Issue 04: NFT Culture

Issue 04
May/June – 1992
Story Title: NFT Culture
Author: Don Slade

The system of Nutrient Film Technique (NFT) was originally designed and developed by Dr Allen Cooper. The concept is described by Dr Cooper as follows: ” A very shallow stream of water containing all the dissolved nutrients required for growth is recirculated past the bare roots of crop plants in a water tight gully…..Ideally, the depth of the recirculating stream should be very shallow, little more than a film of water – hence the name nutrient film. This ensures that the thick root mat, which develops in the bottom of the gully, has an upper surface which, although moist, is in the air. Consequently, there is an abundant supply of oxygen to the roots of the plants. “

The design of systems has altered little but, with continuing experience and understanding of requirements of different crops, NFT is now displacing soil, media bags and other hydroponics related methods for protected cultivation of most suitable crops. While small increases in production have been demonstrated using double gullies with split root systems, together with high and low nutrient concentrations described as ‘ feeding ‘ and ‘ drinking ‘ cycles, most commercial systems retain the simple single gully.

Crop Types NFT systems are used to grow tomatoes, lettuce, endive, Chinese cabbage and similar leafy crops, cucumbers, zucchini and courgettes, beans, sweet peppers, egg plants, chillies, parsley and other herbs, silver beet, strawberries, and many types of ornamentals. The system is not, in its normal form, adapted for the production of root and tuber crops.

The Advantages
The main advantage of the NFT system over other forms of hydroponics – bag, media and soil culture – is that the plant roots are exposed to adequate supplies of water, oxygen and nutrients. In all other forms of production there is a conflict between the supply of these requirements, since excessive or deficient amounts of one results in an imbalance of one or both of the others. NFT, because of its design, provides a system where all three requirements can be met at the same time, providing the simple concept of NFT is always remembered and practised. The result of these advantages is that higher yields of high quality produce are obtained over an extended period of cropping.

The Disadvantages
Flooding and waterlogging of roots, or other problems due to poor design, construction or operation may occur with resulting crop loss. These problems result from producers forgetting the simple concepts of NFT production systems, and constitute the main disadvantages found in the operation of the NFT system. Other disadvantages are associated with the dependence of NFT on reliable supplies of water and power. If breakdowns occur, and suitable back-ups have not been provided, more serious losses can result than in media systems where a degree of buffering is present.

It is often surprising that little thought is given to provision of a back-up on services that are so essential. A standby generator may not be required. If breakdowns are infrequent, a reserve water supply that can be readily introduced into gullies, to keep plants alive during prolonged power or water failures, may be readily and cheaply installed.

The Watertight
Gully Gullies for long term crops with large root systems have been devised using metal, fibreglass, plastic and polystyrene trays, some of which are lined with a plastic film. A wide range of materials have been encountered and include purpose-made plastic gullies. The most practical and cheapest commercial method, at present, requires a level floor or firm support base, upon which folded black and white 800mm wide PVC plastic film is laid, so that the flat base is at least 200m wide and the two equal sides are folded and supported across the top to form an inverted ‘ T ‘.

The nutrient film flows along the floor of the gully. The black inside excludes light and the white exterior reflects light and heat. To assist crop establishment, a small piece of capillary matting that will eventually break down, is often placed under the young seedling, so that survival is assisted while young roots develop.

A support wire, about 200mm above the base, is of assistance in keeping the gully shape formed and for early plant support. During use, the ‘tent’ shape must be maintained to allow adequate aeration. Do not allow the plastic to fall and remain in contact with the top of the roots, excluding air.

Because folded plastic gullies are inconvenient to use for short term crops, the use of rectangular section gullies, for crops such as lettuce, arose. These sections were available as rainwater downpipes, which had planting holes cut in the wider side. In use, they are difficult to clean and expensive to plant up and service. Their excessive height, for young seedlings, requires plants to be raised and set out in small pots with resulting extra costs.

Designs have been registered for trays with removable lids and engineered flow, to keep the nutrient film returning to the young plant roots. Trays may be designed to accept seedlings produced in 5ml plug trays. This helps raising seedlings. The trays were also suited to multi-tier production of leafy green crops. Trays are used for fast growing short term crops.

Round section pipes and trays with curved bottoms have been used but, because of their shape, a nutrient film cannot be established and roots quickly block the flow of nutrient, leading to flooding and waterlogging at an early age. They are not considered further, since they fail to meet the concepts of the NFT system. Other systems have included pumice, rockwool, pea metal, peat and bark placed in the gullies, but these are no longer NFT and again are not considered further.

Design & Layout of Gullies The same design characteristics apply to all conventional NFT gullies. While slopes along the gullies of 1:100 have been recommended, in practice it is difficult to build a base for trays and gullies that is sufficiently true to enable nutrient films to flow without ponding in locally depressed areas. Consequently, it is recommended that slopes of 1:30 to 1:40 are used. This allows for minor irregularities in the surface but, even with these slopes, ponding and waterlogging may occur. The slope may be provided by the floor, or benches or racks may hold the gullies and provide the required slope. Both methods are used and depend on local requirements, often determined by the site and crop requirements.

Sawdust should never be used as a base for gullies, since it breaks down at different rates to provide a most uneven surface. Polystyrene insulating sheets may be used on top of true bases, but will not correct large discrepancies in levels.

Nutrient Flow Rates
As a general guide, flow rates for each gully should be 1 litre per minute. At planting, rates may be half this and the upper limit of 2L/min appears about the maximum. Flow rates beyond these extremes are often associated with nutritional problems.

Length of Gullies
Depressed growth rates of many crops have been observed when gullies exceed 12 metres in length. On rapidly growing crops, tests have indicated that, while oxygen levels remain adequate, nitrogen may be depleted over the length of the gully. Consequently, gully length should not exceed 10-15 metres. In situations where this is not possible, the reductions in growth can be eliminated by placing another nutrient feed half way along the gully and reducing flow rates to 1L/min, through each outlet.

Nutrient Supply Pressure Pipes
Pipes from the pump to the manifolds supplying the gullies, are generally ridged PVC pressure water pipes used in domestic houses. They should be installed to avoid air locks and may be protected from the heat of the sun and insulated if outside the house. Since the solvents in many glues are toxic to plants, allow adequate flushing after gluing, especially when temperatures are below 20°C. Do not plant up until the taste of the glue solvents cannot be detected. Pipe sizes should allow for laminar flow rates within any house or system of manifolds, so that the system is balanced and indivdual manifolds do not vary in their delivery pressures. Ring mains may be used to achieve this on larger establishments. Flow rates and head losses should be determined for each section of the unit.

The nutrient is frequently discharged into a manifold from which narrow tubes of 3-4mm carry it into the gullies. Flow control taps may be placed at the entry of the pressure lines to the manifolds, but can usually be avoided by having a balanced system and a single flow control tap on the pump bypass.

A weak link in many systems is the method used for collection of solution from the gullies for return to the catchment tank. The collection points must be designed so that dirt, dust or water from the house, surface flooding or leaks cannot enter the system. The most satisfactory design is to provide open concreted channels, which are sloped for waste water drainage. The gully drains may be placed in these, raised from the bottom and away from the sides. The size of the collector pipes should be 80mm or more and designed to take the normal flow from the gullies, using open channel flow rates for the fall available, and of sufficient size so that they are never more than 50% full. Once free from the gullies, a dropper of 300mm can be provided and from that point a drain capable of carrying flows calculated for normal pipes can be used.

Entry points for plastic film gullies may be provided by 15x100mm slots cut into the side of the collector pipe, just below the top of the pipe. A seal can be provided with adhesive tape. Growing trays may be discharged into rectangular rainwater collection systems, which are easily sealed.

All drainage systems should be readily dismantled for cleaning. Joints may be sealed with heavy silicone greases made for such applications. Any sections of the drainage system which are underground, should be joined with the correct solvent glue, since water and soil entry cannot be tolerated.

Nutrient Catchment Tanks
Tanks may be constructed of non-toxic plastics, fibreglass or stainless steel. Other unsuitable structures may be lined with plastic. Food grade materials should be used. In most systems the nutrients drain into a catchment tank of a capacity (measured in litres) at least 1 to 4 times the effective crop area of the system (measured in sq. metres). Tanks should be protected with covers and may be housed, with control equipment, in a small shed. The catchment tank should be set in a pit and, unless sub-soil drainage is free, provided with an automatic pump to keep the water table low. If drainage is not adequate, during wet conditions the tank is likely to float out of its hole, damaging connections.

If slopes do not allow this system to operate, smaller catchment tanks can be provided at convenient sites and small automatic pumps used to lift the water to the main nutrient catchment tank and pump.

Nutrient Dosing Systems
‘A’ and ‘B’ nutrients are held in two 100L or 200L plastic drums. These discharge through solenoids directly into the catchment tank. Flow rates must be equal and are adjustable either by cutting the discharge microtube or with suitable taps. pH adjustment may be by air displacement of the acid or alkali, or a small pump or solenoid may be used.

Control Equipment
A range of controllers is available for automatic dosing and control of pH. Some work on 12 volt solenoids, while others use 240 volt systems. When purchasing the solenoids, ensure that the correct wattage and voltage for AC or DC are obtained. Control of units exceeding the rating of the control equipment is possible through a relay.

Operating the NFT System
Nutrient from the A and B tanks is added in equal amounts to the catchment tank, to maintain the correct concentration measured by conductivity. Water enters the tank to replace that lost by transpiration. pH is monitored and corrected, normally by the addition of acid, so the system operates close to pH 6.3. The solution in the catchment tank is circulated through the NFT system and returns, dropping some distance into the tank to assist aeration. The solution is dumped and replaced as unwanted ions, such as sodium, reach unacceptable levels.

Heating the Solution
Plant vigour and root health is enhanced if the solution is heated to 18°C. The temperatures of the solution should normally be run between 15°C and 24°C. Stainless steel electric immersion heaters or heat exchange units may be used.

Alarms & Safeguards
While few growers use alarms, the following are systems where it is desirable to have alarms or safeguards fitted.

1. Dosing system: If not built in, provide timer controls on the solenoids or metering pumps so that dosing cannot be continuous during equipment or control system failures.
2. Conductivity and pH: Provide alarms to warn when acceptable tolerances (say 10%) are exceeded.
3. Leaks: Install a pressure switch (normally on at mains pressure, open when water is entering the system) close to the float valve outlet. A flow restrictor may be required on the mains side of the pressure switch. Wire this pressure switch in series with dosing solenoids so that, if water is continually entering the system, dosing will not occur. No separate alarm is required. Conductivity is lowered and that alarm is triggered.
4. Temperature Control and Heating: Install thermostats to warn of solution and house temperatures above and below requirements.
5. Pump Pressure: Install a pressure switch near the pump to warn of pump failure or large leaks in the pressure system.
6. Power Failures: Install a warning device so that early attention can be provided.

NFT is a hydroponics system that is well established for commercial and domestic short and long term crops, and high yields of quality produce are readily produced by this method. Frequently, problems are encountered when the basic principles of the system’s operation are neglected. These principles are described at the start of this article and it is wise to review them frequently and ensure that the requirements are being achieved requirements are being achieved.