Hydroponics

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NASA researcher checking hydroponic onions with Bibb lettuce to his left and radishes to the right

Hydroponics is crop production with mineral nutrient solutions instead of soil containing silt and clay. Terrestrial plants may be grown with their roots in the mineral nutrient solution only or in an inert medium, such as sand, 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. There is little commercial hydroponic crop production because it is a more expensive method than traditional agriculture.

Contents

History

Researcher with hydroponic strawberries

Contrary to some modern sources, ancient peoples such as the Babylonians and Aztecs did not use hydroponics. The mineral nutrient solutions required for hydroponics were not developed until the 1800s. The so-called Floating Gardens of the Aztecs (chinampas) used soil. The Hanging Gardens of Babylon were supposedly irrigated roof gardens but there is no evidence they used hydroponics.

The earliest published work on growing terrestrial plants without soil was the 1627 book, Sylva Sylvarum by Sir Francis Bacon. Water culture became a popular research technique after that. In 1699, John Woodward published his water culture experiments with spearmint. He found that plants in less-pure water sources grew better than plants in distilled water. Mineral nutrient solutions for soilless culture of plants were first perfected in the 1860s by the German botanists, Julius von Sachs and Wilhelm Knop. Growth of terrestrial plants without soil in mineral nutrient solutions was called solution culture. It quickly became a standard research and teaching technique and is still widely used today. Solution culture is now considered a type of hydroponics where there is no inert medium.

In 1929, Professor William Frederick Gericke of the University of California at Berkeley was first to suggest that solution culture be used for agricultural crop production. He first termed it aquiculture but later found that aquaculture was already applied to culture of aquatic organisms. Gericke created a sensation by growing tomato and other plants to a remarkable size in his backyard in mineral nutrient solutions rather than soil. By analogy with the ancient Greek term for agriculture, geoponics, the science of cultivating the earth, Gericke introduced the term hydroponics in 1937 for the culture of plants in water (from the Greek hydros, water, and ponos, labor).

Reports of Gericke's work and his overzealous claims that hydroponics would revolutionize plant agriculture prompted a huge number of requests for further information. Gericke refused to reveal his secrets because he had done the work at home on his own time. This refusal eventually resulted in his leaving the University of California. In 1940, he wrote the book, Complete Guide to Soilless Gardening.

Two other plant nutritionists at the University of California were asked to research Gericke's claims. Dennis R. Hoagland and Daniel I. Arnon wrote a classic 1938 agricultural bulletin, The Water Culture Method for Growing Plants Without Soil, debunking the exaggerated claims made about hydroponics. Hoagland and Arnon found that hydroponic crop yields were no better than crop yields with good quality soils. Crop yields were ultimately limited by factors other than mineral nutrients, especially light. They developed several formulas for mineral nutrient solutions, known as Hoagland solutions. Modified Hoagland solutions are still used today.

One of the early successes of hydroponics occurred on Wake Island, a rocky atoll in the Pacific Ocean used as a refueling stop for Pan American Airlines. Hydroponics was used there in the 1930s to grow vegetables for the passengers. Hydroponics was a necessity on Wake Island because there was no soil, and it was prohibitively expensive to airlift in fresh vegetables.

In the 1960s, Allen Cooper of England developed the Nutrient Film Technique. The Land Pavilion at Walt Disney World's EPCOT Center opened in 1982 and prominently features a variety of hydroponic techniques. In recent decades, NASA has done extensive hydroponic research for their Controlled Ecological Life Support System or CELSS.

Soilless Culture

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 so represent soilless culture. However, most are not hydroponics because the soilless medium often provides some of the mineral nutrients via slow release fertilizers, cation exchange or 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.

Disadvantages

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Techniques

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 potenially 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. The Land Exhibit at EPCOT Center has aeroponics in vertical sections of large-diameter plastic pipe. Plant 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) 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.

Media

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.

Expanded Clay

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 the nutrients are carefully controlled in the water. Clay pebbles can be reused provided they are cleaned thoroughly between crops. Baked clay pebbles are highly porous, yet irregularly shaped to create a balance of air, space and nutrient solution.

Rockwool

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. 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.

Coco Coir

Coco is a compressed medium created from the husks of coconuts. Coco coir comes also in bags and in slabs. Some types of Coco 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.

Perlite

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.

Vermiculite

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.

Sand

Sand is cheap and easily available. However, it's heavy, it doesn't drain well, can clog roots and must be sterilized between uses.

Gravel

The same type that's used in aquariums, though any small gravel can be used, provided it's washed first. It's inexpensive, easy to keep clean, drains well and won't become waterlogged. However, it's also heavy, and if your system doesn't provide continuous water, the roots may dry out.

Brick Shards

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.

Polystyrene Packing Peanuts

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, of course, will become a sludge.

Nutrient Solutions

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 Ca²⁺ (calcium), Mg²⁺ (magnesium), and K⁺ (potassium); the major nutrient anions in nutrient solutions are NO₃⁻ (nitrate), SO₄²⁻ (sulfate), and H₂PO₄⁻ (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.

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.

Commercial

The largest commercial hydroponics facility in the world is Eurofresh Farms in Willcox, Arizona, which sold 125 million pounds of tomatoes in 2005. Kenney, Brad P. 2006. Success under glass. American Vegetable Grower. May, pages 12-13.[link] Eurofresh has 256 acres under glass and represents about a third of the commercial hydroponic greenhouse area in the U.S. Sorenson, Dan. 2006. Hydroponic tomatoes. Arizona Daily Star [link] In addition to its regular employees, Eurofresh employs 180 Arizona inmates full time and pays them $2/hour. Eurofresh does not consider their tomatoes organic, but they are pesticide-free. They are grown in rockwool with top irrigation.

Some commercial installations use no pesticides or herbicides, preferring Integrated Pest Management Techniques. There is often a price premium willingly paid by consumers for produce which is labeled "Organic". Some States in the USA require soil as a "sine qua non" to obtain Organic Certification. There are also overlapping and somewhat contradictory rules established by the Federal Government. So some food grown with hydroponics can be certified organic. In fact, they are the cleanest plants possible because there is no environment variable and the dirt in the food supply is extremely limited. Hydroponics also saves an incredible amount of water; It uses as little as 1/20 the amount as a regular farm to produce the same amount of food. The water table can be impacted by the water use and run-off of chemicals from farms, but hydroponics may minimize impact as well as having the advantage that water use and water returns are easier to measure. This can save the farmer money by allowing reduced water use and the ability to measure consequences to the land around a farm.

The environment in a hydroponics greenhouse is tightly controlled for maximum efficiency and this new mindset is called Soil-less/Controlled Environment Agriculture (S/CEA). With this growers can make ultra-premium foods anywhere in the world, regardless of temperature and growing seasons. Growers monitor the temperature, humidity, and pH level constantly.

Present and future

With pest problems reduced, and nutrients constantly fed to the roots, productivity in hydroponics is high, plant growth being limited by the low levels of carbon dioxide in the atmosphere, or limited light. To increase yield further, some sealed greenhouses inject carbon dioxide into their environment to help growth (CO₂ enrichment), or add lights to lengthen the day, control vegetative growth etc.

Hydroponics can be used to grow plants anywhere, from Antarctica (where salad vegetables are grown in the 6 month nights) to a coal mine. If vegetables are grown in future space missions, it is likely to be by hydroponic methods.

The fact that plants can be grown almost anywhere, with no natural light by using hydroponics and artificial lighting, has not escaped the notice of clandestine marijuana growers, and a large amount of hydroponics equipment appears to be in use for this purpose. In the UK, theft of high intensity (HID) grow lamps from commercial vegetable growers is a chronic problem. However, this trend is being reduced by the increased availability, especially via the internet, of the specialist hydroponics equipment. Wide availability and low cost of equipment in the U.S. makes theft from greenhouses a rare event. In the UK, growing cannabis remains the criminal offence of trafficking. Large scale growing operations using hydroponics are often detected from abnormally high electricity consumption.

Many hydroponic growers have been working diligently to reduce the stigma of these types of activities, with individuals such as Ray Cogo of the United States promoting outdoor hydroponics in an effort to show average people how simple it is to grow food hydroponically in their own backyards.

References

[Barak, P. 2002. Essential Elements for Plant Growth: Hydroponics.]
Hershey, D.R. 1994. Solution culture hydroponics: history and inexpensive equipment. American Biology Teacher 56:111-118.
[Jensen, M.H. 1997. Hydroponics. HortScience 32]
[Hydroponics as a Hobby: Growing Plants Without Soil. University of Illinois Circular 844]
[Utah State Univerisity Hydroponics]
[Hydroponics at McMurdo Station Antarctica]
[Antarctic Hydroponics]
[Cornell University Commercial Hydroponic Lettuce, Spinach and Pak Choi Grower's Handbooks]
[Hydroponics and Soilless Cultures on Artificial Substrates as an Alternative to Methyl Bromide Soil Fumigation]