Hydroponics is a method of growing plants using mineral nutrient solutions instead of soil. Terrestrial plants may be grown with their roots in the mineral nutrient solution only or in an inert medium, such as perlite, gravel or Rockwool. A variety of techniques exist.
Plant physiology researchers discovered in the 1800s that plants absorb essential mineral nutrients as inorganic ions in water. In natural conditions, soil acts as a mineral nutrient reservoir but the soil itself is not essential to plant growth. When the mineral nutrients in the soil dissolve in water, plant roots are able to absorb them. When the required mineral nutrients are introduced into a plant's water supply artificially, soil is no longer required for the plant to thrive. Almost any terrestrial plant will grow with hydroponics, but some will do better than others. It is also very easy to do; the activity is often undertaken by very young children with such plants as watercress. Hydroponics is also a standard technique in biology research and teaching and a popular hobby.

History

Origin
Gericke originally defined hydroponics as crop growth in mineral nutrient solutions, with no solid medium for the roots. He objected in print to people who applied the term hydroponics to other types of soilless culture such as sand culture and gravel culture. The distinction between hydroponics and soilless culture of plants has often been blurred. Soilless culture is a broader term than hydroponics; it only requires that no soils with clay or silt are used. Note that sand is a type of soil yet sand culture is considered a type of soilless culture. Hydroponics is always soilless culture, but not all soilless culture is hydroponics. Many types of soilless culture do not use the mineral nutrient solutions required for hydroponics.
Billions of container plants are produced annually, including fruit, shade and ornamental trees, shrubs, forest seedlings, vegetable seedlings, bedding plants, herbaceous perennials and vines. Most container plants are produced in soilless media, representing soilless culture. However, most are not hydroponics because the soilless medium often provides some of the mineral nutrients via slow release fertilizers, cation exchange and decomposition of the organic medium itself. Most soilless media for container plants also contain organic materials such as peat or composted bark, which provide some nitrogen to the plant. Greenhouse growth of plants in peat bags is often termed hydroponics, but technically it is not because the medium provides some of the mineral nutrients. Peat has a high cation exchange capacity and must be amended with limestone to raise the pH.

Soilless culture

While removing soil-grown crops from the ground effectively kills them, hydroponically grown crops such as lettuce can be packaged and sold while still alive, greatly increasing the length of freshness once purchased.
Solution culture hydroponics does not require disposal of a solid medium or sterilization and reuse of a solid medium.
Solution culture hydroponics allows greater control over the root zone environment than soil culture.
Over- and under-watering is prevented
Hydroponics is often the best crop production method in remote areas that lack suitable soil, such as Antarctica, space stations, space colonies, or atolls such as Wake Island.
In solution culture hydroponics, plant roots can be seen.
Soil borne diseases are virtually eliminated.
Weeds are virtually eliminated.
Fewer pesticides may be required because of the above two reasons.
Edible crops are not contaminated with soil.
Water use can be substantially less than with outdoor irrigation of soil-grown crops.
Hydroponics cost 20% less than other ways for growing strawberries.
Many hydroponic systems give the plants more nutrition while at the same time using less energy and space.
Hydroponics allow for easier fertilization as it is possible to use an automatic timer to fertilize the plants.
It provides the plant with balanced nutrition because the essential nutrients are dissolved into the water-soluble nutrient solution. Advantages

If timers or electric pumps fail or the system clogs or springs a leak, plants can die very quickly in many kinds of hydroponic systems.
The plants will die if not frequently monitored while soil plants do not require such close attention. Disadvantages
Hydroponics has been widely misconceived as miraculous. Common misconceptions
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, vegetable solids and wood. The containers should exclude light to prevent algae growth in the nutrient solution.

Techniques
In static solution culture, plants are grown in containers of nutrient solution, such as glass Mason jars (typically in-home applications), 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 fugifilm 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 an 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.

Static solution culture
In continuous flow solution culture the nutrient solution constantly flows past the roots. It is much harder to automate than the static solution culture because sampling and adjustments to degree 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 whereby a very shallow stream of water containing all the dissolved nutrients required for plant growth is recirculated past the bare roots of plants in a watertight gully, also known as channels. Ideally, the depth of the recirculating stream should be very shallow, little more than a film of water, hence the name 'nutrient film'. This ensures that the thick root mat, which develops in the bottom of the channel, has an upper surface which, although moist, is in the air. Subsequerntly, there is an abundant supply of oxygen to the roots of the plants. A properly designed NFT system is based on using the right channel slope, the right flow rate and the right channel length. The main advantage of the NFT system over other forms of hydroponics is that the plant roots are exposed to adequate supplies of water, oxygen and nutrients. In all other forms of production there is a conflict between the supply of these requirements, since excessive or deficient amounts of one results in an imbalance of one or both of the others. NFT, because of its design, provides a system where all three requirements for healthy plant growth can be met at the same time, providing the simple concept of NFT is always remembered and practised. The result of these advantages is that higher yields of high quality produce are obtained over an extended period of cropping. 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.
The same design characteristics apply to all conventional NFT systems. While slopes along channels of 1:100 have been recommended, in practice it is difficult to build a base for channels that is sufficiently true to enable nutrient films to flow without ponding in locally depressed areas. Consequently, it is recommended that slopes of 1:30 to 1:40 are used. This allows for minor irregularities in the surface but, even with these slopes, ponding and waterlogging may occur. The slope may be provided by the floor, or benches or racks may hold the channels and provide the required slope. Both methods are used and depend on local requirements, often determined by the site and crop requirements.
As a general guide, flow rates for each gully should be 1 litre per minute. At planting, rates may be half this and the upper limit of 2L/min appears about the maximum. Flow rates beyond these extremes are often associated with nutritional problems. Depressed growth rates of many crops have been observed when channels exceed 12 metres in length. On rapidly growing crops, tests have indicated that, while oxygen levels remain adequate, nitrogen may be depleted over the length of the gully. Consequently, channel length should not exceed 10-15 metres. In situations where this is not possible, the reductions in growth can be eliminated by placing another nutrient feed half way along the gully and reducing flow rates to 1L/min through each outlet.

Continuous flow solution culture

Main article: Aeroponics Aeroponics
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, or gravel. 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.
Additional advantages of these sterile porous media are the reduction of root rotting conditions and the additional ambient humidity provided. These advantages are particularly important in the use of hydroponics for orchid cultivation.
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. 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.

Passive 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

Main article: Deep water culture Deep water culture

Main article: Organoponics Organoponics
One of the most obvious decisions a hydroponicist has to make is which medium they should use. Different media are appropriate for different growing techniques.

Media
Natural sedimentary rock medium. Diahydro consists of the fossilized shells of algae (diatoms) that lived millions of years ago. Diahydro is extremely high in Silica (87-94%), an essential component for the growth of plants and strengthening of cell walls.

Diahydro
Also known as 'Hydroton' or 'leca' (light expanded clay aggregate), trademarked names, these small, round baked spheres of clay are inert and are suitable for hydroponic systems in which all nutrients are carefully controlled in water solution. The clay pellet is also inert, pH neutral and do not contain any nutrient value.
The clay is formed into round pellets and fired in rotary kilns at 1200°C. This causes the clay to expand, like popcorn, and become porous. It is light in weight, and does not compact over time. Shape of individual pellet can be irregular or uniform depending on brand and manufacturing process. The manufacturers considers expanded clay to be an ecologically sustainable and re-usable growing medium because of it's ability to be cleaned and sterilized, typically by washing in solutions of white vinegar, chlorine bleach or hydrogen peroxide (H2O2), and rinsing completely.
Another viewpoint is clay pebbles are best not re-used even when they are cleaned due to root growth which may enter the medium. Breaking open a clay pebble after a crop has been grown will reveal this. However, this view is generally not widely shared.

Expanded clay
Rockwool is probably the most widely used medium in hydroponics. Made from basalt rock it is heat-treated at high temperatures then spun back together like candy floss. It comes in lots of different forms including cubes, blocks, slabs and granulated or flock.
Rockwool is an excellent inert substrate for both 'free drainage' and recirculating systems. In free drainage or run-to-waste systems, the chance of disease spread is greatly lessened. Rockwool is also lightweight and self-contained, which allows plants to be grown at different densities in different stages - young plants can be grown to an advanced stage in a small area before being planted out into the main growing area, thus improving crop turnaround. Its light weight also permits setting up to be quick and inexpensive. Because it is light and rigid it eliminates back-breaking work in preparation and planting and gives substantial labor-saving costs. Rockwool is noted for providing a favourable root environment, thus minimizing plant stress. Root temperature can also be controlled, thus giving substantial energy savings. Rockwool initially causes an increase in pH level. You must adjust the pH level of the nutrient solution to counteract this. A pH level of 5.5-6.5 should suffice to create a suitable pH.
The disadvantages of rockwool are few. Although relatively inexpensive, because of its bulk, transport costs to remote regions can be prohibitive. However, the fact that it can be used several times over will reduce the growers overall costs. Before handling, gloves and long shirt sleeves should be worn to prevent minor skin irritation. This can also be lessened by wetting the rockwool before handling. When this medium is dry, care needs to be taken so as not to inhale any particles; inhaling such particles may carry a health risk.

Rockwool
Coir, from coconut husk fiber, can be used as a compressed medium. Coir comes also in bags and in slabs. Some types of coir are very high in sodium (salt) due to the nature of coconut palms growing on island environments and being processed in the salt air.

Coir
Perlite is a volcanic rock that has been superheated into very lightweight expanded glass pebbles. It is used loose or in plastic sleeves immersed in the water. It is also used in potting soil mixes to decrease soil density. Perlite has similar properties and uses to vermiculite but generally holds more air and less water. If not contained, it can float if flood and drain feeding is used.

Perlite
Like perlite, vermiculite is another mineral that has been superheated until it has expanded into light pebbles. Vermiculite holds more water than perlite and has a natural "wicking" property that can draw water and nutrients in a passive hydroponic system. If too much water and not enough air surrounds the plants roots, it's possible to gradually lower the medium's water-retention capability by mixing in increasing quantities of perlite.

Vermiculite
Sand is cheap and easily available. However, it is heavy, it does not always drain well, and it must be sterilized between use.

Sand
The same type that is used in aquariums, though any small gravel can be used, provided it is washed first. Indeed, plants growing in a typical traditional gravel filter bed, with water circulated using electric powerhead pumps, are in effect being grown using gravel hydroponics. Gravel is inexpensive, easy to keep clean, drains well and won't become waterlogged. However, it is also heavy, and if the system doesn't provide continuous water, the plant roots may dry out.

Gravel
Broken up brick has been used in the place of gravel, works just like it, the disadvantage being that it may alter the pH and if recycled, has to be cleaned first.

Brick Shards
Very lightweight. Cheap, readily available and they drain well. They can be too light, and are mainly used in closed tube systems. Only polystyrene peanuts can be used: the biodegradable ones will become a sludge, although styrene monomer migration may pose health risks.

Polystyrene packing peanuts
Plant nutrients are dissolved in the water used in hydroponics and are mostly in inorganic and ionic form. Primary among the dissolved cations (positively-charged ions) are Ca (phosphate).
Numerous 'recipes' for hydroponic solutions are available. Many use different combinations of chemicals to reach similar total final compositions. Commonly-used chemicals for the macronutrients include potassium nitrate, calcium nitrate, potassium phosphate, and magnesium sulfate. Various micronutrients are typically added to hydroponic solutions to supply essential elements; among them are Fe (iron), Mn (manganese), Cu (copper), Zn (zinc), B (boron), Cl (chlorine), and Ni (nickel). Chelating agents are sometimes used to keep Fe soluble. Many variations of the nutrient solutions used by Arnon and Hoagland (see above) have been styled 'modified Hoagland solutions' and are widely used.
Plants will change the composition of the nutrient solutions upon contact by depleting specific nutrients more rapidly than others, removing water from the solution, and altering the pH by excretion of either acidity or alkalinity. Care is required not to allow salt concentrations to become too high, nutrients to become too depleted, or pH to wander far from the desired value.

Nutrient solutions
Due to its arid climate, Israel has developed advanced hydroponic technology. They have marketed their system to Nicaragua, which uses it to produce more than one million pounds of peppers annually for sale abroad, including the United States.
The largest commercial hydroponics facility in the world is Eurofresh Farms in Willcox, Arizona, which sold 125 million pounds of tomatoes in 2005.

Present and future

Grow box
Growroom
Hydroponics for orchid cultivation