Hydroponics & Fertigation Glossary of Terms

A reference guide to key terms used in hydroponics, soilless culture, plant nutrition, and fertigation. Use this glossary alongside our Fertigation Fertilizers guide, Water Quality reference, and Fertilizer Solubility Table to build a complete understanding of your nutrient program.

---

• 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. (See Absorptive capacity.)
• Aerobic: A condition in which ample O₂ is present in the rooting environment. (See Anaerobic.)
• Aerated standing nutrient solution culture: A method of growing plants hydroponically where plant roots are suspended in a container of continuously aerated nutrient solution. Deep water culture (DWC) is a popular method.
• Aeroponics: A technique for growing plants hydroponically where plant roots are suspended in a container and 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. (See Acidity.)
• Anaerobic: A condition in which O₂ is not present or exists at very low concentration in the rooting environment. (See Aerobic.)
• Anion: An ion in solution that has a negative charge. Common anions in nutrient solutions include BO₃³⁻, Cl⁻, H₂PO₄⁻, HPO₄²⁻, MoO₄²⁻, NO₃⁻, and SO₄²⁻. (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. Greatest when air temperature and movement are high and relative humidity is low.
• Availability: A term used to indicate that an element is in a form and position suitable for plant root absorption.
• Bag culture: A technique for growing plants in a bag of soilless medium (such as coconut coir, sphagnum peat moss, vermiculite, and/or perlite) into which a nutrient solution is applied periodically.
• BATO bucket: A bucket designed with a small reservoir in its base and a drainage nipple so that it can be set on a nutrient solution drainage line.
• Beneficial elements: Elements not essential for plants but which, when present at specific concentrations, enhance plant growth.
• Biological pest control: The use of predator insects or disease organisms to control plant pests, or the use of natural organic substances for control.
• Biopesticide: Pesticides derived from natural materials such as animals, plants, bacteria, and certain minerals (e.g., canola oil, baking soda).
• Biostimulant: Any substance or microorganism applied to plants to enhance nutrition efficiency, abiotic stress tolerance, and/or crop quality traits, regardless of nutrient content.
• Boron (B): An essential micronutrient. Exists in nutrient solution as borate (BO₃³⁻) or boric acid (H₃BO₃). Common reagents: boric acid, Solubor, borax. Shop Boron fertilizers.
• 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 forms a three-carbon carbohydrate; optimum photosynthesis temperature 60–77°F (15–25°C). Examples: tomato, cucumber, lettuce.
• C4 Plants: Plant species whose photosynthetic pathway forms a four-carbon carbohydrate; optimum photosynthesis temperature 85–105°F (30–40°C). Examples: corn, sorghum, rice, sugar cane.
• Calcium (Ca): An essential major element. Exists in nutrient solution as the divalent cation Ca²⁺. Common reagents: calcium nitrate, calcium chloride, calcium sulfate. Shop Calcium Nitrate.
• Calcium Nitrate: A highly soluble source of calcium and nitrogen. Chemical formula (solid): Ca(NO₃)₂·NH₄NO₃·10H₂O. Nutrient content: 15.5% N, 19–22% Ca. Water solubility (20°C): 1,200 g/L. Shop Calcium Nitrate fertilizers.
• Carbon (C): An essential major element obtained from CO₂ in the air, fixed during photosynthesis.
• Cation: An ion in solution that has a positive charge. Common cations in nutrient solutions include Ca²⁺, Cu²⁺, Fe³⁺, H⁺, K⁺, Mg²⁺, Mn²⁺, NH₄⁺, and Zn²⁺. (See Anion.)
• Chelates: Chemical compounds in which a metallic atom (such as Fe) is firmly combined with a molecule by multiple chemical bonds. Most commonly used chelates: EDTA and DTPA. Shop Albion Metalosate chelates.
• Chlorine (Cl): An essential micronutrient. Exists in nutrient solution as the monovalent anion Cl⁻. Not specifically added to nutrient solutions as it is ever-present in the environment.
• Chlorosis: A light-green to yellow coloration of leaves or whole plants, usually indicating an essential element insufficiency or toxicity. Most frequently associated with Fe deficiency.
• Closed hydroponic system: A growing system in which the nutrient solution is circulated and reused (recirculating hydroponics). (See Open hydroponic system.)
• Cocopeat (coir): Organic growing medium made from ground coconut hulls.
• Compost: A mixture of organic (sometimes inorganic) materials used as a rooting medium.
• Continuous flow nutrient solution culture: A method of soilless culture in which plant roots are continuously bathed in a flowing stream of nutrient solution.
• Controlled Environment Agriculture (CEA): Science describing systems of protected agriculture for ultimate environmental control at both aerial and root levels.
• Copper (Cu): An essential micronutrient. Exists in nutrient solution as the cupric cation (Cu²⁺). Common reagent: copper sulfate (CuSO₄·5H₂O). Shop Copper Chelate or Copper Sulfate.
• Cuticle: A very thin waxy film covering the surface of a plant leaf, derived from the outer surfaces of the epidermal cells.
• Deep flow technique: A method of NFT growing in which the depth of the trough is considerable.
• Deficiency: The condition when an essential element is not in sufficient supply or proper form to adequately supply the plant. Plants typically grow poorly and show visual signs of abnormality in color and 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 a concentration gradient.
• 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-flow (flood-and-drain): A hydroponic growing system in which plants are rooted in a watertight vessel containing a coarse inorganic substance, and the nutrient solution periodically floods the growing vessel from a sump.
• Electrical conductivity (EC): A measure of the electrical resistance of a nutrient solution used to determine the level of ions in solution. 1 dS/m = 1 mS/cm = 1,000 µS/cm = 1 mmho/cm. EC (dS/m) × 640 = TDS (mg/L, ppm). (See Specific conductance.)
• Essential elements: Those elements necessary for higher plants to complete their life cycles. Apart from C, H, O, and N found in organic compounds, plants require a range of inorganic elements in varying amounts.
• Feeding cycle: The time period when the nutrient solution is circulated through the root growing medium in systems where plant roots are only periodically exposed to the nutrient solution.
• 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. See our Foliar Fertilizing guide.
• Fungicide: A chemical compound applied to a plant to kill disease organisms.
• Glazing: Light-transmitting materials (glass, polyethylene film, fiberglass, polycarbonates) used to cover a greenhouse.
• Gravel culture: A soilless culture technique where plants are grown in beds containing gravel, periodically bathed in nutrient solution.
• Hoagland/Arnon nutrient solution: A widely used and modified formulation of reagents mixed in water to supply a plant with its essential elements hydroponically.
• Humate: An organic substance with unique physical and chemical properties, obtained by alkaline extraction from soil or organic material such as peat moss.
• Hydrogen (H): An essential major element obtained from water (H₂O); combined with CO₂ to form carbohydrates during photosynthesis.
• Hydroponics: A word coined in the early 1930s by Dr. W.F. Gericke to describe a soilless technique for growing plants. Derived from the Greek: hydro (water) + ponos (labor). Defined as the science of growing plants without soil, in an inert medium to which a nutrient solution containing all essential elements is periodically added.
• Integrated pest management (IPM): A precise system designed to combat the infestation of plants by disease organisms and insects.
• 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 cations and anions. Also refers to the physical–chemical attraction between charged colloidal substances with cations and anions.
• Iron (Fe): An essential micronutrient. Exists in nutrient solution as ferrous (Fe²⁺) or ferric (Fe³⁺) cation. Common reagents: iron chelates (FeEDTA, FeDTPA), ferrous and ferric sulfates. Shop Iron Chelate or Iron Sulfate.
• Leaf analysis: A method of determining the total elemental content of a leaf and relating this concentration to the well-being of the plant. (See Plant analysis.)
• Macronutrient: Refers to those nine essential elements (Ca, C, H, Mg, N, O, P, K, and S) found in the plant at relatively high concentration. (See Micronutrients.)
• Magnesium (Mg): An essential major element. Exists in nutrient solution as the divalent cation (Mg²⁺). Common reagent: magnesium sulfate (MgSO₄·7H₂O). Shop Magnesium Sulfate or Magnesium Nitrate.
• Manganese (Mn): An essential micronutrient. Exists in nutrient solution as the manganous cation (Mn²⁺). Common reagent: manganese sulfate (MnSO₄·4H₂O). Shop Manganese Chelate or Manganese Sulfate.
• Mass flow: The movement of ions as a result of the flow of water; ions are carried in the moving water.
• Micronutrients: The seven essential elements required by and found in relatively small concentrations in plant tissue: B, Cl, Cu, Fe, Mn, Mo, and Zn. Shop Chelated Micronutrient Mixes.
• Mineral nutrition: The study of the essential elements as they relate to the growth and well-being of plants.
• Molybdenum (Mo): An essential micronutrient. Exists in nutrient solution as the molybdate anion (MoO₄²⁻). Common reagent: ammonium molybdate. Shop Molybdenum fertilizers.
• Mycorrhizae: A symbiotic association of the mycelium of a fungus that infests plant roots, providing unique characteristics that benefit the absorption of nutrient elements and protect roots against adverse chemical conditions.
• Necrosis: Dead tissue on plant leaves and stems resulting from poor nutrition, disease damage, overheating, etc.
• NFT (Nutrient film technique): A technique for growing plants hydroponically in which plant roots are suspended in a slow-moving stream of nutrient solution. Developed by Dr. Allen Cooper.
• Nitrogen (N): An essential major element. Found in nutrient solution as nitrate (NO₃⁻) or ammonium (NH₄⁺). Common reagents: ammonium nitrate, potassium nitrate, calcium nitrate, ammonium sulfate, urea. Shop Calcium Nitrate or Potassium Nitrate.
• Nutrient solution (working solution): A water solution containing one or more 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; solution is discarded after a single pass. (See Closed hydroponic system.)
• Osmotic pressure: Force exerted by substances dissolved in water that affects water movement into and out of plant cells. High salt concentrations in nutrient solutions can restrict water movement into root cells.
• Oxygen (O₂): An essential major element obtained from CO₂ in the air, fixed during photosynthesis.
• PAR (Photosynthetically Active Radiation): That portion of the spectrum (400–700 nm) of solar radiation that participates in photosynthesis.
• 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 crushed and heated rapidly to 1,000°C, it forms a white, lightweight aggregate with a closed cellular structure. Average density: 8 lb/ft³ (128 kg/m³). Virtually no cation exchange capacity; devoid of plant nutrients.
• pH: The negative logarithm of the hydrogen ion concentration to the base 10. As pH is logarithmic, the H⁺ concentration increases tenfold when pH is lowered one unit. pH significantly affects the availability and utilization of essential elements.
• Phosphate: Principal component of phosphate fertilizer; usually in the form of ammonium, potassium, or calcium phosphate. Plants take up phosphate ions (H₂PO₄⁻ or HPO₄²⁻) for ATP, DNA, photosynthesis, and respiration. Phosphate does not have fungicidal properties.
• Phosphite: Alkali metal salts of phosphorous acid. The most common is potassium phosphite. Plants take up phosphite ions but they are not used in phosphorus metabolism. Phosphite products have fungicidal properties. Shop Phosphite fertilizers.
• Phosphonate: Broadly, any compound containing a carbon-to-phosphorus bond. More commonly used to describe products made of the salts or esters of phosphorous acid.
• Phosphoric acid: Strong acid used in the manufacture of phosphate fertilizer. Shop Phosphoric Acid.
• Phosphorous acid: Anhydrous solid substance (HPO(OH)₂ or H₃PO₃). The basic ingredient in phosphonate products.
• Phosphorus (P): An essential major element. Exists in nutrient solution as H₂PO₄⁻ or HPO₄²⁻ depending on pH. Common reagents: ammonium or potassium mono- or di-hydrogen phosphate, phosphoric acid. Shop MKP.
• Photosynthesis: The process by which green plants take CO₂ from the air and water from the soil, in the presence of light energy, to form carbohydrates and release oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
• Pinebark: A byproduct of pine lumber processing. Aged and hammer-milled into fractions for addition to organic growing mixes. Has substantial cation exchange and water-holding capacities. 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 relating the concentration found to the well-being of the plant. (See Leaf analysis.)
• Potassium (K): An essential major element. Exists in nutrient solution as the monovalent cation (K⁺). Common reagents: potassium chloride, potassium sulfate. Shop Potassium Nitrate or Potassium Sulfate.
• Raft system: A hydroponic growing method in which a plant is set into an opening in a sheet of material floating on a depth of nutrient solution. Primarily used for lettuce and herbs.
• Relative humidity: The percent of water vapor in air. The amount of water vapor that can be suspended in air depends on air temperature.
• 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.
• Rockwool: An inert fibrous material produced from a mixture of volcanic rock, limestone, and coke, melted at 1,500–2,000°C and extruded as fine fibers. Excellent water-holding capacity. Formed into slabs or cubes for growing use.
• Salt index: A relative measure of the osmotic pressure of a fertilizer solution in relation to an equivalent concentration of sodium nitrate (NaNO₃), whose salt index is set at 100.
• Scorch: Burned leaf margins. Typical of potassium deficiency or chloride excess.
• Slow-release nutrients: A form of fertilizer that has been treated or coated so that its solubility can be controlled.
• Soilless rooting medium: A plant rooting medium that does not contain soil, consisting 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, measured in terms of electrical conductivity. (See EC.)
• Stomata: Minute openings in plant leaves where the exchange of gases (CO₂ and O₂) occurs and water vapor escapes. Surrounded by guard cells that can open and close depending on plant and atmospheric conditions.
• Sufficiency: Designation that an adequate supply of an essential element exists in the plant to satisfy its physiological requirement. The plant will look normal, be healthy, and be capable of high production.
• Sulfur (S): An essential major element. Exists in nutrient solution as the sulfate anion (SO₄²⁻). Common reagents: potassium, magnesium, or ammonium sulfates. Shop Magnesium Sulfate.
• Sump: The reservoir for storage of the nutrient solution in closed, recirculating soilless culture systems.
• Systemic acquired resistance (SAR): A form of induced resistance activated throughout a plant after exposure to elicitors from virulent, avirulent, or nonpathogenic microbes, or artificial chemical stimuli such as chitosan or salicylic acid.
• Tissue testing: A method for determining the concentration of the soluble form of an element in the plant by analyzing sap extracted from a particular plant part, usually stems or petioles. Tests are normally performed using analytical kits.
• Total dissolved solids (TDS): The concentrations of ions in solution measured in mg/L (ppm). EC (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 normal growth and development.
• Trace element: Once commonly used to designate those essential elements now referred to as micronutrients; elements found in plants at low concentration levels, usually at a few or less than 1 mg/kg (ppm) of dry weight.
• Valence: The combining capacity of atoms or groups of atoms. For example, K⁺ and NH₄⁺ are monovalent; Ca²⁺ and Mg²⁺ are divalent. Some elements have more than one valence state, such as Fe (Fe²⁺ or Fe³⁺).
• Vermiculite: An aluminum–iron–magnesium silicate. When heated to 1,000°C, it expands to 15–20 times its original volume, forming a lightweight, high-porosity material. Density: ~5 lb/ft³ (80 kg/m³). High cation-exchange capacity (100–150 meq/100 g). Contains plant-available K and Mg.
• Zinc (Zn): An essential micronutrient. Exists in nutrient solution as the divalent cation (Zn²⁺). Common reagent: zinc sulfate (ZnSO₄·7H₂O). Shop Chelated Micronutrient Mix.