Aggregate Culture.

This method is often referred to as "sand culture" or "gravel culture." Aggregates are used much as soil is used in conventional plantings - to provide anchorage and support for the plants.




The aggregate in the tank or container is flooded with a nutrient solution as required. The advantages of this system of hydroponics over the water-culture method are lack of trouble in aerating the roots, ease of transplanting seedlings into the gravel or other aggregate medium, and less expense.

SYSTEMS OF SOILLESS CULTURE

Water Culture. In the water-culture method, plants are supplied with mineral nutrients directly from a water solution. The chief advantage of this method over aggregate culture is that a large volume of solution is always in contact with the root system, providing an adequate water and nutrient supply. The major disadvantages are the difficulties of providing an air supply (oxygen) for the plant roots and proper support and root anchorage for the plants.

Materials and Equipment. The cost of growing plants through hydroponics depends upon the cost of chemicals and water used in the preparation of the nutrient solutions, the size of the operation, and the amount of mechanization. The cost may be quite low if you have a small setup and use available materials.




For a large setup, you will need a tank or through constructed of concrete or wood. A depth of 6 to 18 inches and a width of 2 to 3 feet are the most common sizes for the larger tanks. If you use wood, be sure that it is free of knots and sealed with asphalt that does not contain creosote or tars. Do not use asphalt that leaves an oil film on the surface of the water. If the system is small, you can use glass jars, earthenware crocks, or metal containers. Metal containers should be well painted on the inside with an asphalt-base paint. Glass jars must be painted on the outside with dark paint to keep out light. A narrow strip should be left unpainted so that the level of the solution can be seen in the glass container.

The seedbed or plant bed should be 3 or more inches deep and large enough to completely cover the trough or tank. To support the litter, cover the bottom of the bed with chicken wire or 1/2-inch-mesh hardware cloth painted with an asphalt-base paint. Fill the bed with litter. The litter may be of wood shavings, excelsior, sphagnum moss, peat, or some other organic material or matter fairly resistant to decay. Germinate the seed in sand or vermiculite and transplant to the water-culture bed. Keep the bed moist until the plants get their roots down into the nutrient solution.

Aeration. The water-culture method often fails because of inadequate aeration of the solution. The space between the seed bed and the nutrient solution may provide enough air for the roots of certain plants. But you must make special provision to allow an exchange of air between this space and the air outside. Prop up the seed bed a fraction of an inch or drill holes in the container or tank just above the highest solution level.

If you have trouble aerating the roots, use an aquarium air pump. Do not stir the solution too vigorously. You may damage the tender roots and cause poor plant growth. Pumping the air through an air stone, a perforated pipe, a porous glass tube, or a hose covered with a fine screen will reduce root damage by breaking down the air bubbles.

Water Supply. An adequate supply of pure water is essential for this system of hydroponics. The mineral content of water varies from place to place. In some areas, water is softened by replacing the calcium and magnesium with sodium. Sodium is toxic to certain plants when present at high levels. Boron and copper may be toxic at very low levels in the water, even though these elements are required in minute quantities for plant growth. Usually the minerals in water are not detrimental to plant growth. Calcium and magnesium, which are often present in water, are beneficial to plants.

Applying Nutrient Solution. Nutrient solution may be added by hand, by means of a gravity feed system, or mechanically. In a small setup, the nutrient solution can be mixed in small containers and added by hand as needed. In a large setup, the gravity-feed system can be used effectively. The nutrient solution is mixed in a vat and tapped from the vat as needed. A large earthen jar or barrel will serve as the vat. If you use a metal barrel or container, paint the inside with an asphalt-base paint.

A pump can be used to transfer the material from the mixing vats to the growing tanks. Use a special non-rusting pump, or wash the pump carefully after each use. This precaution is necessary because the chemicals used in the nutrient solution will corrode metal.

The time to add nutrient solution depends upon the temperature the growth of the plants. When the plants are young, the space between the seedbed and the nutrient solution may be quite small (sometimes one-half inch is sufficient). As the plant roots grow, lower the nutrient level slowly, keeping the level of the solution as constant as possible.

When the temperature is high and evaporation rapid, the plants may need additional solution every day. Keep the roots at the correct level in the water. The roots will die if allowed to dry out.

The container ar tank should be drained completely every two weeks and the nutrient solution renewed from the mixing vats. This operation should be arranged so that it can be accomplished in a short time. If more than a few minutes elapse between the time of draining the tanks and refilling them, the roots will dry out. To delay the drying of the roots, change the solutions on a cloudy day or after the sun has gone down.

Transplanting seedlings or seeding directly into the seedbed will get the plants growing under the solution-culture system. The litter must be kept moist until the roots become established in the nutrient solution. Transplant seedlings carefully. Work the roots through the support netting into the nutrient solution; then build the litter around the plant to support it.

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REQUIREMENTS FOR PLANT GROWTH

The requirements for plant growth in soil culture and nutriculture are the same. The only fundamental difference between the two methods is the manner in which the inorganic nutrients required for growth are supplied to the roots.

Temperature. There is an optimum temperature range for plant growth. Above or below this range, plants will not do well. Warm season crops usually do well between 60o and 75o or 80o F., with 60o F. the night temperature. Cool-season crops do well between 50o and 70o F., with 50o F. the night temperature. Temperatures for best growth should be maintained whenever possible.




Light. Most cultivated plants need large amounts of sunlight. When plants are grown indoors, additional artificial light is sometimes needed. If plants are grown entirely under artificial light, the intensity of the light must be very high without causing the temperature to rise above the optimum range.

Water. Water should be available in adequate amounts in the soil or in soilless culture for proper growth. Too little or too much water will not give optimum growth.

Oxygen. In soil that is not waterlogged, adequate oxygen should be available. In hydroponic systems for growing plants, there may not be sufficient oxygen in the nutrient medium. To provide enough oxygen, it is often necessary to bubble air through the solution surrounding the roots.

Carbon Dioxide. Carbon dioxide, as gas, is taken up through the surface of the leaf and furnishes carbon and oxygen. These elements are required, along with hydrogen, in the manufacture of carbohydrates. Carbohydrates are used by the plants as food. Mineral Nutrients. The plant must absorb certain minerals through its roots to survive. The minerals required in relatively large amounts are nitrogen, potassium, phosphorus, calcium, magnesium, and sulfur. Those required in small amounts are iron, manganese, boron, zinc, and copper. Molybdenum and chlorine are also useful to plants, but the quantities required are so minute that they are usually supplied in the water or along with the other mineral nutrients as impurities.

HYDROPONICS AS A HOBBY

During the past several decades, many amateur and commercial gardeners have become interested in growing plants with their roots in an artificial medium instead of soil. This method of growing plants is commonly known as "hydroponics." It is also sometimes referred to as nutrient-solution culture, soilless culture, water culture, gravel culture, and nutriculture.




Soilless culture of plants is not new. One of the first experiments in water culture was made by Woodward in England in 1699. He was trying to determine whether water or the solid portion of the soil was responsible for plant growth. By the mid-nineteenth century, Sachs and Knop, the real pioneers in this field, had developed a method of growing plants without soil.

In the late 1920's and early 1930's, Dr. W. F. Gericke was able to grow plants successfully on a large scale through the laboratory technique of solution culture. Dr. Gericke used the term "hydroponics" to describe this method of growing plants. Today, hydroponics is used in commercial production, but it is employed mostly in those areas where soil is lacking or unsuitable for plant growth. Hydroponics is also a tool in plant research as well as a fascinating hobby.

SUMMARY COMMENTS ON SPECIFIC ELEMENTS

NITROGEN: Plant requirements for nitrogen are sometimes larger than all of the other elements combined. It can thus be difficult to supply nitrogen in the refill solution without adding excess amounts of other cations. The best solution is to use nitric acid (HNO3) for pH control. This can supply 50% of the nitrogen needs of the crop without adding excess cations. If extra nitrogen is required, ammonium nitrate can be added to the pH control solution. However, because ammonium decreases the uptake of other cations (K, Ca, Mg, and micronutrients) I do not recommend its use in hydroponic solutions unless extra nitrogen is required by the crop for maximum yields.

PHOSPHOROUS and POTASSIUM are rapidly drawn down to µM levels is solution. These low levels do not mean that the plant is starving for these elements, it means that the plant is healthy and actively absorbed these elements from solution.

CALCIUM requirements are almost 3 times higher for dicots than for monocots (grasses). Calcium is nontoxic, even at high tissue concentrations, but it accumulates in solution if too much is added to the refill solution.

MAGNESIUM is highly mobile and can accumulate to toxic levels in upper leaves if the solution concentration is too high.

MICROORGANISMS AND ORGANIC COMPOUNDS IN THE SOLUTION: IS FILTERING USEFUL?

Many people think that filtering the recirculating solution is useful, but we have never filtered our solutions. Our measurements indicate that total organic carbon in the recirculating solution does not exceed 15 mg per liter, even near the end of a 2 month life cycle. About 30% of the organic carbon in the solution is in the chelating agent. Total organic carbon includes the carbon that is in microbial biomass, so it is clear that neither organic compounds nor microorganisms are at high levels in the solution. The solution also appears as clear prior to harvest at 80 days as fresh solution.




Roots leak organic compounds, but there is an equilibrium between microorganisms on root surfaces and the exudates so that compounds are degraded to CO2 at the root surface. Estimates of the quantity of root exudates vary widely, but there is considerable evidence that carbon efflux increases when plants are stressed (Barber and Gunn, 1974; Smucker, 1984; Haller and Stolp, 1985). Bowen and Rovira (1976) found that roots in solution culture produce smaller quantities of exudate than in soil. Trollenier and Hect-Buchholz (1984) found that reduced root growth due to inadequate aeration in hydroponic culture was accompanied by a dramatic increase in root microbe population, which they attributed to increased exudation from roots. The bottom line is that healthy roots in a well aerated hydroponic system should not increase the microorganisms or organics in the solution and filtering is thus unnecessary.