The two main types of hydroponics are solution culture and medium culture. Solution culture does not use a solid medium for the roots, just the nutrient solution. The three main types of solution culture are static solution culture, continuous flow solution culture and aeroponics. The medium culture method has a solid medium for the roots and is named for the type of medium, e.g. sand culture, gravel culture or rockwool culture. There are two main variations for each medium, subirrigation and top irrigation. For all techniques, most hydroponic reservoirs are now built of plastic but other materials have been used including concrete, glass, metal and wood. The containers should exclude light to prevent algae growth in the nutrient solution.
Static solution culture
In static solution culture, plants are grown in containers of nutrient solution, such as glass Mason jars, plastic buckets, tubs or tanks. The solution is usually gently aerated but may be unaerated. If unaerated, the solution level is kept low enough that enough roots are above the solution so they get adequate oxygen. A hole is cut in the lid of the reservoir for each plant. There can be one to many plants per reservoir. Reservoir size can be increased as plant size increases. A homemade system can be constructed from plastic food containers or glass canning jars with aeration provided by an aquarium pump, aquarium airline tubing and aquarium valves. Clear containers are covered with aluminum foil, butcher paper, black plastic or other material to exclude light. The nutrient solution is either changed on a schedule, such as once per week, or when the concentration drops below a certain level as determined with as electrical conductivity meter. Whenever the solution is depleted below a certain level, either water or fresh nutrient solution is added. A Mariotte's bottle can be used to automatically maintain the solution level. In raft solution culture, plants are placed in a sheet of buoyant plastic that is floated on the surface of the nutrient solution. That way, the solution level never drops below the roots.
Continuous flow solution culture
In continuous flow solution culture the nutrient solution constantly flows past the roots. It is much easier to automate than static solution culture because sampling and adjustments to pH and nutrient concentrations can be made in a large storage tank that serves potentially thousands of plants. A popular variation is the nutrient film technique or NFT. In NFT, the plants grow through light-proof plastic films placed over shallow, gently sloping channels. A steady flow of nutrient solution is maintained along the channel, and the roots grow into dense mats, with a thin film of nutrient passing over them (hence the name of the technique). A downside of NFT is that it has very little buffering against interruptions in the flow e.g. power outages, but overall, it is probably one of the more productive techniques.
Static solution culture
In static solution culture, plants are grown in containers of nutrient solution, such as glass Mason jars, plastic buckets, tubs or tanks. The solution is usually gently aerated but may be unaerated. If unaerated, the solution level is kept low enough that enough roots are above the solution so they get adequate oxygen. A hole is cut in the lid of the reservoir for each plant. There can be one to many plants per reservoir. Reservoir size can be increased as plant size increases. A homemade system can be constructed from plastic food containers or glass canning jars with aeration provided by an aquarium pump, aquarium airline tubing and aquarium valves. Clear containers are covered with aluminum foil, butcher paper, black plastic or other material to exclude light. The nutrient solution is either changed on a schedule, such as once per week, or when the concentration drops below a certain level as determined with as electrical conductivity meter. Whenever the solution is depleted below a certain level, either water or fresh nutrient solution is added. A Mariotte's bottle can be used to automatically maintain the solution level. In raft solution culture, plants are placed in a sheet of buoyant plastic that is floated on the surface of the nutrient solution. That way, the solution level never drops below the roots.
Continuous flow solution culture
In continuous flow solution culture the nutrient solution constantly flows past the roots. It is much easier to automate than static solution culture because sampling and adjustments to pH and nutrient concentrations can be made in a large storage tank that serves potentially thousands of plants. A popular variation is the nutrient film technique or NFT. In NFT, the plants grow through light-proof plastic films placed over shallow, gently sloping channels. A steady flow of nutrient solution is maintained along the channel, and the roots grow into dense mats, with a thin film of nutrient passing over them (hence the name of the technique). A downside of NFT is that it has very little buffering against interruptions in the flow e.g. power outages, but overall, it is probably one of the more productive techniques.
Aeroponics
In aeroponics, the roots of a plant are suspended in a darkened chamber and periodically covered with a mist or fog of nutrient solution. No solid medium is used. Traditional aeroponic techniques use pumps and misters more commonly found in micro-irrigation systems, whereas state-of-the-art techniques employ ultrasonic nebulizers which render the nutrient solution into an extremely fine fog.
Ultrasonic foggers mainly consist of piezioelectric disks used to vibrate water in a hydroponic reservoir at ultra high frequencies. This process turns the water and nutrients into droplets 5-15 micron in size. This allows for better nutrient uptake, with the addition of exponentially high oxygen ratios over the already established efficiency of aeroponics. Often ultrasonic foggers are placed in floats and allowed to produce a thick cloud of fog around the base of the plants root structure suspended within the reservoir.
The Land Exhibit at EPCOT Center has aeroponics in vertical sections of large-diameter plastic pipe. Plants are placed through holes drilled in the side of the pipe so roots are inside. The pipe sections are suspended from the greenhouse ceiling and move continuously around the greenhouse on a motorized system. Periodically they pause under a mist nozzle to be irrigated. Aeroponics may be the best method for plants with thick roots such as trees. Thick roots may not get adequate aeration in static or flowing systems.
Passive subirrigation
The medium generally has large air spaces, allowing ample oxygen to the roots, while capillary action delivers water and nutrients to the roots from the base of the medium. The simplest method has the container constantly sit in a shallow layer of nutrient solution or on a capillary mat saturated with nutrient solution. A variety of materials can be used for the medium: vermiculite, perlite, clay granules, rockwool, gravel or Oasis Horticubes. This method requires little maintenance, requiring only occasional refilling and replacement of the nutrient solution. This keeps the medium regularly flushed with nutrient solution and air.
It is important in passive subirrigation to wash out the system from time to time to remove salt accumulation. This may be checked with an electrical conductivity or ppm meter, a good average reading would be about 1500 ppm. Lettuce grows well at about 800 ppm and tomatoes to 3000 ppm but both will grow reasonably well on 1500 ppm. It is important to keep the pH reading at about 6.3 to enable nutrient uptake. Data are available for the optimum settings for most plants.
This is commonly employed for large display plants in public buildings: in Europe a system using small clay granules is marketed for growing houseplants. One method for home use is called semi-hydroponic for growing orchids. A similar subirrigation method, uses a wick. The wick runs from the base of the plant container (e.g. a pot or a tray) down to a bottle of nutrient solution. The solution travels up the wick into the medium through capillary action.
Flood and drain (or ebb and flow system) subirrigation
In its simplest form, there is a tray above a reservoir of nutrient solution. The tray is either filled with growing medium (clay granules being the most common) and planted directly, or pots of medium stand in the tray. At regular intervals, a simple timer causes a pump to fill the upper tray with nutrient solution, after which the solution drains back down into the reservoir. This keeps the medium regularly flushed with nutrients and air.
Top irrigation
In top irrigation, nutrient solution is periodically applied to the medium surface. This may be done manually once per day in large containers of some media, such as sand. Usually, it is automated with a pump, timer and drip irrigation tubing to deliver nutrient solution as frequently as 5 to 10 minutes every hour.
Deep Water Culture (DWC)
Deep Water CultureThe hydroponic method of plant production by means of suspending the plant roots in a solution of nutrient rich, oxygenated water. Traditional methods favour the use of plastic buckets with the plant contained in a net pot suspended from the centre of the lid and the roots suspended in the nutrient solution.
