HYDROPONICS A Glimpse into the Future of Food Cultivation Malcolm Borg
What is Hydroponics? • “The science of growing plants without the use of soil, but by use of an inert medium, such as gravel, sand, peat, vermiculite, pumice or sawdust, to which is added a nutrient solution containing all the essential elements needed by the plant for its normal growth and development.” (Resh, 1995)
Why is Soil needed for Plant Growth? • Anchorage • Water • Nutrients
Hydroponics vs. Soilless Culture • Hydroponics: plant roots suspended in a nutrient solution; • Soilless Culture: plants grown in an inorganic (e.g. sand, gravel, perlite) or organic (e.g. sphagnum peat moss, cocnut fiber) medium
Advantages • Crops cultivated when soil is poor; • Reduced labour (tilling, de-weeding, etc.); • Conservation of water and nutrients (no leaching, recycling of resources) • Limited soilborne plant diseases; • Can be adapted to small areas, indoor places etc. • Vertical space can be used and thus an increased yield can be obtained.
Disadvantages • Original cost • Trained personnel necessary for nutrient mix etc. • Quick spread of disease through water • Not all plants grow in this system • Depends on constant electrical suppl
• Closed vs. Open systems
Standing Aerated Nutrient Solution • Requirements: – Rooting Vessel – Nutrient Solution – Pump to bubble air to add oxygen
• Periodic replacements of solution every 5 to 10 days; • Water loss replaced daily using nutrientfree water or diluted nutrient solution. • Nutrient solution composition is therefore continuously changing.
Nutrient Film Technique (NFT)
• Plant roots are suspended in a trough through which a nutrient solution passes. • Trough set on a slope. • As the nutrient solution slows down the trough, plants at the upper end of the trough reduce the oxygen and/or elemental content of the nutrient solution. Trough should therefore not be too long (approx. 9 metres). • As the root mat becomes denser, water flows around the roots and thus reduce the contact between the roots and the solution.
• NFT systems are usually closed systems: solution is recovered and reused. • Problem with nutrient build-up and variations in pH. • Preferably needs a starting and a top-up solution. • Electrical conductivity will indicate when to add top-up solution and when to discard whole solution (and make a new one).
For Tomatoes
Aeroponics
• Roots growing in air • Good oxygenation of roots • Droplet size and frequency of exposure of the roots to the nutrient solution are the critical factors.
Ebb and Flow
• Made up of: watertight rooting bed, inert rooting medium (e.g. gravel, sand etc.), pump to move nutrient solution from sump to growing bed(s) and a piping system. • This is a closed system and therefore nutrient solution is recirculated until no longer usable.
• Timing schedule for flooding the growing beds will depend on the atmospheric demand and the stage of growth for the crop, as well as the water-holding capacity of the growing medium.
Organic Media Cultures • Nutrients still supplied to plants. • Have absorptive properties and therefore act more like soil. • Examples include sphagnum peat moss, coconut fiber and composted milled pinebark.
• Media have buffering capacity and therefore can store elements.
Peat • High porosity • Good aeration • High water-holding capacity • Low pH • Light (inexpensive to transport) • Low anion exchange capacity
Coir • Mesocarp of coconut • Can have high levels of salts • Has significant amounts of micronutrients
Wood Fibre • Made from wood not bark • Low pH • Low EC • High level of porosity • Low level of available water
Inorganic media can be used… • Either alone or mixed with organic media • Mostly are clay particles and are pest-free • Include – Perlite (light, porous, hold a lot of water, neutral pH, no buffering capacity) – Vermiculite (porous, hold a lot of water, high CEC due to positive charge) – Expanded clay (light, porous, expensive, neutral pH, no cation exchange or buffering capacity)
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PERLITE
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VERMICULITE
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EXPANDED CLAY
• Other media include – Rockwool – Sand – …and others
Leaching • The medium must have a leaching fraction to: (a) maintain EC and (b) keep oxygen levels adequate for plant growth.
Water • All hydroponic/soilless cultures require sizeable quantities of relatively pure water. • Analysis of water is essential. • Water may contain good quantities of Ca and Mg. It may also contain amounts of Na. • Pesticides and herbicides can significantly affect plant growth. • Ions in the water can be removed by a reverse osmosis plant.
• The pH of water varies over a wide range. • pH will vary depending on the amount of carbon dioxide absorbed, ratio of cations to anions, type of ions and their concentration in the solution. • A saturated solution of calcium sulphate will be acidic because this compound is a salt between a strong acid and a weak base.
• Water sterilization: – UV – Ozone
• Costs • Effectiveness debatable for certain pathogens
Nutrient Solution • With continuous use in a closed recirculating system, the nutrient solution is altered with each passage through the root mass and rooting medium, not only chemically by the removal of elements by plant root absorption but also through additions produced by the sloughing off of root material.
• Many researchers have come up with a large number of nutrient solutions for a number of crops. • Nutrient solutions must have the following characteristics: – Properly balanced with all necessary nutrients; – pH should be kept at an optimum range – Must have no ions present in toxic amounts or at levels that may interfere with other ions; – Avoid precipitation; – Total salt concentration must be kept between 1500 and 4000ppm (a high EC will inhibit correct functioning of the plant e.g. poor leaf formation)
Example: Source of N • Urea Nitrite Nitrate • Ammonium content? • Nitrification decreases under low temperature • Cation antagonism
Aquaponics
• Nutrients provided by the mineralization of fish manure. • Nutrients supplied continuously. • Fish used include tilapia, trout, perch and koi. • Sedimentation and biological filtration required. Biological filtration is not needed for mediumbased hydroponics. • Pesticides and other substances toxic to fish cannot be utilized.
