Useful Info - Glossary

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The leaf surface facing away from the stem of the plant.


Non-living, physical or chemical, includes solar radiation, temperature, humidity, and pH, used in context of an effect eg. abiotic damage.

Absolute humidity:

In a system of moist air, the ratio of the mass of water vapour to the total volume of the system. Usually expressed as grams per cubic metre (g/m3).

Absolute temperature scale:

A temperature scale based on absolute zero.

Absolute zero:

A hypothetical temperature characterized by a complete absence of heat and defined as 0 K, -273.15 C, or -459.67 F.

Absorbed water:

Water held mechanically in a soil mass and having physical properties similar to ordinary water at the same temperature and pressure.


The penetration of a substance into the surface layer of a solid with which it is in contact eg. process by which pesticides are taken into plant tissues by roots or foliage (stomata, cuticle, etc.)


A pesticide used to kill mites (an insect relative). A miticide.

Acid soil:

A soil material having a pH of less than 7.0. See also soil reaction.


increased levels of acid (lowered pH) in water bodies which can be a result of changes in land use e.g. forestry.


Any organism that grows best under acidic conditions (low pH).

Active acidity:

The activity of hydrogen ion in the aqueous phase of a soil. It is measured and expressed as a pH value.


Change in an organism resulting from the action of natural selection on variation so that the organism is fitted more perfectly for existence in its environment.

Adaptive enzyme:

An enzyme produced by an organism in response to the presence of its substrate or a related substance. It is also called an induced enzyme.


The leaf surface facing the stem of the plant.

Adiabatic process:

A thermodynamic change of state in a system in which there is no transfer of heat or mass across the boundaries of the system. Compression always results in warming, expansion in cooling.


An ingredient added to a pesticide formulation or mixture to improve its effectiveness. Includes wetting agents, spreaders, emulsifiers, dispersing agents, foam suppressants, penetrants and correctives.


Roots or buds arising from an unusual position on a stem eg. roots on stems, or buds produced elsewhere than in the axils of leaves or the extremities of stems.


To impregnate with a gas, usually air.


The creation of holes in soil or turf by coring, slitting, grooving, hole punching, forking, sliding, spiking or other means to reduce compaction and improve water and air movement through the soil

Aeration, soil:

The process by which air in the soil is replaced by air from the atmosphere.

Aerial roots:

Roots that arise from the above ground


Occurring only in the presence of molecular oxygen, as applied to certain chemical or biochemical processes such as aerobic decomposition.


Particles bound firmly together.


A group of soil particles cohering in such a way that they behave mechanically as a unit.


A mechanical device in a spray tank to ensure uniform distribution of the toxicant during dilution and to prevent sedimentation.


The study of soils.


The branch of agriculture/horticulture that deals with the theory and practice of field-crop production and the scientific management of soil.


Average Grain Size is used to denote the relative grading of sand. AGS is calculated using micron sieve sizes. In general, lower figures denote finer material.


A chemical used to control algae, often referred to as aquatic weeds

Alien species:

An organism that has invaded or been introduced and is growing in a new region.

Alkaline soil:

Any soil that has a pH greater than 7.0.


A group of plant derived substances with toxic properties.


Substance produced by one plant which injures or suppresses the growth of another.


Material such as clay, silt, sand, and gravel deposited by modern rivers. and streams.

Ambient temperature:

The temperature of the surroundings averaged over 24 hours.

Amendment, soil:

An alteration of the properties of a soil with other substances.

Amino acid:

An organic acid containing both amino and carboxyl groups.


Enzyme that breaks down starch.

Anabatic wind:

A local wind which blows up a slope heated by sunshine. It is a feature which is less common than its converse, the kabatic wind


Having no molecular oxygen in the environment.


A general term for instruments designed to measure the speed or force of the wind.


A negatively charged ion. An ion that carries a negative charge of electricity that interacts with cations (positive charges) in ionic bonding reactions


The pollen-bearing part of a stamen.


The state of expansion in a flower; full bloom.


Term describing a fungal disease having characteristic necrotic lesions on leaves.


An insecticide especially effective against sap-sucking insects particularly aphids

Apical meristem:

The tip of a growing plant root or shoot composed of cells from which subsequent growth develops climates.


A species having an articulated external skeleton such as an insect, spider or crustacean.


Clawlike appendages projecting from the base of the leafblade or at the apex of the sheath

Available nutrient:

The portion of any element or compound in the soil that can be readily absorbed and assimilated by growing plants.

Available water:

The portion of water in a soil that can be readily absorbed by plant roots.


Upper angle between a branch or leaf and the stem from which it grows.


Buffer compounds, soil:

The clay, organic matter, and materials such as carbonates and phosphates that enable the soil to resist appreciable change in pH.


Calcareous soil:

Soil containing sufficient calcium carbonate.

Capillary water:

The water held in the small pores of a soil.

Carbon cycle:

The cycle whereby carbon dioxide is fixed in living organisms by photosynthesis or by chemosynthesis, is consumed in carbohydrate, protein, and fat by most animals and plants that do not carry out photosynthesis, and ultimately is returned to its original state when it is freed by respiration and by the death and decay of plant and animal bodies.


A positively charged ion.

Cation exchange capacity (CEC):

The total amount of exchangeable cations that a soil can adsorb.


As a particle-size term


Organic residues, or a mixture of organic residues and soil, that have been piled, moistened, and allowed to decompose. Mineral fertilisers are sometimes added.


The gradual reduction in volume of a soil mass resulting from an increase in compressive stress.


Tillage to prepare land for seeding or transplanting, and later to control weeds and loosen the soil.



To separate the individual components of compound particles by chemical or physical means or both.


The changing of a soil to a more highly leached and weathered state.


Algae having siliceous ceil walls that persist as a skeleton after death.



The study of the relationship between organisms and their environment.


A proteinaceous organic substance, produced within an organism that acts like a catalyst.


The wearing away of the land surface by running water, wind, ice, or other geological agents, including such processes as gravitational creep.



Any organic or inorganic material of natural or synthetic origin that is added to a soil to supply certain elements essential to the growth of plants.

Fertiliser grade:

The guaranteed minimum analysis, in percent, of the major plant nutrient elements contained in a fertiliser material or in a mixed fertiliser.

Fertiliser requirement:

The quantity of certain plant nutrient elements needed, in addition to the amount supplied by the soil, to increase plant growth to a designated optimum.

Field capacity:

The percentage of water remaining in the soil 2 or 3 days after the soil has been saturated and free drainage has practically ceased. The percentage may be expressed in terms of weight or volume.


The process or processes in a soil by which certain chemical elements essential for plant growth are converted from a soluble or exchangeable form to a much less soluble or non-exchangeable form, for example, phosphate fixation.

Fulvic acids:

A term with various meanings that usually refers to the mixture of organic substances that remains in solution when a dilute alkali extract from the soil has been acidified.


The halophytic plants that lack chlorophyll and are filamentous in structure; molds.


Gravitational water:

Water that moves into, through, or out of the soil by gravity.


Humic acids:

A mixture of various dark-coloured organic substances precipitated by acidifying a dilute alkali extract from the soil. The term is used by some workers to designate only the alcohol-insoluble part of this precipitate.


The processes by which organic matter decomposes to form humus. In humus the initial structures or shapes can no longer be recognized.


The fraction of the soil organic matter that is not dissolved when the soil is treated with dilute alkali.


The fraction of the soil organic matter that remains after most of the added plant and animal residues have decomposed


Chemical combination of water with another substance.


The process by which a substrate is split to form two end products by the intervention of a molecule of water.


Impeded drainage:

A condition that hinders the movement of water by gravity through soils.


Resistant to penetration by fluids or roots.


The downward entry of water into the soil.

Infiltration rate:

A soil characteristic determining or describing the maximum rate at which water can enter the soil under specified conditions, including the presence of excess water.


Atom, group of atoms, or compound that is electrically charged as a result of the loss of electrons (cation) or the gain of electrons (anion).

Iron pan:

A thin indurate soil horizon in which iron is a major constituent of the cementing material.



The removal from the soil of materials in solution.


A symbiotic, mutualistic association of an algal type and a fungal type.


A soil textural class.



A chemical element necessary in large amounts, usually greater than 1 ppm in the plant, for the growth of plants and usually applied artificially in fertiliser.

Maximum water-holding capacity:

The average moisture content of a disturbed sample of soil, 1 cm high, that is at equilibrium with a water table at its lower surface.


A microorganism growing best in the presence of small amounts of atmospheric oxygen.


A chemical element necessary in only small amounts, usually less than 1 ppm in the plant, for the growth of plants and the health of animals. Examples of these elements are boron, molybdenum, copper, iron, manganese, and zinc.


A form of life of microscopic size.


A homogeneous naturally occurring phase, sometimes restricted to inorganic, crystalline phases.

Moisture, soil:

Water contained in the soil.


Any material such as straw, sawdust, leaves, plastic film, or loose soil that is spread on the surface of the soil to protect the soil and the plant roots from the effects of raindrops, soil crusting, freezing, and evaporation.


A mass of threadlike filaments branched or composing a network, that constitutes the vegetative structure of a fungus.


A mycorrhizal association in which the fungal hyphae are present on root surfaces.



The biochemical oxidation of ammonium to nitrate.

Nitrogen cycle:

The sequence of biochemical changes by which nitrogen is used by a living organism, liberated upon the death and decomposition of the organism, and converted to its original state of oxidation.

Nitrogen fixation:

The conversion of elemental nitrogen (N2) to organic combinations or to forms readily utilizable in biological processes.



An order of soils that have developed dominantly from organic deposits. The majority of Organic soils are saturated for most of the year, unless artificially drained, but some of them are not usually saturated for more than a few days. They contain 17% or more organic carbon.

over sowing:

cricket square


Particle size:

The effective diameter of a particle measured by sedimentation, sieving, or micrometric methods. Has been called grain size.

Parts per million (ppm):

Weight units of any given substance per one million equivalent weight units of oven dry soil; or, for soil or other solutions, the weight units of solute per million weight units of solution.


Unconsolidated soil material consisting largely of undecomposed or only slightly decomposed, organic matter.


The ease with which a probe can be pushed into the soil.

Perched water table:

A water table due to the "perching" of water on a relatively impermeable layer at some depth within the soil. The soil within or below the impermeable layer is not saturated with water.

Percolation (of soil water):

The downward movement of water through soil.


Chemicals that kill organisms that are injurious to man or to the crops and animals upon which he depends for food, fibre, and shelter. These organisms include insects, mites, microorganisms, weeds, and rodents. Pesticides include insecticides, fungicides, herbicides, and others.

pH, soil:

The degree of acidity or alkalinity of a soil.

Pore space:

The total space not occupied by soil particles in a bulk volume of soil.


The volume percentage of the total bulk not occupied by solid particles.

Porphyrin Ring:

The versatility of the porphyrin ring, its molecular simplicity and its ubiquity in the biosphere suggest that it was developed a very long time ago. At that time life's chemistry was less elaborate than it is now, and one molecule may have performed many functions. A porphyrin molecule is a planar group of four connected rings, each of which contains a nitrogen atom that faces the center of the ring cluster. These four nitrogens provide an ideal environment for the insertion of a metal ion, such as iron or magnesium, which are extremely useful for a variety of oxygen-related reactions. A particular porphyrin ring containing magnesium is the organic molecule called chlorophyll, the substance in green plants that helps harvest the electromagnetic energy of sunlight for use in photosynthesis.

Profile, soil:

A vertical section of the soil through all its horizons and extending into the parent material.



Small heterotrophic bacteria of the genus Rhizobium capable of forming symbiotic nodules on the roots of leguminous plants. In the nodules, the bacteria fix atmospheric nitrogen that is used by the plants. The bacteria receive their energy from the plants.


In the lower plants, one of the unicellular or multicultural root like filaments that serve for attachment and absorption.


The soil surrounding and directly influenced by plant roots.



The process of accumulation of salts in soil.


A soil particle between 0.05 and 2.0 mm in diameter.


To fill all the voids between soil particles with a liquid.


A soil separate consisting of particles between 0.05 and 0.002 mm in equivalent diameter.

Soil structure types:

A classification of soil structure based on the shape of the aggregates or peds and their arrangement in the profile


Directional force acting within a material.

Structure, soil:

The combination or arrangement of primary soil particles into secondary particles, units, or peds.



Saturated with water.

Wilting point (permanent wilting):

The moisture content of a soil at which plants (specifically sunflower plants) wilt and fail to recover their turgidity when placed in a dark, humid atmosphere. The wilting point is commonly estimated by measuring the 15-bar percentage of a soil.



Plants that grow in or on extremely dry soils or soil materials.


Zymogenous flora:

Organisms found in soils in large numbers immediately after the addition of readily decomposable organic materials.