence of gas molecules, ions, or molecules in solution to the surface of solids (ASCE, 1985). The word sorbed is sometimes used to include both absorbed and adsorbed material.

adventitious roots

roots that arise from nonroot tissue, e.g. the "brace roots" of corn.


A spore produced in the aecium of a rust fungus. Each spore has two nuclei and is part of a chain of spores.


Pesticide formulation in which propulsion from a pressurized container suspends pesticide particles or droplets in the air.


Summer dormancy.


The process by which seeds are formed asexually (without fertilization). Does not include vegetative reproduction.


Groups of soil particles that cohere to each other more than to other adjacent particles are called soil aggregates. Aggregates may become peds or parts of peds if they are stable and enduring.

aggregate stability

The ability of soil aggregates to resist rearrangement by various forces that may impact them, especially the effects of water, is called aggregate stability. The stability of aggregates is related to the soil texture, the kind of clay, the kinds of ions associated with the clay, the kind and amount of organic matter present, and the nature of the microbial population.


relative or quantitative, nondifferential ability to cause disease in a host species.


an ecosystem largely created and maintained to satisfy a human want or need.


percentage of incoming solar radiation which is reflected from the surface.

albic horizon

The diagnostic horizon that is equivalent to an E horizon is called an albic horizon. Albic horizons occur at any position in the profile where strong eluviation has removed enough clay, iron oxides, organic matter, and other materials from the soil mass to reveal the underlying color of the sand and silt grains (usually a light gray).


Algae are a group of both macro- and microorganisms that contain chlorophyll and/or other photosynthetic pigments. Most soil algae live at the surface where their ability to use light energy to produce energy-bearing organic compounds is an advantage.


Any of several alternative forms of a gene. An alternative form of a gene at a specific location or locus of homologous chromosomes. See also locus.

allele frequency

>--abundance of a specific allele at a locus in a population

allelic frequency

abundance of a specific allele at a locus in a population


Chemical substances involved in communication between different species.


plant species excrete a substance that inhibits growth of other plant species


The first pyrethroid insecticide synthesized.


Chemical substances involved in communication within the same species that benefits the releaser.


Allophane is the material in the fine- clay fraction that does not have an identifiable structure. The constituents of allophane are too small to be identified but presumably include the building blocks of minerals--tetrahedra containing silicon and octahedra containing aluminum, magnesium, and iron. Allophane is prominent in young soils formed in volcanic ash.


A polyploid having genetically different sets of chromosomes.

alluvial fan

An alluvial fan is a form of stream deposit that occurs where a smaller stream enters the valley of a larger stream. The openness and flatness of the larger valley spreads the water and reduces its velocity so that it deposits sediment in a fan-shaped area. The coarser material is deposited in the upper portion of the alluvial fan and the finer particles are carried down to the toe of the fan or perhaps into the larger stream.


Material deposited by streams is called alluvium. The streams may be large or small, swift or slow. The bulk of alluvial material is transported during flood stage when heavy rains cause water to come pouring from the mountains and hills. The whole action is changeable as the currents come and go. Material of one texture is often deposited on top of material of another texture, thus forming a stratified deposit.

alternative hypothesis

the opposite of the null hypothesis. This is what is usually wanted to be proven true.


See silicate.


development without change in form. Juveniles are identical to adults, but are smaller in size and lack fully formed external genitalia.


The release of ammonium ions from decomposing organic materials is known as ammonification. Most microbes can decompose organic matter; ammonification therefore takes place rapidly when conditions favor microbial activity in general.


A plant possessing the sum of the somatic chromosome number of two species.

Analysis of Variance

Often abbreviated to ANOVA, a general term for a set of statistical procedures to differentiate treatment effects by separating combined variability in an experiment.


(1) Development in which the embryo contains only paternal chromosomes. (2) In cell biology and molecular genetics: Development of plants from the male gametophyte by the culture of anthers or microspores.


Condition of an individual possessing incomplete sets of chromosomes.


(1) The taxon of vascular seed plants; the seed is borne in a matured ovary (fruit). (2) A division of the plant kingdom in which fertilization results in the development of fruit containing seeds. It is a type of vascular plant known as a flowering plant. See also gymnosperm.


unit of length, equal to one tenth of a millimicron or one tenth of a million of a millimeter; primarily used to express electromagnetic wavelengths.


a negatively charged ion such as nitrate (NO3-) or sulfate (SO4-)

anion exchange capacity

The capacity of the clay and humus micelles in soil to hold exchangeable anions is known as the anion exchange capacity. The use of this term is sometimes debated because most anions (chloride, nitrate, sulfate, etc.) are held more loosely than the cations in cation exchange capacity, whereas phosphate anions are held so tightly that their availability for plant us is very low. Anion exchange capacity increases as soils age and their oxide mineral content increases.


condition of absence of oxygen.


Antagonism refers to exceptionally strong competition between pairs of similar ions such as Ca2+ and Sr2+, K+ and Rb+, SO42- and SeO42-, or H2PO4- and H2AsO4-. The specific nature of antagonism indicates that the ions involved enter the root the same way whereas other ions may have other entries.


that part of the stamen that contains the pollen.

anther culture

The in vitro culturing, on a synthetic medium, of anthers containing microspores. The microspores may form haploid callus or develop directly into haploid plants. See also androgenesis.


Pollen shed


Mechanisms that reduce continued usage once insects begin exploiting the plant.

antixenosis (nonpreference)

Characteristics that reduce the plant's attraction of exploiting insects.


invagination of the body wall that strengthens the exoskeleton and provides areas for muscle attachment.


a protein that forms an active enzyme system by combination with a coenzyme and determines the specificity of this system for a substrate.


The separation of the old cuticle from the epidermis; an early step in the molting cycle.


Production of seeds and vegetative propagules by asexual methods. The main features of apomixis are that (i) asexual is substituted for sexual reproduction, (ii) it occurs in parts of the plant normally concerned with the sexual process, and (iii) it occurs without fusion of the egg and sperm cells.


intercellular space


The nonliving continuum of cell walls and free space through which water and minerals move by diffusion in response to a concentration gradient. See also symplast.

apoplastic pathway

Solute or assimilate movement within a tissue through non-cytoplasmic, intercellular space.


the fruit of certain lichens and fungi: usually an open, saucer-shaped or cup-shaped body, the inner surface of which is covered with a layer that bears asci (spores).


the fruit of certain lichens and fungi: usually an open, saucer-shaped or cup-shaped body, the inner surface of which is covered with a layer that bears asci (spores).

apparent photosynthesis

See net photosynthesis.


thick-walled, swollen or flattened terminal or intercalary cell of a germ tube of a fungus, serving for attachment to the host, formed prior to penetration, often with a circular germination pore, found in anthracnose fungi and rusts. Some parasitic seed plants produce a similar structure.

Arcsine Transformation

a transformation useful on data collected of proportions or percentages. These data may be transformed by taking the inverse of the sin or arcsine of the number.

argillic horizon

Argillic horizons are illuvial soil horizons at least one-tenth as thick as the overlying soil and containing at least 3% more, 1.2X as much, or 8% more clay than the A horizon including clay coatings on soil peds, in pores, and/or on sand grains. The argillic horizon is the most common type of B horizon in soils.


An ascus-bearing structure found in the fungi known as ascomycetes. Ascocarps are composed of interwoven hyphae, and in many species they are visible, forming the most prominent part of the fungus. Ascocarps may be cup-shaped, spherical, or flask-shaped.


Any of various fungi belonging to the phylum Ascomycota, characterized by the presence of sexually produced spores formed within an ascus. Like most fungi, ascomycetes also reproduce asexually by the formation of nonsexual spores called conidia at the ends of filaments known as hyphae. Yeasts, many molds that cause food spoilage, and the edible fungi known as morels and truffles, are ascomycetes. A number of serious plant diseases, including ergot, the powdery mildews that attack fruit, and Dutch elm disease, are also caused by ascomycetes.


a type of fungal spore produced by some fungi as a result of sexual reproduction.


A membranous, often club-shaped structure inside which ascospores are formed through sexual reproduction in species of the fungi known as ascomycetes. The ascus is unique to ascomycetes and distinguishes them from other kinds of fungi. Asci are formed when two hyphae that are sexually compatible conjugate. Each ascus typically develops eight ascospores. Asci swell at maturity until they burst, shooting the ascospores into the air.


not involving gametes.

asexual reproduction

>--Reproductive process that does not involve the union of gametes, and which results in individuals with a genotype identical to the maternal parent.


The incorporation of inorganic nutrients into organic forms, e.g. the assimilation of CO2 into carbohydrates, the assimilation of NO3 into amino acids and proteins.


a limiting value of some function. A graph of an asymptotic function will show a line approaching but never reaching a certain function.


is the smallest particle of a chemical element.


An anticholinergic alkaloid drug used as an antidote for organophosphate and carbamate insecticide poisoning.


Direct manipulation of natural enemy numbers by periodic releases of these natural enemies into the environment.


the development of the entire life cycle of a parasitic fungus on a single host or group of hosts.


A polyploid arising through multiplication of the complete haploid chromosome set of a species.


A method for determining the presence and location of radioactively labeled molecules by their effect in creating an image on a photographic emulsion, usually x-ray film, by activating the silver halide grains, which are then reduced to metallic silver when the film is developed. Autoradiography is commonly used to determine whether a radioactive probe molecule has hybridized to denatured DNA or RNA following Southern or Northern transfers, respectively, or in colony hybridization procedures.


An organism that satisfies its need for organic molecules by using solar energy or energy released from the oxidation of inorganic substances to convert inorganic molecules into organic molecules; "self-feeding". In genetics, a mutant that lacks a requirement for its growth required by the wildtype, or a mutant that has acquired resistance to a normally lethal or debilitating condition. Synonym: prototroph. Compare auxotroph, heterotroph


Autotrophic organisms are able to utilize carbon from mineral sources and obtain energy from either mineral sources or light and thereby avoid the necessity of living on organic materials from other organisms.

autumnal dormancy

quiescence in the fall.


are plant growth regulators


A mutant requiring specific compound(s) for its growth that are already present in the wildtype. Useful as a genetic marker and for complementation tests. adj. auxotrophic. Compare autotroph.

available nutrient

The small portion of each essential plant nutrient present in soil that is available to plants. The remainder is unavailable because it is held too firmly in the mineral and organic matter.

available water holding capacity

The amount of water that a soil can store in a form available for plant use is known as the available water-holding capacity of the soil. This is often assumed to be the water held with a capillary potential between 1/3 atmosphere (field capacity) and 15 atmospheres (wilting point).


The downwind portions of large open areas are often subject to increased wind erosion caused by avalanching. Avalanching results from an accumulation of saltating particles--some from upwind areas and some from the local area, including many that would not have saltated had they not been knocked loose by others. See saltation.


1) lack of ability of a pathogen to cause disease in a specific host genotype; 2) a type of pathogen gene, usually dominant, that interacts with a specific host resistance gene.


possessing an avirulence gene that corresponds to a specific host resistance gene; unable to cause disease in a specific host genotype.


in pest management, using management strategies that prevent the host crop from coming in contact with a pest population (same as evasion).


the stiff, bristle like trichome extending from the lemma of a grass flower or seed.


Of or arising in an axil, as in an axillary bud that differentiates and grows in the axil between stem and leaf. In contrast to a terminal bud that arises at the end of stem tissue.

axis of rotation

an imaginary bar extending through the poles of the earth. The earth spins around this imaginary bar. The orientation of this bar is tilted from the vertical in reference to the plane of the earth's orbit.


The part of a neuron that carries impulses away from the cell body.

axonic poison

A chemical that interrupts normal transmission of impulses along the axon of a neuron.

Go to top