PL S 217 Course Notes

Chapter 5
Basic Principals of Hydroponics

ADVANTAGES OF USING HYDROPONICS OVER SOIL CULTURE

1. Crops can be produced on non-arable land including land with poor soils and/or high salinity levels. The grower doesn't have to have good soil since the systems, bags, etc. are placed on top of the ground.
   
   
   
2. Isolation from diseases or insect pests usually found in the soil. The plant roots are contained in systems, bags, etc. and do not grow through soil that might contain diseases or other pests such as insects and nematodes. Additionally, white fabric ground covers can be placed on the greenhouse floor to further isolate the systems and plants from soil-borne pests.
   
   NOTE: The white fabric also reflects light back up into the canopy enhancing photosynthesis, allows for ease of cleaning and helps control humidity and weeds.
   
   
   
3. Direct and immediate control over the rhizosphere. Since the roots are either growing in water or growing through an inert medium, whatever is in the nutrient solution is bathing the roots. Therefore, nutrient concentrations and pH can be adjusted quickly.
   
   
   
4. Higher yields are possible. For field grown tomatoes yields are 10-40 tons per acre compared with around 300 tons per acre for tomatoes grown using greenhouse hydroponics.
   
   
   • Yields are greater due to better control over water, nutrition, EC, pH and diseases (plus control over environmental conditions - see below).
     
     
   • Yields are also higher due to the use of certain varieties bred for hydroponics that are also indeterminant ("vining" - see below).
     
     
     
5. High density planting = minimum use of land area. For tomatoes in the field a typical planting density is 4000 to 5000 plants per acre. Greenhouse hydroponic tomatoes can be 10,000 to 11,000 plants per acre!
   
   Plants can be grown closer together because of the use of indeterminant ("vining") varieties that take up less cross-sectional area than do bush varieties usually used for field cropping. This also takes advantage of the greenhouse volume, so that production is on a "cubic volume basis".
   
   
   
6. Efficient use of water and nutrients. In soil culture water may be lost in wetting the soil beyond the reach of the plant roots or from the surface through evaporation.
   
   In hydroponic culture, since the nutrient solution is enclosed in a bag, tube, etc., there is no loss AND little or no water stress in the plant.
   
   NOTE: When comparing monetary return for water use - for every gallon of water used to irrigate cotton the grower gets 1/10th of a cent. For every gallon of water used to irrigate hydroponic tomatoes, the grower gets over 30 cents! Nutrients (which equate to $money$) are also not lost to the soil but retained in the root zone and in closed systems are replenished and recycled.
   
   
   
7. Ease of cleaning the systems. The aggregate growing media can be steam sterilized, or simply replaced. Whole systems, including the drip irrigation system, can be quickly sterilized using 10% bleach and cleaned of salt build-ups using a mild acid (rinse well).
   
   
   
8. No weeding or cultivation is needed.
   
   
   
9. Transplanting of seedlings is easy - No transplant shock. In soil culture the root mass can be easily disturbed during transplanting causing root breakage, plant stress and stunted growth for up to a week.
   
   In hydroponic culture, seeds are started in Rockwool cubes or plugs, then transplant into larger cubes with holes made for that purpose (see Chapter 9 Seed Germination and Transplanting). There is no disturbance of the root mass, little or no root breakage and therefore minimal plant stress and transplant shock.
   
   
   
10. Fruit of hydroponically grown plants is usually tastier. Hydroponically grown tomatoes, for example, are picked after they have begun to ripen, which includes the typical red color formation of the fruit (lycopene), the formation of gel within the locules and the characteristic taste. The grower can also raise the EC (electrical conductivity measuring salt levels) in the root zone that tends to enhance flavor.
    
    Tomatoes from field grown plants (in many areas) are picked "green", then "gassed" with ethylene which induces lycopene formation but does not enhance the flavor. Therefore, you get nicely colored fruit with little or no taste.

ADVANTAGES OF GREENHOUSE CULTURE

1. Virtual indifference to the seasons.
   In the desert southwest crops can be grown year around including in the winter when field crops are not being produced and prices are higher.
   
   
   
2. Control over the aerial (upper) portions of the plant.
   
   
   • The air temperature and, too some extent, the relative humidity can be regulated to suit the crop under cultivation in the greenhouse environment.
     
     
   • Higher than normal (~330 ppm) levels of carbon dioxide (up to 1000 to 1500 ppm) can be reached using a carbon dioxide generator (burning natural gas) in order to enhance photosynthesis.
     
     
     
3. The greenhouse environment is suitable for mechanization. Includes personnel carts for plant maintenance and picking as well as future designs for automated harvesting "robots" (will require changes in plant structure).

DISADVANTAGES OF GREENHOUSE HYDROPONICS

1. Requires a large capital ($money$) input and energy input.
   
   
   • Any size commercial operation (including injector irrigation systems, computer controls, etc.) will cost about $600,000 per acre with the land itself costing $1000 - $2000 per acre.
     
     
   • Energy costs include those for heating (usually burn natural gas), cooling (usually use evaporative - fan and pad - cooling), electricity to run various types of equipment (some injectors, computer controls, motors, sorting/packing/storage equipment, etc.
     
     
     
2. The grower needs a high degree of competence in plant science, engineering, computer control systems and marketing. Or experts in these fields needs to be hired. This is an intensive form of agriculture where a small problem can escalate to a major disaster very quickly.
   
   
   
3. The technology is limited to crops of high economic value.
   Since the initial cost of a large commercial facility is so high it would not be profitable to grow anything but crops of high economic value including tomatoes, colored bell peppers, cucumbers and even lettuce which, in a hydroponic greenhouse, can yield multiple crops per year.
   
   
   
4. Plant diseases and insect pests may be more difficult to control.
   
   
   • Root pathogens that produce water-borne spores (e.g., zoospores of such fungi as Pythium or Phytophthora) can be devastating to plants growing in a "closed" system where the nutrient solution is recirculated since infected solution will circulate to all plants. NOTE: For treatments see Chapter 4: Plant Protection.
     
     
   • The greenhouse, with its controlled environment, is a perfect habitat for many types of insects, including those considered "pests" on plants such as white flies, aphids, thrips, spider mites, shore flies and fungus gnats. These pests may cause direct damage to the plants or may transmit viruses to the plants. NOTE: Beneficial insects that can be released. See Chapter 4: Plant Protection.
     
     
   • $0.80 - $1.00 per plant can be spent for insect pest control. That adds up!

REVIEW OF PLANT NEEDS
This is critical for understanding how to build hydroponic systems.

1. Water - Critical for metabolic processes, for transport of substances throughout
   the plant body (phloem and xylem) and for transpirational cooling.
   
   
2. Light - Critical for photosynthesis. (Where you put your system is important.)
   
   
3. Inorganic mineral nutrients - at the correct concentrations and pH levels.
   
   
4. Carbon dioxide - Critical for photosynthesis (needed at the leaf surface).
   
   
5. Oxygen - Critical for respiration (needed by all parts of the plant including the
   roots, therefore aeration of the nutrient solution may be required).
   
   
6. The proper temperature and relative humidity (specific to type of plant).
   
   
7. Support systems for the roots and shoots. For plants where the roots hang directly into the nutrient solution and do not provide any support for the plant, mechanical support may be needed. For an indeterminant tomato plant, support for the stem will be needed in the form of twine and vine clips.

TYPES OF HYDROPONIC SYSTEMS

Systems categorized by where the roots are located:

1. Liquid Culture: The roots are in direct contact with the nutrient solution which can be either in the form of a liquid or a mist.
2. Aggregate Culture: The roots grow into an inert medium such as sand, gravel, Rockwool, perlite, vermiculite, peat moss, foam, coconut coir, etc. and are then irrigated with a complete nutrient solution.
   
   

Systems categorized by what happens to the nutrient solution:

1. Open: The nutrient solution is distributed from a reservoir to the plants and is then "drained to waste" (i.e., not used again).
2. Closed: The nutrient solution is distributed from a reservoir to the plants. After passing through the root zone it is collected and reused. In large systems the solution may be analyzed, then modified by the additions of water, acid/base, and/or various inorganic elements to return the solution to the appropriate inorganic mineral composition and pH. The solution may also be sterilized (UV light, ozone treatment, etc.) so that any plant pathogens, picked up in the solution from perhaps one or a few infected plants, are not subsequently spread to all of the plants.

SYSTEM DESIGNS

1. The basic wick: The roots grow down through an aggregate medium. A wick (absorbent material) is laced through the medium and hangs down into a reservoir and draws the nutrient solution up into the root zone.
   Type of system = Aggregate/Closed
   
   
   
   
   
   
   
   
2. The non-recirculating ("air-gap") system: The roots hang into a nutrient solution reservoir, with the upper part of the root mass suspended in air (air roots to take up needed oxygen) and the lower part of the root mass in direct contact with the nutrient solution (water and nutrient roots).
   Type of system = Liquid/Closed
   
   
   
   
   
   
   
   
   
3. The raft, float or deep flow system: Plants are suspended through styrofoam boards which float on the surface of the nutrient solution. Oxygen can be supplied to the roots using an aquarium pump and air stones or a "venturi" system.
   Type of system = Liquid/Closed
   
   
   
   
   
   
   
4. The flood and drain (or ebb and flow) system: The roots grow down through an aggregate. The nutrient solution is pumped into the aggregate medium, floods the root zone for a short time, and is then allowed to drain back into the reservoir.
   Type of system = Aggregate/Closed
   
   
   
   
   
   
5. The top feeder system: The roots grow down through an aggregate. The nutrient solution is delivered to the top of the aggregate medium, percolates through and then either drains to waste or is recirculated into a reservoir.
   Type of system = Aggregate/Closed or Open
   
   
   
   
   
   
   
   
   
6. Nutrient film (flow) technique (NFT): The roots may be growing from Rockwool blocks or through cups filled with an aggregate for support but ultimately hang into a slightly slanted tube or trough. The nutrient solution is pumped to the higher end, flows past the hanging roots and then back to the reservoir.
   Type of system = Liquid-Aggregate/Closed
   
   
   
   
   
   
   
7. Aeroponics: The roots are suspended in an enclosed space and, at regular intervals, sprayed with the complete nutrient solution.
   Type of system = Liquid/Closed or Open

REFERENCE MATERIAL:

1. Hydroponic Food Production. 1991. H.M. Resh. Woodbridge Press Publishing
   Company, Santa Barbara, CA, 93160. ISBN 0-88007-171-0
   
   
2. Hydroponic Gardening. 1991. L. Dalton and R. Smith. Cobb/Horwood
   Publications, Aukland, New Zealand.
   
   
3. Hydroponic Home Food Gardens. 1992. H.M. Resh. Woodbridge Press
   Publishing Company, Santa Barbara, CA, 93160. ISBN 0-88007-178-8
   
   
4. Hydroponics for the Home Gardener. 1992. S. Keynon. Key Porter Books
   Limited, Toronto, Ontario, Canada, M5E 1R2. ISBN 1-55013-375-6
   
   
5. Personal Communications: Members of the Hydroponic Gardeners of Tucson.
   

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