Introduction In recent years growing plants hydroponically--that is, with the roots in a medium other than soil--has stirred the imagination of many persons interested in plant growth and development. Commercial growers have adopted hydroponic methods to produce crops in circumstances that would otherwise be unfavorable. For the plant hobbyist, hydroponics offers an opportunity to learn more about the growth of plants and their interactions with their environment. Gardeners may grow flowers, ornamental plants, and vegetables by hydroponics. Growing your own fresh vegetables out of season can be a special winter treat. Colorful sales campaigns and articles in the popular press have led people to believe that hydroponics is a new discovery that will revolutionize modern agriculture. However, the basic techniques have been used by plant researchers for well over a century to determine the effect of particular nutrients on plant growth and yield. The first recorded experiments were conducted in England in 1699 by Woodward. By the mid-nineteenth century, Sachs and Knop, pioneers in this field, had perfected a method of growing plants without soil. In the late 1920s and early 1930s Gericke was able to grow plants successfully on a large scale by adapting the laboratory technique of solution culture. The widespread use of hydroponics for commercial plant production is a relatively recent occurrence. In areas where soil is lacking or unsuitable for growth, hydroponics offers an alternative production system. However, there is nothing magical about hydroponics. Equally good crops can be produced in a greenhouse in conventional soil or bench systems, often at lower cost. The term hydroponics is used to describe many different types of systems for growing plants without soil. Among the most common are: water culture, aquaculture, or nutriculture: a system in which the plant roots are immersed in water containing dissolved nutrients aggregate culture: in which a material such as sand, gravel, or marbles supports the plant roots aeroponics: in which the plant roots hang in the air and are misted regularly with a nutrient solution continuous flow systems: in which the nutrient solution flows constantly over the plant roots. This system is the one most commonly used for commercial production. Often grouped with these systems is drip or trickle irrigation but it is not a true hydroponic system. The common denominator in all hydroponic systems is the method by which the plants receive their nutrition and water. When plants are grown hydroponically their roots are either immersed in or coated by a carefully controlled nutrient solution. The nutrients and water are supplied by the solution alone, not by the aggregates (if any) that support the roots. A number of packaged hydroponic systems are available to the commercial grower and hobbyist. Individuals, who lack building skills or plant-growing experience can look to these kits as an introduction to a challenging hobby. Similar systems can be built at lower cost, however, by talented hobbyists. Only experienced greenhouse operators should consider hydroponics as a commercial venture.
Water Culture or Aquaculture The water culture method of hydroponics is the simplest to set up on a small scale. In this system the plant roots are totally immersed in a nutrient solution. The major disadvantages of this system are the large amount of water required per plant and the need to aerate the solution continuously. The actual design of the system is limited only by the imagination of the builder. The system must provide means to (1) support the plant above the solution, (2) aerate the solution, and (3) prevent light from reaching the solution (to prevent the growth of algae). A standard tray or tank is shown in Figure 1. The tray may be made of concrete or of plastic-lined or asphaltsealed wood. If you use asphalt to seal the tank, be sure that it does not contain creosote or tars. Do not use asphalt that leaves an oil film on the surface of the water. A typical size is 6 to 12 inches deep, 2 to 3 feet wide, and as long as is convenient. The plants can be supported by inserting them through holes drilled in a plywood top or through holes punched in a l-inch-thick Styrofoam sheet that floats on the surface of the solution. You can make a small system from a child's wading pool, a plastic pail, a fish tank, or a drinking tumbler. A large tomato plant should be grown in a container that holds at least 2 gallons as the solution in a smaller container will be used up too quickly. Lettuce plants, on the other hand, may be grown in smaller containers. Figure 1. Cutaway view of a typical tray for an aquaculture system. Short plants such as lettuce and spinach will usually support themselves. Drill a 1-inch hole in the Styrofoam or wooden cover and insert a transplant. The plant may be held in place by packing a flexible material such as cotton into the hole around the stem. A plant started in sand, perlite, or vermiculite can be transplanted easily to the water culture system because these materials can be washed from the roots readily Figure 2. Cutaway view of a typical tray for an aquaculture system. Vining plants such as cucumbers and tomatoes must be supported by string. When pruned to a single stem they can be wrapped around a loosely hung string as they grow (Figure 2). Aerate the solution continuously by pumping air through a perforated hose or pipe immersed in the solution. For small systems an aquarium pump and porous stone will work. Do not bubble the solution too vigorously because excessive movement may damage the tender roots and impair plant growth. Change the nutrient solution every two weeks when the plants are small and once a week as they begin to mature. Add water daily to keep the solution level constant.
Aggregate Culture Growing plants in aggregates such as sand or gravel is often preferred to the water culture method since the aggregate helps to support the roots. The aggregate is held in the same type of tank as is used for a water culture system. The nutrient solution is held in a separate tank and pumped into the aggregate tank to moisten the roots as needed. After the aggregate has been flooded it is drained to provide aeration. Enough water and nutrients cling to the aggregate and roots to supply the plant until the next flooding (Figure 3). Figure 3. Cross-section of plants growing in aggregate. Figure 4. A manual gravity-feed system. The solution is generally pumped to within 1 inch of the surface and then allowed to drain. If the top surface of the bed is kept dry, the growth of algae will be minimal. To allow rapid drainage, the aggregate must be coarse. Use sand with particles of at least 1/16-inch diameter or gravel of about 1/4- to 3/8-inch diameter. The best aggregates are silica gravel, granite, basalt, or smooth river-bottom rock of the inert type that contains no calcium. Larger aggregates will require more frequent flooding, whereas smaller aggregates will not drain properly. In small, experimental units you may use any of several different substances. Perlite, Styrofoam, and crushed marbles have all been used successfully by hobbyists.
Figure 5. A simple gravity-feed system. The solution flows from vat A into the aggregate material in the growing bed. When the growing bed is flooded, the solution is drained into vat B and then returned to vat A. The aggregate should be flooded for about 10 minutes and allowed to drain for no longer than 30 minutes. Variations of the tray and tank arrangement are shown in Figures 4 through 6. Aeroponics Figure 6. A simple mechanical subirrigation system. The adventurous hobbyist may wish to try an even more exotic method of growing plants. In the aeroponic system the roots of the plant grow in a closed container. A misting system bathes the roots in a film of nutrient solution and keeps them near 100 percent relative humidity to prevent drying. The container may be of almost any design as long as it is moisture proof and dark. Tomatoes may be grown in tall, narrow containers lined with plastic. Lettuce and strawberries have been grown in A-frame containers (Figure 7) to make the best use of available space and light. Figure 7. An A-Frame container for aeroponic culture of small plants. Position the spray nozzles so that at least a portion of each plant's roots are sprayed directly. You may leave the nozzles on at low pressure continuously or operate them intermittently, on for 20 seconds and off for 40 seconds. A fungicide may be added to the solution to avoid root rot pathogens.
Continuous Flow Systems Most commercial hydroponic systems direct a continuous flow of nutrient solution over the plant roots. One continuous flow system uses polyvinyl chloride (PVC) pipe of the type commonly used for household waste plumbing. A 2-inch pipe for lettuce or a 4- to 6-inch pipe for tomatoes may be set up with a slight gradient to allow for flow of the solution. Holes of 1- to 1 1/2-inch diameter are drilled in the pipe, and the plants are inserted into the holes. Lettuce plants will support themselves if they have been started in growing cubes. Tomato plants must be supported with wire or string. The nutrient solution is held in a large tank and pumped or allowed to flow by gravity to the growing pipes. The continuously flowing nutrient solution bathes the roots and then returns to the holding tank. The solution aerates itself as it flows back into the tank. Major problems with using PVC pipe are its relatively high initial cost and the need for cleaning. After a crop has been grown in the pipe, it should be thoroughly cleaned with a 0.5 to 1.0 percent sodium hypochlorite solution (made by mixing one part of household bleach with nine parts of water) to prevent contamination from disease organisms. With the nutrient film technique (NFT) the same methods but less expensive materials are used. A flexible plastic tube supported by a wooden tray is used in place of rigid PVC pipe. The tube is made of black plastic film (much like the plastic film mulch use for gardens) with holes punched at specified intervals. The plants are started in root cubes and then placed in the tube where they are bathed in a continuous flow of nutrient solution. A variation of the continuous flow system is marketed as the Pipe Dream. This system uses 2-inch corrugated plastic drainage pipe placed vertically in a 6-inch drainage pipe for tomatoes or a 2-inch pipe for lettuce (Figure 8). A plastic mesh tube filled with peat moss is placed in the vertical tube and allowed to hang into the horizontal pipe. A nutrient solution flowing in the horizontal pipe supplies water and fertilizer, which move up into the peat moss and thus to the plant roots. Although seeds can be planted directly in the peat moss, it is best to start with transplants. Figure 8. A commercially available type of continuous flow system.
Requirements for Plant Growth Hydroponic systems will not compensate for poor growing conditions such as improper temperature, inadequate light, or pest problems. Hydroponically grown plants have the same general requirements for good growth as field-grown plants. The major difference is the method by which the plants are supported and the inorganic elements necessary for growth and development are supplied. Temperature. Plants grow well only within a limited temperature range. Temperatures that are too high or too low will result in abnormal development and reduced production. Warm-season vegetables and most flowers grow best between 60 and 75 or 80 F. Cool-season vegetables such as lettuce and spinach should be grown between 50 and 70 F. Light. All vegetable plants and many flowers require large amounts of sunlight. Hydroponically grown vegetables like those grown in a garden, need at least 8 to 10 hours of direct sunlight each day to produce wells Artificial lighting is a poor substitute for sunshine, as most indoor lights do not provide enough intensity to produce a crop. Incandescent lamps supplemented with sunshine or special plant-growth lamps can be used to grow transplants but are not adequate to grow the crop to maturity. High intensity lamps such as high-pressure sodium lamps can provide more than 1,000 foot-candles of light. The serious hobbyist can use these lamps successfully in areas where sunlight is inadequate. The fixtures and lamps, however, are very expensive and thus not feasible for a commercial operation. Adequate spacing between plants will ensure that each plant receives sufficient light in the greenhouse. Tomato plants pruned to a single stem should be allowed 4 square feet per plant. European seedless cucumbers should be allowed 7 to 9 square feet, and seeded cucumbers need about 7 square feet. Leaf lettuce plants should be spaced 7 to 9 inches apart within the row and 9 inches between rows. Most other vegetables and flowers should be grown at the same spacing as recommended for a garden. Greenhouse vegetables, whether grown in soil or in a hydroponic system, will not do as well during the winter as in the summer. Shorter days and cloudy weather reduce the light intensity and thus limit production. Most vegetables will do better if grown from January to June or from July to December than if they are started in the fall and grown through the midwinter months. Water. Providing the plants with an adequate amount of water is not difficult in the water culture system, but it can be a problem with the aggregate culture method. During the hot summer months a large tomato plant may use onehalf gallon of water per day. If the aggregate is not kept sufficiently moist, the plant roots will dry out and some will die. Even after the proper moisture level has been restored, the plants will recover slowly and production will be reduced. Water quality can be a problem in hydroponic systems. Water with excessive alkalinity or salt content can result in a nutrient imbalance and poor plant growth. Softened water may contain harmful amounts of sodium. Water that tests high in total salts should not be used. Salt levels greater than 0.5 millions or 320 parts per million are likely to cause an imbalance of nutrients. The amateur chemist may be able to overcome this problem by custom mixing the nutrient solutions to compensate for the salts in the water. Oxygen. Plants require oxygen for respiration to carry out their functions of water and nutrient uptake. In soil adequate oxygen is usually available, but plant roots growing in water will quickly exhaust the supply of dissolved oxygen and can be damaged or killed unless additional air is provided. A common method of supplying oxygen is to bubble air through the solution. It is not usually necessary to provide supplementary oxygen in aeroponic or continuous flow systems.
Mineral Nutrients. Green plants must absorb certain minerals through their roots to survive. In the garden these minerals are supplied by the soil and by the addition of fertilizers such as manure, compost, and fertilizer salts. The essential elements needed in large quantities are nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Micronutrients - iron, manganese, boron, zinc, copper, molybdenum, and chlorine are also needed but in very small amou Support. In a garden the plant roots are surrounded by soil that supports the growing plant. A hydroponically grown plant must be artificially supported, usually with string or stakes.