Glossary Flashcards
gravitational potential
That portion of the total soil water potential due to differences in elevation of the reference pool of pure water and that of the soil water. Since the soil water elevation is usually chosen to be higher than that of the reference pool, the gravitational potential is usually positive.
green manure
Plant material incorporated with the soil while green, or soon after maturity, for improving the soil.
groundwater
Subsurface water in the zone of saturation that is free to move under the influence of gravity, often horizontally to stream channels.
hardpan
A hardened soil layer, in the lower A or in the B horizon, caused by cementation of soil particles with organic matter or with such materials as silica, sesquioxides, or calcium carbonate. The hardness does not change appreciably with changes in moisture content and pieces of the hard layer do not slake in water.
heavy metals
Those metals that have densities of 5.0 Mg/m or greater. Elements in soils include Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Pb, and Zn.
Histosols
An order in Soil Taxonomy. Soils formed from materials high in organic matter. Histosols with essentially no clay must have at least 20% organic matter by weight (about 78% by volume). This minimum organic matter content rises with increasing clay content to 30% (85% by volume) in soils with at least 60% clay.
horizon, soil
A layer of soil, approximately parallel to the soil surface, differing in properties and characteristics from adjacent layers below or above it.
humus
The more or less stable fraction of the soil organic matter remaining after the major portions of added plant and animal residues have decomposed. Usually it is dark in color.
hydration
Chemical union between an ion or compound and one or more water molecules, the reaction being stimulated by the attraction of the ion or compound for either the hydrogen or the unshared electrons of the oxygen in the water.
hydrogen bonding
Relatively low energy bonding exhibited by a hydrogen atom located between two highly electronegative atoms, such as nitrogen or oxygen.
hydrolysis
A reaction with water that splits the water molecule into H+ and OH- ions. Molecules or atoms participating in such reactions are said to hydrolyze.
hygroscopic coefficient
The amount of moisture in a dry soil when it is in equilibrium with some standard relative humidity near a saturated atmosphere (about 98%), expressed in terms of percentage on the basis of oven-dry soil.
igneous rock
Rock formed from the cooling and solidification of magma that has not been changed appreciably since its formation.
illuviation
The process of deposition of soil material removed from one horizon to another in the soil; usually from an upper to a lower horizon in the soil profile.
kandic horizon
A subsurface diagnostic horizon having a sharp clay increase relative to overlying horizons and having low-activity clays.
leaching
The removal of materials in solution from the soil by percolating waters.
Liebig’s law
The growth and reproduction of an organism are determined by the nutrient substance (oxygen, carbon dioxide, calcium, etc.) that is available in minimum quantity with respect to organic needs; the limiting factor.
lime (agricultural)
In strict chemical terms, calcium oxide. In practical terms, a material containing the carbonates, oxides, and/or hydroxides of calcium and/or magnesium used to neutralize soil acidity.
loam
The textural-class name for soil having a moderate amount of sand, silt, and clay. Loam soils contain 7 to 27% clay, 28 to 50% silt, and 23 to 52% sand.
loess
Material transported and deposited by wind and consisting of predominantly silt-sized particles.
luxury composition
The intake by a plant of an essential nutrient in amounts exceeding what it needs. For example, if potassium is abundant in the soil, alfalfa may take in more than it requires.
macronutrient
A chemical element necessary in large amounts (usually 50 mg/kg in the plant) for the growth of plants. Includes C, H, O, N, P, K, Ca, Mg, and S. (Macro refers to quantity and not to the essentiality of the element.)
macropores
Larger soil pores, generally having a diameter greater than 0.06 mm, from which water drains readily by gravity.
matric potential
That portion of the total soil water potential due to the attractive forces between water and soil solids as represented through adsorption and capillarity. It will always be negative.
metamorphic rock
A rock that has been greatly altered from its previous condition thorugh the combined action of heat and pressure. For example, marble is a metamorphic rock produced from limestone, gneiss is produced from granite, and slate is produced from shale.
micronutrient
A chemical element necessary in only extremely small amounts (
micropores
Relatively small soil pores, generally found within structural aggregates and having a diameter less than 0.06 mm.
mollic epipedon
A diagnostic surface horizon of mineral soil that is dark colored and relatively thick, contains at least 0.6% organic carbon, is not massive and hard when dry, has a base saturation of more than 50%, has less than 250 mg/kg P2O5 soluble in 1% citric acid, and is dominantly saturated with bivalent cations.
mollisols
An order in Soil Taxonomy. Soils with nearly black, organic rich surface horizons and high supply of bases. They have mollic epipedons and base saturation greater than 50% in any cambic or argillic horizon. They lack the characteristics of Vertisols and must not have oxic or spodic horizons.
moraine
An accumulation of drift, with an initial topographic expression of its own, built within a glaciated region chiefly by the direct action of glacial ice. Examples are ground, lateral, recessional, and terminal moraines.
mycorrhiza
The association, usually symbiotic, of fungi with the roots of seed plants.
nitrogen fixation
The biological conversion of elemental nitrogen (N2) to organic combinations or to forms readily utilized in biological processes.
ochric epipedon
A diagnostic surface horizon of mineral soil that is too light in color, too high in chroma, too low in organic carbon, or too thin to be a plaggen, mollic, umbric, anthropic, or histic epipedon, or that is both hard and massive when dry.
osmotic potential
That portion of the total soil water potential due to the presence of solutes in soil water. It will generally be negative.
Oxisols
An order in Soil Taxonomy. Soils with residual accumulations of low-activity clays, free oxides, kaolin, and quartz. They are mostly in tropical climates.
parent material
The unconsolidated and more or less chemically weathered mineral or organic matter from which the solum of soils is developed by pedogenic processes.
particle density
The mass per unit volume of the soil particles. In technical work, usually expressed as metric tons per cubic meter (Mg/m3) or grams per cubic centimeter (g/cm3).
peat
Unconsolidated soil material consisting largely of undecomposed, or only slightly decomposed, organic matter accumulated under conditions of excessive moisture.
pedology
The science that deals with the formation, morphology and classification of soil bodies as landscape components.
pH, soil
The negative logarithm of the hydrogen ion activity (concentration) in the soil solution. The degree of acidity (or alkalinity) of a soil as determined by means of a glass or other suitable electrode or indicator at a specified moisture content or soil-to-water ratio, and expressed in terms of the pH scale.
physical weathering
The breakdown of rock and mineral particles into smaller particles by physical forces such as frost action.
plow pan
A subsurface soil layer having a higher bulk density and lower total porosity than layers above or below it, as a result of pressure applied by normal plowing and other tillage operations.
primary mineral
A mineral that has not been altered chemically since deposition and crystallization from molten lava.
Proctor test
A laboratory procedure that indicates the maximum achievable bulk density for a soil and the optimum water content for compacting a soil.
profile, soil
A vertical section of the soil through all its horizons and extending into the parent material.
sand
A soil particle between 0.05 and 2.0 mm in diameter; a soil textural class.
secondary mineral
A mineral resulting from the decomposition of a primary mineral or from the reprecipitation of the products of decomposition of a primary mineral.
sedimentary rock
A rock formed from materials deposited from suspension or precipitated from solution and usually being more or less consolidated. The principal sedimentary rocks are sandstones, shales, limestones, and conglomerates.
silt
(1) A soil separate consisting of particles between 0.05 and 0.002 mm in equivalent diameter. (2) A soil textural class.
slickensides
Stress surfaces that are polished and striated and are produced by one mass sliding past another.
smectite
A group of silicate clays having a 2:1 - type lattice structure with sufficient isomorphous substitution in either or both the tetrahedral and octahedral sheets to give a high interlayer negative charge and high cation exchange capacity and to permit significant interlayer expansion and consequent shrinking and swelling of the clay. Montmorillonite, beidellite, and saponite are in the smectite group.
soil
(1) A dynamic natural body composed of mineral and organic solids, gases, liquids, and living organisms which can serve as a medium for plant growth. (2) The collection of natural bodies occupying parts of the Earth’s surface that is capable of supporting plant growth and that has properties resulting from the integrated effects of climate and living organisms acting upon parent material, as conditioned by topography, over periods of time.
soil alkalinity
The degree or intensity of alkalinity of a soil, expressed by a value of >7.0 on the pH scale.
soil solution
The aqueous liquid phase of the soil and its solutes, consisting of ions dissociated from the surfaces of the soil particles and of other soluble materials.
soil structure
The combination or arrangement of primary soil particles into secondary particles, units, or peds. These secondary units may be, but usually are not, arranged in the profile in such a manner as to give a distinctive characteristic pattern. The secondary units are characterized and classified on the basis of size, shape, and degree of distinctness into classes, types, and grades, respectively.
soil survey
The systematic examination, description, classification, and mapping of soils in an area. Soil surveys are classified according to the kind and intensity of field examination.
soil water potential (total)
A measure of the difference between the free energy state of soil water and that of pure water. Technically it is defined as “that amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermically an infinitesimal quantity of water from a pool of pure water, at a specified elevation and at atmospheric pressure, to the soil water (at the poin under considertation).” This total potential consists of gravitational, matric, and osmotic potentials.
Spodosols
An order in Soil Taxonomy. Soils with subsurface illuvial accumulations of organic matter and compounds of aluminum and usually iron. These soils are formed in acid, mainly coarse-textured materials in humid and mostly cool or temperate climates.
till
(1) Unstratified glacial drift deposited directly by the ice and consisting of clay, sand, gravel, and boulders intermingled in any proportion. (2) To plow and prepare for seeding; to seed or cultivate the soil.
Ultisols
An order in Soil Taxonomy. Soils that are low in bases and have subsurface horizons of illuvial clay accumulations. They are usually moist, but during the warm season of the year some are dry part of the time.
umbric epipedon
A diagnostic surface horizon of mineral soil that has the same requirements as the mollic epipedon with respect to color, thickness, organic carbon content, consistence, structure, and P2O5 content, but that has a base saturation of less than 50%.
vermiculite
A 2:1-type silicate clay, usually formed from mica, that has a high net negative charge stemming mostly from extensive isomorphous substitution of aluminum for silicon in the tetrahedral sheet.
Vertisols
An order in Soil Taxonomy. Clayey soils with high shrink-swell potential that have wide, deep cracks when dry. Most of these soils have distinct wet and dry periods throughout the year.
weathering
All physical and chemical changes produced in rocks, at or near the Earth’s surface, by atmospheric agents.
wilting point (permanent wilting point)
The moisture content of soil, on an oven-dry basis, at which plants wilt and fail to recover their turgidity when placed in a dark, humid atmosphere.
A horizon
The surface horizon of a mineral soil having maximum organic matter accumulation, maximum biological activity, and/or eluviation of materials such as iron and aluminum oxides and silicate clays.
acid cations
Cations, principally Al3+, Fe3+, and H+, that contribute to H+ ion activity either directly or through hydrolysis reactions with water.
adhesion
Molecular attraction that holds the surfaces of two substances (e.g. water and sand particles) in contact.
adsorption
The attraction of ions or compounds to the surface of a solid. Soil colloids adsorb large amounts of ions and water.
aerobic
(1) Having molecular oxygen as a part of the environment. (2) Growing only in the presence of molecular oxygen, as aerobic organisms. (3) Occurring only in the presence of molecular oxygen (said of certain chemical or biochemical processes, such as aerobic decomposition).
aggregate (soil)
Many soil particles held in a single mass or cluster, such as a clod, crumb, block, or prism.
albic horizon
A diagnostic subsurface horizon from which clay and free iron oxides have been removed or in which the oxides have been segregated to the extent that the color of the horizon is determined primarily by the color of the primary sand and silt particles rather than by coatings on these particles.
alfisoils
An order in Soil Taxonomy. Soils with gray to brown surface surface horizons, medium to high supply of bases, and B horizons of illuvial clay accumulation. These soils from mostly under forest or savanna vegetation in climates with slight to pronounced seasonal moisture deficit.
alkaline soil
Any soil that has pH>7. Usually applied to the surface layer or root zone but may be used to characterize any horizon or a sample thereof.
allelopathy
The process by which one plant may affect other plants by biologically active chemicals introduced into the soil, either directly by leaching or exudation from the source plant, or as a result of the decay of the plant residues. The effects, though usually negative, may also be positive.
alluvium
A general term for all detrital material deposited or in transit by streams, including gravel, sand, silt, clay, and all variations and mixtures of these. Unless otherwise noted, alluvium is unconsolidated.
anaerobic
(1) The absense of molecular oxygen. (2) Growing or occurring in the absence of molecular oxygen (e.g., anaerobic bacteria or biochemical reduction reaction).
andisols
An order in Soil Taxonomy. Soils developed from volcanic ejecta. The colloidal fraction is dominated by allophane and/or Al-humus compounds.
anion
Negatively charged ion; during electrolysis it is attracted to the positively charged anode.
arbuscular mycorrhiza
A common endomycorrhizal association produced by phycomycetous fungi and characterized by the development, within root cells, of small structures known as arbuscules. Some also form, between root cells, storage organs known as vesicles. Host range includes many agricultural and horticultural crops. Formerly called vesicular arbuscular mycorrhiza (VAM)
argillic horizon
A diagnostic subsurface horizon characterized by the illuvial accumulation of layer-lattice silicate clays.
aridisols
An order in Soil Taxonomy. Soils of dry climates. They have pedogenic horizons, low in organic matter, that are never moist for as long as three consecutive months. They have an ochric epipedon and one or more of the following diagnostic horizons: argillic, natric, cambic, calcic, petrocalcic, gypsic, petrogypsic, salic, or a duripan.
available nutrient
That portion of any element or compound in the soil that can be readily absorbed and assimilated by growing plants. (“Available” should not be confused with “exchangeable.”)
available water
The portion of water in a soil that can be readily absorbed by plant roots. The amount of water released between the field capacity and the permanent wilting point.
B horizon
A soil horizon, usually beneath the A or E horizon, that is characterized by one or more of the following: (1) a concentration of soluble salts, silicate clays, iron and aluminum oxides, and humus, alone or in combination; (2) a blocky or prismatic structure; and (3) coatings of iron and aluminum oxides that give darker, stronger, or redder color.
base saturation percentage
The extent to which the adsorption complex of a soil is saturated with exchangeable cations other than hydrogen and aluminum. It is expressed as a percentage of the total cation exchange capacity.
blocky soil structure
Soil aggregates with blocklike shapes; common in B horizons of soils in humid regions.
buffering capacity
The ability of a soil to resist changes in pH. Commonly determined by presence of clay, humus, and other colloidal materials.
bulk density, soil
The mass of dry soil per unit of bulk volume, including the air space. The bulk volume is determined before drying to constant weight at 105 degrees Celsius.
C horizon
A mineral horizon, generally beneath the solum, that is relatively unaffected by biological activity and pedogenesis and is lacking properties diagnostic of an A or B horizon. It may or may not be like the material from which the A and B have formed.
capillary water
The water held in the capillary or small pores of a soil, usually with a tension >60 cm of water.
carbon/nitrogen (C/N) ratio
The ratio of the weight of organic carbon (C) to the weight of total nitrogen (N) in a soil or in organic material.
cation
A positively charged ion; during electrolysis it is attracted to the negatively charged cathode.
cation exchange
The interchange between a cation in solution and another cation on the surface of any surface-active material, such as clay or organic matter.
cation exchange capacity
The sum total of exchangeable cations that a soil can adsorb. Sometimes called total exchange capacity, base exchange capacity, or cation adsorption capacity. Expressed in centimoles of charge per kilogram (cmol/kg) of soil ( or of other adsorbing material, such as clay).
chelate
(Greek “claw”) A type of chemical compound in which a metallic ion is firmly combined with an organic molecule by means of multiple chemcial bonds.
chlorosis
A condition in plants relating to the failure of chlorophyll (the green coloring matter) to develop. Chlorotic leaves range from light green through yellow to almost white.
clay
(1) A soil separate consisting of particles 40% clay,
colloid, soil
(Greek “gluelike”) Organic and inorganic matter with very small particle size and a correspondingly large surface area per unit of mass.
colluvium
A deposit of rock fragments and soil material accumulated at the base of steep slopes as a result of gravitational action.
compost
Organic residues, or a mixture of organic residues and soil, that have been piled, moistened, and allowed to undergo biological decomposition. Mineral fertilizers are sometimes added. Usually managed so as to reach thermophilic temperatures.
cover crop
A close-growing crop grown primarily for the purpose of protecting and improving soil between periods of regular crop production or between trees and vines in orchards and vineyards.
cryoturbation
Physical disruption and displacement of soil material within the profile by the forces of freezing and thawing. Sometimes called frost churning, it results in irregular, broken horizons, involutions, oriented rock fragments, and accumulation of organic matter on the permafrost table.
deflocculate
(1) To separate the individual components of compound particles by chemical and/or physical means. (2) To cause the particles of the disperse phase of a colloidal system to become suspended in the dispersion medium.
diagnostic horizons
(As used in Soil Taxonomy) Horizons having specific soil characteristics that are indicative of certain classes of soils. Horizons that occur at the soil surface are called epipedons, those below the surface, diagnostic subsurface horizons.
E horizon
Horizon characterized by maximum illuviation (washing out) of silicate clays and iron and aluminum oxides; commonly occurs above the B horizon and below the A horizon.
earthworms
Animals of the Lumbricidae family that burrow into and live in the soil. They mix plant residues into the soil and improve soil aeration.
ectotrophic mycorrhiza (ectomycorrhiza)
A symbiotic association of the mycelium of fungi and the roots of certain plants in which the fungal hyphae form a compact mantle on the surface of the roots and extend into the surrounding soil and inward between cortical cells, but not into these cells. Associated primarily with certain trees.
edaphology
The science that deals with the influence of soils on living things, particularly plants, including human use of land for plant growth.
eluviation
The removal of soil material in suspension (or in solution) from a layer or layers of a soil. Usually, the loss of material in solution is described by the term “leaching.”
eolian soil material
Soil material accumulated through wind action. The most extensive areas in the United States are silty deposits (loess), but large areas of sandy deposits also occur.
epipedon
A diagnostic surface horizon that includes the upper part of the soil that is darkened by organic matter, or the upper eluvial horizons, or both.
erosion
(1) The wearing away of the land surface by running water, wind, ice or other geological agents, including such processes as gravitational creep. (2) Detachment and movement of soil or rock by water, wind, ice, or gravity.
exchangeable ions
Positively or negatively charged atoms or groups of atoms that are held on or near the surface of a solid particle by attraction to charges of the opposite sign, and which may be replaced by other like-charged ions in the soil solution.
fertility, soil
The quality of a soil that enables it to provide essential chemical elements in quantities and proportions for the growth of specified plants.
fertilizer
Any organic or inorganic material of natural or synthetic origin added to a soil to supply certain elements essential to the growth of plants.
field capacity (field moisture capacity)
The percentage of water remaining in a soil two or three days after its having been saturated and after free drainage has practically ceased.
flocculate
To aggregate or clump together individual, tiny soil particles, especially fine clay, into small clumps or floccules.
fragipan
Dense and brittle pan or subsurface layer in soils that owes its hardness mainly to extreme density or compactness rather than high clay content or cementation. Removed fragments are friable, but the material in place is so dense that roots penetrate and water moves through it very slowly.
friable
A soil consistency term pertaining to soils that crumble with ease.
Gelisols
An order in Soil Taxonomy. Soils that have permafrost within the upper 1 m, or upper 2 m if cryoturbation is also present. They may have an ochric, histic, mollic, or other epipedon.
genesis, soil
The mode of origin of the soil, with special reference to the processes responsible for the development of the solum, or true soil, from the unconsolidated parent material.
gleyed
A soil condition resulting from prolonged saturation with water and reducing conditions that manifest themselves in greenish or bluish colors throughout the soil mass or in mottles
granular structure
Soil structure in which the individual grains are grouped into spherical aggregates with indistinct sides. Highly porous granules are commonly called crumbs. A well-granulated soil has the best structure for most ordinary crop plants.
