As with any subject, there develops a terminology and jargon that can be misleading and confusing even to those who are familiar with the subject. Hydroponics is a soilless method for growing plants, while most soilless methods are not hydroponic. Hydroponics is defined as “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”.
The word hydroponics was coined by Dr. W.A. Setchell, combining two Greek words, hydro meaning water, and ponos meaning work. The word hydroponics first appeared in an article authored by Dr. W.F. Gericke, published in the scientific journal Science (Feb. 178:1, 1937),
There have been other word combinations for identifying a hydroponic growing system, such as aquaculture, solution culture, liquid culture and nutriculture, but none have not been widely accepted.
Hydroponically, the word nutrient refers to the 13 plant essential mineral elements, divided into two categories, the major (high level requirement) elements, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S), and the micronutrients (low level requirement), boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn). The use of the word mineral is also unique when applied to hydroponics as it does not conform to its dictionary definition, but refers just to the 13 of the 16 plant essential elements, those elements that are taken into the plant by adsorption through its roots. The 3 non-mineral plant essential elements, carbon (C), hydrogen (H) and oxygen (O), sometimes referred to as the structural elements, are combined in the process called photosynthesis. The end product of photosynthesis is a carbohydrate, the basic building block for the formation of the plant skeleton.
There have been other words used to identify some of the 13 plant essential elements, secondary for the elements calcium (Ca), magnesium (Mg) and sulfur (S), and both minor and trace elements for identifying the micronutrients, terms that are not in common use today but were used by some in early plant nutrition literature. Recently, the word combination trace elements has taken on a new meaning, referring to those elements found in nature in low concentrations, with some suspected to be beneficial to plants. The early researchers were also wondering about these elements, so they devised an A-Z Solution that contained 26 elements, an aliquot being added to their nutrient solution formulations. Today, there is no justification for the use of such a trace element-containing solution.
A new set of elements, identified as beneficial elements, is coming into common use, elements that do not meet the requirements for essentiality, but their inclusion in a nutrient solution formulation, or presence in the rooting medium, may enhance plant growth (Morgan, 2000). The most commonly referred to element in this category is silicon (Si) whose presence in plant tissues increases their resistance to fungal disease. Therefore its inclusion in some nutrient solution formulation, if not a constituent of the water and/or rooting medium.
The quality of the water for making a nutrient solution is important, although many water sources can be used without treatment since using only pure water is not essential for making a nutrient solution. Calcium (Ca) and magnesium (Mg), found in many water sources, are desirable elements that can contribute to the nutrient solution formulation. The element sodium (Na), and the anions, bicarbonate (HCO3-), carbonate (CO32-), sulfide (S-), chloride (Cl-), and possibility fluoride (F-), if in relatively high concentrations, should be removed, or another water source sought. Reverse osmosis is the most effective treatment method for removing these and other substances from water. Water after reverse osmosis treatment is referred to as “RO” water.
The means of delivery of the 13 mineral elements to the plant roots is by means of a nutrient solution. This word combination is of unknown origin, although it appears in the classic 1950 California Agricultural Experiment Station Circular 347*. The word complete is used to designate a nutrient solution that contains all 13 essential mineral elements. However, the word, incomplete, is not used as a means of describing a nutrient solution that contains only some of the 13 mineral elements. In some hydroponic literature, there appears 2 named-referenced nutrient solution formulations, identifying the name of the formulator, Hoagland (Hoagland and Arnon to be precise, see Hoagland and Arnon, 1950) and Steiner (Steiner, 1961). In the scientific literature, it is not uncommon for the nutrient solution to be identified as being a modified Hoagland’s nutrient solution, with or without indicating what had been modified. It is not uncommon for a nutrient solution formulation to carry a commercial name, such as “Earth Juice”. “MotherPlant”, “FloraGro”, and “Vital Earth’s.” Sometimes a nutrient solution carries a designation as to its primary use, for example, to stimulate vegetable growth, flowering, or fruit set and development, the formulation containing elements in concentrations and ratios thought to enhance such plant growth characteristics.
Nutrient solution concentrates are used when a nutrient solution formulation is being prepared by injecting an aliquot of concentrate into a flowing water stream. Since some of the reagents used to formulate a 13-element containing nutrient solution are not compatible when in concentrated liquid form (they form precipitates when combined), they will be divided into compatible element content concentrates and given a letter designation, such as A, B, C, etc. Other reagents, such as an acid or a specific element-containing solution, that are being added to the flowing water stream, may also carry a letter designation.
There are three aspects for a nutrient solution formulation, its elemental composition, and its use factors, being the volume of nutrient solution per plant and frequency of modification or replacement, use factors that are either not known to the potential user or frequently ignored. However, any change in any one of these parameters (composition and use) will significantly affect plant growth when using a particular nutrient solution formulation, therefore not be ignored.
All hydroponic growing systems can be divided into 2 categories based on how the nutrient solution is handled, open, run-to-waste, being when the nutrient solution is discarded after bathing the plant roots, and closed being when the nutrient solution is collected and re-circulated, either without or with treatment, such as volume adjustment, elemental reconstitution, sterilization and aerated, before being re-circulated.
There are basically 6 hydroponic growing techniques, defined primarily as to how the nutrient solution is delivered to the plant roots or rooting medium. Three of the methods are without the use of a rooting medium, standing aerated, Nutrient Film Technique (NFT) and aeroponics. The standing aerated method is widely used by researchers when control of the plant’s nutrition status is needed. A commercial application of this technique is the growing of lettuce and herbs on rafts floated on a pool of nutrient solution (Morgan, 2002). With the Nutrient Film Technique (referred to mostly by its acronym NFT), plant roots are suspended in a flow of nutrient solution down a slopping enclosed trough or channel, the method primary limited to the growing of lettuce and herbs, or short term crops. Aeroponics is a root chamber method where the plant roots are either continuously or periodically sprayed with a nutrient solution mist. The method is primarily used for the production of herbs when the roots are the harvested portion of the plant.
The other 3 hydroponic growing methods are with the use of a rooting medium, flood-and-drain (sometimes identified as ebb-and-flow), drip irrigation and sub-irrigation. The physio-chemical properties of the rooting medium, whether organic (peatmoss, pinebark, coir, course sawdust) or inorganic (perlite, rockwool, gravel, course sand), will determine its suitability for use with each of these hydroponic growing techniques (Morgan, 2003). The flood-and-drain method was widely used beginning in the 1940s for the production of vegetable crops rooted in beds of either gravel or course sand periodically flooded with a nutrient solution (Eastwood, 1947). Today this method is primary limited to small home garden type growing systems. Drip irrigation, as the means of nutrient solution delivery to the rooting medium, is today the most widely used commercial hydroponic growing technique, primarily for greenhouse vegetable crop production. Both organic (coir) and inorganic (perlite and rockwool) substances are the rooting medium of choice. Vertical growing systems using drip irrigation as the nutrient solution delivery method are commercially available for both greenhouse and outdoor use (see Vertigro.com). Sub-irrigation hydroponic methods have yet to be widely accepted as a means for the commercial production of plants. A hydroponic application of this technique is the commercial Autopot system (Autopot.com). Similar systems are described on the website, Hydrogrosystems.com.
J. Benton Jones, Jr. has a PhD in Agronomy and is the author of several books including Hydropopnics: A Practical Guide for the Soilless Grower and Tomato Plant Culture. Dr. Jones has written extensively on hydroponic growing and has been outdoor vegetable gardening employing sub-irrigation hydroponic growing systems (see: Hydrogrosystems.com), and using domestic water for making his nutrient solution.
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Additional details on all these hydroponic methods can be found in the books authored by Jones (2003) and Resh (2001).
*This is the most frequently cited publication in soil and plant nutrition research literature.
Eastwood, T. 1947. Soilless Growth of Plants. 2nd Edition. Reinhold Publishing, New York.
Hoagland, D.R. and Arnon, D.I. 1950. The Water Cultural Method for Growing Plants without Soil. Circular 347. California Agricultural Experiment Station, University of California, Berkeley, CA.
Jones, Jr., J.B. 2005. Hydroponics: A Practical Guide for the Soilless Grower, 2nd Edition. CRC Press, Boca Raton, FL
Morgan, L. 2000. Beneficial elements for hydroponics: a new look at plant nutrition. The Growing Edge 11(3):40-51.
Morgan, L. 2002. Raft system specifics. The Growing Edge 14(2):46-60.
Morgan, L. 2003. Hydroponic substrates. The Growing Edge 15(2):64-66.
Steiner, A.A. 1961. A universal method for preparing nutrient solutions of certain desired compositions. Plant and Soil 15:134-154.
Resh, H.M. 2001. Hydroponic Food Production, 6th edition. New Concepts Press, Mahwah, NJ.