Nutrient Film Technique (NFT)

NFT 1

The Nutrient Film Technique commonly known as NFT, is a type of hydroponics that relies on the flow of a shallow film of the nutrient solution within a tubular or rectangular structure (gully) where the plant roots are partly covered by the solution while the upper parts of the roots are not.

It is common for the cultivation of smaller leafy plants, usually and for the right reasons not suitable for fruit-bearing ones or generally larger plants (we will explain later why…keep reading).

PROSCONS
No need for a timer to operate the pumpSuitable only for smaller leafy plants such as lettuce, kale, herbs etc – Not suitable for larger plants (e.g. fruit bearing) that grow large roots which can clog the pipes
No need for nutrient solution aeration (although beneficial if done)The water pump needs to be running constantly – danger for plant survival if pump stops operating due to power cut or other fault
Nutrient solution is recirculatedNutrient deficiency can happen if the gullies are too long – unless there is a second nutrient feed half way through
No need for growing mediumGully clogging can happen if not regularly monitored and the mass and volume of the roots are too much for the size of the gullies
Harvesting is easy as there is no growing medium and the plant cups are easily removable
Versatile and expandable set-up; you can easily upsize by adding more gullies and feed pipes

The set-up

Gullies

The characteristic of the NFT is that the plants are placed along gullies/channels in holes/openings that can fit the growing cups. You can use gutter/drainage PVC pipes or bespoke flat-bottomed gullies.

Although the tubular PVC pipes can be used successfully and are widely available and inexpensive, they do not allow for a high ratio of water surface area to the volume of water, so the air above the nutrient film is limited for the given volume of water and the area the roots have available to be submerged into is limited. Thus if tubular PVC pipes are used, the nutrient solution volume in the gully needs to be larger and it needs to be aerated.

Unless a flat-bottomed gully is used (there are bespoke products in the market, usually with grooved bottoms along the length) which offer a large water-to-air surface area, more space for the roots to expand and absorb nutrients and better aeration of the roots.

The plants are supported using holding cups / small starter baskets placed in holes/openings on the top part of the gullies. If the gully is wide to take two rows of plants then just consider what the final size of the plant will be before opening the holes.

Length

Keep the length short (no more than 40 ft or around 12 m), or if you go longer then have a second inlet feed half way through the length. The length of the gullies depends on a number of parameters as described below:

  • Weight: remember that the gullies will be holding plants and most importantly will be carrying water which is heavy. So they need to be supported not only on their two ends, but in between as well, in regular intervals depending on their length; the longer it is the more supports it needs.
  • Space: the shorter the space you will have available the shorter your gullies will be. BUT, this gives you the opportunity to “innovate”. You can build the gullies in a slope arrangement, perhaps all of them in one single loop: water pumped to one end of the top gully and runs through to the far end of the bottom one.
  • Nutrient deprivation: having a long gully means that you are likely to have a large number of plants in it, each one absorbing nutrients as the solution flows through. So naturally, the solution nutrient chemistry (concentration to each of the necessary elements and pH) changes from the inlet to the outlet. And that becomes increasingly problematic the larger the plants are having higher feeding needs.

Gradient

The gullies need to have a gradient for water to flow, usually steeper when the plants are larger and roots bigger so that water keeps on flowing. Imagine big roots in a PVC pipe working as a dam restricting the flow of water…Therefore, the set-up needs to allow for adjustable gradient; from around 2.5% to 4%. That is for every 100 feet (or meters) of gully length, the drop needs to be 2.5 ft (or m) when the plants are young and small to 4 ft (or m) when they get bigger.

So in practice as an example, if you have a room of 10 ft (around 3m) available then the gradient should range from 0.25 ft (around 8 cm) to 0.4 ft (around 12 cm).

Growing medium

While all other hydroponic systems, except Aeroponics, use a growing medium which serves the purpose of supporting the plants and allowing moisture and air to be delivered to the roots, NFT has no growing medium (except in the start of the growth cycle to get the seeds started using either starter cubes or very little growing medium in the small starter baskets).

Flow rate

The more or bigger the plants are in a gully and the longer the gully is, the higher the flow rate needs to be. The plants will be consuming water and nutrients, so you want the concentration of nutrients in the solution and pH not to change a lot by the time the solution reaches the last in row plants.

But the flow rate of the inlet needs to be carefully adjusted so that the gullies do not get flooded. The larger the roots are the more likely this is to happen. So although the inlet flow rate needs to be higher than when the plants are small, this can lead to water flooding the gullies due to the resistance the roots pose to the flow. To overcome this, make the gradient steeper so that water gets more momentum in escaping from the far end (see above section on gradient).

Adjustment of the flow rate at the inlets is done with butterfly valves, so ensure that you fit one in each inlet pipe to the gullies.

Pump

An additional control level of the pressure and flow rate is the pump itself. If the pump can be modulated up or down then this allows you to centrally control the flow rate and then with the butterfly valves to the minor adjustments at each gully. The other benefit of being able to control the pump flow rate and pressure is that you can avoid having pipe bursts and leaks, problems that can occur if the pump flow is not adjustable and the pressure too high because of restricted butterfly valves.

Typical Problems

Algae, bacteria and other microorganism growth in the solution. Avoid using transparent solution containers where light can pass through. Light encourages microorganisms to grow which can

  • change the chemistry of the solution
  • reduce the efficiency of water injection through the nozzles (less so with sprayers though) that can drop the pressure and the effectiveness of the solution mist/spray to reach the inner parts of the roots causing dryness
  • can cause root rotting, ultimately reducing yield.

Thirst. Although this set-up is to have the roots constantly in close contact with the solution, ensure that the pump is working and that the flow rate in each gully is the desired one. A quick check is to observe if there is outflow from each gully.

If the gullies are too long and the plants too many, it is likely the flow is restricted and the plants to the end of the gully are not taking as much water as required. In that case, either increase the flow by adjusting the butterfly valve or add a second supply half way. The second option is better as it provides a more balance solution to the plants downstream.

Overwatering. Overwatering could be an issue with NFT in rare situations. If the solution flow rate in the gullies is too much that the gullies are literally full of water and the roots do not have any parts exposed to air, growth rate will be inhibited and roots likely to rot. So it is important to have adjusted the flow rate to the right level.

Nutrient deprivation. As mentioned in earlier section, it can be caused when the gully is too long and the plants too many; so the solution towards the end has an altered chemistry as the first plants consume water and nutrients from it. So either adjust the flow rate or add a second supply half way.

Remember, that if the solution chemistry is out of the expected range of nutrient concentration, pH and EC, then it is time to rebalance it or flush it and add fresh one.

What you will need

You would need to choose the nozzles to be suitable for producing max 50 microns at 100psi pressure.

  • Solution reservoir (opaque)
  • PVC pipes or bespoke flat-bottomed gullies
  • Starter cubes or growing medium (as little to fit in the small cups)
  • Small cups/baskets to hold the plants
  • Water pump (submersible type most likely)
  • Distribution manifold(s)
  • Butterfly valves
  • Tubing from the pump to the manifolds and from there to each gully
  • pH sensor
  • EC (electrical conductivity) sensor (measures the total salts in the solutio

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