Glossary of Terms and Abbreviations Used in Hydroponics

Glossary of Terms and Abbreviations Used in Hydroponics

It can be quite baffling sometimes these days when reading about some subjects because they frequently use technical terms, abbreviations and acronyms.

Here is a list of the ones often used in hydroponics, along with a brief explanation for each one.


Abscission - The process whereby plants drop their leaves, flowers or fruits.


Acid / Acidity - This describes a solution which has pH of below 7.0 (a pH of 7.0 is called "neutral". A pH of above 7.0 is called alkaline).


Aeroponics - This type of hydroponics system where plants are suspended above an empty chamber where the roots grow into. Periodically, nutrient solution is forced through nozzles creating a fine mist. The high oxygenation in the root zone encourages very fast growth and a large, healthy root mass.


Alkaline / Alkalinity - This is the opposite of acid / acidity. Where acidity describes a solution with a pH of below 7.0, alkalinity describes a solution with a pH of above 7.0.


Aquaponics - This type of hydroponics involves the rearing of fish. The waste from the fish is then used as the main fertiliser for the plants.


Ballast - An electrical device which converts mains power into a form that is suitable for powering lights.


Beneficial Microbes - These are usually either bacteria or fungi that have a symbiotic relationship with plants. They are found naturally in soil and have a mutually beneficial effect for themselves and plant roots.


Biological Control - This is a method of pest or disease control using either certain insects like ladybirds, or bacteria which can kill diseases like powdery mildew. Often used in organic gardening.


CO2 (Carbon Dioxide) - This is a gas which occurs naturally in air. Animals exhale CO2 when they breathe. Plants absorb CO2 through their leaves and use it along with water and light to make sugars via photosynthesis. The sugars are then used to fuel growth.


Capillary Mat - This is a fabric, such as NFT spreader matting that can wick nutrient solution to deliver it to plant roots.


CDM/CMH - Stands for Ceramic Discharge Metal-Halide or just Ceramic Metal Halide. It is the improved version of the older MH HID lamps. They give off a full spectrum white light.


CF - Stands for Conductivity Factor. It is used as a measure of how strong a mineral nutrient solution is. Mineral nutrients are made from salts. As water contains more salts, it becomes more electrically conductive so the conductivity factor goes up. Other units of nutrient strength are E.C (electrical conductivity) and PPM (parts per million).


CFL - Stands for Compact Fluorescent Lamp. A fluorescent bulb-like fixture as opposed to a fluorescent tube


Chlorosis - A reduction in chlorophyll in plants causing them to go from being green to being yellow or brown. Usually caused by a nutrient deficiency.


Cloning - The process of taking cuttings from the side-shoots of a plant and then getting them to sprout their own roots, creating a genetic copy of the donor plant.


Coco Coir - A grow medium made from shredded coconut shells. It offers good drainage and is quite versatile.


Colour Temperature - This is a way of describing the colour of a light. The units are in "Kelvin" which is usually abbreviated to "K". The more red the light is the lower the colour temperature (e.g. 2K). The more blue it is the higher the colour temperature (e.g. 8K). Sunlight is around 5 Kelvin.


Damping Off - This is a fungal infection that usually attacks young plants and cuttings. It causes a darkening and withering usually at the base of the stem.


Dripper System - This is a type of hydroponics system where nutrient solution is periodically fed to plants though dripper stakes inserted into the top of the grow medium.


DWC - This stands for Deep Water Culture. It is a system where small net-pots are suspended above a container with nutrient solution in which the plant roots grow down into it. The nutrient solution needs to be aerated using an air pump and air stone. For that reason the containers are sometimes called "bubblers".


E.C. - Stands for Electrical Conductivity. It is used as a measure of how strong a mineral nutrient solution is. Mineral nutrients are made from salts. As water contains more salts, it becomes more electrically conductive so the E.C. goes up. Other units of nutrient strength are CF (conductivity factor) and PPM (parts per million).


Evaporation - This is where water dries into the air. This action can have a cooling effect on the surface it has evaporated from. This is why sweating helps cool us down.


Flood and Drain, or Ebb and Flow - This is a type of hydroponics system where periodically the pots containing the grow medium are wetted from the bottom upwards with nutrient solution. After a short time, the nutrient solution is drained back out of the pots.


Flushing - This where towards the end of grow cycle, just water is run through grow medium to wash out any nutrients. Then the plant is given just water to force it to use up any nutrients that may be inside it. A flush can also be used to help reset the grow medium if the plant has been over-fertilised or if it has a deficiency.


Fungicide - This is a type of chemical or biological control which works on fungal infections such as powdery mildew.


HID - High Intensity Discharge. This is a family of grow lamps that use an arc-tube filled with a substance that gives off intense light when fired up. Usually need to be driven by a ballast. They include HPS, MH and CDM/CMH lamps


HPS - High Pressure Sodium. This is a type of HID lamp like in the picture above. It gives off a characteristic orange-red light. This used to be the standard type of grow light for flowering due to its efficiency but it is now being surpassed by LED grow lights.


Hydroponics - This is a word that is derived from Greek and it literally means "working water". It is commonly used to refer to any system that automatically feeds nutrient solution to plants.


Insecticide - This is a type of chemical or biological method of pest control.


IPM - Integrated Pest Management. This is where different methods (e.g organic repellants and diatomaceous earth) are used together to control insects or diseases.


LED - Light Emitting Diode. An electronic semiconductor component that emits light when a current is run through it.


Lumen - A measurement of light that is visible to humans. Although it used to be used a lot to rate grow lights it is not really a good way of measuring them as it is not a measurement of PAR.


Macronutrients - These are the elements that plants need in relatively large amounts. They are Nitrogen, Phosphorus and Potassium.


Medium or Media - This is the material in which plants are grow. Examples are soil, coco or clay pebbles. Sometimes they are called "substrates".


MH - Metal Halide. A type of HID lamp which gives off a much whiter light than HPS. It was often used for vegging plants but it has now been surpassed by CDM/CMH.


Micromole - In science, a "Mole" is a number of particles of something like atoms, molecules, photons or even grains of sand. The actual number is 6.022 x 10^23 (6.022 x 10 to the power of 23) which is a very large number. When we talk about the number of photons in light, we use the term micromoles (umols) which is a millionth of a mole (6.022 x 10^17) and gives us a more convenient number, particularly for measuring phoons..


Micronutrients - These are elements that are required by plants in relatively small quantities. They are Boron, Chlorine, Cobalt, Copper, Iron, Manganese, Molybdenum and Zinc.


N-P-K ratio - This is the ratio of the 3 macronutrients - Nitrogen, Phosphorus and Potassium. You'll usually find on the back of a bottle of nutrients.


Nutrient Solution - This water with nutrients in it. For good growth, plants require 3 macronutrients, 3 secondary nutrients and about 8 micronutrients. The nutrient solution is given to plants periodically either by hand-watering or by an automatic feeding system (hydroponics).


PAR - Photosynthetically Active Radiation. Part of the light spectrum (from 400nm to 700nm) that plants use for photosynthesis. Usually measured in micromoles (umols) and used to rate grow lights.


Photon - Light travels in small "packets" called photons. A photon is the smallest possible unit of light. Because they are so small they are often measured in millionths of a mole called micromoles (umols)


Photoperiod - This the number of hours per day that a plant receives light as compared to the number of hours of darkness. Common photoperiods are 18/6 (for veg) and 12/12 (for flowering/fruiting)


Photosynthesis - This is the process whereby plants collect PAR light that hits their leaves and uses the energy in that light to turn carbon dioxide from the air and water, into sugar and oxygen. The sugar is used to fuel growth and the oxygen is released into the air.


PPF - Photosynthetic Photon Flux. This is the total amount of photons in the PAR region of the light spectrum that a grow light puts out per second (umols/sec). It gives an idea of the total amount of photosynthesis a grow light can trigger.


PPFD - Photosynthetic Photon Flux Density. Like PPF, this is a measurement of how much PAR light a grow light puts down but PPFD is measured over a certain area and the units are umols/sec/m2. It is really a measurement of the intensity of light at a certain point.


PPM - Stands for Parts Per Million. It is used as a measure of how strong a mineral nutrient solution is. Mineral nutrients are made from salts. As water contains more salts (dissolved solids), the ppm goes up. Other units of nutrient strength are CF (conductivity factor) and E.C. (electrical conductivity). Slightly confusingly, there are 3 scales of PPM - 500, 650 and 700. Because of this, many growers prefer to use CF or E.C. which only have one scale.


Recirculating System - This is a system where nutrient solution is stored in a reservoir and is fed to plants periodically. After the plants have been fed the nutrient solution is re-stored to be used again.


RH (Relative Humidity) - The amount of water that air can hold depends on the air temperature. Warm air can hold a greater amount of water than cold air. However, if we say that (for any temperature) that the air is holding as much water as it can then the relative humidity (RH) is 100%.


Run-to-Waste System - This is a type of system where nutrient solution is only used once rather being recirculated. It avoids the nutrient solution becoming unbalanced over time.


Secondary Nutrients - These are the elements that are needed in medium quantities. They are Calcium, Magnesium and sulphur.


Stomata - These are the tiny breathing pores on the underside of plant leaves. They take in Carbon Dioxide and release water in the process of transpiration and oxygen produced by photosynthesis.


Substrate - This is another term for media or medium, or the material in which plants grow (soil, coco, clay pebbles etc.)


Systemic - This refers to something that is within a plant like a disease, rather than on its surface.


Transpiration - Plants are a little bit like water pumps. They draw up water (which contains nutrients) via the roots. Some of that water is used for photosynthesis and the rest is lost through breathing pores on the underside of the leaves called stomata. This process is called transpiration. 


Vapour Pressure Deficit (VPD) - This could just as well be called the "drying ability of air". Plants obtain water and nutrients through a process called transpiration (see above). The rate at which this can occur depends upon the drying ability of the air. The drying ability of the air depends on a combination of the temperature and the relative humidity. In cold and humid air plants find it difficult to lose water through their leaves. This means plants find it difficult to uptake nutrients and there is also an increased risk of mold infections. In very warm and dry air plants lose water too quickly which can lead to wilting. There is a sweet-spot where the combination of temperature and humidity is just right and plants can "transpire" optimally, and therefore grow.

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