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A Trial NFT system

The trial scale NFT system described here allows all of the management practices used in larger scale commercial installations to be used and learnt by experience but without the financial risks involved in running a large scale operation. Only a relatively modest outlay is required to set up the trial system. The trial system contains all the components found in large scale systems. The system is large enough to behave in a very similar manner to a larger scale commercial operation.

Layout of the trial system
The trial installation uses a 200litre polythene drum as it's solution sump and this is large enough to be used with upto 60m long rows of trial plants such as tomatoes, cucumbers or peppers. The system can easily be installed to use a few rows of the crop growing in greenhouse otherwise filled with crop in conventional soil culture. A second hand, clean 200 litres polythene drum is required, with its top cut off. This drum has to be sunk in a pit in the ground, with the top edge just below ground level.

The trial rows of crop plants are grown in 600 mm wide polythene film gullies, which must have a fall of between 1 in 40 and 1 in 100 towards the sump. The cheapest way to do this is to slope the soil along the rows towards the sump. The slope must be accurate and it will be best if some timber boxing is set up to the required fall and loose soil within the boxing screeded to the correct fall and well firmed. If several rows are being used then a length of PVC guttering with a fall towards the sump may be needed across the ends of the rows. The tails of the gullies lie in this gutter to return the nutrient solution to the sump. A piece of gully film is laid over the guttering to exclude light and prevent algal growth. New gully film is water repellent, and nutrient solution flows as aserpentine stream through new gullies, so that some plants placed in the gullies would be missed by the stream. This problem is overcome by laying special 150 mm capillary mat in the gullies. The capillary mat is a strong paper product, often called 'chux cloth' because of its similarity to a domestic cloth with that name.

Water supply.
The volume of solution in the system is kept constant by replacing water transpired by the crop plants with fresh water. The fresh water is supplied to the tank through a water meter, and a Jobe trough valve (model J19B). It is important to use this valve and not an ordinary ball float valve, as they do not contaminate the nutrient solution and will only operate at full flow. The nutrient solution is circulated by a small centrifugal pump. This pump must be corrosion proof and must not contaminate the solution with metals or other toxins. A number of moulded plastic pumps and all stainless steel pumps are available which meet these requirements. The pump must have a capacity of at least 20 litres/minute against 6 m head. A tee and plastic or stainless steel ball valve is fitted to the outlet of the pump, with a length of 20 mm polythene pipe delivering the solution to the head of the gullies. The solution is transferred to the gullies through short lengths (300-400mm) of 3mm polythene tube, joined to the 20mm pipe by screw in Wingfield adaptors or similar. Each gully is supplied with 2 litres of solution per minute. The excess solution flowing from the pump passes back through the tee and ball valve into the sump, the ball valve being used to regulate the flow to the gullies.

A pH meter is required for measuring and adjusting the acidity of the solution, while a conductivity meter is needed for regulating the nutrient content of the solution. Some good quality pocket size meters with sufficient accuracy and reliability for this purpose are available. Samples for measurement can be easily be easily obtained by dipping from the sump or taken from the leader tubes. There is more information in the Nutrition section of GHVI)

Solution heating. Solution heating is very desirable if the trial system is to be operated over the winter months. An electric immersion heating element of 2kW capacity could be installed in the sump for this purpose. The heating element must be encased in stainless steel. Ordinary brass, chrome or nickel plated heating elements contaminate the solution with metals which are toxic to crop plants.

Operation Of The System.
It is necessary to have the water supply analysed well before planting out into the system so that the solution can be made up to suit the water supply.

After the systems is fully assembled with capillary mat laid in the gullies, the sump is filled with water and this water circulated through the system for at least 4 hours or overnight to test and clean the system before it is planted. This water is then pumped to waste, and the plants set out in the system. Plants are best raised in peat pots, paper pots or rockwool cubes for use in NFT systems so that they can be planted without disturbing or damaging the root systems. The roots are able to grow through the bases and sides of peat or paper pots. The media used to fill the pots does not appear to be a critical factor, but it should be reasonably open and well drained and pathogen free. It is not necessary to use inert media such as pumice, vermiculite or perlite. Immediately after planting the sump is again filled with water and equal volumes of A & B stock solutions are added in sufficient quantity to raise the CF of the solution to about 15. If necessary the pH is adjusted to between pH 5.5 and pH 5.9 by adding acid.

The solution is not circulated continuously until the plants have established a root mat in the gullies. Continuous circulation could result in the media in the pots becoming too wet, with ensuing risk of root disease. Too free a water supply at this stage could also result in loss of control over plant growth and development with the risks of bolting and aborting the first truss on tomato crops. The plants need checking three or four times each day depending on the weather and the location and the solution is circulated for 15 to 30 minutes each time they show signs of drying out. The pH and CF of the nutrient solution needs checking each time it is circulated and more A & B solution or acid added if necessary. The frequency of circulation will increase as the plants grow and after a few weeks it will be necessary to circulate the solution continuously. In summer intermittent flow might only be necessary 10 days but several weeks might be needed in winter.

The crop will not necessarily remove nutrients from the solution in the same proportions as they are supplied in the A & B solutions, and certain elements supplied in the water may accumulate in the solution over a period of time. Samples of the nutrient solution should be taken monthly and analysed to discover the extent of these changes in the nutrient solution. Changes in the recipes for the A & B or acid solutions may be recommended from the results of these analyses and calculations of crop nutrient uptake. Crop nutrient requirements ( and especially the N:K ratios) change with crop age and season, and these changing requirements will be recognisable in the results and will allow suitable adjustments made to the recipes.

The Learning Experience.
The small scale trial system behaves exactly like a larger scale crop and thus enables its users to develop all the techniques that will be needed for handling a large scale crop. Experience is gained in making up solutions and in monitoring CF and pH changes and keeping solution management records. The analysis and modifications of the solutions on this small scale, will allow a larger scale crop in the following year to be started with recipes that have already been tuned to the water supply. Some of the equipment purchased for the trial system can be used again in much larger systems.


R.A.J.White October 1999