Hydroponics (from the Greek words hydro water and ponos labour) is a method of growing plants using mineral nutrient solutions, in water, without soil. Terrestrial plants may be grown with their roots in the mineral nutrient solution only or in an inert medium, such as perlite, gravel, mineral wool, or coconut husk.
Researchers discovered in the 19th century 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. Hydroponics is also a standard technique in biology research and teaching.
The study of crop nutrition began thousands of years ago. Ancient history tells us that various experiments were undertaken by Theophrastus (372-287 B.C.), while several, extant botanical writings of Dioscorides date from the first century AD.
The earliest published work on growing terrestrial plants without soil was the 1627 book, Sylva Sylvarum by Sir Francis Bacon, printed a year after his death. 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.By 1842 a list of nine elements believed to be essential to plant growth had been made out, and the discoveries of the German botanists, Julius von Sachs and Wilhelm Knop, in the years 1859-65, resulted in a development of the technique of soil-less cultivation. 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 began publicly promoting that solution culture be used for agricultural crop production. He first termed it aquaculture but later found that aquaculture was already applied to culture of aquatic organisms. Gericke created a sensation by growing tomato vines twenty-five feet high in his back yard in mineral nutrient solutions rather than soil. By analogy with the ancient Greek term for agriculture, geo-ponics, the science of cultivating the earth, Gericke introduced the term hydroponics in 1937 (although he asserts that the term was suggested by Dr. W. A. Setchell, of the University of California) for the culture of plants in water (from the Greek hydros, "water", and ponos, "labor").
Reports of Gericke's work and his claims that hydroponics would revolutionise plant agriculture prompted a huge number of requests for further information. Gericke refused to reveal his secrets claiming 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 Soil-less Gardening.
Two other plant nutritionists at the University of California were asked to research Gericke's claims. Dennis R. Hoagland (http://pmb.berkeley.edu/newpmb/faculty/hoagland/NAS_Memoir.pdf) and Daniel I. Arnon (http://pmb.berkeley.edu/newpmb/faculty/deceased.shtml) 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. This research, however, overlooked the fact that hydroponics has other advantages including the fact that the roots of the plant have constant access to oxygen and that the plants have access to as much or as little water as they need. This is important as one of the most common errors when growing is over- and under- watering; and hydroponics prevents this from occurring as large amounts of water can be made available to the plant and any water not used, drained away, re-circulated, or actively aerated, eliminating anoxic conditions which drown root systems in soil. In soil, a grower needs to be very experienced to know exactly how much water to feed the plant. Too much and the plant will not be able to access oxygen; too little and the plant will lose the ability to transport nutrients, which are typically moved into the roots while in solution.
One of the early successes of hydroponics occurred on Wake Island, a rocky atoll in the Pacific Ocean used as a refuelling 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 Centre 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. Hydroponics intended to take place on Mars are using LED lighting to grow in different colour spectrum with much less heat.
In 1978, hydroponics pioneer Dr. Howard Resh published the first edition of his book "Hydroponics Food Production." This book (now updated) spurred what has become known as the 3-part base nutrients formula that is still a major component of today's hydroponics gardening. Resh later went on to publish other books, and is currently in charge of a highly advanced hydroponics research and production facility in the Caribbean.
Some of the reasons why hydroponics is being adapted around the world for food production are the following:
No soil is needed.
The water stays in the system and can be reused- thus, lower water costs.
It is possible to control the nutrition levels in their entirety- thus, lower nutrition costs.
No nutrition pollution is released into the environment because of the controlled system.
Stable and high yields.
Pests and diseases are easier to get rid of than in soil because of the container's mobility.
Today, hydroponics is an established branch of agronomy. Progress has been rapid, and results obtained in various countries have proved it to be thoroughly practical and to have very definite advantages over conventional methods of horticulture. The two chief merits of the soil-less cultivation of plants are, first, much higher crop yields, and second, hydroponics can be used in places where in-ground agriculture or gardening is not possible.
Thus not only is it a profitable undertaking, but one which has proved of great benefit to humanity. People living in crowded city streets, without gardens, can grow fresh vegetables and fruits in window-boxes or on rooftops. By means of hydroponics all such places can be made to yield a regular and abundant supply of fresh greens. Deserts, rocky and stony land in mountainous districts or barren and sterile areas can be made productive at relatively low cost.
Other advantages include faster growth combined with relative freedom from soil disease, and consistency in crops, the quality of produce being excellent. There is also a considerable reduction in growing area. Weeds are practically non-existent, while standard methods and automatic operations mean less labor, lower cost, and less difficult manual labor. As some plants can be raised out of season, better control of crops naturally result.
The hydroponic conditions (presence of fertiliser and high humidity) create an environment that stimulates salmonella growth. Another disadvantage is pathogens attacks including damp-off due to Verticillium wilt caused by the high moisture levels associated with hydroponics and over-watering of soil based plants. Also, many hydroponic plants require different fertilisers and containment systems.
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 rock-wool culture. There are two main variations for each medium, subrogation 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.
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. Once the upper tray fills past the drain stop it begins re-circulating the water until the pump is turned off and the water in the upper tray drains back into the reservoir.
In a Run to Waste type system, nutrient and water solution is periodically applied to the medium surface. This may be done in its simplest form, by manually applying a nutrient and water solution once or more times per day in a container of inert growing media, such as rock-wool, perlite, vermiculite, cocoa fibre, or sand. In a slightly more complex system, it is automated with a delivery pump, a timer and irrigation tubing to deliver nutrient solution with a delivery frequency that is governed by the key parameters of plant size, plant growing stage,climate, substrate, and substrate conductivity, pH, and water content.
One of the most obvious decisions hydroponic farmers have to make is which medium they should use. Different media are appropriate for different growing techniques.
Coir: Coco Peat, also known as coir or coco, is the leftover material after the fibres have been removed from the outermost shell (bolster) of the coconut. Coir is a 100% natural grow and flowering medium.
Sand: Sand is cheap and easily available. However, it is heavy, does not hold water very well, and it must be sterilised between use.
Gravel: 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 power-head 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.
Brick shards: Brick shards have similar properties to gravel. They have the added disadvantages of possibly altering the pH and requiring extra cleaning before re-use.
Wood fibre: Wood fibre, produced from steam friction of wood, is a very efficient organic substrate for hydroponics. It has the advantage that it keeps its structure for a very long time.
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? (sulphate), and H2PO4? (dihydrogen 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 macro-nutrients include potassium nitrate, calcium nitrate, potassium phosphate, and magnesium sulphate. Various micro-nutrients 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 have been styled 'modified Hoagland solutions' and are widely used. Variation of different mixes throughout the plant life cycle, further optimises its nutritional value. 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.
The largest commercial hydroponics facility in the world is Eurofresh Farms in Willcox, Arizona, which sold 125 million pounds of tomatoes in 2005. Eurofresh has 318 acres (1.29 km2) under glass and represents about a third of the commercial hydroponic greenhouse area in the U.S. Eurofresh does not consider its tomatoes organic, but they are pesticide-free. They are grown in rock-wool using the run to waste technique.
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 labelled "organic". Some states in the USA require soil as an essential to obtain organic certification. There are also overlapping and somewhat contradictory rules established by the US Federal Government, so some food grown with hydroponics can be certified organic.
Hydroponics also saves 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 minimise 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.
To increase plant growth, lighting systems such as metal halide for growing stage only or high pressure sodium for growing/flowering/blooming stage are used to lengthen the day or to supplement natural sunshine if it is scarce. Metal halide emits more light in the blue spectrum, making it ideal for plant growth but is harmful to unprotected skin and can cause skin cancer. High pressure sodium emits more light in the red spectrum, meaning that it is best suited for supplementing natural sunshine and can be used throughout the growing cycle. However, these lighting systems require large amounts of electricity to operate.
Hydroponics have been used to enhance vegetables to provide more nutritional value. A hydroponic farmer in Virginia has developed a calcium and potassium enriched head of lettuce, scheduled to be widely available in April 2007. Grocers in test markets have said that the lettuce sells "very well", and the farmers claim that their hydroponic lettuce uses 90% less water than traditional soil farming.
With pest problems reduced, and nutrients constantly fed to the roots, productivity in hydroponics is high, although plant growth can be limited by the low levels of carbon dioxide in the atmosphere, or limited light exposure. 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.