Issue 05: Aeroponics

Issue 05
July/August – 1992
Story Title: Aeroponics
Author: Steven Carruthers

Aeroponics is the most recent development in hydroponic methods, and one that has gained much publicity over recent years. It is defined by the International Society for Soiless Culture as a system where roots are continuously or discontinuously in an environment saturated with fine drops (a mist or aerosol) of nutrient solution.” The method requires no substrate and entails growing plants with their roots suspended in a deep air or growth chamber, with the roots periodically atomised with a fine mist of nutrients. Since their inception some 30 years ago, it is fair to say that aeroponic techniques have proved very successful for propagation, but have yet to prove themselves on a commercial scale. Aeroponics is also widely used in aboratory studies of plant physiology. Both the University of Arizona and Purdue University use aeroponic cultivation techniques to research controlled environment life support systems to be used in space stations of the future, and to support visitors to Mars.

The first aeroponic system was developed at the University of Pia in Italy by Dr Franco Massantini, which led to the development of the colonna di coltura. This consisted of an aeroponic pipe supporting three relatively small cultivation trays fitted with sprinklers and covered with polystyrene lids.

Although several plant types were grown successully, it proved to be expensive. The system was modified to a vertical layout, whereby crops were cultivated in a multi-layered system. A zinc-coated metal frame supported two, three, four and five vertical levels of channels. Plants were arranged in the channels, covered with dark plastic film to protect plant roots from light. Seedlings were then inserted in the film at suitable distances apart. More recent units use vertical PVC pipes with strawberry cultivars the usual crop grown.

The Advantages
Excellent aeration is the main advantage of aeroponics. Trials imply an important relationship between low water level and increased air space – the importance of oxygen in supporting the intensive metabolic processes associated with root formation and subsequent growth is well recognised.

For propagation purposes, aeroponics is well proven. Experiments in Israel to assess aeroponics as a plant propagation method, and to determine the effect of disolved oxygen on the rooting of cuttings, produced some interesting results. Both Chrysanthemum (herbaceous cuttings) and Ficus (woody cuttings), noted as difficult-to-root species, responded to increased dissolved oxygen concentrations. The number of roots and total root length increased as dissolved oxygen increased.

For Chrysanthemum, cuttings rooted over the enture stem surface, while Ficus cuttings only rooted at the cutting base. Nonetheless, rooting percentage and total root length for Ficus were greater in the aeroponic system than in plants from the same stock in a perlite/vermiculite mix or a sand/peat/redwood bark mix.

The Disadvantages
Apart from the relatively high set up costs, the technique is mechanically quite elaborate, susceptible to malfunction, requires precise regulation and control of water and nutrients, and has no buffer capacity to sustain even slight deviations or occasional malfunctions. In the event of blocked nozzles or breakdowns going unnoticed, plants may be irrepairably damaged in a relatively short time.

Ein-Gedi System (EGS)
Against this background, Israel took up the bat with a modified version of aeroponics. The commercial Ein Geidi System (EGS), developed by workers of the Agricultural Research Organisation, is a trough system that utilises the advantages of true water culture systems to overcome the limitations encountered with aeroponics and NFT, in effect combining the good points of all these systems.

The EGS is an aero-hydroponics system, rather than a true aeroponic system – a flowing nutrient solution system in which plant roots are immersed in a deep, circulating, continuously aerated nutrient solution.

The nutrient solution is delivered in two layers – a mist a mist on top of a liquid. The solution is injected into the trough by successive laser-cut apertures in a direction opposing that of the solution flow. This carries oxygen from the spray zone into the nutrient solution, aerating it and providing high oxygen concentrations at the root surface. This permits the roots to be immersed in a deep, large volume of nutrient solution, in troughs of any length. Aeration and delivery of spray varies according to the different designs of Ein geidi systems. The trough system uses a remote mechanical pump and piped spray system.

Roots also grow in the spray zone. The spray is created by injecting nutrient solution, at relatively low water pressure through strategically placed nozzles.

While there are no significant commercial EGS installations in Israel, other than those used for ornamental plants, commercial installations are in operation in Belgium and the USA, with tomatoes and cucumbers the major crops grown.

The Ein Geidi System is also available as a mpropagation, hobby or laboratory unit and consists of an 18 litre cylindrical chamber (half filled with continuously agitated water), motor and housing, and removable cover with collared holes to support plants.

It functions by drawing water up from the bottom centre of the container through a hollow, rotating impeller, driven by an electric motor. The rate of water uptake (2.01/m) is proportional to the rotating speed of the motor (3000 rpm). The drawn water is then thrown horizontally by centrifugal force into the air space above the water reservoir, creating a fine spray of water and, therefore, increasing the water surface area. Then, as the water droplets fall back into the agitated water, gas exchange takes place.

Rainforest
The rainforest system, produced in the USA, is similar to the EGS propgation unit in that it has a deep growth chamber, which is half filled with the nutrient solution, thereby making it an aero-hydroponic system.

It differs in the mist delivery. A submersible pump draws water up a central tube to the top of the root chamber where the jet of water hits a curved plate, causing the solution to be sprayed horizontally onto the roots in the spray zone. The circle of water (not a true spray) created then falls back into the reservoir to provide gas excahnge. It has no extra aeration for the nutrient solution.

Schwalbach System
The Schwalbach System is a commercial aeroponics system developed in Australia. This simple, cost-efective system was developed by Keith Schwalbach of Wilberforce, NSW, to grow lettuce, parsley, silverbeet, celery, cabbage, stringless beans and strawberries.

The system uses a 200 litre plastic nutrient tank which feeds a root chamber constructed of black plastic film supported by a framework of square galvanised pipe. The slow-revving mono pump can service up to 60 jets, with each root chamber fitted with 8 misting jets, enough to feed 48 plants. Each jet delivers 10 litres of nutrient solution per hour.

The growing tops are constructed of 6mm thick structural foam polypropylene which is semi-rigid and resistant to acid and alkaline. It is UV stabilised, ‘food compatible’, and tolerant to a wide range of temperatures. Additionally, the tops are slightly curved to allow rapid rainwater run-off, thus reducing nutrient dilution. The tops also have pockets fitted to allow hoops for an igloo or shadecloth.

The Schwalbach system is fully described in John Mason’s book, Commercial Hydroponics, published by Kangaroo Press.

Aero-Gro System (AGS)
The most recent innovation in aeroponics is the Australian-developed Aero-Gro System (AGS) which adopts ultra-sonic technology to nebuilise or vapourise the nutrient solution, providing a gog-like mist to the plant roots. (This mist is much finer than is achievable in conventional spray and pump systems.

Until the development of the AGS, all aeroponic systems used a spray to deliver the nutrient solution, rather than a true mist or aerosol as defined by Steiner. The spray is delivered using pumps and small nozzels.

The disadvantage of using a high-pressure pump delivery system is miniral build-up and eventually blocked nozzles; for low-pressure sprays the advantage of rapid nutrient absorption is diminsihed.

Another disadvantage of spraying roots with a nutrient solution is that the roots tend to be washed into a matted tangle by the solution run-off. This tends to stifle root growth and can lead to disease problems if the plant roots are not allowed to dry out.

The AGS, developed by Cameron Laurie, an Industrial Design graduate, nebulises the nutrient solution into a fog-like mist, in much the same way that the application of medication is atomised and delivered to the alveoli in the lungs of asthma patients. It was this example that led Cameron to develop the AGS. The application of this technology to aeroponics is currently subject of a patent pending.

One method of nebulising is to force compressed air through a venturi. This requires jets for the nutrient solution which are again prone to blockages.

The other method uses high-energy, ultra-sonic technology – a high frequency sound that blasts the nutrient solution into a fog-like mist.

There are many advantages to nebulising the nutrient solution. Foremost, nutrients are absorbed more rapidly by roots, which are no longer prone to matting; because the nebuliser is soild state, mechanical breakdowns are virtually eliminated; power consumption is low; and maintenance is low because irrigation delivery lines and nozzles are no longer needed to deliver nutrients to plant roots.

The AGS consists of a root chamber, much like the EGS, which also contains the nutrient solution. The ultra-sonic transducer is imbedded in a polystyrene foam which floats on the solution, with the chamber itself aerated to circulate the mist and to remove remove exudants. A prime benefit of this float system is that the water cools the transducer, which in turn warms the nutrient solution.

Preliminary trials indicate that plant growth using the ultra-sonic delivery system is far superior to those grown in what can now be described as a conventional aeroponic delivery system. Marigold seedlings of the same age, and from the same punnet, were planted in the AGS chamber 10 days after those transplanted into a conventional system. At the time of the AGS planting, the marigold in the conventional system were 12 cm high, but only 7 cm high when transplanted into the AGS. After a further 10 days growth in the AGS had more than doubled to 15 cm, while those in the conventional system had only grown 2 cm to 14 cm. Further trials are underway to assess whether long-term commercial crops are viable without incurring the same problems encountered in NFT.

Conclusion
Theoretically, aeroponics is a good system. However, the use of hydroponics overall is dictated by economic considerations and it is here that conventional aeroponics is cost prohibitive for most growing applications. The requirement for expensive timing, irrigation and pumping equipment, and a regular maintenance schedule, put it out of reach for most growers. For propagation and laboratory experiments, however, it is well proven and in wide use.

The development of the Aero-Gro System represents leading-edge technology. High plant performance, low maintenance schedule through the elimination of complex irrigation lines and equipment, and low power consumption, may make ultra-sonic aeroponics an economical proposition for commercial crops of the future.

References(AGS)

Adam J. Savage,
Hydroponics Worldwide: State of the Art in Soilless Crop Production,
International Center for Special Studies, Honolulu, 1985.

Soffer, H. & Burger D.W.;
Research on Aero-Hydroponics,
HSA Proc 9th Annual Conference (p 69-74), 1988.

Soffer, H. & D. Levinger;
The Ein-Gedi System – Research and Development of a Hydroponic System,
ISOC Proc. 5th Int. Cong. on Soilless Culture (p241-2532), 1980.

Vestergaard, B;
Oxygen Supply to the Roots in Different Hydroponic Systems,
ISOSC Proc. 6th Int. Cong. on Soilless Culture (p723- 737), 1969.


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