Nutrient film technique NFT

This is a method of growing plants in a shallow stream of nutrient solution continuously circulated along plastic troughs or gullies. The method is commercially possible because of the development of relatively cheap non-phytotoxic plastics to form the troughs, pipes and tanks (see Figure 22.5). There is no solid rooting medium and a mat of roots develops in the nutrient solution and in the moist atmosphere above it. Nutrient solution is lifted by a pump to feed the gullies directly or via a header tank. The ideal flow rate through the gullies appears to be 4 litres per minute. The gullies have a flat bottom, often lined with capillary matting to ensure a thin film throughout the trough. They are commonly made of disposable black or white polythene set on a graded soil or on adjustable trays. There must be an even slope with a minimum gradient of 1 in 100; areas of deeper liquid stagnate and adversely affect root growth (see aerobic conditions).

The nutrient solution can be prepared on site from basic ingredients or proprietary mixes. It is essential that allowance be made for the local water quality, particularly with regard to the micro-elements, such as boron or zinc, which can become concentrated to toxic levels in the circulating solution. The nutrient level is monitored with a conductivity

Nutrient solution

Dosing pumps

Flow regulator

Probe

Nutrient solution

Flow regulator

Dosing pumps

Probe

Manual Hydroponics
Figure 22.5 Nutrient film technique layout. The nutrient solution is pumped up to the top of the gullies. The solution passes down the gullies in a thin film and is returned to the catchment tank. The pH and nutrient levels in the catchment tank are monitored and adjusted as appropriate.

meter and by careful observation of the plants. Maintenance of pH between 6 and 6.5 is also very important. Nutrient and pH control is achieved using, as appropriate, a nutrient mix, nitric acid or phosphoric acid to lower pH and, where water supplies are too acid, potassium hydroxide to raise pH. Great care and safety precautions are necessary when handling the concentrated acids during preparation.

The commercial NFT installations have automatic control equipment in which conductivity and pH meters are linked to dosage pumps. The high and low level points also trigger visual or audible alarms in case of dosage pump failure. Dependence on the equipment may necessitate the grower installing failsafe devices, a second lift pump and a standby generator. A variation on this method is to grow crops such as lettuce in gullies on suitably graded glass house floors (see Figure 22.6).

Aggregate culture

In aggregate culture the nutrient solution is broken up into water films by an essentially inert solid medium, such as coarse sand or gravel. More commonly today materials such as rockwool, perlite, polyurethane foam, duraplast foam or expanded clay aggregates (see Figure 22.7) are used. These are in the form of polythene wrapped slabs or 'bolsters' of granules sitting on a polythene covered floor graded across the row. Polyurethane slabs are often placed underneath them to help create even slopes and insulate them from the cooler soil below. These growing

Nft System
Figure 22.6 (a) NFT lettuce crop with close up (b) showing gullies and nutrient solution delivery
Tomato Nutrient Film
Figure 22.7 Tomato crop in rockwool growing system

containers, on which the plants sit, are drip fed with a complete nutrient solution at the top with the surplus running out through slits near the bottom on the opposite side. When this method was first developed the NFT systems were copied, i.e. the water was recirculated, but it was soon found to be difficult where the quality of water was poor and there was a risk of a build-up of waterborne pathogens and trace elements. It was found that the surplus nutrient solution was most easily managed by allowing it to run to waste into the soil. However, this open system presents environmental problems and increasingly a closed system has had to be adopted. It is now becoming more usual to run the waste to a storage sump via collection gullies or pipes. Some of this can be used to irrigate outdoor crops if nearby. To recirculate the water it is necessary to have equipment to remove the excess salts or accept a gradual deterioration of the nutrient solution and then flush it out to a sump when it becomes unacceptable. Sources of infection such as Pythium are minimized by isolation from soil and using clean water; the risks of recirculating pathogens is addressed by using one of the four main methods of sterilization (see water quality).

Nutrient Film Technique

Water level indicator

Suitable growing medium e.g. Leca Attractive outer box

Flower pot Water level

Nutrient battery (ion exchange resins)

Figure 22.8 Plant pots with water reserves. Plants grown in a variety of growing media can be fitted with reservoirs that supply water by capillarity. A water level indicator is frequently incorporated and in some systems the nutrients are supplied from ion exchange resins. While this system can be used for any pot size it is particularly attractive in large displays

Water level indicator

Suitable growing medium e.g. Leca Attractive outer box

Flower pot Water level

Nutrient battery (ion exchange resins)

Figure 22.8 Plant pots with water reserves. Plants grown in a variety of growing media can be fitted with reservoirs that supply water by capillarity. A water level indicator is frequently incorporated and in some systems the nutrients are supplied from ion exchange resins. While this system can be used for any pot size it is particularly attractive in large displays

Rockwool slabs are a very successful way of growing which lend themselves to a modular system. It is widely used for a range of commercial crops, such as tomatoes, cucumbers, peppers, melons, lettuce, carnations, roses, orchids and strawberries, in protected culture. It is not biodegradable so the vast quantity of rockwool now utilized has produced a serious disposal problem. The slabs can be used successfully several times, if sterilized on each occasion, but eventually they lose their structure. Tearing them up and incorporating them in composts or soils can deal with a limited amount, but far more can now be recycled in the production of new slabs.

Several types of expanded clay aggregates used in the building industry, such as Leca or Hortag, have been used particularly in interior landscaping (see Figure 22.3). Smooth but porous granules 4-8 mm in diameter, giving a capillary rise of about 100 mm, are used to create an ideal rooting environment with a dry surface which makes it an attractive method of displaying house plants (see Figure 22.8). All forms of aggregate culture require feeding with all essential minerals. Trace element deficiencies occur less frequently when clay aggregates are used. Ion exchange resins are an ideal fertilizer formulation in these circumstances because the nutrients are released slowly, remove harmful chlorides and fluorides from irrigation water, and aid pH control.

Sports surfaces

The specifications for sports playing surfaces are such that turf has increasingly given way to artificial alternatives, typified by the trend toward playing 'lawn' tennis on 'clay' courts. This is partly attributable to maintenance requirements, but at the higher levels of sport it is because the users or the management expect play to continue with a minimum of interference by rainfall. The usual problem is that the soil in which the turf grows does not retain its structure under the pounding it receives from players and machinery, especially when it is in the wet plastic state. Turf is still preferred by many, but to achieve the high standards required it has to be grown in a much modified soil (see also sand slitting) or, increasingly, in an alternative such as sand. The most extreme approach is to grow the turf in pure sand isolated from the soil, sometimes within a plastic membrane. The high cost of these methods is such that it is only used to create small areas such as golf greens.

Normally the existing topsoil is removed from the site and the subsoil is compacted to form a firm base and graded to carry water away to drains. Drainage pipes are laid, above which is placed a drainage layer usually

Root zone 25-30 cm deep: Closely graded sand, 0.1-0.6 mm diameter

Hortag Drainage

Root zone 25-30 cm deep: Closely graded sand, 0.1-0.6 mm diameter

Drainage layer: Pea gravel to drainage pipes

Foundation: Compacted subsoil

Figure 22.9 Pure sand root zone used for sportsground surfaces

Drainage layer: Pea gravel to drainage pipes consisting of washed, pea-sized gravel, as shown in Figure 22.9 . Because it is considerably coarser than the sand placed on it, this layer prevents the downward percolation of water (see water films) and creates a perched water table. This helps to give the root zone a large reserve of available water whilst ensuring that gravitational water, following heavy rain or excess irrigation, is removed very rapidly.

Foundation: Compacted subsoil

Figure 22.9 Pure sand root zone used for sportsground surfaces

A 25-30 cm root zone of free-draining sand is placed uniformly over the drainage layer, evenly consolidated. Allowance has to be made for continued settling over the first year. It is essential that the sand used has a suitable particle size distribution, ideally 80-95 per cent of the particles being between 0.1 and 0.6mm diameter. A minimum of 'fines' is essential to avoid clogging up of the pores in the root zone (see Figure 22.2). Sometimes a small amount of organic matter is worked into the top 5 cm to help establish the grass, although success is probably as easily achieved with no more than regular light irrigation and liquid feeding.

Some very sophisticated all-sand systems, such as the cell system, are constructed so that the root zone is sub-divided into bays with vertical plastic plates and supplied with drains that can be closed so that the water in each of them can be controlled. Tensiometers are used to activate valves that allow water back into the drainage pipes to sub-irrigate the turf.

Check your learning

  1. State the main disadvantages of growing in soils.
  2. Describe what is required of a material to be used in a compost.
  3. State the advantages and the disadvantages of loam based composts.
  4. State the advantages of loamless compost.
  5. Explain why alternatives are being sought to replace peat in growing plants.
  6. State the advantages of hydroculture growing systems.

Further reading

Bragg, N. (1998). Grower Handbook 1 - Growing Media. Grower Books. Bunt, A.C. (1988). Media andMixesfor Container Grown Plants. Unwin Hyman. Cooper, A. (1979). The ABC ofNFT. Grower Books.

Handreck, K.A. and Black, N.D. (2002). Growing Mediafor Ornamentals and Turf.

Revised 3rd edn. New South Wales University Press. McIntyre, K. and Jakobsen, B. (1998). DrainageforSportsturfandHorticulture.

Horticultural Engineering Consultancy. Mason, J. (1990). Commercial Hydroponics. Kangaroo Press. Molyneux, C.J. (1988). Practical Guide to NFT. Nutriculture Ltd. Pryce, S. (1991). The PeatAlternatives Manual. Friends of the Earth. Smith, D. (1998). GrowerManual 2 Growing in Rockwool. Grower Books.

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Responses

  • neil
    How to irrigate using a dosing pump?
    6 years ago
  • OLIWIA
    How to grow tomato in nft?
    5 years ago

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