Below is a list of definitions that pertain to plant nutrition and its related studies.

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Absorption - The process by which a substance is taken into and included within another substance, e.g., intake of water by soil, or intake of gases, water, nutrients or other substances by plants.  Practically all of the fertilizer nutrients and water requirements of plants are absorbed through their roots.  Much of this absorption is through the root hairs that are located near the growing tips of the young roots.  It is believed that simple diffusion and active absorption (energy from living cells) are the processes utilized by plants in their absorption of nutrients.

Acid fertilizer - A fertilizer that causes a decrease in pH (i.e., increased acidity or decreased basicity) when applied to the soil/growing media.  Acidity is primarily due to the application of ammonium, ammonia, and/or urea in the fertilizer.

Acidification - The process of making a substance or solution more acid, i.e., lower in pH.  In irrigation, it refers to adding acid (sulfuric, phosphoric, or nitric acid) to the irrigation water to create a slightly acid 5.8 to 6.2 pH or reduction to a certain level of alkalinity.

Acid Soil - A soil with a pH value below 7.0.
A soil containing more hydrogen ions (H⁺) verses hydroxyl ions (OH-) in the soil solution (active acidity) and held to the surface of soil particles (reserve or potential acidity). Soil pH values are below 7, depending on the concentration of hydrogen ions in the soil solution.
Soils become more acid as a result of leaching of calcium (Ca⁺²), magnesium (Mg⁺²), and potassium (K^-) cations from the topsoil into the subsoil, and through the removal of cations by growing crops, and by nitrification of ammonium (NH₄⁺ ) nitrogen.  As cations are removed from the soil particles, they are replaced with hydrogen and acid-forming aluminum ions.  Soil acidity affects availability of essential plant nutrients.

Acidity and basicity of fertilizers - Fertilizers have neutral, acidic (lower soil pH), or basic (increased soil pH) effects when added to the soil.  This effect is commonly expressed in terms of the amount of pure calcium carbonate that would be required to either offset the acid-forming reactions of 100 pounds of fertilizer materials or the amount of calcium carbonate required to equal the acid-neutralizing effects of 100 pounds of fertilizer.

Actinomycetes - A category of soil bacteria that includes thread-like microorganisms forming  elongated cells with a tendency towards branching. Actinomycetes are heterotrophic bacteria utilizing fixed carbon sources and their presence is therefore conditioned by the availability of organic substrates (organic matter).  Their activities aid the decomposition of certain disease resistant components of plant and animal tissue, formation of humus, and organic transformations at high temperatures, particularly in the rotting of green manures, hay, compost piles and animal manures.

Activated alumina - An oxide of aluminum used in water treatment to remove impurities, such as fluoride.

Activated carbon - A form of carbon used in water treatment to remove impurities, such as fluoride.

Activated Sewage Sludge - An organic fertilizer made from sewage, free from grit and coarse solids, and aerated after being inoculated with micro-organisms.  The resulting flocculated organic matter is withdrawn from tanks, filtered with or without the aid of coagulants, dried, ground and screened.

Active Acidity - The hydrogen ion concentration of the soil solution is designated as active acidity.
Active acidity is measured by soil pH but does not indicate the actual magnitude of lime that is required to correct soil acidity.  Lime requirements are based on the amount of reserve or potential acidity held on the soil's exchange complex. See Also: Acid Soil; Reserve Acidity .

Adjusted sodium adsorption ratio (Adj. Rna) - A mathematical term used to estimate the sodium hazard in irrigation water.  It is an equation that takes into account the sodium content of water in relation to the salinity, calcium, magnesium, and bicarbonate content. Similar measures of sodium hazard that are reported are: sodium absorption ratio (SAR), adjusted sodium absorption ratio (SARadj), exchangeable sodium percentage (ESP), sodium percentage, and residual sodium carbonate.

Adsorption - The increased concentration of molecules or ions at a surface, including exchangeable cations and anions on soil particles.

Aeration Soil - The exchange of air in soil with air from the atmosphere.  The composition of the air in a well-aerated soil is similar to that in the atmosphere; in a poorly aerated soil, the air in the soil is considerably higher in carbon dioxide and lower in oxygen than the atmosphere above the soil.

Aerial Shoots - An aerial stem may grow upright, climbing or in a prostrate position.

Aggregate - A group of soil particles cohering so as to behave mechanically as a unit.

Air porosity or air space - The percent volume (volume/volume) of soil/growing media that is filled with air when irrigating media to container capacity.  The air in media is primarily in the large macropores.

Alkaline - A basic reaction in which the pH reading is above 7.0, as distinguished from acidic reaction, in which the pH reading is below 7.0.

Alkaline - Indicates a pH above 7.0.  Alkaline is also called basic.

Alkaline Soil - A soil having a high degree of alkalinity (pH 8.5 or higher) or high in exchangeable sodium (15% or higher) - or both, so that growth of most crops is reduced. A soil having a pH greater than 7.0: practically, one having a pH above 7.3.

Alkaline Soil/Media - A soil/media that has an alkaline reaction, i.e., a soil for which the pH reading of the saturated soil paste is above 7.0.

Alkaline Solution - Aqueous solution of base.

Alkalinity - A measure of water's capacity to neutralize acids.  True alkalinity is the sum of the dissolved bicarbonates, carbonates, hydroxides, ammonia, borates, organic bases, phosphates, and silicates in the water.  Most of the time in practice, alkalinity is the sum of the bicarbonates and carbonates (total carbonates) expressed as me/1 or ppm.

Alakali Soil/Media - See Sodic Soil/Media.

Alum - Either aluminum sulfate [Al₂(SO₄)₃ ·18H₂O] or a double salt of potassium aluminum sulfate [KAl(SO₄)₂·(12H₂ O)] used to treat irrigation water and recycled water to flocculate and remove colloidal clay.  Aluminum sulfate is more commonly used because of its lower costs and its availability in both liquid and dry forms.

Amendment - Any material, such as lime, gypsum, sawdust or synthetic conditioners, that is worked into the soil to make it more productive.  Strictly, a fertilizer is also an amendment, but the term amendment is used more commonly for added materials other than fertilizer.

Amino Acids - Nitrogen-containing organic compounds, large numbers of which link together in the formation of the protein molecule.  Each amino acid molecule contains one or more amino (-NH₂ ) groups and at least one carboxyl (-COOH) group.  In addition, some amino acids (cystine and methionine) contain sulfur.

Ammonia - A form of inorganic nitrogen (NH₃ ).  It can be toxic at concentrations as low as 2.5 p.m. (0.15 mM).  Toxicity increases as pH increases above pH 7.3.

Ammoniacal - Consisting of, containing, or producing ammonia.  When referring to a fertilizer, it indicates a fertilizer that contains ammonia or ammonium (such as ammonium upon breakdown such as urea).

Ammoniated Superphosphate - A product formed by ammoniating superphosphate.

Ammoniation - A process wherein ammonia (anhydrous, aqua or a solution containing ammonia and other forms of nitrogen) is used to treat superphosphate to form ammoniated superphosphate, or to treat a mixture of fertilizer ingredients (including phosphoric acid) in the manufacture of a multinutrient fertilizer.

Ammonification - Formation of ammonium compounds or ammonia. The conversion of organic nitrogen to ammonium-nitrogen by microorganisms in the soil.

Ammonium - An inorganic nitrogen fertilizer (NH₄⁺ ).  Ammonium will always contain a small amount of ammonia in equilibrium.

Ammonium Citrate [(NH₄)₃C₆H₅O₇] - A salt formed ammonia and citric acid.  A neutral ammonium citrate solution, prepared by the official methods of the AOAC, is used as a reagent in the determination of "available" phosphoric acid in fertilizers.  After a sample is washed with water to remove the water-soluble phosphoric acid (P₂O₅), the residue is treated with the neutral ammonium citrate solutions, as prescribed by the official methods, and the phosphoric acid removed by this extraction is termed "citrate-soluble".  The sum of the water-soluble plus the citrate-soluble phosphoric acid is termed "available."

Analysis - Investigation of chemical compound and determination of its composition. The percentage composition as found by chemical analysis, expressed in those terms that the law requires and permits.  Although "analysis" and "grade" sometimes are used synonymously, the term "grade" is applied only to the three primary plant foods - nitrogen (N), available phosphate (P₂O₅) and potash (K₂O) - and is stated as the guaranteed minimum quantities present.  (See also Grade ).

Anchorage - Another function of roots is to hold the stem part of the plant upright.  This anchorage role may be carried out by the regular roots or by special brace roots.

Animal Manures - The excreta of animals - dung and urine with the straw or other materials that may have been used in the confinement areas. The plant nutrients in animal waste solids must usually be mineralized before those nutrients are available to plants.  Nutrients in urine, on the other hand, are immediately available.  Most of the urinary nitrogen is present as urea.  Potassium in both solids and urine is readily available. It is difficult, if not impossible, to control the amount or the timing of N release from animal manures.  This difficulty with manures actually increases the environmental risk, compared to that of commercial fertilizer, when they are used as the primary source of plant nutrients.

Anion - Negatively (electrically) charged particle, e.g., NO₃^- (nitrate ion), HPO₄^2- (hydrogen phosphate ion). A negatively charged ion.

Anion exchange resin - A positively charged ion exchange resin that removes anions (negatively charged ions) from water.

Atom - Smallest particle of chemical element.

Annual - Horticulturally, a plant that completes its entire life cycle in a single growing season.

Antagonism - When one nutrient in high concentration suppresses the activity or another nutrient and causes an induced nutrient deficiency.

Apatite - (rock phosphate) - A mineral phosphate having the type formula Ca₁₀(X₂) (PO₄)₆ where X is usually fluorine, chlorine or the hydroxyl group, either singly or together.  Fluorapatite is widely distributed as the crystalline mineral and as amorphous phosphate rock, both forms of which are important fertilizer materials.  Crystalline fluorapatite contains from 38.0 to 41.0 percent phosphoric acid (P₂O₅) and from 3.2 to 4.3 percent fluorine.  Calcium hydroxyapatite or calcium hydroxy-phosphate, Ca₁₀(OH)₂(PO₄)₆ , may be formed to a small extent in ammoniated superphosphate.

Arboriculture - Cultivation of woody plants, particularly those used for decoration and shade.

Artificial Media - A mixture of various organic and inorganic constituents, such as perlite, vermiculite and peat moss, but not including soil, which is used for growing plants in containers or beds.

Assimilation - The process of assimilation is the absorption and incorporation of nutrients into the cell protoplasm.  This process can take place in any cell which has growth or manufacturing function.  It is especially active in the leaf and root cells.

Atomic weight - Weight of atom, referred to weight of hydrogen atom.

Autotrophic Bacteria - A category of soil bacteria that obtain their energy from the oxidation of mineral constituents, such as ammonium, sulfur, and iron and obtain most of their carbon from carbon dioxide. The numbers of these bacteria are much smaller than those classified as heterotrophic but their involvement in nitrification and sulfur oxidation reactions makes them tremendously important in higher plant nutrition.

Available - In general, a form capable of being assimilated by a growing plant.  Available nitrogen is defined as the nitrogen that is water-soluble plus what can be made soluble or converted into free ammonia.  Available phosphoric acid is that portion which is water-soluble plus the part which is soluble in ammonium citrate. Available potash is defined as that portion soluble in water or in a solution of ammonium oxalate.

As applied to nitrogen, phosphorus, and potassium, this term has somewhat different meanings.  In each case all that is soluble in water is available.  In addition, however, some of each that is not soluble in water is available to plants.  In general, a form of nutrient capable of being assimilated by a growing plant. Available nitrogen is defined as nitrogen that is water-soluble plus that which is readily solubilized or converted to free ammonia.  Available phosphorus (expressed as P₂O₅) is that portion which is water-soluble plus that which is soluble in ammonium citrate.  Available potassium (expressed as K₂ O) is defined as that portion soluble in water or a solution of ammonium oxalate.

Available Nutrient in Soil/Media - The part of the supply of a plant nutrient in the soil that can be taken up by plants at rates and in amounts significant to plant growth.

Available water capacity (AWC) - The percent volume (volume/volume) of soil/growing media occupied by water that is available to the plant.  It is calculated by determining the container capacity, then subtracting the unavailable water (PWP).

Available Water in Soil/Media - The part of the water in the soil that can be taken up by plants at rates significant to their growth; usable; obtainable.

Bagasse - An organic growing media component made from sugar cane fiber.

Banded Fertilizer - Placement of fertilizer in a concentrated zone either on or below the soil surface.
Concentrated zones or bands of fertilizer tend to minimize fixation of added nutrients by the soil or crop residues and make the fertilizer more available to plant roots.

Banding - Method of fertilizer application.  Banding is a general term that implies applications which concentrate fertilizers into narrow zones that are kept intact to provide a concentrated source of nutrients.  Applications may be made prior to, during, or after planting. See Also: Deep Banding Fertilization; Dribble Fertilization; Starter Fertilizer .

Bark - An organic growing media component made from the bark of hardwood or softwood trees.  It is usually hammer milled, screened to size, and composted or aged prior to use.

Base - Compound that forms hydroxide ions (OH^-) in aqueous solutions, in a wider sense also metals, e.g., K, Ca (see alkaline solution ).

Base Exchange - The replacement of cations, held on the soil complex, by other cations.  (See also Cation Exchange Capacity )

Basic - Indicates a pH above 7.0.  Basic is also called alkaline.

Basic fertilizers - A fertilizer that causes an increase in pH (i.e., decreased acidity or increased basicity) when applied to soil/growing media.  A fertilizer is basic primarily due to its nitrate content.

Basic Slag - A by-product in the manufacture of steel , containing lime, phosphate and small amounts of other plant food elements such as sulfur, manganese and iron.  Basic slags may contain from 10 to 17 percent phosphate (P₂O₅ ), 35 to 50 percent calcium oxide (CaO) and 2 to 10 percent magnesium oxide (MgO).  The available phosphate content of most American slag is in the range of 8 to 10 percent.

Basic Soil/Media - See Alkaline Soil/Media .

Best management practices (BMP) - The practice of minimizing production inputs that contribute to environmental contamination. :  Best management practices (BMPs) are those practices which have been proven in research and tested through farmer implementation to give optimum production potential, input efficiency, and environmental protection.

Bicarbonate - Partially dissociated carbonic acid in the form HCO₃^- .  A water pH between 7.4 and 9.3 will have bicarbonate as the main form of carbonic acid present.

Bonemeal - Raw bone meal is cooked bones ground to a meal without any of the gelatin or glue removed.  Steamed bone meal has been steamed under pressure to dissolve out part of the gelatin.

Boom system - An overhead irrigation system where the nozzles are mounted on a boom that moves above the plants.

Boron (B) - Boron (B) is an essential non-metallic micronutrient and exists in the soil in a number of primary and secondary nutrients.  It is absorbed by plants as boric acid (H₂BO₃ ) or one of the borate anions.Boron is essential for germination of pollen grains and growth of pollen tubes and is essential for seed and cell wall formation.  Boron forms sugar/borate complexes associated with sugar translocation and effects protein formation.  Boron deficiency generally results in stunted plant growth - the growing point and the younger leaves first because of lack of mobility in the plant.  In many crops the symptoms of boron deficiency are well defined and quite specific such as crooked and cracked stem in celery, corky core in apples, black heart in beets, hollow heart in peanuts, and ringed or banded leaf petioles in cotton.  Alfalfa, especially susceptible to boron deficiency, shows a rosetting (yellow top), then death of the terminal bud.  Coarse-textured sandy soils low in organic matter are typically low in minerals that contain boron and boron availability.  Organic matter is an important source of soil boron.  High soil pH also limits boron availability.  Deficiencies of this element can be determined by soil and plant analysis.

Brand - The trade name assigned by a manufacturer to a particular fertilizer product.

Brimstone - Sulfur.

Brine water - A water very high in solutes.  It may also refer to the byproduct or waste water of water purification systems.

Broadcast Application - Application of either solid or fluid fertilizer to the soil surface with or without subsequent incorporation by tillage.  No specific location relative to the plant is implied.  Nutrients may be applied prior to or after the crop is planted. See Also: Weed-and-Feed; Top-Dressed Application .

Buds (Herbaceous) - The growth of herbaceous stems is largely from buds which are a mass of meristematic tissues that develop into branches from nodes or joints on the main stem.  They are mostly naked buds and not covered with scales for protection.  Herbaceous buds may develop into stems, leaves or flowers.

Buds (Woody) - The elongation growth of woody stemmed plants is from buds.  Also they develop in the axils of leaves, junction of leaves and twigs, ends of branches and at nodes.  However, in woody plants the secondary growth or cambium tissues cover up the nodes of the young elongating stems.  Woody stemmed buds are usually covered with scales for protection.

Buffer - A system of substances, usually a mixture of weak acids and their salts, which tends to resist changes in pH.  In soils, organic matter, clays, and free calcium carbonate tend to buffer the system against pH changes.

Buffer Capacity of Soil - The ability of a soil to resist a change in soil solution hydrogen ion concentration (soil pH), resisting the tendency to become more acid upon the addition of an acid or an acid-forming material or more alkaline upon addition of basic materials such as lime. The ability of the soil to resist a change in its pH (hydrogen ion concentration) when acid-forming or base-forming materials are added to the soil.

Builder's lime - See hydrated lime .

Bulbs - A few plants, such as the tulip, have bulbs very similar to the onion except all of the bulb portion of the stem is below the ground.

Bulk Blending - The practice of mixing dry, individual, granular materials or granulated bases.  The product is a mixture of granular materials rather than a granulated mixture.

Bulk density - The ratio of the mass (weight) of dry soil/growing media to its bulk volume, expressed as grams per cubic centimeter (g/cc) or pounds per cubic foot (lb/cu ft).

Bulk Density - The ratio of the mass of water-free soil to its bulk volume.  Bulk density is expressed in pounds per cubic foot or grams per cubic centimeter and is sometimes referred to as apparent density.  When expressed in grams per cubic centimeter, bulk density is numerically equal to apparent specific gravity or volume weight.

Calcareous Soil - A soil containing calcium carbonate, a soil alkaline in reaction because of the presence of free calcium carbonate.
Properties:  Treatment with dilute hydrochloric acid causes such soils to effervesce (fizz), giving off carbon dioxide gas. A soil containing calcium carbonate, or a soil alkaline in reaction because of the presence of calcium carbonate; a soil containing enough calcium carbonate to effervesce (fizz) when treated with dilute hydrochloric acid.

Calcareous soil - A field soil containing large amounts of free calcium carbonate (i.e., lime), usually with a high pH of 7.6 to 8.3.  Common in the southwestern states and areas of low rainfall.

Calcined clay - An inorganic growing media component made from clay that is fired (calcined) to harden it, then crushed and screened to size.

Calcitic lime - A lime composed of calcium carbonate (CaCO₃ ) derived from calcite.  Pure calcitic lime contains 40% Ca.  It is also called calcitic limestone.

Calcium (Ca) - Calcium (Ca) is an essential secondary nutrient grouped with sulfur (S) and magnesium (Mg).  It exists in the soil and is absorbed as the Ca⁺² ion. Calcium stimulates root and leaf development and forms compounds which are part of the cell walls.  Physiological roles include the activation of several enzyme systems, reduction of nitrate, and neutralization of organic acids.  Calcium deficiency symptoms are not often seen in the field because secondary effects, associated with high soil acidity, limit growth first.  Leaves may be cupped-shaped and crinkled, and the terminal buds deteriorate with some breakdown of petioles.  Fruits may break down at the blossom end.  Calcium deficiency is associated with "blossom-end rot" in tomatoes and other crops.  Deficiencies can be determined by soil and plant analysis.

Calcium carbonate (CaCO₃ ) equivalent - On fertilizer labels it is used as the basis for expressing potential acidity (how much calcium carbonate could 1 ton of the

Calcium Carbonate Equivalent - The amount of calcium carbonate required to neutralize the acidity produced by a given quantity of fertilizer product.

Cambium Section of the Root - As the root increases in diameter, meristematic tissue develops between existing xylem and phloem tissues.  These meristematic tissues are responsible for and are called cambium.

Capillary action - Absorption and movement of water through capillary pores due to the forces of capillary attraction.  Water from subirrigation systems and some drip systems wet growing media by capillary action.

Capillary mat irrigation system - A subirrigation system where container plants are placed on a moistened mat or absorbent material from which water moves into the container by capillary action.

Carbohydrate - A compound containing carbon, hydrogen and oxygen.  Usually the hydrogen and oxygen occur in the proportion of 2 to 1, such as in glucose (C₆H₁₂O₆ ).

Carbonate - Completely dissociated carbonic acid in the form CO₃⁼ .  At pH 10.3 or above, carbonate is the predominate form present.

Carbonic acid - Dissolved carbon dioxide in the hydrated form H₂CO₃ .  At pH of 6.4 or below, carbonic acid is the predominate form present.

Carbon Cycle - The cycling of carbon in the environment including the fixation of atmospheric carbon dioxide (CO₂ ) by plants in the presence of sunlight and water to form sugar (photosynthesis), conversion of sugars to plant structural components, consumption of plants by man and animals, and the return of carbon dioxide to the atmosphere by decay of plant and animal residues.

Carbon:Nitrogen Ratio - The ratio obtained by dividing the percentage of organic carbon by percentage of nitrogen.

Cation - A positively charged ion. An ion carrying a positive charge of electricity.  Common soil cations are calcium, magnesium, sodium, potassium and hydrogen. Positively (electrically) charged particle, e.g., K⁺ (potassium ion), Mg²⁺  (magnesium ion), NH₄⁺ (ammonium ion). A charged form of an atom or molecule carrying one or more position charges of electricity (valence). The most common soil cations are calcium (Ca⁺²), magnesium (Mg⁺²), sodium (Na⁺), potassium (K⁺), ammonium (NH₄⁺), and hydrogen (H⁺) and aluminum (Al⁺³).

Cation Exchange - The exchange of cations held (adsorbed) at the surface of soil clay and organic matter particles for other cations in the soil solution.  Cation exchange is an important reaction in soil fertility, in correcting soil acidity and alkalinity, in changes altering soil physical properties and as a mechanism in purifying or altering percolating waters.  For example, when an acid soil high in exchangeable hydrogen ions is treated with a liming material such as calcium hydroxide (Ca(OH)2), calcium ions from the lime replace hydrogen ions at the surface of the soil particles.  The hydrogen ions are neutralized by hydroxyl ions and soil pH increases. The plant nutrients calcium (Ca), magnesium (Mg), and potassium (K) are supplied to plants in large measure from exchangeable forms.  The usual soil test to predict a soil's ability to furnish potassium to the plant is a measure of the soil's exchangeable potassium content.  The amounts of cations in the soil solution are intimately related to the exchangeable ions.  Any change in the concentration of cation in the soil colloids.  Cation exchange capacity (CEC) is the amount of exchangeable cations per unit weight of soil (dry basis).

Cation exchange capacity (CEC) - The milliequivalents (1/1000's of an equivalent) of cation exchange sites present per 100 grams (me/100 g) of dry soil. In soilless organic-based growing media, it is often expressed as milliequivalents per 100 cubic centimeters (me/100 cc)  because of the very low weight of dry soilless media.  The CEC is a measure of the nutrient holding capacity of cationic nutrients, such as K⁺, NH₄⁺, Ca⁺⁺ , etc. The total quantity of cations which a soil can adsorb by cation exchange, usually expressed as milliequivalents per 100 grams.  Measured values of cation exchange capacity depend somewhat on the method used for the determination.

Cation exchange resin - A negatively charged ion exchange resin that removes cations (positively charged ions) from water.

Cation Exchange Sites - Locations of the surface of soil colloids (clay, organic matter) with negative charges capable of attracting and holding positively charged cations. Cations exchange sites are more abundant in fine soils having high contents of clay and organic matter than in sandy soils that are low in clay and organic matter.  Divalent cations such as calcium (Ca⁺²) are held more firmly at the surface of soil colloids than monovalent cations such as potassium (K⁺ ).

Cell Structure - Most plant cells contain three major parts: the cell wall, protoplasm and inclusions.

Cellulose acetate-type membranes - A membrane composed of cellulose acetate used in reverse osmosis water purification systems.  These membranes are sensitive to pH, but resistant to chlorine.

Cell Wall - A cell wall consists of cellulose or lignin-like material which may be covered with a gelatinous pectin and/or waxy substance.  Also, it may be impregnated with oils and resins.  The cell wall gives form and support to the plant tissue.  It encloses and protects the living substances, or protoplasm, inside the cell.  There may be thin areas (pits) and perforations in a cell wall through which fluids, salts, protoplasmic strands, etc., move to an adjacent cell.

Chelate - Derived from the Greek word "chele" meaning claw.  A large organic molecule, called  a chelating agent, that contains one of the micronutrient heavy metals - iron (Fe), zinc (Zn), manganese (Mn), copper (Cu)-or divalent cations-calcium (Ca), magnesium (Mg)-held in the center by ligand bonds.  Used as fertilizers in alkaline soil/media/water to keep Fe, Zn, Mn, and Cu soluble.

Chelates - Certain organic chemicals, known as chelating agents, form ring compounds in which a polyvalent metal is held between two or more atoms.  Such rings are chelates.  Among the best chelating agents known are ethylenediaminetetraacetic acid (EDTA), hydroxyethylenediaminetriacetic acid (HEDTA) and diethylenetriaminepentaacetic acid (DTPA).  Citric acid is also used as a chelating agent.

Chelating agent - A large organic molecule that is able to bond to heavy metal micronutrients-  iron (Fe), zinc (Zn), manganese (Mn), copper (Cu),- or divalent cations-calcium (Ca), magnesium (Mg)-by ligand bonds to form a chelate.  The most commonly used chelating agents in fertilizers are DTPA, EDDHA, HEDTA, EDTA, citrate, and lignosulfonates.

Chemigation - Application fertilizers and/or pesticides in irrigation water to fertilize crops and control pests. Application of fertilizer, particularly nitrogen, in irrigation water is an accepted means of application close to the time of plant need.  Sulfur can also be easily applied in this manner.  This technique is also known as fertigation.   Herbicide and insecticide application through sprinkler irrigation systems can be an effective means of pest control.  Special precautions are required to prevent fertilizers and pesticides from siphoning into the well.
See Also: Fertigation .

Chemoautotrophic Bacteria - Microorganisms are divided into two broad classes with respect to their energy and carbon sources: heterotrophic forms which require preformed organic nutrients to serve as sources of energy and carbon, and autotrophic microorganisms, which obtain their energy from sunlight or by the oxidation of inorganic compounds and their carbon by the assimilation of carbon dioxide.  Autotrophic bacteria are of two general types: photoautotrophs , whose energy is derived from sunlight, and chemoautotrophs, which obtain the energy needed for growth and biosynthetic reactions from the oxidation of inorganic materials.Some species of bacteria are limited exclusively to inorganic oxidations and are considered to be obligate chemoautotrophs.  Significant obligate chemoautotrophs include the genus Nitrosomonas, which oxidizes ammonium nitrate to nitrite; Nitrobacter, which oxidizes nitrite-nitrogen to nitrate; and Thiobacillus, which oxidizes certain inorganic sulfur compounds.

Chlorination - To treat with chlorine as a disinfectant; used in recycled water treatment.

Chlorine (Cl) - Plants utilize this non-metallic micronutrient in the form of chloride (Cl^- ), the only form in which this element exists in the soil. Chloride is involved in energy reactions in the plant, specifically involved in the chemical breakdown of water in the photosynthesis reaction.  It also activates several enzyme systems.  It is involved in transporting several cations - potassium, calcium, magnesium - within the plant, regulating the actions of stomatal guard cells, thus controlling water loss and moisture stress while maintaining plant turgor.  Research has shown that chloride diminishes the effects of fungal root and leaf diseases in small grains.  Lowered incidence of stalk rot in corn has been related to adequate chloride.  Chloride is very mobile in the soil and leaches readily.  Deficiencies are most likely on sandy soils but can occur on any soil texture.  Deficiencies can be determined by soil and plant analysis.

Chloroplast - The green plastids contain chlorophyll and are called chloroplasts.  The chloroplasts are the food-making bodies of the protoplasm of plants.  These green bodies have the ability to capture, hold and utilize the energy of the sun (it is not known just how this function is carried out) in the synthesis of the many plant chemical compounds.

Chlorosis - Loss of the green color in plants indicated by yellowing of the leaves. A sign of nutrient deficiency.  Specific patterns of chlorosis are characteristic of individual nutrients. Pale green to yellow coloration.  The term is usually applied to leaves and is a common symptom of many nutrient deficiencies. Yellowing of green portions of a plant, particularly the leaves.

Chromoplast - The red, yellow and orange plastids are called chromoplasts.  They contain zanthophyll and carotene and give color to flowers and fruits.

Citrate - A organic acid that is sometimes used as a micronutrient chelating agent of iron, zinc, copper, and manganese.  Citrate possesses weak chelating ability.

Citrate-Soluble Phosphoric Acid - That fraction of the phosphoric acid insoluble in water but soluble in neutral ammonium citrate.  However, since that soluble in water is also soluble in ammonium citrate, "citrate-soluble" may be used to indicate the sum of water-soluble plus citrate-soluble phosphoric acid.  (See also Available .)

Clarified water - Partially purified water free of heavy sediments and floating debris.

Clarifier - A pit, basin, or tank for removing colloidal material in water treatment systems. Chemicals are added to flocculate the colloidal material, then polymers are added to aggregate the floc and cause it to settle to the bottom.  The clear water is then recovered from the top layer.

Clay - A minute soil particle less than 0.002 millimeter in diameter. Naturally occurring inorganic crystalline particles in soils and other parts of the earth's crust.  Clay particles are less than 0.002 millimeters in diameter. Properties:  Contains compounds of silicon, aluminum, hydrogen, and oxygen.

Climbing Stems - Some plants have their stems and branches encircle or twine around an upright object, as a post or tree, for an upright support.  The pole bean and morning glory vines are examples of climbing stem plants.

Coagulant - A chemical that causes flocculation, such as alum which acts as a coagulant to flocculate colloidal clay.

Coated Fertilizers - Fertilizer materials, generally urea, that are coated to slow the release of the fertilizer.  Coating material is most commonly sulfur, but resins and thermoplastics are also used.

Coefficient of uniformity (Q) - A measure of how uniformly an irrigation system delivers water, with 1 being perfect and below 0.8 indicating a poorly performing system.

Coir fiber - An organic growing media component made from coconut husk fiber.

Collenchyma - Another strengthening stem tissue similar to the sclerenchyma tissue is the collenchyma tissue.  It is composed of thick-walled cells which have thickened corners.  The collenchyma cells stay alive longer than the sclerenchyma tissue cells.

Colloid - Soil particles (organic or inorganic) having small diameters ranging from 0.10 to 0.005 micron. Characterized by high cation exchange capacity.  Colloids have a vast surface area per unit mass, which accounts for their high adsorptive capacity and their high cation exchange capacity in soils.

Colloidal solution - Mixture of very fine solid particles with liquid (e.g., protein colloid), either in liquid (sol) or jelly-like state (gel).

Complete Fertilizer - A fertilizer containing all three of the primary fertilizer nutrients (nitrogen, phosphate and potash) in sufficient amounts to be of value as nutrients.

Complex Group - The complex plant groups have a more complex or advanced stage of development and are found mostly on land.  They have much more complex structure, including conducting tissue and embryos.  The seed producing plants of this group dominate the vegetative production found on the earth's land mass.  Also they provide most of the food and fiber for man.

Complex Permanent Tissues - When a permanent tissue is composed of several kinds of cells which make simple tissue, they form a complex permanent tissue.  The two important complex permanent tissues are xylem and phloem.

Compost - A mixture that consists largely of decayed organic matter and is used for fertilizing and conditioning soil.
A mixture that consists largely of decayed, relatively stable plant and animal wastes. Used for fertilizing and conditioning the soil.

Composting - A process that partially decomposes organic matter.

Compound - Substance consisting of several elements, e.g., water (H₂ O).

Conditioner (of fertilizer) - A material added to a fertilizer to prevent caking and to keep it free-flowing.

Conditions for Photosynthesis - The direct conditions necessary for photosynthesis are: carbon dioxide (from air), water (mostly from roots), favorable temperature (5 to 40 C or 40 to 105 F), light or energy from the sun (only about 3 to 5% of the sunlight energy is utilized by the leaves) and chlorophylls (the green pigments in the chloroplasts).

Conductivity, Electrical - A physical quantity that measures the readiness with which a medium transmits electricity.  Commonly used for expressing the salinity of irrigation waters and soil extracts because it can be directly related to salt concentration.  It is expressed in decisiemens per meter (dS/ m), or in millisiemens per centimeter (mS/ cm) or millimhos per centimeter (mmhos/ cm), at 25°C.

Conductivity meter - An instrument used to measure electrical conductivity of solutions.  Also called a solubridge or salts meter.

Conservation Tillage - Any tillage system that maintains at least 30 % of the soil surface covered by residue after planting to reduce soil erosion by water; or where soil erosion by wind is the primary concern, maintains at least 1000 pounds of flat small grain residue equivalent on the surface during the critical wind erosion period. Many different types of tillage systems can be used to meet the requirements of this .   Emphasis should be placed on the achievement of the desired goal and development of a systematic approach to that goal.  There is no reason to replace or eliminate a piece of equipment from a tillage system if it is helping to achieve the goals established for that field.  Several specific types of tillage systems are included under the general heading of conservation tillage.  These terms include:
   No-till: The soil is left undisturbed from harvest to planting except for nutrient injection.  Planting or drilling is accomplished in a narrow seed bed or slot created by coulters, row cleaners, disk openers, in-row chisels or roto-tillers.  Weed control is accomplished primarily with herbicides.  Cultivation may be used for emergency weed control.  In addition to deep placement of nutrients, starter fertilization is advisable and widely practiced.  Side-dressed nitrogen for row crops and top-dressed nitrogen for small grains with dribble techniques may improve use efficiency.  Fertigation is an option.
  Ridge-till: The soil is left undisturbed from harvest to planting except for nutrient injection.   Planting is completed in a seedbed prepared on ridges with sweeps, disk openers, coulters or two cleaners. Residue is left on the surface between ridges.  Weed control is accomplished with herbicides and/or cultivation.  Ridges are rebuilt during cultivation.
Methods of fertilizer application include knife placement into the ridge prior to planting use of starters, side-dressed applications and fertigation.
   Mulch-till : The soil is disturbed prior to planting.  Tillage tools such as chisels, field cultivators, disks, sweeps or blades are used.  Weed control is accomplished with herbicides and/or cultivation.  Fertilizer application methods include broadcast applications; knifed applications with tillage equipment including sweeps; starter application; side-dressing; top-dressing, and fertigation.

Container capacity - The percent volume (volume/volume) of soil/growing media filled with water after it has been saturated and allowed to drain.  It is sometimes called the water holding capacity and is the maximum amount of water that soil/growing media can hold.  In field soils, it is called field capacity.

Container Stock - Nursery plants grown entirely in containers rather than being dug from a field.

Controlled-release fertilizer (CRF) - A fertilizer that is not immediately soluble and available to plants because the nutrients are released over time, from weeks to months.  Release is based on low solubility, biological breakdown, or a semipermeable coating.

Controlled-Release Fertilizer - "Slow or Controlled Release Fertilizers.  A fertilizer containing a plant nutrient in a form which delays its availability for plant uptake and use after application, or which delays its availability to the plant significantly longer than a reference 'rapidly available nutrient' fertilizer such as ammonium nitrate or urea, ammonium phosphate, or potassium chloride.  Such delay of initial availability or extended time of continued availability may occur by a variety of mechanisms.  These include controlled water solubility of the material (by semipermeable coatings, occlusion, or by inherent water insolubility of polymers, natural nitrogenous organics, protein materials, or other chemical forms), by slow hydrolysis of water-soluble low molecular weight compounds, or by other unknown means."  (AAPFCO).  Also called controlled-availability fertilizers and slow-release fertilizers. Limited solubility may be an inherent characteristic of the fertilizer such as in urea-formaldehyde reaction products and magnesium ammonium phosphate; or it may be imparted to a soluble fertilizer by coating the particles with such materials as molten sulfur, waxes, and plastics.

Conventional Agriculture - Conventional agriculture is composed of modern, site-specific, progressive, science-based production systems in which available and applicable technologies and inputs are used effectively and efficiently.  Conventional agriculture shows concern for the environment as well as profitability. Production systems have developed over time because they are more efficient and profitable.  Today's conventional systems are the results of extensive research and education programs on the development, adaptation, and implementation of technology in production practices in a competitive system.  Any inefficient, resource-wasting practices are constantly being eliminated and replaced with more efficient, best management practices (BMPs).

Conventional Tillage - Conventional tillage systems vary widely from region to region and crop to crop.  The term conventional tillage originally implied use of the moldboard plow, disking, and harrowing to level the soil surface prior to seeding.  In actuality, however, conventional tillage systems have now evolved to the use of other tillage implements including widespread use of the chisel plow or other primary tillage implements.  Conventional tillage today is actually a reduced tillage system compared to past practices.

Copper (Cu) - Copper (Cu) is an essential metallic micronutrient and is absorbed by plants from the soil in the form of the Cu⁺² ion. copper is necessary for chlorophyll formation in plants and catalyzes several other plant reactions although it is not usually a part of the products formed by those reactions.  Organic soils are most likely to be copper deficient, since copper is fixed in unavailable forms in these soils.  High soil pH also decreases copper availability.  Mobility in the plant is low.  Common symptoms of copper deficiency include dieback in citrus and blasting of onions.  Leaves of copper-deficiency vegetable crops lose turgor and develop a bluish-green shade before becoming chlorotic and curling.  Plants may fail to flower.  Small grains fail to develop heads when copper is deficient.  Deficiencies of this element can be determined by soil and plant analysis.

Cork - Another protective simple tissue is cork.  This tissue is composed of cells which have their walls water-proofed by suberin.  Since the function of this tissue is to protect the inner tissue from excessive evaporation or a physical protection, their cells die shortly after they are formed.  Tree bark and the skins of many tubers are examples of cork tissue.

Corms - Other plants, such as a crocus, have a globose perennial stem under the soil surface.  The bulb portion is small and has very thin papery leaves on its surface.  Corms function as a storage of food and reproductive organ.

Cortex - Underneath the epidermis layer is the cortex tissue.  This area is a mass of irregular shaped parenchyma cells with many intercellular spaces. The cortex tissues are largely a storage place for water and nutrients.

Coulter Injection - Use of a narrow coulter and high pressure to place a fluid fertilizer in a vertical band from the soil surface to the depth of coulter penetration.  A variation of banded fertilizer application.

Critical deficiency limit - The lowest level of tissue nutrient content from which you can expect adequate growth.  Below this, deficiencies are likely.

Critical toxicity limit - The highest level of tissue nutrient content from which you can expect adequate growth.  Above this limit, toxicities are likely.

Crop Nutrient Budget - A balance sheet showing the nutrient applied to the crop and the nutrients removed by the crop. Only a portion of the nutrients needed by a crop is removed from the field at harvest.  Grain crops remove less nutrients per unit of harvest than forage crops.

Crop Nutrient Recycling - Returning to the soil the nutrients contained in plant residues.

Crop Nutrient Removal - Nutrients removed in harvested crops. Perhaps the most critical factor when evaluating the sustainability of a farming system.  Simply put, if the nutrients removed from a field are not replaced, the system is not sustainable.  When nutrients removed in harvested crops are not returned, the result is a decline in soil productivity.
The two nutrients most susceptible to depletion through crop removal are phosphorus (P) and potassium (K).  Unlike nitrogen (N), which can be partially replenished by rotation with legume crops, there is no biological method of replacing P and K.  Once soil supplies are depleted through crop removal, the only method of replacement is through importation of outside sources.  The source can be organic residues, manures, or commercial fertilizers.

Crop Nutrient Sources - Plants obtain essential nutrients from a number of sources including soil minerals, organic matter, commercial fertilizers, legumes (N), animal wastes, sewage sludge, and other wastes. Plants do not distinguish between these sources.  All nutrients are taken up in identical inorganic forms regardless of source.

Crop Nutrient Uptake - Large quantities of nutrients are contained in the above ground portion of crops.  Total nutrient demand is often referred to as crop nutrient uptake.

Crop Residue Management (CRM) - A year-round system beginning with the selection of crops that produce sufficient quantities of residue and may include limited secondary harvest of residue.  CRM includes all field operations that affect residue amounts, orientation and distribution throughout the period requiring protection.  Site-specific residue cover amounts needed are usually expressed in percentage but may also be in pounds.

Cross Section of Herbaceous Stems - Since herbaceous stems are mostly of primary growth tissues, its cross section is very much like the cross section of a growing root.  It has an epidermis, cortex, stele with phloem and xylem tissues.  There is often considerable pith in the stems of these annual plants.

Cross Section of Root - A vertical or cross section of a root shows a structure which is composed of epidermis, cortex, endodermis, pericycle, xylem, phloem and parenchyma tissues.  In time, the older portions of the roots develop cambium tissues.

Cross Section of Woody Stems - ross section of woody stemmed plants shows both primary and secondary tissue.

Cytoplasm - l of the living substances in a cell outside of the nucleus is called the cytoplasm.  It is not uniform in its appearance and contains many types of structure, such as cytoplasmic membranes and plastids.

Cytoplasmic Membrane - The cytoplasmic membrane surrounds the cytoplasm and exists just inside the cell wall.  It controls the entry and exit of materials into and from the cytoplasm.

Damping-Off - Sudden wilting and death of seedling plants resulting from attack by microorganisms.

DeciSiemen per meter (dS/m) - The preferred unit to express electrical conductivity.  One dS/m  = one mS/cm = one mmhos/cm = (p.m./700).

Deep Banding Fertilization - Deep banding refers to preplant applications of nutrients placed 2 to 6 inches below the soil surface. Some applications are deeper, as much as 15 inches.  The applied nutrients may be in solid, fluid, or gaseous forms. Concentrated zones of nutrients are produced, either streams sheets or points, depending on the design of the applicator.  In some areas this fertilization technique is performed many months before the next crop is seeded, often in conjunction with a tillage operation.  Reduced tillage grain drills have been adapted for one-pass seeding and deep placement of fertilizer either between and rows or below the seed. Other common terms for deep banding include "deep placement," "dual placement," "dual banding," "knifing," "preplant banding," "double shooting," "triple shooting," "root zone banding," and "tillage implement application."  Dual application implies simultaneous application of anhydrous ammonia as the main nitrogen source and either fluid or solid phosphorus, potassium, and sulfur fertilizers.  Otherwise, deep banding terminology can imply the use of either fluid or solid fertilizers.  See Also: Banded Fertilizer; Coulter Injection; Double Shooting; Dual Placement; Knifed application; Triple Shooting .

Deficiency - When the concentration of a nutrient becomes low enough in a plant to cause decreased growth or tissue damage.  It is often accompanied by visual symptoms.

Deionization - A water purification process for removing ions (cations and anions) from water using ion exchange resins.

Denitrification - The process by which nitrates or nitrites in the soil or organic deposits are reduced to lower oxides of nitrogen by bacterial action.  The process results in the escape of nitrogen into the air.

Differentiation Zones - The older meristematic cells of a young root change to mature tissues. They develop xylem, phloem and other tissues to carry absorbed nutrients up into the stems, etc., and return synthesized material to the growing sections of the root.

Diffused Roots - When a primary root branches quickly into many slender roots it develops a diffused or fibrous root system.  There may be several main roots which are about equal in diameter and have many branching root systems.  When the length of all of the roots and rootlets are added together, a mature well-developed plant such as a native prairie grass plant, may have a root length of thousands of miles.

Diffusion - Diffusion is the equalization of the concentration of nutrients solutions which exists between the root cells and soil solution.  The stronger salt solutions move from the soil to the less concentrated solutions inside the cells.  This movement takes place easily and rapidly when the root membranes, root hairs, are permeable to the soil solutions and its solutes.

Digestion - The process, which converts water insoluble food into water soluble foods, or changes complex food into simpler foods for utilization, is called digestion.  It usually proceeds transportation, assimilation and respiration.  Also, it requires an enzyme to aid in this process.  Chemically, it is known as hydrolysis.

Distillation - A water purification process where water is heated into vapor form (which leaves impurities behind) and is then condensed to produce purified water.

Dolomite - A lime composed of calcium-magnesium carbonate [CaMg(CO₃)₂].  Pure dolomite contains 13.1% Mg.  Dolomite is often used interchangeably and incorrectly for dolomitic lime. A material used for liming soils in areas were magnesium and calcium are needed.  Made by grinding dolomitic limestone, which contains both magnesium carbonate, MgCO₃, and calcium carbonate, CaCO₃.  (See also Lime). A lime composed primarily of calcium carbonate (calcite, CaCO₃), with a lesser content of calcium-magnesium carbonate [dolomite, CaMg(CO₃)₂].  Any lime that contains some Mg from dolomite is called dolomitic lime, but the concentration of Mg varies greatly, from 1.3 to 11.7%.

Double Shooting - Synonymous with dual placement or dual application. Placement of two fertilizer materials in subsurface bands using separate delivery tubes.
See also: Dual Placement.

Dribble Fertilization - Dribbling or strip banding is a form of band placement that involves application of solid or fluid fertilizers in bands or strips of varying widths on the soil surface or on the surface of crop residues. Zones of high nutrient concentration are produced which improve nutrient use efficiency.  Typically, the fertilizer material contacts 25% to 30 % of the soil surface.  If these surface strip applications are followed by tillage, the concentration effect is diluted to something between broadcast application and deep banding where the concentrated zones remain intact.
See also: Broadcast Application; Deep Banding Fertilization .

Drip Irrigation - See Low-Volume Irrigation

Drip irrigation system - An irrigation system where water is delivered to the soil/growing media surface below the plant canopy, usually by small nozzles, emitters, or tubes.

DTPA - Diethylenetriaminepentaacetic acid.  A chelating agent that is used to chelate ferric (Fe⁺³) iron.  DTPA is used on acid to slightly alkaline soils and in many chelated liquid fertilizer formulations.

Dual Placement of Application - Placement of two fertilizer materials in subsurface bands.  Application: Usually accomplished through injection of the two materials from two tubes at two points on an applicator shank. For example, anhydrous ammonia and fluid ammonium polyphosphate (10-34-0) or a mixed liquid fertilizer containing other nutrients many be dual applied. See Also: Deep Banding Fertilization; Double Shooting.

Ebb-and-flow flooded floor system - An ebb-and-flow subirrigation system that uses molded concrete floors as the flood container.

Ebb-and-flow subirrigation system - A subirrigation growing system where containerized plants are placed in watertight trays or molded concrete floors that are flooded when needed with nutrient solution, then drained.  Usually the nutrient solution is collected and recirculated.  Also called ebb & flow, ebb-and-flood, flood irrigation, and pulsed subirrigation.

Ebb-and-flow tray system - An ebb-and-flow subirrigation system that uses watertight trays, usually placed on benches, as the flood container.

ECe - Electrical conductivity of a saturated soil extract.

Ecology - The branch of biology that deals with the mutual relations among organisms and between organisms and their environment.

Ecw - Electrical conductivity of water.

EDDHA - Ethylenediaminetetraacetic-o-hydroxyphenylacetic acid, a chelating agent used to chelate ferric iron (Fe⁺³ ); used on highly alkaline soils/media.  Very expensive.

EDTA - Ethylenediaminetetraacetic acid, a chelating agent used to chelate ferric iron (Fe⁺³ ), manganese, copper, and zinc.  Used on slightly acid soils/media and in many chelated liquid fertilizer formulations and hydroponic nutrient solutions.

Electrical conductivity (EC) - The ability of a solution to conduct electricity due to dissolved or  suspended ionic solutes.  Used as a measure of soluble salt content of water.  Preferred to be expressed as deciSiemen/meter (ds/m), but often expressed as milliSiemen/centimeter (mS/cm), millimhos/centimeter (mmhos/cm), EC ´ 10-3, or micromhos/centimeter (µmhos/cm).  Each dissolved salt has its own unique conductivity, hence, EC is only an approximation of the actual salt content of a solution [ppm = (ds/m)(700)].

Electrodialysis - A water purification process where water is passed over electrically charge membranes to remove ionic solutes.

Electrolytes - Any compound or material that forms ions when dissolved in water, thus forming a conductor of electricity, such as all soluble salts.

Element (chemical) - Basic chemical substance consisting of chemically uniform atoms; there are  92 natural elements arranged in the periodic system.

Elemental sulfur - The elemental form of sulfur used to decrease soil pH (increase acidity) in alkaline soil/growing media.

Elongation Region - As new cells develop, they elongate and push the meristematic section and root cap ahead.  With this elongation, more protoplasm and vacuoles accumulate to nourish and stimulate the meristematic region.  This active growing region is not more than a few millimeters in length.

Emulsion - Heterogeneous mixture of liquids (e.g., fat droplets in water).

Environment - All external conditions that may act upon an organism or soil to influence its development, including sunlight, temperature, moisture and other organisms.

Enzymes
Protein substances produced by living cells which modify the rate of chemical reactions.  They are organic catalysts.

Epidermis of the Leaves - The tissue which forms a covering layer over leaves, the softer portions of stems and roots, etc., is called epidermal tissue.  Its thickness is usually one cell.  On the stems and leaves it often has a waxy substance, cutin, on its outside walls.  The function of this epidermis is largely for protection to prevent excessive evaporation of water and aid the root in its absorption of substances from the soil.   On leaves and some stems, this epidermal tissue contains openings or pores called stomata, where gases are exchanged.

Epidermis of the Roots - The outer or surface layer of the root tissues is the epidermis.  In the young root, its function is largely that of protection and absorption of water and nutrients.  As the roots become larger and older, this layer may be sloughed off and replaced by a layer of cork.

Epidermis - The epidermal tissues are the single layers of cells on the upper and lower surfaces of a leaf.  The ordinary epidermal cells are tightly packed and colorless.  They serve as a protection of the inner tissues from mechanical injury, parasite invasions and from dehydration.

Epsom salt - Magnesium sulfate (MgSO₄.7H₂O); used primarily as a magnesium fertilizer, but it also supplies sulfur.  Used to supply Mg without altering soil/growing media pH.

Eq.- Amount of substance, numerically equal in grams to equivalent weight, e.g.,
for KCl (potassium chloride): 1 eq.= 74.6 g (Univalent cation),
for MgSO₄ (magnesium sulfate):   1 eq.= 60 g(bivalent cation),
See also mole, equivalent weight, valence .

Equilibrium reaction (pHc) - A mathematical measure similar to pH, but it also takes into account the alkalinity, calcium, magnesium, carbonate, and bicarbonate  content of water.

Equivalent - A mole (Avogadro's number) of charges (positive or negative).  One mole of a  monovalent cation would contain one equivalent of positive charges, whereas one mole of a divalent cation would contain two equivalents of positive charges.

Equivalent weight - Molecular weight of substance, divided by valence (see eq .).

Erect Stems - When the shoots grow upright, such as corn, cotton, alfalfa, shrub and tree plants, they are called plants with erect stems.  Most of the branching shoots of such plants have a tendency to grow upright.

Erosion - The wearing away of the land surface by detachment and transport of soil and rock materials through the action of moving water, wind or other geological agents.

Essential element - One of the 17 elements that plants need for normal growth and development and to complete their life cycle.  They are: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), boron (B), molybdenum (Mo), chlorine (CI), and potentially nickel (Ni).

ESP - see Exchangeable Sodium Percentage

Eutrophication - A condition in stagnant pools and lakes usually characterized by an abundant accumulation of nutrients that supports a dense growth of plant and animal life, the decay of which depletes the shallow waters of oxygen in summer.

Evapotranspiration (ET) - The sum of water evaporation and transpiration. The loss of water from a soil by evaporation and plant transpiration.

Exchangeable Base - A basic cation (Ca⁺², Mg⁺², K⁺, NH₄⁺) adsorbed on a soil colloid, but which can be replaced by hydrogen (H+) or some other cation.
See Also: Cation Exchange .

Exchangeable Ions - Ions held on the soil complex that may be replaced by other ions of like charge.  Ions which are held so tightly that they cannot be exchanged are called nonexchangeable.

Exchangeable Sodium Percentage - The degree of saturation of the soil exchange complex with sodium.  It may be calculated by the formula:
ESP = Exchangeable sodium (me/100 g soil)/Cation exchange capacity (me/100 g soil) * 100

Exchangeable Sodium Percentage (ESP)
The degree of saturation of the soil exchange complex with sodium (Na).
May be calculated by the following formula: Soil cation exchange capacity (CEC) = 12 milliequivalents (meq)/100 grams
 Calcium (Ca⁺²) = 7 meq/100 grams
 Potassium (K⁺) = 2 meq/100 grams
 Magnesium (Mg⁺²) = 2 meq/100 grams
 Sodium (Na⁺) = 1 meq/100 grams
ESP = 1 meq Na/12 meq/100 soil x 100 = 8.33%
Reducing exchangeable Na to 5% of the soil's CEC (cation exchange capacity) is a target of many reclamation plans.

Expressed sap tissue testing - The use of analytical techniques to determine the nutrient content of plant sap.

Fallow - Cropland left idle in order to restore productivity, mainly through accumulation of water, nutrients or both.  Summer fallow is a common stage before cereal grain in regions of limited rainfall.  The soil is tilled for at least one growing season to control weeds, to aid decomposition of plant residues and to encourage the storage of moisture for the succeeding grain crop.  Bush or forest fallow is a rest period under woody vegetation between crops. The practice of allowing cropland to be left idle in order to enhance productivity, mainly through accumulation of water, release of nutrients from organic matter, or both.
Summer fallow is a common practice in cereal grain production systems in regions of limited rainfall.  The soil is kept weed-free for one growing season by tillage or herbicides in order to store moisture and nutrients for the following grain crop.  However, moisture storage is relatively inefficient.

Ferric - The form of iron in the +3 valency form (Fe⁺³ ).  Ferric iron is the insoluble form found in most soils.

Ferrous - The form of iron in the +2 valency form (Fe⁺²).  Ferrous iron is the primary form of iron plants absorb.

Fertigation - A term often used to describe the application of soluble fertilizers in the irrigation water.  Also called liquid feed. : Application of fertilizer in irrigation water. Nitrogen is the most common plant nutrient applied in irrigation water. Both sprinkler and furrow irrigation systems can be utilized.  Anhydrous ammonia, urea-ammonium nitrate (UAN) solutions and solid nitrogen sources such as urea have been effectively applied in this manner.  Care has to be given to the injection of anhydrous ammonia into water containing large amounts of dissolved carbonates and bicarbonates to avoid precipitation of salts within the irrigation system.  Ammonia application through a sprinkler irrigation system can lead to substantial volatilization losses.  Sulfur can also be easily applied in this manner.  Common sulfur sources for fertigation would include ammonium thiosulfate, ammonium sulfate, ammonium polysulfide and potassium thiosulfate.
See Also: Chemigation .

Fertilizer - Any natural or manufactured material added to the soil in order to supply one or more plant nutrients.  The term is generally applied to manufactured materials other than lime or gypsum. AAPFCO officialis: "Any substance containing one or more recognized plant nutrient(s) which is used for its plant nutrient content and which is designed for use or claimed to have value in promoting plant growth, except unmanipulated animal and vegetable manures, marl, lime, limestone, wood ashes, and other products exempted by regulation." Fertilizer is food for plants.  It contains nutrients needed to supplement the soil's supply, which is often less than that required to satisfy demands for optimum crop production. The most common fertilizer nutrients are nitrogen (N), phosphorus (P), and potassium (K).  These three nutrients are needed in largest amounts by crops and are most often the ones first limiting in soils.  The composition of fertilizer varies depending on the crop for which it will be used and the nutrient status of the soil.

Fertilizer analysis - A sequence of three numbers on all fertilizer labels that gives the percent composition, on a weight basis, of N-P2O5-K2O in the fertilizer.

Fertilizer Application - Various methods of delivery of chemical fertilizer to the soil.

Fertilizer Formula - The quantity and grade of materials used in making a fertilizer mixture.

Fertilizer Grade - An expression that indicates the weight percentage of plant nutrients in a fertilizer.  Thus a 10-20-10 grade contains 10 percent nitrogen (N), 20 percent phosphoric acid (P₂O₅) and 10 percent potash (K₂ O).

Fertilizer injector - A device attached to the irrigation line to inject a small amount of concentrated fertilizer into the water stream to yield a dilute concentration of  fertilizer in the irrigation water.  The injector ratio or proportion varies from1:15 (i.e., dilutes 1 part fertilizer concentrate to every 15 parts of irrigation water) to 1:400.  On some injectors the proportion can be varied.

Fertilizer Placement - Concentration fertilizer into a band or strip at a specific location on or below the soil surface.  Examples: starter, dribble fertilization, deep banding.

Fertilizer ratio - The fertilizer analysis reduced to the least common denominator, which yields the ratio of N-P₂O₅-K₂ O.  For example, a 18-6-12 analysis has a 3-1-2 ratio.

Fertilizer Soil Reaction Zone - That volume of soil containing fertilizer components and/or their reaction products before they are dispersed by soil water or tillage.  The size of the fertilizer-soil reaction zone is determined by fertilizer physical form (solid or fluid), rate of application, particle size, and method of application as well as soil physical and chemical properties.

Fertilizer Use Efficiency - An expression of the units of yield per unit of nutrient provided for the crop.  Common expressions include bushels of grain per pound of applied nutrient or pounds of yield per pounds of applied nutrient.
Any production practice which improves the final crop yield directly affects fertilizer use efficiency.  When a superior variety or hybrid increases grain yields by 5%, for example, this translates directly to a similar increase in fertilizer use efficiency.  Rotation, planting date, seeding rate, and method of fertilizer application can have similar effects.

Field Moisture Capacity - The moisture content of soil in the field two or three days after a thorough wetting of the soil profile by rain or irrigation water.  Field capacity is expressed as moisture percentage, dry-weight basis.

Fifteen-Atmosphere Percentage - The moisture percentage, dry-weight basis, of a soil sample which has been wetted and brought to equilibrium in a pressure-membrane apparatus at a pressure of 221 psi.  This characteristic moisture value for soils approximates the lower limit of water available for plant growth.  (See also Permanent Wilting Percentage.)

Fixation - Processes by which available plant nutrients are rendered unavailable by reaction with soil components.  Generally, refers to reactions of phosphorus, ammonium, and potassium leading to decreased availability. The more acid the soil and the higher its clay content, the greater its capacity to fix phosphorus.  On such soils, liming and applying the phosphate in bands to lessen its contact with the soil have been the conventional remedies to lessen fixation and improve nutrient use efficiency. Soils differ in their capacity to fix potassium, depending on the kinds and amounts of clay minerals they contain.  Fixed potassium ions are trapped between the silica sheets of certain soil clay minerals, including especially vermiculite and illite.  These trapped ions may be slowly released over time.
Ammonium ions from ammonia-based fertilizers, legumes, and wastes also can be fixed by the same clay minerals that fix potassium, and in the same manner. (See also Reversion and Nitrogen Fixation .)

Flocculation - The aggregation of particles into larger masses.  Flocculation is used in water treatment to aggregate contaminants, which are then precipitated.

Flooded floor - See ebb-and-flow flooded floor system .

Floriculture - Production of foliage or flowering ornamental plants in fields or greenhouses for commercial sales.

Flotation Applicator - A type of fertilizer applicator equipped with large, low pressure tires intended to spread the weight of the vehicle over a large soil surface area.

Flow Divider - Mechanical device used for splitting a stream of liquid fertilizer to achieve uniform distribution to individual application points.

Flower Parts - When these flowering parts develop into a perfect or complete flower, there are four kinds of floral organs: sepals, petals, stamens and pistil.

Flowers - The flower part of seed plants is a highly modified shoot which develops into reproductive organisms.  The development of a bud into a flowering shoot depends on many factors such as heredity, photoperiods, moisture, food supply, temperature, etc.

Fog system - A device that dispenses fine droplets of water that stay suspended in the air.  Used for temperature and humidity control in greenhouses and propagation.

Foliar analysis - Nutrient analysis of leaf tissue.

Foliar Diagnosis - Estimation of the nutrient status of a plant or the nutrient requirements of the soil for producing a crop through chemical analysis or color manifestations of plant leaves, or by both methods.

Foliar Fertilization - Supplying plant nutrients through leaves, with absorption taking place through the stomata of leaves and leaf cuticles.

Foliar Fertilization - Application of soluble fertilizers in the form of spray to the foliage of plants.

Forage - Unharvested plant material which can be used as feed by domestic animals.  Forage may be grazed or cut for hay or silage.

Frit - A slow-release fertilizer with the nutrients impregnated into powdered glass.  Frit is most commonly used for potassium or iron.

Fruit - A fruit is a mature ovary of a flower such as an apple, tomato, etc.  There are many kinds of fruits.

Function of Leaves - The important function of most leaves is the manufacture of food for photosynthesis.  Associated with photosynthesis are other physiological processes in leaves such as transpiration, respiration, digestion and assimilation.  All of these chemical transformations involve the physiological and functional processes which take place in living cells are called metabolism.

Function of Plant Roots - There are four important functions of plant roots; namely, absorption of plant foods, transportation of foods to and from stems, storage of foods, and anchorage.

Function of Stems - The chief function of stems is to conduct materials and produce support for leaves and reproduction organs.  Raw materials or nutrients absorbed from the soil solution are transported from the roots to the leaves by the stem tissues. After these plant food materials are manufactured into plant materials in the leaves, these synthesized materials are transported by the stems to the growing regions of the plant, to the roots, to the storage tissues. The bud or growing part of a stem produces more stem tissues for the leaves, which manufacture plant materials.  Also stems provide support for the reproductive organs or flowers.
 The stems of some plants provide storage for absorbed nutrients and synthesized plant materials.  Often, these stored materials are used for new growth later in the season or next year.

Geographic Information Systems (GIS) - Technology that links satellite positioning data to on-board map information for variable rate applications, variable rate planting and yield mapping.

Global Positioning Systems (GPS) - The term global positioning system (GPS) refers to a network of U.S. Defense Department satellites that provide exact location coordinates to computers on board tractors, fertilizer applicators, trucks, combines and other vehicles.
The GPS system is an integral part of variable rate fertilization systems that allows site-specific fertilizer application and permits the applicator operator with an on-board computer to know precisely the equipment's location within the field at all times.  The GPS system is also utilized with variable-rate planting equipment and field-mapping harvesting equipment. A cursor on the on-board computer screen relates the equipment location to a field map.  The applicator operator is then able to change rates of application while traveling across the field to adapt to soil test information stored in the on-board computer.  The equipment does not have to be operated in straight lines.  A trail line on the screen lets the operator know where the applicator has been. GPS also benefits application on the increasing number of fields farmed on the contour or terrace.

Grade - The guaranteed analysis of a fertilizer containing one or more of the primary plant nutrient elements.  Grades are stated in terms of the guaranteed percentages of nitrogen (N), available phosphate (P₂O₅) and potash (K₂O), in that order.  For example, a 10-10-10 grade would contain 10 percent nitrogen, 10 percent available phosphate, and 10 percent potash.
 (See also Analysis ).

Gray Water - See Recycled Water .

Green Manure - Crops grown especially to be plowed under for the benefit of succeeding crops.
Common green manures are alfalfa, clovers, buckwheat, cowpeas, small grains, and other crops.

Growing media - The substrate in which plant's roots grow.  Usually it is a highly amended or totally soilless mixture made from organic and inorganic components.  Also called potting soil, soilless media, mix, or substrate.

Ground Cover - Plants grown for their low, spreading habit, to protect soils, to prevent the growth of weeds and for aesthetic purposes.

Guano - The decomposed dried excrement of birds and bats, used for fertilizer purposes.  The most commonly know guano comes from islands off the coast of Peru and is derived from the excrement of seafowl.  It is high in nitrogen and phosphate and at one time was a major fertilizer in this country.

Guarantees - The AAPFCO official regulation follows: The statement of guarantees of mixed fertilizer shall be given in whole numbers.  All fertilizer components with the exception of potash (K₂O) and phosphoric acid (P₂O₅ ), if guaranteed, shall be stated in terms of the elements.

Gypsum - Calcium sulfate .  Gypsum is used primarily as a calcium fertilizer, and it also supplies sulfur.  Most often used as a calcium fertilizer when you do not alter soil/growing media pH.

Gypsum (CaSO₄.2H₂O) - The common name for calcium sulfate, a mineral used in the fertilizer industry as a source of calcium and sulfur.  Gypsum also is used widely in reclaiming alkali soils in the western United States.  Gypsum cannot be used as a liming material, but it may reduce the alkalinity of sodic soils by replacing sodium with calcium.  Another common name is landplaster.  When pure it contains approximately 18.6 percent sulfur.

Half-life - The length of time for one-half of a compound to disappear or be used.

Hardpan - A hardened or cemented soil horizon or layer.  The soil material may be sandy or clayey and may be cemented by iron oxide, silica, calcium carbonate or other substances. A hardened or cemented soil horizon or layer.  The soil material may be sandy or clayey and may be cemented by iron oxide, silica, calcium carbonate, or other substances.

Hard water - A water high in calcium and/or magnesium.

HEDTA - hydroxyethylethylenediaminetriacetic acid, a chelating agent used to chelate ferric iron (Fe³⁺ ).  HEDTA is used under moderately alkaline soil conditions.  Also abbreviated HEEDTA.

Herbaceous Stems - Herbaceous stems are soft and green.  They consist largely of primary tissues which develop annually.  They are usually slender or narrow in diameter with an outer covering of epidermal tissue.

Heterotrophic Bacteria - A category of soil bacteria that obtain their energy and carbon directly from soil organic matter.  Most soil bacteria fit in this category. The general-purpose decay and ammonifying bacteria are heterotrophic.  Fungi and actinomycetes are also heterotrophic in character.
See Also: Actinomycetes .

High Pressure Injection - A stream or pulse of fluid fertilizer forced below the soil surface at 2000 to 6000 psi without prior opening of the soil by some mechanical means.  Pressures for this type of application are many times higher than those used in coulter injection.

Hoagland Solution - Nutrient solution containing all essential plant nutrients, and used for hydroponically grown plants.  Original nutrient solution was developed by Professor Hoagland at the University of California.

Hollow-fiber membranes - A type of membrane used in water purification systems.

Horizon Soil - A layer of soil, approximately parallel to the soil surface, with distinct characteristics produced by soil-forming processes.

Horticulture - The science of producing and using ornamental plants, fruits and vegetables.

Humification - The process of forming humus, well decomposed organic matter, in soils. Humification in soils is controlled mainly by factors which are favorable for microbial activity.  Among these are optimum soil moisture, temperature and aeration, as well as a source of energy.  Incorporation of crop residues and appropriate tillage also promote soil organic matter formation.  Humification is needed to maintain soil organic matter levels since decomposition of soil humus occurs during much of the year.  Loss of soil organic matter leads to low water holding capacity, soil compaction, increased erosion, and decreased productive capacity.

Humus - Dark brown or black substances consisting of well decomposed organic matter that provides nutrients for plants and increases the water retention of soil.  Humus constitutes from one fifth to one half of the organic matter in peat, compost, leaf mold, and rotted animal manures. The importance of humus to the growth of crops is due principally to its high buffer capacity over a considerable range of pH values.  It tends to stabilize soil structure and has a high cation exchange capacity. The well-decomposed, more or less stable portion of the organic matter in mineral soils.

Hydrated lime - Lime composed of calcium hydroxide [Ca(OH)₂ ].  Hydrated lime is used to raise media pH quickly.

Hydrogen Ion Concentration - See pH .

Hydrolysis - Reaction of a salt with ions of water, e.g., CaCO₃ + 2(H⁺ + OH^-)  » Ca(OH)₂ + H₂CO₃ .

Hydroponics - Cultivation of plants by placing the roots in liquid nutrient solutions rather than in soil/growing media.  The soilless culture or water culture of plants. The production of plants in a liquid solution or gravel medium supplemented with all required nutrients for proper growth. The production of plants in a liquid solution or gravel medium supplemented with all required nutrients for proper growth. AAPFCO official is " . . . a system in which water-soluble nutrients are placed in intimate contact with the plant's root system, being grown in an inert supportive medium which supplies physical support for the roots but which does not add or subtract plant nutrients."

Hydroxide - Compound containing (OH^- )-ions; hydroxides of metals are also called bases.

Hygroscopic - Capable of taking up moisture from the air.

Immobile - A term used to describe translocation of a nutrient in plants when the nutrient only travels up the stem and into organs via the xylem, but it cannot be loaded into the phloem to travel out of organs or down stems.  Deficiencies of immobile nutrients always occur on the young leaves first.

Impregnation - Thorough mixing or spraying of a small amount of herbicide, fungicide, or other pesticide in a large amount of fertilizer. Normally done through bulk blending operations.  Onboard impregnation during application is growing in importance.

Inclusions - Also inside a plant cell are solid inclusions and solutions which may be either waste products or synthesized foods for the living protoplasm and vacuoles.  These inclusions may include starch grains, oil drops, protein bodies, and salt crystals.

Incorporation - Mechanical mixing of fertilizer materials with the surface soil.

Injection - Placement of fluid fertilizer or anhydrous ammonia in the soil either through use of pressure or nonpressure systems.
See Also: Deep Banding Fertilization; Double Shooting; Dual Placement; Knifed Application; Point Injection; Spoke Injection .

Inorganic - A compound or substance that is not carbon based.  When used to describe fertilizers, it includes the mineral forms, such as potassium nitrate, superphosphate, etc.  When used to describe growing media components, it includes mineral, synthetic, or non- biological forms, such as perlite, vermiculite, sand, etc. Substances occurring as minerals in nature or obtainable from them by chemical means.  Refers to all matter except the compounds of carbon, but includes carbonates.

Insoluble - Not soluble.  As applied to phosphoric acid in fertilizer, that portion of the total phosphoric acid which is soluble neither in water nor in neutral ammonium citrate.  As applied to potash and nitrogen, not soluble in water.

Integrated pest management (IPM) - The use of multiple approaches to control pest damage, including variety selection, economic considerations,  cultural control, sanitation, timing of planting, biological control, pesticides, etc.

Internal Structure of Leaves - Under a microscope, a cross section of a leaf shows three types of tissues: epidermis, mesophyll and veins.

Interveinal - Between the veins of the leaves.

Ion - An electrically charged particle.  As used in soils, an ion refers to an electrically charged element or combination of elements resulting from the breakage up of an electrolyte in solution.  Since most soil solutions are very dilute, many of the salts exist as ions.  For example, all or part of the potassium chloride (muriate of potash) in most soils exists as potassium ions and chloride ions.  The positively charged potassium ion is a cation, and the negatively charged chloride ion is an anion. Any atom or molecule that is electrically charged due to loss of electrons, which results in a positively charged cation; or the gain of electrons, which results in a negatively charged anion.

Ion exchange resin - A solid matrix or beads containing fixed positive and/or negative charges that remove ions from water by ion exchange.

Iron (Fe) - Iron (Fe) is an essential metallic micronutrient and is absorbed by plants as the ferrous (Fe⁺²) ion. Iron is a catalyst in chlorophyll formation and acts as an oxygen carrier.  It also helps form certain respiratory enzyme systems in the plant.  The nutrient is immobile in plants.  Iron deficiency shows up as a very light pale leaf color with veins remaining green, usually first appearing on younger leaves; but severe deficiency may result in the entire plant showing such symptoms.  Iron deficiency may be caused by an imbalance with other metals such as molybdenum (Mo), copper (Cu), or manganese (Mn).  Other factors contributing to iron deficiency include high availability of soil P, high soil pH, wet and cold soil conditions, low soil organic matter, and plant genetic differences.  Deficiencies of this element can be determined by soil analysis.

Kelp - Any of several species of seaweed sometimes harvested for use as a fertilizer.  Dried kelp will usually contain 1.6 to 3.3 percent N, 1 to 2 percent P₂O₅ and 15 to 20 percent K₂O.

Knifed Application - Process where fertilizer materials are banded into the soil with a slender knifing tool.
See Also: Deep Banding Fertilization; Double Shooting; Dual Placement; Injection.

Leaching - Removal of nutrients, salts, pesticides, or other water soluble compounds from growing media with water.  Irrigation and rain leaches compounds from soil/growing media, and propagation mist leaches compounds from cuttings. The removal of materials in solution by the passage of water through soil.  In agriculture, leaching refers to the downward movement of free water (percolation) out of the plant root zone.  It occurs when the amount of rainfall or irrigation water entering the soil becomes greater than its water holding capacity.  Most likely to occur on coarse textured soils.
Leaching of nutrients, particularly nitrate-nitrogen (NO₃^- ), can cause decreased nutrient use efficiency, lower yields, and environmental problems including nitrate accumulation in groundwater.  Best management practices minimize this type of nutrient loss, usually through multiple nutrient applications close to the time of nutrient need and through the use of nitrification inhibitors.  In some cases, leaching is intentionally practiced to remove accumulations of soluble salts from soils.

Leaching potential - A measure of a pesticide's tendency to move in solution with water and leach below the root zone into deep percolation in the soil.

Leaching requirement (LR) - The fraction of water applied during each irrigation that passes through the root zone and out the bottom of the container or soil profile.  An LR of 0.5 means 50% of the irrigation water would pass through the container or soil profile.  You can estimate the LR needed to maintain a certain soil salinity level by LR =Ecw/[5/(ECe-ECw)]; where ECw is the electrical conductivity of the irrigation water and ECe is the electrical conductivity of a saturated soil extract.

Leaching Requirement - The fraction of the water entering the soil that must pass through the root zone in order to prevent soil salinity from exceeding a specified value.  Leaching requirement is used primarily under steady-state or long-time average conditions.

Leaf Blades - Leaf blades are usually flat and thin to permit good penetration of light and CO2.  They vary in shape, size, venation, margins, etc., for the different plants.

Leaf Margins - There are three types of leaf margins; entire, toothed and lobed.  The entire margins are the smooth and unindented blade edges.  The toothed margins are indented as saw teeth.  These indentations may be small, large and variously shaped.  Lobed margins have the large indentation of irregular shape.  These indentations vary with the plant species.

Leaf Size - Leaves may vary in length from a fraction of an inch to over fifty feet.  Their width may range from a fraction of an inch to over 24 inches.

Leaf Shape - In shape, leaf blades vary from long narrow blades, such as grass blades, to round or circular forms. There are many variations between these extremes in shape.

Leaves - Leaves are the lateral outgrowth of buds which arise at nodes in the stems.  They develop from meristematic tissues as lateral protuberances of the bud tip.  These protuberances enlarge into leaves when they grow.  Most leaves are flat, however there are various modifications and specialized kinds of leaves.  Food manufacture is the important function of most leaves.  One or several leaves may develop at a node.  They can have several arrangements, such as alternate, opposite, spiral or whirled.  They may last for only one growing season (deciduous) or several seasons (evergreen).

Legumes - Plants of the family Leguminosae characterized botanically by fruit called a legume or pod that opens along two sutures when ripe.  Some samples of legumes are alfalfa, soybeans, peas, clovers, and vetches.
Legumes in symbiotic relationship with Rhizobium bacteria fix atmosphere nitrogen in the nodules on the plant's roots.  It has been estimated that from 100 to 300 pounds of nitrogen per acre may be fixed annually be a leguminous crops.

Leucoplast - The colorless food-producing plastids are called leucoplasts.  They collect sugars and transform them into starches.  They are abundant in roots, tubers and similar storage organs.

Lignosulfonate - A mixture of sulfonated lignins derived as byproducts of the pulp/paper process. Used as a chelating agent for iron, manganese, copper, and zinc.

Lime - Technically, calcium oxide (CaO).  Practically or agriculturally, it is any material containing the carbonates, oxides, and/or hydroxides of calcium and/or magnesium used to neutralize soil/media acidity.  The most common ones used are calcitic lime, dolomitic lime, dolomite, and hydrated lime.  Also called limestone.

Lime - Generally the term lime, or agricultural lime, is applied to ground limestone (calcium carbonate), hydrated lime (calcium hydroxide) or burned lime (calcium oxide), with or without mixtures ofmagnesium carbonate, magnesium hydroxide or magnesium oxide, and to materials such as basic slag, used a amendments to reduce the acidity of acid soils.  In strict chemical terminology, lime refers to calcium oxide (CaO), but by an extension of meaning it is now used for all limestone-derived materials applied to neutralize acid soils.

Lime Requirement -The amount of standard ground limestone required to bring a 6.6-inch layer of an acre (about 2 million pounds in mineral soils) of acid soil to some specific lesser degree of acidity, usually to slightly or very slightly acid.  In common practice, lime requirements are given in tons per acre of nearly pure limestone, ground finely enough so that all of it passes a 10-mesh screen and at least half of it passes a 100-mesh screen. Lime requirement is the amount of good quality agricultural limestone required to establish the desired soil pH range for the cropping system being used.  Lime requirements are determined in the laboratory using a buffer pH in equilibrium with the soil.  Lime requirement is NOT determined by the soil pH alone but is primarily by the soil's cation exchange capacity.  The specific requirement is usually expressed in terms of effective calcium carbonate equivalent (ECCE) which is based on the calcium carbonate equivalent content of the liming agent and its fineness of grind.  Smaller particles react faster in the soil.  Different liming materials have different neutralizing values depending upon their chemical composition.

Limestone - See lime .

Liquid Fertilizer -A fluid in which the plant nutrients are in true solution.

LISA - Acronym for Low Input Sustainable Agriculture.
Farming systems that economize on the use of manufactured inputs, notably fertilizers and pesticides, but also chemicals used to prevent livestock diseases or promote growth.  The emphasis is on minimizing the use of agricultural chemicals without drastically reducing crop yields or livestock production.

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 less than 52% sand.
See Also: Soil Texture .

Loam - The textural class name for soil having a moderate amount of sand, silt and clay.  Loam soils contain 7 to 27 percent clay, 28 to 50 percent silt, and less than 52 percent sand.  (In the old literature, especially English literature, the term loam applied to mellow soils rich in organic matter, regardless of the texture.  As used in the United States, the term refers only to the relative amounts of sand, silt and clay; loam soils may or may not be mellow.)

Longitudinal Section - As a root develops and elongates it develops four regions of cells, the root cap, meristematic region, elongation region and maturation region.

Low-Volume Irrigation -Irrigation system including drip, micro sprinklers, misters or foggers, or any system that is designed to apply water in or near the rooting zone in relatively precise amounts with respect to the plants needs.

Luxury Consumption-The uptake by a plant of an essential nutrient in amounts exceeding what is needs.  Thus if potassium is abundant in the soil, alfalfa may take in more than is required. A phenomenon brought about by the availability to plants of nutrients from any source which causes uptake of nutrients beyond those amounts required for normal growth and function.
The normal physiological function of plants may be upset by excessive accumulation of some nutrients, e.g., excessive nitrogen (N) may prolong the vegetative cycle and delay maturing or it may even prevent flowering, excessive phosphorus (P) may decrease plant uptake of zinc (Zn) and other micronutrients, and excessive potassium (K) may decrease plant uptake of magnesium (Mg).

Macronutrients - Nutrients that plants require in relatively large amounts; nitrogen (N), phosphorus (P), and potassium (K).
Magnesium is a constituent of chlorophyll and is actively involved in photosynthesis.  Much of the magnesium in plants is found in the chlorophyll.  Seeds are also relatively high in magnesium, though grain crops such as corn have low magnesium levels in the seed.  Magnesium aids in phosphate metabolism, plant utilization of sugars, and the activation of several enzyme systems.  Magnesium is mobile in the plant and deficiency symptoms first appear on the lower (older) leaves.  It appears first as a light, yellowish, faded discoloration with the veins remaining green.  In crops such as corn, the leaves are yellowish or very light green striped while veins remain green.  In some crops, as the deficiency progresses, a reddish-purple color develops with green veins.  Deficiencies can be determined by soil and plant analysis.

Macronutrient - One of the six essential elements required by plants in larger quantities (0.2 to 7%).  They are: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca),  magnesium (Mg), and sulfur (S).

Magnesium (Mg) - Magnesium (Mg) is an essential secondary nutrient classed with calcium and sulfur.  It exists in the soils and is absorbed by the plant as the Mg⁺² ion.  Its oxidation state in the plant does not change. Magnesium is a constituent of chlorophyll and is actively involved in photosynthesis.  Much of the magnesium in plants is found in the chlorophyll.  Seeds are also relatively high in magnesium, though grain crops such as corn have low magnesium levels in the seed.  Magnesium aids in phosphate metabolism, plant utilization of sugars, and the activation of several enzyme systems.  Magnesium is mobile in the plant and deficiency symptoms first appear on the lower (older) leaves.  It appears first as a light, yellowish, faded discoloration with the veins remaining green.  In crops such as corn, the leaves are yellowish or very light green striped while veins remain green.  In some crops, as the deficiency progresses, a reddish-purple color develops with green veins.  Deficiencies can be determined by soil and plant analysis.

Major element - See macronutrient .

Manganese (Mn) - Manganese (Mn) is a metallic micronutrient existing in the soil in several oxidation states of which the Mn⁺² ion is the form most commonly absorbed by plants. Manganese functions primarily as a part of enzyme systems in plants.  It activates several important metabolic reactions and plays a direct role in photosynthesis by aiding chlorophyll synthesis.  Manganese accelerates germination and maturity, while increasing the availability of phosphorus and calcium.  Because manganese is not translocated (mobile) in the plant, deficiency symptoms appear first on the younger leaves with yellowing between the veins - and sometimes brownish-black specks.  In small grains, grayish areas appear near the base of younger leaves.  Manganese deficiencies occur most often on high organic matter soils, and on those soils with neutral-to-alkaline pH and naturally low in manganese content.  Deficiency symptoms are most severe during cool spring months when soils are sometimes waterlogged.  On some soils, an extremely acid pH (5.0 or lower) may cause manganese toxicity to crops.  Deficiencies of this element can be determined by soil and plant analysis.

Manure - Generally, the refuse from stables and barnyards, including both animal excreta and straw or other litter.  In some other countries the term manure is used more broadly and includes both farmyard or animal manure and "chemical manures," for which the term fertilizer is nearly always used in the United States.

Marl - An earthy deposit, consisting mainly of calcium carbonate, commonly mixed with clay or other impurities.  It is formed chiefly at the margins of freshwater lakes.  It is commonly used for liming acid soils.

Matric tension - The negative suction pressure or tension developed in soil/growing media due to the attraction of water into capillary pores or adsorbed onto surfaces.  It is measured in Pascals (preferred method), bars, or atmospheres of pressure.  Also called moisture tension.  It can be related to water content by a moisture retention curve.

Maturation Region - The root section back of the elongation region is the maturation region. This section of roots is divided into the root hair zone and differentiation zone.

Meg - 1/1000 eq. (See eq .).

Meristematic Region - Back of the root cap is a mass of cubical thin-walled cells which contain much protoplasm.  These meristematic cells divide rapidly by mitosis (a cell wall develops across the center of a cell and splits the cell and nucleus into two cells) and then start rapid growth and elongation.

Meristematic Tissues - Embryonic or meristematic tissues are made up of small, thin-walled and frequently cubical shaped cells which are densely packed with protoplasm.  These cells divide rapidly into new cells and provide the elongation of roots and stems.  Thus, meristematic tissues are found at the tips of roots and stems, between food and water conducting tissues, and in various other growth zones in the plant.  Their development among permanent tissues increases the diameter of the roots and stem or cross sectional growth.  On maturity, much of meristematic tissues change to permanent tissue.

Mesophyll - In the central portion of the leaf is the mesophyll tissue.  It is a layer or two of palisade tissue over a spongy tissue.  All of these palisade tissues are high in their chlorophyll content as they are the main food manufacturing parts of the leaf and plant.  These mesophyll tissues contain xylem and phloem tissues to bring in the plant nutrients and carry away the synthesized plant materials.  Also there are large intercellular spaces to conduct and diffuse gases such as CO₂ and O₂ to these manufacturing cells.

Micronutrient - One of the eight essential elements required by plants in smaller quantities (1 to 600 ppm); iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), boron (B), molybdenum (Mo), chlorine (Cl), and possibly nickel (Ni).

Micronutrient mix - A commercial fertilizer that supplies more than one of the micronutrients.

Milliequivalent (me) - One-thousandth of an equivalent; see equivalent .  Used to express nutrient and salt concentrations in growing media, soil, and plant  analyses. One-thousandth of an equivalent.  In the case of sodium chloride, 1 me would be 0.023 gram of sodium and 0.0355 gram of chloride in 1 liter of water.

Milligram per liter (mg/1) - One-thousandth of a gram per liter.  The metric equivalent to parts per million (ppm).

Millimhos per centimeter (mmhos/cm) - One-thousandth of a mhos per centimeter.  The unit most commonly used to express electrical conductivity in the past or on older conductivity meters.  The current trend is to use deciSiemen/meter (dS/m), which is numerically equal to mmhos/cm.

Minerals - Substances from mineral contents of earth's crust or soil (minerals are inorganic, mostly crystalline substances).

Mobile - A term used to describe the translocation of a nutrient in plants where the nutrient can travel up the stem and into organs via the xylem and also can be loaded into the phloem to travel out of organs or down stems.  Deficiencies of mobile nutrients always occur on the old leaves first.

Mobility - Term that describes whether a nutrient is mobile or immobile.

Model-based irrigation control - An automated irrigation system that is turned on and off based on mathematical models that predict water loss.  The model takes into account environmental and plant factors.

Moisture content - See matric tension .

Moisture Retention -The ability of a soil/media to retain moisture.  Retentiveness depends upon the type and percentage of materials contained in the soil/media.  Generally expressed as a percentage. The availability of a soil or growth medium to retain moisture.  Moisture retention is dependent upon soil composition of sand, silt and clay, organic matter content, and soil structure.

Moisture tension - The tension, or more precisely the negative pressure, under which water is held in soils/growing media and plants.  It is measured in bars, Pascals, or atmospheres of pressure.

Mole - Amount of (chemically homogeneous) substance, containing as many grammes as the numerical value of the molecular weight, e.g., for KCl (potassium chloride), 1 mol =  74.6 g, for MgSO₄ (magnesium sulfate), 1 mol=120 g.

Molar solution - Solution containing 1 mol per liter (unit designation: M).

Molecules - Atoms bound to one another.

Molecular weight - Weight of one molecule (sum of atomic weights).

Molybdenum (Mo) - Molybdenum (Mo) is a metallic micronutrient that is absorbed as a molybdate anion (MoO₄^-2 ).  Plants require molybdenum in the smallest quantities of all the essential elements. Molybdenum is required for the synthesis and activity of the enzyme nitrate reductase.  This enzyme system reduces nitrate-nitrogen to ammonium-nitrogen in the plant.  Molybdenum is also vital for the process of symbiotic nitrogen fixation by Rhizobia bacteria in legume root nodules.  Molybdenum deficiency symptoms show up as a general yellowing and stunting of the plant.  A deficiency can cause nitrogen deficiency symptoms in legume crops because symbiotic soil bacteria must have molybdenum to fix nitrogen from the atmosphere.  Molybdenum availability goes up as soil pH rises, the opposite of most other micronutrients.  Deficiencies are more likely to occur on acid soils.

Muck - Highly decomposed organic soil material developed from peat.  Generally, muck has a higher mineral or ash content than peat and is decomposed to the point that the original plant parts cannot be identified.

Mulch - A material applied to the ground to prevent excessive drying of the soil surface, to prevent rapid changes in soil temperature, as a soil amendment, for decorative purposes or to prevent weed growth. A material applied to or left on the soil surface to slow water loss by evaporation, to prevent rapid changes in soil temperature, to prevent erosion, to suppress weed growth, to add organic matter to the soil, and for decorative purposes. The main value of mulches is to reduce loss of water through evaporation, help control weeds, and reduce soil erosion.  Mulches also help moisture penetration into the soil and help maintain a more even temperature in the soil. Mulching materials, besides crop residues, include sawdust, leaves, grass clippings, compost, etc., as well as paper and plastic specially prepared for mulching purposes.

Mulch-Till - see Conservation Tillage

Muriate of Potash-Potassium chloride.

Nature of Roots - When a seed germinates, its embryo sends out a cylindrical tube that is a mass of meristematic cell.  This tube grows both downward and upward.  The downward growing portion is called the radicle or first root.

Nature of Stems - Most stems are classified as aerial or grow above the soil.  There are some stems, which grow in the soil or at the surface and are called subterranean shoots.  There are several types of these two classes of stems.

Necrosis - Brown, scorched, or dead areas on plant organs.  Necrosis is a common symptom of many nutrient deficiencies.

Nephelometer - An instrument using diffused light to determine the turbidity or clarity of water. Sometimes referred to as a turbidimeter.

Nephelometric turbidity units (NTU) - A standard unit of measure of the turbidity or clarity of water based on scattering of light by suspended particles of colloids, silica, bacteria, or mineral precipitates.  NTU is measured with a nephelometer using formazin as a reference standard.

Nesting - Pockets or very large granules of solid fertilizers placed below the soil surface. Research has indicated superior performance for subsurface placement of large urea granules or briquettes for rice.  Nesting or deep placement of urea increase efficiency in lowland rice by minimizing losses due to ammonia volatilization and microbial oxidation.

Net Venations - In the net form of venations, the veins branch out many ways and form a network of veins in the leaf.  The net venation may have only midrib or main vein from which the smaller veins originate (pinnate).  Also there may be several main ribs from which the smaller veins branch out into the blades (palmate).

Neutral fertilizers - A fertilizer that does not significantly alter pH when applied to soil/growing  media.

Neutral Soil - A soil with a high percentage (80% to 90%) of the exchange capacity occupied by calcium and magnesium ions and a soil pH near 7.0. There are 5 to 10 times as many calcium ions as magnesium ions on the exchange sites.  Potassium ions occupy about 2% to 5% of the capacity and hydrogen ions the remainder.  These ranges are well established but critical values are not firm for all soils.  The theoretical pH is 7; practically it is 6.8 to 7.3.

Neutralization - Combination of acid and base to form a (neutral) salt.

Nitrate - An inorganic nitrogen fertilizer (NO₃^-).  The form of nitrogen found in most soils/growing media.

Nitrification - The formation of nitrates and nitrites from ammonia (or ammonium compounds), as in soils by microorganisms. A two-step process of converting ammonium to nitrate by bacteria in soil/growing media.  In the first step, ammonium (NH₄⁺) is converted to nitrate (NO₂^-) by the bacterium Nitrosomonas, and in the second step nitrite (NO₂^-) is converted to  nitrate (NO₃^-) by the bacterium Nitrobacter. The formation in soils of nitrites and nitrates from ammonium ions through the activities of certain soil bacteria. Nitrification is a two-step biological process in which the ammonium ion (NH₄⁺) is first converted to nitrite (NO₂^-) by bacteria of the genus Nitrosomonas, and then the nitrite is converted to nitrate (NO₃^-) by bacteria of the genus Nitrobacter.  The process is the same regardless of where the ammonium ion originates, whether from soil organic matter, legumes, wastes, or ammonium containing fertilizers. Nitrification is most rapid in warm, moist and approximately neutral pH soils.  The process halts entirely when the temperature drops to about 37° F.
In an effort to reduce nitrification of fall applied ammonium producing fertilizers and thus reduce the possibilities of nitrate losses by leaching, many state, provincial, and commercial laboratories recommend delaying N application until the soil temperature reaches 45° F or 50° F.  Nitrification inhibitors are now available to slow the rate of nitrification.  These inhibit the growth or the activity of the nitrification bacteria but are eventually decomposed and nitrification continues.
See Also:  Nitrification Inhibitor, Nitrobacter, Nitrogen Cycle, Nitrosomonas.

Nitrogen Fixation - Generally, the conversion of free nitrogen to nitrogen compounds.  Specifically in soils, the assimilation of free nitrogen from the soil air by soil organisms and the formation of nitrogen compounds that eventually become available to plants.  The nitrogen-fixing organisms associated with legumes are called symbiotic; those not definitely associated with the higher plants are non-symbiotic or free-living.

Nitrification Inhibitor - Compounds such as 2-chloro-6 (trichloromethyl) pyridine (nitrapyrin) and dicyandiamide that delay bacterial oxidation of the ammonium ion (NH₄⁺) to nitrite (NO₂^-) and thus slow production of nitrate (NO₃^-).  The objective of use of these compounds is to control leaching of nitrate by keeping nitrogen in the ammonium form longer to prevent denitrification of nitrate-nitrogen and to provide ammonium-N to plants over a longer period of time.

Nitrobacter - A genus of obligate aerobic chemoautotrophic soil bacteria which oxidizes nitrate ions to nitrate in the final stage of the nitrification process. Nitrobacter and Nitrosomonas are the major soil nitrifying chemoautotrophic bacteria and are essential for the production of the main form of nitrogen absorbed by plants - nitrate.  Numbers of both organisms are stimulated by the addition of ammonium ions to the system.  High concentrations of ammonium ions, however, can inhibit the activity of Nitrobacter.  Oxidation of nitrite ions to nitrate by Nitrobacter yields energy for the organism.  Rate of conversion of nitrite to nitrate is affected by soil temperature, soil pH, and the amount of molecular oxygen in the soil.  The specific reaction mediated by Nitrobacter is:
   NO₂^-      +      ½O₂      == Nitrobacter ==>       NO₃^-
   Nitrite           Oxygen                                         Nitrate
See Also: Nitrification; Nitrosomonas .

Nitrogen (N) - Nitrogen (N) is an essential nutrient, a constituent of every living cell, plant or animal.  In plants it is a part of the chlorophyll molecule, amino acids, proteins, and many other compounds. Large amounts of nitrogen are needed by all growing crops.  Nitrogen is necessary for chlorophyll synthesis and, as a part of the chlorophyll molecule, is involved in photosynthesis.  Lack of adequate N and chlorophyll diminishes plant utilization of sunlight as an energy source, decreases production of carbohydrates, and limits essential functions such as nutrient uptake and protein synthesis.  As a result, growth is stunted, yields are reduced, and crop quality is impaired. Nitrogen deficiency produces a yellowing of leaves (chlorosis) beginning at the bottom of the plant.  Nitrogen is mobile and is moved from older leaves to new growth areas under deficient conditions.  As the deficiency becomes more severe, chlorosis can extend over the entire plant.  As a result, plants are stunted, water use efficiency is diminished, and yields decline.
See Also:  Ammonification; Denitrification; Nitrification.

Nitrogen Cycle - The routes taken by nitrogen from the atmosphere through soils, plants, animals, and man, back to the atmosphere.  Nitrogen can take three routes from the atmosphere to the soil:
1.  Nitrogen oxides and nitric acid produced by lighting plus ammonia and ammonium compounds in the air are carried to the soil by rainfall;
2.  Rhizobia bacteria in the root nodules of legumes fix nitrogen from the atmosphere, and some forms of algae are also capable of fixing atmospheric nitrogen and;
3.  Commercial fixation of atmospheric nitrogen and application of nitrogen fertilizers.
Bacterial fixation (Rhizobia) furnishes a direct route for nitrogen from the atmosphere to reach legume plants.  Nitrogen reaching the soil by either of the other two routes first enters the soil solution in the form of ammonium or nitrate ions, which are then assimilated by plants through their root systems.
Organic matter in the soil usually contains 5% to 6% nitrogen, largely in the form of protein, that is unavailable to crops until it has undergone mineralization by the action of bacteria, actinomycetes, and fungi.  Mineralization (or ammonification) produces ammonium ions which are either converted to nitrate by soil bacteria or directly absorbed by higher plants.
Fertile soils having adequate supplies of decaying organic matter, a carbon:nitrogen ratio of 30 or greater, and a high microbial population are capable of storing enormous quantities of nutrients for future plant use.  This is possible because of microbial organisms that consume nitrogen in the ammonium form.  In soils with a low organic matter content, a carbon:nitrogen ratio of less than 15, and a quantity of ammonium ions in excess of that which can be absorbed by either the plant or microbes, the excess ammonium nitrogen is oxidized by nitrification, first to nitrate by Nitrobacter.  In soils containing oxidizable organic matter and inadequate oxygen, nitrate may be reduced to nitrogen gas by microbes that use the nitrogen of the nitrate ion in place of their need for oxygen.  The nitrogen eventually finds its way back to the atmosphere. On permeable soils having low organic matter content, nitrates may be moved into drainage water, especially under conditions of high rainfall.

Nitrogen Fixation - The conversion of elemental atmospheric nitrogen (N₂) into organic or inorganic forms.  Specifically in soils, fixation refers to the assimilation of N₂ from the soil air by soil organisms and the formation of nitrogen compounds that are available to plants.  The nitrogen-fixing process associated with legume nodules-roots is known as symbiotic nitrogen fixation; nitrogen fixation by soil organisms not associated with higher plants is termed non-symbiotic.  Symbiotic nitrogen fixation is many hundreds of times greater than non-symbiotic processes.

Nitrogen Management - Nitrogen management implies use of controllable factors to maximize nitrogen use efficiency, maximize return to investment in N from whatever source, and minimize impacts of N use on the environment.
Several nitrogen management factors are listed in the discussion of nitrogen use efficiency.  Commercial sources of N are easier to manipulate in terms of release of N for crop use and control of nitrate leaching than are organic N sources such as legumes, animal wastes, and sewage sludge. Mineralization of organic N sources by biological reactions is difficult to predict both in terms of amount of N release and time of N release.

Nitrogen Stabilizer - A compound that slows the process of nitrogen oxidation in soil by inhibiting the Nitrosomonas bacteria, the organisms which oxidizes ammonium nitrogen to nitrite.  Consequently, production of nitrate is also restricted, and leaching of nitrate and denitrification are diminished.
See Nitrification Inhibitor: Nitrosomonas.

Nitrogen Use Efficiency - Nitrogen use efficiency, as well as use efficiency of other essential nutrients, is an expression of the units of yield per unit of nutrient provided for the crop.  Common expressions include bushels of grain per pound of applied N - pounds of yield per pound of applied N. Increasing nutrient use efficiency is the goal of best management practices for all types of crop production.  Higher yields per unit of applied N, for example, help cut production costs per unit of production and increase crop profitability.  The objective of higher use efficiency and higher yields is reducing the amount of carryover nitrate-nitrogen which might leach into groundwater.  Improved nitrogen use efficiency should also be targeted toward organic N sources, the most difficult sources of N to manage. Nitrate (nutrient) use efficiency can be improved through matching of application rates to yield goal, proper timing of applications, proper placement for particular tillage systems, matching hybrid/variety nutrient demand with nutrient availability, use of nitrification inhibitors, availability of adequate amounts of other nutrients (particularly phosphorus, potassium, and sulfur), control of soil acidity, and many other factors.
See Also:  Fertilizer Use Efficiency.

Nitrosomonas - A genus of obligate aerobic chemoautotrophic soil bacteria which oxidize ammonium ions to nitrite in the first stage of the nitrification process.
The specific process mediated by Nitrosomonas is:
 NH₄⁺         +         1½O          ==Nitrosomonas ==>        NO₂^-         +         2H⁺         +         H₂O
ammonium             oxygen                                                  nitrite                hydrogen               water
    ions
Nitrogen inhibitors such as nitrapyrin specifically inhibit the activity of these organisms.  The reaction listed above is the acidifying reaction resulting from the nitrification process.  Source of the ammonium ion has nothing to do with the reaction; the results are identical, regardless of the source of the ammonium.
See Also: Nitrification; Nitrobacter.

Nonsaline-Sodic Soil -A soil, which contains sufficient exchangeable sodium to interfere with the growth of most plants but does not contain appreciable quantities of soluble salts.  The exchangeable sodium percentage is greater than 15, the conductivity of the saturation extract is less than 4 decisiemens per meter (at 25°C) and the pH of the saturated soil usually ranges between 8.5 and 10.0.

Normal solution - Solution containing 1 equivalent (eq.) of a substance per liter water (unit designation: N)

No-Till - see Conservation Tillage

Nucleus - A plant nucleus has thread-like bodies called chromosomes which gives the inheritance and hereditary traits to the cells.  Surrounding nucleus and inside its membrane is a clear liquid nuclear sap.  The nucleus controls the activity and life of the living cell.

Number of Leaves - Some plants have only one or a single leaf per petiole.  Other plants have several leaves on a main petiole.  These multi-leaves are usually called leaflets.

Nutricote - Trade name for controlled release fertilizer (usually containing N, P, and K) composed of a pellet of soluble fertilizer surrounded by a semipermeable coating.

Nutrient - A term for the essential elements.

Nutrient film technique (NFT) - A subirrigation growing system.  Plants are rooted into porous cubes or containers and placed in sloped troughs where a  nutrient solution continuously flows down the troughs in a thin film to supply water and nutrients.  This technique is used most in greenhouse vegetable production.

Nutrient Management - The utilization of best management practices (BMPs) which maximize nutrient use efficiency and minimize nutrient losses to surface or groundwater. Nutrient management is affected by a wide range of crop management practices and methods of application for supplemental nutrients.  Higher nutrient use efficiency can be achieved through matching application rates to yield goals, timing of nutrient applications, proper fertilizer placement for particular tillage and cropping systems, use of nitrification inhibitors, specific phosphorus placement, matching hybrid/variety nutrient demands with nutrient availability, and the provision of adequate amounts of other nutrients. See Also: Fertilizer Use Efficiency; Nitrogen Use Efficiency .

Nutrient, Plant -Any element taken in by a plant which is essential to its growth and which is used by the plant in elaboration of its food and tissue.

Nutrient Solution-See Hoagland Solution .

Nutrient Uptake - The process of plant absorption of nutrients, usually through the roots.  Small amounts of nutrients may be absorbed through the leaves following foliar application of nutrients.  Root nutrient uptake is affected by expendable energy supplies in the plant, soil temperature, soil aeration, soil moisture, soil structure, soil pH, concentrations of nutrients in the soil, interactions of various nutrients, plant rooting patterns, and a wide range of other plant physiological and morphological factors. Nutrient uptake is obviously essential for plant growth.  Efficient utilization of available plant nutrients depends on many factors listed above, some of which are manageable and have to be taken into account in trying to design a system with highest ability to promote plant growth.  Tillage systems, amount of crop residues in and on the soil, soil compaction, rainfall, irrigation water quality, method of fertilizer application, and hybrid/variety nutrient demands are some of the factors that have to be considered.

Nutritional charting - Conducting tissue analysis on a routine basis (for example, weekly) and plotting the results on graph paper to monitor how the level changes over the life of the crop.

Organic - A compound composed of a carbon backbone.  Usually used to describe something of biological origin. When used to describe fertilizers, it includes those of synthetic origin (for example, urea) or natural plant or animal products (manure, lignosulfonates).  When used to describe growing medium components, it applies to components usually of plant origin, such as peat moss, composted bark, etc.

Organic -Compounds of carbon other than the inorganic carbonates.

Organic Farming - A system of farming or home gardening which utilizes organic wastes and composts with minimal or no use of commercial fertilizers and pesticides.

Organic Matter - An important soil component composed of carbonaceous material of plant or animal origin containing essential plant nutrients, providing adhesives for soil particles, and characterized by a high cation exchange capacity and absorptive capacity for water. Organic matter is an important part of the soil's ability to maintain structure, retain air, water and nutrients, and stimulate microbial activity.  Loss of organic matter leads to low fertility, low water holding capacity, compaction, increased erosion, and diminished productive capacity.
See Also: Humus .

Organic Soil -A general term applied to a soil or to a soil horizon that consists primarily of organic matter, such as peat soils, muck soils and peaty soil layers.

Ornamental Horticulture -The branch of horticulture specializing in the areas of floriculture, turfgrass management, nursery stock production and landscaping.

Orthophosphate -A salt of orthophosphoric acid such as ammonium, calcium or potassium phosphate.  Each molecule contains a single atom of phosphorus.

Orthophosphoric Acid-H₃PO₄ .

Osmocote - A trade name for controlled release fertilizer (usually containing N, P, and K) composed of pellet of soluble fertilizer surrounded by a semipermeable plastic polymer coating.  The rate of release is several months to over a year, depending on the coating.

Other Tissues - There are many other plant tissues which are composed of various combinations of these simple and complex tissues.  Some of these other tissues are the cortex, stelar and cambium tissues.  Each of these tissues has its special function.

Overhead irrigation system - An irrigation system that sprays water into the air above plants.

Oxide - Compound of an element and oxygen, e.g., CaO (calcium oxide).

Oxidation - Combination of element with oxygen or increase of its valence (loss of electrons),   e.g., Ca (calcium) + O (oxygen)  »  CaO (calcium oxide): Fe²⁺ (bivalent iron ion)   »   Fe³⁺ (trivalent iron ion) + 1 electron.

Ozonization - To treat with ozone as a disinfectant.  Used in recycled water treatment.

Parallel Venations - The veins running parallel to each other from the base of the blade to the tip, such as grasses, corn, lilies, bananas, etc., is parallel venation.  In some of these parallel designs the veins run parallel to each other at a slight angle from the midrib.

Parenchyma - A common tissue found throughout the plant, and especially around the conducting tissues, is the parenchyma tissue.  These tissues are thin-walled cells which may be spherical, cylindrical or ovoid in shape and contain large vacuoles.  Sometimes these cells contain chloroplasts and manufacture food.  The majority of these parenchyma tissues are composed of non-green cells which function chiefly for food and water storage.

Parent Material -The unconsolidated mass of rock material (or peat) from which the soil profile develops.

Particle Density -The average density of the soil particles.  Particle density is usually expressed in grams per cubic centimeter and is sometimes referred to as real density or grain density.

Parts Per Million (ppm) -A notation for indicating small amounts of materials.  The expression gives the number of units by weight of the substance per million weight units of another substance, such as oven-dry soil.  The term may be used to express the number of weight units of a substance per million weight units of a solution.  The approximate weight of soil is 2 million pounds per acre-6 inches.  Therefore, ppm X 2 equals pounds per acre-6 inches of soil, or ppm X 4 equals pounds per acre-foot of soil.

Parts per million (ppm) - Unit of concentration used to describe substance concentration in a million; may be expressed on a weight, volume, or number basis.  The metric equivalent is mg/l.

Pascal - A unit used to express pressure or tension (negative pressure); for example, matric or moisture tension in soil/growing media.  In soil, matric or moisture tension is usually expressed as megaPascals (Mpa, i.e., 1 million Pa).  For conversion, 1 megaPascal = 10 bar = 9.87 atmospheres = 7500 mm Hg = 145 psi of pressure.  In soilless growing media,   moisture tension is usually expressed as kiloPascals (Kpa, i.e., 1 thousand Pa).  For conversion, 1 kiloPascal = 0.01 bar = 0.00987 atm = 7.5 mm Hg = 0.145 psi.

Peat-The AAPFCO has adopted as official the following : "Peat is partly decayed vegetable matter of natural occurrence.  It is composed chiefly of organic matter that contains some nitrogen of low activity."

Peat - An organic growing media component composed of partially decomposed mosses, reeds, and sedges from wet, swampy areas.  The most commonly used and highest quality peat is Sphagnum peat from northern bogs (Canada, northern Europe), but hypnum and reed-sedge peat is also used.

Peat moss - See peat .

Peat-lite mix - A common name used to describe soilless growing media mixes containing peat and either perlite or vermiculite.  The original peat-lite mixes were developed by Boodley and Sheldrake at Cornell University in the 1970's.

Perched water table - A layer of growing media in the bottom of a container that remains saturated with water after irrigation and all drainage from the force of gravity has occurred, i.e., a saturated layer in the bottom of the container at container capacity.

Percolation - The downward movement of water through soil.

Perennial Plant - A plant that lives for more than two years.

Pericycle Section - Inside the second layer are the endodermis and pericycle tissues. This pericycle section is usually only a one cell layer.  It is the tissue in which branch roots originate.

Perlite - An inorganic growing-media, component made from alumino silicate volcanic rock that is mined, crushed, screened to size, then is heated to 1,800°F (3215°C), which causes it to expand into a light-weight, white aggregate.

Permanent Tissues - Permanent tissues do onto change into other kinds of tissue with maturity.  Such tissues are classified into two groups, namely simple and complex.

Permanent Wilting Percentage - The moisture percentage of soil at which plants wilt and fail to recover turgidity (15 atmospheres).  It is usually determined with dwarf sunflowers.  The expression has significance only for non saline soils.

Permeability Soil - The quality of a soil horizon that enables water or air to move through it.  It can be measured quantitatively in terms of rate of flow of water through a unit cross section in unit time under specified temperature and hydraulic conditions.  Values for saturated soils usually are called hydraulic conductivity.  The permeability of a soil is controlled by the least permeable horizon even though the others are permeable.

Petals - Inside the sepals, there is another layer of leaf-like structures which are called the petals.  These petals are often highly colored to attract insects for pollination.  Their number is usually the same as the sepals.  These petals often secrete aromatic substances and nectar (sugar solution) to aid in attracting insects.  The group of petals for a flower is called the corolla.

Petioles - The petiole is usually a short and narrow cylindrical organ, which contains vascular bundles or conducting tissues that connect to the xylem and phloem of the stem and to the veins of the leaf.  Some petioles are in a sheath form around the stem such as grasses.  Some leaves have no petioles and the blade connects to the stem tissues (sessile leaves), as a snake root plant.  Petioles permit the leaves to orient their position for better utilization of the sunlight.

Petiole Analysis - A method of determining adequacy of nutrient levels in plants by chemical analysis of the petiole portion of the leaf structure.
Petiole analysis has been correlated to the levels of plant nutrients required for optimum plant growth and yield.  Petiole analysis is not used on all crops because of differences in plant structure.  It is particularly useful in long season crops or perennial crops such as trees and vines where adequate time allows supplemental nutrients to be supplied when needed.
See Also:  Plant Sample; Tissue Analysis.

pH - A numerical designation of acidity and alkalinity.  Technically, pH is the common logarithm of the reciprocal of the hydrogen ion concentration of a solution.  A pH of 7.0 indicates precise neutrality; values between 7 and 14 indicate increasing alkalinity, and values between 7 and 0 indicate increasing acidity. Soil pH is used as an initial diagnostic tool in establishing optimum growing condition for crops.  Low soil pH (acid soil) is an indication of a need for liming to diminish acidity and enhance nutrient availability but is not exactly correlated with the amount of lime needed to correct the problem.  Soil testing is the key to correction of soil acidity problems.  Soil alkalinity is much more difficult to change.  Unless soil analysis show high concentration of sodium associated with alkaline soil conditions, attempting to decrease alkaline soil pH is unnecessary and probably uneconomical. pH - (latin: potentia hydrogenii) negative logarithm to the base 10 of the hydrogen-ion  concentration in eq./ liter.  For hydrogen ions (H⁺) 1 eq. - 1.g.  pH -A numerical designation of acidity and alkalinity as in soils and other biological systems.  Technically, pH is the common logarithm of the reciprocal of the hydrogen ion concentration of a solution.  A pH of 7.0 indicates precise neutrality; higher values indicate increasing alkalinity, and lower values indicate increasing acidity.

Phloem - The downward conducting tissues in the leaves, stems and roots are called phloem.  Such tissues are composed of sieve tubes, companion, fiber and parenchyma cells.  The sieve tubes are the important conducting cells of the phloem.  They are composed of vertically elongated rows of cylindrical cells, which have their end walls perforated.  When they are living, these cells contain cytoplasm.  The sieve cells are surrounded with companion cells which strengthen and help the sieve cells in their conduction of synthesized foods.  The fiber cells are long thick-walled strengthening cells.  The parenchyma cells of the phloem tissue are storage cells.

Phosphate -A salt of phosphoric acid made by combining phosphoric acid with ions such as ammonium, calcium, potassium or sodium.

Phosphate Rock -Phosphate-bearing ore composed largely of tricalcium phosphate.   Phosphate rock can be treated with strong acids or heat to make available forms of phosphate.  Finely ground rock phosphate is sometimes used in long-time fertility programs.

Phosphoric Acid-A term that refers to the phosphorus content of a fertilizer, expressed as phosphoric acid (P₂O₅).  The AAPFCO has adopted as official the following : "The term phosphoric acid designates P₂O₅."  Phosphoric acid also refers to the acid H₃PO₄ .

Phosphorus (P) - Phosphorus (P) is one of the 17 essential nutrients required by plants and classified as one of the three macronutrients (nitrogen, phosphorus, potassium).  Phosphorus is present in soils in organic forms and as iron, manganese, aluminum and calcium phosphates. Phosphorus is taken up by plant from the soil solution as the primary orthophosphate ion (H₂PO₄^-), in smaller amounts as the secondary orthophosphate ion (HPO₄⁼ ).  Soil pH greatly influences the ratio of these two ions.  Only very small quantities of phosphorus are present in the soil solution, however, and these quantities must be continually replenished from other forms of soil phosphorus.  Many soils are deficient in phosphorus and these deficiencies can be determined by soil analysis.  Phosphorus, a mobile plant nutrient, plays key roles in photosynthesis respiration (utilization of sugars), energy storage and transfer, cell division, cell enlargement, genetic coding, and many other plant processes.  Adequate phosphorus promotes early root formation and growth improves fruit, vegetable, and grain yield and quality, hastens plant maturity, increases resistance to winter kill, improves use efficiency of other nutrients such as nitrogen (N), and increases water use efficiency.  Deficiency symptoms develop early in plant growth.  Plants are stunted and may develop purple coloring due to the accumulation of sugars in the plant.  Yields are severely diminished.

Photosynthesis -  This term is defined as the manufacture of carbohydrates, from carbon dioxide and water with the aid of energy from the sunlight through the mechanism of chlorophyll.   The process by which green plants capture light energy by combining water and carbon dioxide to form carbohydrates.  The pigment chlorophyll is required for the conservation of light energy into chemical energy. The process by which green plants combine water and carbon dioxide to form carbohydrates under the action of light.  Chlorophyll is required for the conversion of light energy into chemical energy.

Pistil - In the center of the open flower structure is the pistil.  There may be only one pistil (simple) or several closely packed pistil stalks (compound).  A pistil consists of a slender stalk (style) with a slightly enlarged opening at its top (stigma) and a hollow swollen base (ovary).  Inside the ovary are division (ovules) which develop into seeds. The essential parts of the flower are the stamens and pistils.  The sepals and petals are accessory parts.

Plant Cells - All living tissues have units of structural development.  These units are miniature box-like compartments called cells.

Plants - There are about 350,000 known species of plants.  These species vary in their structure, habits of growth, size methods of reproduction, rate of growth, longevity, physiological activities and environmental preferences.  As a result, they may be classified in many ways.  Plants are usually classified into simple (Thallophyta) and complex (Embryophyta) groups with several subdivisions in each of these two groups.

Plants - Plants are living organisms which have their own characteristic features, even to likes and dislikes.  They may be a single celled organism or a complex structure of trillions of cells. Plants have the power of assimilating non-living substance (plant nutrients) into living substances (protoplasm).  Also they have the power of reproduction.  They can break up (digest) organic compounds and use this process to release energy for metabolism.  Most useful agricultural plants have the ability to adjust to changes in environment. Most plants are able to synthesize their food from the raw materials which they absorb from the soil and air. Also most plants contain a green pigment, chlorophyll.  Their structural frames are generally some form of cellulose.  Most plants are stationary or move very little.  In their plan of growth they are greatly affected by the availability of their food and their environment.

Plant Sample - A representative sample of plant tissue utilized for chemical analysis to determine plant nutrient status.   Specific instructions have been developed for collection of tissue samples from various crops.

Plant Tissues - A group of plant cells which have similar sizes, structures, and functions is called a tissue.  The various kinds of plant tissues are classified on the basis of their structural and physiological differences, such as meristematic and permanent tissues.

Plastids - Inside the cytoplasmic fluid are larger bodies of ovoid and spherical mass called plastids, which may be colorless or have various colors.

Plowdown Fertilizer - Fertilizer applied to the soil surface prior to tillage with a moldboard plow. Broadcast applications of nutrients, particularly phosphorus (P) and potassium (K), are often incorporated into the soil by moldboard plowing or some other type of deep tillage operation.  Nutrient applications prior to plowing or deep cultivation can be incorporated more deeply into the soil to build soil nutrient availability levels (soil tests) and improve subsoil nutrient availability.

Point Injection - Use of a spiked wheel to inject fluid fertilizer into the rooting zone (4 to 5 inches) at points about 8 inches apart. Where the potential exists for positional unavailability in dry surface soils or immobilization of nutrients in high residue soils, point injection of liquid fertilizers can increase nutrient use efficiency.  Under dryland conditions in the northern Great Plains and Prairies, postemergent point injection has been shown to improve winter wheat yields and substantially increase the efficiency of nitrogen fertilizer.  Nitrogen use efficiency can be increased by point injection directly into the rooting zone just prior to the period of maximum assimilation by the crop.  Advantages to point injection have also been reported under rain fed and irrigated conditions for small grains and row crops.
See Also: Spoke Injection.

Point-source pollution - Pollution that is generated from a discrete, relative small area, such as a greenhouse range or nursery.

Polyamide-type membranes - A membrane composed of polyamide polymers used in reverse osmosis water purification systems.  They are resistant to pH but sensitive to chlorine.

Polyphosphate - A polymer of orthophosphates that is used as a phosphorus fertilizer or as a weak chelating agent of the heavy metal micronutrients (Fe, Zn, Cu, Mn). A salt of polyphosphoric acid such as ammonium, calcium or potassium polyphosphate.  Poly means "many" and refers to multiple linkages of phosphorus in each molecule.

Polyphosphoric Acid-Condensed phosphoric acid ranging in P₂O₅ content from 68 to 83 percent.

Polystyrene - An organic polymer that is used to make styrofoam.

Pop-Up Fertilizer - Fertilizer applied at planting in direct seed contact. A form of starter fertilizer. Pop-up fertilization rates are necessarily low to avoid seeding damage from soluble salts.  Pop-up formulation should involve only very small amounts of urea to avoid ammonia damage to germinating seedlings.
See Also: Starter Fertilizer .

Porometer - An instrument developed by the North Carolina State University Horticultural Substrates Laboratory to determine the physical of soil/growing media.

Porosity -The fraction of soil volume not occupied by soil particles.

Postplant Fertilizer - A fertilizer application made to the plant/growing media after sowing seed or transplanting. Fertilizer applied after planting without specific references to method of application.  Side-dressing, top-dressing, and weed-and-feed applications are forms of postplant fertilization.

Potash-The AAPFCO has adopted as official the following : "The term potash designates potassium oxide (K₂ O)."

Potassium (K) - Potassium (K) is one of the three macronutrients, nitrogen, phosphorus, and potassium.  It is required by most plants in approximately the same amounts as nitrogen. About 90% of potassium is soils is present in unavailable forms in primary silicate minerals.  Another 2 % to 10% is held in slowly available forms between the sheets of clay minerals.  Available potassium, 1% to 2% of the total, is held on the surface of soil clay and organic matter colloids and in soil solution.  Many soils are potassium deficient, which can be detected by soil analysis.
Movement of K⁺ ions to plant roots is primarily by diffusion through soil water.  Anything which interferes with that movement such as moisture stress or cold soil temperatures affects plant uptake of potassium.  Plant uptake and metabolism of potassium is in that same ionic form, K⁺ . Potassium has important roles in activation of many enzyme systems in the plant.  It is vital to photosynthesis and to the formation and utilization of sugars (respiration).  Potassium is also essential to protein synthesis and maintenance of protein structure.  Potassium helps the plant use water more efficiently and helps control the loss of water from plant leaf surfaces.   Adequate potassium helps plants resist diseases and develop strong stems.  Potassium deficiency symptoms, like nitrogen deficiency symptoms, are usually noted first at the bottom of the plant because of its mobility.  Chlorosis develops around leaf margins.  High yielding cotton varieties frequently show characteristic potassium deficiency symptoms at the top of the plant brought on by late season demands of potassium during boll development.

Potential acidity - The pounds of calcium carbonate (CaCO₃ ) estimated to be required to neutralize the acidity caused by adding 1 ton of an acid-forming fertilizer to soil/growing media.  The higher the potential acidity of a fertilizer, the more likely it will cause the soil/growing media pH to decrease over time (i.e., become more acid).

Potential basicity - The pounds of calcium carbonate (CaCO₃ ) estimated to be equal to the addition of 1 ton of a base-forming fertilizer to the soil/growing media.  The higher the potential basicity of a fertilizer, the more likely it will cause the soil/growing media pH to increase over time (i.e., become more basic or alkaline).

Potting Mix -A mixture of various organic and inorganic constituents, including soil, which is used for growing plants in containers or beds.

Preplant fertilizer - A fertilizer application incorporated into growing media prior to sowing seed, transplanting, or potting.

Primary Nutrients - Nutrients required by plants in relatively large amounts and are frequently applied as fertilizers. The macronutrients nitrogen (N), phosphorus (P), and potassium (K).

Primary Plant Nutrients (plant foods) -Nitrogen (N), phosphate (P₂O₅) and potash (K₂ O).

Primary Root - This radicle is the beginning of the root system and develops into the primary or first root.  It generally grows downward.

Primary Tissue - In woody stemmed plants the primary tissues (meristematic tissues) develop in the early part of the first season's growth of a stem.  Growth after this first year's growth is secondary. This primary tissue growth is similar to that of the herbaceous stem.  It consists of an epidermis, cortex and stelar tissues.

Processes of Photosynthesis - No one knows the exact steps or chemical transformations which leaves follow in photosynthesis.  It is known that the raw materials - CO₂, water and minerals are provided by the roots and atmosphere.  The chloroplast transforms and regulates the energy to combine CO₂ and water into carbohydrates and release oxygen.  The mineral elements aid in controlling the energy requirements of the different food compounds and other related processes as assimilation, digestion, transportation, respiration, and enzymatic functions.  The proper combinations of these mineral nutrients greatly stimulate photosynthesis.  The problem is to provide the right combination of these nutrients and related conditions.
 Associated with these photosynthetic factors are oxygen and mineral salts.  It may be that these materials are more directly related to some of the other processes, such as respiration and transportation which accompany photosynthesis.

Productivity -In simplest terms, the ability of the soil to produce.  It differs from fertility to the extent that a soil may be fertile and yet unable to produce because of other limiting factors.

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

Prostrate Stems - Some plants send their shoots along the ground.  Melons, cucumbers, etc., are examples of such prostrate stem plants.

Protein -Any of a group of high-molecular-weight nitrogen-containing compounds that yield amino acids on hydrolysis.  Protein is a vital part of living matter and is one of the essential food substances of animals.

Protoplasm - The key to life in the cell is a more or less transparent, slimy and mucilaginous liquid, called protoplasm. This protoplasm contains a nucleus and cytoplasm.

Puddled Soil -Dense, massive soil artificially compacted when wet and having no regular structure.  The condition commonly results from tillage of or heavy traffic on a clayey soil when it is wet.

Pulsed subirrigation - See ebb-and-flow system.

Quick lime - See hydrated lime .

Quick Test - Simple and rapid chemical tests of soils and plant tissues designed to give an approximation of nutrients available to plants. Simple and rapid chemical tests of soils designed to give an approximation of the nutrients available to plants.

Radical - Stable groups of atoms reacting as a whole.

Ratio-See  Fertilizer Ratio .

Ray Fungi - See Actinomycetes

Reaction (chemical) - Process in which substances react with one another and are thus altered.

Reaction (of solution) - Acidity expressed as pH (e.g., a solution has acid "reaction").

Recirculating system - A term often used to describe a closed irrigation system where the plants or containers are placed in trays or troughs, and the irrigation solution is recirculated between the trays/troughs and storage tanks; usually applied to a zone in a production family.

Reciprocal - Mathematical expression. Equal to the number 1 divided by a specific value.  In relation to acidity, pH is expressed as the logarithm (base 10) of the reciprocal of the hydrogen ion concentration (1/[H⁺ ]).

Reclamation-The process of restoring lands to productivity by removing excess soluble salts or excess exchangeable sodium from soils.

Recycled Water -Water that has been used for domestic, industrial, or agricultural purposes, but not exposed to extensive treatment prior to reuse.

Recycling system - A term often used to describe a system where the irrigation runoff from the entire production facility (greenhouse or nursery) is collected, treated, stored, then reused for irrigation.

Redox Reaction - Combined reaction of oxidation and reduction, e.g.,
    oxidizing         reducing
     agent              agent
        |                      |
     MnO₂       +  2 Fe ²⁺  +  Fe₂O₃  + 2H⁺
    reduction                     oxidation

Reduced Tillage - A system of land preparation for crop planting that implies omitting one or more of the operations from a conventional tillage system for that region.  Specific varies widely, however.  In recent years, economics and concerns for environment as well as availability of alternatives for pest control, have resulted in most farmers reducing tillage operations to some degree, further complicating the question of what is conventional.
Methods of fertilizer application for reduced tillage systems include pre-plant, deep-banding of nutrients below the surface residue, dribbling or strip banding, point injection of liquid fertilizers, starter fertilization, side-dressing, top-dressing or fertigation, depending upon the cropping system.  Broadcast applications of nutrients in significantly reduced tillage systems tend to be subjected to reduced availability because of the large amounts of crop residue on the soil surface.

Reducing agent - Substance that combines with oxygen or loses electrons and becomes oxidized in the process.

Reduction - Removal of oxygen or lowering of valence (absorption of electrons).  Reversal of oxidation, e.g., Mn⁴⁺ (tetravalent manganese ion) + 2 electrons   »   Mn²⁺ (bivalent manganese ion).

Reserve Acidity - The exchangeable hydrogen ions held on the soil colloids are referred to as reserve or potential acidity of the soil.
In an acid soil, two groups of hydrogen ions are involved - those of the soil solution itself and those held as adsorbed cations by the colloidal complex.  These groups are in dynamic equilibrium and consequently both must be considered in any attempt to alter the pH of the soil solution.  For convenience of distinction, the hydrogen ion concentration of the soil solution is designated as active acidity while exchangeable hydrogen ions on colloids constitute reserve or potential acidity.  The relative magnitude of the two types of acidity varies tremendously.  Active acidity constitutes only a very small part of soil acidify. Conservative calculations suggest that reserve acidity may be 1000 to as much as 100,000 times greater (for a clay soil) than active acidity.  Liming to correct soil acidity must then neutralize residual acidify.
See Also: Acid Soil; Active Acidity; Residual Acidity.

Residual Acidity - The ultimate acidity that develops from fertilizer in a particular soil horizon after the residual salts are removed from that horizon by leaching.  The level of residual acidity depends on the extent that any ammonium nitrogen in the fertilizer nitrifies, the extent that plants differentially absorb cations and anions, and the initial composition of the fertilizer.

Residual Basicity - The ultimate basicity that develops from fertilizer in a particular soil horizon after the residual salts are removed from that horizon by leaching.

Residual Fertility - Available nutrient content of a soil carried over to the next crop after fertilizing the previous crop.

Residual Value - The value of the fertilizer to succeeding crops after it has been in the soil for one or more cropping seasons.
Residual values of plant nutrients affect soil test levels for nutrients such as phosphorus and potassium.  Residual values, then, affect requirements for additional nutrients in the future.

Respiration -  The chemical process used to liberate the energy of foods for growth, reproduction, assimilation, transportation, etc., is called respiration.  It is a slow and enzymatically controlled process of oxidation.  The end products are CO2 and water.  Respiration is influenced by many external factors.

Retention Zone - Soil zone where nutrients are concentrated following a fertilizer application.  Usually refers to some sort of banded application.

Reverse osmosis system - A water purification system where the water is forced under high pressure through a very fine membrane that filters out dissolved solutes.

Reversion-The interaction of a plant nutrient with the soil which causes the nutrient to become less available.  In fertilizer manufacturing, the excessive use of ammonia in ammoniation of phosphates results in phosphate reversion.  (See also Fixation.)

Rhizobium/Rhizobia - Genus of cylindrical, aerobic, mesophilic bacteria that infect the roots of leguminous plants causing the formation of nodules.  When in a symbiotic relationship with a legume, the plant supplies the bacteria with energy, water, nutrients and the bacteria fixes atmospheric nitrogen proteins into amino acids for use by the plant.
Inoculation of legumes (seed) with specific Rhizobia is an essential practice for proper nitrogen nutrition of the crop.  Amount of nitrogen fixed is affected by many factors including type of legume, soil nutrient levels, soil pH, availability of water, and soil aeration.

Rhizomes - Some plants have horizontal perennial shoots growing at or beneath the soil surface, which are called rhizomes.  Many grasses have slender rhizomes to aid in producing new plants.

Ridge-Till - see Conservation Tillage

Rock wool - Inorganic growing media component made from basalt rock, steel mill slag, or other minerals that are liquefied at high temperatures and then spun into fibers.  Rock wool in its pure form may also be used as cubes (stock plant production) or slabs/cubes (cut flower production).

Rock wool culture - A subirrigation growing system where plants are rooted into bags or containers of rock wool and irrigated with drip tubes or in troughs that are periodically flooded.

Roots - The root portion of a plant grows normally below the soil surface.  There are many kinds of roots.  They vary in their structure and have different functions.

Rootbound -Having a closely packed mass of roots, as on a plant that has grown too large for its container.

Root Cap Region - The apex of the root, or root cap, is a thimble-shaped mass of average sized cells which protect the meristematic cells which develop just back of it.  If any of these root caps are broken off as they are pushed through the soil mass, they are replaced by some of the meristematic cells back of them.  They serve as a protection for the cells of the meristematic region.

Root Hair Zone - The younger portion of the mature region, or just back of the elongation region, is the root hair zone.  In this zone, the epidermal cells develop hair-like growths or protrusion out into the soil mass.  These root hairs are active in absorbing water and nutrients from the soil.  They increase the absorbing power of a root many times.  Many have estimated that the root hairs absorb 95% of the materials absorbed by plant roots.  As the root elongates, the older root hairs sluff off and new root hairs develop on the newer epidermal cells.

Runoff potential - A measure of a pesticide's tendency to move with sediment in runoff water.

Saline-Alkali Soil - A soil containing a high proportion of soluble salts and either a high degree of alkalinity or a high amount of exchangeable sodium (Na), or both, so that the growth of most crops is less than normal.
Soils high in exchangeable sodium often have poor structural characteristics and may require extensive treatment with gypsum or acid followed by leaching to remove the high sodium concentrations from the root zone.

Saline-Sodic Soil/Media -A soil/media 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 greater than 15, and the electrical conductivity of the saturation extract is greater than 4 decisiemens per meter (at 25°C).  The pH reading of the saturated soil is usually less than 8.5.

Saline Soil - A soil containing enough soluble salts to impair its productivity, but not containing an excess of exchangeable sodium.  Its pH is less than 8.5. High concentrations of soluble salts in soil can severely limit plant growth.  Special treatment including leaching with good quality water may be necessary to remove the excess salts from the root zone.

Saline Soil/Media -A soil/media containing enough soluble salts to impair its productivity for plants but not containing an excess of exchangeable sodium.

Salinity - A term used to describe the salt content of soil, growing media, or water.

Salt - Combination of one cation and one anion, sometimes easily soluble in water (common salt,  or "potassium salt"), sometimes practically insoluble (e.g., Ca-silicate, apatite). A compound that dissolves in water and dissociates into cations (positively charged ions) and anions (negatively charged ions).  All soluble inorganic fertilizers dissolve to form salts.

Salt Index -An index used to compare solubilities of chemical compounds.  Most nitrogen and potash compounds have high indexes, and phosphate compounds have low indexes.  When applied too close to seed or on foliage, the compounds with high indexes cause plants to wilt or die. An index used to compare solubilities of chemical compounds used as fertilizers.  Most nitrogen and potassium compounds have high indexes, and phosphorus compounds have low indexes.  When applied too close to seed or on foliage, the compounds with high indexes cause plants to wilt or die because of the compounds' high affinity for water. The lower the salt index, the lesser the risk of crop injury with localized placement of fertilizers, specifically close to or in direct seed contact (pop-up fertilization).

Salting Out -Precipitation of the dissolved salts continued in a solution when the temperature drops to a certain point (the salting out point).

Salts-The products, other than water, of the reaction of an acid with a base.  Salts commonly found in soils break up into cations (sodium, calcium, etc.) and anions (chloride, sulfate, etc.) when dissolved in water.

Sand -Individual rock or mineral fragments in soils having diameters ranging from 0.05 millimeters.  Usually sand grains consist chiefly of quartz, but they may be of any mineral composition.  The textural class name of any soil that contains 85 percent or more sand and not more than 10 percent clay.One classification of soil particles ranging in size from 0.05 to 2.0 millimeters.  Sand grains may be round or quite irregular depending on the amount of abrasion they received in weathering.  Composition of sand is not implied by its particle size; however, the main component is quartz. nd, when dominant in a soil, imparts properties known as sandy or light.  Sand particles exhibit practically no plasticity and stickiness and as a consequence have little influence on changes in soil moisture content.  Water holding capacity is low, and because of the large size of the spaces between the separate particles, passage of percolating water is rapid.  Sandy soils are of open charter, posses good drainage and aeration, and are usually in a loose, friable condition.

Sand filter - A filter composed of layers of different sized sand and gravel that is used to filter particulate material out of water.

Saturated media extract (SME) - A method of soil/growing media extraction where the soil/growing medium is mixed with just enough water to make  a paste, and then some of the water is vacuumed out or squeezed out for testing.

Saturated Soil Paste -A particular mixture of soil and water commonly used for measurements and for obtaining soil extracts.  At saturation the soil paste glistens as it reflects light, flows slightly when the container is tipped and slides freely and cleanly from a spatula for all soils except those with high clay content.

Saturation Extract -The solution extracted from a soil at its saturation percentage.

Saturation Percentage -The moisture percentage of a saturated soil paste, expressed on a dry-weight basis.

Scaling - The buildup of deposits of calcium and/or magnesium carbonates, hydroxides, or hemihydrates. Occurs in water lines, tanks, and other plumbing.

Sclerenchyma - The sclerenchyma tissue is composed of thick-walled cells.  They may be either elongated and tapering fiber cells or shorter stone cells. The filled compact cells found in many fruit skins, nut shells, barks, etc., are examples of sclerenchyma stone cells.  When full grown most sclerenchyma cells will die. They are the common strengthening tissues of plant stems

Secondary nutrients - The macronutrients calcium (Ca), magnesium (Mg), and sulfur (S). Calcium (Ca), magnesium (Mg), and sulfur (S) are called secondary nutrient elements because they are essential to plant growth but less frequently deficient than the macro- (primary) nutrients and in greater quantity than the micronutrient elements. Sulfur has frequently been called the fourth major nutrient because of the frequency of sulfur deficiencies and the relatively large quantities used by plants.

Secondary Plant Nutrients -Calcium, magnesium and sulfur.

Secondary Roots - As the plants grows, branch roots develop on this primary root.  They are called secondary roots and generally grow outward and downward from where they emerge from the primary root.

Secondary Tissue - The secondary tissues in woody plants involve the cambium and cork cambium layers.  The cambium tissues contain xylem and phloem tissues but in a different position than in the primary tissues.  The cork cambium layer is a secondary meristematic tissue and gives rise to cork cells. As the woody stems grow older, there is a physical and chemical change in them.  Most of the conducting tissues are plugged up with many kinds of metabolic or waste products.

Sedimentation pit - A reservoir, pit, or tank used to produce clarified water by removing sediment and floating debris from waste or recycled water.

Seed - Fertilized and ripened ovules become the seeds.  Also there are many kinds, shapes, sizes and colors of seeds.

Seed Plants - A typical seed plant is made up of four parts: roots, stems, leaves and flowers.  The roots, stems and leaves are the vegetative parts which are primarily concerned with the absorption of raw nutrient materials, transportation of these foods, and their manufacture into plant materials.  The flowers are the reproductive parts of seed plants.

Sensor-based irrigation control - An automated irrigation system that is turned on and off by  moisture sensors, called tensiometers, placed in the soil/growing media.

Sepals - The outermost leaves of the flower bud are the sepals.  They may be green or the same color as the petals.  They protect the outer or inner parts of the flower.  All of the sepals of one flower bud is called the calyx.

Separate, Soil -One of the individual-size groups of mineral soil particles-sand, silt or clay.

Series, Soil -A group of soils that have soil horizons similar in their differentiating characteristics and arrangement in the soil profile, except for the texture of the surface soil, and are formed from a particular type of parent material.  Soil series is an important category in detailed soil classification.  Individual series are given proper names from place names near the first recorded occurrence.  Thus, Yolo, Panoche, Hanford and San Joaquin are names of soil series that appear on soil maps, and each denotes a unique combination of many soil characteristics.

Sewage Sludge-An organic product resulting from the treatment of sewage.  The composition varies widely depending on the method of treatment.

Side-Banded Fertilizer - Placement of fertilizer in bands on one or both sides of the seed or seedlings. Placement of starter fertilizers is often termed side-banded when fertilizer material is placed to the side and below the seed on one or both sides of the row.  The term may also refer to placement of fertilizers in a side-dressed application after plants are established.

Side-Dressed Fertilizer - Application made to the side of crop rows after plant emergence. Side-dressing usually refers to application of nitrogen (N), although any other nutrient may be applied in a similar manner.  The term usually refers to application on row crops, particularly corn, cotton, and grain sorghum. Applications may be on or below the soil surface.

Siemen - The currently accepted unit for expressing electrical conductivity.  Replaces the formerly and most commonly used term: mhos.  Reported as deciSiemen/meter, which is abbreviated ds/m.

Silt -(1) individual mineral particles of soil that range in diameter between the upper size of clay, 0.002 mm, and the lower size of very fine sand, 0.05mm.  (2) Soil of the textural class silt containing 80 percent or more silt and less than 12 percent clay. (3) Sediments deposited from water in which the individual grains are approximately the size of silt, although the term is sometimes applied loosely to sediments containing considerable sand and clay. Classification of soil particles ranging from 0.002 to 0.05 millimeter.  Silt particles are irregularly fragmental, diverse in shape, and seldom smooth or flat. They are actually micro-sand particles, quartz being the dominant mineral. Silt particles possess some plasticity, cohesion, and adsorption due to an adhering film of clay. The presence of silt and especially clay in a soil imparts to it a fine texture, and a slow water and air movement.  Much of the windblown material called loess occurring over wide areas of the central U.S. is composed of a high percentage of silt-sized particles.

Silt density index (SDI) - A measure of the clarity or turbidity of water due to suspended colloids, silica, bacteria, or mineral precipitates.

Silviculture -A branch of forestry dealing with the development and care of forests.

Simple Groups of Plants - The simple groups of plants are the most primitive and usually are associated with water.  They will range from a single cell to millions of cells.  They may or may not contain chlorophyll.  Also, some of these simple plants do not manufacture their own food, as this class includes, the parasites (live off the foods synthesized by other plants, as the mistletoe) and saprophytes (live off dead tissue, as toad stools).  Algar and fungi belong in this class of plants.

Simple Permanent Tissues - When a tissue is composed of similar permanent cells it is called a simple permanent tissue. There are five subdivisions of this group of tissues, namely: epidermis, parenchyma, sclerenchyma, collenchyma and cork.

Single superphosphate - See superphosphate .

Site-Specific Management - Management of nutrient inputs, pesticide applications, crop population and other cropping system practices according to changes in soil character and composition. Site-specific management is a developing technology which has been predicted on intensive soil sampling and nutrient management by individual, small land blocks or soil types.  Application and planting equipment has been developed with on-board computers, digitized soil maps and the use of global positioning satellites to facilitate changes in application rates of plant nutrients as the equipment moves across the field.  See Also: Variable Rate Application .

Slow-release fertilizer (SRF) - See controlled-release fertilizer

Slurry Fertilizer -A fluid mixture containing dissolved and undissolved plant nutrient materials which requires continuous mechanical agitation to assure homogeneity.

Sodic Soil - The term sodic refers to a soil that has been affected by high concentrations of salt and sodium.  Sodic soils are relatively low in soluble salts but are high in exchangeable sodium. saline, sodic and saline-sodic soils are terms used to define arbitrary limits of salt content and exchangeable sodium percentage in salt-affected soils.  Sodic soils with low soluble salts but high exchangeable sodium tend to remain in dispersed condition, almost impermeable to both rain and irrigation water.  They are of poor tilth - plastic and sticky when wet and prone to form hard clods and crust upon drying.  When wet they have a characteristic smooth, slick look caused by the dispersed condition of clay and humus.  Sodic soils are very poor for the growth of plants.  Exchangeable sodium exceeds 15% of the cation exchange capacity.

Sodic Soil/Media-A soil/media that contains sufficient exchangeable sodium to interfere with the growth of most plants, either with or without appreciable quantities of soluble salts.  (See also Nonsaline-Sodic Soil and Saline-Sodic Soil / Media .)

Sodium adsorption ratio (SAR) - Similar to adjusted sodium absorption ratio, except it does not take the water's bicarbonate content or salinity into account.

Sodium Adsorption Ratio -A ratio for soil extracts and irrigation waters used to express the relative activity of sodium ions in exchange reactions with soil.

Sodium Percentage -The percent sodium of total cations.  Calculations are based on milliequivalents rather than weight.

Soil - The unconsolidated mineral and organic material on the immediate surface of the earth that serves as a natural medium for the growth of plants. Unconsolidated mineral matter on the surface of the earth has been subjected to and influenced by original composition and environmental factors including climate, macro and microorganisms, and topography acting over a period of time and producing a product - soil - that differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics.  Organic matter is an important part of soil and soil characteristics.

Soil Acidifier - A material or mixture used, especially in semi-arid areas, to neutralize soil alkalinity. Sulfuric acid, elemental sulfur, phosphoric acid, liquid sulfur dioxide, and ammonium polysulfide are soil acidifiers.  In several cases, the acidity produced from the addition of these compounds to the soil is through the oxidation of sulfur compounds.  Ammoniacal nitrogen (N) fertilizers also have a strong acidifying effect on the soil through the process of nitrification.

Soil Acidity - see Acid Soil

Soil Aeration - The process by which air in the soil is replaced by air from the atmosphere. In a well aerated soil, the soil air is similar in composition to the atmosphere above the soil.  A well aerated soil is one in which gases are available to growing organisms (particularly higher plants) in sufficient quantities and in the proper proportions to encourage optimum rates of the essential metabolic processes of these organisms.  Poorly aerated soils contain a much higher percentage of carbon dioxide and a correspondingly lower percentage of oxygen than in the atmosphere above the soil.

Soil Amendment - Any material such as lime, gypsum, sawdust, or synthetic conditioner, that is worked into the soil to make it more amenable to plant growth. The AAPFCO official of soil amendment implies any substance which is intended to improve the physical characteristics of the soil, except commercial fertilizers, agricultural liming materials, unmanipulated animal manures, unmanipulated vegetable manures, pesticides, and other materials exempted by regulation.
Soil amendments may contain important fertilizer elements but the term commonly refers to added materials other then those used primarily as fertilizer.  An important use of a soil amendment would be the incorporation of gypsum into sodic soils to displace sodium from the exchange complex and improve soil permeability.

Soil Conditioners - Any material added to a soil for the purpose of improving its physical condition. Examples of soil conditioners include crop residues, animal manures, sewage sludge, polyelectrolytes such as complex vinyl and acrylic compounds and certain cellulose and lignin derivatives.  Soil conditioners tend to agglomerate soil colloids and produce a crumb structure in the soil increasing the permeability of the soil to air and water and reducing crusting of dry soil.

Soil Conservation - The practices of protecting soil from wind and water erosion and maintenance of its productive capability through protection of structure, water holding capacity, and ability to supply plant nutrients.
Soil conservation today implies the collective use of number of practices intended to protect soil and maintain its productive capabilities.  Soil conservation practices include contour cropping construction of terraces and waterways.  Utilization of various types of reduced tillage to maintain surface residues and adequate utilization of supplement plant nutrients from any source to maintain soil fertility levels.  Soil conservation is saving the soil, making maximum use of it and improving it while allowing only minimum waste.

Soil Erosion - The wearing away of the land surface (soil) by running water, wind or other geological agents including such processes as gravitational creep. Erosion of the soil has always existed in nature and is called geologic erosion (natural erosion losses).  Acceleration of this loss by cultivation, burning, overgrazing and other removal of protective surface cover results in severely diminished soil productive capability.

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, when other growth factors such as light, moisture, temperature and physical condition of the soil are favorable. Understanding and preservation of soil fertility are key to mankind's survival.  Adequate amounts of plant nutrients are one characteristic of a productive soil.  But a fertile soil is not necessarily a productive soil unless other limiting factors are controlled.  Soil fertility and the replenishment of needed nutrients removed in crop production have been recognized, written about, and discussed for thousands of years.  Extensive research has helped us to a better understanding of plant nutrition and the importance of fertility, but much remains to be learned.

Soil Moisture Stress -The sum of the soil moisture tension and the osmotic pressure of the soil solution.  It is the force plants must overcome to withdraw moisture from the soil.

Soil Moisture Tension -The force by which moisture is held in the soil.  It is a negative pressure and may be expressed in any convenient pressure unit.  Tension does not include osmotic pressure values.

Soil Nutrient Status - The level or state of adequacy or inadequacy at which available plant nutrients are present in a soil.
The level of availability of soil nutrients is commonly determined by soil testing, complemented by plant tissue analysis.  Soil testing and plant analysis are best management practices (BMPs) essential for efficient, profitable, and environmentally protective crop production.

Soil Permeability - The property of a soil to allow water to percolate through. The characteristic of a soil horizon that enables water or air to move through it.  Permeability can be measured quantitatively in terms of rate of flow of water through a unit cross section per unit of time under specified temperature and hydraulic conditions.  Values for saturated soils usually are called hydraulic conductivity. The permeability of a soil is controlled by the least permeable horizon even though the others are permeable.

Soil Profile - A vertical section of soil extended form the surface through all its horizons and into the parent material.

Soil sorption index - A measure of a pesticide's tendency to attach to soil particles as measured in Koc value.  Higher Koc indicates stronger attachment, and lower Koc indicates a greater tendency for leaching.

Soil Structure - The physical arrangement of the soil particles.  Soil structure controls soil porosity and subsequently the ability of the soil to store water and exchange gases with the atmosphere.

Soil Test - A chemical analysis of soil composition, usually intended to estimate availability of plant nutrients but also including measurements of soil acidity or alkalinity and physical measurements of soil electrical conductivity.

Soil Texture - The relative proportion of various sized particles making up the soil.  These particles are frequently referred to as soil separates and include sand, silt, and clay, all of which vary in size through a particular range. Texture is an important soil characteristic because it will determine water intake rates, water storage in the soil, the ease of tilling the soil, and the amount of aeration, and will influence soil fertility.  Textural names are given to soils based upon the relative proportions of each of the three soil separates - sand, silt, and clay.  Soils that are predominantly clay are clay (textural class); those with high silt content are silt (textural class): those with a high sand are sand (textural class).  The textural triangle can be used to determine the soil textural name after the percentage of sand, silt, and clay are determined from a laboratory analysis.
See Also: Clay: Sand: Silt .

Solenoid valve - An electrically activated valve used to control water flow in irrigation systems.

Solubility - The amount of substance that will dissolve at a given temperature in a specified water is known as solubility. To be available to plants a nutrient must be at least slightly soluble in the soil solution.  Solubility of compounds such as urea, ammonium nitrate, potassium chloride increases rapidly with temperature.  The presence of other substances in the solution may either increase or decrease the solubility.

Soluble salts - A salt that is soluble in water.  Salts include most inorganic fertilizers (such as amonium, nitrate, potassium, sulfate, phosphate) and mineral salts dissolved in irrigation water (such as sodium, bicarbonate).

Solution - Mixture of solvent (e.g., water) and dissolved substance (e.g., salt).

Solution Fertilizer - A clear, liquid fertilizer in which all nutrients have been completely dissolved in water.

Spaghetti irrigation system - A type of drip irrigation where the water is delivered in small diameter (spaghetti) tubes that are often weighted at the end to keep them in place when irrigating pots or hanging baskets.

Special Stems - Some aerial stems have unusual growth habits or special functions.  These shoots are called special stems, such as aerial bulbs, stolons, storage stems and thorns.

Split Application - Fertilizer applied two or more times during the crop growing season.  Preplant and one or more postplant applications are common. The intent of split fertilizer applications is to enhance the use efficiency of a particular nutrient. Usually, the terminology applies to nitrogen (N) utilization, particularly on soils which may be subject to nitrogen leaching. Application of nutrients nearest the time of greatest plant demand tends to improve use efficiency.

Spoke Injection - Application of liquid fertilizer at defined points in the soil through a wheel which has hollow spokes extending about 6 inches beyond the wheel radius.  Fertilizer is injected under pressure only when the spoke is pointing straight down.
See Also: Point Injection .

Stamens - Inside the petals are slender filament stalks or stamens.  These stalks have pollen bearing anthers at their apex or head.  The pollen grains cause fertilization which initiates the development of the ovule and seed.

Starter Fertilizer - Fertilizer applied at planting either in direct seed contact or to the side and below the seed.  Exact position is not implied. Liquid or solid fertilizer, placed near or in contact with the seed or the roots of new transplants, is commonly considered as starter fertilizer.  In early practice, the low-analysis fertilizer then on the market was usually placed with the seed.  Later, with increased concentration and higher rates of application, the fertilizer was banded approximately 2 inches below and 2 inches to the side of the seed to avoid germination damage and seedling injury from soluble salts.  Starter fertilizers provide high concentrations of nutrients near developing seedlings which can overcome nutrient uptake problems associated with low soil nutrient content, low soil temperature and compaction. See Also:  Pop-Up Fertilizer; Side-Banded Fertilizer .

Stems - The upward elongation and growth of the embryo axis becomes the stem.  When a stem develops its first leaf, it is called a shoot.  All of the shoots, branches and leaves, are called the shoot system.  Stems have many types, structures and functions.

Stoma - Intermingled with these ordinary epidermal cells are guard cells.  They exist in pairs and provide a small opening or stoma for the exchange of CO2, O2 and H2O vapors.  Often these outer surfaces of the epidermal cells have a waxy protective substance called cutin.  Usually the upper epidermal layer has fewer guard cells than the lower layers and a thicker cutin layer.

Structure of Roots - Roots are usually cylindrical in form and colorless.  They have no nodes (joints) and internodes as occur in many stems of plants.  Branch roots originate in an internal tissue (pericycle tissue) and not from buds as stems do.

Structure of Stems - The external structures of stems are classified as Herbaceous and Woody.

Structure, Soil -The physical arrangement of the soil particles.

Styrofoam - An inorganic growing media component made from expanded and solidified polystyrene foam that is formed into small flakes or beads.

Strip Fertilization - Fertilizer applied in surface bands that may be incorporated by tillage or remain on the soil/residue surface.
See Also:  Surface Band Application .

Stolons - Plants, as the strawberry, develop shoots which grow above and horizontal to the soil.  Such stems are often called runners.  They will root and produce a new plant at their nodes when they touch the soil.

Storage - Many plants synthesize more plant materials than they require for their immediate growth. This excess food may be stored in the embryo, seed or other parts of the plant for further use.  Some plants store this food in enlarged parenchyma tissues which are associated with the phloem tissues of the root, such as beets, carrots, etc.  Some plants, especially the perennials, store absorbed nutrients in the parenchyma tissues associated with the xylem tissues for next spring use.

Storage Roots - Sometimes the upper part of the primary root expands below the soil line and produces an enlarged space to store synthesized plant food.  This is called a storage root.  Examples of the storage root are the beet, carrot, turnips, rutabagas, etc.

Storage Stems - There are prickly plants which develop a fleshy stem with scale-like branches that end in spines.  These plants store water and food in these fleshy stems to carry the plant through dry periods.  The cactus is an example of a storage stem plant.

Subirrigation system - An irrigation system that delivers water onto a surface where containerized plants are placed.  Water enters the container through drain  holes by capillary action from below.  The surface may be an absorbent mat (capillary mat), through, tray, or molded concrete floor (ebb-and-flow).

Subsoil -Roughly, that part of the soil below plow depth.

Subsoil - The underlying layers of the soil beneath the topsoil which may contain less organic matter and more characteristics of the soil's parent material. Subsoils tend to be lower in available nutrient supply, particularly nutrients such as phosphorus and micronutrient.  The actual plant nutrition depend upon a number of factors including the degree of weathering of the subsoil and the climate of the region.  Subsoil sampling for accumulated nitrate-nitrogen is an important part of nitrogen management practices in many areas, particularly those with lower rainfall.

Substance (chemical) - Chemical elements and combinations of such.

Substrate - See growing media .

Sub-Surface Irrigation-The distribution of irrigation water through conduits below the surface of the soil. May also apply to water available to plants from a water table. See subirrigation system.

Subterranean Stems - The shoots of many plants grow along or beneath the soil surface.  Such stems are called subterranean stems.  There are several types of subterranean stems.

Sufficiency range - The recommended range of nutrients in plant tissue that yields acceptable growth.  Also called normal, acceptable, or optimum range by various analytical labs.

Sulfur (S) - Sulfur (S) is an essential secondary plant nutrient classed with calcium and magnesium.  It exists in the soil in a number of oxidation states and is absorbed by the plant in the sulfate ion (SO₄^-2 ) form.  Sulfate-sulfur is reduced in the plant before incorporation into plant components. Sulfur is essential in forming plant protein because it is part of certain amino acids.  As a part of plant protein it is essential for enzyme activity.  Sulfur is also involved in nodule formation and nitrogen fixation in legumes.  Sulfur is essential in chlorophyll formation although it is not a constituent of the chlorophyll formation and nitrogen fixation in legumes.  Sulfur is essential in chlorophyll formation although it is not a constituent of the chlorophyll molecule.  Sulfur-deficient plants are pale green.  Symptoms look very much like nitrogen deficiency. Sulfur is not mobile and symptoms generally appear first on the upper leaves, while nitrogen deficiency shows up first on the lower leaves.  However, in sulfur deficiency the entire plant can take on a pale green appearance.  Sulfur deficiencies occur most often in sandy soils low in organic matter.  Deficiencies can be determined by soil and plant analysis.

Sulfur-coated fertilizer - A slow-release fertilizer composed of a pellet of soluble fertilizer coated with a layer of sulfur that causes slow release.  The most common is sulfur-coated urea (SCU), but other nutrients are available.  Some sulfur-coated urea products may also include a light polymer coating to further control release.

Superphosphate - A granular phosphorus fertilizer derived by dissolving raw rock phosphate with acid.  Use of sulfuric acid produces single superphosphate (0-20-0, plus Ca and S), and use of phosphoric acid produces treble (or triple) superphosphate (0-45-0).

Superphosphate -the AAPFCO has adopted as official the following : "Superphosphate is a product obtained by mixing rock phosphate with either sulfuric acid or phosphoric acid or with both acids.  (The grade that shows the available phosphoric acid shall be used as a prefix to the name.  Example: 20 percent superphosphate.)"

Superphosphoric Acid-See Polyphosphoric Acid

Surface Band Application - Placement of a liquid or solid fertilizer as either a dribble or forced stream on the soil surface. The techniques of surface band application have been developed primarily to improve use efficiency of nitrogen (N) and phosphorus (P).  Some data also show an enhanced use efficiency for potassium (K) applied in this manner.  Surface band applications increase nutrient concentration in the area of the band which may enhance penetration of heavy surface residues or diminish soil fixation reactions for phosphorus.  Surface band applications are frequently more effective than broadcasting treatments but less nutrient-use efficient than knifed or sub-surface banding.
See Also: Strip Fertilization .

Suspension - Suspension of solid particles in liquid, e.g., lime in water.

Suspension Fertilizer -A fluid containing dissolved and undissolved plant nutrients.  The suspension of the undissolved plant nutrients may be inherent to the materials or produced with the aid of a suspending agent of non-fertilizer properties.  Mechanical agitation may be necessary in some cases to facilitate uniform suspension of undissolved plant nutrients. A fluid containing dissolved and undissolved plant nutrient compounds.  Suspension of the undissolved materials is usually produced with the aid of a suspending agent of non-fertilizer properties (clay).  Mechanical or air agitation may be necessary to facilitate uniform suspension of undissolved plant nutrients.

Sustainable Agriculture - Integration of soil and crop management technologies to produce quality food and fiber while maintaining or improving soil productivity, farm profitability and environmental quality.

Symbiosis - To dissimilar organisms living together in intimate association resulting in mutual benefit, such as Rhizobia species and leguminous plants and the association of algae and fungi in lichens. In symbiosis, the symbiont organisms rely upon one another and both benefit by the relationship.  In the symbiotic relationship of legumes and Rhizobia species, the host plants provide fixed carbon sources for the bacteria which in turn fix atmospheric nitrogen for the host plant.  The mycorrhizal fungus-plant root association is another instance of a symbiosis.  Plant uptake of such nutrients as phosphorus and zinc can be benefited by this type of relationship.The living together of two different organisms with a resulting mutual benefit.  A common example is the association of rhizobia with legumes.; the resulting nitrogen fixation is sometimes called symbiotic nitrogen fixation.  Adjective: symbiotic.

Symbiotic - The relationship of two living organisms in which both benefit, such as nitrogen fixation by Rhizobia in nodules on legume roots.

Symbiotic Bacteria - In agriculture, usually relates to bacteria in nodules growing on the roots of legumes which have the ability to fix free nitrogen from he atmosphere into forms which can be utilized by the host legume plant. Agronomically, the Rhizobium-legume symbiosis is most important as it results in appreciable nitrogen gains for he system.  This is a true symbiosis as neither plant nor bacterium utilizes atmospheric nitrogen (N₂ ) in the absence of the second organism.

Synthesis - Formation of chemical compound.

Tap Root - Some plants have one main root with only a few small branch roots off of it.  Such a root is called a tap root.  Many vegetables, such as carrots, beets, dandelions, etc., have tap root systems.  The roots of alfalfa and many clovers approach the tap root systems.  Some of these plants will send their tap roots down 40 to 50 feet to reach water.  Also some trees and shrubs have a tap root system.

Tankage -Dried animal residue.  Process tankage is made from leather scrap, wool and other inert nitrogenous materials by steaming under pressure with or without addition of acid.  This treatment increases the availability of the nitrogen to plants.

Tensiometer -A device used to measure the tension with which water is held in the soil. A sensor with a porous ceramic tip that when placed in the soil/growing media measures moisture tension (matric tension) of the water in the soil/growing media. This can be related to moisture content with a moisture tension curve.

Texture, Soil/Media -The relative proportions of the various size groups of individual soil grains in a mass of soil/media.  Specifically, it refers to the proportions of sand, silt and clay.

Thin-film composite membrane - A very thin, layered membrane containing a polyamide layer used in reverse osmosis water purification systems.  They are resistant to pH but sensitive to chlorine.

Thorns - Some plants, such as a locust tree, produce thorny stem branches.  These thorns are for protection.

Tilth-The physical condition of a soil with respect to its fitness for the growth of plants.

Tissue Analysis - A diagnostic technique in which plants are sampled and their tissues analyzed for nutrient concentrations to evaluate the soil-plant nutrient status during the growing season and thereby confirm a sufficiency or insufficiency of these plant nutrients. Concentrations of plant nutrients in plant tissues reflect the available nutrient status in the soil.  When properly interpreted such information along with soil test data can be effectively used in developing fertilizer recommendations for optimum crop production.
See Also:  Petiole Analysis .

Tissues - Inside the pericycle layer are many other tissues that are largely for storage and conduction. They consist of xylem, phloem and parenchyma tissues.  These tissues that are bounded by the pericycle, are called stellar tissues.

Top-Dressed Application - Surface application of fertilizer to the soil after crop has been established. Top-dressing is frequently used to describe the application of top-dressed fertilizer.  Top-dressing is an important management practice in fertilization of established crops such as legumes, forage grasses and small grains.  The terminology implies surface application which is most effective in an environment with adequate moisture.  For small grains, the terminology usually refers to nitrogen and sulfur applications.  For grasses and alfalfa, all nutrients are usually applied as top-dressed applications.

Topsoil - Topsoil refers to the surface layer of a soil including most of the organic matter content of the soil profile.  Technically, this layer is considered as the dark-colored A horizon of the soil profile. Much of the available nutrient supply for crops is associated with the topsoil.  This portion of the soil horizon is usually sampled for determination of nutrient availability.  This emphasis on topsoil as a nutrient source, however, tends to ignore the important contributions of lower soil levels and their ability to provide the plant with water and nutrients.

Total carbonates (TC) - The sum of bicarbonates and carbonates.  Often used to express alkalinity.

Total dissolved solids (TDS) - The total of nonvolatile solutes dissolved in water.  Often called total dissolved salts.  For most water it reflects the soluble salt content.  Usually expressed as ppm or mg/1.

Total porosity - The percent volume (volume/volume) of soil/growing media comprised of  pores.  The pores are filled with air and/or water.

Toxicity - When the concentration of a nutrient or chemical becomes high enough in a plant to cause damage.

Trace Elements-See Micronutrients .

Transpiration - The process of transpiration is the loss of water from the aerial portions of the plant, mostly the leaves.  It aids in pulling the sap to the leaves and cools the leaves.  It can cause plant wilting.  Factors which would normally influence surface water evaporation affect transpiration.  Plants react to reduce transpiration when necessary by increasing the cutin coating, position of leaf, shedding leaves and closing of leaf stomata.

Transportation - As a root grows in diameter and length, it develops cells, tubes, tissues or pipe lines which conduct absorbed and synthesized materials.  The upward conducting tissues which carry absorbed nutrients to the stems, etc., are called the xylem tissues.  The downward conducting tissues, the phloem, transport the plant materials that are synthesized in the leaves, to the growing parts of the roots and to the storage tissues.

Triple Shooting - Placement of three fertilizer materials in separate subsurface bands. Accomplished through the injection of incompatible materials from three tubes on an applicator shank.  For example, anhydrous ammonia, ammonium polyphosphate or other mixed liquid fertilizer and ammonium polysulfide are sometimes used with this application technology in the western U.S.

Triple Superphosphate-A product that contains 40 to 50 percent available phosphoric acid.  Triple superphosphate differs from ordinary superphosphate in that it contains very little calcium sulfate.  In the fertilizer trade, the product is also called treble superphosphate, concentrated superphosphate, double superphosphate and multiple superphosphate. See superphosphate

Trough culture - A subirrigation growing system where containerized plants are placed in sloped troughs, down which nutrient solution flows as needed.  Usually the nutrient solution is collected and recirculated.

Tubers - Some plants, as the Irish potatoes, have enlarged tips on their rhizomes that develop into tubers.  These tubers are largely storage places.  Buds or "eyes" develop on these tubers that can grow into aerial stems.

Turf -Any grassy area maintained by frequent mowing, fertilization and watering used for lawns, roadsides or playing fields.

Turfgrass -A species or cultivar of grass, usually of spreading habit, which is maintained as a mowed turf.

Types of Roots - Different species of plants produce roots that vary in extent, form, depth, number, etc.  The entire mass of roots is called the root system.  There are two general types of root systems, namely, the diffused and tap root system.

UC mix - A group of media mixes containing various ratios of sand to peat.  Developed by Baker at the University of California in the 1950s.

Unavailable water (PWP) - The percent volume (volume/volume) of soil/growing media occupied by water that is unavailable to the plant.  It is also called  the permanent wilting percentage (PWP) and is the water content at which the plant permanently wilts and will not recover unless additional water is added to the soil/growing media.  It is determined by measuring the percent of water present at a moisture tension of 1.5 megaPascals (approximately 15 bars or atm).

Uncoated Organic Compounds -Generally, organic nitrogen compounds, usually based on urea, that release nitrogen slowly over given periods of time.

Unit -The AOAC has adopted as official the following : "A unit of plant food is twenty (20) pounds, or one percent (1 percent) of a ton."

Urea - An organic nitrogen fertilizer [CO(NH₂)₂ ] that is soluble and can be taken up directly by the plant, thus acting like an inorganic fertilizer.  In the soil/growing media, the urease enzyme of bacteria readily converts it to ammonium and carbon dioxide, so it behaves like an ammonium fertilizer.

Urea formaldehyde - A slow-release nitrogen fertilizer composed of a polymer of urea and formaldehyde.  Availability is based on low solubility and biological breakdown.  Also called ureaform.

Vacuoles - The water solutions of sugars, salts, etc., which are usually found in the center of the cell are called vacuoles.  Such areas are the storage reservoirs for food.  A vacuolar membrane surrounds each vacuole.

Valence - Number of (positive or negative) charges of ion (univalent: K⁺, Na⁺; bivalent: Mg²⁺, SO₂^4-; trivalent: PO₃^4-).

Variable-Rate Application - Variable-rate fertilization is a technique which changes nutrient application rates according to changes in available nutrient levels in soil as the applicator moves across the field. This technology requires soil survey maps, aerial infrared photographs and/or grid maps to delineate soil types and other physical features.  Soils are sampled according to the soil maps, and that information is stored in an applicator's on-board computer.  As the applicator moves across the map shown on an on-board monitor, impulses from the computer change the rate of application from the applicator's bins or tanks.  To do this, the computer has to know precisely where the applicator is at all times and that job is handled by information from a global positioning satellite.

Variation of Flower Structure - There are many variations in the arrangement of the flowering parts.  All these parts may be in one unit such as a complete, perfect or bisexual flower.  Some plants have the stamens in one flower and the ovule bearing organ, the carpel, in another flower and are called imperfect, incomplete or unisexual.  In some species of plants, the male and the female flowers are on separate plants.

Veins - In each blade, the vascular tissues of the petiole continue as veins to conduct the water and mineral salts to the mesophyll and transport the manufactured foods back to the petioles and stems.

Venation - There is a definite arrangement of the conducting tissues or veins in leaves.  The major types of venation are parallel and net.

Vermiculite - An inorganic growing media component made from a micalike ore of aluminum-iron-magnesium silicate that is fired (heated) to cause the layers to expand into an accordion-like structure.

Volatilization-The evaporation or changing of a substance from liquid to vapor.

Water-Holding Capacity -The amount of water held by a given quantity of absolutely dry soil / media when saturated.

Water Pollution Control Acts - The laws set by Congress (Public Law 84-660 and 92-500) that set the standards for clean water.

Water softener - A water treatment system where a cation exchange resin is used to remove the hard water salts calcium and magnesium and replace them with the soft water salt sodium, and in some applications potassium.

Water Table -The upper surface of ground water.

Water Table, Perched -The upper surface of a body of free ground water in a zone of saturation separated from underlying ground water by unsaturated material.

Weathering-The physical and chemical disintegration and decomposition of parent material as in soil formation.

Weed-And-Feed - A term used in the agricultural chemical industry to denote mixing and application of fertilizers and herbicides. Combination fertilizer and herbicide applications have been developed as a crop production cost cutting practice.  These dual applications potentially reduce equipment, labor, and time requirement and reduce soil compaction by elimination of operations.  In some cases, performance of the herbicide may be enhanced by the combined application.  However, fluid mixtures should always be tested on a small scale for compatibility prior to large scale mixing to avoid physical, chemical, and performance problems.

Wilting Percentage-See Permanent Wilting Percentage

Windbreaks -A group of plants placed in locations where they might screen out winds and snow drifting around a house or landscape.

Winter Hardiness -The plant characteristic of being able to successfully withstand the rigors of winter.

Woody Stems - Woody stems are tough and non-green with a covering of corky bark.  They may become quite thick in diameter by enlarged secondary tissue growth.  Woody stem plants are largely perennial.  A tree is a woody stemmed plant with a single main stem or trunk.  A shrub is a woody stemmed plant with several stems of about equal size.

Xylem - The conducting tissues, which transport water and absorbed nutrients upward from the roots to the stems, leaves and flower stalks, are called xylem.  Such tissues consist of tracheids, vessels, fibers, and parenchyma cells.  Tracheids are elongated and tapering cells with pitted walls.  They die at maturity.  Their function is to give strength to the stem and aid in conduction. The vessel cells are long continuous tubes.  They are formed from vertically elongated cells, which have the ends of the cells dissolved to form a long tube.  These vessels or long tubes are primarily for conduction.  The fiber cells are similar to the tracheid cells except that they have thicker walls and fewer pits.  At maturity they have lost their protoplasm.  The parenchyma cells in the xylem tissue are storage cells.  All of these different kinds of cells may not be found in some xylem tissues.

Zero-leach - A term used to indicate no or very minimal drainage (i.e., leaching) from a container after irrigation, i.e., a leaching fraction of zero or near zero.

Zinc (Zn) - Zinc (Zn) is a metallic micronutrient present in the soil and absorbed by plants as the Zn⁺² ion.  Its oxidation state in the soil remains the same. Zinc was one of the first micronutrients recognized as essential for plants.  Zinc aids synthesis of plant growth substances and enzyme systems and is essential for promoting certain metabolic reactions.  It is necessary for production of chlorophyll and carbohydrates.  The metal is not translocated within the plants, so deficiency symptoms appear first on the younger leaves and other plant parts.  Zinc deficiency in corn is called white bud" because new growth turns white or light yellow.  Corn leaves may develop broad yellow bands (chlorosis) on one or both sides of the center mid-rib.  Other symptoms include bronzing of rice, rosette of pecans, "little leaf" of fruit trees, and severe stunting of corn, sorghum, dry beans, and soybeans.  Zinc becomes less available as soil pH increases.  High soil P availability can increase the severity of zinc deficiency.  Much of the soil's available zinc is associated with low soil organic matter.   Deficiencies of this element can be determined by soil and plant analysis.