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.