Putting Nutrient Film Technique to the Test

In the 1980s, nutrient film technique, better known as NFT, was hailed as a major advancement in hydroponic technology.

NFT and Commercial Growers

Many commercial growers quickly put the NFT growing system to the test, but found it had several significant flaws, the primary one being that as plant roots fill the trough, the flow of nutrient solution may be disrupted, followed by the development of pockets of anaerobic conditions within the root mass that could adversely affect plant growth.

Since then, trough design was changed to minimize these effects. Today, NFT is more or less confined to the growing of short-term plants, such as herbs and lettuce.

Earlier this year, I visited a man growing lettuce commercially in a greenhouse complex year-round using the standing aerated hydroponic system. Lettuce plants are fitted into holes in a Styrofoam sheet floated on 9 in. of an oxygen-enhanced nutrient solution. A 40-hp electric pump is required to move the nutrient solution from the storage tank into the pond and keep it circulating. Pure oxygen is introduced through nozzles spaced in the nutrient solution pond.

His lettuce seeds are germinated in pressed peat moss germination cubes. When the seedling roots begin to grow out of the base of the germination cube, the cube is placed into an opening in the Styrofoam sheet, which is then placed on the nutrient solution pond. No additional action is taken until the plants are harvested, usually between five to eight weeks from planting the seed, depending on sunlight conditions during the growing period.

(Thinking of getting into NFT growing? Check out Nutrient Film Technique: Tips for Hobby NFT Growers)

The Nutrient Solution

The nutrient solution is then drawn from a storage tank and circulated through the nutrient solution pond, and then drawn back into the storage tank after passing through a two-stage filter system. Sterilizing the nutrient solution has not been necessary, although it is being considered.

The nutrient solution is assayed about every three days and adjusted to maintain constancy in elemental content and the addition of water to keep the nutrient solution volume constant.

The nutrient solution has been continuously used for more than two years without replacement. This growing system generates high-quality bibb lettuce with the entire plant harvested (including the root) and placed into a clamshell for marketing as a live plant.

Looking for ways to reduce costs, mainly those associated with the electrical requirement to operate the 40-hp water pumps (each nutrient solution pond requires one pump) and the requirement to inject pure oxygen continuously into the nutrient solution pond, the NFT hydroponic growing system was put to use on a trial basis.

It was found that only one 40-hp water pump is required to move the nutrient solution from the storage tank to the head of all the NFT trough arrays. Also, it was not necessary to oxygenate the nutrient solution.

The NFT troughs are 4-in. wide, 1.75-in. deep and 20-ft. long. The spacing between plants in the trough and between troughs is 8 in. The troughs are placed to create a 2% slope from head to foot.

Nutrient solution is dispensed at the head of the trough for 30 seconds followed by a four-minute delay, and then the cycle is repeated continuously day and night. The amount of nutrient solution dispensed in the 30-second time period increases with the age of the plant from initially 0.13 gal. up to a maximum of 0.5 gal., based on the stage of plant growth and sunlight conditions.

The nutrient solution is the same as that used with the aerated-standing method. Also, the same procedure is followed for germinating the seedling for placement in the NFT trough openings as that used for placement in the Styrofoam sheet.

The lettuce plant growth cycle in the trough is the same as if the plants were growing in the aerated nutrient solution pond. However, the lettuce heads growing in the trough are generally tighter and the whorl leaves denser, resulting in a lettuce head weight 15 to 20% greater than the heads growing in the nutrient solution pond.

The reason for this difference and its consistency has yet to be determined. Currently, the lettuce plant is marketed as unit based, not by plant weight. However, the consumer may be able to detect the difference when making a comparison among other sources of hydroponically grown lettuce. The lettuce root characteristics for both growing systems are similar.

Although electrical power costs are less for the NFT system, a power failure or flow restriction that interrupts the introduction of nutrient solution can quickly result in plant wilting, which in turn will adversely affect growth. The result is the possible death of the lettuce plant, particularly if the atmospheric demand is high.

Loss of electrical power has a minimal effect on the lettuce plants growing in the ponds of aerated nutrient solution. What effects the physical characteristics of the NFT system, such as trough size, width, height, length and slope, plus the operating parameters, such as nutrient solution formulation and maintenance, and volume of nutrient solution dispensed and application frequency, has on the rate of plant growth and the physical characteristics of the harvested plant have yet to be thoroughly investigated.

There is much we still need to learn about the NFT growing system to obtain the maximum genetic plant growth potential as well as the production of a quality, commercially viable product to cover the NFT system’s operating costs and generate a financial profit.

(Still wanting to learn more about NFT? We can help. Check out the article, Analyzing Nutrient Film Technique)