What factors are important in setting up an efficient NFT system for different types of plants?
Nutrient film technique (NFT) is a recirculating hydroponic system where nutrient solution flows down a set of channels (also known as gullies). The solution is pumped from a holding tank, through irrigators at the top of every sloping channel and the run-off from the bottom of the channels is returned to the tank. A simple fundamental layout is shown in Figure 1. (Note that this schematic has the pump above the tank, in which case it would need to be a self-priming pump. Usually the pump would either be external to the bottom of the tank—to give a positive suction head; or else, a submersible pump within the tank.)
Plant roots require oxygen in order to respire, that is, to make use of the energy input from photosynthesis. In the process of respiration, plant cells take in oxygen and release carbon dioxide. If roots cannot respire (often because they are waterlogged with water that has run out of dissolved oxygen), they will die.
The thing that is unique about NFT is the fundamental requirement that the plant roots are in a flowing thin film of nutrient solution. The impact of the thin film is twofold. Firstly, some of the roots in the channel will be directly in contact with the air. Secondly, when dissolved oxygen in the water is taken up by the submerged plant roots, this oxygen can be replaced by absorption through the large surface area of thin water film.
A well designed NFT system will never have a problem with a lack of oxygen in the root zone. Bad design will eventually lead to plants dying from lack of oxygen, often resulting in total loss of the crop.
By its nature, a basic principle of NFT is that it is a ‘closed’ recirculating system.
The technique was first developed in the 1960s as a research tool by Dutch researcher H.C.M. de Stigter of the Plant Physiological Research Centre. The commercial potential of this technique was recognised and its further development was led by Dr Allen Cooper of the Glasshouse Crops Research Institute (GCRI) in the UK. NFT development continued over the 1970s and a number of large-scale UK commercial glasshouse growers converted completely to NFT, mainly growing tomatoes and lettuce. However, almost all have now changed to media-based systems, mostly using rockwool or cocopeat.
Since then the technique has spread across the world, especially in hobby-sized systems. However, commercially it is mainly used to grow short-term vegetative crops such as lettuce, herbs and Asian greens. Of commercial hydroponics (soilless culture) around the world, NFT contributes probably only about 3% of the total area, by far the largest proportion of which is in Australia using NFT channels on tables and mobile gully systems.
The basic requirement is that the shape of the cross-section of the channel should allow the solution flow to have a basically flat profile. Consequently, the worst profile is a circular tube; although growers who have nursery channels to get lettuce started before transplanting will sometimes use small circular pipes).
The other aspect of shape is the width of channel. This needs to allow for the size of the root mat of the mature crop intended to be grown in the channel. Typical widths are: 100mm (4 inch) for short-term crops such as lettuce and herbs, etc; 150mm (6 inch) for longer term, but relatively small plants such as strawberries; 200mm (8 inch) or preferably wider, for longer term large crops such as tomatoes. To ensure that water flow contacts small young plants, specialist channel profiles have a dip or small ribs running down the base. For large channels, a narrow strip of capillary matting can be placed across the channel under the new plant.
The GCRI recommendations were initially for a minimum slope of 1 in 100 down the channel, later increased to 1 in 75. Both of these were intended for layflat plastic channels placed on accurately smoothed concrete floors. In Australia, for rigid channels placed as tables on supports we recommend 1 in 40 (or 2.5%) to allow for some sagging between the supports. A general principle is that ‘dead spots’ are to be avoided.
Length interacts with slope, but for a slope of 1 in 40 the usual maximum length recommended is 12m (40ft). For flatter slopes, it is recommended to have a maximum length of only 6m (20 ft).
In Australia, the growing of hydroponic lettuce, herbs and Asian greens is usually done on tables at a convenient working height as shown in Figure 2. The produce is often sold as living plants.
When using wide channels to grow large plants such as tomatoes, a flow rate of up to 2 litres per minute per channel is used. For the 100mm-wide channels growing much smaller plants, a flow rate of about 0.5 litres per minute per channel is used. For steeper slopes, a higher flow rate may be needed. For insurance against blockages, two irrigators are often put into the top of each channel.
The working capacity of the tank determines how much nutrient solution is held per plant. The smaller the volume of solution held per plant, the more unstable the system can become. For systems without automatic pH and EC control, obviously these properties can be affected. However, other major properties to change significantly, whether there is automatic control or not, are solution temperature and nutrient balance, which can be the unrecognised cause of problems. For example, a downside to pH control is that it can add substantial amounts of acid and hence significantly change the solution nutrient balance.
Put another way—to save money by buying a small tank can come back to bite you through solution instability. For small plant systems I recommend 0.5 litre, or preferably more, per plant working capacity; and for large plants, at least 2 litre per plant.
Here is an example of what can happen if the NFT design is bad:
A multi-million dollar hydroponic strawberry investment scheme set up NFT systems as follows: 100mm-wide lettuce channels (better at 150mm), length of channels = 30m (should have been no more than 12m), slope = flat (should have been nearer 1 in 40).
My prognosis was that as the plants matured, especially once they started fruiting, the root oxygen demand would increase, using up the oxygen in the solution. The last plants in the channels would then get oxygen starvation and die, soon leading to the death of all the plants in that system. This is what predictably happened. The company blamed it on herbicide sabotage, but it was simply appallingly bad design. Another poor aspect of their design was that the channels were only raised to knee height, thus negating the major benefit of having a convenient working height. Ω
PH&G October 2014 / Issue 148