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Glossary Of Terms

 

  • Absorptive capacity A measure of the capacity of a substance used as a growing medium in soilless culture to take (absorb) into pores and cavities nutrient solution. The trapped solution is a potential future source of water and essential elements. The composition of the nutrient solution is unaffected by this absorption. (See Adsorptive capacity .)

  • Acid injection The addition of a strong mineral acid to an irrigation water to lower the concentration of alkalinity.

  • Acidity Refers to the pH of the nutrient solution or growth medium when the pH measures less than 7.0. An increasing hydrogen (H+) ion concentration leads to increasing acidity as the pH decreases from 7.0. (See Alkalinity .)

  • Active absorption Refers to the process of ion uptake by plant roots requiring the expenditure of energy. This process is controlled and specific as to the number and types of ion species absorbed. (See Passive absorption .)

  • Adsorptive capacity A measure of the capacity of a substance used as a growing medium in soilless culture to selectively remove from the nutrient solution essential elements by precipitation, complexing, or ion exchange. Adsorbed elements may be released and therefore available to plants at a later time. The adsorptive capacity of a substance will significantly affect the composition of the nutrient solution through time, depending on the degree of adsorption or release. (See Absorptive capacity .)

  • Aer obic A condition in which ample O2 is present. In a rooting environment and/or rooting medium, O2 is not lacking. (See Anaer obic .)

  • Aerated standing nutrient solution cultur e A method of growing plants hydroponically where the plant roots are suspended in a container of continuously aerated nutrient solution. The usual procedure is to maintain the volume of the solution by daily addition of water and to replace the nutrient solution periodically with fresh.

  • Aeroponics A technique for growing plants hydroponically where the plant roots are suspended in a container and the roots are either continuously or periodically bathed in a fine mist of nutrient solution.

  • Alkalinity Refers to the pH of the nutrient solution or growth medium when the pH measures greater than 7.0. A decreasing hydrogen (H+) ion concentration leads to increasing alkalinity as the pH increases from 7.0. (See Acidity .)

  • Anaerobic A condition in which O2 is not present or exists at a very low concentration. In a rooting environment and/or rooting medium, O2 is lacking. (See Aer obic .)

  • Anion An ion in solution that has a negative charge. When applied to the composition of the nutrient solution, it designates ions, such as BO3 3–, Cl–, H2PO4 –, HPO4 2–, MoO4 2–, NO3 –, and SO4 2–, which are common forms for these essential elements in solution. In chemical notation, the minus sign indicates the number of electrons the compound will give up. (See Cation .)

  • Aquaculture A system of hydroponics and production of fish in which the fish are cultured in the nutrient solution.

  • Atmospheric demand The capacity of air surrounding the plant to absorb moisture. This capacity will influence the amount of water transpired by the plant through its exposed surfaces. Atmospheric demand varies with changing atmospheric conditions. It is greatest when air temperature and movement are high and relative humidity is low. Atmospheric demand is lowest under the opposite conditions.

  • Availability A term used to indicate that an element is in a form and position suitable for plant root absorption.

  • Bag cultur e A technique for growing plants in a bag of soilless medium (such as mixtures of sphagnum peat moss, composted milled pinebark, vermiculite, and/or perlite) into which a nutrient solution is applied periodically.

  • BATO bucket A bucket that is especially designed with a small reservoir in its base and a drainage nipple in its base so that the bucket can be set on a nutrient solution drainage line.

  • Benefi cial elements Elements not essential for plants but which, when present in the nutrient solution at specific concentrations or in rooting media, enhance plant growth (see Chapter 6).

  • Biological pest contr ol The use of predator insects or disease organisms to control plant pests or the use of natural organic substances for control.

  • Biostimulant: A plant biostimulant is any substance or microorganism applied to plants with the aim to enhance nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrients content.

  • Bor on (B) An essential element classed as a micronutrient. Boron exists in the nutrient solution as either the borate (BO3 3–) anion or molecular boric acid (H3BO3). The common reagents for making a nutrient solution are boric acid, H3BO3; solubor, Na2B4O7•4H2O + Na2B10O16•10H2O; and borax, Na2B4O7•10H2O.

  • Buffer capacity The ability of the nutrient solution or growth medium to resist a change in pH during the period of its use.

  • C3 Plants Plant species whose photosynthetic pathway results in the formation of a three-carbon carbohydrate; such plants reach maximum efficiency at lower temperatures and light intensities than C4 plants do. The optimum temperature range for photosynthesis of C3 plants is between 60 and 77˚F (15 and 25˚C). Tomato, cucumber, and lettuce are C3 plants.

  • C4 Plants Plant species whose photosynthetic pathway results in the formation of a four-carbon carbohydrate; such plants reach maximum efficiency with increasing temperature and light intensities. The optimum temperature range for photosynthesis of C4 plants is between 85 and l05˚C (30 and 40˚F). Many grain crops, such as corn, sorghum, rice, and sugar cane, are C4 plants.

  • Calcium (Ca) An essential element classed as a major element. Calcium exists in the nutrient solution as the divalent cation Ca2+. The common reagents for making a nutrient solution are calcium nitrate, Ca(NO3)2•4H2O; calcium chloride, CaCl2; and calcium sulfate, CaSO4•2H2O.

  • Carbon (C) An essential element classed as a major element. Carbon is obtained from carbon dioxide (CO2) in the air fixed during photosynthesis.

  • Cation An ion in solution that has a positive charge. When applied to the composition of a nutrient solution, it designates ions such as Ca2+, Cu2+, Fe3+, H+, K+, Mg2+, Mn2+, NH4 +, and Zn2+, which are common forms for these essential elements in solution. In chemical notation, the plus sign indicates the number of electrons the element will accept. (See Anion .)

  • Chelates A type of chemical compound in which a metallic atom (such as Fe) is firmly combined with a molecule by means of multiple chemical bonds. The term refers to the claw of a crab, illustrative of the way in which the atom is held. The most commonly used chelates are: EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid).

  • Chlorine (Cl) An essential element classed as a micronutrient. Chlorine exists in the nutrient solution as the monovalent anion Cl–. Since the chloride anion is ever-present in the environment and in chemicals commonly used to prepare nutrient solutions, it is not specifically added to the nutrient solution.

  • Chlorosis A light-green to yellow coloration of leaves or whole plants that usually indicates an essential element insufficiency or toxicity. Chlorosis is most frequently associated with Fe deficiency.

  • Closed hydroponic system Designates a growing system in which the nutrient solution is circulated and reused. (See Open hydr oponic system .)

  • Cocopeat (coir) Organic growing medium made from the grinding of coconut hulls.

  • Compost A mixture of organic (sometimes includes inorganic) materials that is used as a rooting medium.

  • Continuous flow nutrient solution cultur e A method of soilless culture in which the plant roots are continuously bathed in a flowing stream of nutrient solution.

  • Controlled Envir onment Agricultur e (CEA) Science that describes systems of protected agriculture for the ultimate in environmental control at both the aerial and root levels.

  • Copper (Cu) An essential element classed as a micronutrient. Copper exists in the nutrient solution as the cupric cation (Cu2+). The common reagent for making a nutrient solution is copper sulfate, CuSO4•5H2O.

  • Cuticle A very thin waxy film covering the surface of a plant leaf, derived from the outer surfaces of the epidermal cells.

  • Deepfl ow technique A method of NFT growing in which the depth of the trough is considerable.

  • Deficiency Describes the condition when an essential element is not in sufficient supply or proper form to adequately supply the plant or is not in sufficient concentration in the plant to meet the plant’s physiological requirement. Plants therefore usually grow poorly and show visual signs of abnormality in color and plant architecture.

  • Diffuse radiation Light radiation that is scattered after passing through a transparent material.

  • Diffusion The movement of an ion in solution from a high concentration to an area of lower concentration due to the existence of a concentration gradient. Movement continues as long as the concentration gradient exists.

  • Drip nutrient solution culture A method of soilless culture in which the nutrient solution and/or water is slowly applied as drops onto the rooting medium.

  • Ebb-and-fl ow A system of hydroponic growing, sometimes also referred to as flood-and-drain, in which plants are rooted in a watertight vessel containing a coarse inorganic substance, and the nutrient solution, housed in a sump, periodically floods the growing vessel.

  • Electrical conductivity (EC) A measure of the electrical resistance of a nutrient solution, or effluent from a growing bed or pot, used to determine the level of ions in solution. Conductivity may be expressed as specific conductance in mhos (micro- or milli-) or decisiemens (dS) or as resistance in ohms. 1 dS/ m = 1 mS/cm = 1000 μS/cm = 1 mmho/cm; EC (in dS/m × 640 = TDS [in mg/L (ppm)]. (See Specifi c conductance .)

  • Essential elements Those elements that are necessary for higher plants to complete their life cycles; also refers to the requirements established for essentiality by Arnon and Stout. Apart from the elements found in organic compounds (i.e. carbon, hydrogen, oxygen, and nitrogen), plants, animals, and microorganisms all require a range of elements in inorganic forms in varying amounts, depending on the type of organism.

  • Ethyl phosphonate Organic (carbon-based) compound bonded to an alumninum ion forming aluminum tris (O-ethyl phosphonate) or fosetyl Al; the active ingredient in Aliette and Chipco Signature fungicides.

  • Feeding cycle The time period when the nutrient solution is circulated through the root growing medium in those systems where plant roots are only periodically exposed to the nutrient solution.

  • Flood-and-drain (See Ebb-and-fl ow .)

  • Foliar feeding The application of a fertilizer solution to the foliage of a plant as a means of correcting a nutrient element deficiency or supplying a nutrient element needed by the plant to sustain growth.

  • Footcandle (fc) A unit of illuminance when the foot was taken as the unit of length. It is the illumination on a surface, one square foot in area, on which there is uniformity distributed energy level of one lumen.

  • Footcandle = one lumen per square foot (fc = 1 Lm /ft2)

  • Fruit truss: A plant structure on which fruit is set.

  • Fungicide: A chemical compound that is applied to a plant to kill disease organisms.

  • Glazing: Light-transmitting materials, such as glass, polyethylene film, fiberglass, or polycarbonates, which are used to cover a greenhouse.

  • Gravel cultur e A soilless culture technique where plants are grown in beds containing gravel, which are periodically bathed in nutrient solution. The gravel serves as a root support for the plants.

  • Hoagland/Ar non nutrient solution A formulation of reagents mixed in water to form a solution for supplying a plant with its essential elements hydroponically, frequently only referred to a Hoagland nutrient solution. Designates a nutrient solution that has been widely used and modified.

  • Humate An organic substance that has unique physical and chemical properties and is obtained by alkaline extraction from soil or an organic material, such as peat moss.

  • Hydr ogen (H) An essential element classed as a major element. Hydrogen is obtained from water (H2O) and after splitting is combined with carbon dioxide (CO2) to form a carbohydrate in the process called photosynthesis. (See Photosynthesis .)

  • Hydroponics A word coined in the early 1930s by Dr. W.F. Gericke (a University of California researcher) to describe a soilless technique for growing plants. The word was derived from two Greek words: hydro meaning water, and ponos meaning labor — literally, working water. Hydroponics has been defined as the science of growing plants without the use of soil or in an inert medium to which a nutrient solution containing all the essential elements needed by the plant for normal growth and successful completion of its life cycle is periodically added. In this text, hydroponics refers only to those systems of soilless growing that do not use a rooting medium.

  • Insect predators Insects that are cultured and brought into the greenhouse to combat insect infestations that are adversely affecting a plant.

  • Integrated pest management (IPM) A precise system designed to combat the infestation of plants by disease organisms and insects.

  • Intermittent flow nutrient solution culture A method of soilless culture in which the nutrient solution is only periodically brought into contact with plant roots.

  • Ion An atom or group of atoms having either a positive or negative charge from having lost or gained one or more electrons. (See Anion and Cation. )

  • Ion exchange A method of water purification in which water is passed through a resin bed to remove both cations and anions from the water. Ion exchange also refers to the phenomenon of physical–chemical attraction between charged colloidal substances with cations and anions. Ions of the essential elements can be removed from or released into the nutrient solution by ion exchange characteristics of sphagnum peat moss, pinebark, vermiculite, and clay colloids adhering to sand and gravel particles. Plant roots also have ion exchange properties.

  • Iron (Fe) An essential element classed as a micronutrient. Iron exists in the nutrient solution as either the ferrous (Fe2+) or ferric (Fe3+) cation. The common reagents for making a nutrient solution are iron tartrate, iron citrate, and the chelate forms, FeEDTA and FeDTPA. There are also inorganic compounds that can be used as a Fe source, such as ferrous and ferric sulfates, iron ammonium sulfate, and iron citrate and tartrate.

  • Langley A unit of incident solar radiation equal to one calorie per square centimeter.

  • Leaf analysis A method of determining the total elemental content of a leaf and relating this concentration to the well-being of the plant in terms of its elemental composition. (See Plant analysis .)

  • Lux = one lumen per square meter (lux = 1 Lm /m2) One footcandle equals about 10.76 lux

  • Macronutrient Refers to those nine essential elements (Ca, C, H, Mg, N, O, P, K, and S) that are found in the plant at relatively high concentration. (See Major essential elements and Micr onutrients .)

  • Magnesium (Mg) An essential element classed as a major element. Magnesium exists in the nutrient solution as the divalent cation (Mg2+). The common reagent for making a nutrient solution is magnesium sulfate (MgSO4•7H2O).

  • Major essential elements The nine essential elements found in relatively large concentrations in plant tissues. These elements are Ca, C, H, Mg, N, O, P, K, and S.

  • Manganese (Mn) An essential element classed as a micronutrient. Manganese exists in the nutrient solution as the manganous cation (Mn2+). The common reagent for making a nutrient solution is manganese sulfate (MnSO4•4H2O).

  • Mass flow The movement of ions as a result of the flow of water; the ions are carried in the moving water.

  • Medicinal plants Those plants that have medicinal properties that are used for human health maintenance.

  • Micronutrients The seven essential elements required by and found in relatively small concentrations in plant tissue. These elements are B, Cl, Cu, Fe, Mn, Mo, and Zn.

  • Mil A unit of thickness equal to 0.0001 of an inch (0.0254 mm); it is used to define the thickness of glazing material. (See Glazing material .)

  • Mineral nutrition: The study of the essential elements as they relate to the growth and well-being of plants.

  • Mineral Salts: Inorganic salts that need to be ingested or absorbed by living organisms for healthy growth and maintenance. They comprise the salts of the trace elements in animals (see essential element) and the micronutrients of plants.

  • Mist nutrient solution cultur e See Aer oponics .

  • Molybdenum (Mo) An essential element classed as a micronutrient. Molybdenum exists in the nutrient solution as the molybdate anion, MoO4 2–. The common reagent for making a nutrient solution is ammonium molybdate, (NH4)6Mo7O24•4H2O.

  • Mycorrhizae A symbiotic association of the mycellium of a fungus that infests plant roots and provides unique characteristics that benefit the absorption of nutrient elements and protects roots against adverse chemical conditions.

  • Necrosis The dead tissue on plant leaves and stems that results from poor nutrition, disease damage, overheating, etc.

  • Neem An oil extracted from neem (Azadirachta indica) tree seeds that has both fungicidal and insecticidal properties. Azadirachtin is the active ingredient in neem oil.

  • NFT See Nutrient film technique.

  • Nitrogen (N) An essential element classed as a major element. Nitrogen is found in the nutrient solution as either the nitrate (NO3 –) anion or the ammonium (NH4 +) cation. The common reagents for making a nutrient solution are ammonium nitrate, NH4NO3; potassium or calcium nitrate, KNO3 and Ca(NO3)2 . 4H2O, respectively; ammonium sulfate, (NH4)2SO4; and ammonium mono- or di-hydrogen phosphate, (NH4)2HPO4 and NH4H2PO4, respectively. Urea, CO(NH2)2, is also a commonly used N source, but it has only very special uses for the hydroponic and soilless grower.

  • Noncirculation hydroponic system A hydroponic growing system in which the nutrient solution is not reused.

  • Nutrient fi lm technique (NFT) A technique for growing plants hydroponically in which the plant roots are suspended in a slow-moving stream of nutrient solution. The technique was developed by Dr. Allen Cooper.

  • Nutrient solution (working solution)A water solution that contains one or more of the essential elements in suitable form and concentration for absorption by plant roots.

  • Open hydroponic system A growing system with one-way passage of the nutrient solution through the rooting medium or trough. After this single passage, the solution is discarded. (See Closed hydroponic system .)

  • Osmotic pr essur e Force exerted by substances dissolved in water that affects water movement into and out of plant cells. The salts dissolved in nutrient solutions exert some degree of force, which can restrict water movement into plant root cells or extract water from them.

  • Oxygen (O 2) An essential element classed as a major element. Oxygen is obtained from carbon dioxide (CO2) in the air; it is fixed during photosynthesis. (See Photosynthesis .)

  • PAR See Photosynthetic active radiation .

  • PAR The radiation in the wavelength range of 400 to 800 nm controlling the phytochrome reaction, which governs development and differentiation of growth responses (vegetative growth, flowering, reproduction, elongation, dormancy).

  • PAR, Photosynthetically Active Radiation The total radiation in the wavelength range of 400 to 700 nm contributing to photosynthetic productivity in relation to the relative quantum efficiency of the spectral quality of the radiation.

  • Passive absorption The movement of ions into plant roots carried along with water being absorbed by roots. (See Active absorption .)

  • Perlite An aluminosilicate of volcanic origin. When this natural substance is crushed and heated rapidly to 1000˚C, it forms a white, lightweight aggregate with a closed cellular structure. Perlite has an average density of 8 pounds per cubic foot (128 kg/m3), has virtually no cation exchange capacity, is devoid of plant nutrients, contains some fluoride (17 mg/kg, ppm), and is graded into various particle sizes for use as a rooting medium or added to soilless mixes.

  • Pesticide A chemical applied on or around plants to kill pests.

  • pH The negative logarithm of the hydrogen ion concentration to the base 10: pH = log10 × 1/[H+] As pH is logarithmic, the H+ concentration in solution increases ten times when the pH is lowered one unit. The pH of the nutrient solution and rooting medium will significantly affect the availability and utilization of the essential elements.

  • Phosphate Principle component of phosphate fertilizer; usually in the form of ammonium phosphate, potassium phosphate, or calcium phosphate. Plants take up and used phosphate ions (H2PO4- or HPO4=) for ATP, DNA, photsynthesis, respiration, and other metabolic functions. Phosphate does not have fungicidal properties.

  • Phosphite Alkali metal salts of phosphorous acid. The most common phosphite is potassium phosphite, and is made by mixing a solution of potassium hydroxide with phosphonic acid. Potassium phosphite is also referred to as mono- and di-potassium salts of phosphorous acid on some phosphonate product labels. Plants take up phophite ions (H2PO<sub3-) but they are not used in phophorus metabolism. Phosphite products have fungicidal properties.

  • Phosphonate Broadly, any compound containing a carbon to phosphorus bond. More commonly, used to describe products made of the salts or esters of phosphorus acid.

  • Phosphonic acid Strong acid produced by dissolving phosphorous acid in water. The term phosphonic acid is often used synonymously with phosphorous acid.

  • Phosphoric acid Strong acid used in the manufacture of phosphate fertilizer.

  • Phosphorous acid Anhydrous solid substance, often cited by its chemical formula HPO(OH)2 or H3PO3-. The basic ingredient in phosphonate products.

  • Phosphorus (P) An essential element classed as a major element. Phosphorus exists in the nutrient solution as an anion, either as H2PO4 – or HPO4 2–, depending on the pH. The common reagents for making a nutrient solution are ammonium or potassium mono- or di-hydrogen phosphate, (NH4)2HPO4, K2HPO4, NH4H2PO4, and KH2PO4, respectively, and phosphoric acid, H3PO4.

  • Photosynthesis A process by which green plants take carbon dioxide from the air, and water and inorganic nutrients from the soil, in the presence of light energy, to form carbohydrates and to release oxygen as a byproduct.

  • Photosynthesis The process that occurs in green plant leaves by which chloroplasts in the presence of light split water (H2O) and combine with carbon dioxide (CO2) to form simple carbohydrates and release oxygen (O2): 6CO2 + 6H2O —> chloroplasts in light —> C6H12O6 + 6O2.

  • Photosynthetic active radiation (P AR) That portion of the spectrum (450 to 700 nm) of solar radiation that participates in photosynthesis.

  • Pinebark A byproduct of the processing of pine, usually southern yellow pine, for lumber. Bark stripped from the tree is allowed to age (compost) in the natural environment for 6 months to 1 year and is then passed through a 1- inch screened hammer mill. The resulting material is screened into fractions of various sizes for addition to organic growing mixes. Pinebark has substantial cation exchange and water-holding capacities. Pinebark contains substantial quantities of Mn.

  • Plant analysis A method of determining the total elemental content of the whole plant or one of its parts and then relating the concentration found to the wellbeing of the plant in terms of its elemental requirements. (See Leaf analysis .)

  • Plant nutrient elements Those elements that are essential to plants. (See Major essential elements ; Macr onutrients ; Micronutrients .)

  • Plant nutrition The study of the effects of the essential as well as other elements on the growth and well-being of plants.

  • Plant requir ement That quantity of an essential element needed for the normal growth and development of the plant without inducing stress from a deficiency or an excess.

  • Potassium (K) An essential element classed as a major element. It exists in the nutrient solution as a monovalent (K+) cation. The common reagents for making a nutrient solution are potassium chloride (KCl) and potassium sulfate (K2SO4).

  • Radiation The emission and propagation of electromagnetic waves or particles through space or matter.

  • Radiation The process in which energy is emitted as particles or waves. In relation to plants, it can refer to amount of solar energy impacting the plant. For greenhouse topics, it is the amount of energy that is either received into the greenhouse or that which may be lost from it.

  • Raft system A method of hydroponic growing in which a plant is set into an opening in a sheet of material floating on a depth of nutrient solution with the plant roots extending into the nutrient solution. This method of growing is confined to growing lettuce and herbs.

  • Relative humidity The percent of water vapor in air. The amount of water vapor that can be suspended in air will depend on the temperature of the air.

  • Reverse osmosis A method of water purification in which ions are removed from water by an electrical potential placed on either side of a membrane that acts to extract ions from a passing stream of water.

  • RNCR, Room Capacity Ratio A number indicating room cavity proportions calculated from length, width, and height.

  • Rockwool An inert fibrous material produced from a mixture of volcanic rock, limestone, and coke, melted at 1500 to 2000˚C, extruded as fine fibers, and then pressed into loosely woven sheets. rockwool has excellent water-holding capacity. For growing uses, the rockwool sheets are formed into slabs or cubes.

  • Salt index A relative measure of the osmotic pressure of a solution of a fertilizer material in relation to an equivalent concentration of sodium nitrate (NaNO3) whose salt index is set at 100. (See Table 7.28.)

  • Sand cultur e A soilless culture technique where plants are grown in a bed containing sand, which is periodically bathed in nutrient solution.

  • Scorch Burned leaf margins. This visual symptom is typical of potassium deficiency or chloride excess.

  • Secondary elements Obsolete term once used as a classification term for three of the major essential elements, Ca, Mg, and S.

  • Siderophor es A substance released by plant roots to assist in the absorption of Fe. So-called “iron-sufficient” plants are those that release such substances.

  • Silicic acid, the plant available form of silicon. Soluble silicon in solution.

  • Slow-release nutrients A form of fertilizer that has been treated or coated so that its solubility can be controlled.

  • Sodium Absorption Ratio (SAR) A ratio used to express the relative activity of sodium (Na+) ions in relation to calcium (Ca2+) and magnesium (Mg2+) ions, expressed in milliequivalents per liter: SAR = Na+/ .

  • Soilless gar ening A term used to describe plant growing in other than soil.

  • Soilless rooting medium A plant rooting medium that does not contain soil, but consists of inorganic (gravel, sand, vermiculite, perlite, rockwool , pumice) and/or organic (sphagnum peat moss, pinebark, coir, sawdust, rice hulls) substances.

  • Soluble salts A measure of the concentration of ions in water (or nutrient solution) used to determine the quality of the water or solution, measured in terms of its electrical conductivity. [See Specifi c conductance ; Electrical conductivity (EC) .]

  • Specific conductance The reciprocal of the electrical resistance of a solution, measured using a standard cell and expressed as mhos per centimeter (mho/ cm) or decisiemens per meter (dS/m) at 25˚C: Specific Conductance This value is equal to θ /R where θ is the cell constant and R is the resistance in ohms. [See Electrical conductivity (EC) .]

  • Steiner nutrient solution A specifically formulated nutrient solution that contains the ions in solution in their ratio for ease of absorption by plant roots, a formulation developed by Dr. Steiner.

  • Stomata Minute openings in plant leaves where the exchange of gases (CO2 and O2) occurs and water vapor escapes. It is believed that the process of photosynthesis occurs in cells surrounding these openings. Stomata are surrounded by guard cells that can open and close the stomata depending on plant and atmospheric conditions.

  • Subirrigation A method of supplying water to plant roots by its introduction under plant roots.

  • Sufficiency Designation that an adequate supply of an essential element exists in the plant; also, an adequate concentration of an essential element in the plant to satisfy the plant’s physiological requirement. The plant in such a condition will look normal in appearance, be healthy, and be capable of high production.

  • Sulfur (S) An essential element classed as a major element. Sulfur exists in the nutrient solution as the sulfate (SO4 2–) anion. The common reagents for making a nutrient solution are potassium, magnesium, or ammonium sulfates, K2SO4, MgSO4•7H2O, and (NH4)2SO4, respectively.

  • Sump The reservoir for storage of the nutrient solution in closed, recirculating soilless culture systems.

  • Tissue testing A method for determining the concentration of the soluble form of an element in the plant by analyzing sap that has been physically extracted from a particular plant part, usually from stems or petioles. Tests are usually limited to the determination of nitrate, phosphate, K, and Fe. Tissue tests are normally performed using analytical kits, and the elemental concentration found is related to the well-being of the sampled plant. (See Plant analysis .)

  • Total dissolved solids (TDS) The concentrations of ions in solution measured in mg/L (ppm). TDS is related to the EC: EC (in dS/m) × 640 = TDS (mg/L, ppm). (See Electrical conductivity .)

  • Toxicity The condition in which an element is sufficiently in excess in the rooting medium, nutrient solution, or plant to be detrimental to the plant’s normal growth and development.

  • Trace element Once commonly used to designate those essential elements that are currently referred to as micronutrients; designates those elements found in plants at low concentration levels, usually at a few or less than 1 mg/kg (ppm) of the dry weight.

  • Tracking A technique of following through time the essential element content of the rooting medium or plant by frequent time-spaced analyses.

  • Valence The combining capacity of atoms or groups of atoms. For example, potassium (K+) and ammonium (NH4 +) are monovalent, whereas calcium (Ca2+) and magnesium (Mg2+) are divalent. Some elements may have more than one valance state, such as Fe, which can be either divalent (Fe2+) or trivalent (Fe3+). This change from one valance state to another involves the transfer of an electron.

  • Vermiculite An aluminum–iron–magnesium silicate. When heated for about one minute to 1,000˚C, this platelike, naturally occurring substance expands to 15 to 20 times its original volume, forming a lightweight, high-porosity material that has a density of about 5 pounds per cubic foot (80 kg/m3). Vermiculite has a fairly high cation-exchange capacity (100 to 150 meq/100 g) and contains plant-available K and Mg. Normally, vermiculite is added to an organic mix to increase the water-holding capacity of the mix, particularly for germinating mixes.

  • Zinc (Zn) An essential element classed as a micronutrient. Zinc exists in the nutrient solution as the divalent cation (Zn2+). The common reagent for making a nutrient solution is zinc sulfate, ZnSO4•7H2O.

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