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Deep flow technique (DFT), also known as deep water technique, is the cultivation of plants on floating or hanging support (rafts, panels, boards) in containers filled with 10—20 cm nutrient solution (Van Os et al. 2008) (Fig. 4.3). In AP this can be up to 30 cm. There are different forms of application that can be distinguished mainly by the depth and volume of the solution, and the methods of recirculation and oxygenation.
Fig. 4.3 Illustration of a DFT system with floating panels
One of the simplest systems comprises 20—30 cm deep tanks, which can be constructed of different materials and waterproofed with polyethylene films. The tanks are equipped with floating rafts (several types are available from suppliers) that serve to support the plants above the water whilst the plants' roots penetrate the water. The system is particularly interesting as it minimizes costs and management. For example, there is a limited need for the automation of the control and correction of the nutrient solution, particularly in short duration crops such as lettuce, where the relatively high volume of solution facilitates the replenishment of the nutrient solution only at the end of each cycle, and only the oxygen content needs to be monitored periodically. Oxygen levels should be above 4—5 mg Lsup-1/sup; otherwise, nutrient deficiencies may appear due to root systems uptake low performance. Circulation of the solution will normally add oxygen, or Venturi systems can be added which dramatically increase air into the system. This is especially important when water temperatures are greater than 23 ˚C, as such high temperatures may stimulate lettuce bolting.
The NFT technique is used ubiquitously and can be considered the classic hydroponic cultivation system, where a nutrient solution flows along and circulates in troughs with a 1—2 cm layer of water (Cooper 1979; Jensen and Collins 1985; Van Os et al. 2008) (Fig. 4.4). The recirculation of the nutrient solution and the absence of substrate represent one of the main advantages of the NFT system. An additional advantage is its great potential for automation to save on labour costs (planting & harvesting) and the opportunity to manage the optimal plant density during crop cycle. On the other hand, the lack of substrate and low water levels makes the NFT vulnerable to the failure of pumps, due to e.g. clogging or a failure in the power supply. Temperature fluctuations in the nutrient solution can cause plant stress followed by diseases.
Fig. 4.4 Illustration of NFT system (left) and a multilayer NFT trough, developed and marketed by New Growing Systems (NGS), Spain (right)
The development of the root system, part of which remains suspended in air above the nutrient flow and which is exposed to an early ageing and loss of functionality, represents a major constraint as it prevents the production of longcycle crops (over 4—5 months). Because of its high susceptibility to temperature variations, this system is not suitable for cultivation environments characterized by high levels of irradiation and temperature (e.g. southern areas of the Mediterranean basin). However, in response to these challenges, a multilayer NFT trough has been designed which allows for longer production cycles without clogging problems (NGS). It is made of a series of interconnected layers placed in a cascade, so that even in strong rooting plant species, such as tomatoes, the nutrient solution will still find its way to the roots by by-passing the root-clogged layer via a lower positioned layer.
The aeroponic technique is mainly aimed at smaller horticultural species, and has not yet been widely used due to the high investment and management costs. Plants are supported by plastic panels or by polystyrene, arranged horizontally or on inclined tops of growing boxes. These panels are supported by a structure made with inert materials (plastic, steel coated with plastic film, polystyrene boards), in order to form closed boxes where the suspended root system can develop (Fig. 4.5).
Fig. 4.5 Illustration of the aeroponics technique
The nutrient solution is directly sprayed on the roots, which are suspended in the box in air, with static sprinklers (sprayers), inserted on pipes housed inside the box module. The spray duration is from 30 to 60 s, whilst the frequency varies depending on the cultivation period, the growth stage of the plants, the species and the time of day. Some systems use vibrating plates to create micro droplets of water which form a steam which condenses on the roots. The leachate is collected on the bottom of the box modules and conveyed to the storage tank, for reuse.