Soil Terminology

absorption - Uptake of matter or energy by a substance. absorption, active - Movement of ions and water into the plant root because of metabolic processes by the root, frequently against an electrochemical potential gradient. absorption, passive - Movement of ions and water into the plant root from diffusion along a chemical potential gradient.

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. adsorption complex - Collection of various organic and inorganic substances in soil that are capable of adsorbing ions and molecules. adsorption isotherm - A graph of the quantity of a given chemical species bound to an adsorption complex, at fixed temperature, as a function of the concentration of the species in a solution that is in equilibrium with the complex. Called an isotherm only because adsorption experiments are done at constant temperature. inner sphere adsorption - Adsorption of ions that occurs with the elimination of water of hydration in the space between the adsorbed ion and the surface. The force of retention of ions involves both ionic and covalent bonding. Strong adsorption of anions and cations at variable charge sites in organic matter, oxides, and phyllosilicate edges involves inner sphere adsorption. outer sphere adsorption - Adsorption of ions that occurs with the retention of waters of hydration between the surface and the adsorbed ion where the force that retains the ion is only electrostatic attraction. Ions that are retained by outer sphere adsorption are readily exchangeable. See also exchangeable cation and exchangeable anion. sodium adsorption ratio (SAR) - A relation between soluble sodium and soluble divalent cations which can be used to predict the exchangeable sodium fraction of soil equilibrated with a given solution.

sodium adsorption ratio, adjusted - The sodium adsorption ratio of a water adjusted for the precipitation or dissolution of Ca²⁺ that is expected to occur where a water reacts with alkaline earth carbonates within a soil. specific adsorption - The strong adsorption of ions or molecules on a surface. Specifically adsorbed materials are not readily removed by ion exchange.

aeration, soil - The process by which air in the soil is replaced by air from the atmosphere. In a well-aerated soil, the soil air is very similar in composition to the atmosphere above the soil. Poorly aerated soils usually contain a much higher content of CO₂ and a lower content of O₂ than the atmosphere above the soil. The rate of aeration depends largely on the volume and continuity of air-filled pores within the soil. soil aeration - The condition, and sum of all processes affecting, soil pore-space gaseous composition, particularly with respect to the amount and availability of oxygen for use by soil biota and/or soil chemical oxidation reactions.
alkali soil - (no longer used in SSSA publications) (i) A soil with a pH of 8.5 or higher or with a exchangeable sodium ratio greater than 0.15. (ii) A soil that contains sufficient sodium to interfere with the growth of most crop plants. See also saline-sodic soil and sodic soil. saline-alkali soil - (no longer used in SSSA publications) (i) A soil containing sufficient exchangeable sodium to interfere with the growth of most crop plants and containing appreciable quantities of soluble salts. The exchangeable-sodium percentage is >15, the conductivity of the saturation extract >4 dS m^-1(at 25 ° C), and the pH is usually 8.5 or less in the saturated soil. (ii) A saline-alkali soil has a combination of harmful qualities of salts and either a high alkalinity or high content of exchangeable sodium, or both, so distributed in the profile that the growth of most crop plants is reduced. See also saline-sodic soil.

alkalinity, soil - The degree or intensity of alkalinity in a soil, expressed by a value >7.0 for the soil pH.

ammonia volatilization - Mass transfer of nitrogen as ammonia gas from soil, plant, or liquid systems to the atmosphere.

ammoniation - The process of introducing various ammonium sources into other fertilizer sources forming ammoniated compounds. Ammonium polyphosphates and ammoniated superphosphate are ammoniated compounds.

ammonification - The biological process leading to ammoniacal nitrogen formation from nitrogen-containing organic compounds.
ammonium fixation - The process of entrapment of ammonium ions in interlayer spaces of phyllosilicates, in sites similar to K⁺ in micas. Smectites, illites and vermiculites all can fix ammonium, but vermiculite has the greatest capacity. The fixation may occur spontaneously in aqueous suspensions, or as a result of heating to remove interlayer water. Ammonium ions in collapsed interlayer spaces are exchangeable only after expansion of the interlayer. See also potassium fixation.

anaerobic - (i) The absence of molecular oxygen. (ii) Growing in the absence of molecular oxygen (such as anaerobic bacteria). (iii) Occurring in the absence of molecular oxygen (as a biochemical process). anaerobic respiration - The metabolic process whereby electrons are transferred from a reduced compound (usually organic) to an inorganic acceptor molecule other than oxygen. The most common acceptors are carbonate, sulfate, and nitrate. See also denitrification.

anion exchange capacity - The sum of exchangeable anions that a soil can adsorb. Usually expressed as centimoles, or millimoles, of charge per kilogram of soil (or of other adsorbing material such as clay).

antagonism - Production of a substance by one organism that inhibits one or more other organisms. The terms antibiosis and allelopathy have also been used to describe such cases of chemical inhibition. nutrient antagonism - The depressing effect caused by one or more plant nutrients on the uptake and availability of another nutrient in the plant.
application rate - (i) (irrigation) Rate at which water is applied per unit area; usually in mm per hour, (ii) weight or volume of a fertilizer, soil amendment, or pesticide applied per unit area.

assimilation - The incorporation of inorganic or organic substances into cell constituents.

available nutrients - (i) The amount of soil nutrient in chemical forms accessible to plant roots or compounds likely to be convertible to such forms during the growing season. and (ii) The contents of legally designated "available" nutrients in fertilizers determined by specified laboratory procedures which in most states constitute the legal basis for guarantees.

available water (capacity) - The amount of water released between in situ field capacity and the permanent wilting point (usually estimated by water content at soil matric potential of -1.5 MPa). It is not the portion of water that can be absorbed by plant roots, which is plant specific. See also nonlimiting water range.

banding - A method of fertilizer or other agrichemical application above, below, or alongside the planted seed row. Refers to either placement of fertilizers close to the seed at planting or subsurface applications of solids or fluids in strips before or after planting. Also referred to as band application.

bar - (i) A generic term for ridge-like accumulations of sand , gravel, or other unconsolidated material formed in the channel, along the banks, or at the mouth of a streams or formed by waves or currents as offshore features in large lakes or oceans. (ii) A unit of pressure equal to one million dynes per square centimeter. Megapasal is the preferred unit for pressure in SSSA publications. fifteen-bar percentage - (no longer used in SSSA publications) The percentage of water contained in a soil that has been saturated, subjected to, and is in equilibrium with, an applied pressure of 15 bars. Approximately the same as the fifteen-atmosphere percentage. See also soil water. one-third-bar percentage - (no longer used in SSSA publications) The percentage of water contained in a soil that has been saturated, subjected to, and is in equilibrium with, an applied pressure of one-third bar. Approximately the same as one-third-atmosphere percentage. See also soil water, soil water potential. point bar - One of a series of low, arcuate ridges of sand and gravel developed on the inside of a growing meander by the slow addition of individual accretions accompanying migration of the channel toward the outer bank.

base saturation - The ratio of the quantity of exchangeable bases to the cation exchange capacity. The value of the base saturation varies according to whether the cation exchange capacity includes only the salt extractable acidity (see cation exchange capacity) or the total acidity determined at pH 7 or 8. Often expressed as a percent.

basic fertilizer - One that, after application to and reaction with soil, decreases residual acidity and increases soil pH.

bioassay - A method for quantitatively measuring a substance by its effect on the growth of a suitable microorganism, plant, or animal under controlled conditions.

biodegradable - A substance able to be decomposed by biological processes.

biomass - (i) The total mass of living organisms in a given volume or mass of soil. (ii) The total weight of all organisms in a particular environment. See also microbial biomass. microbial biomass - (i) The total mass of living microorganisms in a given volume or mass of soil. (ii) The total weight of all microorganisms in a particular environment.

bioremediation - The use of biological agents to reclaim soil and water polluted by substances hazardous to the environment or human health.

biuret - H₂NCONHCONH₂ A product formed at high temperature during the manufacturing of urea. It is toxic to plants. Also called carbamoylurea.

block (thinning, checking) - To remove plants from a row with hoes or other cutting devices as a means of reducing and uniformly spacing plants.

broadcast - The application of solid or liquid fertilizer or other agrichemical on the soil surface. Usually done prior to planting and normally incorporated with tillage but may be unincorporated in no-till systems.

broadcast application - The application of material scattered or sprayed on surface of the soil.

buffer power - The ability of solid phase soil materials to resist changes in ion concentration in the solution phase. Can be expressed as where represents the concentration of ions on the solid phase in equilibrium with , the concentration of ions in the solution phase. Includes pH buffering as well as the buffering of other ionic and molecular components.

bulk blending - Mixing dry, individually granulated materials to form a mixed fertilizer.

bulk-blended - A physical mixture of dry granular fertilizer materials to produce specific fertilizer ratios and grades. Individual granules in the bulk blended fertilizer do not have the same ratio and content of plant food as does the mixture as a whole. bulk fertilizer - Solid or liquid fertilizer in a non-packaged form.
calcareous soil - Soil containing sufficient free CaCO₃ and other carbonates to effervesce visibly or audibly when treated with cold 0.1M HCl. These soils usually contain from 10 to almost 1000g kg^-1 CaCO₃ equivalent.

calcitic lime - Limestone containing mostly CaCO₃.

calcium carbonate equivalent - The content of carbonate in a liming material or calcareous soil calculated as if all of the carbonate is in the form of CaCO₃. See also lime, agricultural.

calcium:magnesium ratio - A statement of the relative proportions of available calcium and magnesium in the soil.
carbon cycle - The sequence of transformations whereby carbon dioxide is converted to organic forms by photosynthesis or chemosynthesis, recycled through the biosphere (with partial incorporation into sediments), and ultimately returned to its original state through respiration or combustion.

carbon-organic nitrogen ratio - The ratio of the mass of organic carbon to the mass of organic nitrogen in soil, organic material, plants, or microbial cells.
cation exchange - The interchange between a cation in solution and another cation in the boundary layer between the solution and surface of negatively charged material such as clay or organic matter. cation exchange capacity (CEC) - The sum of exchangeable bases plus total soil acidity at a specific pH, values, usually 7.0 or 8.0. When acidity is expressed as salt extractable acidity, the cation exchange capacity is called the effective cation exchange capacity (ECEC) because this is considered to be the CEC of the exchanger at the native pH value. It is usually expressed in centimoles of charge per kilogram of exchanger (cmol_ckg^-1) or millimoles of charge per kilogram of exchanger. See also acidity, total. effective cation exchange capacity (ECEC) - See cation exchange capacity(CEC).

chelates - Organic chemicals with two or more functional groups that can bind with metals to form a ring structure. Soil organic matter can form chelate structures with some metals, especially transition metals, but, much metal ion binding in soil organic matter probably does not involve chelation. Artificial chelating compounds are sometimes added to soil to increases the soluble fraction of some metals.

chemically precipitated phosphorus - (no longer used in SSSA publications) Relatively insoluble phosphorus compounds resulting from reactions of phosphorus with soil constituents: e. g. calcium and magnesium phosphates which are precipitated above a pH of about 6.0 to 6.5 (if calcium and magnesium are present); and, iron and aluminum phosphates which are precipitated below a pH of about 5.8 to 6.1. See also phosphorus fixation.
chemigation - The process where fertilizers, pesticides and other agrichemicals are applied into irrigation water to fertilize crops, control pests or amend soils.
citrate-soluble phosphorus - The fraction of total P in fertilizer that is insoluble in water but soluble in neutral 0.33 M ammonium citrate. Together with water-soluble phosphate, this represents the readily available P content of the fertilizer. See also phosphate.
clay loam - Soil material that contains 27 to 40% clay and 20 to 45% sand. clay - Soil material that contains 40% or more clay, <45% sand, and <40% silt. sandy clay loam - Soil material that contains 20 to 35% clay, <28% silt, and >45% sand. sandy clay - Soil material that contains 35% or more clay and 45% or more sand. silty clay loam - Soil material that contains 27 to 40% clay and <20% sand. silty clay - Soil material that contains 40% or more clay and 40% or more silt. attapulgite clay - See palygorskite. cat clay - Poorly drained, clayey soils, commonly formed in an estuarine environment, that become very acidic when drained due to oxidation of ferrous sulfide. clay - (i) A soil separate consisting of particles <0.002 mm in equivalent diameter. See also soil separates. (ii) A textural class. See also soil texture. (iii) (In reference to clay mineralogy) A naturally occurring material composed primarily of fine-grained minerals, which is generally plastic at appropriate water contents and will harden when dried or fired. Although clay usually contains phyllosilicates, it may contain other materials that impart plasticity and harden when dried or fired. Associated phases in clay may include materials that do not impart plasticity and organic matter. clay coating - Same as clay film. clay films - Coatings of oriented clay on the surfaces of peds and mineral grains and lining pores. Also called clay skins, clay flows, illuviation cutans, or argillans. clay flows - See clay films. clay loam - A soil textural class. See also soil texture. clay mineral - A phyllosilicate mineral or a mineral that imparts plasticity to clay and which harden upon drying or firing. See also phyllosilicate mineral terminology. clay mineralogy - See phyllosilicate mineral terminology. clay skins - See clay films. sandy clay - A soil textural class. See also soil texture. sandy clay loam - A soil textural class. See also soil texture. silty clay - A soil textural class. See also soil texture. silty clay loam - A soil textural class. See also soil texture.

colloid - A particle, which may be a molecular aggregate, with a diameter of 0.1 to 0.001 µm. Soil clays and soil organic matter are often called soil colloids because they have particle sizes that are within, or approach colloidal dimensions.

compaction - (i) To unite firmly; the act or process of becoming compact. (ii) (geology) The changing of loose sediment into hard, firm rock. (iii) (soil engineering) The process by which the soil grains are rearranged to decrease void space and bring them into closer contact with one another, thereby increasing the bulk density. (iv) (solid waste disposal) The reducing of the bulk of solid waste by rolling and tamping. soil compaction - Increasing the soil bulk density, and concomitantly decreasing the soil porosity, by the application of mechanical forces to the soil.

compost - Organic residues, or a mixture of organic residues and soil, that have been mixed, piled, and moistened, with or without addition of fertilizer and lime, and generally allowed to undergo thermophilic decomposition until the original organic materials have been substantially altered or decomposed. Sometimes called "artificial manure" or "synthetic manure." In Europe, the term may refer to a potting mix for container-grown plants.

composting - A controlled biological process which converts organic constituents, usually wastes, into humus-like material suitable for use as a soil amendment or organic fertilizer.

concentration - The amount of suspended or dissolved particles, or elements in a unit volume or unit mass as specified at a given temperature and pressure. critical nutrient concentration - The nutrient concentration in the plant, or specified plant part, above which additional plant growth response slows. Crop yield, quality or performance are less than optimum when the concentration is less. critical soil test concentration - That concentration at which 95% of maximum relative yield is achieved. Usually coincides with the inflection point of a curvilinear yield response curve. nutrient concentration vs. content - Concentration is usually expressed in grams per kilogram (g kg^-1) or milligrams per kilogram (mg kg^-1) of dry or fresh weight; content is usually expressed as weight per unit area (e.g., kg ha^-1). These terms should not be used interchangeably with regard to nutrients in plants. soil test critical concentration - The concentration of an extractable nutrient above which a crop response to added nutrient would not be expected. symmetry concentration - (no longer used in SSSA publications) That quantity of cations (or anions) equivalent to the exchange capacity of a soil. For example, if the cation exchange capacity of a soil is 10 cmol_c kg^-1of soil, then 1 symmetry concentration is 10 cmol of any monovalent cation or 5 cmol of any divalent cation.

cover crop - Close-growing crop, that provides soil protection, seeding protection, and soil improvement between periods of normal crop production, or between trees in orchards and vines in vineyards. When plowed under and incorporated into the soil, cover crops may be referred to as green manure crops.

critical soil test concentration - That concentration at which 95% of maximum relative yield is achieved. Usually coincides with the inflection point of a curvilinear yield response curve. soil test critical concentration - The concentration of an extractable nutrient above which a crop response to added nutrient would not be expected.

clean tillage (clean culture, clean cultivation) - A process of plowing and cultivation which incorporates all residues and prevents growth of all vegetation except the particular crop desired during the growing season. cross cultivation - The tillage of a field, orchard, etc., in which the field is cultivated in one direction followed by cultivation at some angle between 10 and 90° from the preceding tillage. cultivation (weeding) - Tillage action which lightly tills the surface 1-2 cm of soil for the purpose of destroying weeds. cultivation - Shallow tillage operations performed to create soil conditions conducive to improved aeration, infiltration, and water conservation, or to control weeds. cross cultivation - See tillage, cross cultivation. cultivation - See tillage, cultivation.

Darcy's law - (i) A law describing the rate of flow of water through saturated porous media. (Named for Henry Darcy of Paris, who formulated it in 1856 from extensive work on the flow of water through sand filter beds.) As formulated by Darcy, the law is

Q = KS(H + e)/e

where Q is the volume of water passed in unit time, S is the area of the bed, e is the thickness of the bed, H is the depth of water on top of the bed, and "K is a coefficient dependent on the nature of the sand," and for cases "when the pressure under the filter is equal to the weight of the atmosphere." (ii) Generalization for three dimensions: The rate of viscous flow of water in isotropic porous media is proportional to, and in the direction of, the hydraulic gradient. (iii) Generalization for other fluids: The rate of viscous flow of homogenous fluids through isotropic porous media is proportional to, and in the direction of, the driving force.

denitrification - Reduction of nitrogen oxides (usually nitrate and nitrite) to molecular nitrogen or nitrogen oxides with a lower oxidation state of nitrogen by bacterial activity (denitrification) or by chemical reactions involving nitrite (chemodenitrification). Nitrogen oxides are used by bacteria as terminal electron acceptors in place of oxygen in anaerobic or microaerophilic respiratory metabolism.

desorption - The migration of adsorbed entities off of the adsorption sites. The inverse of adsorption.
diatoms - Algae having siliceous cell walls that persist as a skeleton after death. Any of the microscopic unicellular or colonial algae constituting the class Bacillariaceae. They are abundant in fresh and salt waters and their remains are widely distributed in soils.

diffusion (nutrient) - The movement of nutrients in soil because of a chemical activity gradient.

dolomitic lime - A naturally occurring liming material composed chiefly of carbonates of Mg and Ca in approximately equimolar proportions.

drain tile - Concrete, ceramic, plastic, or other rigid pipe or similar buried structure used to collect and conduct profile drain-water from the soil in a field. tile drain - Concrete, ceramic, plastic etc. pipe, or related structure, placed at suitable depths and spacings in the soil or subsoil to enhance and/or accelerate drainage of water from the soil profile.

drip irrigation - Irrigation whereby water is slowly applied to the soil surface through small emitters having low-discharge orifices. subsurface drip irrigation - Application of water below the soil surface through emitters, with discharge rates generally in the same range as drip irrigation. This method of water application is different from and not to be confused with subirrigation where the root zone is irrigated by water table control.

EC - See electrical conductivity. EC_e - The electrical conductance of an extract from a soil saturated with distilled water, normally expressed in units of siemens or decisiemens per meter at 25° C. adenylate energy charge ratio (EC) - A measure of the metabolic and growth state of microorganisms and microbial communities. The energy charge ratio is calculated using the formula: EC = (ATP + ½ADP)/(ATP + ADP + AMP). The denominator represents the total adenylate pool; the numerator, the portion charged with high energy phosphate bonds. electrical conductivity (EC) - Conductivity of electricity through water or an extract of soil. Commonly used to estimate the soluble salt content in solution.

ectomycorrhiza(e) - A mycorrhizal association in which the fungal mycelia extend inward, between root cortical cells, to form a network ("Hartig net") and outward into the surrounding soil. Usually the fungal hyphae also form a mantle on the surface of the roots.

effective precipitation - That portion of the total rainfall precipitation which becomes available for plant growth.

compensating emitter - Designed to discharge water at a constant rate over a wide range of lateral line pressures. continuous flushing emitter - Designed to continuously permit passage of large solid particles while operating at a trickle or drip flow thus reducing filter fineness requirements. emitter - A small microirrigation dispensing device designed to dissipate pressure and discharge a small uniform flow or trickle of water at a constant discharge, which does not vary significantly because of minor differences in pressure head. Also called a "dripper" or "trickler". flushing emitter - Designed to have a flushing flow of water to clear the discharge opening every time the system is turned on. line-source emitter - Water is discharged from closely spaced perforations, emitters, or a porous wall along the tubing. long path emitter - Employs a long capillary-sized tube or channel to dissipate pressure. multi-outlet emitter - Supplies water to 2 or more points through small diameter auxiliary tubing. orifice emitter - Employs a series of orifices to dissipate pressure. vortex emitter - Employs a vortex effect to dissipate pressure.

endomycorrhiza - A mycorrhizal association with intracellular penetration of the host root cortical cells by the fungus as well as outward extension into the surrounding soil. See also arbuscule, vesicles, and vesicular arbuscular.
enzyme - Any of numerous proteins that are produced in the cells of living organisms and function as catalysts in the chemical processes of those organisms. restriction enzyme - A class of highly specific enzymes which make double stranded breaks in DNA at specific sites near where they combine.
evapotranspiration - The combined loss of water from a given area, and during a specified period of time, by evaporation from the soil surface and by transpiration from plants.

exchangeable anion - A negatively charged ion held on or near the surface of a solid particle by a positive surface charge and which may be easily replaced by other negatively charged ions ( e.g. with a Cl^- salt).

exchangeable bases - Charge sites on the surface of soil particles that can be readily replaces with a salt solution. In most soils, Ca²⁺, Mg²⁺, K⁺ and Na⁺ predominate. Historically, these are called bases because they are cations of strong bases. Many soil chemists object to this term because these cations are not bases by any modern definition of the term. See also base saturation and exchangeable cation.

exchangeable cation - A positively charged ion held on or near the surface of a solid particle by a negative surface and which may be replaced by other positively charged ions in the soil solution. Usually expressed in centimoles or millimoles of charge per kilogram. exchangeable cation percentage - (no longer preferred in SSSA publications) The extent to which the adsorption complex of a soil is occupied by a particular cation.

exchangeable nutrient - A plant nutrient that is held by the adsorption complex of the soil and is easily exchanged with the anion or cation of neutral salt solutions.

exchangeable sodium fraction - The fraction of the cation exchange capacity of a soil occupied by sodium ions.
exchangeable sodium percexchangeable sodium ratio (ESR) - The ratio of exchangeable sodium to all other exchangeable cations.
exoenzyme - Enzymes that are excreted by organisms into the surrounding environment and carry out their metabolic or catabolic activity in that location.

extractable soil nutrient - The quantity of a nutrient removed from the soil by a specific soil test procedure.

exudate, root - Low molecular weight metabolites that enter the soil from plant roots.

fermentation - The metabolic process in which an organic compound serves as both an electron donor and the final electron acceptor.
fertigation - Application of plant nutrients in irrigation water.

fertility, soil - The relative ability of a soil to supply the nutrients essential to plant growth. soil fertility - The quality of a soil that enables it to provide nutrients in adequate amounts and in proper balance for the growth of specified plants or crops.

fertilization, foliar - Application of a dilute solution of liquid fertilizers to plant foliage.

acid-forming - Fertilizer that, after application to and reaction with soil, increases residual acidity and decreases soil pH. blended - A mechanical mixture of different fertilizer materials. bulk-blended - A physical mixture of dry granular fertilizer materials to produce specific fertilizer ratios and grades. Individual granules in the bulk blended fertilizer do not have the same ratio and content of plant food as does the mixture as a whole. complete - A chemical compound or a blend of compounds that contains significant quantities of all three primary nutrients, N, P, and K. It may contain other plant nutrients. compound - A fertilizer formulated with two or more plant nutrients. controlled-release - A fertilizer term used interchangeably with delayed release, slow release, controlled availability, slow acting, and metered release to designate a controlled dissolution of fertilizer at a lower rate than conventional water-soluble fertilizers. Controlled-release properties may result from coatings on water-soluble fertilizers or from low dissolution and/or mineralization rates of fertilizer materials in soil. granular - Fertilizer particles sized between an upper and lower limit or between two screen sizes, usually within the range of 1 to 4 mm and often more closely sized. The desired size may be obtained by agglomerating smaller particles, crushing and screening larger particles, controlling size in crystallization processes, or prilling. injected - Placement of fertilizer into the soil either through use of pressure or nonpressure systems. inorganic - A fertilizer material in which carbon is not an essential component of its basic chemical structure. liquid - Fertilizer wholly or partially in solution that can be handled as a liquid, including clear liquids and liquids containing solids in suspension. mixed - Two or more fertilizer materials blended or granulated together into individual mixes. The term includes dry mix powders, granulated, clear liquid, suspension, and slurry mixtures. organic - A material containing carbon and one or more plant nutrients in addition to hydrogen and/or oxygen. pop-up - Fertilizer placed in small amounts in direct contact with the seed. salt index - The ratio of the decrease in osmotic potential of a solution containing a fertilizer compound or mixture to that produced by the same weight of NaNO₃ x 100. sidedressed - A fertilizer application usually banded to the side of crop rows after plant emergence. slow-release - See fertilizer, controlled-release. starter - A fertilizer applied in relatively small amounts with or near the seed usually during planting for the purpose of accelerating early growth of the crop plants. suspension - A fluid fertilizer containing dissolved and undissolved plant nutrients. The undissolved plant nutrients are kept in suspension with a suspending agent, usually a swelling type clay. The suspension must be flowable enough to be mixed, pumped, agitated, and applied to the soil in a homogeneous mixture. top-dressed - A non-incorporated surface application of fertilizer to a soil after the crop has been established.

basic fertilizer - One that, after application to and reaction with soil, decreases residual acidity and increases soil pH. bulk fertilizer - Solid or liquid fertilizer in a non-packaged form. fertilizer - Any organic or inorganic material of natural or synthetic origin (other than liming materials) that is added to a soil to supply one or more plant nutrients essential to the growth of plants.

· fertilizer analysis - The percent composition of a fertilizer as determined in a laboratory and expressed as total N, available phosphoric acid (P₂O₅) equivalent, and water-soluble potash (K₂O) equivalent.

· fertilizer fixation - See fixation.

· fertilizer grade - The guaranteed minimum analysis in percent of the major plant nutrient elements contained in a fertilizer material or in a mixed fertilizer. The analysis is usually designated as N-P₂O₅-K₂O; but it may be N-P-K where permitted or required as specified by state law. Grades must be expressed in percent N-P-K for SSSA publications (oxide values may be included in parentheses). See also fertilizer analysis.

· fertilizer ratio - The relative proportions of primary nutrients in a fertilizer grade divided by the highest common denominator for that grade, e.g., grades 10-6-4 and 20-12-8 have a ratio 5-3-2.

· fertilizer recommendation - See soil test interpretation.

· fertilizer requirement - The quantity of certain plant nutrients needed to increase nutrient availability in the soil with the objective of increasing plant growth to a designated level.

· organic fertilizer - By product from the processing of animals or vegetable substances that contain sufficient plant nutrients to be of value as fertilizers.

flocculation - The coagulation of colloidal soil particles due to the ions in solution. In most soils the clays and humic substances remain flocculated due to the presence of doubly and triply charged cations.

foliar diagnosis - An estimation of plant mineral nutrient status from the chemical composition of selected plant parts, and the color and growth characteristics of the plant foliage.
functional nutrient - Chemical elements that function in plant metabolism whether or not their action is specific.

fungistat - A compound that inhibits or prevents fungal growth.

gravity flow - Water flow which is not pumped but flows due to the acceleration forces of gravity. Used in irrigation, drainage, inlets, and outlets. gravity sprinkler - A sprinkler irrigation system in which gravity furnishes the desired head. gravity - Irrigation in which the water is not pumped but flows and is distributed by gravity.

green manure - Plant material incorporated into soil while green or at maturity, for soil improvement. green manure crop - Any crop grown for the purpose of being turned under while green or soon after maturity for soil improvement.

greenhouse effect - The absorption of solar radiant energy by the earth's surface and its release as heat into the atmosphere; longer infrared heat waves are absorbed by the air, principally by carbon dioxide and water vapor, thus, the atmosphere traps heat much as does the glass in a greenhouse.

guano - The decomposed dried excrement of birds and bats, used for fertilizer.
gypsum - CaSO₄ · 2H₂O. The common name for calcium sulfate, used to supply calcium to ameliorate soils with a high exchangeable sodium fraction. gypsum requirement - The quantity of gypsum or its equivalent required to reduce the exchangeable sodium fraction of a given amount of soil to an acceptable level where dispersion of soil colloids does not take place.

hardpan - A soil layer with physical characteristics that limit root penetration and restrict water movement.

harvest index - Thc quantity of harvestable biomass per unit total biomass produced. If used in relation to nutrients it would be the quantity of biomass produced per unit input of plant nutrient.
heavy metals - Those metals which have densities >5.0 Mg m^-3. In soils these include the elements Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, and Zn.

heavy soil - (colloquial) A soil with a high content of the fine separates, particularly clay, or one with a high drawbar pull and hence difficult to cultivate, especially when wet.

soil horizon - A layer of soil or soil material approximately parallel to the land surface and differing from adjacent genetically related layers in physical, chemical, and biological properties or characteristics such as color, structure, texture, consistency, kinds and number of organisms present, degree of acidity or alkalinity, etc

humic acid - The dark-colored organic material that can be extracted from soil with dilute alkali and other reagents and that is precipitated by acidification to pH 1 to 2.

humic substances - A series of relatively high-molecular-weight, yellow to black colored organic substances formed by secondary synthesis reactions in soils. The term is used in a generic sense to describe the colored material or its fractions obtained on the basis of solubility characteristics. These materials are distinctive to soil environments in that they are dissimilar to the biopolymers of microorganisms and higher plants (including lignin). See also humic acid, fulvic acid, and humin.

humification - The process whereby the carbon of organic residues is transformed and converted to humic substances through biochemical and abiotic processes.

humin - The fraction of the soil organic matter that cannot be extracted from soil with dilute alkali.
humus - Total of the organic compounds in soil exclusive of undecayed plant and animal tissues, their "partial decomposition" products, and the soil biomass. The term is often used synonymously with soil organic matter. humus form - A group of soil horizons located at or near the surface of a pedon, which have formed from organic residues, either separate from or intermixed with, mineral material.

hydrated lime - A liming material composed mainly of calcium and magnesium hydroxides that reacts quickly to neutralize acid soils.

hydroxy-aluminum interlayers - Polymers of general composition which are adsorbed on interlayer cation exchange sites. Although not exchangeable by unbuffered salt solutions, they are responsible for a considerable portion of the titratable acidity (and pH-dependent charge) in soils.

immobilization - The conversion of an element from the inorganic to the organic form in microbial or plant tissues.
impervious - Resistant to penetration by fluids or by roots.

incorporation - Mixing of materials found on or spread upon the soil surface (e.g. fertilizers, pesticides, or crop residues) into the soil volume via tillage.

cumulative infiltration - Total volume of water infiltrated per unit area of soil surface during a specified time period. Contrast with infiltration flux (or rate). infiltration - The entry of water into soil. infiltration capacity - See infiltration flux. infiltration flux (or rate) - The volume of water entering a specified cross-sectional area of soil per unit time [L t^-1]. infiltration, cumulative - See cumulative infiltration.

inner sphere adsorption - Adsorption of ions that occurs with the elimination of water of hydration in the space between the adsorbed ion and the surface. The force of retention of ions involves both ionic and covalent bonding. Strong adsorption of anions and cations at variable charge sites in organic matter, oxides, and phyllosilicate edges involves inner sphere adsorption.

ions - Atoms, groups of atoms, or compounds, which are electrically charged as a result of the loss of electrons (cations) or the gain of electrons (anions).

ion selectivity - (i) The relative adsorption of an ion by the solid phase in relation to the adsorption of other ions. (ii) The relative absorption of an ion by a root in relation to absorption of other ions.

ionic strength - A parameter that estimates the interaction between ions in solution. It is calculated as one-half the sum of the products of ionic concentration and the square of ionic charge for all the charged species in a solution. It is needed for calculation of single ion activity.

irrigation-induced erosion - Erosion caused by irrigation, in which water quality, decreasing downslope runoff and rapid hydration in the furrow runoff stream affect the expression of erosion processes. advance time - The time it takes the first water applied to a dry irrigation furrow to travel the length of the furrow. alternate set irrigation - A method of managing irrigation whereby, at every other irrigation, alternate furrows are irrigated, or sprinklers are placed midway between their locations during the previous irrigation. alternate side irrigation - The practice of furrow irrigating one side of a crop row (for row crops or orchards) and then, at about half the irrigation time, irrigating the other side. border dikes - Earth ridges built to guide or hold irrigation water within prescribed limits in a field; a small levee. border ditch irrigation - A ditch used as a border of an irrigated strip or plot, water being spread from one or both sides of the ditch along its entire length. border-strip irrigation - The water is applied at the upper end of a strip with earth borders to confine the water to the strip. center-pivot irrigation - Automated sprinkler irrigation achieved by automatically rotating the sprinkler pipe or boom, supplying water to the sprinkler heads or nozzles, as a radius from the center of the field to be irrigated. Water is delivered to the center or pivot point of the system. The pipe is supported above the crop by towers at fixed spacings and propelled by pneumatic, mechanical, hydraulic, or electric power on wheels or skids in fixed circular paths at uniform angular speeds. Water is applied at a uniform rate by progressive increase of nozzle size from the pivot to the end of the line. The depth of water applied is determined by the rate of travel of the system. Single units are ordinarily about 1250 to 1300 feet long (381 to 397 m) and irrigate approximately a 130-acre (52.7-ha) circular area. check irrigation - Modification of a border strip with small earth ridges or checks constructed at intervals to retain water as the water flows down the strip. check-basin irrigation - The water is applied rapidly to relatively level plots surrounded by levees. The basin is a small check. conjunctive water use - The joining together of two sources of irrigation water, such as groundwater and surface water, to serve a particular piece of land. consumptive irrigation requirement - The centimeters per hectare of irrigation water, exclusive of precipitation, stored soil moisture, or ground water, needed consumptively for crop production. continuous delivery - A system by which an irrigator receives his allotted quantity of water at a continuous rate throughout the irrigation season. contour ditch - Irrigation ditch laid out approximately on the contour. contour flooding - Method of irrigating by flooding from contour ditches. contour-furrow irrigation - Applying irrigation water in furrows that run across the slope with a forward grade in the furrows. contour-level irrigation - Irrigation of areas bounded by small contour levels; cross levels are completely flooded. controlled drainage - (irrigation) Regulation of the water table to maintain the water level at a depth favorable for optimum crop growth. conveyance loss - Loss of water from delivery systems during conveyance, including operational losses and losses due to seepage, evaporation, and transpiration by plants growing in or near the channel. corrugate irrigation - The water is applied to small, closely spaced furrows called corrugates, frequently in grain and forage crops, to confine the flow of irrigation water to one direction. cutback irrigation - Water applied in furrow irrigation at a faster rate at the beginning of the irrigation period and then reduced or cutback to a lesser rate, usually one-half the initial rate or that amount to balance with the intake rate. demand system of irrigation - System of irrigation water delivery where each irrigator may request irrigation water in the amount needed and at the time desired. discharge curve - (i) Rating curve showing the relation between stage and rate of flow of a stream. (ii) Curve showing the relation of discharge of a pump and the speed, power, and head. drainage curves - Design curves giving prescribed rates of surface runoff for different levels of crop production, and which may vary according to size of drainage area. drip irrigation - Irrigation whereby water is slowly applied to the soil surface through small emitters having low-discharge orifices. dynamic head - The total of the following factors: a) the total static head, b) friction head in the discharge pipeline, c) head losses in fittings, elbows, and valves, and d) pressure required to operate lateral lines. flood irrigation - Irrigation in which the water is released from field ditches and allowed to flood over the land. flume - (i) Open conduit for conveying water across obstructions. (ii) An entire canal elevated above natural ground. An aqueduct. (iii) A specially calibrated structure for measuring open channel flows. furrow irrigation - Irrigation in which the water is applied between crop rows in furrows made by tillage implements. gravity flow - Water flow which is not pumped but flows due to the acceleration forces of gravity. Used in irrigation, drainage, inlets, and outlets. gravity sprinkler - A sprinkler irrigation system in which gravity furnishes the desired head. gravity - Irrigation in which the water is not pumped but flows and is distributed by gravity. gross duty of water - The irrigation water diverted at the intake of a canal system, usually expressed in depth on the irrigable area under the system; diversion requirement. See also irrigation, net duty of water. gross irrigation water requirement - The net water requirement plus distribution and application losses in operating the system. irrigable area - Area capable of being irrigated, principally as regards to availability of water, suitable soils, and topography of land. irrigation application efficiency - Percentage of irrigation water applied to an area that is stored in the soil for crop use. irrigation canal - A permanent irrigation canal constructed to convey water from the source of supply to one or more farms. irrigation check - Small dike or dam used in the furrow alongside an irrigation border to make the water spread evenly across the border. irrigation efficiency - Variously defined, including: (i)The ratio of the water actually consumed by crops on an irrigated area to the amount of water applied to the area; (ii) the ratio of water infiltrated to total water applied; (iii) the ratio of water profile storage increase to total water applied. irrigation frequency - Time interval between irrigations. irrigation hose - A closed conduit for supplying water to moving irrigation systems, flexible when subjected to normal operating pressure and may be collapsible to a flat cross section when purged of water. irrigation lateral - A branch of a main canal conveying water to a farm ditch; sometimes used in reference to farm ditches. irrigation methods - The methods and/or manner in which water is intentionally applied to an area. irrigation period - The number of hours or days that it takes to apply one irrigation to a given design area during the peak consumptive-use period of the crop being irrigated. irrigation set - The area irrigated at one time within a field. irrigation tailwater recovery system - A water runoff collection and storage system to provide a constant quantity of water back to the initial system or to another field. Water is applied to the rows at the same rate for the entire irrigation period. Advance time should equal irrigation recession time as nearly as possible. Recession time is usually one-fourth of the entire irrigation period. lagtime - (flood irrigation) The period between the time that the irrigation stream is turned off at the upper end of an irrigated area and the time that water disappears from the surface at the point or points of application. lath box - Preferred term is spile. See irrigation, spile. length of run - Distance water must run in furrows or between borders over the surface of a field from one head ditch to another, or to the end of the field. limited irrigation - Management of irrigation applications to apply less than enough water to satisfy the soil water deficiency in the entire root zone. Sometimes called "deficit" or "stress irrigation". line source - Continuous source of water emitted along a line.

microirrigation - The frequent application of small quantities of water and drops, tiny, streams, or miniature spray through emitters or applicators placed along a water delivery line. Microirrigation encompasses a number of methods or concepts such as bubbler, drip, trickle, mist, or spray.

K₂O - Potassium oxide, designation on the fertilizer label that denotes the percentage of available potassium reported as K₂O. See also potash.

law of diminishing returns - When other factors in production do not change, successive increases in the input of one factor will not proportionately increase product yield.
law of the minimum - See Liebig's law

leaching - The removal of soluble materials from one zone in soil to another via water movement in the profile. See also eluviation. leaching fraction - The fraction of infiltrated irrigation water that percolates below the root zone. leaching requirement - The leaching fraction necessary to keep soil salinity, chloride, or sodium (the choice being that which is most demanding) from exceeding a tolerance level of the crop in question. It applies to steady-state or long-term average conditions.
Liebig's law - The growth and reproduction of an organism is dependent on the nutrient substance that is available in minimum quantity.

lime, agricultural - A soil amendment containing calcium carbonate, magnesium carbonate and other materials, used to neutralize soil acidity and furnish calcium and magnesium for plant growth. Classification including calcium carbonate equivalent and limits in lime particle size is usually prescribed by law or regulation.

lime concretion - An aggregate of precipitated calcium carbonate, or of other material cemented by precipitated calcium carbonate.

lime requirement - The amount of liming material as calcium carbonate equivalent required to change a volume of soil to a specified state with respect to pH or soluble Al content.
macronutrient - A plant nutrient found at relatively high concentrations ( >500 mg kg^-1) in plants. Usually refers to N, P, and K, but may include Ca, Mg, and S.

artificial manure - (no longer used in SSSA publications) In European usage denotes commercial fertilizers. green manure - Plant material incorporated into soil while green or at maturity, for soil improvement. green manure crop - Any crop grown for the purpose of being turned under while green or soon after maturity for soil improvement. manure - The excreta of animals, with or without an admixture of bedding or litter, fresh or at various stages of further decomposition or composting. In some countries may denote any fertilizer material.

mass flow (nutrient) - The movement of solutes associated with net movement of water.
mature soil - A soil with well-developed soil horizons produced by the natural processes of soil formation and essentially in equilibrium with its present environment.

microclimate - (i) The climatic condition of a small area resulting from the modification of the general climatic conditions by local differences in elevation or exposure or other local phenomena. (ii) The sequence of atmospheric changes within a very small region.

microbial biomass - (i) The total mass of living microorganisms in a given volume or mass of soil. (ii) The total weight of all microorganisms in a particular environment.
mineral - A naturally occurring homogeneous solid, inorganically formed, with a definite chemical composition and an ordered atomic arrangement. mineral soil - A soil consisting predominantly of, and having its properties determined predominantly by, mineral matter. Usually contains <200 g kg^-1organic carbon (< 120-180 g kg^-1 if saturated with water), but may contain an organic surface layer up to 30 cm thick.

mineralization - The conversion of an element from an organic form to an inorganic state as a result of microbial activity

minor elements - See micronutrients.

mixed fertilizers - Two or more fertilizer materials mixed or granulated together.

mulch farming - A system of tillage and planting operations which maintains a substantial amount of plant residues or other mulch on the soil surface. mulch tillage - Tillage or preparation of the soil in such a way that plant residues or other materials are left to cover the surface; also, mulch farming, trash farming, stubble mulch tillage, plowless farming; operationally, a full-width tillage or tillage and planting combination that leaves >30% of the surface covered with crop residue. mulch - (i) Any material such as straw, sawdust, leaves, plastic film, loose soil, etc., that is spread or formed upon the surface of the soil to protect the soil and/or plant roots from the effects of raindrops, soil crusting, freezing, evaporation, etc. (ii) To apply mulch to the soil surface. stubble mulch tillage - See tillage, mulch tillage; tillage, plowless farming. stubble mulch - The stubble of crops or crop residues left essentially in place on the land as a surface cover before and during the preparation of the seedbed and at least partly during the growing of a succeeding crop. dust mulch - A very loose, finely granular, or powdery condition on the soil surface.

myco - Prefix designating an association or relationship with a fungus (e.g., mycotoxins are toxins produced by a fungus).

mycorrhiza (pl. mycorrhizae) - Literally "fungus root". The association, usually symbiotic, of specific fungi with the roots of higher plants. See also endomycorrhiza and ectomycorrhiza.

neutral soil - A soil in which the surface layer, at least in the tillage zone, is in the pH 6.6 to 7.3 range. See also acid soil, alkaline soil, pH, and reaction, soil.

neutron probe - Probe, with radioactive source, that measures soil water content through reflection of scattered neutrons by hydrogen atoms in soil water.

nitrate reduction (biological) - The process whereby nitrate is reduced by plants and microorganisms to ammonium for cell synthesis (nitrate assimilation, assimilatory nitrate reduction) or to nitrite by bacteria using nitrate as the terminal electron acceptor in anaerobic respiration (respiratory nitrate reduction, dissimilatory nitrate reduction). Sometimes used synonymously with "denitrication."

autotrophic nitrification - Oxidation of ammonium to nitrate through the combined action of two chemoautotrophic bacteria, one forming nitrite from ammonium and the other oxidizing nitrite to nitrate. heterotrophic nitrification - Biochemical oxidation of ammonium and/or organic nitrogen to nitrate and nitrite by heterotrophic microorganisms. See also nitrification. nitrification - Biological oxidation of ammonium to nitrite and nitrate, or a biologically induced increase in the oxidation state of nitrogen.

nitrogen cycle - The sequence of biochemical changes undergone by nitrogen wherein it is used by a living organism, transformed upon the death and decomposition of the organism, and converted ultimately to its original oxidation state.

critical nutrient concentration - The nutrient concentration in the plant, or specified plant part, above which additional plant growth response slows. Crop yield, quality or performance are less than optimum when the concentration is less. crop nutrient requirement - The amount of nutrients needed to grow a specified yield of a crop plant per unit area. diffusion (nutrient) - The movement of nutrients in soil because of a chemical activity gradient. exchangeable nutrient - A plant nutrient that is held by the adsorption complex of the soil and is easily exchanged with the anion or cation of neutral salt solutions. extractable soil nutrient - The quantity of a nutrient removed from the soil by a specific soil test procedure. functional nutrient - Chemical elements that function in plant metabolism whether or not their action is specific. mass flow (nutrient) - The movement of solutes associated with net movement of water. nutrient - Elements or compounds essential as raw materials for organism growth and development. nutrient antagonism - The depressing effect caused by one or more plant nutrients on the uptake and availability of another nutrient in the plant. nutrient balance - An undefined theoretical ratio of two or more plant nutrient concentrations for an optimum growth rate and yield. Nitrogen and sulfur is an classic example that can be defined because both nutrients are metabolically related in the protein fraction. nutrient concentration vs. content - Concentration is usually expressed in grams per kilogram (g kg^-1) or milligrams per kilogram (mg kg^-1) of dry or fresh weight; content is usually expressed as weight per unit area (e.g., kg ha^-1). These terms should not be used interchangeably with regard to nutrients in plants. nutrient deficiency - A low concentration of an essential element that reduces plant growth and prevents completion of the normal plant life cycle. nutrient efficient plant - A plant that absorbs, translocates, or utilizes more of a specific nutrient than another plant under conditions of relatively low nutrient availability in the soil or growing media. nutrient interaction - A term usually used to describe the response from two or more nutrients applied together that deviates from additive individual responses when applied separately. This term may also be used to describe metabolic or ion-uptake phenomenon. nutrient stress - A condition occurring when the quantity of nutrient available reduces growth. It can be from either a deficient or toxic concentration. nutrient toxicity - Quality, state or degree of harmful effect from an essential nutrient in sufficient concentrations in the plant. nutrient-supplying power of soils - The capacity of the soil to supply nutrients to growing plants from the labile, exchangeable, and the moderately available forms. See also fertility, soil. plant nutrient - An element which is absorbed by plants and is necessary for completion of the normal life cycle. These include C, H, O, N, P, K, Ca, Mg, S, Cu, Fe, Zn, Mn, B, Cl, Ni, and Mo.

organic farming - Crop production system that reduces, avoids or largely excludes the use of synthetically compound fertilizers, pesticides, growth regulators, and livestock feed additives.

organic - A material containing carbon and one or more plant nutrients in addition to hydrogen and/or oxygen. organic fertilizer - By product from the processing of animals or vegetable substances that contain sufficient plant nutrients to be of value as fertilizers.

organic soil - A soil in which the sum of the thicknesses of layers containing organic soil materials is generally greater than the sum of the thicknesses of mineral layers. organic soil materials - Soil materials that are saturated with water and have 174 g kg^-1 or more organic carbon if the mineral fraction has 500 g kg^-1 or more clay, or 116 g kg^-1 organic carbon if the mineral fraction has no clay, or has proportional intermediate contents, or if never saturated with water, have 203 g kg^-1 or more organic carbon. soil organic matter - The organic fraction of the soil exclusive of undecayed plant and animal residues. See also humus. soil organic residue - Animal and vegetative materials added to the soil of recognizable origin.

outer sphere adsorption - Adsorption of ions that occurs with the retention of waters of hydration between the surface and the adsorbed ion where the force that retains the ion is only electrostatic attraction. Ions that are retained by outer sphere adsorption are readily exchangeable. See also exchangeable cation and exchangeable anion.

P₂O₅ - Phosphorus pentoxide; designation on the fertilizer label that denotes the percentage of available phosphorus reported as phosphorus pentoxide.

parts per million (ppm) - (no longer used in SSSA publications) (i) The concentration of solutions expressed in weight or mass units of solute (dissolved substance) per million weight or mass units of solution. (ii) A concentration in solids expressed in weight or mass units of a substance contained per million weight or mass units of solid, such as soil.

percent area wetted - Area wetted by irrigation as a percentage of the total crop area.

permanent wilting point - The largest water content of a soil at which indicator plants, growing in that soil, wilt and fail to recover when placed in a humid chamber. Often estimated by the water content at -1.5 MPa soil matric potential.

percolation, soil water - The downward movement of water through soil. Especially, the downward flow of water in saturated or nearly saturated soil at hydraulic gradients of the order of 1.0 or less.

permeability, soil - (i) The ease with which gases, liquids, or plant roots penetrate or pass through a bulk mass of soil or a layer of soil. Since different soil horizons vary in permeability, the particular horizon under question should be designated. (ii) The property of a porous medium itself that expresses the ease with which gases, liquids, or other substances can flow through it, and is the same as intrinsic permeability k. See also intrinsic permeability, Darcy's law, and soil water.

pH , soil - The pH of a solution in equilibrium with soil. It is determined by means of a glass, quinhydrone, or other suitable electrode or indicator at a specified soil-solution ratio in a specified solution, usually distilled water, 0.01 M CaCl₂, or 1 M KCl.

pHc - The calculated pH that a solution would have if it were in equilibrium with calcium carbonate. Numerically, is equal to (pK₂ - pK_c) + p(Ca) + pAlk, where p(Ca) and pAlk are the negative logarithms of the molar concentrations of Ca and of the equivalent concentration of (CO₃ + HCO₃), respectively, and pK₂ and pK_c are the negative logarithms of the second dissociation constant of H₂CO₃ and the solubility constant of CaCO₃, respectively, both corrected for ionic strength. It is used in conjunction with the measured pH of a water to determine if CaCO₃ will precipitate from the water, or if the water will dissolve CaCO₃ as it passes through a calcareous soil.

dependent charge - The portion of the cation or anion exchange capacity which varies with pH.

ammonium phosphate - A generic class of compounds used as phosphorus fertilizers. Manufactured by the reaction of anhydrous ammonia with orthophosphoric acid or superphosphoric acid to produce either solid or liquid products. phosphate - In fertilizer trade terminology, phosphate is used to express the sum of the water-soluble and the citrate-soluble phosphoric acid (P₂O₅); also referred to as the available phosphoric acid (P₂O₅). phosphate rock - A microcrystalline, calcium fluorophosphate of sedimentary or igneous origin of varying P content. It is usually concentrated and solubilized to be used directly or concentrated in manufacture of commercial phosphate fertilizers. water-soluble phosphate - That part of the phosphorus in a fertilizer that is soluble in water as determined by prescribed chemical tests.

phosphoric acid - In commercial fertilizer manufacturing, it is used to designate orthosphorphoric acid, H₃PO₄. In fertilizer labeling, it is the common term used to represent the phosphate concentration in terms of available P, expressed as percent P₂O₅.

phosphorus, fixation - (no longer used in SSSA publications) The immobilization of phosphorus by strong adsorption or precipitation.

phototropic - The response of a biological organism to the presence of light.

plant analysis - The determination of the nutrient concentration in plants or plant parts with analytical procedures.

plant food - The inorganic compounds elaborated within a plant to nourish its cells; a frequent synonym for plant nutrients, particularly in the fertilizer trade.

nutrient efficient plant - A plant that absorbs, translocates, or utilizes more of a specific nutrient than another plant under conditions of relatively low nutrient availability in the soil or growing media. plant nutrient - An element which is absorbed by plants and is necessary for completion of the normal life cycle. These include C, H, O, N, P, K, Ca, Mg, S, Cu, Fe, Zn, Mn, B, Cl, Ni, and Mo.

potassium fixation - The process of converting exchangeable or water-soluble potassium to that occupying the position of K+ in the micas. They are counter-ions entrapped in the ditrigonal voids in the plane of basal oxygen atoms of some phyllosilicates as a result of contraction of the interlayer space. The fixation may occur spontaneously with some minerals in aqueous suspensions or as a result of heating to remove interlayer water in others. Fixed K⁺ ions are exchangeable only after expansion of the interlayer space. See also ammonium fixation.

quantity intensity ratio - The change in quantity sorbed with change in quantity in solution. It is determined from the slope of the plot of concentration in solution vs. the quantity sorbed. See sorption.

redox - Reduction-oxidation. redox concentrations - Zones of apparent accumulation of Fe-Mn oxides in soils. redox depletions - Zones of low chroma (2 or less) where Fe-Mn oxides alone or both Fe-Mn oxides and clay have been stripped out of the soil. redox-potential - See E_H and pe.

reflectance - The ratio of the radiant energy reflected by a body to that incident upon it. The suffix (-ance) implies a property of that particular specimen surface.

remote sensing - Refers to the full range of activities that collects information from a distance, e.g., the utilization at a distance (as from aircraft, spacecraft, or ship) of any device for measuring electromagnetic radiation, force fields, or acoustic energy. The technique employs such devices as the camera, lasers, and radio frequency receivers, radar systems, sonar, seismographs, gravimeters, magnetometers, and scintillation counters.

restriction enzyme - A class of highly specific enzymes which make double stranded breaks in DNA at specific sites near where they combine.

rhizosphere - The zone of soil immediately adjacent to plant roots in which the kinds, numbers, or activities of microorganisms differ from that of the bulk soil.

runoff - That portion of precipitation or irrigation on an area which does not infiltrate, but instead is discharged from the area. That which is lost without entering the soil is called surface runoff. That which enters the soil before reaching a stream channel is called ground water runoff or seepage flow from ground water. (In soil science runoff usually refers to the water lost by surface flow; in geology and hydraulics runoff usually includes both surface and subsurface flow.) surface runoff - See runoff. underground runoff (seepage) - Water that seeps toward stream channels after infiltration into the ground

saline soil - A nonsodic soil containing sufficient soluble salt to adversely affect the growth of most crop plants. The lower limit of saturation extract electrical conductivity of such soils is conventionally set at 4 dS m^-1(at 25 ° C). Actually, sensitive plants are affected at half this salinity and highly tolerant ones at about twice this salinity. saline-alkali soil - (no longer used in SSSA publications) (i) A soil containing sufficient exchangeable sodium to interfere with the growth of most crop plants and containing appreciable quantities of soluble salts. The exchangeable-sodium percentage is >15, the conductivity of the saturation extract >4 dS m^-1(at 25 ° C), and the pH is usually 8.5 or less in the saturated soil. (ii) A saline-alkali soil has a combination of harmful qualities of salts and either a high alkalinity or high content of exchangeable sodium, or both, so distributed in the profile that the growth of most crop plants is reduced. See also saline-sodic soil. saline-sodic soil - (no longer used in SSSA publications) A soil containing sufficient exchangeable sodium to interfere with the growth of most crop plants and containing appreciable quantities of soluble salts. The exchangeable sodium ratio is greater than 0.15, conductivity of the soil solution, at saturated water content, of >4dS m^-1(at 25 ° C), and the pH is usually 8.5 or less in the saturated soil. See also saline-alkali soil.

salinity, soil - The amount of soluble salts in a soil. The conventional measure of soil salinity is the electrical conductivity of a saturation extract.

salinization - The process whereby soluble salts accumulate in the soil.
salt tolerance - The ability of plants to resist the adverse, nonspecific effects of excessive soluble salts in the rooting medium.

saturate - (i) To fill all the voids between soil particles with a liquid. (ii) To form the most concentrated solution possible under a given set of physical conditions in the presence of an excess of the solute. (iii) To fill to capacity, as the adsorption complex with a cation species; e.g., H⁺-saturated, etc.

saturated soil paste - A particular mixture of soil and water. At saturation, the soil paste glistens as it reflects light, flows slightly when the container is tipped, and the paste slides freely and cleanly from a spatula.

saturation content - The mass water content of a saturated soil paste.
saturation extract - The solution extracted from a soil at its saturation water content.

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. See also primary mineral.

secondary nutrients - Refers to Ca, Mg, and S in fertilizers.

sensor - Any device which gathers electromagnetic radiation (EMR) or other energy and presents it in a form suitable for obtaining information about the environment. Passive sensors, such as thermal infrared and microwave, utilize EMR produced by the surface or object being sensed. Active sensors, such as radar, supply their own energy source. Aerial cameras use natural or artificially produced EMR external to the object or surface being sensed.

silt - Soil material that contains 80% or more silt and <12% clay.

slow-release - See fertilizer, controlled-release. slow release - A fertilizer term used interchangeably with delayed release, controlled release, controlled availability, slow acting, and metered release to designate a rate of dissolution (usually in water) much less than is obtained for completely water-soluble compounds. Slow release may involve either compounds that dissolve slowly or soluble compounds coated with substances relatively impermeable to water.

sodium adsorption ratio (SAR) - A relation between soluble sodium and soluble divalent cations which can be used to predict the exchangeable sodium fraction of soil equilibrated with a given solution.

soil amendment - Any material such as lime, gypsum, sawdust, compost, animal manures, crop residue or synthetic soil conditioners that is worked into the soil or applied on the surface to enhance plant growth. Amendments may contain important fertilizer elements but the term commonly refers to added materials other than those used primarily as fertilizers. See also soil conditioner.

soil auger - A tool for boring into the soil and withdrawing a small sample for field or laboratory observation. Soil augers may be classified into several types as follows: (i) those with worm-type bits, uninclosed; (ii) those with worm-type bits inclosed in a hollow cylinder; and (iii) those with a hollow cylinder with a cutting edge at the lower end.

soil compaction - Increasing the soil bulk density, and concomitantly decreasing the soil porosity, by the application of mechanical forces to the soil.

subsoiling - Any treatment to non-inversively loosen soil below the Ap horizon with a minimum of vertical mixing of the soil. Any treatment to fracture and/or shatter soil with narrow tools below the depth of normal tillage without inversion and with a minimum mixing of the soil. This loosening is usually performed by lifting action or other displacement of soil dry enough so that shattering occurs.

substrate - (i) That which is laid or spread under an underlying layer, such as the subsoil. (ii) The substance, base, or nutrient on which an organism grows. (iii) Compounds or substances that are acted upon by enzymes or catalysts and changed to other compounds in the chemical reaction.

sulfur cycle - The sequence of transformations undergone by sulfur wherein it is used by living organisms, transformed upon death and decomposition of the organism, and ultimately converted to its original oxidation state.

ammoniated - A product obtained when superphosphate is treated with NH₃ or with solutions containing NH₃ and/or other NH₄-N containing compounds. concentrated - Also called triple or treble superphosphate, made with phosphoric acid and usually containing 19 to 21% P (44 to 48% P₂O₅). enriched - Superphosphate made with a mixture of sulfuric acid and phosphoric acid. This includes any grade between 10 and 19% P (22% and 44% P₂O₅), commonly 11 to 13% P (25 to 30% P₂O₅). normal - Also called ordinary or single superphosphate. Superphosphate made by reaction of phosphate rock with sulfuric acid, usually containing 7 to 10% P (16 to 22% P₂O₅). ordinary - See superphosphate, normal. single - See superphosphate, normal. superphosphate - A product obtained when phosphate rock is treated with H₂SO₄, H₃PO₄, or a mixture of those acids.

surfactant - A substance that lowers the surface tension of a liquid.

suspension - The containment or support in fluid media (usually air or water) of soil particles or aggregates, allowing their transport in the fluid when it is flowing. In fluids at rest, suspension follows Stoke's Law. In wind this usually refers to particles or aggregates <0.1 mm diameter through the air, usually at a height of >15 cm above the soil surface, for relatively long distances. suspension - A fluid fertilizer containing dissolved and undissolved plant nutrients. The undissolved plant nutrients are kept in suspension with a suspending agent, usually a swelling type clay. The suspension must be flowable enough to be mixed, pumped, agitated, and applied to the soil in a homogeneous mixture. colloidal suspension - Suspension in water of particles so finely divided that they will not settle under the action of gravity, but will diffuse, even in quiet water, under the random impulses of Brownian motion. Particle sizes range from about 1 mm to about 1nm; however, there is no sharp differentiation by size between coarse ("true") suspension and colloidal suspension or between colloidal suspension and solution.

synergism - (i) The nonobligatory association between organisms that is mutually beneficial. Both populations can survive in their natural environment on their own although, when formed, the association offers mutual advantages. (ii) The simultaneous actions of two or more factors that have a greater total effect together than the sum of their individual effects.

tensile strength - The load per unit area at which an unconfined cylindrical specimen will fail in a simple tension test.

tensiometer - A device for measuring the soil-water matric potential in situ; a porous, permeable ceramic cup connected through a water-filled tube to a manometer, vacuum gauge, pressure transducer, or other pressure measuring device.

topsoil - (i) The layer of soil moved in cultivation. Frequently designated as the Ap layer or Ap horizon. See also surface soil. (ii) Presumably fertile soil material used to topdress roadbanks, gardens, and lawns.

volumetric water content - The soil-water content expressed as the volume of water per unit bulk volume of soil.

water table - The upper surface of ground water or that level in the ground where the water is at atmospheric pressure.

wetting front - The boundary between the wetted region and the dry region of soil during infiltration.
windbreak - A planting of trees, shrubs, or other vegetation, usually perpendicular or nearly so to the principal wind direction, to protect soil, crops, homesteads, roads, etc., against the effects of winds, such as wind erosion and the drifting of soil and snow.

relative yield - The harvestable or biomass yield with or without supplementation of the nutrient in question expressed as a percentage of the yield with the nutrient in adequate amounts. yield - The amount of a specified substance produced (e.g., grain, straw, total dry matter) per unit area. yield curve - A graphical representation of nutrient application rate or availability versus crop yield or nutrient uptake. yield goal - The yield that a producer expects to achieve, based on overall management imposed and past production records. yield, sustained - A continual, annual, or periodic yield of plants or plant material from an area; implies management practices which will maintain the productive capacity of the land, be economically feasible, and maintain environmental integrity of the ecosystem.