Critical Analysis: The Nutrient Film Technique (NFT) for Hydroponics

 Introduction:

The Nutrient Film Technique (NFT) for hydroponic crop production is a conventional method/system that has been widely used by many commercial greenhouse growers for the past few decades. It contributes to significant water and fertilizer usage reduction over field crop production in soil through the recirculation of the irrigation solution as a shallow film (2 to 3 millimeters deep).

The main concept that allows NFT systems to function in creating a favorable root zone environment is the nutrient film itself. By design, a shallow film of water can most efficiently dissolve oxygen by passive diffusion from air, which allows roots to respire and grow healthy within short growth cycles under ideal temperature ranges. In addition, a venturi component is often added to the system to use the pump suction power to pull air bubbles into the solution as it flows through the pump. To function properly, a venturi must be installed on the intake side of the irrigation pump. Though this can achieve efficient oxygenation of the solution at cool temperatures (under 21C), it reduces pump efficiency and in some cases can cause pump cavitation and damage.

Using NFT Systems:

When using an NFT system, growers are able to exert some control over the flow rate and velocity of the constantly recirculating fertilizer solution by adjusting the incline of the troughs (typically 2 to 5%) and controlling the flow through the main valve for each feed manifold. With no individual flow controls to each trough, the common approach to stopping the flow of individual troughs is to clip an unused micro-tube to the nearest trough or connect two together to stop their flow. This potentially makes it a bit of a wet process, and it is advised that users dress for the job. Harvesting usually involves removing one trough at a time and transporting it to the harvesting and processing/packing area, during which troughs are liable to drip over the surrounding plant canopy out one side or the other. Growers using NFT systems need to account for enough space for maneuvering, cleaning, set-up etc. to accommodate whatever length of trough they are using.

Troughs can be bought or made with various plant spacings for different applications, and the plant spacing in a given section of an NFT system would be determined by which troughs were used. It is advisable to block light from any vacant and open planting sites to avoid inhibiting root growth and promoting algal films on the inside surfaces. The inside bottom surface of the troughs must also serve to direct water to the plants’ roots or else they create a high risk of localized dry outs. This is achieved with grooves, ridges, or a slight concave shape. Round pipes are not ideal for use in NFT systems as they do not allow for the creation of a shallow film of solution, and this throws off oxygen solubility dynamics and complicates the system by requiring the addition of net pots and active aeration of the solution. Round pipes seen operating successfully as hydroponic systems are most likely being employed as DFT (deep flow technique) with active aeration, ebb-and-flow, or dry hydroponic systems.

NFT systems can be very effective at growing numerous crops under the right conditions and are only limited physically by the lack of built-in space and structural support for larger plant shoots and roots. Some growers overcome this by employing a nutrient film below containers filled with substrate, simulating aspects of wicking systems and ebb-and-flow sub-irrigation. In said application, however, a nutrient film does not allow for the precision irrigation that deeper flooding and complete drainage that ebb-and-flow irrigation achieves with simpler hardware.

NFT systems can be automated as well, and crops can be made to move through the greenhouse in space as they move through their growth cycle in time. In such cases, robotics are generally used for seeding, transplanting and harvesting, and ideal conditions as well as biosecurity must be strictly maintained as scouting can become virtually impossible without access to the central crop canopy.

In extreme environments such as tropical climates, NFT systems suffer from warm irrigation solution with reduced oxygen solubility, and this contributes to poor root health and subsequently poor nutrient uptake. This causes a shortage of immobile elements, such as calcium, at the growing meristems and in turn causes severe tip burn. The most skilled growers will, in this case, increase air-flow, ramp up environmental control, chill the water, and pulse irrigate the system to allow the roots to breathe. While these measures do help achieve better results, they are energy intensive and hindered by the fact that plants down the troughs receive irrigation solution that has already passed through other plants' roots and been stripped of some of its oxygen and nutrients. Where relative humidities are also high by default, evaporative cooling is ineffective and the nutrient film itself can contribute to excessively high humidity through evaporation at such a high surface area of water exposed to air.

What NFT Looks Like in Nature:

In nature, we do see NFT hydroponics working quite well for those wild plant species that have evolved the ability to have continuously submerged roots, provided that the water is shallow and moving. The shallow flow maximizes natural diffusion of oxygen into the first 2 to 3 millimeters, and the movement ensures that the roots do not deplete the dissolved oxygen around them. Watercress is a great example of such a plant that thrives with submerged roots, as is rice (though rice has a higher tolerance for low dissolved oxygen) and many riverside tree species. In nature, however, you are unlikely to see many of the relatives of our favorite food crops growing in such a way, as they appear to be healthier when given a root zone moisture gradient or at least wet and dry cycles.

Advantages of NFT:

-Systems are light-weight (important in rooftop farms) and can be stacked when circumstances allow

-Systems are modular and easy to deploy and expand

-Many manufacturers supply NFT systems worldwide, making options and forms numerous and costs diverse

-Passively maintain good dissolved oxygen levels under optimal environmental conditions that can sometimes require strong environmental control

-Gaps between troughs can contribute to air flow around the crop

Disadvantages of NFT:

-Limited to short cycle cultivation and small root masses, such as greens, baby-greens, and herb production, unless additional containers are used

-Irrigation and drainage plumbing are relatively complex with more parts per unit than most other systems

-Micro-tubes' small inside diameter causes system to be prone to clogging and localized crop damage

-Pressure compensation across irrigation manifold is challenging and must be monitored to ensure even irrigation

-Algal growth is promoted when planting holes are too large and troughs are open ended

-The nutrient film functions as a perfect heat exchanger, therefore the system lacks thermal stability and this affects dissolved oxygen levels and root health

-Moving and cleaning troughs are not ergonomic processes due to common trough dimensions (long and thin)

-No capacity for precision irrigation, as system is usually constantly recirculating water and wetting plant roots

Conclusions and Future Directions:

It should go without saying that we owe the creators of the Nutrient Film Technique of hydroponic crop production a great deal of gratitude for creating an adoptable root zone management tool that has served to save many farmers a lot of water, fertilizer, and plant protection products. As a system, NFT has provided the capacity for maintaining a root zone free from pathogens and nematodes, and this has contributed to success in growing the high value crops that might normally be especially susceptible to attack when grown in certain soils.

The nutrient film itself is not without weaknesses, unfortunately. As a first generation hydroponic system, however, certain limitations and design flaws have eventually come to light through widespread commercial, residential, and experimental use. The extreme ratio of surface area to volume of the nutrient film recirculating through the troughs not only serves to dissolve oxygen, but also to increase relative humidity in the air around the system. In addition, the shallow film functions as an effective heat exchanger between the nutrient solution and the ambient air. Therefore, if the greenhouse environment is allowed to cool excessively (less than 15 C), then the solution will also be cooled by heat exchange with the ambient air, and this will slow plant enzymatic reactions and thereby overall growth rates for most vegetable and fruit crops. If, however, the greenhouse environment heats up excessively (more than 25 C), then water significantly decreases in its capacity to dissolve gases, including oxygen, and it can become hypoxic despite active aeration. One means to partially alleviate this latter problem in NFT systems is to use micro and nano bubble generators and infusers that can suspend tiny oxygen/air bubbles in the nutrient solution so that the roots are aerated regardless of what dissolved oxygen levels would normally be. This requires additional power usage and reduces pump efficiencies, while the pump also destroys a portion of the suspended bubbles.

Insulation and sealing can allow for nutrient solution conservation and temperature control, affecting dissolved oxygen and enzymatic growth rates positively and helping to reduce the probability of tip burn and bolting symptoms in cold weather vegetable crops. NFT also lacks the ability to irrigate precisely, causing water stress to the roots of some plant species and causing them to adjust their physiology to be more sensitive to drought stress, often resulting in rapid wilting if irrigation is interrupted for more than a few minutes. Removing the incline and allowing the same irrigation solution to arrive to each plant's roots simultaneously would significantly improve the system design. Therefore, it seems likely that the traditional NFT model will evolve to include fewer problematic components, a more sealed and secure root zone, insulation for temperature control, solenoid valves and control systems for precision irrigation, and more ergonomic units that facilitate the greenhouse growing process and make it safer.