Nutrient Film Technique: Tips for Hobby NFT Growers

There are several approaches to hydroponic plant culture that can be applied by both home hobbyists and the commercial grower. Hobbyists striving to produce their own food often focus efforts on short-lived, quick production cycles of leafy annual greens and herbs. While these crops can be successfully grown with most hydroponic production methods, nutrient film technique (NFT) is one of the more common ways home gardeners go about achieving bountiful yields with short turnaround times. The method was popularized in the early seventies in England and has since become a staple of crop production in the growroom and commercial greenhouse alike.

Over my years as a hydroponic enthusiast and farmer, I have delved into this method while trying to produce certain quick-cycle vegetables and herbs with a lessened cost of production and improved quality of crop. When setting up an NFT garden in a home hobby setting, there are certain facets of design growers can incorporate from a commercial agricultural perspective. By mirroring important aspects of a commercial NFT garden, risk can be mitigated while quality of product and return on investment increased. Much of these principles can be considered universal for most hydroponic and greenhouse cultures, but as with any system, NFT must be approached from a specific lens. Certain limiting factors, if not kept in check, become more problematic when growing with NFT.

Precise Delivery of Nutrient Solution

The key advantage of this system is the precise, consistent delivery of dissolved oxygen and nutrient solution in the root zone. A thin mat of solution, or “film,” consistently being fed to the root zone of plants is a key differentiator between this system and something like an ebb and flow sub irrigated garden.

The system relies on a series of troughs, sometimes referred to as gullies or channels, to house plants in cells that vary in size depending on the crop. Depending on the set-up, plants may be suspended in net cups or placed directly in the cell to rest on the channel; in any case, the goal is to allow initial root system capillary wicking access to the film of oxygenated solution running down the trough. Many lettuce and herb growers will propagate in jiffy or stone wool plugs, then directly transplant these seedlings into the main trough for finishing. Troughs are aligned along a header manifold continuously fed nutrient solution from a primary reservoir. The troughs are always sloped on the side of initial solution intake, with run-off being collected and eventually returned to the primary reservoir in a variety of systematic approaches controlled by the grower.

It is important to truly understand all general limiting factors outside of the NFT system when coming at crop production from any commercial standpoint. When all these factors are met, the system can truly thrive. Most hobby growers will first buy or build a system, and then work backwards to implement lighting, air movement, air quality, and other important environmental considerations. Depending on the given crop, most NFT growers will base inputs on several factors such as cost of production and more horticulturally aligned metrics like photosynthetically active radiation (PAR) rating and visible/invisible color spectrum (lighting inputs). When discussing lighting, spectrum of light source is usually measured in nanometers, and should be aligned with the crop grown. The PAR rating refers to the actual amount of energy being used from a light source directly related to photosynthetic production of plant. This unit of measure can also be applied when considering greenhouse glazing material if the NFT set-up is within a greenhouse.

Most commercial growers will also take advantage of environmental monitoring and corrective software that allows for precise control over factors such as air temperature, relative humidity, light intensity, and nutrient quality control/remediation. In a commercial hydroponic crop production, before the actual grow system is even considered, all other limiting factors must be kept in consideration.

In a home approach, most can get by with lighting kits from the hydroponic store. These range from T5 fluorescents and compact fluorescent lamps to specific LED panels made for vegetative growth. In the commercial approach, growers often use similar lighting technology, but they must calculate necessity of light intensity by analyzing crop requirements. It is important for a grower to fully understand the crop they intend to produce and work backwards from the crop’s cultural requirements. How far will a given intensity of nanometers penetrate in a given crop canopy? What is the PAR rating per square foot that a crop needs? Commercial growers also calculate rates of evapotranspiration, photosynthesis, and specific conductive needs of each macro- and micronutrient fed to the given crop. Commercial NFT set-ups factor in air movement (intake, exhaust, filtration, etc.) based off these findings. For example, crops that go through evapotranspiration at a faster rate may require a different fan system with a higher cubic foot per minute (CFM) rating.

Crop Selection and Consistency in NFT Production

As mentioned earlier, most home and commercial NFT production is better suited for quick-grown leafy greens like spinach, kale, chard, head lettuce, and herbs. Commercial crops of peppers, tomatoes, and other horticulture-intensive vegetables are still grown in systems resembling NFT set-ups, though this is usually a hybrid of different technologies like top-fed slabs coupled with NFT principles. Growers should pick easy varieties and strive for consistency within the NFT channels. Growing vastly different crops in terms of essential element requirements with support from the same reservoir is not advised. For example, home growers can get away with having crops like basil, lettuce, arugula, and spinach spread out among different individual troughs aligned with the same primary reservoir but would see poor results growing bell peppers next to oak leaf lettuce.

As tempting as it is to repurpose, buy used, and make due when planning out materials for an NFT system, a commercial system relies on consistency when planning production. Step away from using plastic tables and consider ordering stainless steel NFT supports from a commercial supplier. Use consistent commercial varieties of pipe, hose, and emitter tubing. Avoid trying different emitters on different troughs, or different troughs on different supports. The goal is to ensure all materials outside of central grow components like pumps, reservoirs, and testing equipment are universal and adhere to an obvious system. Have the same number of feeder tubes in every trough (usually two in a commercial set-up). Don’t be the grower who has a plastic picnic table with a PVC pipe next to a rain gutter next to a commercial trough from the local hydroponics store; it never works out.

When considering the purchase of plastic inputs like troughs, reservoirs, and plumbing, aim for commercial hydroponic suppliers who have prefabricated kits available to further capture and emphasize the points made above. If buying these inputs as needed, growers must assume basics like grade of plastic, shape, size, ergonomics, and functionality. Most hydroponic plastics are considered food-grade, but not all food-grade is technically suitable for hydroponics, primarily NFT. Plastics in hydroponics are constantly exposed to fertilizer and other chemical inputs, UV radiation, and other stressors and must be able to maintain rigidity and not threaten or contaminate the crops grown with them. A lot of commercial NFT growers use high-density polyethylene plastics (HDPE) or altered forms of PVC. Check with the given vendor when buying these inputs, as many NFT channels marketed on sites like Alibaba would not be suitable under American or Canadian production standards.

When it comes to trough size and functionality, avoid PVC pipes (unless propagating), conduits, and rain gutters. Not only are the materials not suitable, but the shape of these potential troughs allows for pooling and uneven distribution of water. With a proper square, smooth bottom trough, growers will achieve a larger surface area of film for roots to form cohesion and wick from. A grower should also ensure the correct depth and width of trough is selected for given crop. A leafy green crop like kale may require a larger cubic area for root expansion to avoid clogging troughs, while a short-lived crop like Boston lettuce may do well in a narrower shallower trough.

Nutrient Solution: Delivery and Return

While there is a plethora of approaches the home grower can apply, many commercial NFT producers usually adhere to a couple of fundamental principles regarding nutrient solution intake and return. Growers will usually house the primary feed tank either underground, outside of the actual grow environment, or a combination of both. This makes it easier to maintain proper reservoir temperatures, which are critical for capturing and utilizing dissolved oxygen in the nutrient solution film and atmospheric oxygen (air existing around root system within channel). Many growers will use solution input dosers, which commonly rely on peristaltic pumps or injectors. These dosing systems are configured with probes that continuously monitor crucial data related to temperature, electrical conductivity, and acidity/alkalinity. A freshwater float is also used to maintain a consistent volume of total solution to supply the grower’s predetermined number of cells in the series of troughs.

Many growers will also dose individual elements or groupings of elements by means of a nutrient delivery system. These systems are adaptable and are partly the same as the injectors mentioned above but are usually autonomously connected with other control system software, which monitor all cultural parameters beyond the contents of the reservoir. The solution is commonly returned to the primary feed reservoir through a series of one-inch HDPE plastic pipes. The recycled solution can be set up with additional probes and prebuffered/corrected in terms of nutrient and pH quality prior to return, but many opt to have this happen in the primary reservoir, as the solution is being pumped on an ongoing basis.

Trough Layout, Aeration, and Pumps

To maximize aeration in a commercial NFT system, one should be mindful of a couple of considerations. The length of troughs should not exceed 40 feet because the original dissolved oxygen content created in the primary reservoir starts to deplete and become unavailable to roots beyond that length. As previously emphasized, the system is critical to the delivery of oxygen to the root zone. Roots breathe in oxygen, leading to the transpirational release of water and intake of carbon in leaf stomata. If dissolved oxygen drops off, processes considered significant outside of and connected to the root zone start to crumble.

Many home growers will also think to use an external air pump and air stone, but in a large set-up, this can become costly and logistically problematic for reasons of tubing layout and energy consumption. Given the size of reservoirs usually being employed in the commercial space, air stones usually do not offer up enough equal diffusion. Many commercial NFT growers use one form of bypass aeration or another, usually add-on valves or pumps, to properly address this limiting factor. Simply put, pumps can be aligned with valves that re-release water from primary tubing back into the reservoir. This acts to build up dissolved oxygen via turbulence and actively ensures better recirculation and diffusion of air in the system. The grower ends up with increased oxygen content by harnessing the free atmospheric oxygen available through a splash. One should also ensure the selected pump is magnetic drive or made with some other means of mechanical heat avoidance. Because the NFT system runs continuously, the pump selected must not give off any additional heat to cause temperature imbalances.

The number of plants a grower plans to grow directly affects the size of the reservoir and potential of the pump. A good rule of thumb is to allow one gallon of reservoir potential for every five cells in the system. For example, if a grower is using a 1,000-gallon reservoir, they could get away with supporting 5,000 cells (plants). This ratio will vary by crop and grower; a grower may require more working capacity if plants are growing faster to ensure proper functioning of dosing probes. So, it is important to monitor rates of growth in relation to transpiration and photosynthetic activity.

When measuring oxygen levels, this can be highly contested amongst growers depending on who you talk to. Some growers get away with as low as 5.0 parts per million (ppm) of oxygen, while other system operators, such as aeroponic, claim an oxygen ppm in the hundreds if not thousands. Aim for as much aeration as possible. Another rule of thumb I personally seem to have had success with is basing aeration on total dissolved liters of air per minute, but this is more crucial when using external air pumps. A grower should aim for around one-sixth total volumetric potential (related to size of reservoir in liters) of air. For example, if a reservoir is 1,000 liters, an external air pump rated at 165 liters of air per minute will suffice. This would allow for more than 5.0 ppm of dissolved oxygen.

Other Important Considerations

Growers should also remember the working flow rate per trough when assessing a pump and reservoir. Many commercial producers operate with a flow rate of half a liter per minute per trough. This may seem small, but the intention is to not flood the trough, but rather create a minimal thin film for roots to perform capillary wicking. One can easily determine the number of troughs and work backwards in terms of needed liters per minute or gallons per hour pump output. Many growers will quadruple this capacity and control output flow through in-line valves to address head height issues and height needed by pump.

Slope is also contested amongst growers. Two commercial growers contacted for this article both indicated different slopes, with one recommending one to three per cent and the other recommending three to five per cent. This is something best addressed by the grower given their specific environment. Growers with troughs on the longer side may choose to run with a higher slope over 2.5 per cent, while shorter channels will work with 1.75-2.0 per cent. I usually run the trough slopes in my hobby set-up at two per cent, with each trough receiving just under two liters per minute.

When designing the system, it is crucial to keep everything as consistent and minimalistic as possible. Special attention needs to be applied to oxygen levels in the troughs and in the nutrient solution. Poor oxygen levels can be mitigated through precise monitoring and control of temperature, humidity, trough design and alignment, and the inclusion of other appropriate controls for environmentally limiting factors of growth.