Understanding and Using NFT Hydroponic Systems
Developed more than 50 years ago, NFT systems can quickly produce large yields of produce with minimum water usage. However, these systems can present problems — for both home and commercial growers — if they aren’t monitored properly.
Nutrient film technique (NFT) was the world’s first method of crop production that didn’t use a solid rooting medium. Developed in the 1960s by Dr. Allen Cooper at the Glasshouse Crops Research Institute in England, the system was initially met with skepticism until studies proved NFT was not only viable but had considerable potential for many crops. Nowadays, NFT systems have gained worldwide acceptance, mostly in producing small-framed, rapid-turnover crops such as lettuce, herbs, strawberries, green vegetables, fodder, and micro greens, although longer-term plants like tomatoes will grow in these systems in larger channel sizes. NFT allows for producing ‘living greens’ crops — those harvested and sold with roots intact and wrapped in plastic sleeves to prolong shelf life post-harvest. NFT, being a solution culture method often captures the imagination of many indoor and hobby gardeners, as well as those planning a commercial venture as a method of growing plants with minimal requirements with efficient water usage. However, NFT is a system that needs to be well understood for maximum growth rates and yields with minimal problems.
The Principals of NFT
As the name suggests, NFT is a system of solution culture where a thin film (two to three millimeters depth) continually flows along the base of small channels in which the root systems sit. Channels are often constructed of rigid materials such as high-density polyethylene, specifically designed and manufactured for hydroponic crop production, or formed from thick plastic film which is folded up to form a triangular shaped channel. With NFT, the objective is that part of the developing root mat is in the nutrient flow, but many of the other roots are sitting up above this in the moist air, accessing oxygen without being submerged. Seedlings or young plants in NFT are typically supported by either propagation cubes of stonewool, oasis or coconut fiber, or in small lattice pots which slot into the top surface of the growing channels. A central nutrient reservoir holds the nutrient solution and from there it is pumped up into the growing channels, flows past the root systems, and is piped back to the reservoir for monitoring and adjustment either manually or automatically via electronic doser units.
One of the main advantages of NFT, apart from the cost and labor savings of not requiring large volumes of a growing substrate, is it allows production of salad greens, lettuce, and herbs without the risk of grit contamination which can occur in soilless substrate systems. NFT systems may incorporate heating or cooling of the nutrient solution as is required for year-round cropping and allow a very ‘clean’ system to be run, with sterilization of equipment between crops if necessary.
The main disadvantage of NFT is that power outages, which halt the continual flow of nutrient, can quickly result in crop death, particularly under warm growing conditions.
Flexibility of NFT systems
NFT systems may seem more complex than substrate-based types of hydroponics, however, they are extremely flexible with regards to size, design, and equipment. A small basic system can comprise a single growing channel for production of a few plants, a plastic bucket or bin for a nutrient reservoir, and a small aquarium pump to circulate the nutrient solution. NFT growing channels with a flat base profile can be purchased, however, over the decades, many innovative materials have been used where these are not available. These include rainwater downpipes, large diameter bamboo, channels formed from timber, metal, or concrete and lined with plastic and long rolls of plastic film itself folded into channels. These types of basic NFT systems require manual checking and adjustment of the electrical conductivity (EC) and pH of the nutrient solution, and regular water level adjustments.
Commercial NFT systems are typically far more automated than hobby systems and make use of electronic and computer-controlled EC, pH adjustment, and float valves to replace water used in the system. Advanced commercial NFT systems may also utilize dissolved oxygen and solution temperature measurements to monitor these root zone variables. An NFT-based growing operation often has both seedling and finishing NFT benches through which the plants progress during production. Seedling systems have plants at a much higher density, which are moved to a wider spacing in the finishing system to maximize the growing area inside greenhouses. Other NFT systems include moveable channel systems where the distance between adjacent channels is increased as the plants grow. Vertical NFT systems are also popular and optimize growing space for lower-light crops such as lettuce and some herbs. NFT systems can also be installed outdoors or under basic shade covers.
Important NFT Factors to Consider
Constructing and running a successful NFT system is reliant on a few vital factors. These include slope, flow rate, nutrient reservoir size, channel volume, and pump capacity. For those purchasing an NFT system kitset, choose a reliable supplier with a good reputation and these variables will already have been factored into the design. For home-built systems, some consideration needs to be given to all components and equipment. The channels must be positioned on a slope so the nutrient solution flows steadily down at a good flow rate, which for most systems is around one liter/minute. Some systems allow the channel slope to be adjusted and increased as the root system develops and expands inside the channels, ensuring nutrient ponding does not occur. The dimensions of the NFT channels are dependant on the type of plants being grown. Small lettuce and herbs may be grown in standard NFT channels of four inches wide and two inches high made from UV stabilized FDA approved high density polyethylene. Larger plants like cucumbers and tomatoes require a wider and taller channel to accommodate their considerably larger root system.
Reservoir size is also important and many smaller NFT systems are set up with undersized nutrient tanks. A buffer capacity volume of at least 50 per cent is a good rule when selecting a reservoir. This means when all the channels are carrying nutrient solution at the desired depth, sufficient nutrient remains in the tank to half fill it or the tank is large enough to hold twice the amount of nutrient in the gullies. This is considered a minimum and many systems have larger tanks than this, as a greater volume slows the rate of change in EC, pH, and temperature in the reservoir nutrient solution.
NFT pumps are pieces of equipment that are often misjudged and don’t provide the capacity required. Not only is it important to consider the capacity of the pump in terms of liters or gallons per hour, but also the head height which is the pump’s ability to move nutrient solution up to a certain height (i.e. the top of the NFT system). Most NFT systems use centrifugal pumps capable of producing constant flow of nutrient solution. These pumps will either pump a small volume of nutrient solution to a high head, or a large volume of nutrient to a low head height. In between the two extremes the characteristics of the pump will vary and ‘pump performance’ curves should be used to select the pump with the correct head height verses flow rate characteristics for a particular NFT system.
Managing NFT Nutrient Solutions
NFT systems continually recirculate nutrient solution, thus they are more prone to nutrient imbalances, build up of unwanted elements, and changes in nutrient ratios over time as compared to ‘drain to waste’ substrate-based systems. For this reason, a high-quality, low-mineral water supply is required that doesn’t contain high levels of unwanted elements, like sodium, which may accumulate over time. Where plant usable elements such as calcium, magnesium, and some trace elements are present in the water supply, these can be adjusted with a customized nutrient solution. A build-up of unwanted minerals or a significant change in the nutrient balance over time requires the NFT solution to be regularly changed either partially or completely to maintain good growth rates. NFT systems also require regular monitoring and adjustment of pH and EC as changes in these can occur rapidly with some crops. For smaller systems, once-daily checking and adjustment is usually sufficient for manually monitored systems. For systems automatically adjusting EC and pH, the system still requires occasional manual checks as a backup to equipment failures and errors.
NFT systems, like all hydroponic methods, are not without their potential problems. While disease outbreaks do sometimes occur in NFT, as they can in all hydroponic systems, careful management and following the principals of NFT with a high rate of oxygenation, minimal flow depth, and solution temperature control all assist with minimizing these issues. An area of increasing research and understanding with NFT, particularly with the relatively new development of organic NFT systems, is the role that microbial populations, present in the root system, growing channels and nutrient solutions, play in disease prevention and crop health. Beneficial microbes develop in NFT systems just like in soil and soilless substrates. Growers may choose to increase these microbes via slow sand filtration or the use of inoculant products.
Nutrient management is also an area of potential problems as it is possible for a rapidly growing crop to strip out an essential element from the recirculating system (often nitrogen or potassium) faster then it is being replenished via stock solution additions. Use of nutrient solution analysis is a common practice with commercial growers; it allows rapid adjustment of the recirculating solution as required so imbalances are prevented. Growth may be slowed when NFT solutions start to get out of balance and may go unnoticed until a severe and visible deficiency becomes obvious, so being proactive with solution monitoring and adjustment for larger growers is also recommended. Smaller growers who don’t want to pay for solution analysis testing can simply carry out regular partial or complete solution changes to ensure all nutrient elements remain in balance.
NFT is a great learning tool for students and inexperienced growers as it allows the root system to be viewed as plants develop and allows for gaining skills in nutrient control and running an efficient system. While NFT can seem complicated with the equipment required, a basic system can be easy and quick to set up with the added benefit of not requiring large volumes of substrate to be regularly brought in and disposed of.
Written by Lynette Morgan | Author, Partner at SUNTEC International Hydroponic Consultants
Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette has authored five hydroponic technical books and is working on her sixth.