Glossary on Soil Science （1）

Acid sulfate soil: A soil which is potentially extremely acidic (pH<3.5), because large amounts of

reduced forms of sulfur are oxidized to sulfuric acid when soil is drained or excavated.

Active acidity: The activity of hydrogen ions in the aqueous phase of a soil. It is measured and

expressed as a pH value.

Adhesion: Molecular attraction that holds the surfaces of unlike substances in contact. For ex-

ample, water is attracted to clay particles by adhesion.

Adsorption : The process by which atoms, molecules, or ions are taken up from the soil solution or soil atmosphere and retained on the surfaces of solids by chemical or physical binding.

Aerobic Respiration: A form of respiration in the presence of oxygen, in which the terminal elec-

tron acceptor is oxygen, leading to the formation of water. This process yields maximum energy.

Anaerobic Respiration: A form of respiration in the absence of oxygen, in which the terminal

electron acceptors may be nitrate or sulfate ions or other substances. This process yields less energy than aerobic respiration.

Arable Soil: A soil which is suitable for the production of cultivated crops in an economical and

practical manner.

Available Nutrients: The amount of soil nutrient in chemical forms accessible to plant roots, or the compounds likely to be convertible to such forms during the growing season.

Biocycling : The cycling of materials through microorganisms, plants and animals.

Biogeochemical Cycle: The movement of chemical elements from organism to physical environment to organism in more or less circular pathways.

Capillary Action: (capillarity) The process by which soil moisture may move in any direction

through the fine (i.e., capillary) pores of the soil, under the influence of surface tension forces

between the water and individual soil particles. Soil moisture in this state is called capillary mois-

ture. It exists as a film or skin of moisture on soil particles, and may be drawn above the water-table by capillary action and into plant roots by the process of osmosis.

Capillary  Pores: In these pores the shape of the interface between air and water is determined by the configuration of pores and by forces on the interface. The resulting interface is called the capillary meniscus. The flow of water in these pores is considered to be laminar (movement of fluid particles in a direction parallel to each other) and dominant in soils.

Capillary Water: Water that is left in the soil, along with hygroscopic moisture and water vapor,

after the gravitational water has drained off. Capillary water is held by surface tension as a film of

moisture on the surface of soil particles and peds, and as minute bodies of water filling part of the

pore space between particles.

capillary Zone: (capillary fringe) The zone immediately above the water table, in which water may be drawn upwards as a consequence of capillary action.

Cation Exchange: A process in which cations in solution are exchanged with cations held on the

exchange sites of mineral and organic matter, particularly on the surfaces of colloids of clay and

humus.

Cation Exchange Capacity: The total amount of exchangeable cations that a particular material or soil can adsorb at a given pH. Exchangeable cations are held mainly on the surface of colloids of

clay and humus and are measured in centimoles of charge per kilogram (cmol /kg) of soil (or of

other adsorbing material such as clay).

Clay : As a particle-size term: a size fraction less than 0.002 mm in equivalent diameter.

Clay Loam: Soil material that contains 27 to 40% clay and 20 to 45% sand.

Cohesion : The molecular attraction that holds the surfaces of like substances in contact. For ex-

ample, the attraction between water molecules is cohesion.

Colloid : A substance in a state of fine subdivision, whose particles are 10-4 to 10-7 cm in diameter.

Condensation : The conversion of a substance (as water) from the vapor state to a denser liquid or solid state, usually initiated by a reduction in the temperature of the vapor.

Decomposition : The breakdown of complex organic molecules of dead protoplasm and cell walls

into simpler organic and inorganic molecules, which may be used again by other organisms.

Diffusion : The movement of individual species of gas in response to a concentration gradient.

Dipole : A pair of separated opposite electrical charges.

Dissociation : The breakdown of a molecule, ion, etc., into smaller molecules, ions, etc. For ex-

ample, carbonic acid disassociates into bicarbonate and hydrogen ions

Ecosystem : A community and its environment treated together as a functional system of comple-

mentary relationships, with the transfer and circulation of matter.

Eluviation : The transportation of soil material in suspension or in solution within the soil by the

downward or lateral movement of water.

Equilibrium : A state of balance between opposing forces or actions that is either static (as in a body acted on by forces whose resultant is zero) or dynamic (as in a reversible chemical reaction when the velocities in both directions are equal).

Erosion : The wearing away of the land surface by running water, wind, ice, or other geological

agents, including such processes as gravitational creep. Erosion can also involve the detachment

and movement of soil or rock by water, wind, ice, or gravity.

Food Chain: The transfer of energy from the primary producers (green plants) through a series of

organisms that eat and are eaten, assuming that each organism feeds on one other type of organism.

Food Web: A diagram that represents the feeding relationships of organisms within an ecosystem.

It consists of a series of interconnecting food chains.

Gravel : Rock fragments 2 mm to 75 mm in diameter.

Groundwater : Water that is passing through or standing in the soil and the underlying strata. It is free to move by gravity.

Humic Substances: A series of complex, relatively high molecular weight, brown-to black-colored organic substances that make up 60 to 80% of the soil organic matter and are generally quite resistant to microbial attack.

Humus :

1. The fraction of the soil organic matter that remains after most of the added plant and animal

residues have decomposed. It is usually dark colored.

2. Humus is also used in a broader sense to designate the humus forms referred to as forest humus.

They include, principally, mor, moder, and mull.

3. All the dead organic material on and in the soil that undergoes continuous breakdown, change,

and synthesis.