Acid: A sour substance: An acid or sour soil or solution has a low pH.

Aeration: Supplying soil and roots with air or oxygen.

Alkaline: Refers to soil or solution with a high pH. Any pH over 7 is considered alkaline.

Aeroponics: a variation of hydroponics that involves the misting of plant roots with nutrient solution.

Allele: different forms of the same gene; allele "A" may produce a tall plant, while allele "a" gives a short plant.

Anther: part at the top of the male flower that produces the pollen.

Aquaponics: the integration of aquaculture (the raising of marine animals, such as fish) with hydroponics; the waste products from the fish are treated and then used to fertilize hydroponically growing plants.

Bacteria: Very small, one-celled organisms that have no chlorophyll.

Bacterial soft rot. See Botrytis.

Ballast: The electrical components (usually a transformer and capacitor) that energize a high intensity bulb.

Beneficial Insect: A good insect that eats bad flower and vegetable-munching insects.

Biodegradable: Able to decompose or break down through natural bacterial action: Substances made of organic matter are biodegradable.

Bloom Booster, Blossom Booster: Fertilizer high in phosphorus (P) that increases flower yield.

Bolting: for a plant to prematurely begin the development of a flowering stalk and, subsequently, seed.

Boron (B): The function of this micronutrient is not well understood, but it is suspected that it might aid carbohydrate transport.

Botrytis: any of several fungal diseases that afflict plants; commonly called bacterial soft rot or gray mold.

Buffering: The ability of a substance to reduce shock and cushion against pH fluctuations.

Burn: Leaf tips that turn dark from excess fertilizer and salt burn.

Calcium (Ca): Calcium is vital in all parts of plants to promote the translocation of carbohydrates, healthy cell wall structure, strong stems, membrane maintenance and root structure development. Calcium is a macronutrient.

Carbon Dioxide(CO2): A colorless, odorless, tasteless gas in the air necessary for plant life. Occurs naturally in the atmosphere at .03%.

Capillary action: when the surface of a liquid is in conact with a solid, the liquid is elevated or depressed depending upon the relative attraction of the molecules of the liquid for each other or for those of the solid. This is similar to how plants seemingly defy gravity when they transport liquid from the roots upward through the plant.

CF: conductivity factor: see Conductivity

Chelate: Combining nutrients in an atomic ring that is easy for plants to absorb.

Chlorine (Cl): This micronutrient is essential for photosynthesis, where it acts as an enzyme activator during the production of oxygen from water.

Chlorophyll: the green material in plants that is created in the presence of light and is instrumental in photosynthesis.

Chlorosis: The condition of a sick plant with yellowing leaves due to inadequate formation of chlorophyll. Chlorosis is caused by a nutrient deficiency, usually iron or nitrogen; nutrient deficiencies are themselves often caused by a pH that is out of the acceptable range.

Cladosporium: any of several fungal diseases that afflict plants; commonly called leaf mold.

Clone: A plant produced through asexual reproduction including, but not limited to, cuttings, layering and tissue culture.

Closed system: a hydroponic system, like nutrient film technique (NFT) systems, that recirculates the nutrient solution.

Color Temperature: (also known as Kelvin temperature or correlated color temperature)  A measure of color of light emitted by a bulb in comparison to black.  This is used as a general meaure of a bulb's coolness (whiter light) or warmness (redder light).

Conditioning: To soak new Rockwool in an acidic solution to lower the pH from 8.0 to 5.5.

Conductivity: the scale, described as electrical conductivity (EC) or conductivity factor (CF), that is used to measure the strength of nutrient solution. Pure water doesn’t conduct electricity. But as salts are dissolved into the pure water, electricity begins to be conducted. An electrical current will begin to flow when live electrodes are placed into the solution. The more salts that are dissolved, the stronger the salt solution and, correspondingly, the more electrical current that will flow. This current flow is connected to special electronic circuitry that allows the grower to determine the resultant strength of the nutrient solution.

Copper (Cu): This micronutrient is an internal catalyst and acts as an electron carrier; it is also believed to play a role in nitrogen fixation.

Cross-pollination: transferring pollen from the flowers of one plant of a species to the stigma of another plant of the same species.

Damping-off Fungus: Disease that attacks young seedlings and cuttings, causing stems to rot at the base; overwatering is the main cause of damping-off.

Deficiency: See mineral deficiency.

Dioecious: varieties or species with male and female flowers on separate plants.

Dissolved Solids: The amount of dissolved solids, usually fertilizer salts, that are measured in water in parts per million.

Drip Aeration: A hydroponic method wherein air pressure from a small air pump is used to percolate nutrient solution out through a ring of feeder tubing which encircles the plant.

Drip Method: A very efficient watering system that employs a main hose with small water emitters. Water is metered out of the emitters, one drop at a time. The most popular among commercial applications: leaching of salt build up, smaller reservoirs, and less nutrients are just a few of the positive traits of a drip system.

Dry rot: See Fusarium.

Ebb & Flow: A hydroponic system in which the medium, usually aggregate pebbles, is periodically flooded with nutrient solution and then drained again, feeding and aerating the medium and root system.

EC: electrical conductivity: See conductivity.

F1, F2, F3, etc.: the F1 generation is the result of crossing two different varieties; a cross of two F1 plants produces F2 seed; and so on.

Fertilizer Burn: Over fertilization: First leaf tips burn (turn yellow or brown) then the leaves curl.

Filament: see stalk.

Foliar Feeding: Misting plants with fertilizer solution which is absorbed by the foliage.

Footcandle: A unit of illumination equal to the intensity of one candle at a distance of one foot.  Footcandles are usually used as a measure of light received.

Fungicide: A product that destroys or inhibits fungus.

Fungus: A lower plant lacking chlorophyll which may attack green plants: Mold, rust, mildew, mushrooms and bacteria are fungi. Common fungal diseases that attack plants are "damping-off," Botrytis, and powdery mildew.

Fusarium: any of several fungal diseases that afflict plants; commonly called dry rot or wilt.

Geolite: One of several brand names/varieties of clay aggregate medium (also known as LECA for light expanded clay aggregate). It is a lightweight, porous substrate with excellent aeration. Because it does not really wick water effectively, Geolite and other LECA mediums are favorites in ebb-and-flow and drip hydroponic systems.

Germination: the activation of a seed causing it to start to grow; also the production of a pollen tube by a pollen grain

Gray mold: See Botrytis.

Growing medium: materials that are sometimes used in hydroponic growing to support the plant's roots and, sometimes, to hold nutrient.

Harden-off: To gradually acclimatize a plant to a more harsh environment. A seedling must be hardened-off before planting outdoors.

H.I.D. (High Intensity Discharge) Lights: Metal Halide (MH) and High Pressure Sodium (HPS) lamps belong to the HID family of lights. They are used for garden lighting because of their high light output per watt, and the spectral distribution of their light.

Hormone: Chemical substance that controls the growth and development of a plant. Root-inducing hormones help cuttings root.

Humidity (Relative): Ratio between the amount of moisture in the air and the greatest amount of moisture the air could hold at the same temperature.

Hybrid: The offspring from two plants of different breeds, variety or genetic make-up.

Hydrated Lime: Instantly soluble lime, used to raise or lower pH.

Hygrometer: Instrument for measuring relative humidity in the atmosphere: A hygrometer will save time, frustration and money.

Insects: a variety of insects attack plants. These include aphids, caterpillars, cutworms, fungus gnats, leaf miners, nematodes, spider mites, thrips, and whiteflies.

Iron (Fe): This micronutrient acts as a catalyst in the photosynthesis/respiration process, and is essential for the formation of sugars and starches. Iron also activates certain other enzymes.

Leach: Dissolve or wash out soluble components of soil by heavy watering.

Leaf Curl: Leaf malformation due to over watering, over fertilization, lack of magnesium, insect or fungus damage or negative tropism.

Leaf mold: See Cladosporium.

Leggy: Abnormally tall, with sparse foliage: Leggyness of a plant is usually caused by lack of light.

Light Mover: A device that moves a lamp back and forth across the ceiling of a garden room to provide more even distribution of light.

Lumen: A unit of illumination; a measurement of light output.  One lumen is equal to the light emitted by one candle that falls on one square foot of surface located one foot away from the candle.

Macronutrients: the major minerals that are used by plants in large amounts, consisting of nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg).

Manganese (Mn): This micronutrient activates one or more enzymes in fatty acid synthesis; it also activates the enzymes responsible for DNA and RNA production. Closely associated with copper and zinc, manganese also participates directly in the photosynthetic creation of oxygen from water.

Medium: The substrate or soilless material which supports the plant and absorbs and releases the nutrient solution in hydroponic horticulture.

Micronutrients: the minor minerals that are used by plants in small amounts, consisting of boron (B), copper (Cu), cobalt (Co), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn).

Mineral deficiency: when a plant is not receiving a required nutrient--at all or in an insufficient amount--a disorder will result.

Molybdenum (Mo): This micronutrient is essential for nitrogen fixation and nitrate reduction.

Monoecious: varieties or species with separate male and female flowers on each plant.

Necrosis: The dying of plant tissue, usually the result of serious nutrient deficiency or pest attack.

Nitrogen (N): Nitrogen is used in various forms to promote rapid vegetative growth, leaf, flower, fruit and seed development, and chlorophyll development; and to increase the protein content in all plants.

Nutrient Film Technique: NFT is the method of using shallow troughs with lids where a thin nutrient solution film streams along the bottom of the channels. Ideal for lettuce, herbs, strawberries and flowers.

Nutrient Solution: minerals dissolved in water that are used to feed hydroponically grown plants.

Nutients: The elements needed by plants for normal growth and health. The major nutrients (MACRONUTRIENTS) are nitrogen (N), phosphorus (P), and potassium (K), but there are numerous MICRONUTRIENTS (also called TRACE ELEMENTS) which also have integral roles in maintaining plant health. A good quality hydroponic nutrient formula will contain all of the major nutrients and micronutrients needed by the vast majority of plants.

Nutients, Secondary: The elements other than N, P and K which are considered nearly as important as N, P and K but are not considered micronutrients. Calcium (Ca), magnesium (Mg), and sulphur (S) are the elements usually referred to by this term.

Osmosis: the flow or diffusion that takes place through a semipermeable membrane typically separating a solvent and a solution that strives to bring about a condition of equilibrium.

Parts per million (ppm): a ratio figure that represents the amount of one substance that is in one million parts of another substance; commonly used to describe the relative concentrations of nutrient solutions.

Perlite: Sand or volcanic glass which has been expanded by heat; perlite holds water and nutrients on its many irregular surfaces.

pH: a measurement of a nutrient solution's relative concentration of positive hydrogen ions: a pH of 7 is considered neutral; below 7 is called acidic; above 7 is called alkaline.

Photoperiod: The relative hours of light and darkness in a 24-hour period.  Some plants respond to a change in day length (photoperiod) in order to grow or flower.

Phosphorus (P): Phosphorus promotes and stimulates early growth and blooming and root growth. It hastens maturity and seed growth, and contributes to the general hardiness of plants. Phosphorus is a macronutrient.

Photosynthesis: the formation of carbohydrates from carbon dioxide (CO2) and a source of hydrogen (H)--such as water--in chlorophyll-containing cells exposed to light involving a photochemical release of oxygen through the decomposition of water.

Pistil: the entire female section of the flower, including the eggs, ovary, style, and stigma.

Pollen: the mass of microspores in a seed plant that usually appears as a fine dust and is the agent of pollination.

Pollination: the transfer of pollen from the male part of a flower (stamen) to the female part of a flower (the style and stigma).

Potassium (K): Potassium promotes disease resistance and good development of carbohydrates, starches and sugars, and it increases fruit production. Potassium is a macronutrient.

Primary Nutrients: N (nitrogen), P (phosphorus, K (potassium).

Propagate: 1. Sexual–produce a seed by breeding different male and female flowers 2. Asexual–to produce a plant by taking cuttings.

Pyrethrum: Natural insecticide made from the blossoms of various chrysanthemums. Pyrethrum is the most effective natural spider mite exterminator.

Reservoir: The container in a hydroponic system which holds nutrient solution in reserve for use.

Reverse osmosis: the process of removing minerals from water, which is forced by pressure through a differentially permeable membrane, filtering out the minerals; can happen when growers accidentally apply too strong of a nutrient to a plant's roots, leeching life out of the plant.

Rockwool: Inert, soilless growing medium consisting of woven, thin strand-like fibers made from molten volcanic rock and limestone, which is heated to over 2900 degrees F, extruded, and formed into slabs, cubes and blocks.

Secondary Nutrients: Calcium (Ca) and magnesium (Mg).

Stalk: on a male flower, the portion of the stamen that supports the anther.

Stamen: the basic part of the male flower that includes the stalk (or filament), anther, and pollen.

Sterilization: the act of rendering something free from living cells. In hydroponics it is essential that all materials (especially any growing medium) used are sterile to avoid contaminating the hydroponic system. Steam and chemical agents are often used in this process.

Stigma: the surface at the end of the pistil on a female flower where pollen lands and germinates.

Style: the part of a female flower that supports the stigma.

Systemic: Used in reference to a disease within the plant tissue, not initiated from the external cells. Also refers to materials and compounds which are taken up or absorbed by the plant and designed to fight disease (e.g. systemic fungicide).

Trellis: Frame of small boards (lattice) that trains or supports plants.

Vermiculite: Mica which has been processed and expanded by heat. Vermiculite has excellent water-retention qualities and is a good soil amendment and medium for rooting cuttings.

Verticillium: any of several fungal diseases that afflict plants; commonly called wilt. See also Fusarium.

Wick: woven fiber used in some hydroponic systems to draw nutrient to a plant's roots through capillary action.

Wilt: See Fusarium and Verticillium.

Zinc (Zn): Like copper and manganese, zinc is linked to chlorophyll synthesis.