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Hydroponics, term applied to cultivation of plants in nutrient solutions without use of soil. Soilless growing of cultivated plants began in the 1930s as an outgrowth of the culture techniques used by plant physiologists in plant nutrition experiments. More recent successful methods of soilless growth differ in particulars but have two common features: (1) nutrients are supplied in liquid solutions; and (2) plants are supported by porous material, such as peat, sand, gravel, or glass wool, that acts as a “wick” in relaying the nutrient solution from its source to the roots.

Through photosynthesis, green plants manufacture their own organic food, using carbon dioxide and oxygen as raw materials. The nutrients usually supplied to plants by soil are almost entirely mineral salts. Plant physiologists have discovered that plants require carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, magnesium, sulfur, calcium, iron, manganese, boron, zinc, copper, and probably molybdenum. Carbon, hydrogen, and oxygen are obtained in large quantities from water and air, but the remaining elements are ordinarily supplied as salts by the soil. The relative amount of each of these elements required for normal growth is different in each plant, but all plants require relatively large proportions of nitrogen, phosphorus, potassium, magnesium, sulfur, and calcium. Iron, manganese, boron, zinc, copper, and molybdenum are supplied in minute quantities, and are called micronutrients or trace elements. The specific salts used to supply these elements may be varied at the discretion of the grower; a typical solution of primary minerals is composed of distilled water containing potassium nitrate, KNO3, calcium nitrate, Ca(NO3)2, potassium acid phosphate, KH2PO4, and magnesium sulfate, MgSO4. In solution, the salts dissociate into ions; potassium nitrate, for example, is available to plants as the ions K+ and NO3-. A solution of micronutrient salts is added to the solution of primary elements to complete the nutrient solution. A small amount of fungicide is usually added to prevent the growth of mold.

Several culture techniques are used. The most practical commercial method is subirrigation, in which plants are grown in trays filled with gravel, cinders, or other coarse materials, and periodically flooded with nutrient solution. The solution is allowed to drain off after each flooding and may be reused as long as sufficient minerals remain in it. The water-culture method is used widely for botanical experimentation. A common type of water culture consists of glazed porcelain jars filled with solution; the plants are placed in beds of glass wool or similar material that are supported at the surface of the solution. Roots of the plants penetrate the beds and remain in the solution. The least exact method, commonly called the slop method, is the easiest to operate. Coarse, clean sand is used in place of soil, and nutrient solution is poured on the sand in approximately equal amounts at regular intervals. A refinement of this practice is the drip method, in which a steady, slow feed of nutrient is maintained. Excess nutrient solution is allowed to drain off in both slop and drip methods.